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-154 ,,I~+ When perchloric acid solutions of Zr'" are diluted from IZr'"l= lo-' M to [Z?l c lom4M and the 'HClO, concentration is increased from 0.5 M to 2-4M, [Zr4(OHj8(H;0)16]8+ is converted to the mononuclear ion, presumably [Zr(Hz0),]4+. The kinetics of this process, abbreviated Zr4+Zr, follow the simple rate law k,[H+][Zr4]; at ionic strength 2.0M (NaC104): kH(25"C) = 0.95 X MI' s-l, AH* = 55.2 kJ mol-', and AS*= -117 f 8 J K-l mol-'. Acids H X , such as H3P04, HZCzO4 and HF, also induce Zr4+ Zr conversion, giving the rate law kH[H+][Zr4] kHx[HX][Zr4].'56 X-Ray diffraction studies have established the presence of coordinated water molecules and bridging hydroxo ligands in normal and basic Zr (Hf) salts and in certain neutral and anionic complexes. Water- and hydroxo-containing compounds that consist of discrete complexes are listed in Table 6. Averaged metal-oxygen bond lengths to coordinated water molecules and bridging hydroxo ligands are in the ranges 2.20-2.29 and 2.10-2.14 A, respectively. Structures of these complexes, mostly dodecahedral, are discussed in later sections dealing with the other ligands.
+
Table 6 Structural Data for Aqua and Hydxoxo Complexes of Zircanium(1V) and Hafnium(IV)
Structural formula
Coordination polyhedron a
DD DD DD DD
DD DD DD BTP DD DD DD
PBP DD
Averaged bond length (A) Zr-OH, Zr-OH
2.272 2.26 2.196 2.28 2.21 2.24 2.23 2.29 2.22
2.142
1 2 3 4 5 6 7
a
2.10 2.11 2.283 2.271 2.27
Re5
9 10 11 12 13 14
DD = dodecahedron;BTP = bicapped trigonal prism; PBP = pentagonal bipyramid. 1. T. C. W. Mak, Can. J. Chern., 1%8,46,3491. 2. D. L. Rogachev, L.M. Dikareva, V. Ya. Kuznetsov and G. G. Sadikov, 3. Struct. Chem. (Engl. Transl.), 1981,22, 470. 3. D. L. Rogachev, L. M. Dikareva, V. Ya. Kuznetsov, V. V. Fomenko and M .A. Porai-Koshits, J . Sfrucr. Chem. (Engl. Tram/.), 1982, 23, 765. 4. D. L. Rogachev, V. Ya. Kumetsov, L. M. Dikareva, G. G. Sadikov and M. A. Potai-Koshits, J . Struct. Chem. (EngZ. T r u d . ) , 1980. 21, 118. 5 . I. J. Bear and W. G . Mumme, Acta Crystallogr., Sect. B, €969,25, 1566. 6. I. I. Bear and W. G. Mumme, Acta Crystallogr., Sect. B, 1969, 25, 1572. 7. I. J. Bear and W. G. Mumme, Acra Crystallogr.. Sect. B, 1969, 25, 1558. 8. W. G. Mumme, Acta Crystallogr., Sect. B , 1971, 21, 1373. 9. M. A. Porai-Koshits, V. I. Sokol and V. N. Vorotnikova,J. Sfrucr. Chcm. (Engl. Trans/.), 1972, W, 815. 10. Yu. E. Gorbunova,V. G . Kuznetsov and E. S. Kovaleva, 1. Sfruct. Chem. (Engl. Transl.), 1968,9,815. 11. A. Clearfield, Inorg. Ckim. Acta, 1970,4,166. 12. F. Gabela, B. Kojic-Prodic, M. Sljukic and 2. Ruzic-Toros,Acta Crystullogr., Sect. B, 1977, 33, 3733. 13. R. L. Davidovich, M. A. Medkov, V. B. Timchenko and B. V. Bukvetskii, BuZl. Acnd. Sci. USSR,Diu. Chem. Sci., 1983, 2181. 14. A. I. Pozhidaev, M. A. Porai-Koshits and T. N. Polynova, J . Struct. Chem. (Engl. Transl.), 1974, 15, 548.
a
Very little X-ray data is available for zirconium or hafnium complexes that contain oxo ligands. The only X-ray evidence for the existence of the zirconyl or hafnyl group, M=Ozc, comes from a structural study of KzZr03.157This compound contains chains of ZrO pyramids that share basal edges. The Zr-0 distance to the apical oxygen atom (1.92 ) is appreciably shorter than the sum of ionic radii (2.20 A), the sum of the covalent radii (2.22 A)
386
Zirconium and Hafnium
and the Zr-0 distances to the four basal, bridging oxygen atoms (2.13 A). In his 1973 review of the structural chemistry of zirconium compounds, MacDermott3 identifies the apical Zr-0 bond in K2Zr03 as the only probable example of a zirconium-oxygen double bond yet reported. He adds, 'There is no doubt that the 'zirconyl' group as a persistent species in solution is mythical.' Planalp and Ander~on'~'have recently reported the structure of [{ZrME[N(SiMe3)2]2}20], a centrosymmetric oxo-bridged dinuclear complex having a linear Zr-0-Zr bridge and a short Zr-0 bond distance (1.950( 1)A). p-Oxo compounds of the type [ { MX(q -C5H5)2},O] (M-Zr or Hf; X=C1, Me or SPh) are common in organometallic chemistry.14 These compounds exhibit a characteristic IR band associated with the M-0-M unit at -750800 cm-' and M-0 bond distances in the neighborhood of 1.95 A. The Zr-0 bond lengths (average 1.959(3) A) in the cyclic trinuclear complex [{ZrO(g-C5H5)2)3] are similar to those in the dinuciear complexes, indicative of an appreciable amount of zlrconium-oxygen dn-pn bonding. lS9 Numerous attempts have been made to identify the zirconyl or hafnyl group in solid compounds on the basis of vibrational spectroscopy. Barraclough, Lewis and Nyholmlm found that compounds known to have an M=O bond exhibit a strong, sharp IR band in the region 900-1100 cm-' while polymers with an -M-0-M-0chain structure give a broad band at somewhat lower frequency. Kharitonov and Zaitsev161 suggested the range -800-1000 cm-l for ~ ( Z F O ) and v(Hf==O), and noted that d(M0H) deformation modes can give rise to strong bands in the same region, Thus, study of deuteriated compounds is necessary in order to avoid incorrect assignments. A case in point is ZrOC12-4H20.This compound exhibits a sharp IR band at 918 cm-l, assigned to Y(Z-0) by Spasibenko and Goroshchenko.162 However, Powers and Gray155showed that the 918 cm-' band is displaced to 697 cm-l in the spectrum of ZrOCl2-4Dz0,and therefore it cannot be assigned to the zirconyl group. The IR spectrum of anhydrous Zr0Cl2, prepared by reaction of ZrCld with ClzO, exhibits an intense band at 877 cm-l and a broad band at 538 cm-'. The 877 cm-' band was assigned to the Zr02+ cation, and the 538cm-' band to the -Zr-O-Zr-% chain of a polymeric (ZrOClq-}, anion.163The same bands have been observed in the IR spectrum of an amorphous form of anhydrous ZrOClz obtained from the reaction of ZrC14 with Sb203.1a On the other hand, anhydrous ZrOClz prepared by (i) thermal decomposition of Zr20C16.4MeCN'65or (ii) partial dehydration of ZrOC12.8H20 with thionyl chloride followed by heating in vacuo at 200 'C1& showed no bands that could be assigned to the zirconyl group. Several oxochloro complexes of zirconium have been reported. These compounds are listed in Table 7 along with frequencies of IR bands that have been assigned to zirconium-oxygen stretching modes. The acetonitrile and dioxane adducts of Zr20C16, [ZrzOCls]2- and [Zr20Cf10]2- are believed to contain Zr-0-Zr bridging units. The strong, sharp band at group, 1104cm-' in the spectrum of Zr20Cb.4POC13 has been cited as evidence for a Z-0 and both ZFO groups and -Zr-&Zr-Ochains have been suggested for (C5H5NH)2ZrOC14.
Table 7 Zirconiurn4xygen Stretching Frequencies for Oxo-Chloro Complexes of Zirconium(1V)
Compound Zr,OCI,.4Me.CN ZrzOCl,~5C,H,0z Zr,OCl6~4POCl, [NOl*[Zr2OC18l [NEt414[Zr*O%I (C5H,NH),ZrOCl4*
Y(ZQ
(cm-')
800 740 1104 605, 584
750 1O00, 770
Ref.
1 1
1 2 3 4
Isolated from methanol. Another form of (C,W,NH)3ZflCI,, isolated from carbon tetrachloride, shows a v(Zr0) band at -920cm- (ref. 5). 1. A. Feltz, 2. Anorg. Allg. Chem., 1965, 335, 304. 2. H. Prim, K. Dehnicke and U.Muller, Z.Anorg. Aflg. Chem., 1982.448,49. 3. A. Feltz, Z. Anorg. Allg. Chem., 1968, 358, 21. 4. G. M. Toptygha, I. B. Barskaya and I. 2. Babievskaya, Rrrss. 1. Inorg. Chem. (Engl. TrmZ.), 1972, 17, 1104. 5 . G . M. Toptygim, I. B. Barskaya and I. Z . Babievskaya, Russ. J . Inorg. Chem. (Engl. Transl.), 1971, 16, 686.
Zirconium and Hafnium
387
Kharitonov and coworkers have presented IR evidence for the existence of the M=O group in two classes of zirconyl (hafnyl) compounds: (i) oxyfluorides such as ZrOF2, KZrOF3-2H20, Zr4F1003, HfOF2, Hf4FI4O and Hf4F1202;'B7 (ii) isothiocyanates such as A[ZrO(NCS)3(H~O)].H20 (A = NH4, C5H5NH, K, Rb or Cs), A[HfO(NCS)3(H20)]-H20 However, Selbin" has been (A = C5H5NH or Cs) and [CSH~NH]~[(H~O)Z(NCS)~]~H~O.~~~~ unable to duplicate the oxyfluoride work and ClearfieldS2has suggested that the intense, but rather broad, IR band reported for the oxyfluorides in the region 833-896cm-' could be assigned to an M-0-M vibration instead of an M=O mode. The isothiocyanate complexes display a sharp, rather intense band at 913-927 cm-' (M = Zr) or 934-940 cm-' (M = Hf),99,'oo and this has been confirmed by Selbin53 for A[ZrO(NCS)3(H20)].H20 (A = Cs or NMe4). However, the corresponding deuteriated compounds have not been studied, and therefore it is possible that the 913-927 and 934-940 cm-' band arises from a S(M0H) vibration rather than a v(M=O) mode.16' Lipatova and Semenova'6Rhave reported that this band disappears when CS[MO(NCS)~(H~O)].H~O (M = Zr or Hf) is dissolved in acetone, and reappears when the solvent is evaporated. This observation was interpreted in terms of a reversible hydrolysis of the M=O group of the solid compound, giving an M(OH)2 moiety in solution. The mode of attachment of the oxygen atom in the A[MO(NCS),(H,O)].H,O complexes remains uncertain, and X-ray studies are needed to establish the structure of these compounds. The zirconyl group has also been said to be present in oxo zirconium(1V) complexes of the type [ZrOX2L] (X=Cl, Br, NO3 or NCS), [ZrOL4]12, and [ZrOL6][C1O4I2,where L is an oxygen donor ligand such as diphenyl s ~ l f o x i d e , ' ~ ~tetramethylene ~''~ s~lfoxide,'~'pyridine N-oxide,17' 2-methylpyridine N - o ~ i d e , '2,6-lutidine ~~ N - o ~ i d e , 'triphenylphosphine ~~ and in the analogous 2,2'-bipyridine 1,l'-dioxide complexes or hexamethylpho~phoramide'~~ [ZrOXz(bipyO2)j*2H20 (X= C1, Br, NO3 or NCS), [ZrO(bipyOz)z]12-2HZ0 and [ZrO(bipy02]3[C104]2.2H20.17h,277 In general, the chloro, bromo, nitrato and isothiocyanato complexes are monomeric nonelectrolytes in ionizing organic solvents, while the iodide and perchlorate compounds behave as 1:2 electrolytes. IR spectra of these compounds show that (i) the nitrate ligands are bidentate, (ii) the NCS ligands are attached to zirconium through the nitrogen atom, and (iii) the perchlorate compounds contain uncoordinated C10, ions. A weak IR band in the 900-980cm-' region has been assigned to the v(Zr=O) mode, but the low intensity of this band argues against such an assignment. On the other hand, the molecular weight and conductance data are consistent with mononuclear structures that contain a Zr=O group. Unfortunately, no X-ray structural data are available. Other compounds of unknown structure that may possibly contain a Z-0 group include oxozirconium(1V) complexes with N-heterocyclic ligands (Section 32.4.2.2), tridentate Schiff bases (Section 32.4.9.1.i) and 1,lO-phenanthroline mono-N-oxide (Section 32.4.9.3). Previous reviewers 51-54 have maintained a healthy skepticism concerning the existence of the Zr=02+ and Hf==Oz+ions. There is no evidence for the existence of these species in aqueous solution and IR evidence for the presence of Z-0 and H F - 0 groups in solid compounds is equivocal. These structural units appear to be rare, and only X-ray studies can provide really convincing evidence for their existence. Relatively insoluble, high-melting oxometal complexes are likely to have polymeric structures based on -M--0-M-O chains. IR bands assignable to v(Zr--0-Zr) vibrations suggest structures of this type for Zr0(02CR)2.nRC02H(R = Me, Et, Pr, CH2CI or Ph),17' Zr0(02SeC6H4X)2 (X = 4-C1, 3-C1 or 3-Br), and ZrO(OzSeC6H4-3-N02)2.2H20.17y The carboxylato complexes exhibit a broad band of medium intensity in the 800-865 cm-' region, and the benzeneseleninato complexes show a medium or strong band at 739-750 cm-I . 32.4.4.2 Peroxide and hydroperoxide
A variety of peroxo and hydroperoxo complexes of zirconium(1V) and hafnium(1V) have been isolated from aqueous or aqueous methanolic hydrogen peroxide solutions that contain additional ligands such as sulfate, oxalate or fluoride. Examples of recently reported complexes are listed in Table 8 dong with characteristic vibrational frequencies and the pH employed in the aqueous preparations. Earlier work on peroxo compounds has been reviewed by Cannor and Ebsworth'80 and by L a r ~ e n . ~ The peroxo complexes are white solids, soluble in mineral acids, but insoluble in common organic solvents. They are relatively stable at room temperature; however, they lose active oxygen at elevated temperatures. Several of the compounds in Table 8 were isolated with
Zirconium and Hafnium
388
Table 8 IR and Raman Data and Synthesis Conditions (pH) for Peroxo and Hydroperoxo Complexes of Zirconiurn(1V) and Hafnium(1V)
Compound IR
R IR R 1R IR
IR IR R IR
865111,84Ovw 872s, 840s 862111 867s 839w
600s 4%, bd
500w 4955 555w
85Om
M I S
84Om 850s 850m
5%
7
4 5, 6 1
7
590s
536111
850s
538m
IR
87Ovw 867vs, 846s 862w
500m
8%
7
6oas
2-3
8
6-7
8
5-6
8
6-7
8
7-8
5-6
8 R
1
8
5655
R
84Ovw, 8 4 5 ~ 861111, 84Ovs 844m 837vs
IR
835m
R
838vs
IR
84om
IR
2.3
5, I
R
R lR K
0.2-0.3
536m
850s
IR
1
840
R IR R
0.2-0.3
528m
1. G. V. Jere and G. D. Gupta, J . Inorg. Nucl. Chem., 1970, 32, 537. 2. G. D. Gupta and G. V. Jere, Indian I . Chem., 1968,654. 3. G . D. Gupta and G. V. Jere, Indian 3. Chem., 1972, 10, 102. 4. W. P . Griffith and T. D. Wickins, J . Chem. Soc. (A), 1968, 397. 5. M. T. Santhamma and G. V. Jere, Synrh. React. Inorg. Metal-Org. Chem., 1977,7, 413. 6 . G. V. Jere, G. D. Gupta, V. Raman and M. T. Santhamma, Indian .IChem., . Sect. A , 1978, 16, 435. 7. G .V. Jere and M. T. Santhamma, Inorg. Chim. Acta, 1977, 24, 57. 8. S. 0. Gerasimova,Yu. Ya. Kharitonov, S. A. Polishchuk and L. M. Avkhutskii,Sow. J . Coord. Chem. (Engl. Trams[.),1981. 7, 267.
variable amounts of lattice-held water of crystallization. The distinction between lattice-held and coordinated water molecules was made on the basis of therrnogravimetric analysis. These complexes exhibit characteristic IR and Raman bands in the regions 840-880 and 500-600 cm-' (Table 8). The band at 840-880crn-', observed in both IR and Raman spectra, has been assigned to the 0-0 stretching mode of the bidentate peroxo ligand in the triangularly linked MOz group (20). 181-184 Two bands in the 500-600cm-1 region have been attributed to the symmetric and asymmetric metal-oxygen stretching vibrations, v,(M02) and vas(MOZ), although this assignment is somewhat uncertain for the peroxofluoro complexes owing to the presence of v(M-F) modes in the same frequency region, 181,185
Jere and Gupta"* have reported two ~ ( 0 - 0 )bands for [Zr2(02)3(S04)(H20)4]~(4-6) a band at -870 cm-', fairly strong in both IR and Raman spectra, which has been assigned to a triangularly linked bidentate peroxo ligand, and a band at MOcm-', strong in the Raman spectrum but very weak in the IR, which was attributed to a bridging peroxo group. In principle, one should be able to distinguish these two modes of attachment since ~(0-0) of a triangular M 0 2 group is both IR and Raman active, while v ( G 0 ) of an M-0-0-M group is active in the Raman spectrum but inactive (Ci syrnmetr ) or only weakly allowed ( C , symmetry) in the IR spectrum. However, Gerasimova et ~ 1 .1' have ~ noted that ~ ( 0 - 0 )of a
Zirconium and Hafnium
389
triangular MO2 group may also be very weak in the IR, and thus distinguishing the two modes of peroxide attachment may be problematic. Structures have been suggested for some of the peroxo complexes on the basis of the vibrational spectra.18Z-184However, no X-ray structures are yet available. The hydroperoxo complexes K2MF5(02H) (M = Zr or Hf) may be distinguished from the peroxo complexes on the basis of a unique IR band of moderate intensity at - 1 4 5 0 cm-*. This band has been assigned to S(O2H).'*' X-Ray powder patterns indicate that [NH4I3[MF6(02H)] and [NH4I3[MF7](M = Zr or Hf) are isostructural. Since [MF7I3-has a pentagonal bipyramidal structure , the [m6( O,H)] ions may be assigned an analogous seven-coordinate structure, with the OzH- group behaving as a monodentate ligand.185 324.43 Alkoxitks, triulkysilyloxides and ary loxides
Metal alkoxides have been extensively reviewed by Bradley61~'Pb188 and by Mehrotra.'@-I9l Zirconium and hafnium tetraalkoxides may be prepared by (i) reaction of alcohols with the anhydrous metal chlorides or pyridinium hexachlorometallates in the presence of ammonia (equations 26 and 27), (ii) alcoholysis or alcohol interchange (equation 28), (iii) transesterifications (equation 29) or (iv) reaction of alcohols with metal dialkylamides or metal acetylacetonates (equations 30 and 31). A preferred method for preparation of many M(OR), (M = Zr or Hf) compounds in a high state of purity involves alcoholysis of M(OCHMe2)4.HOCHMe2in benzene solution; the isopropanol solvate can be purified by recrystallization from isopropanol, and the isopropanol produced in the alcoholysis is conveniently removed by fractional distillation of the benzene-isopropanol azeotrope. This approach, however, is inefficient for synthesis of the f-butoxides because of the small (-0.2 "C) difference in boiling points for isopropanol and t-butanol. The t-butoxides are better prepared from the tetrakis(diethy1amide) (equation 30) or by transesterification between M(OCHMe2)4-HOCHMe2and t-butyl acetate.
---
+ 4 R O H + 4NH3 [C,H,NH],[MCl,] + 4 R O H + 6NH3 M(OR), + 4R'OH M(OR)4 + 4MeCOOR' Zr(NEt,), + 4Me3COH Zr(acac), + 4ROH MCl,
M(OR), + 4 N w C 1 M(OR), + 6NH4Cl + 2CSH5N M(OR'), + 4 R O H
(26)
(27)
+ 4MeCOOR Zr(QCMe,), + 4Et2NH M(OR'),
Zr(OR),
+ 4Hacac
The reaction of alcohols with ZrCL, in the absence of ammonia yields chloride alkoxides ZrCl3(0R).nROH and ZrC12(OR)2*nROH.Mixed alkoxides such as Zr(OR)(OCMe3)3 and Zr(OR)2(0CMe3)2(R = Me or Et) have been obtained from alcoholysis reactions between (i) Zr(OR), and t-butanol or (ii) Zr(OCMe3)4 and ROH. The monomethoxy complex is dimeric, [{Zr(OMe)(OCMe3)3}2], and probably has a methoxy-bridged structure. Boiling points and molecular association data for the tetraalkoxides are listed in Table 9. The methoxides are insoluble, polymeric solids of low volatility; Zr(OMe), sublimes in a molecular still at 280°C. Ti(OMe), is a tetramer, but the structures of Zr(OMe), and Hf(OMe)* remain unknown. Most of the other tetraalkoxides are volatile liquids, which are soluble in organic solvents. Two forms of zirconium is0 ro oxide (liquid b.p. 160 OC/O.l mmHg and crystalline m.p. 135 "C) have been described. 92 For tetraalkoxides that contain the same number of carbon atoms, the boiling point and the degree of association (n) in boiling benzene decrease with increasing branching of the alkyl chain. Thus, in the case of the eight isomeric zirconium amyloxides, the primary amyloxides have values of n in the range 3.7-2.4, while the secondary amyloxides are dimers and the tertiary amyloxide Zr(OCMe2Et), is a monomer. The tertiary alkoxides are monomeric because steric hindrance prevents alkoxide bridging, which causes oligomerization of the primary and secondary alkoxides. Careful vapor pressure measurements on M(OCMe3), and M(OCMeZEt), have shown that volatility increases as M varies in the order Ti < Zr < Hf; the increase in volatility with increasing molecular weight has been attributed to an entropy effect. 193,194 Ti(OCMe,Et), and Hf(OCMe,Et), have been separated by gas chromatography, but attempts to separate Zr(OCMe,Et), and Hf(OCMe,Et), were unsuccessful. 'H NMR spectra of the trimeric isopropoxides M(OCHMe2)* (M = Zr or Hf) indicate rapid exchange of terminal and bridging isopropoxide groups; 'HNMR spectra of mixtures of
P P
390
Zirconium and Hafnium
Table 9 Boiling Points and Degree of Association for Zirconium(1V) and Hafnium(1V) Alkoxides and Trialkylsilyloxides B.p.
Compound Zr(OMe), Zr(OEt),
wow4 WOCHM%)4 WOBu), Zr(OCHMeEt), Zr(OCMc3), Zr(OCH2CH2CH2CH2Me), Zr(OCH,CH,CHMe,), Zr(OCH,CHMeEt), Zr(@-3v3fe3)4 Zr(OCHEt,), Zr(0CHMe Pr), Zr(OCHMeCHMe,), Zr(OCMe,Et), Zr(OCHMeBu), Zr(OCHMeCMe,), Zr(OCMeEt,), Zr(OCMe,Pr), Zr(OCMe,CHMe,),
na
280b
18+200/0.1 208/0.1 160/0.1 243/0.1 1fiqO.l 8Y.1/5.0
27610.1 24710.1 238/0.1 18810.2 181/0.1 178/0.1
17610.1 95/0.1 190/0.1 12810.1 171.4/5.0 161.6/5.0
Zr(OCHPr2),
133/0.1 16yo.1
Zr{OCH(CHMe,),), zr(C=Et,),
Zr(OCMe,CMe,), Zr(OCMeEtCHMe,), w3CH*CF,), a{ocH(cF3)2}4 Zr(OCMe,CCl,),
B.p.
(“C/mmHg)
3.6
Re$
1 2,3
(“C/mmHg)
Hf(OMe), Hf(OEt),
180-200/0.1
Hf(OCHMe,),
170/0.35
Hf(OCMc,), Zr(OSiMe3),
135/0.1b
Hf(OCMe,Et), Zr(OSiMe,Et),
M.p.
na
Ref.
z 2
3
3.0,3.6‘ 3.4 2.5
4.5
1.0 3.2 3.3
6
3.I 2.4 2.0
2.0 2.0 1.0 1.o
1.0
.o
2,5
4 3 7 7 7
4 4 4 7 4 4 6 6 6
4
15810.1
1.0
4
18010.4 168/0.5
1.o
6 6 6
3.0 1.8
8
1.o
3.3‘
3
1
194/0.7 146/0.8 110/0.8
Compound
9016.5
2
152
2.0
12
105/0.1
105
1.2
2 12
Zr(OSiMeE&), Zr(OSiMe,Pr), Zr(OSiMe,CHMe,),
120/0. 1
30
1.0
12
103/0.05
60
1.1
12
110/0.1
1.0
12
Zr(OSiEt,),
147/0.1
1.0
12
92/0.1
9,lO 11
10
Degree of association, in boiling benzene unless indicated otherwise. Sublimes. In freezing benzene. 1. D. C. Bradley and M. M. Factor, N a m e (London), 1959,184, 55. 2. D. C. Bradley, R . C. Mehrotra and W. Wardlaw,J . Chern. Soc., 1953, 1634. 3. D . C. Bradley, R. C. Mehrotra, J. D. Swanwick and W. Wardlaw,J . Chem. Soc., 1953, 202.5. 4. D. C . Bradley, R. C. Mehrotra and W. Wardlaw,J . Chcm, Soc., 1952, 5020. 5 . D. C. Bradley and C. E. HoJloway, J. Chem. Soc. ( A ) , 1968, 1316. 6. D. C . Bradley, R . C. Mehrotra and W. Wardlaw,J. Chem. Soc., 1952, 4204 7. D. C. Bradley, R. C. Mehrotra and W. Wardlaw,J. Chem. Soc., 1952, 2027. 8. P. N. Kapoor and R. C. Mchmtra, Chem. Id.( h n d o n ) , 1966, 1034. 9. P. N. Kapoor, R. N. Kapoor and R. C. Mehrotra, Chem. Ind. (London), 1968, 1314. 10. K. S. Mazdiyasni, B. J. Schaper and L. M. Brown, Inorg. Chem., 1971, 10, 889. 11. D . C. Bradley, R. N. P. Sinha and W . Wardlaw, J. Chem. Soc., 1958, 4651. 12. D. C. Bradley and I. M. Thomas, J . Chem. Soc., 1959, 3404.
a
M(OCMe3)4and t-butanol evidence rapid intermolecular exchange of t-butoxide ligands.196IR spectra of zirconium and hafnium tetraalkoxides exhibit a strong v(C-0) band near 1000 cm-’ and one or more v(M-0) bands in the region 520-590 cm-1.197*198 Calorimetric measurements have afforded the following values for mean thermochemical bond energies (kJmol-’) in Zr(OCHMe2)4 and Hf(OCHMe&: Zr-0, 552; Hf-0, 573.68No X-ray structural data are available for the tetraalkoxides except €or a preliminary report of a study of the isopropanol solvates M(OCHMe2)4.HOCHNLe2(M = Zr or Hf). These complexes have a dimeric structure in which two MOs octahedra share an edge.199 Zirconium and hafnium tetradkoxides are highly reactive compounds. They react with water, alcohols , silanols, hydrogen halides, acetyl halides, certain Lewis bases, aryl isocyanates and other metal alkoxides. With chelating hydroxylic compounds HL, such as P-diketones, carboxylic acids and Schiff bases, they give complexes of the type ML,(OR)4-,; these reactions are discussed in the sections dealing with the chelating ligand. Partial hydrolysis of zirconium alkoxides yields polymeric oxozirconium alkoxides such as Zr304(OR)4 (R = Et or CMe3), Zr203(OCHMe2)2 and Zr203(0CMe3)2.4HOCMe3;m720 complete hydrolysis gives hydrated zirconium dioxide. An X-ray study2mof an oxo zirconium methoxide hydrolysis product has shown that the crystals contain molecules of [Zr130s(OMe)36].This complex has a % z ( D ~ structure ~) (Figure 3) consisting of a cubic close packed arrangement of 13 Zr atoms, three above and three below a plane of seven. The central
Zirconium and Hafaium
39 1
Zr atom is surrounded by a slightly distorted cube of eight oxygen atoms (at 2.19 and 2.23 A), each of which is attached to three of the other Zr atoms. The 12 peripheral Zr atoms are seven-coordinate, being bonded to two bridging oxygen atoms (at 2.15-2.19 A), four bridging methoxide ligands (at 2.12-2.21 A) and one terminal methoxide ligand (at 2.00 and 2.03 A). An alternate route to oxozirconium alkoxides is the reaction of alcohols with ZrOCl2.2MeCO2H in the presence of piperidine. This approach has been used to prepare compounds of the t pe ZrO(OR)2.nROH and ZrOCl(0R)-nROH (R = Me, Et, CHhfe2 or CMe3; n = 0.5-2.0. 2x4 n
Figure 3 The arrangement of zirconium atoms (solid circles) and oxygen atoms (open circles) in molecules of [Zr,,O,(OMe),,]; O(2) and O(3) are bridging oxygen atoms while the other oxygen atoms are part of methoxide groups (reproduced by permission from ref. 202)
The synthetic utility of alcoholysis reactions of Zr( OCHMe2),.HOCHMe2 has been mentioned already. Additional examples are the preparation of glycoxides of the type Zr{O(CR2)xO} (OCHMe&, Zr{O(CR2)x0}2 and Zr{O(CR2)xO}{O(CR2)xOH}2 (R = H or Me)’’, and N-methylaminoalkoxides of the type Z T ( O R ) ~ ( O C H M ~(R ~ )= ~ CH2CH2NHMe, -~ CH2CH2NMe2 or CHMeCH2NMe2; n = 1, 2, 3 or 4)m5 from stoichiometric amounts of ZrO(OCHMe2),.HOCHMe2 and the appropriate glycol or aminoalcohol. The N methylaminoalkoxides are monomeric in boiling benzene except for Zr(OCH2CH2NHMe),(OCHMe2)4-n (n = 1 or 2), which are dimeric. Silanolysis of Zr(OR), (R = CHMe2 or CMe3) (equation 32) gives the corresponding trialkylsilyloxides Zr(OSiRj), (Table 9). These compounds are colorless liquids or wlute solids that can be distilled or sublimed under reduced pressure. They are less susceptible to hydrolysis and possess greater thermal stability than the corresponding t-alkoxides. It is interesting io note that the trialkylsilyloxides are slightly more associated than the t-alkoxides, presumably because steric effects of the R3Si0 group are smaller that those of the R3C0 group; Zr(OSiMe3), is a dimer in boiling benzene. Zr(OR),
+ 4R;SiOH
-
Zr(OSiR;),
+ 4ROH
(32)
Halide alkoxides ZrX,(OR)4-n ( X = C1 or Br) are formed in reactions of zirconium alkoxides with hydrogen halides or acetyl halides. Thus Zr(OCHMe2),.HOCHMe2 reacts with acetyl chloride in 1:1, 1:2 and 1:3 mole ratios yielding isopropyl acetate and ZrC1(OCHMe2)3.HOC€€Me2, ZrC12(OCHMe2)2-HOCHMe2 and ZrC13(0CHMe2), respectively. When an excess of acetyl chloride is used, the product is ZrC14.2MeCOzCHMe2.20B Relatively few coordination compounds are known in which zirconium or hafnium alkoxides bond to Lewis bases. Zirconium isopropoxide forms 1:1 adducts with isopropanol, pyridine, hydrazine and ethylenediamine, but it does not bond to donors such as diethyl ether, dimethylamine, triethylamine, 2,2’-bipyridyl or t h i ~ u r e a . ~ ’ *Halogen ~’~ substitution in the alkoxide ligand appears to increase the Lewis acid character; thus Zr(OCH2CC13), forms a 1:2 adduct with acetone. Chloride alkoxides ZrC1n(OR)4-, (n = 1, 2 or 3) and ZrOCl(0R) are better Lewis acids than the corresponding ZrIOR), and ZrO(OR)2 compounds. ZrOC1(OCHMe2)y2HOCHMe2 reacts in dichloromethane with a variety of Lewis bases yielding adducts of the type ZrOCl (OCHMe2).L (L = py, bipy, quinoline, DMF, dimethylacetamide or phthalimide) and ZrOC1(OCHMe2).3DMS0, whereas the corresponding dialkoxides ZrO(OR)2-ROH (R =Me, Et or CHMe2) failed to react.209 Zirconium tetraalkoxides Zr(OR), (R = Pr, CHMe2 or CMe3) react with stoichiometric
Zirconium and Hafnium
392
amounts of phenyl or naphthyl isocyanate to give the insertion products 2r{NR‘C(0)0R},,(0R),~, (R’ = Ph or C10H7; n = 1, 2, 3 or 4).’1° Reaction of zirconium and hafnium alkoxides with alkali metal, alkaline earth metal, aluminum and gallium alkoxides yields bimetallic alkoxides of the type: M’Zr(OCMe&, M‘Mz(0R)g and M$M3(OR),, (M’ = alkali metal; M = Zr or Hf; R = alkyl); M”{M2(0CHMe2)9}2and M”M3(OCHMe2)14(M” = Mg, Ca, Sr or Ba); M”‘M(OCHMe& and hf;’M(OCHMe2)lo ( M = A1 or Ga).188,190The bimetallic alkoxides are volatile, covalent compounds that can be sublimed or distilled under reduced pressure. Most of these compounds are monomeric in boiling benzene, though some tend to dimerize. Structures have been suggested on the basis of molecular weight data, ‘HNMR spectra, and the products of alcoholysis reactions, but no X-ray structural data are yet available. A recent study of solubility in the Al(OCHMe2)3-Zr(OCHMe2)4-L (L = THF or HOCHMe2) system has confirmed the existence of AIZr(OCHMeJ7, but failed to provide evidence for A12Zr(OCHMe2)10.2’1The has been established in the existence of bimetallic alkoxides M’”Zr3(0CHMe2)15.3HOCHM~ M(OCHM~Z)~-Z~(OCHM~~)~-HOCHM~~ (M’” = Y or La) s y ~ t e m s . ~ ~ ~ , ~ ~ ~ Compounds of composition [CSH5NH]2[MC14(OMe)z] (M = Zr or Hf), [CSHSNHIZ[ZrCIS(OEt)] and [C5H5NH]2~ZrClz(OEt)4] have been prepared by reaction of the metal tetrachlorides with methanol or ethanol in the presence of pyridine.’14 where The reaction of ZrC4 with Li(tritox) in diethyl ether yields [ZrC13(tritox)2-Li(OEtz)2], tritox is the sterically bulky tri-t-butylmethoxide ligand, (Me,C),CO. An X-ray study of this compound revealed trigonal bipyramidal coordination about the zirconium atom, with the two tritox ligands in equatorial sites and three chlorine atoms in the remaining sites. The equatorial chlorine atom and one of the axial chlorine atoms bridge to the lithium atom, which is also attached to two diethyl ether molecules. The Zr--0 bond lengths (average 1.895A) are exceptionally short and the Zr-0-C angles (average 169”) are nearly linear, indicative of strong pn-dn bonding. It has been suggested that the neutral (Me3C)3C0. ligand may function as a five-electron donor, like a cyclopentadienyl group. Extraction of [ZrC13(tritox)z-Li(OEt2)z] with hexane yields ZrClz(tritox)z, which can be converted to ZrMe2(tritox)z by reaction with MeLi.’15 Conductometric titration of ZxCL with KOPh in nitrobenzene indicates the formation of Zr(OPh),, ZrC13(OPh) and ZrClz(OPh)2, which have been isolated and characterized by chemical analysis. The Lewis acidity of Zr(OPh), and ZrClz(OPh)z has been demonstrated by isolation of adducts with py, bipy, phen and their N-oxides.216 ZrCI4 and HfC1, react with LiOAr (OAr = 2,6-di-t-butylphenoxide) in benzene or diethyl ether to give the aryloxide complexes [MC1(OAr)3] (M = Zr or Hf). Attempts to prepare the corresponding MC12(0Ar)2 compounds were unsuccessful. [HfCl(OAr),] has a sterically congested, tetrahedral structure with bond distances H f - C l = 2.365(1) A, Hf-0 = 1.938(3), 1.925(2) and 1.917(3) A, and Hf-0-C bond angles in the range 152-159”. Low-temperature ‘H NMR spectra exhibit inequivalent t-butyl groups proximal and distal to the chlorine atom bonds. Because the sterically owing to a slowing at - -60 “C of rotation about the Hf-0-Ar bulky t-butyl groups tend to protect the metal atom from nucleophilic attack, the [MCl(OAr),] complexes are not easily alkylated though [HfCl(OAr),] does react with MeLi to give [HfMe(OAr)3].217
32.4.44 B-Ketoenolates, tropolonates, catecholates and quinones; ethers, aldehydes,
ketones and esters
(i) p- Ketoenolates Tetrakis(acety1acetonato) complexes of zirconium and hafnium were reported in 1904218and 1926,219 respectively, and a large number of p-diketonate derivatives have been described [M(dik)3X], [M(dik)2X2], subsequently (Table 10). These compounds are of the type [M(~lik)~], M(dik)X3 and [M(dik)?]Y (dik = P-diketonate anion; X = C1, Br, I, NO3, or alkoxide; Y = [FeCl,], [AuCl,], ?[PtCl6] or +[Zr(S04)3]),Additional P-diketonate compounds include the anionic complex [NELt][Zr(bzbz)F4] (bzbz = dibenzoylmethanate)220 and the 1:1 ZrC14diketone adducts [ZrC14(MeCOCRzCOMe)] (R = H or Me).ZZ’-ZW Early work on zirconium and hafnium P-diketonates has been reviewed by L a r ~ e nBradley ,~ and Thorntoq6 FacklerE3 and Mehrotra et aLZz4 The following methods have been employed for preparation of zirconium and hafnium
Zirconium and Hafnium
393
___
Zirconium and Hafnium
394
W
4
u' a w Go w w buNbV>Mo" has been rationalized in terms of increasing ligand (nu)+ metal ( d X ~ - yd,,)n ~ , bonding.492
1.
?
Figure 8 A perspective view of the [Zr(S,C6H4)3]'- anion. The complex is located on a crystallographic twofold axis
Tris(o-mercaptophenolato) complexes, [NEt4]2[M(SOC&14)3](M = Zr or Hf) ,280 have been prepared by a method similar to that used for the analogous dithiolates. A compound of composition Zr(SOC6&)(HSOC6H& has been obtained from the reaction of ZrCb with three equivalents of o - m e r ~ a p t o p h e n o l . ~ ~ ~ 32.4.5.4 Other sulfur donor ligands
Reaction of Na{S2P(OEt)2} with ZrCL in toluene affords the tetrakis(diethy1dithiophosphate) [Zr{S2P(OEt)2}4].4x An X-ray has established that the corresponding dkopropyl derivative [Zr(S2P(OCHMe2)2}4] has an eight-coordinate DU-mmmm dodecahedral structure, with Zr-Sn = 2.746(3) and Zr-SB = 2.653(3 A. This structure is somewhat surprising461in view of the rather large S.4 bite (3.178(5) ) of the dithiophosphate ligand; the normalized bite is 1.177 A. Zirconium and hafnium tetrachlorides form 1:1 and 1:2 adducts with thiourea. IR spectra indicate that the 1:1 adducts contain terminal and bridging chlorine atoms and S-bonded thiourea ligands. A chlorine-bridged dimeric structure [{H2N)2CS}C13MC12MC13{SC(NH2)2}] has been proposed.496 The IR spectra and calorimetric measurements suggest that only one thiourea ligand is coordinated to the metal in the 1:2 adducts; the second ligand appears to be attached to the complex via hydrogen bond^.^'^,^^ The reaction of ZrCL with triphenylphosphine sulfide in benzene at reflux affords a 1:l adduct, which has been assigned a p2-chlorine-bridged structure on the basis of molecular weight, conductance and IR data.498An insoluble red-brown thiophene adduct of composition ZrC14(C4H4S)2has been obtained by prolonged shaking of a 4: 1 mixture of thiophene and ZrC4 in benzene .292
B
32.4.6
Selenium Ligands
The reaction of ZrC1, with A1(SePh)3 in ether-benzene Zr(SePh)4 as a turquoise, microcrystalline
yields the phenyiselenolate
Zirconium and Hafnium
422
32.4.7
Halogens as Ligands
324.7.1 Tetrahalides Zirconium(1V) and hafnium(IV) halides are high-melting crystalline solids, which do not contain discrete complexes. a-ZrF2°0 and /3-ZrF4501have extended three-dimensional structures in whch ZrF8 dodecahedra (aform) or square antiprisms ( p form) are linked together by sharing fluorine atoms. Crystalline ZrCL consists of infinite zigzag chains of ZrC16 octahedra which share non-opposite edges; ZrBr4, HfC14 and HfBr4 are isostructural with ZrC14.5mIn zirconium(1V) iodide, ZrIh octahedra share non-opposite edges so as to give a helical chain structure with an identity period of six octahedra.503Hafnium(1V) iodide has yet another type of chain structure in which sharing of non-opposite edges of HfIb octahedra results in folded chains with an identity period of four octahedra.504Much useful information on the preparation and properties of zirconium(1V) and hafnium(1V) halides may be found in previous reviewss36 and in the monograph by Canterford and colt or^.^'^ In the gas phase, the tetrahalides exist as monomeric regular tetrahedral molecules. This has been established by electron diffraction and vibrational spectroscopy. Bond lengths and vibrational frequencies are listed in Table 16. Force constants for M-X bond stretching are slightly larger for the hafnium tetrahalides than for the zirconium analogues; the force constants decrease appreciably as X varies in the order C1> Br > I.506 Table 16 Bond Lengths
Compound
HfCI, ZrBr, HfBr, ZrI, Hfl4
(A)
and Vibrational Frequencies" (cm-') in Gaseous Zirconium(1V) and liafnium(W) Halides
Bond length
y1 ( a , )
1.902(4) 1.909(5) 2.32(1) 2.32(2)
633 (ED)b 659 (ED) 377 380.5
2.316(5) 2.33(2) 2.465(4) 2.450(4) 2.660( 10) 2.662(8)
382
225.5 235.5 158 158
v2
(e)
185 (ED) 187 (ED) 98 101.2
60 63 43 55
v2
(td
668 (IR) 645 (IR) 418 423 (IR) 421 (IR) 390 393 (IR) 315 273 254 224
y't
(t,)
Ref.
187 (ED) 179 (ED) 113
1-3 1-3 3-8
112
3,5,6,9
72 71 55 63
6,lO 6, 10 6,lO 6,lO
Raman frequencies, unlcss indicatcd otherwise. Calculated from electron diffraction data and the v3(fz) IR frequency. 1. V. M. Petrov, G. V. Girichev, N. 1. Giricheva, 0 . K. Shaposhnikova and E. Z. Zasorin,J. Struct. Chem. (Engl. Trawl.), 1979,20, 110. 2. G. V. Girichev, N. I. Giricheva and T. N. Maiysheva, Rms. J . Phys. Chem. (Engl. Trans/.), 1982,56, 1120. 3. A. Biichler, J. B. Berkowitz-Mattuck and D. H. Dugre, J . Chem. Phys., 1961,34, 2202. 4. M. Kimura, K. Kimura, M. Aoki and S . Shibata, Bull. Chem. Soc. Jpn., 1956, 29,95. 5. V. P. Spiridonov,P. A. Akishin and V. I. Tsirel'nikov, J. Struct. Chem. (Engl. Trunsl.), 1962, 3, 311. 6. R. J . H. Clark, B. K. Hunter and D. M. Rippon, Inorg. Chem., 1972,11,56. 7. W. Brockner and A. F. Demiray, J. Raman Spectrosc., 1978, 7, 330. 8. J. K. Wilmshurst, J . Mol. Spectrosc., 1960,5, 343. 9. G. V. Girichev, V. M. Petrov, N.I. Giricheva, A. N. Utkin and V. N. Petrova, J. Smut. Chem. (Engl. Trunsl.), 1981, 22,694. 10. G. V. Girichev, E. Z. Zasorin, N. I. Giricheva, K. S . Krasnov and V. P. Spiridonov, I. Smccr. Chem. (En$ Trans[.), 1977,18,34.
Metal tetrahalide adducts with Lewis bases (cf. Tables 3, 4, 13, 15 and 23) are discussed in other sections. The tetrafluorides form stable hydrates MF4(H20), (n = 1 or 3), in contrast to the other tetrahalides which react with water to give oxyhalides. It is interesting to note that ZrF4(H20)3and HfF4(H20)3have rather different structures. The zirconium c o m p ~ u n d ~ , ~ ~ consists of discrete, centrosymmetric [ZrzF8(H20)6]complexes in which two dodecahedral ZrFs(H20)3 units are joined by sharing an F...F m edge. The three water molecules (Zr-0 = 2.263-2.323 .$) and a bridging fluorine atom (Zr-F = 2.214 A) occupy the dodecahedral A sites, while the other bridging fluorine atom (Zr-F = 2.118 A) and the three terminal fluorines (Zr-F = 1.996-2.032 A) take the B sites. Occupation of the B sites by the better HfF6(H20)2 n-donor ligand is in accord with Orgel's rule.458r4s9In the hafnium units share two F..-F polyhedral edges giving a chain structure with repeating unit HfF4(H20)2; the uncoordinated water molecule is held between the chains by hydrogen bonds. The monohydrate ZrF4(H20) has a three-dimensional network structure510 in which ZrF6(H20)2 dodecahedra share six corners (four fluorine atoms and two water molecules) with six adjacent dodecahedra.
Zirconium and Hafnium
423
32.4.7.2 Flutlromediates
Zirconium and hafnium form a vast array of complex fluorides, M;M,F,, where M is Zr or Hf, and M' is an alkali, alkaline earth, divalent transition metal, or lanthanide cation, or the conjugate acid of a nitrogen base. The number of fluorine atoms per Zr (Hf) atom is commonly eight, seven, six or five, but non-integral ratios are also found, as in Na3Zr2FI1, Na5Zr2FI3, Li3Zr4F19,Rb,Zr&, and Na,Zr,F,, . The variety of alkali fluorometallates is illustrated in Table 17. Table 17 Alkali Fluorornetdates Alkali cation ( M ' )
Compound
Na,3 K6 Na,>5 K6 Li? Na3 Rh'*
1. Yu. M. Korenev and A. V. Novoselova, Inorg. Muter. (Engl. Trawl.), 1965,1, 546. 2. R. E. Thoma, H. Insley, H. A. Friedman and G. M. Hebert, J. Nucl. Muter., 1968,27, 166. 3. C. J . Barton, W .R . Grimes, H. Insley, R. E. Moore and R. E. Thoma, J. Phys. Chem., 1958, 62, 665. 4. I. D. Ratnikova, A. A. Kosorukav, Yu. M. Korenev and A. V. Novoselova, Russ. J . Inorg. Chern. (Engl. Truml.), 1975, 20, 782. 5. I. V. Tananaev and L. S. Guzeeva, Russ. J . Inorg. Chem. (EngL Trawl.), 1966,11,590. 6. A . V. Novimlovd, Yu. M. Korenev and Yu. P.Simanov, Dokl. Chern. (Engl. Truml.), 1961, w, 771. 7. H. M. Haendler, C. M. Wheeler, Jr. and D. W. Robinson, J . Am. Chem. Soc., 1952, 74, 2352. 8. B. Gaudreau, Rev. Chim. Min., 1965, 2 , 1. 9. H. Hull and A . G. Turnbull, J . Inorg. Nucl. Chem., IY67, 29,951. 10. H. M. Haendler. F. A . Johnson and D. S. Crocket,J . Am. Chem. Soc., 1958, 80,2662. 11. D. S. Crucket and H. M. Haendler, J . A m . Chem. Soc., 1960, 32, 4158. 12. G. D. Robbins?R. E, Thoma and H. Insley, J . ltwrg. N w I . Chem., 1965, 27, 559. 13. A. A. Kosorukov,V. Ya. Frenkel', Yu. M. Korenev and A. V. Novoselova, Russ. J . Inorg. Chem. (Engl. Transl.), 1973, 18, 1025. 14. H. Bode and G. Teufer, Z . Anorg. Allg. Chern., 1956, 283, 18. 15. H. M. Haendler and D. W. Robinson, 1.Am. Chem. SOC., 1953, 75,3846. 16. H. Hull and A. G. Turnbull, J . Inorg. Nucl. Chem., 1967, 29, 2903. 17. D. Avignant, I. Mansouri, R. Chevalier and J. C. Cousseins, J. Solid State Chem., 1981,38, 121. 18. G . Brunton, Acta Crystdlogr., Sect. B, 1971,21, 1944. 19 I. N. Sheiko, V. T. Mal'tsev and G. A. Bukhalova,Sou. Progr. Chem., 1966,32, 978. 20 I. V. Tananacv and L. S . Guzeeva, Rms. J . Inorg. Chem. (Engl. Trawl.), 1966,11,587. 21 B. Gaudreau, L R . Hehd. Seances Acud, Sci., Ser. C , 1966, 263, 67. -.m LL. A. A . Opalnvskii, T. F. Gudimovich, M. Kh. Akhrnadeev and L. D. Ishkova, Russ. J . Inorg. Chem., 1982,27,664. 23. N. P. Sazhin. B . V. Shchepochkin and G. A, Yagodin, R d . Acud. Sci. USSR,Div. Uhem. Sci., 1965, 1097.
24. A. A. Opalovskii, T. F. Gudimovich, N. Kh. Akhmadeev and L. D. Ishkova, Russ. 3. Inorg. Chem. (EngZ. T r m l . ) , 1983, 28, 1065.
Table 18 summarizes structural data for fluorozirconates whose structures have been determined by diffraction methods. In some cases, the analogous fluorohafnates are known to be isomorphous with the fluorozirconates. The compounds are grouped according to the coordination number of the Zr atom (six, seven or eight); individual structures are discussed in the following sections. In general, there is no simple relation between structure and stoichiometry. Fluorine bridging between Zr atoms is common, resulting in oligonuclear, chain, layer and extended three-dimensional structures in which the coordination number of the metal exceeds that expected on the basis of the simplest formula. A variety of coordination polyhedra have been found in fluorozirconate structures; the more common ones are the octahedron for coordination number six, the pentagonal bipyramid and Czu monocapped trigonal prism for
Zirconium and Hafnium
424
coordination number seven, and the DM dodecahedron, D4d square antiprism, and Czv bicapped trigonal prism for coordination number eight. Averaged Zr-F bond lengths (Table 18) are in the ranges 1.98-2.04, 2.04-2.08 and 2.10-2.13 for coordination numbers six, seven and eight, respectively. In compounds that contain Zr-F-Zr bridges, the averaged Zr-F (bridging) bond length is as much as 0.1 8, larger than these values, and the averaged Zr-F (terminal) bond length is correspondingly smaller. The following discrete anions have been characterized structurally: [ZrFSl2-, [ZrF7I3-, [ZrF8I4-, [Zr2FI2l4-, [Zr2Flo(H20)2]2-, [Zr2F1$, [ZrzFI4l6-and [Zr4FX]'-. Table Is Structural Data for Complex Fluorozirconates
Compound
Average Zr-F bond length (A)
Structural description'
Kej.
Isolated [ZrF6]'- octahedra Isolated [ZrF,f- octahedra Isolated [ZrF,]'- octahedra [ZrF,]'- octahedra Three-dimensional network; ZrF, and FeF, octahedra share comers Three-dimensional network; ZrF, and CuF6 octahedra share corners Three-dimensional network; ZrF, octahedra and SmF, polyhedra share corners
2.016 2.04 2.04 1.996 1.991 (cubic) 1.995 (trigonal) 1.995 (cubic) 2.006
6
isolated [ZrF7I3- PBP; dynamically disorderd Isolated [ZrF7I3- CTP Centrosymmetric dimer; two ZrF,(H,O) FBPS share equatorial F...F edge Centrosymmetric dimer; two ZrF, PBPs share equatorial edge Centrosymmetric dimer; two ZrF, PBPS share equatorial edge Centrosymmetric [Zr2F,,l4-; two ZrF7 CTPs share edge C, symmetry; two ZrF, CTPs share the capping site ZTF, irregular polyhedra share two F atoms
an
2.02 2.059 2.045
7, n 9 10
an
2.062
11
an
2.063
12
an
2.082
13
2.06 2.065
14
2.10 2.103 2.109 2.105 2.114 2.12 2.126 2.10 2.12 2.112 2.11
16
Isolated [ZrF,I4-DD Isolated [ZrF,f- SAP Centrosymmetric dimer; two ZrF, SAPs share an s edge Chain structure; ZrF, DDs share m edges Chain structure; ZrF, DDs share a edges Chain structure; ZrF, DDs share a edges Isostructural with @-BaZrF, Zigzag chain structure; ZrF, BTPs share tXedges Layers of (ZrF;)"; ZrF, BTPs share edges and corners Layer structure; ZrF, SAPs and MnF,(H,O), PBPs share F...F edges IxKtructural with Mn$rF8.6H,0 Three-dimensional network; ZrF, SAPS share corncrs and edges
2.114 2.113
1
2 2 3 4
5
15
17 18 19 20 21,22 23 24 25 26
27 28
29
Coordination numbers 6, 7 and 8 Rb,Zr4Fz,
Cross-linked structure containing: ZrF6 octahedra ZrF, polyhedra (CO/PBP) ZrF, polyhedra (CO/PBP) ZrF, irregular antiprisms
30 1.98 2.06 2.08 2.13
PBP =pentagonal bipyramid; CTP = C, monocapped trigonal prism; CO = monocapped octahedron; DD =dodecahedron; SAP = square antiprism; BTP = bicapped trigonal prism. 1. G. Brunton, Acta Crystallogr., Sect. B, 1973,29, 2294. 2. H. Bode and G. Teufer, 2. Anorg. AUg. Ckm., 1956, 283, 18. 3. J. Fischer and R. Weiss, Acta Crystallogr., Sect. 3,1973, 29, 1955. 4. P. Kohl, D. Reinen, G. Decher and B. Wanklyn, 2. Krirtallogr., KrirtdZgeorn., KriFtnllphys., Krirtullchem., 1980,153,211. 5 . V. Propach and F. Steffens, Z. N ~ t ~ r f a t ~Teil d r ,E , 1978,33, 268.
Zirconium and Hafnium
425
Table 18 (footnotes continued) 6. M. Poulain, M. Poulain and J. Lucas, J . Solid Stare Chem., 1973, 8, 132. 7. H.J. Hurst and 5. C. Taylor, Acta Crystdlogr., Secr. 13, 1970, 26, 417. 8. H. J. Hurst and J. C. Taylor, Acta Cqsrdlogr., Sect. B, 1970,26,2136. 9. I. P. Kondratyuk, B. V. Bukvetskii, R. L. Davidovich and M. A. Medkov, Sou. J . Coord. Chem. (Engl. Trawl.), 1982, 8, 113. 10. R. L. Davidovich, M. A . Medkov, V. B. Timchenko and B. V. Bukvetskii, Bull. Acud. Sci, USSR, Div. Chem. Sci., 1983, 32, 2181. 11. I. P. Kondratyuk, M. F. Eiberman, R. L. Davidovich, M. A. Medkov and B. V. Bukvetskii, Sov. J . Coord. Chem. (Engl. T m l . ) , 1981, 7, 549. 12. J. Fischer and R. Weiss, Acra C y t d l o g r . , Sect. B , 1973, 29, 1958. 13. J.-P. Laval, R . Papiernik and B. Frit, Acta Crysrullogr., Sect. B, 1978, 34, 1070. 14. R. M. Herak, S. S. Malcic and L.M. Manojlovic,Acta Crystullogr., 1965, D,520. 15. G. Brunton, Acto Crystullogr., Sect. B, 1969, 25, 2164. 16. D. R. Sears and J . H. Burns, J. Chcm. Phys., 1964,41,3478. 17. J. Fischer, R. Elchinger and R. Weiss, Acta Crystullogr., Sect. B, 1973,29, 1967. 18. J. Fischer and R. Weiss, Actu CrystdZor., Sect. B, 1973, 29, 1963. 19. T. S. Chernaya, L. E. Fykin, V. A. Sann, N. I. Bydanov, N. M. paplash, B. V. Bukvetskii and V. I. Simonov, Sou. Phys. Crystullogr. (Engl. Truwi.), 1983, 28, 393. 20. R. Hoppe and B. Mehlhorn, Z . Anorg. Allg. Chem., 1976,425, 200. 21. L. P. Otroshchenko, R. L. Davidovich and V. I. Simonov, Sou. J. Coord. Chem. (Engl. Trawl.), 1978,4, 1079. 22. L. P. Otroschenko, V. I. Simonov, R. L. Davidovich, L. E. Fykin, V. Ya. Duderov and S. P. Solov'ev, Sou. Phys.-Crystullogr. (Engl. Truml.), 1980, 25, 416. 23. B. Melhorn and R. Hoppe, 2.Anorg. Allg. Chem., 1976,425, 180. 24. J.-P. Laval, D. Mercurio-Lavaudand B. Gaudreau, Rev. Chim. Min., 1974, 11,742. 25. B. Kojic-Prodic, S. Scavnicar and B. Matkovic, Acta Crystullogr., Sect. 8, 1971, 27, 638. 26. D. Avignant, I. Mansouri, R. Chevalier and J. C. Cousseins, J . Solid State Chem., 1981, 38, 121. 27. L. P. Otroshchenko, R. L. Davidovich, V. I. Simonov and N. V. Belov, Sov. Phys. Crysrullogr. (Engl. Trawl.), 1983, t6,675. 28. M. F. Eiberman,T.A . Kaidalova, R. L. Davidovich, T. F. Levchishina and B. V. Bukvetskii, Koord. Khim., 1980,6,1885 (Chern. Abutr., 1981, 94, 112794). 29. J. H. Burns, R. D. Ellison and H. A. Levy, Actu Crystullogr., Sect. E , 1968, 24, 230. 30. G. Brunton, Acta Crysrullogr., Sect. B, 1971, 27, 1944.
( i ) Alkali metal salts Alkali fluorometallates may be obtained by crystallization of MF4-M'F melts (M = Zr or Hf; M' = alkali metal), or by reaction of a suitable Z P or Hf'" compound with an alkali fluoride in hydrofluoric acid, water, acetic acid or methanol. References to the preparation of specific compounds are given in Table 17. Phase equilibria in MF4-M'F systems are rather complex, and polymorphism is common. For example, the following compounds have been identified in the ZrF4-NaF system: NaZrFS, Na3Zr2Fll, Na3Zr4Fi9, Na7Zr6F3', two olymorphic forms of Na3ZrF7 and Na5Zr2FI3,and at least three polymorphs of Na2ZrF6.5117s1' From aqueous MF4-M'F systems, the usual products are MjMF,, M;MFs or M'MF,-H,O depending on the stoichiometry of the reaction mixture. The hexa- and penta-fluorometallates can be recrystallized from water, but crystallization of the heptafluorometallates requires the presence of excess alkali fluoride. For the aqueous MF4-CsF system, the highest fluorometallate obtained, even in the presence of excess CsF, is CszMF6.513,514 Synthesis of the cesium heptafluorometallates Cs3MF7is accomplished by crystallization of a 1:3 MF4-CsF melt. The pentafluorozirconate that crystallizes from aqueous ZrF4-NHa-HF solutions depends on temperature and the concentration of HF; at 20 "C and HF concentrations less than 5 wt%, (NH4)ZrF5.H20 is obtained, while at higher HF concentrations or at elevated temperatures, anhydrous (NH4)ZrF5 is formed.515 (NH4)ZrF5 may also be prepared by thermolysis of (NH4)3ZrF716,517 and by reaction of (NI-&)2ZrF6 with XeF2.518Other routes to anhydrous (ii) fluorometallates include (i) reaction of HfF4 with M'F in anhydrous acetic reaction of ZrBr4 with M'F in r n e t h a n ~ l ~and ~ ' .(iii) ~~~ reaction of Z r0 2 or Hf02 with NH$ at 1300~~517,523 The thermodynamic properties of ammonium fluorozirconates have been determined515s516 from heat of solution measurements; values of AI$ (kJmol-') at 298K are -3386f:3 for F ~ , f 3 for (NH4)ZrF5 and -2728 f 3 for (NH4)3ZrF7, -2918 f 3 for B - ( N H ~ ) ~ Z ~-2217 (NH4)ZrF5-H20.The heat of dehydration for the pentafluorozirconate (equation 49) is 54 f2 kJ mol-', and the equilibrium water vapor pressure associated with this reaction is 16.1mmHg at 20°C. These values indicate very weak bonding of the water. Thermal decomposition of ammonium fluorozirconatess24 occurs in a stepwise manner, with the decomposition temperatures depending on pressure; the steps in equation (50) have been identified at 1atm pressure.
-
(NH4)ZrF5.H20(s) (NH4),ZrF,
297"c
(NJUZrF6
(NH,)ZrF,(s) 357 T
'~
+ W,O(g) 410°C
~ , ) Z r ~ 5
(49) ZrF4
(50)
Zirconium and Hafnium
424
Raman s ectra of aqueous solutions of (NH4)3HfF7and (NH&HfF6525are identical ( v l 589, v5230 cm- ), and are similar to spectra of crystalline cs2W6 (M = Zr or Hf) salts, which are
P
known to contain [MF$ anions. Therefore, [HfF7I3- dissociates in solution into [HfF6I2- and F-. Paper chromatographic studies5’, have shown that [ZrF7I3- is similarly dissociated. Raman spectra of Zr02 and HfOz in 5 M HF indicate that the {MF6I2- anions are the predominant species in solution.152 19FNMR spectra of 1M aqueous solutions of (NH4)2ZrF6 and (NH4)2HfF6525exhibit a single, rather broad resonance (line width -8 Hz for [ZrF6]2- and -40 Hz for [HfF6I2-), which broadens and shifts upfield upon addition of excess F-. These observations have been interpreted in terms of rapid fluoride exchange, probably proceeding via an [MF,I3intermediate. For a solution 0.25 M in [ZrF6I2- and 1.5 M in F-, which gave a 19Flinewidth of 1550 Hz, the average lifetime of a fluorine nucleus in the [ZrF,]’- and F- sites was estimated to be -1.4 X lop5s. The ”Zr NMR spectrum of (NH4)2ZrF6in D z O ~ ~ also ’ consists of a single, broad line (minimum linewidth 50Hz); no Zr-F coupling is observed, in accord with rapid fluoride exchange. Because is -1.7 times more soluble in water than K2ZrF67s28zirconium and hafnium can be separated by fractional crystallization of the hexafluorometallates. This approach is used on an industrial scale in the USSR.286Conductance measurements on aqueous solutions of M;MF6 (M’ = K, Rb, or Cs} indicate very little hydrolysis of the EMF6]’- ions.529Alkaline hydrolysis of potassium and ammonium fluorozirconates ields crystalline M’ZrF3(OH)2.H20 complexes, which are easily dehydrated to M’ZrF3(OH),. 2 0 The coordination number of the Zr (Hf) atom in the cr stalline hexafluorometallates varies from six to eight depending on the alkali cation. Li2ZrF6””Yand the isomorphous rubidium and cesium salts M;MF6s32 contain the octahedral [MI?$ anion. The Zr atom in y-NaZZrFs is surrounded by an irregular polyhedron of seven fluorine atoms, two of which are The isomorphous K2MF6 salts532 contain chain anions which are built up from MF8 dodecahedra that share opposite m edges, Le. all four bridging fluorine atoms are located on one trapezoid of the dodecahedron (Zr-F terminal = 2.038, 2.077; Zr-F bridging = 2.111, 2.231A).534 Still another structure (space group Pmma) is adopted by the isomorphous (NH&MF6 and T12MF6salts. In these compounds, the Zr (Hf) atom may have a pentagonal bipyramidal environment; however, the locations of the fluorine atoms are not well defined by the X-ray data.532 The isomorphous, cubic heptafiuorornetallates (NH4)3MF7and K3ZrF7 contain d namicaily disordered [MF7I3- ions. Both capped octahedrals3’ and pentagonal bipyramidaly36 models have been suggested, and the latter is supported by single-crystal X-ray and neutron diffraction studies of (NH4)3ZrF7.s37,538 The averaged Zr-F bond length in (N€&)3ZrF7 (2.02 A) appears to be a bit too short in comparison with Zr-F bond lengths in other seven-coordinate structures (Table 18). The sodium salts Na3MF7 crystallize in the tetragonal system (space group I4/mmm) and are also dynamically disordered; the Zr(Hf) atom appears to be surrounded by a cube of F-atom sites, one of which is vacant.539The dynamics of F-atom motion in alkali heptafluorometallates have been studied by ”F NMR s p e ~ t r o s c o p yand ~~~,~~ by perturbed angular correlation m e a s u r e m e n t ~ . ~ ~ ~ , ~ ~ ~ The coordination environment of the Zr atom in Li4ZrFs is unknown, but the [ZrFRI4-anion has been found in Li6BeZrF12, This compound, which is obtained from a LiF-BeFrZrF4 melt of stoichiometric composition, contains tetrahedral [BeF4I2- and dodecahedral [ZrFsI4anions (Zr-FA = 2.16, Zr-FB = 2.05 A$).544 Eight-coordinate Zr atoms are also found in the pentafluorozirconate TlZrF,. In this compound, ZrFs bicapped trigonal prisms share one edge and four corners so as to give (ZrF;), layers that are held together by TIf cations.545 The sodium salt Na5Zr2FI3contains [Zr2F13]5- anions of Cu, symmetry in which two ZrF7 monocapped trigonal prisms share the F atom that occupies the capping site (Zr-F terminal = 2.00-2.10, Zr-F bridging = 2.10 A).546 Rb5Zr4F2,has a complex cross-linked chain structure that contains four crystallographically independent Zr atoms having three different coordination numbers. The structure is built up from edge- and/or corner-shared ZrF, octahedra, ZrFs irregular antiprisms and two independent ZrF7 pol h e d ~ a .Application ~~~ of the dihedral angle criteria of Muetterties and GuggenbergerZx indicates that both ZrF, polyhedra are midway between a capped octahedron and a pentagonal bipyramid. Na7ZrhF3,contains octahedral arrays of six ZrF, square antiprisms which share corners in such a way as to give Zr6F36units. These units in turn share external edges with six neighboring Zra3, units, giving an extended three-dimensional anion with formula (Zr6@;)n. The ‘extra’
Zirconium and Hafnium
427
F- ion is located within the cuboctahedral cavity surrounded by the six ZrFs square
anti prism^.^^' Mixed-ligand fluorometallates that contain peroxide, sulfate, selenate, carbonate or oxalate ligands are discussed in Sections 32.4.4.2, 32.4.4.5 .iii, 32.4.4.6.iii and 32.4.4.6.i~. ( i i ) Hydrazinium, alkylammonium and guanidinium salts
~
Hydrazinium(2+) salts, (N2H6)MF6and (N2&)3M2F14 (M = Zr or Hf), have been isolated from hydrofluoric acid solutions of the metal oxides and hydrazinium(2+) f l ~ o r i d e . ~ ~ , ~ ~ ~ (NzH6)ZrF6contains infinite zigzag chains of ZrFs bicapped trigonal prisms which share the two tl edges on the quadrilateral faccss2 The reaction of (N2&)MF6 with XeF6 at room temperature yields thermally unstable compounds n XeF6-MF4 (n G 6) which decompose in vacuo at room temperature to white, moisture-sensitive solids of composition XeF6-MF4. Vibrational spectra suggest that these compounds contain [XeF5]+ cations and polymeric (MFF), anions.s53 A hydrazinium(l+) salt of composition (N2H5)[email protected] been prepared by reaction of ZrF4 and hydrazinium(l+) fluoride in aqueous solution.55 Large, colorless, prismatic crystals of [NMe4]z[ZrzFlo(H20)z]have been isolated from dilute HF solutions of ZrF4(H20)3and tetramethylammonium fluoride. The [ZrzF10(H20)2]2-anion is a centrosymmetric dimer in which two ZrF6(HzO) pentagonal bipyramids share an equatorial F...F edge (Zr-F terminal = 1.972-1.997; Zr-F bridging = 2.143, 2.184 A); the water molecule occupies an equatorial site (Zr-0 = 2.271 A). The following metal-ligand stretching bands are observed in the IR spectrum: v(Zr-F terminal) 520, 480, 420; v(Zr-F bridging) 325; v(Zr-0) 385 cm-'.554 The hexafluorometailates [NEt4]2[MF6].2Hz0have been prepared by direct electrochemical oxidation of zirconium or hafnium anodes in PhCN solutions that contain [NEt4]F-3HF.555 The ethylenediammonium fluorozirconates (H3NGH4NH3)(ZrF~)z.Hz09 (H3NG&NH3)ZrF6 and (H3NC2&NH3)3(ZrF7)z-2Hz0have been prepared in aqueous solution by reaction of various molar ratios of (H3NCzH4NH3)Fz.HFand H2ZrF6.2H20. The presence of v(Zr-F terminal) and v(Zr-F bridging) in the 600-380 cm-l region of IR spectra of the pentafluorozirconate indicates that this compound probably contains polymeric (ZrF;), anions.556Crystals of (H3N&H4NH3)ZrF6 contain centrosymmetric [ZrzF12]4- ions in which two pentagonal bipyramidal ZrF7 groups share a common equatorial edge (Zr-F terminal = 1.960-2.073; Zr-F bridging = 2.146, 2.154 while (H3NC&NH3)3(ZrF7)2.2Hz0 contains two crystallographically independent monocapped trigonal prismatic [ZrF7J3- ions.55x The following guanidinium and aminoguanidinium fluorozirconates have been isolated from aqueous media and have been characterized by IR spectroscopy and X-ray diffraction: (CN,H,),ZrF, (CN3H&ZrF7, (CN4H7)2ZrF6,(CNfi@F,.O.5&0 and (cN4Hp)ZrF6.H20. The IR and X-ray studies indicate the presence of [ZrF6lz- in (CN4H7)2ZrF6,[Zr2F1J4- in ( C N ~ H ~ ) Z Z Tand F ~ [Zr&.,]'ions in (CN4H8)ZrF6-0.5H20. Comparison of IR spectra of (CN3H6)3ZrF7and (CN4J&)ZrF6.H20 with spectra of compounds of known structure suggests that the former compound contains [ZrF7I3- ions while the latter probably contains (Zre-), chain anions like those in K2ZrF6. Thermal decomposition of the (CN.&)zrF6-nHz0 compounds involves loss of water at 100-120 "C and elimination of HF at 130-190 "C; the latter process yields the pentafluorozirconate (CN4H7)ZrF5.559 (iii) Divalent rnetul fluorometallates
Several types of divalent transition metal fluorornetallate hydrates have been obtained by slow, room-temperature evaporation of aqueous HF solutions of M 0 2 or MF, (M = Zr or Hf) and the divalent metal fluoride M"F2. A 1:1 molar ratio of M"F2 and MOz gives well-formed crystals of M"MF6.6H20 (M" = Fe, c o , Ni or zn), MnMF6.5H20 or cubi.F6.4H20, while a 2: 1 ratio of M"F2 and MF4 in water, slightly acidified with HF,yields crystals of M';MFg12H20 (M" = Co, Ni, Cu or Zn). From HF solutions of CdF2 and MOz, crystals of CdZMFg6HzO are obtained, regardless of the molar ratio of CdFz and M02.560,561 The MMF6-6Hz0compounds are isostructural with FeSiF6.6Hz0(space group R h ) , which is known to contain octahedral [Fe(H20)6]2' and [SiF6I2- ions in a slightly distorted CsCl s t r ~ c t u r e . The ~ ~ -structure ~ ~ ~ of CUZrF6.4H20 consists of square planar [CU(H~O)~]~+ and octahedral [ZrF6j2- ions (Zr-F = 1.982-2.007 A), which are arranged in infinite
428
Zirconium and Hafnium
.[ZrF6I2-...[CU(H~O)~]" * *.[ZrF6Iz-. . chains. Thus the coordination polyhedron about copper is a tetragonally distorted octahedron, with two rather long (2.246 A) trans Cu.--F interaction^.'^^ The structure of MnZrF6.5H20 contains infinite chains of ZrF, dodecahedra, which are threaded on the twofold (c) axis of the monoclinic crystal (Figure 9). Each ZrF, dodecahedron shares the two a edges with neighboring ZrFs dodecahedra and shares two B vertices with two MnFz(H20)4 octahedra. An additional uncoordinated water molecule is present in the lattice. Averaged bond lengths are Zr-FA = 2.21 and Zr-FB = 2.03 &563*564
Figure 9 Projection of part of the structure of MnZrF,.SH,O on the ac plane showing ZrF, dodecahedra and MnF,(H,O), octahedra
The isostructural octafiuorozirconates MIZrF8-6H20(M" = Mn or Cd) have a layer structure in which ZrFs square antiprisms and M"F4(H20)3pentagonal bipyramids are linked together by sharing F.-.F polyhedral On the other hand, Cu2ZrF8.12H20 contains isolated square antiprismatic [ZrF#- anions and [CU(H,O)~]'+cations.567 The heptafluorozirconate C U ~ ( Z ~ F ~ ) isolated ~ + ~ ~from H ~ O40% , hydrofluoric acid, contains [CU(H~O)~]'+,[ C U ~ ( H ~ O ) ~and ~ ] ~[ZrzF'14]6+, ions, which are held together by a threedimensional network of hydrogen bonds. The [2r2FJ- ion is a centrosymmetric dimer in which two ZrFs square antiprisms are joined together by sharing an s polyhqdral edge (Zr-F terminal = 2.048-2.107; Zr-F bridging = 2.171, 2.185 4.5:h635m The structure of K2Cu(ZrF6)2.6H20 consists of K+, [ C U ( H , ~ ) ~ ] and ~ + , centrosymmetric [Zr2F12]4-ions, ID [Zr2FI2l4-,two ZrF7 pentagonal bipyramids share an equatorial edge (Zr-F terminal = 1.9672.064; Zr-F bridging = 2.156, 2.161 A).57o Anhydrous alkaline earth and transition metal hexafhorometallates Mf'MF6 are generally prepared from M"F2 and MF4 by crystallization of a melt or by prolonged annealing of the metal fluorides. Most of these compounds exist in two polymorphic forms, depending on the temperature. The low temperature (a)form of BaZrF6 contains centrosymmetric [Zr2FI2l4anions in which two ZrF, monocapped trigonal prisms share the F-9-Fedge that joins the site to an adjacent vertex (Zr-F terminal = 1.999-2.079; Zr-F bridging = 2.155, 2.254 ).571 The structure of the high-temperature ( p ) form of BaZrF, contains infinite chains of a-edge-shared ZrFs dodecahedra, similar to the chains in MfirF6-5HzO (Figure 9); bond lengths in P-BaZrF, are Zr-FA = 2.242 and Zr-FB = 2.010 A.572C E - S ~ Z ~a-EUZrF6, F~, and PbZrF6 are isostructural with P-BaZrF6; a single-crystal study of the lead compound gave Zr-FA = 2.19 and Zr-FB = 2.00 A.573In studies of phase equilibria for the SrF2-MF4 (M = Zr or Hf) systems, the compounds a-and p-Sr2MFs and Sr3MF,, have been identified in addition to a- and P-SrMF6.574 M"MF6 (M" = Mn, Fey Co, Ni or Cu) exist in several crystalline forms, the most common being a high-temperature, cubic ordered R eo 3 structure and a low-temperature, trigonal LiSbF6 structure. Both modifications consist of octahedra which share corners in three dimensions and which are centered alternately by M" or M.575577The isostructural lanthanide fluorometallates LnMF, 578 also have an extended three-dimensional structure; an X-ray study of SmZrF7579has shown that the structure is built on corner sharing of ZrF6 octahedra and irregular SmF8 polyhedra.
cappiY
Zirconium and Hafnium
429
(iu) Vibrational spectra of fluorometallates The vibrational spectra of alkali fluorometallates have been examined in considerable detail. In the hexafluorometallatesLi2ZrF6and MaMF, M’ = Rb or Cs; M = Zr or Hf) the octahedral [MF6I2- anion occupies a site of D3d symmetry,‘3L,532 and consequently the triply degenerate normal modes [v3 (flu), v4 (tlu), v5 (t2,) and Y g (t%)in Oh]are split into a and e components. Vibrational frequencies (Table 19) have been assigned on the basis of polarized Raman580,s81 and polarized IR reflectances8’ spectra of single crystals, and assignments have been confirmed by normal coordinate analyses. TnMe 19 Vibrational Frequencies for Solid Alkali Hexafluorornetallates
~~
Li2ZrF6 Rb2ZrF6 cs2zrF6 CS2HfF6
585
580(?)
589 579 572
585(?)
560 537 535 498
498 522 502 490
246 240 229 217
187 192 190 187
303 258 245 257
251 244 233 247
142(?)
1 2 1 2
1. L. M.Toth and J. B. Bates, Spectrmhim. Acta, Part A , 1974,30, 1095. 2. 1. W.Forrest and A. P. Lane, Inorg. Chem., 1976,15,265.
Far IR spectra of solid K2MF6, (NH4)3ZrF7,(N&)2ZrF6, (N&)ZrF5 and (NH4)ZrF5.Hz0 exhibit broad band systems in the region 550-15Ocm-l. These spectra are more complex and The broad bands have been attributed to less well defined than spectra of metal-fluorine vibrations of complex anions of low site ~ y m m e t r y The . ~ ~close ~ ~ ~similarity ~ between the spectra of (N&)ZrFs and (NI&)ZrF5-H20 in the Zr-F stretching region strongly suggests that the water molecule is not attached to the Zr atom.’83 Attempts have been made to correlate the frequency of the Raman-active symmetric stretching mode of crystalline fluorozirconates of known structure with the coordination number of the Zr atom.5&1,s85The data in Table 20 show that, for the alkali metal salts, v,(Zr-F) decreases with increasing coordination number, although the interpretation of this trend is complicated by fluorine-bridging in the seven- and eight-coordinate compounds. No correlation exists when all of the data are considered; values of v,(Zr-F) for the isostructural eight-coordinate P-BaZrF6, PbzrF6 and ru-SrZrF6vary considerably among themselves and are more similar to v,(Zr-F) frequencies for the six- and seven coordinate fluorozirconates of alkali metals. Raman spectra of LiF-NaF-ZrF4 melts have been examined, and compositiondependent systematic variations in Raman frequencies have been interpreted in terms of an equilibrium distribution of [ZrFSl4-, [zrF,]’-,$ZrF,]’and possibly species in which the Zr atom has a coordination number less than six.’ Table u) Coordination Numbers of the Zirconium Atom and v,(Zr-F) Rarnan Frequencies for Crystalline FIuorozirconates Coordination number
6
Compound Li,ZrF, Rb,ZrF,
v, (Zr-F) (cm-’)
(&zrF6
585 589 579
7
Na,ZrF, a-BaZrF,
556 560
8
K2ZrF, Na7Zr6bt B-BaZrF, PbZrF, a-SrZrF,
525 548 562 567 578
1 . L. M.Toth, A. S. Quist and G. E. Boyd, J. Phys. Chem., 1973,77,1384. 2. I. W.Forrest and A. P. Lane, Inorg. Chem., 1976, W, 265. 3. Y. Kawamoto and F. Sakaguchi, Bull. Chem. Soc.Spn., 1 9 8 3 , s 2138.
Ref.
1 3 3 3
Zirconium and Hafnium
430
An IR spectral criterion for identification of dinuclear anions having enta onal bipyramidal coordination about zirconium has been proposed by Davidovich et aLR4 Thgese workers have noted that the [Zr2FI2l4- anion in [H3NC2H4NH3I2[Zr2FI2] and [CN3H6I4[Zr2Fl2]and the [Zr2Flo(H20)2]2-anion in [NMe4]2[Zr2F,o(H20)2]exhibit an intense v(Zr-F bridging) band in the 300-400cm-' region and a characteristic triplet of v(Zr-F terminal) bands in the 400-600 cm-' region; the central band of this triplet, located at 470-490 cm-', is more intense than the other two.
3247.3 Chlommetallates, bromometallates and iodometralluies
In contrast to the great variety o€ fluorometallates, nearly all of the known chloro-, bromoand iodo-metallates are salts that contain the simple, octahedral [M&]'- anion. The best characterized alkali metal salts are listed in Table 21. The hexachloro complexes MSMCls may be prepared (i) by fusing a stoichiometric mixture of M'Cl and MCL at elevated t e r n p e r a t ~ r e s , ~(ii) ~ ~by , ' ~heating ~ M'Cl in the presence of MCL vapor at -1 atm p r e s s ~ r e ' ~ ~ , ~ or (is) by reaction of stoichiometric amounts of M'Cl and M C 4 or MOCl2.8H20 in concentrated hydrochloric acid saturated with hydrogen chloride The recently reported MIMX, (X = Br or I) complexes were obtained by fusing or sintering a stoichiometric mixture of M'X and M X . Analogous ammonium salts (NH&MC16 have been isolated from hydrochloric acid solutions of W C 1 and MOC12.8Hz0.' 9 0 , ~ ~ ~
Table 21 Physical Properties of Alkali Hexachloro, Hexabromo and Hexaiodo Complexes of Zirconiurn(lV) and Hafnium(1V) AH; (298 K)
Compound
M . p . ("C)
535l 557l
w3
AH; (kJ mol-')
("C)
39 f 7l 37 f 5l 17 f 5l
501' 513' 634'
23*d
831'
6605
7991 80S6
7a8 795'O 809'
826'
'Enthalpy of fusion.
AHc' (kJ mol-')
-32 f 2' -46 f 34 -79 + z4 -96 + 3'
(kJ mol-')
1040' 9532
(4
-1835 4~4'
-1857
f
12'
- 1932 f 44
-1957 f lz4
9042 73 f 2"
ad
10.082(3)3 10.036(3)' 10.178(4)9
-145 rt 2'
-1992 f 4'
-59 f 512
-1603 f 612
-116 f 2j2 -167 i 4"
-1665 f 312
-179312
10.407(5)9 10.395(4]: 10.62(1) 10.58(1)l2 10.91(1)'* 10.91(1)'2
Decomposition temperature at Thich the equilibrium pressure of MCl,(g) reaches 1 atm. e Enthalpy of the Cubic lattice parameter. complexing reaction: 2M'X(s) + MX,(s)+ M;M&(s). 1. J. E. Dutrizac and S. N. Flengas, Can. 3. Chem., 1967,45,2313. 2. D . A. Asvestas, P. Pint and S. N. Flengas, Cnn. J . Chem., 1977, 55, 1154. 3. R. L. Lister and S. N. Flengas, Can. J . Chem., 1964,42, 1102. 4. P. Gelbman and A. D. Westland, J . C h m . Soc., Dalton Trans., 1975, 1598. 5. I. S. Morozov and S. In'-Chzhu, Russ. I . Inorg. Chem. (Engl. Transl.), 1959, 4, 307. 6. G. I. Kipouros and S. N. Hengas, Can. 3. Chem., 1978,56,1549. 7. G . J. Kipouros and S. N. Flengas, Can. J. Chem., 1983,61, 2183. 8. G . M. Toptygina and I. B. Barskaya, RILES. J . Inorg. Chem. (Engl. Transl.), 1965,10, 1226. 9. G. Engel, 2.Kristallogr., Kristallgeom., KristaIlphys., Kristallchem., 1935, 90, 341. 10. I. B. Barskaya and G. M. Toptygina, RWS. J . Inorg. Chem. (Engl. Transl.), 1967, U ,14. 11. A. S. Kucharski and S. N.Flengas, Can. J. Chem., 1974, 52, 946. 12. R. Makhija and A. D. Westland, I. C k m . SOC., Dalton Tram., 1977, 1707. 13. V. V. Chibrikin, A. V. Nevzorov, 2.B.Mukhmetshina,V. P. Seleznev and G, A. Yagodin, Rurs. J. Inorg. Chem. (Engl. Tmml.), 1981,26, 1216. 14. V. V. Chibrikin, Yu. V. Shabaev, Z. B. Mukhametshina, V. P. Selemev and G. A. Yagodin, Russ. J . Inorg. Chem. (EngI. Transl.), 1981, 26, 1376. 15. D.Sinram, C. Brendel and B. Krebs, Inorg. Chim. Acta, 1982, 64, L131.
Zirconium and Hafnium
431
The M i m compounds are moisture-sensitive, high-melting, white (X = C1 or Br) or yellow (X = I) crystalline solids that decompose at elevated temperatures to solid M'X and gaseous Mx,. The thermal stability of the MiMC16 compounds, as measured by the decomposition temperature Tdec (Table 21), increases with increasing size of the alkali cation. Values of AH,, the enthalpy of the complexing reaction in equation (511, indicate that [HfC16j2- is slightly more stable than [ZrC16]*-, but [HfBr6I2- is apgreciably more stable than [ZrBr6I2-, thus illustrating the enhanced class b character of Hf . The thermodynamic data in Table 21 and data for related NblV and SnIV complexes suggest that halogen px+metal dx bonding is important in [Zr&]'- and [HfX$ c o m p ~ e x e ~ . ~ ~ ~ ~ ~ ~ ~ 2M'X(s)
+ M&(s)
-
M;M&(s)
(51)
X-Ray powder patterns indicate that MiMCl6 (M' = K , Rb, Cs, or NH4),591 MiMBr6 (M' = K or C S ) , and ~ ~ C~ S ~ M Ihave ~ , ~the ~ cubic K2PtC16structure (space group Fm3m), which contains regular octahedral anions. The powder data afford Z r - C l bond lengths of 2.44(10 A for RbzZrC16 and 2.45(10) A for Cs2ZrC16?94An averaged H f - C l bond length of 2.446 in the octahedral [HfCl6I2- ion has been obtained from a single-crystal study of Bi+[Bi$+][HfC12-]3.595The Hf-I distance in Cs2Hf16 is 2.829(2) A.593 Interestingly, the NazMC16 complexes are not cubic; X-ray powder data for Na2HfC16 have been indexed assuming a tetragonal cell with a = 15.99 and c = 13.21 A.596 Quite a number of hexachlorozirconates and some hexachlorohafnates, hexabromozirconates and hexabromohafnates of nitrogen bases have been reported. These compounds are of the type A2M& (A = R4N, R3NH, R2NH2, RNH3 or C,H,NH; R = alkyl or aryl). They ma be prepared by (i) reaction of MX4 with a substituted ammonium halide in thionyl c h l ~ r i d e ? ~ - ~ ~ acetyl chloride,600benzoyl chloride601@2' or acetonitrile;603(ii) reaction of [ m ( E t C N I 2 ] with ;~~ (iii) reaction of Mx, with an amine in ethanol the ammonium halide in c h l o r o f ~ r m or saturated with hydrogen chloride [NEt4I2[MBr6]complexes have been obtained by electrochemical oxidation of zirconium or hafnium metal in the presence of bromine and [NEt4]Br in benzene; however, analogous oxidation in the presence of chlorine and [NPr4]C1in thionyl chloride gave the pentachlorometallates [NPr4][MClS].'ls [C5H5NHI2[ZrCb].reacts with NaOEt in ethanol at reflux yielding the substitution product [C5H5NH]2[ZrC15(OEt)] ,605 Aminolysis reactions of alkylammonium hexachlorozirconates were mentioned in Section 32.4.2.1. Vibrational spectra of cS#Vfc16],s99 [NEt4]2[m],599,mand [Et2NH&WIm (X = CI or Br) have been reported. Raman (Y,, v2 and vs) and IR (vg and v4) bands have been assigned and force constants determined. Frequencies for the NEt: salts, taken from the most complete study to date,5w are listed in Table 22. Far IR spectra of MiZr16 (M' = Li, Na, K, Rb or Cs) display a strong band in the re ion 195-175 cm-l; the analogous M1Hf16 complexes exhibit a similar band at 165-145 cm-'J7 Electronic spectra of [Et2NH2lZ[Zr&] (X = C1 or Br) have been recorded, and UV bands have been assigned to halogen px+metal d charge-transfer transitions.
d
Table 22 Vibrational Spectral Data for Solid [NEt,],[M&]
[~t'ilm~l [NEt,l,[HfC&I [NEt4l*[ZrBr,l [NEt412iHfBr,l
321s 326s 194s 197s
25Ow, sh 257w, sh 144w, sh 142w, sh
293s 275s 223s 189s
(X= C1 or Br) Complexes'
152s 145s 106m 102m
151s
156s 99s 101s
1. W. van Bronsayk, R. J . H.Clark and L. Maresca, Inorg. Chem., 1969,8, 1395.
r
Other hexachlorornetallates that have been characterized by vibrational s ectroscopy include [SbPh4]2[ZrC16],m [No]~[Mc16](M = Zr or Hf) ,609 and [SCi3]2[ZrC16].61 The [NO],[MC16] complexes were prepared by treating a concentrated solution of the metal tetrachloride in thionyl chloride at 0 "C with NOCl gas; their IR and Raman spectra exhibit the bands expected for [MC16I2- and show a strong v(NO+) band at 2191-2199 cm-l. [SCl3I2[ZrCl6]was obtained by reaction of sulfur, chlorine and ZrC14 in a sealed tube; Raman frequencies of [ZrC16]2- in this compound were identified at 324 (q), 258 ( v 2 ) and 154cm-' ( ~ 5 ) .
432
Zirconium and Hafiiuin
Raman spectra of ZrC14-PC15 mixtures reveal the presence of at least one chlorozirconate species in addition to [ZrC&J2-. The new Raman frequencies have been attributed to tri onal bipyramidal [ZrC15]-, which is formed as a result of the equilibrium in equation (52).6 IR evidence for the presence of solvated [ZrClJ in acetonitrile solutions of ZrC14 and [NEt4]C1 has been presented by Olver and Bessette," and [NEt4][ZrCI5(MeCN)]has been isolated from acetonitrile by Feltz.612 This compound loses acetonitrile in vacuo at 90°C yielding [NEt4][ZrC15].
9
[ZrC1,I2-
+ [PCI4]+ e [ZrC15]- + PC1,
152)
The reaction of ZrCld with t-butyl chloride and phosphorus(1IE) chloride in carbon disulfide gives [Me3CPC13][Zr2C19] as a moisture-sensitive, insoluble solid. IR and Raman spectra of the anion are consistent with a D3hstructure in which two ZrCb octahedra share a common face; v(Zr-Cl terminal) 350-387, v(Zr-C l bridging) 240-315 L
32.4.8 Hydroborates and Hydrides as Ligands Zirconium(1V) and hafnium(1V) tetrakis(tetrahydrob0rates) M(B&)4 are of interest as extremely volatile, covalent complexes that contain tridentate B&- Li ands and exhibit rapid intramolecular exchange of bridging and terminal hydrogen atoms.6'4,615 These compouncts were prepared initially from NaMF5 (M = Zr or Hf) and excess Al(BH& (equation 53),616but they are obtained more conveniently from the reaction of the anhydrous metal tetrachloride with excess lithium tetrahydroborate (equation 54), either in the solid state617i618 or in the presence of a small amount of diethyl ether.619
Zr(BH+,)4 and Hf(BH& have very similar properties. Both are low-melting (m.p. 29"C), colorless crystalline solids that exhibit a vapor pressure of 15mmHg at 25 "C. They are highly reactive, spontaneously inflaming in dry air, detonating on contact with water, and slowly decomposing with liberation of hydrogen gas on standing at 25°C. The solid Gomplexes are very soluble in nonpolar solvents. The chemical reactions of IvI(BH~)~ complexes have received very little attention. Both compounds behave as Lewis acids in reactions with tetraalkylammonium and lithium tetrahydroborates, yielding [(C8H17)3NPr][M(B&)5], [NBu4][M(BH4),] and LiM(B&)5.6U' Z T ( B H ~reacts )~ with LiAH, in ether to give Zr(Al&),, an unstable white solid, which decomposes within several hours at room temperature to a pyrophoric black solid.617 Tridentate coordination of the BHB ligand in M(B&)4 complexes was first established by a low-temperature (- 160 "C) single-crystal X-ray study of Zr(BIT,)4.621A very recent siylecrystal neutron diffraction study (at -163 "C) of the isostructural Hf(B&), analogue6= has afforded accurate positions for the bridging hydrogen atoms, The regular tetrahedral Hf(BH.& molecule (Figure 10) occupies a site of crystallographic 43m{Td) symmetry, and the BE& ligands adopt the conformation in which the Hf-H bonds to one B€& ligand are staggered with respect to the Hf-B bonds to the other three BH.- ligands. Bond lengths are Hf--B = 2.281(8), Hf-Hb = 2.130(9), B-Hb = 1.235(10) and B-H, = 1.150(19) A, where Wb and Ht denote the bridging and terminal hydrogen atoms, respectively. An electron diffraction study623 has afforded the following bond lengths for the gaseous Z T ( B H ~ )molecule: ~ Zr-B = 2.308 f0.010, Zr-H,, = 2.21 f0.04, B-Hb = 1.27 f 0.05 and B-Ht = 1.18 f 0.12 A. The electron diffraction data suggest that the gaseous molecule may have only T symmetry, with the tridentate BHT ligands being rotated out of the exactly staggered conformation (Figure 10) by an averaged torsion angle of 22". IR and Raman s ectra of Zr(BH& and Hf(BIT,)4 have been assigned in terms of both Td and T and assignments have been supported by deuterium isotopic substitution studies62w27and normal coordinate analyses.62M29The vibrational spectra have not provided unequivocal evidence for a choice between these two closely related point groups. However, the spectra strongly support a tridentate attachment of the B&- l i g a r ~ d s . ~ Characteristic ~~.~~' IR and Raman bands for Z T ( B H ~ and ) ~ Hf(B&)4 occur in the following frequency regions:
Zirconium and Hafnium
433
Figme 10 The structure of Hf(BH,), (reproduced by permission from ref. 622)
v(B-H,) 2560-2580, v(B-Hb) 2100-2200, bridge deformation 1200-1300 and v(M-B&) 480-560 cm-' . The Raman studies626and normal coordinate ana lyse^^^,^^^ suggest that direct metal-boron bonding interactions may be important. He(1) and He(I1) photoelectron spectra of M(B&)4 (M = Zr or Hf) have been reported. Assignments were made on the basis of ualitative molecular-orbital model^^^',^^ and an LCAO-HFS(Xa) calculation on Zr(B&)4. 6% 'H and "B NMR spectra of M(BH4), (M = Zr or Hf)618p63"636and 91ZrNMR spectra of Zr(BH4)4636at 25°C indicate the equivalence of all of the hydrogen atoms owing to rapid intramolecular exchange between the bridging and terminal positions. 'H and "B spectra of the zirconium and hafnium compounds are closely similar. The 'H spectra consist of a quartet of equally intense lines due to spin-spin splitting by boron-11 (I = $; J("B-H) = 90 Hz),with some additional weaker lines due to splitting by boron-10 (I= 3; J("B-H) = 30 Hz). The '*B spectra exhibit a 1:4:6:4: 1 quintet (J("B-H) = 90Hz). The inner nine lines of the anticipated 17-line spectrum have been observed in the llB-decoupled 91Zr spectrum of Z T ( B H ~ )the ~ ; averaged coupling constant J(Zr-H) is 28 Hz. On cooling solutions of M(BH4)4to -80 "C, the 'H resonance lines are markedly broadened. This effect was originally attributed to a slowing of the rate process that exchanges hydrogen atoms between the bridging and terminal sites.634However, it was later shown that the observed line-shape changes are due to quadrupolar effects and that the hydrogen-atom exchange rate is still rapid (probably > lo4 s-l) at -80 Ca5 Variable temperature, solid-state 'H NMR studies of M(BH4)4637have provided evidence for two intramolecular motional processes. The higher temperature process has been assigned to exchange of bridging and terminal hydrogen atoms; for M = Z r , E a = 2 1 . 8 i 1 . 3 and AG* (-73 "C) = 30.5 W mol-'; for M = Hf, E, = 35.1 f 1.3 and AG* (-59 "C) = 33.9 kJ mol-l. The lower temperature process has been interpreted in terms of rotation of the BH; ligands about the threefold (M-B-HJ axis; for M = Zr, E, = 22.6 f 0.8 and AG* (-149 "C) = 18.0 W mol-'; for M - Hf, E, = 19.2 f 0.8 and AGz (-140 "C) = 19.2 kJ mol-*. The methyltrihydroborato complex Zr(BH3Me)4has been prepared by reaction of ZrC14 and LiBH3Me in chlorobenzene. It has a tetrahedral structure, like Z T ( B H ~ )with ~ , tridentate attachment of the BHJMe- ligands (Zr-B = 2.335(3), average Zr-H,, = 2.06, average 1.14 The mixed-ligand complexes [MC1(BH4){N(SiMe3)2)2] (M = Zr or Hf) and [Hf(BH4)3{N(SiMe2CH2PMez)2}] were mentioned in Sections 32.4.2.4.ii and 32.4.3.2 respectively. IR and 'HNMR studies of the [MC1(BH4){N(SiMe3)2}2]complexes indicate that the BH; ligand is tridentate and exchange of bridging and terminal hydrogen atoms is fast on the NMR time scale at -80 0C.81On the other hand, [Hf(BH4)3{N(SiMe2CH2PMe2)2}] contains bidentate BHT ligands as evidenced by two v(B-H,) (2470 and 2420 cm-') and two v(B-Hb) (2110 and 1950cm-') IR bands. 'H and "BNMR spectra of this complex indicate rapid exchange of ine uivalent B&- ligands as well as rapid exchange of bridging and terminaf hydrogen atoms.940 Treatment of [Hf(BHJ3{N(SiMe2CH2PMe&}]with Lewis bases (NEt3 or PMe3) yields the This complex is fluxional; 'H and dinuclear trihydride [JHm(SiMe2CH2PMe2)2]}2(H)3(BH4)3]. selectively decoupled 'P NMR spectra show the presence of three equivalent hydrides coupled to four equivalent phosphorus n u ~ 1 e i . l ~ ~
Zirconium and Hafnium
434
32.4.9 Mixed Donor Atom Ligands
32.4.9.1 Open polydentate ligands
Zirconium(1V) forms a rather large number of complexes with ONO- and ONS-tridentate Schiff base ligands, ONNO- and SNNS-tetradentate Schiff base ligands, and related species such as diacyl hydrazines. Relatively few hafnium analogues have been reported thus far.
( i ) ONO- and ONS-Tridentate Schiff base ligands Zr(OCHMe2)4.HOCHMe2reacts in benzene at reflux with a variety of dibasic tridentate Schiff bases H2L in 1 :1 and 1:2 mole ratios to give complexes of the type Zr(OCHMe&(L) and Zr(L),, respectively. The Schiff bases that undergo these reactions include aldimines and ketamines derived from carbonyl compounds and hydroxyalkyl amines (35)63p-641 or mercapazines (37),643semicarbazones (38) ,614 thiosemicarbazones (39) ,645 and toalkyl amines (36) ,@’ Schiff bases derived from S-alkyldithiocarbazates (40);646one example of each type of ligand is shown in (35)-(40). The carbonyl compounds from which these Schiff base ligands are derived are commonly salicylaldehyde, 2-hydroxy-l-naphthaldehyde, acetylacetone or 2hydroxyacetophenone. With but a few exceptions, the Zr(L), complexes are monomeric in solution, while the Zr(OCHMe2)2(L) analogues are dimeric. IR spectra indicate that (L)’behaves as an ONO- or ONS-tridentate ligand, and six-coordinate structures have been suggested, for example (41) and (42). The Zr(OCHMe&(L) complexes undergo alkoxide exchange reactions with alcohols such as t-butanol, 2-methylpentane-2,Cdiol (C6HI4O2),or benzene-1,2-diol (C&02) yielding the [Zr(OCMe&(L)], [Zr(C6HI2O2)(L)] and IZr(C6H4O2)(L)] derivatives, respectively; these complexes are monomeric in solution and presumably five-coordinate.
TNLTI R
(35) H,L; R = H or Me, E = 0 (36) HZL;R = H or Me, E = S
Me
’
yNbg H/
N-i
(37) H J -
‘OH
Me
NH?
(38) HZL;E = 0 (39) H2L; E = S
(42) R=CHMe,
Mixed-ligand complexes [Zr(L)(L’)] that contain dinegative anions of two different ONOtridentate Schiff base ligands have been prepared in benzene at reflux by reaction of a 1:1 : 1 mole ratio of Zr(OCHMe2)4-HOCHMe2, H2L and H2L’.647 Several [ Zr(L)z] [ {Zr(OCHMe2)2(L)}2], [Zr(C&II2O2)(L)] and [Zr(C6H402)(L)] complexes that contain the dinegative anions of ONS-tridentate Schiff bases have been synthesized from benzot hiazolines.@* The reaction of Hf(OCHMe2)4.HOCHMe2 with stoichiometric amounts of the Smethyldithiocarbazate (40) or the dibasic benzoyl hydrazones (43) yields analogous hafnium complexes Hf(L)2 and Hf(OCHMe2)2(L).@9*650 Only the disubstitution products HfC12(L)could be obtained from the reaction of HfC14 with (40) or (43). The HfC12(L complexes are moisture-sensitive, insoluble, yellow solids and are probably polymeric.6517s2 On the other hand, a monomeric trigonal bipyramidal structure with trans chlorine atoms has been suggested for thiosemicarbazone complexes of the type ZrCl2(L); these compounds exhibit a single v ( Z r 4 1 ) IR band at -290 cm-1.653Unfortunately, both the HfC12(L) and ZrC12(L) complexes are too insoluble for molecular weight measurements.
b
Zirconium and Hafnium
435
\
Ph
H
(43) H,L; R = H or Me
Zirconyl chloride (or acetate) reacts in methanol at reflux with tridentate Schiff bases prepared by condensation of substituted salicylaldehydes with ethan~lamine$~ uhydroxyben~ylamine,~~ o-aminobenzyl or ~alicylhydrazide~~~ to give 1:2 meta1:ligand complexes of the type [ZrO(HL),]. IR spectra indicate that the Schiff bases tbehave as monobasic ONO- tridentate ligands; for example, (35) coordinates to zirconium through the phenolate oxygen, azomethine nitrogen and hydroxylic oxygen atoms. The complexes are monomeric nonelectrolytes in solution, and they exhibit a medium intensity IR band in the region 880-945 cm-l, which has been attributed to v(Z-0). If these compounds do contain a Z-0 group, the Zr atom would be seven coordinate. Dibasic tridentate Schiff bases derived from salicylaldehydes and 2-aminobenzoic acid658or l-amin0-2-rnercaptobenzene~~~ react with aqueous zirconium nitrate to give monomeric complexes of the type [Zr(OH),(L)(H,O)]. IR spectra of these compounds support an ONOor ONS-tridentate attachment of the (L)2- ligands. (ii) ONNO- and SNNS-Tetradentate Schiff base ligands The eight-coordinate complex [Zr(dsph], where (dsp)’- is the dianion of the tetradentate Schiff base (U),has been prepared by condensation of tetrakis(salicyla1dehydato)zirconium(1V) [Zr sal with o-phenylenediamine, and its structure has been determined by X-ray difhaction?’ T L i complex has a dodecahedral coordination polyhedron with nitrogen atoms in the A sites and oxygen atoms in the B sites. The tetradentate ligands span the mum polyhedral edges, arid the trapezoidal planes are very nearly perpendicular (dihedral . The Zr-N bonds (mean 2.43 A) are appreciably longer than the Zr-0 bonds ). The corresponding [Zr(d~peb)~] complex, where (dspeb),- is the dianion of (45), is of interest as a possible starting material for synthesis of coordination polymer^.^ A glossy red polymer of approximate molecular weight 20000-40000 has been prepared by condensation of [Zr(sal),] and 1,2,4,5-tetraaminoben~ene.@~
R (44) H,dsp; R = H (45) H,dspeb; R = CO,Et
The reaction of Zr(OCHMe&.HOCHMez with dibasic ONNO-tetradentate Schiff base ligands derived from diaminoalkanes and salicylaldehyde, 2-hydroxy-l-naphthaldehyde,acetylacetone or 2-hydrox acetophenone yields dimeric [{ Zr(OCHMe2)2(L)}2] and monomeric [Zr(L),] complexes,Janalogous to the ONO-tridentate Schiff base complexes discussed in the previous section. Monomeric [Zr(OCHMe2)2(L)]and [zX(L)z] complexes, where (L)’- is the dianion of the SNNS-tetradentate Schiff bases (46) have been prepared from Zr(OCHMe2),-H0CHMe2 and the appropriate benzothiazoline.648 A few compounds of composition ZrC12(L)663g664and ZrO(L)(H20)3665have been synthesized in methanol by reaction of ONNO-tetradentate Schiff bases and zirconyl chloride. CoC3-0
Zirconium and Hafnium
436
R R (46)H2L; R = H or Me
(izi) Diacyl hydrazines With diacyl hydrazines, zirconium tetrafluoride and tetrachloride form 2 :1, 1:1 and 1:2 adducts of the type (ZrF&(RCONHNHCOR’) (R = 4 -C & N, R’ = Me or Ph),666 ZrF4(RCONHNHCOR> (R = R’ = H or Me; R = Me, R’ = Ph),667 and ZrCL(MeCONHNHCOPh)2. In addition, zirconium tetrachloride undergoes substitution reactions yielding ZrC12(RCONNCOR’) (R = R’ = H, Me or Ph; R = Me, R‘ = Ph; R = 4-C5H4N, R ‘ = M e or Ph).666,668All of these compounds may be polymeric since they have high decomposition temperatures and are insoluble in common organic solvents. IR spectra suggest that the diacyl hydrazines function as ONNO-tetradentate ligands. 3249.2 Aminopolycurbmylutes
Zirconium(1V) and hafnium(1V) form numerous stable chelate complexes with the anions of aminopolycarboxylicacids. Among the compounds that have been isolated in the solid state are K2[Zr(nta)2],669[M(edta)(Hz0)2]-2H20670 and H[Zr(dtpa)]-3H20671(M = Zr or Hf; nta = nitrilotriacetate; edta = ethylenediaminetetraacetate; dtpa = diethylenetriaminepentaacetate). These compounds are prepared by concentration of hot, acidic aqueous solutions that contain stoichiometric amounts of an MIV salt and the chelating ligand. Solid complexes of the type M‘Hf(OH)(edta).2H20 and M’Hf(OH)(cdta).2H20 (M’ = Na or N a ; cdta = 1,2cyclohexanediaminetetraacetate) are also known.672 Eight-coordinate dodecahedral structures have been established for [Zr(nta),]’- 673 and [Zr(edta)(H,O),]”“ by single-crystal X-ray studies. [Zr(nk&I2- exhibits a ligand wrapping pattern (Figure 11) in which the two nitrogen atoms occupy dodecahedral A sites, in accord with Orgel’s rule; the three glycinate groups of each tetradentate nta ligand span a, g and rn polyhedral edges. In comparison with cubic, square antiprismatic, and other dodecahedral stereoisomers, the observed stereoisomer is ideally suited for minimizing nonbonded repulsions and chelate-ring strain. Z r - 0 bonds to the dodecahedral A sites (2.251(7) A) are significantly longer than Zr-0 bonds to the B sites (2.124(9), 2.136(8)&, and the Zr-N bonds are extraordinarily long (2.439(9) A). In [Zr(edta)(H20),] (Figure ll), the two nitrogen atoms and the water molecules occupy the dodecahedral A sites, while the four oxygen atoms of the hexadentate edta ligands take the €3 sites, again in accord with Orgel’s rule (Zr-N = 2.43(2): Zr-0 = 2.12(2), 2.14(2); Zr--OH2= 2.27(2) A).
Io)
F i g u r e 11 Ligand wrapping pattern in (a) [Zr(nta),12- and
t b)
(b) [Zr(edta)(H,O),J. Primed and unprimed symbols are
related by the indicated crystallographictwofold axes
Zirconium and Hafnium
437
The existence of a wide variety of zirconium(1V)-aminopolycarboxylate complexes in solution was established by the potentiometric titration studies of Intorre and Marte11.675-677 This work and related solution studies have been reviewed by Larsen.' Zirconium(1V) forms 1:1 and 1:2 complexes with tetradentate aminopolycarboxylates, 1:1 complexes with hexadentate and octadentate aminopolycarboxylates, and mixed ligand complexes with aminopolycarboxylates and various bidentate ligands, for example 1:1: 1 Zr-edta-Tiron and 1:1:2 Zr-nta-Tiron complexes (Tiron = 1,2-dihydroxybenzene-3,5-disulfonate).The stoichiometries of these complexes are consistent with the tendency of zirconium to achieve a coordination number of eight. 'H NMR spectra of the 1:1 Zr-edta and Hf-edta complexes in the pH range 1-3.5 exhibit an AB pattern for the inequivalent glycinate protons and a single resonance for the ethylene The spectra are not as complex as might have been expected on the basis of the low (C,)symmetry of the dodecahedral [Zr(edta)(H20)z]molecule; evidently th4 inequivalent glycinate groups undergo rapid exchange, but the rate of Zr-N bond rupture is slow on the NMR time scale. Changes in the NMR spectra above pH 3.5 have been interpreted in terms of hydrolysis and polymerization. Thermodynamic parameters for formation of M(edta) and M(nta)+ (M = Zr or Hf) from M4+ and (edtaJ4- or (nta)3- have been determined at 15, 25 and 35°C from calorimetric 3 for Zr(ed1a) at 25 "C are AGO = -187.2 f 0.3 kJ mol-l, AH" = measurements. 8 ~ 8 Results -2.6 f 0.6 kJ mol-' and AS" = 618.8 dz 2.5 J mol-' deg-I. The large positive entropy change is associated with the effects of dehydration of the highly charged reacting ions. 32.4.9.3
NO-,NS-and US-bidentate ligands
Zirconium(1V) and hafnium(1V) halides from a variety of 1:1 and 1:2 adducts with NO- and NS-bidentate ligands (Table 23). These compounds are generally prepared by mixing solutions or suspensions of the components in a polar organic solvent at room temperature. They are moisture-sensitive, white or yellow solids, insoluble in most organic solvents. Structures of the adducts have not been established; however the mode of coordination of the ligands is suggested by frequency shifts in characteristic IR bands upon complexation. Most of the ligands in Table 23 behave as NO- or NS-bidentate ligands, but 8-quinolinol and the N arylsalicylaldimines appear to coordinate only through the nitrogen atom. IR spctra suggest that only one of the two benzaldehyde thiosemicarbazone ligands is attached to the metal in MC14(PhCH=NNHCSNH& (M = Zr or Hf). Coordination of the hydroxylic oxygen atom of 3- and 4-aminophenol in the 1:4 adducts MCl.,(H2NC&,OH), is uncertain. Anhydrous zirconyl chloride forms insoluble, yellow 1:2 adducts, ZrOC12(2HOC6&CH=NC&R), (R = H, 4-Me, 4-C1 or 4-OMe), with N-arylsalicylaldimines. Fivecoordinate structures in which the salicylaldimines behave as N-bonded monodentate ligands have been suggested on the basis of IR spectra.684 The reaction of ZrCh with two equivalents of ethanolamine or diethanolamine in ethyl acetate yields the substitution products ZrC1z(OGH$JH2)2 or ZrClz(0C&NH~&OH),, respectively, while the analogous reaction with triethanolamine gives the 1 :4 adduct ZrCl4{N(GH40H)3).+.IR spectra indicate that (OQH4NHz)- behaves as an NO-bidentate ligand.68' The following moisture-sensitive substitution products have been prepared by prolonged heating (in refluxing chloroform, THF or dichloromethane) of stoichiometric amounts of ZrCL or HfC14 and benzoyl hydrazine, 4-pyridinecarboxylic acid hydrazide, benzoylhydrazones or Schiff bases derived from S-methyldithiocarbazate: ZrC12(PhCONNH2)2,668 ZrC13(4NCs&CONNHz),666 HfC13(L) and HfC1z(L)2 (L = RFt'C=NNC(O)Ph or RR'C=NNC(S)SMe).651*652 IR spectra indicate that these ligands act as monobasic NO- or NS-bidentate ligands, forming five-membered chelate rings upon complexation. The complexes are insoluble in most organic solvents, and they may be polymeric. The N-phenylsalicylaldiminatoderivative ZrC12(2-OC6H4CH=NPh)2 has been synthesized by condensation of aniline and dichlorobis(sa1icylaldehydato)zirconium(IV) .686 ZrC14 and HfC14 react with 8-quinolinol (eight equivalents) in THF at reflux or in the absence of solvent at 140 "C to give the hydrolytically and thermally stable, isomorphous tetrakis chelates [M(&O-q~in)~]( M = Z r or H , These compounds can be sublimed at -350 T / O . O S mmHg.@' Precipitation of Zrr9 from aqueous solutions with 8-quinolinol often gives products of variable composition, but pure [Zr(8-O-q~in)~] can be obtained from hot
Zirconium and Hafnium Table 23 Complexes of Zirconium(1V) and Hafniurn(1V) Halides with NO- and NS-Bidentate Ligands Ligund Sernicarbazide Thiosemicarbazide Acetone thiosemicarbazone Benzaldehyde thiosemicarbazone Benzoyl hydrazine 4-Pyridinecarboxylicacid hydrazide Aminopyrine 2-Pyrazine carboxamide 8-Ouinolinol
N-Arylsalicylaldimines Amiiophenols
complex“
Rf$
Ligund donor atom@)
[MCl,( H,NNHCONH,)] [MC14(HZNNHCSNH,)] [MCI,( Me,C=NNHCSNH,)] [MC&(PhCH=NNHCSNH,)] MCl,(PhCH=NNHCSNH,), ZrF,(PhCONHNH,), ZrCI,(PhCONHNH,), ZrF4(4-NC,H,CONHNH,)
0 and hydrazine pri-amino N S and hydrazine pri-amino N S and azomethine N S and azomethine N S and azomethine N 0 and pri-amino N 0 and pri-amino N 0 and pri-amino N
I~Cl,(C,,H,,N,O)l tZr~,(C,W,O)I ZrF4(8-HO-quin) MC14(8-HO-quin), MC14(2-HOC6H,CH=NC6H,R), (R = H, 4-Me, 4-NO,) MCI,(2-H,NC6H,0H), MClJ 3-H,NC,H,OH), MC1,(4-H,NC6H,0H),
0 and dimethylamino N
8
0 and pyrazine N N N Azomethiie N
9 10 10 11
OandN
12,13 12,13 12,13
N,0 (?I N,0 I?)
aM=ZrorHf. 1. Ts. B. Konunova, A. Yu.Tsivadze, A. N. Smirnov and S. A. Kudritskaya, Rurs. I. Inorg. Chem. (Engl. Trunrl.), 1982, 27, 807. 2 . Ts. E. Konunova, A. V. Abluv, S. A. Kudritskaya and V. D. Brcgd, Sou. J. COOrd. Chew. (Engf. Traml.), 1976, 2, 589. 3. Ts.B. Konunova, A. Yu. Tsivadze, A. N. Smirnov and S. A. Kudritskaya, Rws. I . Inorg. Chem. (Engf. TransL), 1983,28,1282. 4. Ts.B. Konunova, A. V. Ablov, S. A. Kudritskaya and V. D. Brega, Sov. J . Coord. Chern. (Ens[. Transl.), 1979,5, 658. 5 . R. C. Agganval, B. N. Yadav and T. Prasad, Indurn J. Chem., 1972, 10, 672. 6. R. C. Agganval, B. N. Yadav and T. Prasad, J. Inorg. Nucl. Chem., 1973,35,653. 7. R . C. Agganval, T. Prasad and B. N. Yadav, J. Inorg. Nucl. Chem., 1975,37,899. 8. B. P. Hajela and S. C. Jain, Indian 1. Chern., Sect. A , 1982,21, 530. 9. S. C. Jain, M. S. Gill and G. S. Rao, J . Indian Chem. SOC., 1976, 53, 537. 10. M. J. Frazer and B. Emmer, J . Chem. SOC.( A ) , 1968, 2273. 11. V. A. Kogan, V. P. Sokolov and 0. A. Osipov, Russ. J . Inorg. Chem. (Engl. Trawl.), 1968,33, 1195. 12. Ts. B. Konunova, T. N. Popova, 2. P. Bumasheva and V. D. Brega, Sou. J. Ccmrd. Chem. (Engf. Trumf.), 1978,4,341. 13. Ts. B. Konunova, T. N. Popova, 2.P. Bumasheva and V. D. Brega, Sou. J. Coord. Chem. (Engl. Transl.),1977, 3,278.
solutions that contain an excess of oxalate.68xThe structure of tZr(8-0-q~in)~J is dodecahedral with the nitrogen atoms in the A sites and oxygen atoms in the B sites, in accord with Orgel’s rule (Zr-N = 2.405(8), Zr-0 = 2.106(6) A). The molecule adopts the rather uncommon D2-gggg ligand wrapping pattern, presumably to avoid steric crowding of o-hydrogen atoms along the a edges of the more common D2d-mmmmstereoi~omer.~~’ Tetrakis(N-ethylsalicylaldiminato)zirconium(IV) [Zr(2-OC&CH=NEt)4] has a closely related dodecahedral structure with nitrogen atoms in the A sites and oxygen atoms in the B sites, but in this case the bidentate ligands span the g edges so as to give the very rare S4-gggg stereoisomer. The Zr-N bond length (2.539(9) A) is extraordinarily long, and the Zr-0 bond length (2.055(7) A) is unusually short. [Zr(2-OC6H4CH=NEt)4] and the N-isopropyl and N-t-butyl analogues were prepared by reaction of the appropriate salicylaldimine with Zr(NMe2)4.690 The N,N-diethylhydroxylamido(1- ) complex [Zr(ONEtz)4] has been prepared from Zr(OCHMez)4.HOCHMezand excess Et2NOH. An X-ray study of the analogous titanium(IV) compound shows that the complex exists as the expected dodecahedral Du-rnmrnm stereoisomer with nitrogen atoms in the A sites and oxygen atoms in the B sites. Coalescence of the NMR resonances of the diastereotopic methylene protons of [Zr(ONEt,),] at 42°C indicates rapid cleavage of the Zr-N bonds (AG* = 69 f 6 kJ Numerous NO-bidentate Schiff base complexes and a few NS-bidentate Schiff base complexes of the type M(OCHMe&,(L), (M= Zr or Hf; n = 1, 2, 3 or 4) have been synthesized by reaction of stoichiometric amounts of M(OCHMe2)4-HOCHMe2and the Schiff base (HL) in refluxing benzene. The known zirconium complexes contain ligands such as N-arylsalicylaldimines, N-arylnaphthylaldirnine~~~~*~~~and N-(2-pyridyl)sali~ylaldimine.~~~”‘ In some cases, only two or three of the isopropoxy groups could be replaced by the Schiff bast ligand. The Zr(OCHMeZ)(L>3 and Zr(L)4 complexes are monomeric in solution, bur association, presumably involving isopropoxy bridges, is sometimes observed for the Zr(OCHMe&(L) and Zr(OCHMe2)2(L)2 complexes. Benzoyl hydrazone and S. methyidithiocarbazate Schiff base complexes of hafnium, Hf(OCHMe2)4-n{RCH=
Zirconium and Hafnium
439
NNC(O)Ph}, and Hf(OCHMe2)4-,{RCH=NNC(S)SMe}, (R = Ph or C4H30), have been reported by Verma et al.64g*a50 Oxozirconium(1V) N-arylsalicylaldiminato complexes of composition ZrOCl(L) and ZrO(L)Z (L = 2-OC6H4CH=NAr) have been prepared by reaction of 1:1 and 1:2 mole ratios of ZrOC12.8H20 and the Schiff base in MeOH-EtzO. The ZrOCl(L) complexes can also be obtained from a 1:1:1 molar mixture of ZrOC1243H20,salicylaldehyde and aromatic amine.= Treatment of a hot methanol solution of appropriate zirconyl salts with 1,lo-phenanthroline mono-N-oxide (phenNO) yields the oxozirconium(1V) complexes [ZrO(phenNO)X2] (X = C1, Br, NO3 or NCS), [2rO(pl1enNO)~]I~ or [ZrO(phenNO)3][C104]2.697 These compounds are analogues of the 2,2'-bipyridine-l ,l'-dioxide complexes discussed in Section 32.4.4.1. Insertion of phenyl isocyanate or methyl isothiocyanate into the metal-nitrogen bonds of M(NMe2)4gives the tetrakis (NO- or NS-bidentate) chelates [M{NPhC(0)NMe2}4](M = Zr or Hf) or [Zr{NMeC(S)NMe2}4].72Analogous insertion into the Zr-C bonds of Zr(CH,Ph), yields [Zr{NPhC(0)CHzPh},] or [Zr{NMeC(S)CH2Ph>4],698 while insertion of phenyl or naphthyl isocyanate into the Zr-0 bonds of zirconium alkoxides gives Zr{NR'C(0)OR},(OR)4-, (R' = Ph or C10H7;R = Pr, m e z or CMe3; n = 1, 2, 3 or 4):" A few complexes have been reported in which zirconium(1V) or hafnium(1V) is attached to an OS-donor bidentate ligand. Alkyl thioglycolates, Zr(OCHMe2)2(L) and Zr(L)2 (L = [SCH=C(OR)0I2-; R = Me or Et), have been synthesized from stoichiometric amounts of Zr(OCHMe2)4 and the alkyl thioglycolate, H2L. The isopropoxy groups of Zr(OCHMe&(L) are replaced by t-butoxy groups when the isopropoxy complex is heated under reflux with an excess of Me3COH in benzene. Molecular weight measurementts and IR spectra indicate that the Zr(OR)2(L) complexes have alkoxy-bridged dimeric structures.699 Related Zr(OCHMe2),(L) and Zr(L)z (L = ECH2CH20]2-, [SCHZCH~CO~]~or [ ~ - S C ~ H ~ C O Z ] ~ - ) complexes have been r e p ~ r t e d . ~ ~Reactions ~.~ of stoichiometric amounts of zirconium isopropoxide and 2-mercaptopropionic acid yield Zr(OCHMe2)(L)(HL) and Zr(L)2.471 Tetrakis(thioacetylacetonato)zirconium(IV) [Zr(Sa~ac)~] has been prepared by reaction of stoichiometric amounts of ZrCL and Na(Sacac) in dichloromethane. [Zr(Sacac),] has a square antiprismatic structure (47) in which the ligands span the s polyhedral edges in such a way as to cluster the sulfur atoms in all-cis positions. The observed Cz-ssss stereoisomer is distorted in the direction of the dodecahedral C1-rnrngg and bicapped trigonal prismatic C1-tltlp2p2 stereoisomers. Consistent with the former distortion, the averaged Zr-0 and Z r 4 bond lengths fall into two classes (Zr-OA = 2.185, Zr--OB = 2.132, Zr-SA = 2.724, Zr--SB = 2.665 A).7o1
The sulfur atoms are also clustered in all-& positions in the C1tlpzpz bicapped trigonal prismatic structure of tetrakis(N-methyl-p-thiotolylhydroxamato)hafnium(IV) [Hf{MeCc;H4C(S)N O)Me},]. Averaged metal-ligand bond distances are Hf-0 = 2.150 and H f 4 = 2.678 .702 Monothiocarbamato complexes are discussed in Section 32.4.5.2.
d
32.4.10 Multidentate Macrocyclic Ligands 32.4.10.1 Porphyrins Octaethylporphinato complexes of the type [M(OEP)(02CMe)2](M = Zr or Hf) have been prepared by reaction of H20EP and [M(acac),] in molten phenol at 210-240°C followed by crystallization from pyridine-acetic acid-water mixtures. The complexes have been characterized by IR and mass s p e ~ t r a ' ~and ~ , by ~ ~electronic absorption and emission spectra.7D5They have an eight-coordinate square antiprismatic structure (48) in which the four porphinato nitrogen atoms occupy the coordination sites on one square face and the two bidentate acetate
Zirconium and Hafnium
440
ligands span s edges on the opposite (rectangular) face. Bond lengths in the hafnium complex of C,, symmetry are Hf-0 = 2.278(3), Hf-N1 = 2.266(5) and Hf-N2 = 2.248(3) A; the zirconium and hafnium complexes are nearly i s o d i m en ~i o n al . ~~
The red-violet mixed-ligand complex [Zr(OEP)(acac)(OPh)] has been isolated from the reaction of H20EP and [Zr(acac),] in molten phenol, and has been converted to bis(/3diketonato) derivatives [Zr(OEP)(dik)2] (dik = acac or bzbz) by reaction with an excess of the B-diketone in boiling pyridine. The hafnium analogues [Hf(OEP)(dik)2] (dik = acac or bzbz) were prepared from the @-diketone and [Hf(OEP)(02CMe)2]. 'H NMR spectra of the [M(OEP)(bzbz),] complexes at 0 "C (M = Zr) or -40 "C (M = Hf) exhibit two triplets and two quartets for the porphinato ethyl groups, consistent with a C, structure like The meso-tetraphenylporphyrin complex [Zr(TPP)Clz] has been prepared by reaction of HzTPP with ZrC14 in boiling benzonitrile. The electronic spectrum of [Zr(TPP)C12] and the facile kinetics of TPP dissociation in acidic media suggest that the zirconium atom lies considerably out of the plane of the TPP ligand with both chlorine atoms being located on the same side of the TPP plane."' 32.4.10.2 Phthalocyanines
The bluish chlorophthalocyanine complexes [M(PcC1)C12] (M = Zr or Hf; PcCl = [C32H15C1N8]2-)have been synthesized by reaction of the metal tetrachlorides with ophthalonitrile at 280 "C (equation 55)."'9 Treatment of the [M(PcCl)Cl,] complexes with boilin acetic acid gives the dichroic red-violet/dark blue acetato analogues [M(PcCl)(02CMe)2]?o' while treatment of the [M(PcC~)~] complexes with ethanol and then warm water yields the blue hydroxo derivatives M(PcC1)(OH),.2Hz0 .710 These complexes exhibit enormous thermal stability. [ Z ~ ( P C C ~ ) ( O ~ C M can ~ ) ~be] sublimed in high vacuum at 500 0C.709 [Hf(Pc),] (Pc = [C32H16N8]2-)sublimes at 540-560 "C and does not decompose until -686 0C.711 ZrCl,
+ 4GH4N2
-
[Zr(PcCl)Cl,] + HCl
(551
32.410.3 Other polyaza mamcycles ZrOC12.8H20reacts with 2,6-dipicolinoyl dihydrazine (H2dpdh) yielding a 1: 1 complex that has been formulated as ZrO(dpdh)(H,O), or Zr(OH)z(dpdh)(H20) (49). The free NH2 groups of compound (49) condense with the carbonyl groups of P-diketones affording compounds (50) that are believed to contain 12-membered macrocyclic rings.712
?\
~ N - N H ~
d'
0
R'
Zirconium and Hafnium
441
32.5 REFERENCES 1. 2. 3. 4. 5. 6.
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Zirconium and Hafnium
45 1
H. G. Langer, J. Inorg. Nucl. Chem., 1964, 26, 59. R. E. Sievers and J. C. Bailar, Jr., Inorg. Chem., 1962, 1, 174. A. Marques-Netto and J. C . Abbe, J. Inorg. Nucl. Chem., 1975, 37,2235. J. L. Hoard, E. W. Silverton and J. V. Silverton, J. Am. Chem. SOC., 1968,90,2300. A. I. Pozhidaev, M. A. Porai-Koshits and T. N. Polynova, J. Struct. Chem. (Engl. Transl.), 1974, 15, 548. B. J. Intorre and A. E. Martell, J . Am. Chem. SOC., 1960, 82, 358. B. J. Intorre and A. E. Martell, J. Am. Chem. SOC., 1961, 83, 3618. B. J. Intorre and A. E. Martell, Inorg. Chem., 1964, 3, 81. Y. 0. Aochi and D. T. Sawyer, Inorg. Chem., 1966, 5 , 2085. N. A. Kostromina, V. P. Shelest, T. V. Ternovaya and Ts. B. Konunova, Russ.J. Znorg. Chem. (Engl. Transl.), 1977, 22, 1659. 680. V. P. VasiYev, V . P. Lymar and A. I. Lytkin, Russ. J. Inorg. Chem. (Engl. Transl.), 1978, 23,29. 681. V. P. Vasil’ev, V. P. Lyrnar and A. I. Lytkin, Russ.J. Inorg. Chem. (Engl. Transl.), 1978, 23, 525. 682. V. P. Vasil’ev, V. P. Lymar and A. I. Lytkin, Russ. J . Inorg. Chem. (Engl. Transl.), 1978, 23, 683. 683. V. P. Vasil’ev, V. A. Borodin, V. P. Lymar and A. I. Lytkin, Russ. J . Inorg. Chem. (Engl. Trawl,), 1981, 26, 1306. 684. N. S. Biradar, A. L. Locker and V. H. Kulkarni, Rev Roum. Chim., 1974,19,45. 685. Ts. B. Konunova, E. 1. Toma and V. D. Brega, Sou. J. Coord. Chem. (Engl. Trunsl.), 1976,2,800. 686. V. A. Kogan, V. P. Sokolov and 0. A. Osipov, J. Gen. Chem. USSR (Engl. Transl.), 1970,40,292. 687. M. J. Frazer and B. Emmer, J. Chem. SOC. ( A ) , 1968,2273. 688. A. V. Vincogradov and V. S. Shpinel’, Russ.J. Inorg. Chem. (Engl. Transl.), 1961, 6, 687. 689. D. F. Lewis and R. C. Fay, J . Chem. SOC., Chem. Commun., 1974, 1046. 690. D. C. Bradley, M. B. Hursthouse and I. F. Rendall, J. Chem. Soc., Chem. Commun., 1970, 368. 691. K. Wieghardt, I . Tolksdorf, J. Weiss and W. Swiridoff, Z. Anorg. Allg. Chem., 1982, 490, 182. 692. S. R. Gupta and J. P. Tandon, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 1973, 21, 911. 693. J. Uttamchandani and R. N. Kapoor, Monatsh. Chem., 1979,110,841, 694. M. Singh, B. L. Mathur, K. S. Gharia and R. Mehta, Inorg. Nucl. Chern. Lett., 1981, 17, 291. 695, M.Singh, Pol. J . Chem., 1982, 56, 393. 696. A, Kriza and M. Nasta, Rev. Roum. Chim., 1981, 26, 693. 697. A. K. Srivastava, R. K. Agarwal, M. Srivastava and T. N. Srivastava, J. Znorg. Nucl. Chem., 1981, 43, 2144. 698. J. F. Clarke, G. W. A. Fowles and D. A. Rice, J. Organornet. Chmn., 1974, 74, 417. 699. D. Bhatid, A. M.Bhandari, R. N. Kapoor and S. K. Sahni, Synth. React. Inorg. Met.-Org. Chem., 1979, 9, 95. 700. M. Hasan, B. S. SankhIa and R. N. Kapoor, A u t . J . Chem., 1968, 21, 1651. 701. M. E. Silver, H. K. Chun and R. C. Fay, Inorg. Chem., 1982,21,3765. 702. K. Abu-Dari and K. N. Raymond, Inorg. Chem., 1982,21, 1676. 703. J. W. Buchler, G. Eikelmann, L. Puppe, K. Rohbock, H. €3. Schneehage and D. Weck, Liebigs Ann. Chem., 1971, 745, 135. 704. J . W. Buchler and K. Rohbock, Inorg. Nucl. Chem. Lett., 1972, 8, 1W3. 705. M. Gouterman, L. K. Hanson, G.-E. Khalil, J. W. Buchler, K. Rohbock and D. Dolphin, J. Am. Chem. SOC., 1975, 97,3142. 706. N. Kim, J. L. Hoard, J. W. Buchler and R. Rohbock, unpublished results cited by J . L. Hoard in ‘Porphyrins and Metalloporphyrins’, ed. K. M. Smith, Elsevier, Amsterdam, 1975, pp. 347-348. 707. J, W. Buchler, M. Folz, €3. Habets, J . van Kaam and K. Rohbock, Chem. Ber., 1976,109, 1477. 708. B. D. Berezin and T. N. Lornova, Russ.J . Inorg. Chem. (Engi. Trunsl.), 1981, 26, 203. 709. P. Muhl, 2. Chern., 1967, 7 , 352. 710. V. E. Plyushchev, L. P. Shklover and I. A. Rozdin, Rms. J . Znorg. Chem. (Engl. Transl.), 1964, 9 , 68. 711. P. N. Moskalev, V. Ya. Mishin, E. M. Rubtsov and I. S. Kirin, Russ. J. Znorg. Chem. (Engl. Transl.), 1976, 21, 1243. 712. S. Kher, S. K. Sahni, V. Kurnari and R. N. Kapoor, Synth. React. Inorg. Metal-Org. Chem., 1980,10,431.
670. 671. 672. 673. 674. 675. 676. 677. 678. 679.
33 Vanadiurn LUIS VILAS 8OAS and JOAO COSTA PESSOA lnstituto Superior Tecnico, Lisbon, Portugal 33.1 GENERAL INTRODUCTION
454
33.2 LOW OXIDATION STATES (BELOW +2)
451 457 451 457 457 458 459 460
33.2.1 Introduction 33.2.2 Cyanide Complexes 33.2.3 Nitrogen Ligan& 33.2,3.1 Sipyridyl, a-phenanthrohe and terpyridyl 33.2.3.2 Nitrosyl 33.2.3.3 Dinittogen complexes 33.2.4 Phosphorus Ligands 33.2.5 Tris(dithio1ene) and Tris(diseleno1ene)Complexes 33.2.6 Hydrido Complexes
460 462 462 462 462
33.3 VANADLUM(I1) 33.3.1 Introduction 33,3.2 Cyanides and Isocyanides 33.3.3 Nitrogen Ligan& 33.3.3.1 Ammonia and amines 33.3.3.2 N-heterocyclic ligands 33.3.3.3 Isothiocyanotes 33.3.3.4 Acetonitrile 33.3.4 Phosphines 33.3.5 Oxygen Ligands 33.3.5.1 Water 33.3.5.2 Alcohols and ethers 33.353 b-Diketones 33.3.5.4 Oxalate and carbomte 33.3.6 Halides 33.3.7 Macrocyclic Ligands 33.3.8 Vanadium(II) as Reducing Agent 33.3.8.1 The reduction of N2and related compounds 33.3.8.2 The reduction of HzO,H,Oz and 0, 33.3.8.3 The reduction of metal complexes 33.3.9 Biological Dinittogen Fixation
463 463 463 464
4a 465 465 465 466 466 467 467 469 469 469
471 472 413
473 473 474 414 474 475 475 476 476 476 477 477 478 478 478 479 479 480 480 481 481 481 481 482 483 483 484 485 486
33.4 VANADIUM(1LI) 33.4.1 Introduction 33.4.2 Cyanide and Isocyanide 33.4.3 Nitrogen Ligands 33.4.3.1 Ammonia, amines and amides 33.4.3.2 N-heterocyclic ligands 33.4.3.3 Thiocyanate, selerwcyanate and azide 33.4.3.4 Nitriles 33.4.3.5 Silaranes 33.4.4 Phosphines and Arsinex 33.4.5 Oxygen Ligands 33.451 Aqua species, O H 33.4.5.2 Diaxygen 33.4.5.3 Alcohols and ethers 33.4.5.4 /3-Ketoenolates and catecholate 33.4.5.5 Oxoanwns 33.4.5.6 Carboxylic acids 33.4.5.7 Hydroxy acids 33.4.5.8 Dimethyl sulfoxide and amides 33.4.6 Sulfur Ligands 33.4.6.1 Thioethers 33.4.6.2 Dithiocarbamates 33.4.6.3 Dithiolate and dithiophosphinates 33.4.7 Halides 33.4.8 Mixed Donor Alum Ligan& 33.4.8.1 Schiff bases 33,4.8.2 Amino acidr 33.4.8.3 Complexones 33.4.9 Vanadium in Tunicates
453
454
Vanadium
33.5 VANADIUM(1V) 33.5.1 Introduction 33.5.2 Group IV Ligands 33.5.3 Nitrogen Ligands 33.5.3.1 Ammonia and aliphatic amines 33.5.3.2 N-heterocyclic l i g a d 33.5.3.3 Hydrazine and related ligands 33.5.3.4 Thiocyanate 33.5.3.5 Nitriles 33.5.3.6 Biguanide and related ligands 33.5.4 Phosphorus and Arsenic Ligands 33.5.5 Oxygen Ligands 33.5.5.1 Oxovanadiurn(PV) in aqueous solutions 33.5.5.2 Peroxide ligand 33.5.5.3 Alcohols 33.5.5.4 Catecholate, polyketonate, hydroxyaldehydateand hydroxykctonate ligands 33.5.5.5 Oxoanions as ligands 33.1 5.6 Carboxylates 33.5.5.7 Hydroxycarboxylates 33.5.5.8 Other oxygen ligands 33.5.6 Sulfur Ligands 33.5.6.1 Thiovanadyl complexes 33.5.6.2 CS,-based and related ligands 33.5.6.3 Dithiolates 33.5.6.4 Other S-containing ligands 33.5.7 Selenium Ligands 33.5.8 Halogen Ligands 33.5.8.1 Oxovanadium(W} complexes 33.5.8.2 Vanadium(N) complexes 33,5.9 Mixed Donor Atom Ligands 33.5.9.1 Schiffbase ligands 33.5.9.2 Amino acids 33.5.9.3 Complexones 33.5.9.4 Other polydentate ligands 33.5.10 Mixed-ligand Complexes 33.5.11 Multidentate Macrocyclic Nitrogen Ligands 33.5.11.1 Porphyrins 33.5.31.2 Phthalocyanines 33.5.11.3 Tetraaza[l4]annulenesand other macrocycles 33.5.12 Polynuclear Complexes 33.5.12.1 Binucleating and cvmpartmental ligands 33.5.12.2 Heteropolynuclear complexes 33.5.13 Nnturally Occurring Ligands
487 487 489 489 489 4B 494 495 4% 496 496 498
33.5 REFERENCES
569
498 501 502
502 511 513 517 522 523 523 524 528 529 529
529 529
531 531 531 544 547 551 555 557 557 559 559 561
56I 56.5
567
33.1 GENERAL INTRODUCTION The element was recognized in 1831,when N. G. Sefstrom was able to isolate and characterize the oxide; the name vanadium was derived from Vanadis, a goddess in Scandinavian mythology. The beautiful colours of vanadium compounds had been observed as early as 1801, by A. M. del Rio in his experiments with a new element he called erythronium because of the red colour after treatment with acid.’ Some tribulations in the experimental work and the variety of colours observed in early vanadium chemistry can now be explained as due to the formation o€ various coordination compounds, early symptoms of a rich and challenging chemistry. There are vanadium compounds with formal oxidation states from -3 up to +5 with the exception of -2. Under ordinary conditions, the most stable states are +4 and +5. Suitable sources of information for the early chemistry of vanadium are Melior's' and Sidgwick’s3 books. More recent work is covered by several The coordination chemistry of vanadium is strongly influenced by the oxidizingheducing properties of the metallic centre, and the chemistry of vanadium ions in aqueous solution is limited to oxidation states +2, +3, i - 4 and +5, although V’ can reduce water. Redox potentials are given in Table 1 and an Evs. pH diagram is shown in Figure 1. The oxidation state -3 has been observed only in organometallic compounds, and until now there have been no reports of compounds with oxidation state -2, although there is no reason
Vanadium
455
Table 1 Standard Potentials of Vanadium Couples in Aqueous Solutions at 25 "C
Strongly acid solutions
Couple
Stanahrd potentids (V) in Weakly acid soluiions
Neutral and basic solutions
~~~~
VV-VIV vlv-vl"
0.723 0.481 -0.082 -1.13
1.000 0.337 -0.255 -1.13
v"I-vII
V"-P
0.991 0.542 -0.486 -0.820
Data from Y. Israel and L. Meites, in 'Standard Potentials in Aqueous Solution', rd. A. I. Bard, R.Parsons and J. Jordan, Dekker, New York, 1985.
2 16
I2 38
->
04
1
4
c -04
-08 -12 -I E.
0
2
4
6
8
1 0 1 2
14
PH
Figure 1 Potcntial versu~pH diagram for the vanadium-water system at 25 "C.The dashed lines indicate the domains of relative predominance of the dissolved forms of the metal, but the various dissolved forms for each oxidation state are not explicit. The solid lines correspond to saturated solutions with a total vanadium concentration of 0.51 gdm-3. The long dashed lines correspond to oxidation and reduction of water (for E" values of 1.23 and 0.00 V respectively) (adapted from E. DeItombe, N. Zoubov and M. Pourbaix, in 'Atlas d'Equilibres Electrochimiques', ed. M. Pourbaix, Gauthier-Villars, Paris, 1963)
to believe that such compounds cannot exist. Coordination compounds with formal oxidation states from -1 up to +2 are usually octahedral; for some it is not possible to ascertain a formal oxidation state as the bonding scheme may alternatively be explained by coordination of the reduced ligand. The ability of V2+to reduce water and the instability of most of the complexes in these low oxidation states explains the small number of compounds. On the other hand, the reducing properties of vanadium(I1) ions and their complexes have been used in many preparative methods, and among such reactions, the dinitrogen reduction deserves special attention. Although many vanadium(II1) complexes are unstable towards air, there are quite a few compounds in this oxidation state, most with octahedral geometry. However, remarkable seven-coordinate complexes were also characterized. High concentration of vanadium in the blood of some tunicates has been a long-standing problem of biochemistry. That vanadium(II1) ions are part of the respiratory pigment has been ruled out recently. The efficient mechanism used to concentrate vanadium from sea water is now understood but the utility of vanadium for these living organisms is still an intriguing question. The +4 oxidation state is the most stable under ordinary conditions and the majority of vanadium(1V) compounds contain the V02+ unit (vanadyl ion), which can persist through a variety of reactions. Its complexes typically have square pyramidal or bipyramidal geometry
Vanadium
456
with the vanadyl oxygen apical and the V atom lying above the plane defined by the equatorial ligands. Trigonal bipyramidal complexes are also known. The V02+ entity bonds most effectively to the more electronegative atoms, e.g. F, C1, 0, N (and also S and P). Many vanadyl complexes are not air stable and generally they can be easily hydrolyzed and/or oxidized to the +5 oxidation state. Table 2 gives examples of the coordination geometry adopted by each oxidation state. Table 2 Oxidation States and Stereochemistries of Vanadium Complexes Oxidation state -3
Coordination number
1
Low but not defined 2 3
Exumples
6
7 -1 0
Stereochemistry
6 6 6 6
6 3
Monocapped octahedral Octahedral Octahedral Octahedral Trigonal prismatic Octahedral
Planar
6
Tetrahedral Trigonal bipyramidal Octahedral Pentagonal bipyramidal Tetrahedral Trigonal bipyramidal Square pyramidal Octahedral
8 4
Dodecahedral Tetrahedral
5 6
Trigona1 bipyramidal Square pyramidal Octahedral
7
Pentagonal bipyramidal
8
Dodecahedral
4 5 6
7 4 5
Many vanadium(V) compounds are oxo complexes containing the V03+ or the V02+ entity, and the cis geometry in dioxo complexes has been confirmed by structure determinations. A great number of oxo complexes containing halides, alkoxides, peroxide, hydroxamates and aminocarboxylate have been characterized. The oxidation of ligands by vanadium(V) prevents the isolation of a larger number of complexes. On the other hand, the oxidizing properties of vanadium(V) compounds are useful for many preparative reactions, namely for the catalysis of oxidations. Important examples are catalysts used for the oxidation of SOz to SO3 in the industrial production of sulfuric acid, and there is great concern over the presence of small concentrations of vanadium in fuels (coal and petroleum) because of this ability of vanadium to catalyze the oxidation of sulfur dioxide. Some similarities between vanadate and phosphate explain the biological activity of the metal in the oxidation state +5. The most remarkable example is the strong inhibition of the Na+ pump (Na+, K+-ATPase). This and other problems in the bioinorganic chemistry of vanadium are likely to continue as areas of active research. The great recent development in electrochemical techniques will certainly be helpful for the study of redox processes of a metal which can occur in so many oxidation states. Multinuclear NMR spectrometers will allow increased use of 51V resonance as a routine method for the characterization of complexes in solution. Other recent developments are the study of polynuclear complexes, metal clusters (homo and hetero-nuclear) and mixed valence complexes, and it can be anticipated that these topics will soon become important areas of vanadium coordination chemistry, although the isolation of compounds with such complex
Vanadium
457
structures is again symptomatic of the difficulties that may be found in the chemistry of this metal.
33.2 LOW OXIDATION STATES (BELOW +2) 33.2.1 Introduction The low oxidation states (from -3 to +1) are less usual; ligands in these complexes are those capable of stabilizing low oxidation states of other metals. The derivatives of the pentacarbonylvanadate(-111) ion have the lowest formal oxidation state known for vanadium. Solutions of [Na(diglyme)2][V(C0)6] are reduced by sodium to [V(CO)5]3-. Various salts were isolated by using the alkali iodide or onium ha1ide;'O Rbf and Cs+ salts are rather thermally stable. [V(CO)5]3- with R3EX (R= alkyl or aryl; E = Sn, Pb; X = halide) leads to [(R3E)V(C0)5]2- and [(R3Sn)2V(C0)5]-. The IR spectra of crystalline (Et4N)[(Ph3Sn)2V(CO)5]and its solutions are inconsistent with a pentagonal bipyramidal geometry and X-ray methods showed'' that the anion structure is approximately a monocapped octahedron: the Sn-V lengths are 2.757(3) and 2.785(3) A with LSnVSn 137.9(1)". The corresponding compounds with oxidation number -2 have not been isolated.
33.2.2 Cyanide Complexes VBr3, potassium and potassium cyanide react in liquid ammonia." The brown product, K2V(CN)2.0.5NH3, was paramagnetic (1.86 BM) and extremely pyrophoric. v(CN) at 1905cm-' is consistent with a low oxidation state; the U V reflectance spectrum was recorded: 45.0, 36.0 sh, 28.0 sh, 20.0 br (cm-' x lo3).
33.2.3 Nitrogen Ligands
33.2.3.1 BipyfidyI, oghenanthroline and terpyridyl Complexes with formal oxidation states from +2 to -3 were reported by Herzog and co-workers in classic experiments; those for the bipyridyl complexes are in Scheme 1. Solutions of [V(bipy)s]12 in 50% methanol on reduction by magnesium or zinc yield [ V ( b i ~ y ) ~ ]This .'~ had peff= 1.9 BM and a two-dimensional X-ray study'4 indicated that the nitrogen atoms were at the corners of a distorted octahedron with V-N 2.10 f 0.03 A and LNVN 73.6 f 1.5". ESR spectral5 also suggested trigonal symmetry and an electronic structure analogous to that for [VS6C6Ph6](Section 33.2.5).
1 equivalent I2 of in 1, py in py
/
h
e
y
1 isocyanide
Li(V(bipy),] a4THF black diamagnetic crystals Scheme 1 Preparation and interconversions of bipy complexes in low oxidation states
458
Vanadium
Solutions of [V(bi~y)~]+ can be obtained by partial oxidation of [ V ( b i ~ y ) with ~ ] iodine but the solid obtained with empirical formula [V(bipy),]I.Jpy was suggested'' to be a mixed crystal of the similar compounds of vanadium(0) and vanadium(I1). Na3[V(bipy)3].7THF was ~ b t a i n e d 'on ~ reducing the tris(bipy) complex with sodium. This air-sensitive compound has peff= 2.76 BM. The corresponding tris(phenanthro1ine) complexes were obtained'' on reducing a [V(~hen)~]Iz solution in THF with Li2benzophenone to [V(p h e~~)~] which could be reoxidized +. reduction of [V(phen),] with with two equivalents of I2 back to [ V ( ~ h e n ) ~ ] ~Further Lizbenzophenone led to [V(phen),]- and Li[V(phe11)~3.3.5THFwas isolated. Similarly, black [V(terpy);?]was isolated when [ V ( t e r ~ y ) ~was ] I ~reduced with magnesium or LiA1H4.19 The reduction of VC13 by sodium amalgam in the presence of a suitable ligand has been proposed'" as an improved method for the preparation of several zerovalent complexes VL3 (L = 2,2'-dipyridyl disulfide, 2,2'-dipyridylamine, di-t-butyldiimine) The magnetic moments were 1.82, 1.71 and 1.79BM. Such similarity of magnetic properties is in favour of similar bonding. An alternative description considers that the electrons of the reduced species are located in n* orbitals of the ligand. Reduced forms of bipy (bipy- and bipy2-) are known. The UV-vis spectra of [ V ( b i ~ y ) ~ were ] interpreted" as indicating the existence of partially reduced ligand (bipy- or bipy2-) at least in the excited states, but were also interpreted22 as indicating VO bonded to neutral bipy. By comparing IR spectra of [V(bi~y)~]' ( z = 2+, 0) and Li(bipy), [ V ( b i ~ y ) ~should ] be considered23 as a complex of the reduced ligand bipy- or a complex of collectively reduced ligands." The ESR spectrum of [ V h ] [L = benzilbis(N-phenylimine)] showsZShyperfine splitting due to "V, corresponding to a d5 low-spin configuration with 80-90% of the charge of the unpaired electron at the metal. Polarography and cyclic voltammetryz6 of [V(bipy)3]"( z = 2+, 1+, 0, 1-, 2-, 3-) gave Eln values of -0.8, -1.01, -1.11, -1.55 and -2.23V for these five steps, and smaller polarographic waves at -2.10 and -2.6V were attributed to the reduction of free bipy. From comparison of the values for the bipy complexes with those for 4,4'-dimethyl-2,2'-bipy and 5,5'-dimethyl-2,2'-bipy,the electron added (or removed) in the reactions of [V(L)3]z ( z = 2+, 1+, 0) was at t2, orbitals.26 On the other hand, for V(bipy)JV(bipy)Y and V(bipy);/V(bipy)? the added electron would occupy orbitals with a predominant n* character. However, [V(bipy),] is diamagnetic, and an intermediate character has been proposed for it. I
33.2-3.2 Nitrosyl Using [Co(X)(NO)& as a nitrosylating agent, Rehder and co-workers synthesized various nitrosyl and dinitrosyl complexes as depicted in Scheme 2.27728 An extensive study of 51VNMR of solutions of nitrosylvanadium species has been published." THF molecules in cis[V(N0)2(THF)4] are easily replaced by other L leading to C~S-[V(NO)~(THF),-,L,]X (L = several ligands such as py, phen, MeCN, OPEt,, thiophene, acetone; n = 2 to 4; X = C1, Br, I). For L = C N R (R=cyclohexyl, Pri, But), [VX(N0)2L3] are also formed and the presence of coordinated X was e ~ t a b l i s h e dby ~ ~the normal halogen dependence (C1< Br < I) of 51Vshielding in 51VNMR.
Scheme 2 Preparation of dinitrosyl complexes
Vanadium
459
A cyanonitrosyl complex, K3[V(NO)(CN),].2H20, was obtainedz9 from VOi- and CN- in alkaline medium using hydroxylamine as the source of NO. Its structure3' is tetragonally distorted octahedral: the mean V - C distance is 2.17%i,there is a short V-N bond (1.66A) and the VCN and VNO linkages are almost linear. By conducting nitrosylation in H2S,31 &[v(No)(cN)6].Hz0 was isolated and K4[V(NO)(CN)6].~KOH.~H20 studied by d i f f r a c t i ~ n . ~ ~ [V(CN)@O)]*- has approximately pentagonal bipyramidal geometry with the nitrosyl group axial. The mean V-C bond length is 2.15A and the V-N bond length is 1.68A. These complexes have been used to prepare K2[V(NO)(CN)&] from &iV(NO)(CN)6] with L = bipy and [V(NO)2(L)z]' derivatives from K3[V(NO)(CN)5].3 or Using an excess of h droxylamine in the reductive nitrosylation of VOi- forms the dinitrosyl anion [V(NO),(CN).&, and on treatment with bipy (or phen), complexes of formula [V(N0)z(CN)2L].H20were isolated.35 Magnetic moments (peE= 1.5 and 1.6BM for bipy and hen derivatives) confirmed the existence of one unpaired electron and ESR (no 51V-15N or '1V-13C superhyperfine splitting) indicated that it is located in a metal orbital. Two strong v(N-0) bands at 1650 and 1520 cm-' confirm the cis NO groups. A brown polynuclear dichlorotrinitrosylvanadium(-I) was isolated36 from a solution of [VCl,] in carbon tetrachloride by reaction with dry NO, and Beck and co-workers reported3' that the same product was obtained from [v(co)6] with NOC1. A trigonal bipyramidal arrangement with terminal and bridging chloro ligands was proposed for this compound [V(N0)3C12],, which reacts with solvents losing NO and forming mononitrosyls [V(NO)L4Cl2] (L = MeCN, Ph3P0, iphen). Herberhold and Trampisch38 used sodium amalgam or zinc powder to prepare [V(NO),LXl], (L = acetonitrile, methyl isonicotinate) and [V(NO)~(BU'NC)~~PF~. IR and "VNMR indicate that a single cis dinitrosyl species is the intermediate. The extensive work with 'lV NMR is summarized in Figure 2.
300
0
-300
-600 -300 -1200 -1500
p.p ma
Figure 2 51VNMR shift ranges of nitrosyl vanadium complexes. Ligand abbreviations: N = NR3, NCR; 0 = OCRz, OR,, OPR,; P = PR,, As = AsR,, Sb = SbR,, E = P, As,Sb; S = SR2; X = CI, Br, I, PF,. (a) "V shieIding increases from left to right; shift values are measured taking VOCl, as standard. (b) I = [V(nitrilotripropanolato)(NO)L](L = 0 ligand); I1 = [V(dipicolinato)(NO)(ONH,)(H,O)]-; 111= [V(CN),(NO)(ONH,)]3-. (c) L = P, As or CO. (d) L = O ligand or acetonitrile. (e) L=THF or acetonitrile. ( f ) I and I1 are respectively the di- and mono-nitrosyl derivatives formed in acetonitrile solution; 111, resonance measured in CD,NO, solution (data taken from refs. 27 and 38)
33.2.3.3 Dinitrogm cornplats Complexes of vanadium(0) with dinitrogen were obtained by co-condensations. [V(N&] has been prepared3' in frozen matrices; this formulation was based on the similarity of IR and UV-vis spectra with the corresponding [V(CO)6J. By increasin the vanadium concentration, changes in the IR were explained as due to a dimer, [V2(N2)1z].3Dinitrogen is acting as a weak ligand compared with CO, Le. a poorer u donor and a poorer n acceptor.40
8
Vanadium
460
33.2.4 Phosphorus Ligands The dropwise addition of sodium naphthalenide to THF containing vanadium trichloride and 1,2-bis(dimethylphosphino)ethane (dmpe) causes chan es of colours suggesting a stepwise reduction from +2 to 0. Brown [V(dmpe),] was isolated$1 ( p e =~ 2.10 BM) and IR data suggest octahedral coordination. The same complex was synthesized by a metal vapour technique .42 The ESR was that expected and the unit cell is cubic with a = 11.041(3) A. by UV irradiation of Na diglyme)2][V(CO)6]in diglyme [Et,N][V(PF&] saturated with PF3was and studied45 by multinuclear (19F, P, (5Iv ) NMR. The well-resolved spectra confirmed a (non-ideal) cubic geometry for the complex and V-PF3 is weaker than V-CO Other V-P bonds may be found in phosphinehydrido complexes. The heptacoordinate [V(H)(PF3),] was prepared43 from [V(PF&] on heating with phosphoric acid. Reaction of a THF slurry of [V2(p-C1)3(THF)6]2ZnzC16 with PMePh2 followed by addition of LiBH4 yielded a dark red air-sensitive material, a Zn2V2 aggregate with a V=V double bond and hydrogen bridging V to Zn atoms (Figure 3).&
I
1
P Figare 3 Structure of the phosphinohydrido complex V,ZnZH4(BH4),(PMePh,),. The V-V was interpreted as corresponding to a double bonda
33.2.5
distance of 2.400(2) 8,
Tris(dithio1ene) and Tds(diseleno1ene) Complexes
‘Dithiolene complexes’ was the name suggested by McCleverty4’ for complexes of unsaturated 1,2-dithiols without implying any particular ligand structure or valence formalism. The dithiolene complexes described differ from the vanadium(II1) dithiolates (Section 33.4.6) reported by three independent groups.48 The charge at the vanadium in the usual dithiolene complexes may be estimated49 as less than 2, and these complexes are therefore included in the section on low valence states. Various tris(dithio1ene) complexes have been isolated and one-electron oxireductions studied by polarographic and voltammetric techniques. Table 3 summarizes methods of preparation and electrochemical behaviour. Schrauzer and co-workers5’ correlated electrochemical data with Taft’s constants: the observed linear correlation reflected the ligand JG orbital origin of the orbitals involved in the redox process. [V(S2C2Ph2)3] is trigonal prismatic, slightly distorted by a trigonal twist of 8.5”.51 [V(S2&Ph2)3]- was also trigonal prismatic as its electronic spectrum is similar to the isoelectronic [Cr(S2&Ph2)3],51 which was isomorphous with [V(S2C2Ph2)3]. (Me4N)2[V(S2C2(CN)2)3] was studied by d i f f r a ~ t i o nand ~ ~ a structure between octahedral and trigonal prismatic was proposed for the anion [V(S2G(CN)&I2-. The C=C stretching frequencies of the S2C2Ph2and S2GH2 complexes increase with the negative charge of the complexes, i.e. the sulfur ligands become more ‘dithiolate’ in The dithiolene complexes have several intense transitions in the visible, whose assignment is not yet complete. Using previous work as well as the resonance-Raman spectra for [V(SzC2Ph2)3]0>1and [V(S,C,(CN)&]”, a set of assignments was produced (Figure 4).54 The ESR of [V(S2C2(CN)2}3]’- doped in single crystals of isomorphous
Vanadium
461
Table 3 Preparation and Electrochemical Studies of Tris(dithio1ene) and Tris(diseleno1ene) Complexes
Method of preparation of isolated solid complex
Formula
R
Z
El,z values (VI
H
~ ~ ~ ~ Z G R Z ~ 3 1 "-2
-0.72Y
-1
NazSzGHz
+0.25"
NazSzGHz+ VO(acac),
Ref-
0 -3
CN
-0.4gb
4 4
+0.66b
4 5
-2
-1 -3
1 2 1 3
CF3 -1.10*
-2
5
+0.47,b +0.Ogd
-1 0 -2
5
Ph -0.71b
-1 +0.30b
Acyloin + P4S10 method
3 5 5 5
5
0
-2
5
p-MeC6H4 -0.745"
1 3
+0.323"
1
-0.783"
1 3
+0.260a
1
-0.12b
5 5
-O.Bb
5 5
-1 0
-2 Acyloin + P,S,, method
p-MeOC,H,
-1 0
[V(SzC&R)31Z Na,(S,C,H4)
Na&C,H3Me
+ VCl, + VCl,
-3
-2 -3
Me
-2 [V(SezG(cF,)*)3)1= -3
(z = -1 complex) + [AsPhJCl
-2
s%G(cF3)2 + [AsPh41!V(Co)61
-1
-1.05* +0.07d
6 6 6 6 6
Potentials measured in DMF solutions us. Ag/AgCl electrode in an aqueous 0.10 M LiCl solution." Potentials measured in acetonitrile solutions at room temperature us, an aqueous calomel electrode saturated with NaCl.' The difference in potentials measured with techniques (a) and (b) was estimated as -0.03 V, i.e. potentials in DMP may be estimated by adding 0.03 V to the correspondingvalues measured in acetonitrile. Potentials measured in CH,CI, vs. saturated calomel electrode.
a
1. D. C. Olson, V. P.Mayweg and G. N. Schrauzer, J . Am. Chem. Soc., 1966,%8,4876. 2. G.N. Schrauzer, V. P.Mayweg and W . Heinrich, Chem. Znd. (London), 1965, 1464. 3. G. N. Schrauzer and V. P. Mayweg, J . Am. Chem. Soc., 1966,88,3235. 4. A. Davison, N. Edelstein, R.H. Holm and A. H. Maki, J . Am. Chem. Soc.,1964,86,2799. 5 . A. Davison, N. Edelstein, R. H. Holm and A. H. Maki,Znorg. Chem., 1965,4,55. 6. A. Davison and E.T. Shawl, Imrg. Chem., 1970,9, 1820.
[AsPh4]2[Mo{SzCz(CN)z}3] was fitted by an axially symmetric spin Hamiltonian with g,,= 2.000, gl = 1.974, A,,= 10 G and A* = -100 G, interpreted in terms of a 2A1ground state in D3 symmetry.49The unpaired electron has predominantly metal d,z character. A molecular orbital scheme (Figure 4) was proposed for the dithiolene c ~ m p l e x e sThis . ~ ~ scheme has been used in the discussion of ESR results,49and electronic and resonance-Raman ~pectra.'~ Tris(diseleno1ene) complexes derived from Se&( CF& have been prepared by similar methods .56
462
Vanadium
Metal orbitals
Ligand orbitals
Figure 4 Electronic configurations of vanadium tris(l,2-dithiolene) complexes (from ref. 54, based on the scheme in ref. 55)
33.2.6 Hydrido Complexes Hydridophosphino complexes have been described revioudy (Section 33.2.4). The bond dissociation energy of 209 kJ mol-' has been measured R by ion beam mass spectrometry using the reaction in equation (I). The bond energies between V' and other atoms were compared with the corresponding carbon bonds: the energy of the bond to vanadium is about one half that of the corresponding carbon bond.
33.3 VANADIUM(I1)
33.3.1 Introduction The small number of vanadium(I1) compounds may be explained by (i) difficulties in keeping the compounds (they are very often easily oxidizable) and (ii) difficulties in characterization (because of contamination by higher oxidation state(s)). Although the easy oxidation may be inconvenient in the isolation and handling of compounds of vanadium(II), there are many examples of applications of vanadium(I1) as a reducing agent for a large variety of both organic and inorganic species (see Section 33.3.8), usually behaving as a one-electron reducing agent. Vanadium(I1) compounds are usually prepared by reduction of acidic solutions of higher oxidation states. The usual geometry is octahedral and the electronic spectra often show three d-d spin-allowed bands. These s ectra, as well as those of tetrahedral and linear complexes, have been re~iewed.~' For this dI: configuration, a spin-only magnetic moment of 3.87BM is expected. Slow substitution can be predicted for vanadium(I1) complexes but such reactions are not as slow as those for the isoelectronic Cr"' ions. 33.3.2
Cyanides and Isocyanides
A solution of VC13 reduced with zinc and added to saturated KCN gave yellow &[V(CN)6 The structure59 is almost a regular octahedron with V-C having a mean value of 2.161(4) and C-N a mean value of 1.153(7) A, comparable to other cyano complexes of vanadium. The value 3.78 BMm for the room temperature magnetic moment is consistent with vanadium(I1) and the spectrum showedm two d-d bands at 22.3 X lb and 27.7 X lo3cm-l. A purple powder14 K,V(CN), was obtained by reaction of K2V(CN)2-0.5NH3with NH4CN in liquid ammonia. Its low magnetic moment (3.2BM) and high v(CN) were due to a polymeric cyanide-bridged structure.
8;
Vanadium
463
[v(cNBU')6]2' has been prepared by reduction of V[CNBut)3C13or oxidation of [V(CO),]- in the presence of an excess of t-butyl isocyanide. In red [ V ( C N B U ~ ) ~ ] [ V ( C Othe ) ~ ]octahedral ~,~~ cation has V-C 2.10(1) A comparable to 2.161(4) 8, for the hexacyanide. 33.3.3 Nitrogen Ligands
33.3.3.1 Ammonia and amines Vanadium@) chloride does not react with liquid ammonia at -33"C, but at room temperature, it reacts slowly giving purplish brown V(NH3)5.4C11.9with a magnetic moment of 3.70 BM.6Z[V(NH3),]Br2 was isolated63from solutions of [V(H20)6]Br2in ethanol by bubbling dry NH3, or by condensing ammonia on to the hexahydrate. The magnetic moment is -3.9 BM and the powder photograph has been indexed in terms of a face-centered cubic unit cell of side 10.5 A, isomorphous with [Ni(NH3)6]Br2.The three bands in the diffuse reflectance spectrum (32000, 21200 and 14800cm-') have been interpreted in terms of octahedral symmetry. Methylamine and VClz in a Carius tube at 160"C formed purple-brown v(MeNH2)6C12.* Complexes [VL3]X2 (L = en, l,Zdiaminopropane, 13-diaminopropane; X = C1, Br, I) and [V(dien)z]Xz were prepared by mixing ethanolic solutions of amine and of vanadium(I1) halide. The magnetic moments (3.66-3.91 BM) and electronic spectra were typical of octahedral vanadium(I1) and there was no halogen coordination.65
33.3.3.2 N-heterocyclic ligands Vanadium(I1) alcoholates in ethanoP or hydrated vanadium(I1) halides in n-butano16' were used to prepare complexes [vL6]x2 and [VL4X2](L = pyrazole, imidazole, dimethylimidazole, 1-methylimidazole, 2-methylimidazole7 benzimidazole and isoquinoline; X = CI, Br, I). [V(benzimidazole)2C1z]+0.5butanolhas been obtained67 and the reflectance spectrum was consistent with an essentially octahedral environment. The magnetic moment (2.41 BM at 298 K decreasing with temperature) also suggests a polymeric structure. On heating the green v(1-methylimida~ok)~Cl~ under vacuum, violet compounds formulated as V( 1methylimidazole),Ci2 (n = 4, 5) were obtained.
(i) Polypyrazaiyl ligands Mani and co-workers68 synthesized complexes of polypyrazolylborates H,B(pz)& ( n = 0, 1, 2) with vanadium(I1). For n = 0 or 1, [VL] were obtained with electronic spectra (and magnetic moments) consistent with an octahedral geometry and HB(pz), and B(pz); as tridentate ligands. For n = 2, K[V(H,B(~Z)~),].E~OH was isolated: the anion has a slightly distorted octahedral geometry with a mean V-N of 2.169(19) A.68The isoelectronic ~ l y ( 1 pyrmoly1)methane ligands have been used to synthesize vanadium(I1) complexes. The potentially bidentate ligands H2C(pz), and HzC(Mezpz)z react with vanadium(I1) salts giving different stoichiometries, depending on the coordinating power of the anions: Q X 2 (L = H2C(pz),, X = C1, Br, I, NSC; L = HzC(Me,pz),, X = Br, NCS) and [VL$J]Y (L = HzC(pz)2, Y = BPh.,; L = HzC(Me2pz),, Y = PF6, BPh4). They were octahedral with the exception of [V(HzC(Mezp~)2)zC1]Y (Y= PF6 or BPh): these two have moments of 3.82-3.83 BM, virtually independent of temperature, and are monomeric and five-coordinate. The potentially tridentate H C ( ~ Zformed )~ [VL]Y2 (Y= Br, PF6) and [VL]2[V(NCS)6],with octahedral coordination, Complexes of tris(1-pyrazolylethy1)amines (shown in Figure 5 and abbreviated as tpea) have been characterized. Complexes [V(NCS)zL] (L = tpea, R = H,70 Me71) possess a slightly distorted octahedral geometry, the complex [VClLIBPh, (L = tpea, R = H) (peE= 3.54 BM at 295 K and 2.93 at 90 K) has a polymeric structure with bridging chlorine atoms. (ii) Pyridine and related ligands [v(py)6][v(co)6]2 precipitated by adding solid [v(CO),] to a toluene solution containing an excess of pyridine. By dissolving it in CH2C12, an isocarbonyl-bridged complex
464
Vanadium
N
t 3.) CH,CH,N,
~ .. .
Figure 5 General formula of the tris( 1-pyrazolylethy1)amineligands
[v(py)4][v(co)& is formed.72 [vL4][v(Co)& (L = B- or y-picoline) were obtained by reaction of V(CO)6 and the ligand in benzene.73 V(py)4C12has a trans structure with a mean V-N of 2.189 A.74On heating it at 170"C under vacuum, red-brown V(py),C12 was detected and finally V(py)Clz is formed. The formulae of complexes with pyridine, p-picoline and y-picoline are listed in Table 4 with magnetic moments and thermal decomposition data. Table 4 Complexes of Vanadium(I1) with Pyridine and Picolines
Compound
Product 3.8513.73 3.9213.75 3.91f3.86 3.8413.78 3.4112.28 3.9413.85
V(6pfc),Br2 V(/J-plc)Brz V( y-pic)Br, VI,
Thermal decomposition Temperature range (K) 435-470 475-575 430-475 505-550 340-425 410-470 495-555 420-500 505-550
450-560 470-567
AH (kJ mol-') 97f2 59zt 1 104rt2 71f4 120 f 2 133 f 2 70f3 166f4 71 f 4 200 f 10 194 f 2
Data taken from M. M. Khamar, L. F. Larkrvocthy, K. C. Patel, D. J. Phillips and G . B e d , Ausf. J . Chem., 1974, 27, 41.
Complexes like [V(L),]I, (L = bipy, phen) and [V(terpy)z]lz (already mentioned as starting materials for low-valent compounds) were isolated from solutions of VS04 by adding the corresponding ligand and iodide. The spectrum of V(bipy)s(C104)z shows'' three bands due to d-d transitions with much metal-to-ligand charge-transfer character. The fine structure was due to vibrational interactions, evidence for electron delocalization within the excited states. [V(arninoq~inoline)~]X~~2H~O (X = C1, Br, I) were ~y n t h es i zedand ~~ showed spectra resembling those of the bipy complex. Pyridine (and derivative) complexes [VL4(NCS),] have been isolated from aqueous ethanol; IR spectra indicated trans structures. On the other hand, [VL&WS),] with L=phen or bipy exhibit split CN absorptions suggestive of cis structure^.^^
33.3.3.3 Isothioeyanafes &[V(NCS)6].EtOH was prepared from VSO, solution in the presence of an excess of potassium t h i ~ c y a n a t e and , ~ ~ other counterions (NMe;, NEt;, Hpy+) have been reported.77 Thiocyanato groups are N-bonded and the moments are near the spin-only value. The electronic spectra have been interpreted on the basis of an octahedral d3 ion although the splitting observed in the vZ band makes the assignment uncertain. On the other hand, X-ray powder photographs showed77that [NEt4]4[V(NCS)6]is isomorphous with the nickel complex, suggesting an octahedral geometry. In aqueous solutions, [v(&o)6]2+ reacts with the NCSion;'' the results from the thermodynamic and kinetic studies of this equilibrium have been summarized in Table 5. 333.3.4 Acetonitrile
The [V(NCMe)6I2+ion was first obtained73 in a solid [v(NcMe)6][v(co)6]2as a product of the disproportionation of [v(co)6]. More recently, the structure of the
465
Vanadium Table 5 Data on the Complexation of Vanadim(I1) with Thiocyanate in Water at 25°C Ref. Composition of the complex"
V(HzO)2'
+ NCS- +V(Hz0)5NCS+
I
Thermodynamic data
K = 21 f 3 , b 27*8' AH = 22 i 3 kJ mol-', AS = 46 f 13 J K-' mol-'
2d
1"
Kinetic data
k AHf (kJ mol-') AS* (J K-' mol-') AVf (cm3 mol-')
Forward reaction (M-I s-')
Reverse reaction
22.3 f 1.3 15f2 66.6 f 1.6 4.9 f 5.4 -2.1 f 0.8
1.06 f 0.09 1.4f0.2 62.7 f 2.6 -35.0 f 9.4 -11.5 f 0.9
6-7 2' 38 2 2 2
Mechanism of reaction and estimated volume changes (cm' mol-') [V(H,O),]'+
+3 2
[V(H,O),,NCS]+
-5.?
i-N C X
Job's method of continuous variationsp From kinetic data. From spectrophotometric data. Ionic strength 0.5M (LiCIO,). Ionic strength 0.84M (LiClO,). 'Stopped flow method. 'Temperature jump method, 2 4 T , ionic strength 1 MCIO;/Cl- medium. 1. J. M. Malin and J. H. Swinehart, Inorg. Chem., 1968, 7,250. 2. P. J. Nichols, Y. Ducommun and A. E. Merbach, Inorg. Chem., 1983,22,3993. 3. W. Kruse and D. Thllsius, Inorg. Chem., 1968,7,464. a
hexakis(acetonitrile)vanadium(II) ion has been determined79 in [V(NCMe)6][ZnC14]obtained from the reduction of VC13 with ZnEt, in acetonitrile. The vanadium is surrounded by the six acetonitrile nitrogen atoms in near octahedral geometry (with mean V-N of 2.11 A) and the VNC bond angle (173.4' av.) shows a slight deviation from linearity. 33.3.4 Phosphines
Bis(triethylphosphine)vanadium(II) chloride may be prepared by the zinc reduction of VC13(PEt3), or from 'VCl2(TlfF); upon addition of triethylphosphine. The solid has a low value of 1.98 BM consistent with a polymeric structure and pcffremained unchanged in benzene; cryoscopy in benzene showed a dimer.80 The spectra of such solutions contain three d-d transition bands at 10 040 ( E = 21), 15 120 ( E = 19) and 22 500 cm-I ( E = 30). [V,(p-Cl),(THF)6]z[Zn2C16] was used to synthesize other dinuclear compounds with bridging chlorine atoms, bridging diphosphine ligands and bidentate tetrahydroborates: [V(p-Cl)(por bis(dimethy1phosphinodiphos)Bhl2 (diphos = bis(diphenylphosphino)methaneg1 methane82). The structures of these two were very similar; the bisdimethylphosphine complex is represented in Figure 6. The V-V distances (respectively 3.112(3)A and 3.124(2) A) are too long to suggest any significant metal-metal bonding. Using 1,2-bis(dimethylphosphino)ethane (dmpe) mononuclear truns-VXz(dmpe)2 (X= C1 or Me) were obtained83 and the trans structure of the dichloro complex was confirmed: the mean V-C1 and V-P were 2.44 and 2.508, respectively. 33.3.5
Oxygen Ligands
33.3.5.1 Water A hydrolysis constant of lop7 has been assumedw from the colour changes in VC12 titrated with base, and a pX value of 6.49 was determined from pH measurements in solutions of
466
Vanadium
(a 1 Figure 6 Structure of the phosphine complexes. (a) The complex with bis(dimethy1phosphino)methane. A similar structure is found in the corresponding complex with bis(diphenylphosphino)methane.82 (b) The complex with
bis(diphenylphosphino)ethaneS3
VSo4.85Polarography suggested dimeric species .86 Baes and Mesmer" criticized the value for the hydrolysis constant, since it is difficult to avoid oxidation of the metal by water. Contamination with higher oxidation states would lead to high values of the constant. Molecular orbital studiess8 of hexahydrated divalent metals show that pK > 10 comparabie to manganese(I1) ions should be expected. Exchange in [V(H,0),J2+ has been studied by " 0 NMR:89 ICE'= 87 4 spl, AH* = 61.8 f0.7 kJ mol-', AS* = -0.4 f 1.9 J mol-' K-' and AV* = -4.1 f 0.1 cm3 mol.-' This negative volume of activation was evidence of an associative interchange mechanism. On reducing V205 in sulfuric acid by electrolysis, sodium amalgam or zinc, [V(H20)6]S04 may be isolated, isomorphous with zinc sulfate. Tutton's salts are double sulfates M2[V(H20)6I(S04)2 (M = W*,K+,Rb+, CS'); in ( N H ~ ) Z [ V ( H ~ O ) ~ ] ( the S ~ ~[V(H20)6I2+ )Z, have mean V-0 of 2.15 A.90 Several hydrated halides have been prepared" by dissolving V205 in acid (hydrochloric, hydrobromic or hydroiodic) and reducing by electrolysis: [V(H20)6]X2(X = Br or I), [V(H,O),X2] (X = C1, Br or I) and [V(H20)2X2](X = C1 or Br). The hexa- and tetra-hydrates are magnetically dilute, with moments near the spin-only value and invariant with temperature. On the other hand, the dihydrates have moments which indicate a polymeric structure.
*
33.3.5.2 Alcohols and ethers Green [V(MeOH)61[V(CO)6]2 was prepared by reacting methanol with [v(cO)6].= Vanadium(I1) chloride methanolates were prepared by electrolytic reduction of VC13 in methanol.93 The hexasolvated vanadium(I1) complex was converted to tetrakis and bis methanolates by careful heating. [VC12(MeOH)z]has pefflower than the spin-only value and a polymeric structure (with C1 bridges) may be In [V(MeOH)6]C12, the V 0 6 unit has almost ideal octahedral geometry with average V-0 2.132(3) The electronic spectrum confirms a slightly weaker field for methanol compared to water. An alternative routeM for vanadium(I1) alcoholate synthesis involved treatment of ethanolic solutions of hydrated vanadium(I1) salts with triethyl orthoformate. Several solids were isolated (VBr2.6EtOH, VClz*4EtOH,VC12.2EtOH). The V" cation in [V(THF)4][V(CO)6]2 was shown to have octahedral coordination:y6 four THF molecules are in a planar array with V-0 2.170 A. The axial positions are occupied by oxygen atoms of the two hexacarbonylvanadate(-I) anions with V - O ( C 0 ) 2.079 A.
33.3.5.3 fi-Diketones A basic solution with a large excess of acetylacetone is strong blue. Complexes have been studied (log PI = 5.383, log B2 = 10.189, log p3 = 14.704). A potential of -1.0 V ZIS. SHE was measured for the reduction of the V"I tris complex.84 Mixed complexes V(L),(py), (L = acac, tduoroacetylacetonato or dibenzoylmethanato) isolated from solutions containing VS04 and the appropriate ligandw show intense absorption in the region 700-300 nm.
Vanadium
467
33.3.5.4 Oxalate ond carbonate
V(Cz0,).2H20 may be obtained9* from VClz solution and sodium oxalate or oxalic acid; its thermal decomposition under a C 0 2 atmosphere (-3000bar) was shown99 to occur at 262 f 2 "C and by 280 "C the conversion into V2O3 is complete. The vanadium(I1) carbonate was prepared from a VS04 aqueous solution and Na2C03.99 Amorphous VC03.2H20 was isolated. When this was heated in COz (-3500bar), the water content was lowered and crystalline products were obtained. 33.3.6 Halides
A compound obtained from reduction of VCl, by zinc metal in THF was shown to be [V2(y-c1)3(THF)61~[Znz(~-cl)2cl,].Its structure has been determined at - 105 oC1OO,lO1 and - 160"C102avoiding the extensive and detrimental disorder in the THF ligands. Figure 7 shows the [V2(y-C1)3(THF)6]+ion. At -105 "C, the mean V - C l and V-0 distances are 2.478(3) and (2.99 A) corresponds to some interaction, and antiferromagnetic 2.147(7) A. The V-V behaviour was observed with an exchange integral -J=75cm-l. The visible spectra in dichloromethane with a few drops of THF show a strong sharp doublet at ca. 25 OOOcm-','o' assigned as a double spin-flip transition of the coupled pair of V2+ions.
F i n r e 7 Structure of the [V,(pa-CL)3(THF)d' cation'O'-loz
Vanadium(I1) dihalides and complex halides are prepared by high temperature dry routes. The preparation, structure and properties were reviewed.'" Blue crystals of V F 2 have a rutile-type structure: the V-F distances (2.073 and 2.092A) are near the sum of the ionic Complex fluorides MVF3 (M = Na, K or Rb) have been obtained from VFZand the appropriate fluoride by heating at 1200°C and separating the crystals by means of a microscopelo6or heating at 800 "C and treating the crystals with dilute acid solution.'o7 NaW3 has a distorted perovskite-t e s t r u c t ~ r eand ~ ~ KVF, ~ ~ ' ~and ~ R b W 3 have a cubic structure of the ideal perovskite type.'"By re lacing some v' ions by V3+ in KVF3, new compounds with structures similar having the general formula K,VirV$xF3 or KxVF3may be to the potassium tungsten bronzes. VClz can be prepared by such methods as reduction of VU3 with hydrogen at 675 oC,lloor the mixing of stoichiometric quantities of VC13 and V powder in a uartz tube under vacuum and slowly heating to 800 OC.lll Crystals have a Cd1,-type structure. 1 2 Using dry high temperature procedures, several complexes have been isolated: KVC13,983113 C S V C ~ ~ , NI&Vc13114 ~ * * ' ~ ~ and T1VC13.114RbVC13 has been obtained by careful dehydration of RbVC13-6H20.'lS All these have CsNiC13-type structure^^^^^'^'^^ with a slightly distorted octahedron of chloride ions. VBr2 can be prepared by reduction of VBr3 with hydrogen'16 and VI2 by reaction of stoichiometric amounts of the elements at 160 "C.'16 The diiodide may form black crystals by using an excess of iodine but red crystals are obtained by sublimation at 750 "C with vanadium in excess.117The dibromide118and the red form of vanadium diiodide1l7 have structures of the Cd12 ty e while the black form of VI2 exhibits a CdBr2-type ~ t r u c t u r e . ~The ' ~ complexes CsVBrJ3 and CsV13,'19 prepared by heating stoichiometric amounts of dihalide and the Cs halide at 700°C in sealed quartz tubes, have structures similar to the chloride complexes (Le. coc3-P
Vanadium
468
CsNiC1,-type) . Table 6 summarizes results on electronic spectra. The three spin-albwed transitions are usually observed and extensive vibrational contribution has been observed in single crystal spectra.1m~12*Intense spin-forbidden transitions may also be observedx2' due to exchange interactions in the solid phase. The diiodideluF1" and the complex halides"* show trigonal distortion. Table 6 The Absorption Spectra of Vanadium Dihalides and Complex Halides Compound
vF*
VCl,
I
KVCl, RbVCI, CSVCI, VBr2 CsVBr, VI2 CSVI,
v1 (cm-')
v2 (an-')
30 300
9200
16 450 14 270
22 220
. 8980
13 920
21 840
7300 to 97700
13 200 to 14 900
21 900 to 25 600
v3
8600
13 330
20 330
7870 7870 7700
12 270 12 280 12 OOO
18 920 20 330 18 700
D q (cm-')
B (cm-')
Re&
1030 920 800 1010 1029 950 860 870 790 787 770 790
687 615 755 640
1 2 3 4
640 530 625 510 540 515 590
5 6 2 6 2 7 8 6
1. M. W. Shafer, Mater. Res. Bull., 1969, 4, 905;B value calculated in C. Cros, Rev. Inorg. Chem., 1979, 1, 163.
2. W. E. Smith, J . Chem. Soc., Dulron Tram., 1972, 1634. 3. S. S. Kim, S. A. Reed and J . W. Stout, Inorg. Chem., 1970, 9, 1584. 4. W. E. Smith, J . Chem. Soc. ( A ) , 196!3,2677. 5. C. Cros, Rev. Inorg. Chem., 1979, 1, 163. 6. A. Hauser and H. U. Giidel, Chem. Phys. Letf., 1981,82,72. 7. W. van Erk and C. Haas, Phys. Sfut. Solidi B , 1975,71,537. 8. G. L. McPherson, L. J. Sindel, H. F. Quark, C. B . Frederik and C. Doumit, Inorg. Ckem.,1975,14, 1831.
The magnetic properties have been studied by means of magnetic susceptibility and specific heat measurements; some results are in Table 7. In VF, there are strong exchange interaction! (antiferromagnetic) between the vanadium a t o r n ~ . In ' ~ the ~ ~lamellar ~~ dihalides (VCl?, VBr: and VI2), the predominant interactions take place between atoms in the same plane. There art relatively strong antiferromagnetic interactions at low temperature, which decrease from thg chloride to the iodide .l18 In the complex fluorides there are strong antiferromagnetic interactions due to superexchange couplings between the vanadium ions. The vanadium(I1 complex halides of the CsNiC1, type (chloride, bromide and iodide) do not obey tht Curie-Weiss law. The molar susceptibilities are weak and increase significantly from chloridc to iodide. The strongly negative antiferromagnetic interactions in these halides result fron either direct t2g-t2gor superexchange t2g-t2g-t2gand eg-s-eg coupling.113,1'6
Table 7 Magnetic Moments and Curie-Weiss Law Parameters of Vanadium(I1) Dihalides and CompIex Halides perp (BM)
Halide
, C
(cm3 mol-' K)
B (K)
Ref.
~
VF, NaVF3 KVF, RbVF,
va2 vBr2
VI2
3.79 3.70 3.82 3.94 3.93 3.96 4.07 4.07
1.79 1.72 1.82 1.94 1.93 1.96 2.07 2.07
-90 -52.5 -59 -220
-190 -437 -335 - 143
1 2 3 2 2 4 4 4
1. C. Cros, R. Feurer and M. Pouchard,J . Flwrine Chem., 1975, 5, 457. 2. C. Cros,R. Feurer and M. Pouchard, Muter, Res. Bull, 197611, 117. 3. R. W. Williamson and W. 0. J. Boo, Inorg. Chem., 1977.16, 646. 4. M. Niel, C. Cros, G . Le Hem,M. Pouchard and P. Hagenmullet, Nom. J. Chim., 1977,1, 127; Physicu B + C (Amterdum), 1977,M B , 702.
Vanadium
469
33.3.7 Macrocyclic Ligands By reacting the appropriate halide with the ligand in anhydrous DMF, VLX2(L= Me2[14]aneN4,X = C1; L = Me6[14]aneN4,X = C1, Br, I) were obtained.12' All have moments close to spin-only , the reflectance spectra can be interpreted assuming a tetragonally distorted octahedral structure, and the IR spectra confirm that the ligands adopt a planar arrangement of the four nitrogen atoms, the halides occupying the trans axial positions. The chemistry of vanadium porphyrins has been reviewed.12* Reduction of dihalogenovanadium(1V) porphyrins produced the corresponding vanadium(I1) complexes trans[V(por)L] (por=OEP, TPP, TpTP, TmTP; L=THF, PPhMe2). The structures of [V(OEP)(THF)z] and [V(OEP)(PPhMe2),] have been determined.12' In both, the vanadium is in the plane defined by the four porphyrin nitrogens and V-N are 2.051(4)A in [V(OEP) PPhMe&] and 2.046(4)A in [V(OEP)(THF),]. V - 4 in the THF complex is 2.174(4) and the V-P bond in the phosphine complex is 2.523(1) A, comparable with lengths of V-0 in other THF complexes (Sections 33.3.5.2 and 33.3.6) and V-P in complexes with diphosphine ligands (Section 33.3.4).
8,
33.3.8 Vanadinm(1I) as Reducing Agent Vanadium(I1) has been the reducing agent for a large variety of organic and inorganic substrates and both aqueous and nonaqueous solutions have been used. The solutions of vanadium(I1) can be conveniently obtained by electrolysis (Section 3 3 . 3 3 , from reduction of V2OS with zinc metal" and by reduction of VC13 with lithium tetrahydroaluminate in anhydrous THF.I3' The reduction of organic compounds by low valent metals has been reviewed13' and Table 8 shows examples for vanadium(I1). Vanadium acts as a one-electron reducing agent and usually the reactions involve the formation of organic radicals. For instance,'33 [VC12(py),] was rapidly oxidized by several aryl and alkyl halides RX to vanadium(II1) and an almost quantitative yield of R-R obtained. Initial attack on the V" complex by the aralkyl halide via an outer-sphere mechanism probably forms a radical which rapidly reacts with a further V" yielding an aralkyl vanadium(II1) intermediate. The coupled products are formed by oxidation of this by another molecule of RX.The reduction of organic radicals has also been studied: the rapid reduction of methyl radical by vanadium(I1) was observed'34 in the reaction of V2+ with hydrogen peroxide and Me2SO: methane (-25% based on the amount of Hz02) was formed with traces of ethane (
(v.o,)2+
2 x lo-, M, an increase in absorptivity indicates dimerization. The nature of the dimer has been discussed.204 33.4.3-3 Thiocyanate, selenocyanate and azide From spectrophotometry, the formation constant of V(NCS)'+ is Complexes containing [V(NCS)6J3- have been isolated and the structure of K3[V(NCS),].12H2O determined by X-ray diffraction,206confirming that thiocyanate coordinates via the N atom (the mean V-N distance in this complex is 2.044(2) A). When VC13 in acetonitrile reacts with ammonium thiocyanate, a progressive exchange occurs.2o7 Complexes with various stoichiometries were isolated: VC12(NCS)-3MeCN, VC1(NCS)2-3MeCN, V(NCS)3.5MeCN and Ct3[v(Ncs)6).2MeCN (Ct = K+, N&+, pyH+, coc3-P'
Vanadium
476
piperidinium, Ph4P+, MePh3As+ or Me3S+) Similar complexes containing bromide and thiocyanate were also isolated as well as a mixed complex of NCS- and NCSe- formulated as M3[V(NCS>,(NCSe),].2MeCN(M = K+ or Na+). Complexes Cts[V(NCSe)6,1 (Ct = Ph4As+, Me4N+, Et4N+ or K+) have also been prepared.208.2w Several azido species have been detected in acetonitrile: V(N3)2+,V(N3)3 and V(N3)2-.2'0 (BU~N)~[V(N,)~] has been isolated.208As expected for octahedral VI'', the magnetic moments are slightly lower than spin-only values and there is a marked temperature dependence at low temperatures .'"
33.4.3.4 Nitriles Anhydrous vanadium trichloride or tribromide reacts with acetonitrile to yield green or red-brown solutions. Complexes [VX3(MeCN),] (X = C1 or Br) crystallize with varying amounts of acetonitrile in the lattice,212where the IR spectra show the free (2253.5 cm-') and coordinated (2292 cm-') acetonitrile. In acetonitrile, vanadium trichloride reacts with large organic chlorides to yield cream-yellow complexes of the type Ct[VC14(MeCN)2] (Ct = E t a + , MePh&+ or Ph4As+). Bromidecontaining complexes have also been isolated: (Et4N)[V13r4(MeCN)2] and (Et4N)[VC13Br(MeCN)2]. The acetonitrile molecules of (Ph4As)[VC14(MeCN)2]may be removed at 100 "C under vacuum.213 Acrylonitrile and propionitrile complexes have been described.214
33.4.3.5 Silazanes
Hexamethylcyclotrisilazane, (Me2SiNH)3, and octamethylcyclotetrasilazane, (Me2SiNH)4 (shown in Figure 10 and abbreviated as HMT and OMT respectively), form octahedral complexes. VC13 reacted with HMT in THF to form VC13(HMT),215which dissolves in dichloromethane giving a nonconducting solution. The ligand is terdentate and two V-Cl bands (at 358 and 326cm-') confirm the facial geometry for the chlorine atoms. On reacting [VC13(THF),] with OMT in benzene [(VC13)2(0MT)(THF)2] was obtained,'16 with both coordinated and uncoordinated NH groups and with bridging chlorine atoms. Me
Me
: * k
Me
H
(a)
Me
H
Me
(b)
Figure 10 Formulae of siiazanes: (a) hexamethylcyclotrisilazane (HMT), (h) odamethylcyclotetrasilazane (OMT)
33.4.4
Phosphines and Arsines
VC13 reacted with Et,P or Pr3P in THF to form trigonal bipyramidal [VC13L].21'
[VC13(Me2PCH2CH2PMeJ(THF)] was prepared by reaction of [VC13(THF)3]with ligand.218 Five-coordinate [VC13(PMePh2)2]has been synthesized from [VC13(THF)3] and phosphine upon reflux in ben~ene.''~X-Ray diffraction has shown that the crystalline solid contains two independent but nearly identical molecules, each of them trigonal bipyramidal (Figure 11).'" On refluxing in THF, VCI3 reacted with triarsines to yield red-brown complexes [VCl,(TAS)] (TAS = bis(dimethylarsinopheny1)rnethylarsine or 1,l,l-tris(dimethylarsinomethyl)ethane).220 These are non-electrolytes, and the ligand is terdentate. Both have a facial structure.
Vanadium
477
Figure 11 Structure of {VCl,(PMePh,),]. The V-41 distance to the underlined atom (2.178,) is shorter than the average of the other V 4 I bonds (2.258,). Another distortion of the trigonal bipyramid is the observed P-V-P bending (166.6"). The small difference in V-P bond lengths (2.547 and 2.525 A) has no chemical significane219
33.4.5
Oxygen Ligands
33.4.5.1 Aqua species, O H -
The characterization of [V(H20)6]3+has been done on the cesium alum C S V ( S O ~ ) ~ - ~ ~ H ~ O ~ ~ and on [V(H20)6](H502)(CF3S03)4;222 the V 0 6 unit has an octahedral geometry. When hydrogen atoms of the coordinated water molecules are considered, the symmetry is lowered from Oh to D3d. Two remarkable features are underlined in Figure 12: (i) the water molecules are nearly flat, i.e. for each coordinated water the vanadium centre, the oxygen and the two hydrogen atoms are lying in the same plane; (ii) the six hydrogen atoms of three mutually cis water molecules are essentially coplanar (the deviations from the mean plane are less than 0.1 A with an average absolute deviation of 0.056 A).
Io) Figure 12 Structure of
(bl ion. The oxygen atoms are octahedrally coordinated with an avera e V-0 viewed down the C, axis; (b) a view perpendicular to the C3 axis'
H
In the TR spectrum of the cesium alum, two bands were assigned to antisymmetric '~ metal-ligand stretch v3 (532 cm-') and antisymmetric metal-ligand bend v4 (607 ~ m - ~ ) , ' and in the oriented single-crystal Raman spectrum, the totally symmetric stretching mode of the cation occurs at 525 The electronic spectra of the cesium alum and of vanadium(II1) in perchloric acid solutions are very similar (vI = 17 800 and v2 = 25 700 cm-') and it was concluded that vanadium(II1) existed as the hexaaqua ion in such solution^.'^^ The coordination number six was confirmed by NMR.226 Water exchange has been studied by "ONMR spectroscopy as a function of temperature (255-413 IS) and pressure (up to 250 MPa):227 k,, = (5.0 f0.3)x lo's-' (at A V + = -(8.9 f AH* = (49.4 k 0.8) kJ mol-', AS* = -(28 k 2) J K-' mol-', 298 K), 4) cm mol-' and the compressibility coefficient of activation A/3# = -(1.1 f 0.3) X cm3mol-' MPa-'.
Vanadium
478
Table 9 Data on Hydrolysis and Oxoreduction of Vanadium(II1) Ref.
Reaction V(H,O);+-+ V(H,0),(OH)2+ + H+ 2V(H,O);++ V,(H,O),o(OH)~ + 2H+ 2V(H,O)Z++ V,(H,O),(OH), + 3H+ V 0 2 + $H2+ H++ 'V3"
+
V(H,O);'
+ e--+ V(H,O)Zt
log /3 = -3.07 f 0.03 log /3 = -3.94 f 0.02 logp = - 7 . 8 7 f 0 . 0 9 log /3 = 6.192 f 0.004 = 366.3 f0.3mV E,,, = -475 m V (us. SCE) AS" = 155 J K-' mol-'
1 1 1 1 2
1. F. Brita and J. M. Gonphes, An. Quim., 1982, 78, 104; solutions 3 M in KCI at 25 "C with total vanadium concentration in the range 0.006-0.160M and O IO7 s-1 Figure 20 Different substitution rates at various coordination sites of oxovanadium(1V) ion (see text), with approximate first-order rate constants at room temperature
(ii) Hydrolysis in solution there is agreement From potentiometry, calorimetry (Table 16) and ESR about the species in moderately acidic solutions. On the other hand, the nature of species present^ in neutral or basic solutions is still controversial. In moderately acid solutions, oxovanadiurn(1V) exists mainly as [VO(H20)5]2+.As pH COC3-Q
500
Vanadium Table 16 Hydrolysis of Oxovanadiurn(1V) at 25 "C Medium
logs,,
NaClO, (3 M) NaClO, (3 M) NaClO, (3 M) NaCIO, (3 M) NaC10, (1 M) KNO, (0.1 M) LiClO, (0.1 Mf KCl(3 M)
logB22
-6.0 -5.77 -6.1 -5.67 -6.07 -5.44 -5.04 -6.4
-6.88 -6.90 -6.91 -6.67 -6.59 -7.18 -6.72 -7.45 -7.5
-
MgSO, (1 M)"
log012
logB21
Ref.
-
-
1 2 3 4 5 6 7 8 9
L
-10.05 -
-
-
-
- 10.00
3.75
I
'Temperature is probably 25 "C. 1. F. J. C. Rossotti and H. S. Rossotti, Acta Chem. Scand., 1955,9, 1177. 2. F. Brito, An. Quim., 1966.628, 123. 3. F. Brito, An. Quim., 1966, 62E, 193. 4. R. P. Henry, P. C. H. Mitchell and J. E. Prue, J . Chem. Soc., Dalton T M ~ s .1973, , 1156. 5. I. F6byPn and I. Nagyp61. Inorg. Chim. Acta, 1982, 62, 193. 6. M. M. Taqui Khan and A. E. Martell, J . Am. Chem. Soc., 1968,90,6011. 7. A. Komura, M. Hayashi and H.Imanaga, Bull. Chem. SOC.Jpn., 1977,50,2927. 8. S . Mateo and F. Brito, An. Quim., 1971, 68, 37. 9. A. Diaz, Dr. Ph. Thesis, Facultad de Ciencias, UCU, Caracas, 1%.
increases (but pH 5 4), evidence for the formation of [VO(OH)]+ and [(V0)2(OH)2]2+is strong. Table 16 summarizes the relevant constants (B, is defined according to equation 27).
Oxovanadium(1V) is amphoteric but only a few quantitative studies exist on vanadium(1V) in neutral or basic solutions. Ducret explained his observations by the existence at p Hr8 -9 of [(VO)2(OH)5]-."" Britton and Welfordm also suggested [(V0)4(OH)lo]2- to explain titration in basic solutions and Lingane and Meitesa9 interpreted polarographic data in terms of [(VO)2(OH)6]2- and [(VO)4(OH),o]2-. Ostrowetsky also assumed the formation of [ ( V O ) ~ ( O H ) I ~in ~ - neutral and basic solutions and obtained brown reddish cr stals, (NH&V4o9.' Chasteen remarked that species in the H range 7-11 are ESR silent4Y and suggested that polymeric [VO(OH)3]; exist at pH 7.3 5 B Virtually all amphoteric oxides are converted to monomeric anions in sufficiently strong basic media. Rieger and co-workers studied strongly basic oxovanadium(1V) solutions by ESR, optical and Raman spectro~copy.~'~ Raman absorption at 987 cn-l confirms the presence of the vanadyl group in solution, and from ESR spectra and titration, the authors concluded that vanadium(1V) exists as a monomer [VO(OH),]- for pH k 12. In Figures 21(a) and 21(b), speciations of solutions with two different total oxovanadium(1V) concentrations are presented assuming that the predominant soluble species in the pH range ca. 6-11 is [(V0)2(0H)51-. Further research is required to sort out the species and equilibria
-
90
-
70
-
% 50% 3010
I
I
I
L
I
I
,
I
I
I
I
I
FH (0I
21 Speciation in oxovanadium(1V) aqueous solutions: (a) C,, = M and (b) C,, = 2 x loT3M. Formation constants Bll, BZzand Bm are those reported by Komura et al. for 0.1 M LiCIO, and 25 oC.467 For pH L 5 these speciations should only be used as a guide in order to give an idea of what may probably be the relative importance of the total concentrations of species [(V0),(OH);ln and [(VO),(OH)~-],. For pH z 12 apparently only [VO(OH);] predominates. Note that the ratio Cmonomcric: Cpalymsric increases as the C, decreases and that in (b) K,, 5 [V02+][OH-]2 in the pH region ca. 4-8. In the calculations Is,, was assumed to be 1 X -re
Vanadium
501
(iii) Redox behattiour of oxovanadium(N) solutions
Because of the multiple and slow equilibria, the elucidation of all the species present and the hydrolysis and redox equilibria is a difficult task. Redox behaviour of oxovanadium(1V) will not be discussed here: detailed information is given by P o ~ r b a i x Hepler , ~ ~ ~ and c o - w o r k e ~ s , ~ ~ ~ ‘Gmelins Handb~ch’”~ or Baes and M e ~ m e r . ~ ~ The most important half-cell reactions are shown in equations (28) and (29).476 Oxovanadium(1V) can be oxidized by 02,more easily as pH increases; in acidic solutions, the blue V02+ solutions are stable to air. Air oxidation in alkaline solutions (0.006-3.8 M NaOH) is rapid at 15 0C.477 The initial rate is proportional to [OH-]. Fe’I‘ catalyzes the reaction while Cr”’ inhibits it. With [VI > 0.002 M, the rate is controlied by diffusion of oxygen. When V02+ is chelated, oxidation may be considerably retarded and-as a rule-the more stable the complexes, the more pronounced this ‘retardation effect’. However, in 1:l solutions of oxovanadium(1V) and tartrate at pH 8, the oxidation is rapid.478The dissolved oxygen was depleted within a few seconds.
-
+ 2H+ + e- e V 0 2 + ( a q )+ H20 V02’(aq) + 2H+ + e- # V”(aq) + H,O VO;(aq)
Eo= 1.00 V
(28) (29)
Eo = 0.359 V
A study of several VO’+-complexone-aqua complexes with a few outer-sphere oxidants proved the importance of an equatorial H 2 0 or OH- ligand.47gThe products certainly have a hexacoordinate cis-dioxo structure. Presumably the aqua ligand can change into a new oxo ligand without changing its site, and the other donors are retained without rearrangement or partial dissociation. Further, significant regioselectivity exists:479cis-oxo-hydroxo complexes are much more rapidly converted into cis-dioxo complexes (V” compounds) than cis-oxo-aqua complexes or those without hydroxo or aqua cis to the vanadyl oxygen.
(iv) Mixed-valence species Ostrowetsky studied solutions containing Vv and VIv and concluded:461(i) at pH 2 10, no mixed-valence complex forms; (ii) at 8 5 pH 5 10, one forms with a VIV/Vvratio of 2, and at least six vanadium atoms; a solid, Na3[VyV:v015H], was isolated; (iii) at 4 5 pH 5 6.5, six different mixed-valence complexes form but only those with VIV/Vv ratios of 3/7 and 713 are stable; (iv) a compound containing eight Vv and two VW transforms into the more stable 7/3 compound in a few minutes; (v) all these compounds contain a Vlo group in the molecule. Deep violet crystalline [Et4N]2[V5014H2]was re ared from CH2C12 (or similar) solutions p59,4’!2 Why it forms is not understood. It was containing [VO(acac),], Et4NCl and [Cu(a~ac)~]. formulated as [Et.$l]4[V~V$v028H4]. Two types of mixed-valence vanadium compounds are known: the very inert bronzes and the aqueous decavanadates. This compound appears intermediary, somehow related to both types.482Its colour, hardness and metallic lustre are similar to the bronzes. Yet it is soluble in water, producing a green solution with a spectrum similar to one of the 7/3 decavanadates described by Os t ro ~et s k y . ~’~ Robin and Day also discuss the formation of the stable 7/3 species proposed by Ostrowetsky and assume that all these anions almost certainly retain the VloOzsskeleton.483
33.5.5.2 Peroxide ligand Orange-red oxoperoxovanadates(1V) (Ct)[VO(OZ)F(H20),] (Ct = N&+, K+,Rb+, Cs’) were synthesized from (Ct)[VOF4] with H202in the molar ratio 1:12 (pH 4).484Insoluble in common organic solvents, they decompose in water. Redox titrations indicate the presence of 0;- coordinated to the VIv of VO”. The p:; are 1.70-1.75BM and the optical spectra showed three absorptions at ca. 476 (or lower), 565 and 855 nm, the first obscured by strong charge v(V=O) of compounds (Ct)[V0(02)F(H20)2] was in the range 950970 cm-l.
-
Vanadium
502
33.5.5.3 Alcohols Alcohols and phenols cleave two bonds of V C 4 forming dichloroalkoxides. Those formed by aliphatic alcohols are dark green and dimeric in boiling benzene.6 At 150 "C (0.1 mmHg) they yield [V20C13(OR)3].The t-butyl and t-amyl alkoxide chlorides are made by alcohol exchange on the isopropoxides.6 QVC12(0R)2.ROH],where R = n- or t-butyl, give well-resolved ESR spectra with g , = 1.95.4 While alkali metal alkoxides do not usually effect complete cleavage of V-Cl bonds, pale blue [V(OSiPh3)4]may be obtained in the reaction of VCl, with sodium triphenyl~ilanolate.~~~ The liquid analogue [V(OSiEt3)4] forms from [V(NEt2),] and triethylsilanol in benzene.420 Complete replacement of the halogen atoms in VCl, can be achieved using lithium dialkylamides. As described in Section 33.4.3.1.i, V C 4 with LiNEt, gives the dark green liquid [V(NEQ4]. This and [V NMe2)4] react vigorously with alcohols to give moisture-sensitive tetraalkoxides [V(OR),] .4 Molecular weight measurements in benzene show that the tertiary alkoxide complexes are monomeric liquids, the secondary are predominantly monomeric and the primary are associated.487An eight-line partially resolved ESR spectrum was obtained with pure V(OBU')~at 303 K.4877419The ESR parameters from room temperature487 and frozen s o l ~ t i o n sindicate ~ ~ ~ , ~distorted ~~ symmetry with a d , ~ - ~ 2ground state; the compound is magnetically dilute with peff= 1.69 BM independent of temperature. Solid vanadium tetramethoxide gave a very broad ESR spectrum. The results are consistent with a trimeric species [V3(OMe)12].It shows significant temperature variation of Dimeric [V2(0Et),] also gave a broad ESR spectrum consistent with an orbital singlet ground state with magnetic dipole interaction. NMe; and NHZ salts of [VCl5(0R)]'- (R = Me, Et, Pr", Bun) ave peffvalues near the spin-only value for a d' system with negligible temperature variation. 485 VOCl2*3MeOHmay be prepared when VC14 reacts with MeOH490(cf. Section 33.5.3.2). Several other complexes [VO(L),(X),] where L is a relatively strongly bound ligand e.g. Cl-) and X is alkoxide or alcohol have been included in early review^,^.^^$ ,356 e.g. [NMe4][VOC14(EtOH)]. Some are discussed in other sections according to the nature of L. Glycerol inhibits VIv oxidation in alkaline solutions and prevents precipitation in the whole pH range. Absorption spectra indicate coordination of glycerol.49' The stability constants with ascorbic (31) and 5,6-O,0'-isopropylideneascorbic acids (32) have been estimated and are almost independent of the nature of the ligands,492which are unidentate, the hydroxyl groups in the lactone ring being involved in the coordination. A vanadyl ascorbate has been synthesized, formulated [VO(C&0BOH)].2H20.493
1
33.5.5.4 Catecholate, polyketonate, hydroxyaldehydate and hydroxyketonate ligands
(i) Cutecholutes Aromatic polyalcohols act as strong coordinating agents and Table 17 summarizes reported formation constants. The complexes are quite stable; this behaviour has been used for the qualitative and quantitative determination of vanadium (e.g. refs. 494 and 495). At pH 3-4, an initial vanadyl catechol complex slowly converts to a tris complex.496In fact complexes with 1: 3 metal-ligand stoichiometry have been isolated (see below), but since in the equilibrium (30) no protons are consumed or liberated, [VO(cat),]'- and [ V ( ~ a t ) ~ ]are ~ - not distinguishable by potentiometric studies. VO(cat)i- + H,cat e V(cat):- + H 2 0 (301 HO OH m
Vanadium
503
Table 17 Formation Constants Bxyzof Oxovanadium(1V) Complexes with Aromatic Polyalcohols
Catechol (1,2-dihydroxybenzene) 1,ZDihydroxybenzene4-sulfonic acid 1,2-Dihydroxybenzene3,5-Disulfonic acid (Tiron) Pyrogallol
(33)
(34)
2.5-Dihvdroxv-l.4' benzoquindne'( = HzD) 3,6-Dibromo-H2D 3 ,6-Dichloro-HzD
25 "C, 0.06 M NaCIO, 35 "C, 0.2 M NaCIO, 25"C, 1 = 0 2 0 T , O.lMKNO,
1 2 3 4,5
31.2 31.2
20 "C, 0.1 M K N 0 3 20 "C, 0.1 M KNO,
4,5 4,s
32.79 28.7 28.57 32.66
2 5 T , O.lMKN03 2 0 ° C 0.1 M KNOa 35 "C, 0.2 M NaCIO,
5,6
21.27' 18.98' 18.82d
25 "C, and I = 1.0 M (acetate)
15.76 16.85 16.69= 17.7
29.42 31.48 30.78' 33.5
16.7 16.8 16.74. 15.0 15.15 17.6 17.31
-
-
10.83" -
25"C, 1 = 0
4 2 597 8 9 9 9
"The constants were reported in terms of H,L. They were converted to prYzusing the p K , values for I = 0.1 M KNO,. The value correspondsto [VO(L)J/([VO(OH)L][H+]). 'The values correspond to BlU studied in the pH range 5-7. dThe valuc corresponds to f1122 studied in the pH range 6.5-8.5. 1. R. Trujillo, F. Brito and J. Cabrera, An. Fk. Quim., 1956, 528, 5x9. 2. G.M. Husain and P. K. Bhattacharya, J . fndian Chem. Soc., 196Y, 46, 875. 3. R. P. Henry, P. C. H. Mitchell and J. E. Prue, 1. Chem. Soc., Dalton Trans., 1973, 1156. 4. J. Zelinka and M. Bartdek, Collect. Czech. Chem. Commun., 1971, 36, 2615. 5. R. M. Smith and A. E. Martell, 'Critical Stability Constants', Plenum, New York, 1976. 6. G. E. Mont and A. E. Martell, J. Am. Chem. Soc., 1966,88, 1387. 7. S. Chaberek, Jr., R . L. Gustafson, R. C. Courtney and A. E. Martell, J . Am. Chem. Soc., 1959, 81,515. 8. H. J. L. Lajunen and S. Parhi, Finn. Chem. Len., 1979, 140 (Chem. Absir., 1979, 91, 2177428). 9. J. F. Verchere and J. M. Poirier,J . Inorg. Nucl. Chem., 1980, 42, 1514.
Although 1:2 complexes may be isolated, formation of 1:3 complexes and displacement of the vanadyl oxygen seem to be favoured. This effect is probably due to the exceptional chelating ability of 1,2-dihydroxybenzenes which appear particularly good 0 and n donors, an effect that stabilizes metal and ligand without a change in formal change. An interesting comparison can be made for K3[V"'(cat),] ,497 (Et3NH)2[V'V(~at)3]256 and Na[VV(DTBcat)3]*56 (cat = C6&Ot-, H2DTBcat = 35); d(V-0), are 2.013, 1.942 and 1.91 A, and the ionic radii are 0.78, 0.72 and 0.68 A, respectively. From equation (30), whether a tris chelate is formed depends on the donor ability of the ligand. Several vanadium(IV) complexes with 1,2-dihydroxybenzenes formed from solutions of the ligand, VOS04 and thallium(1) acetate.498 The compounds are of two types: (I) T12[V(RC6H302)3](R = H , 3-Me, 4-Me, 3-Me0) and (11) T12[VO(RC6H302)2](R = CHO). The crystal and molecular structures of (Et,NH),[V(cat),].MeCN (37) and K2[VO(cat)2].EtOH.H20 (36) were determined. Complex (36) has a square pyramidal geometry; d(V=O) = 1.4168, (longer than usual in complexes with geometry of type (41)) and d(V-O(cat)) = 1.956 A. Complex (37) has an approximately octahedral coordination geometry. (EtfiH)2[VO(L)2].2MeOH (L = 35 and 37) were prepared by displacement using A similar method was used for [VO(acac),] and addin the appropriate Na,[VO(DTBcat),] (38)m! Its absorption spectrum in CH2Clz is dominated by intense change transfer which results in a bronze-coloured solution. When one equivalent of SOCl, is added to one of (38) in CH2CI2, the solution immediately changes to blue. The resulting blue Na2[VC12(DTBcat)z](39) can also be formed by the same procedure as for the synthesis of (38), but with VC12(acac)2in lace of [VO(acac),]. In contrast with catecho1"981501~M2 and norepinephrine"' complexes where VIv seems to be the stable oxidation state, VI" and VIv DTBcat complexes are oxidized to Vv by protic solvent^.^‘"' However, observation of VIv ESR does not necessaril imply complete reduction of Vv by catechol;B6 either partial reduction or even adventitious Vyv could yield a similar result. Redox chemistry of vanadium-catechol systems is complicated; References 256, 497 and 499-508 discuss this subject in detail. In complexes, the metal centre may be in the + 5 , +4, +3 (and +2) formal oxidation state and quinones complex in three localized electronic forms:
504
Vanadium
catecholate, semiquinolate and quinolate. A complex containing mixed charged ligands, [VVO(DTBSQ)(DTBcat)], (DTBSQ = semiquinonate of 35), has been characterized structurally and the formation of [V(CL&SQ)(Cl,Q),]"' (CLQ = tetrachloroquinonato and CkSQ = tetrachlorosemiquinonato) has also been suggested.
(if) P-Diketonato ligands In addition to the very stable, commercially available, crystalline blue-green [VO(a~ac)~], several other [VO(/3-diketonato)2]complexes have been prepared;mb51othey vary from yellow to green and brown. The pk& values are in the range 1.71-1.76BM and normally decrease slightly in the range 300-380 K.509 [VO(acac),] may be prepared by several methods (e.g. by one-electron reduction of Vv by ethanol or mixing the diketone with VOSO,) and its crystal and molecular structure has been determined.511It consists of discrete molecules of [VO(acac),] in which vanadium presents a nearly square pyramidal geometry (Figure 22). Some internuclear distances are in Table 18. The crystal structure of [VO(bz a~) ~] (bzac = l-phenyl-l,3-butanedionato) has also been determined.512a The ligands are arranged in a square pyramidal geometry with a cis configuration and each phenyl ring is nearly coplanar with its chelate ring. Some distances are in Table 18. IR and Raman spectra of [VO(acac),] were
Figure 22 Perspective view of VO(acac), molecule.511Internuclear distances are given in Table 18
Table 18 Internuclear Distances for Several Oxovanadium[IV) 0-Diketonato Complexes
comgound
V4(1)
V4(2)
[vo(acac)zl
1 .S6 1 S71 1.612 1.58 1.62
1.97 1.974 1.952 a -1.99
IvO(bW,l [VO(acac),(4-PhPY)l [vO(a~ac)~(dioxane)]
Internuclear distances (A) V-0(3) V-0(4) 1.96 1.955 1.986 a -1.99
1.96 1.962 1.982 a -1.99
V4(5)
V-X
1.98 1.983 1.946 a -1.99
a a
Rd 1 2 3 4
5
* Not reported. 1. R. P. Dodge, D. H. Templeton and A. Zatlun, J . Chem. Phys., 1%1,35, 55. 2. P. K. Hon, R . L. Belford and C. E Pfhger, 1. Chem. Phys., 1965,43, 3111. 3. P. K. Hon, R. L. Belford and C. E. PRugtr, J. Chem. Phys., 1965,43,1323. 4. M.R. Caira, J. M. Haigh and L. R . Nassimbeni, Inarg. Nucl. Chem. Len., 1972,8, 109. 5. K. Dichmann, G. Hamer, S. C. Nyburg and W. F. Reynolds, Chcm. Commun., 1970, 1295.
In many solvents, particularly water and alcohols, [VO(@-diket~nato)~] are slowly oxidized by atmospheric O2,'l but very rapidly with H202.355 The constant for reaction (31) for P-diketones in tautomeric keto (HK) and enol (HE) forms, can be partitioned between K K and K E (reactions 32 and 33). At equilibrium, equations (34) and (35) hold, and values obtained are given in Table 19.514Kinetics showed that the reaction occurs through the enol form by parallel acid-independent and inverse-acid paths (Scheme 15).514 VO"+HL
VOZL+ HK VO*'+HE
A VO(L)'+H'
& VO(E)' + H i & VO(E)'+H'
(31)
(32) (33)
505
Vanadium
Table 19 Equilibrium Constants for Formation of Mono-b-diketone Complexes of VOz+ in Aqueous Solution at 25 "C ~~
acacb tfacacb tftacacb
Kla
KK"
0.55
0.67 0.32 0.11
3.14 73.05 10.39
-
-
0.32 0.11 -
bzac'
logolzo
Ref.
-
15.8
-
-
10.5
20.5
1,2 1 1 3
lOgBll0
%a
8.7
-
~
tfacac = 1,1,l-trifluoropentane-2,4-dione; tftacac = 4,4,4-trifluoro-1-(2-thienyl)butane-l,3dione. a Constants K,, KK and K , are defined by equations (31)-(33). Except for log /31zo, I = 1.0 M NaCIO,. ' I = 0.1 M. 1. M. J. Hynes and B. D. ORegan, J . Chem. Sm., Dalton Trans., 1980,?. 2. R . Trujillo and F. Brito, And. Fis. Quim., 1956,52B,407. 3. N. V. Melchakova, N. A. Krasnyanskaya and V, M. Peshkova, J . And. Chern. USSR (Engl. Trunsl.), 1970, 25, 1756.
VO"
+ HE
& V02' + E- + H' K
VO(HE)*+ e VO(E)*
+ H'
Scheme 15
In dichloroethane the ligand exchange of [VO(acac),] with Hacac ["C] occurs without side reactions and the rate may be expressed b R - k2[V0(acac),][Hacac], where [Hacac] is the concentration of the free acac in enol form.TI5 Electrochemistry of [VO(a~a c)~] in DMSO has been studied by cyclic voltammetry and controlled-potential coulometry at a platinum e l e c t r ~ d e . ~[VO(a~ac)~] '~ is irreversibly reduced by one electron at - 1.9 V vs. SCE (saturated calomel electrode) to a stable V"' product. In the presence of an excess of ligand, [VO(a~ac)~] is reduced by two electrons to [V(acac),]- with the V"' species mentioned above and [V(aca$,] as intermediates. The one-electron oxidation of [VO(acac),] at +0.81 V 21s. SCE gives a V product. The room temperature ESR spectrum of [VO(acac),] in toluene consists of the eight lines expected for an atom of spin with g = 1.971 and an average coupling constant of 120 G.517,518 In frozen (77 K) toluene solution, molecular tumbling ceases, different molecular axes of the axially symmetric [VO(a~ac)~] are oriented along the applied field and the spectrum splits into two overlapping eight-line spectra. The single crystal spectra of cis-[VO(bzac)2] show anomalous fine structure.519 Solvent effects and adduct formation of [VO(acac)2.and other [VO(B-diket~nato)~] complexes have been studied by several m e t h o d ~ ~ ~ 532 l , ~and ~ , in ~ coordinating solvents [VO(acac)2] is known to add a sixth ligand according to equation (36). Older reports include a spectrophotometric and calorimetric study of [VO(acac),] and [VO(tfacac),] add~cts.~''With [VO(a~ac)~] in nitrobenzene, the enthalpy change for reaction (36)ranges from 44.3 kl mol-' for n-decylamine to 24.3 kJ mol-' for methanol. Equilibrium constants K were between lo00 and -0.6. [VO(acac),] is not a sensitive indicator of relative base strength.521For [VO(a~ac)~], A. decreases by -3 G (and go increases by -0,0004) as amine adducts are formed.5u
z,
VO(acac), + ligand
&
VO(acac),(ligand)
(36)
Equilibrium constants for reaction (36) have been estimated from spectrophotometry. In some studies, visible spectra of solutions containing [VO(a~ac)~] at constant concentration and base in variable concentration show an isosbestic point, which suggests but does not prove that
SO6
Vanadium
3
rn
Vanadium
507
there are only two absorbing species. However, in all cases the results were interpreted according to equation (36). Table 20 summarizes results from several spectroscopic equilibrium studies of adduct formation [ v O ( a c a ~ ).525,532-534 ~] In early work, solvent effects were explained by coordination at the vacant axial position. This was questioned by Guzy et al. who measured spectra of [VO(acac),] in numerous solvents and pointed out that ,especially for hydrogen-bonding solvents, there might be interaction with vanadyl oxygen.361Selbin concluded that for alcohols (and CHC13 and CH2Cl2), the H-bonding effect is greater with the vanadyl oxygen than with the acac oxygen^.^^^ The interaction of [VO(a~ac)~] with solvents has been measured by the 51V hyperfine coupling in mixtures of ethanol and CCL as a function of solvent composition.529 IR studies with pyridine derivative^^^^,'^^ showed the existence of adducts with the sixth ligand bound cis rather than trans to the oxygen; an X-ray study of the 4-phenylpyridine (Fhpy) adduct confirmed this (Figure 23a) .s37 Interaction of ligands (solvent or a dissolved molecule) with [VO(acac),] may involve (i) coordination in a sixth position trans to the vanadyl oxygen, (ii) coordination in a sixth position cis to the vanadyl oxygen, and (iii) hydrogen bonding to the oxygen (or atoms of the ligand). While type (ii) was observed with the 4-phenylpyridine adduct, interaction of type (i) exists between [ VO( aca ~)~] and 1,4-dioxane (diox) or 4-picoline. With diox, a substantial change in the v(V=O) stretch is observed when it is added to [VO(a~ac)~] in CHC1238but only small effects for the visible spectra in nitr~benzene.~~’ In the complex [VO(acac)Z(diox)],from X-ray analysis, the oxygen atoms of diox are coordinated trans to the vanadyl oxygen (Figure 23b).539The crystal structure of [VO(aca~)~(4-picoline)l consists of discrete molecules with 4-picoline in the ~ a position m and a very short V-0 of 1.557(17) C
c
*
0
c
C
4
O 3
c
i:> (Ql Figure 23 Schematic representation of the molecular structure of (a) [VO(acac),(4-phenylpyridine)]?37 (b) [VO(acac),(l,4-dio~ane)]~~~ (a dioxane molecule bridges two [VO(acac)J groups) and (c) [ V O ( 0 M e ) ( a c a ~ ) Some ]~~~~ internuclear distances are included in Table 18
Besides the structure of [ V O ( a ~ a c ) ~ ( P h p yand ) ] ~ ~IR ~ e ~ i d e n c e , ~ENDOR ~ ~ . ~ , ~studies (see Section 33.5.1) suggest formation of cis adducts and/or the existence of cis and tram isomers.527 For [VO(acac)2] with methanol and substituted pyridines in frozen CHC13/toluene solutions, there must be a well-defined binding site for CHC13, most likely via a hydrogen bond to the vanadyl oxygen, and methanol is at the vacant axial position. For [VO(a~ac)~] and substituted pyridines in CHCl,, complexation induced constant shifts of 31 f3 cm-’ and/or 51 f 3 cm-I (Table 21)53s attributed to cis (31 cm-l shift) and/or trans (51 cm-’ shift) adducts. However, the IR of solid [VO(acac)2(substituted py)] (Table 22) were interpreted on the reverse assumption that cis coordination produces the larger shift of V ( V = O ) ? ~ ~Kink and van Willigen consider that the group of pyridines that coordinate in the cis position induces a larger shift in v(V=O);~” accepting this, they find a good correlation between the solution IR data535 and their own ENDOR. In a number of instances, the isomer that is present in the solid state536 and frozen solution. Other investi ations emphasize subtle may not predominate in 41 For example, Mannix factors controlling equilibrium between different onf formation.^^^^^^^, ! and Zipp526studied the visible spectra of [VO(acacX),] [X= C1 or Br) in several solvents with a
Vanadium
508
wide range of donor properties and obtained a linear relationship between the energy differences of the first two visible bands, D1,,, and the solvent donor number.542However, the [VO(acacX),] compounds were found to be poorer acceptors than [VO(a~ac)~]; this is surprising. Table 21 v(V=O) (cm-') in Solutions Containing [VO(acac),] and Substituted Pyridines (1:5) in CHC1, at Room Temperature ~~
~~
~
v(V=O) bands and
Substituent
relatiue interairy
1003 975 !n2 >969 >969 >>971
>>971 -975
J. J. R. Frahto da Silva and R. Wootton, Chem. Commun., 1969, 421.
Table 22 v(V=O) (cm-') of [VO(acac),L] (L = Substituted Pyridines) Substituent No adduct 4-COzEt
v(V=O) 999 955 957 955 959 959 959 962 953 958 974 973 971 971 975
4-COPh 4-Ph 3,5-Me, 4-Et 3-m* 4-Bu' 4-NMe2 4-NHZ
4-CN H 4Pr" 4Me 3,4-Me2
M. R. Caira, J. M. Haigh and L. R. Nassimbeni, J. Inorg. Nucl. Chem., 1972, 34, 3171.
A rather different study of the kinetics of decomposition of solid complexes of [VO(dbm);?(L)] (dbm = dibenzoylmethanato, L = py and several methyl-, dimethyl- and amino-pyridines) used differential scanning calorimetry (DSC).531Using the temperature that corresponds to the loss of the molecule L in equation (37), a linear relationship was found between it and the basicity of L, except for 4-amino- and 4-methyl-pyridine. VO(dbrn),L
-
VO(dbm)z
+L
(37) 1
In the coordination of 4-picoline N-oxide (MPNO) to a series of [VO(/3-diketonato)2]
Vanadium
509
complexes, IR spectra and equilibrium constants for adduct formation followed the order: tftacac > tfacac > dbm > bzac > acac > dpm (dipivaloylmethanato). A similar order was found with several other neutral bases.524For [VO(a~ac)~], the MPNO coordinates trans but for [VO(b~ac)~] and others the coordination is cis. A similar trend was found in compounds [VO(/3-diketonato)z(L)] with acac or tfacac and L = py, phen, a-and P-naphthoquinoline (aand [VO(aca~)~(phen)]-2MeOHwas prepared from an excess of phen with [VO(aca~)~] in methanol. The same reaction in CH2C12 results in VIv oxidation and [V02(acac)(phen)] is obtainedY5*while [VO(tfa~ac)~(phen)] is produced in both solvents if the complexes, phen was starting material is [VO(tfa~ac)~]. In the two [VO(fI-diketonat~)~(phen)] said to be unidentate and a cis octahedral structure was proposed for [VO(tfacac)2(L)] complexes (L = py, phen, a-nq, Early publications on [VO(P-diket~nato)~] have been reviewed.355More recently, complexes with benzoyl rn-nitroacetanilide, benzoyl acetanilide545and 1,1‘-(1,3-phenylene)-bis(butane1 , 3 - d i 0 n e ~have ~ ~ been synthesized. Other [VO(/3-dik)2] adducts have been isolated, for example [VO acac adducts with a series of pyridine N - o ~ i d e sand ~ ~ ~several pyridine c a r b o ~ a m i d e s ja~k!~ [VO(b~ac)~] adducts with pyridine , methylamine, isoquinoline and 4 - p i ~ o l i n eEquilibrium .~~~ constants of 1:1and 2 :1 adducts of pyrazine with [VO(tfacac),] have been determined (equation 38).550 In the 2: 1 complex, the pyrazine bridge between two equatorial sites of adjacent vanadium atoms promotes a weak exchange interaction. The nitroxide radical 2,2,6,6-tetramethylpiperidinyl N-oxide also forms an adduct with [VO(hfa~ac)~] in which there is a strong interaction between the electrons on the metal and nitroxide.551
*VO(tfacac)p
VO(tfacac)z+ pz
K = I 5x106
K=2xlb
[{VO(tfa~ac)~),pz]
(38)
Oxidation of alkenes, sulfides, sulfoxides and amines by alkyl hydroperoxides (ROOH) is catalyzed by [VO(acac),] (equations 39-42) ,552 and mechanisms involving association of ROOH with [VO(acac),] forming Vv compounds have been suggested.552The reactions of phenoxyl, iminoxyl, nitroxyl, peroxyl and alkoxy1 radicals with [VO(aca~)~] in solution were studied by kinetic ESR spec tros~op$~~ and the net reaction was found to be catalytic reduction of the radical, probably also involving initial formation of a Vv compound.
-S-
-tROOH %v
II -S-
+ RQH
0 -
II
ROOH
-S-+
a ‘SO, , + ROH
(41)
’ 0
-N-+
I
t
-N-+
ROOH
I
ROH
The reaction in alcoholic medium of a P-diketone, a base and V0C12 in 1:2 : 1 molar ratios leads to compounds [VO(alko~y)(P-diketonato)]~.~~~ This is an interesting result since [VO(P-diketonato)z] form if the reaction with the same species is carried out in molar ratios of 2:2:1 or 1:l:l. Therefore, the stoichiometry in which the reagents are mixed will drive the reaction towards one product or the other. This behaviour was rationalized assuming Scheme 16 where an intermediate [VO(OH)L] (not isolated) forms and is irreversibly transformed to the corresponding [VO(OR)L] compound. VOC12+ HCI + 20H-
-2CI-, -H20
VO(0H)L
VO(0R)L
-
iVOCI, + fVOL2+ OH[VO(OR)L],
Scheme 16
Several complexes [VO(OR)2(J3-dik)]2were prepared including [VO(OMe)(acac)12; for its crystal structure?s0 see Figure 2 3 ( ~ ) . ” ~It consists of dimeric units with a chair-like
Vanadium
510
arrangement and the geometry around the vanadium atom was best described as tetragonal bipyramidal. The vanadium lies 0.60A above the plane of two P-diketone oxygens and two bridgin alkoxy oxygen atoms. The p $ is 1.80BM, i.e. magnetic exchange is much greater in the CUIB analogues. V-V is 3.102 A. Although the compositions, homogeneity and molecular weights of the polymers obtained using [VO(OR)(P-dik)l2 complexes as Ziegler-Natta catalysts are almost the same as those obtained using the commercial catalysts [V(acac),] and [VOC13],the latter show higher activities. A few [VO(OMe)(/3-diketonat0)]~and [V0(L)I2 complexes (L = polydentate 0-coordinating ligands) also have a high catalytic activity for the oxygenation of catechols (namely 3,5-di-t-b~tyl-l,2-dihydroxybenzene)~~* and this reaction was considered similar to the one catalyzed by pyrocatechase. A different group includes vanadiumIV hexacoordinate complexes with no vanadyl bond. Complexes [VO(L),] (L = tetradentate or two bidentate ligands) react under mild conditions with S0C12 or SOBrz producing [V(L),C12] or [V(L),Br2] hexacoordinate VIv complexes.429 The same deoxygenation can be accomplished in dioxane by PC15, which is even more reactive than S0C1Z.555Complexes [V(@-diketonato)Xz](X = C1, Br) have been studied with acac, bzac, dbm, tfacac: cis and trans isomers may form, in relative abundances which are solvent dependent. In non-polar solvent, the abundance of the trans-dichloro isomer decreases in the order tfacac > acac > bzac > dbm.429All these dark compounds are rapidly hydrolyzed in air to vanadyl complexes.
(iii) Hydroxyaldehydato rand hydroxyketonato ligands Several V 0 2 + salicylaldehydato (and derivative) complexes [VO(L)2] have been prepared although practically no work is recent. The early work has been reviewed355,357b and includes salicylaldehyde (Hsal) and the derivatives 5-C1-, 5-Br-, 3,5-Br2-, 3,5-Cl2-, 4-phenyl- and 4-amino-salicylaldehyde. The oxovanadium(1V) complex of B-hydroxynaphthaldehyde was also r e p ~ r t e d .More ~ ~ ~recent ,~~~ studies include the synthesis of compounds formulated as [VO(L)2] and 4,6with L = (46; R = 3-Me0, 5-C1),557,558(47), 3,7-dimethy1-7-hydroxyo~tan-l-a1~~ dihydroxycoumaran-3-one (40) .561 IR spectra of the last suggests that coordination involves the 3- and 4-oxygen atoms. No magnetic data are given and v(V=O) = 965 cm-'.
0I
0 I1
After mixing for a few hours a solution of (42) in CH2Clzwith a solution of Vo S o 4 (pH 3) L
H20under N2,a chelate with the P-ketoaldehyde (41), formulated as [VOL], was obtainer from the organic phase.= ESR showed A. = 105 G. Similarly formulated complexes with thl nitroxides (43) and (44) were observed in solution. [VO(tropol~nato)~] (tropolone = 45) anfir the deoxygenated compounds obtained by reaction with S0Clz and SOBrz (Section 33.5.5.4.i) [VX2(tropolone)2] (X = CI, Br), have been studied.429 A trigonal prismatic structure i proposed with the chelate ligands occupying two vertical edges.
Vanadium
511
33.5.5.5 Oxoanions as ligands
(i) Surfate Hydrated sulfates VOSO4-xH20 are one of the most common starting materials for the preparation of oxovanadium(1V) compounds. Several forms have been characterized; some are in Table 23, which shows that V=O is 1.58-1.59 8, and is longer in a-and p-VOSO, where the V02+ group is involved in * -V=O- - .V=O. - chains; the H20 molecules are either free or coordinated to the vanadium atom and the oxygens of V 0 6 octahedra belong to water molecules, to SO, groups or to other V 0 6 octahedra. Table 23 Anhydrous and Hydrated Forms of Vanadyl Sulfates VOS0,.xH20: X-Ray Diffraction Data V - 0 infernuclear distances
X
Vanadyl oxygen
Equatorial oxygens
(A) Axial oxygen
0 (B form) (unstable)
1.594
2.056,2.005 2.05,2.016
2.284
0 (aform)
1.63
2-04"(x 4)
2.41
Type of vanadium environment Three-dimensional network with chains of VO, distorted octahedra connected pairwise by an SO, group via corner sharing A VO, octahedron has four SO, tetrahedra in the equatorial plane
Ref.
1
2
3
3
1.576
2.033,2,048 2.003",2.001"
5
1.591
5
1.584
2.048,2.037 2.035, 1.983a 2.008,2.019 2.006,1.994"
5
(p form)
unstable) 6
1.591
2.031,2.031
2.018,2.018 1.586
2.023,2.021 2.160,2.004
Oxygen atoms of SO, groups. 1. P. Kierkegaard and J. M. Longo, Acra Chem. Scand., 1965,19,1906. 2. J. M.Longo and R. J. Arnott, J. Solid Srare Chem., 1970,1, 394. 3. G.Ladwig, 2.Anorg. AUg. Chem., 1969,364,225. 4. F. Thkobald and S. Galy,Acta Crysrallogr., Sect. E , 1973,29, 2732. 5. M.Tachez and F. Thhbald, Acru Cyvstallogr., Sect. E , 1980,36,2873. 6. C.J. Ballhauscn, B. F. Djurinskij and K. J. Watson, .I. Am. Chem. Sac., 1968,90,3305. 7. M.Tachez, F. Theobald, K. J. Watson and R.Mercier, Acta Ciystullogr., Sect. 8, 1979,3S, 1545. 8. M. Tachez and F. ThWald, Aclu Crysrallogr., Sect. E, 1980,36, 1757. 9. M.Tachez and F. Theobald,Acta Crystaibgr., Sect. E , 1980,36,249. a
6 7
512
Vanadium
Neutron powder data on VOS04.3D20 improved the location of hydrogen in the structure.563In 2VOS04-H2S04,X-ray techniques show V 0 6 octahedra linked by monodentate SO4 tetrahedra into polymeric [VOS04], layers joined together by sulfuric acid mole~ules.~* Several (V0)2H2(S04)3hydrates have been reported.357a Oxovanadium(1V) sulfate solutions were studied by Ducret who concluded that [VO(SO,)] and [VO(S04)2]2- form and reported [VOS04]/([VO"][SO~-]) = 63.466 Others obtained 2.4 X 102.565 Pressure jump relaxation techni ues on VOS04 solutions were explained according to equation (43): K = (3.0 f 0.5) x 16PMGri or'eva studied vanadium(1V) solutions over a wide range of H2S04and Sot- concentration^.^
P
VOz'-tSO:-
very fast
VO(I-I,O)SO, (outer sphere complex)
k
- lo3
VOSO,
K=
[VO(H,0)S04] + [VOS04]
(inner sphere
[vo'+][So:-]
(43)
complex)
(ii) Phosphoric acids There are many old reports of solid vanadyl phosphates.355Complex formation in aqueous acidic (pH 5 3) vanadyl phosphate solutions was explained by formation of [VOHP04], [HVO(HP04)2]- and [VO(HP04)2]2-.s6R Orthophosphate coordinates as bidentate HPOi-. The most important is [VOHP04]. The formation of 1:3 complexes was considered unlikely. These568and spectrophotometric results569include a total of five monomeric species and were later discussed on the basis of Scheme 17.570[VO(H2PO&] is dominant in 1M H2P04 solution at pH2,570 and the rate of loss of H2PO; was determined from 31PNMR. Aquation and anation rate constants at 25 "C were estimated to be 2.5 x lo4s-l and 2 X lo6 r n ~ l - l s - ~ , respectively. The anation rate constant indicates that coordinated water in [VO(H2P04)]+ is more labile than the equatorial H,O molecules in the complex VO(HzO)Z+.
+
VOZf H2P0T I VO(H2P04)' VO(H,PO,)+
+ H,PO; e
VO(H,PO,),
VO(H2PO4)' e VO(HPO4) + Hf VO(HZP0JZ e VO(H,PO,)(HPO4)- + H+ VO(H,PO,)(HPO,)- e VO(HP0,):- + H' Scheme 17
The 22-line ESR s ectra of oxovanadium(1V) pyrophosphate (5 5 pH 5 6) suggested a pyrophosphate trimer!' Na6(VO)3(P207)3.18H20had p = 3.6 BM and its solution ESR also exhibits a 22-line signal. Osmometry supported the trimeric nature.573 Addition of base to its solution causes a decrease in the intensity of the ESR and at pH 2 11 no signal was detected. The explanation assumed (i) at pH7-8, disproportionation of trimer to monomeric [VO(P207)2]6-and an anionic species [(VO)4(OH)10]2-and (ii) at pH 9.5-10.5, base hydrolysis of the monomeric complex to [(V0)4(OH),0]2-.57 In vanadium phosphorus oxides, the oxidation state and the phosphorus :vanadium ratio can be varied. They act as heterogeneous catalysts in the oxidation of n-butane and n-butene to maleic The best catalysts have vanadium in the +4 state and a P :V ratio of -1. Understanding mechanisms requires crystal structures. That of [(VO)2P207]is built of double chains of V 0 6 octahedra that share edges across the chain.577Pyrophosphates link the double chains into a three-dimensional network by sharing oxygen corners with vanadium. This compound forms at 400 "C from [(VOJzH4P209], was studied by several technique^^^^,^^^ and characterized as VO(HP0,).0.5Hz0 .57 Vanadyl hydrogen phosphate layers stacked along the c axis are held together by H bonding. The layers contain €ace-sharing V 0 6 octahedra; one face-shared 0 atom is from an H20molecule that bridges the two vanadyl groups. The four remaining 0 atoms of each octahedron are shared with phosphate tetrahedra. Each
Vanadium
513
tetrahedron has three oxygens shared with vanadium and one OH group. (V0)2P207contains double chains of V(IV)cations, and VO(HP04).0.5Hz0contains isolated V(’v)-V(rv) (iii) Perchlorate-oxovanadium (ZV) solutions and salts
Solutions of the perchlorate have been prepared by several methods (e.g. ref. 357a, p. 250): (ii) start (i) precipitate VOZ in the absence of O2 and dissolve the solid with perchloric with VOS04 and precipitate with Ba(C104)2;580(iii) reduce V(v) compounds, e.g. reduce NH4V03 with HCI and KI and precipitate halides with AgC104;4M1 (iv) reduce a slurry of v205 in perchloric acid at a platinum cathode.581SOz has also been used.582Oxovanadium(1V) in stock solutions has been determined mainly by oxidation by KMn04 but several have used spectro h~ tor netry.~~’ Frausto da Silva and co-workers used titrations of 1:l V02+ and edta,379,378 Tapscott and Belford titrated 1:2 vanadyl perchlorate-sodium oxalate583 and other^^^^,'^' used Grads method.586 The blue, very hygroscopic VO(H20)5](C10,)2 has been shown to involve no C104 coordination in solution or soli&87 and has p = 1.70 BMSS7(see also ref. 357a, p. 250). It is easier to get solids with ligands other than water, for example [VO(DMSO)5]-(C104)2.355 Benali-Baitich studied spectra of V02+-perchloric acid (0 6 [HC104]6 13.3 M).461For high acid concentrations, VIv is oxidized; others observed rapid oxidation in solutions containing -2.2 M NaC104 for pH 22.5.58x From the visible spectra, ClOh coordinates.461However, such changes may arise from VOH+ (see Section 33.5.5.1.i), not C10, c ~ o r d i n a t i o n . ~ ~
(iv) Nitrate-oxovanadium(KV) solutions Although dilute solutions of nitrate ma be prepared, the instability of concentrated solutions and the reaction of HN03 with VOJ+ have been r e ~ o r t e d . ~ Oxidation ~ ~ , ’ ~ ~ of VOS04 in acid at 80°C by nitrate is rapid, after a long induction which resembles an autocatalytic reaction.’” ( v ) Miscellaneous anions
A few sulfite, arsenate, selenite and selenate compounds were but should be reinvestigated. VOz+ forms a deep purple complex with phosphotungstic in contrast with the yellow complex with Vv. Spectrophotometrically, the formation constant is 1.3 x 10s. The kinetics with 1Ztun stovanadophosphates were analogous to those with 12molybdotungstophosphates.5 9 i
33.5.5.6 Carboxylates (i) Monocarboxylares Table 24 summarizes most known VO(RCOZ)2 complexes. The magnetic properties of formate hydrates differ from those of higher carboxylates. p:& values are normally 1.451.80 BM and these complexes obey the Curie-Weiss hw.358,595A m agnetically normal hydrated formate, VO(HC02)2-H20,is known’96 and the compound Kz[VO(HC02)4]consists of infinite zigzag chains in which the V atoms are linked in an axial-equatorial manner by formate bridges (Figure 24).597The geometry is pseudo-octahedral, the V atom lying 0.27A out of the equatorial plane. pt& = 1.79 BM, indicating little or no ma netic interaction between the V atoms. Formation constant studies have been performed.591-56 Subnormal magnetic oxovanadium(1V) complexes that include this type of ligand have been reviewed by Syamd (Table 24).3’8 v(V=O) values appear --1Oo~m‘-~lower than commonly found. Vanadyl acetate and halogenoacetates VO(CH3-,,X,C02)2, (n = 1,2, 3; X = C1 or Br) axe characterized by one v(V=O) at 895 f 5 cm-’, by very low p:S (Table 24) and by antiferromagnetic spin-spin interactions. These results have been interpreted on the basis of a polymeric structure (52) m358t598 The same characteristics and interactions exist in many [VO(salnpn)],599 polymeric with complexes of other aliphatic and aromatic carboxylic
Vanadium
514
Table 24 Vanadyl Monocarboxylate Complexes VO(RC02)2 Ligand ( R in RCO;) H (sesquihydrate) H (monohydrate) H (hemihydrate) Me Et pr"
PhCH,
p$
(BM) 1.70 1.65 1.45 1.24 1.19 1.24 1.20 1.25 1.20 1.33 1.23 1.21 1.23 1.26 1.22 1.25 1.23 1.25 1.25 1.38 1.32 1.25 1.19
V(V=O) (cm-.') 300K 100K
902
891
893
885
891
883
891
883
893
884
893
886
s93
886
Ligand (R in RCO;)
Ref. 1 1
CH$l
1
CHCl,
2 1 3 1 3 1 3 1 1 3
1 1 1 1 1 1 1 3 1 3
CCI, CH,Br CHBr, CBI~ Me(CH,),, Me(CH,),CH=CH (CH,), PhCH=CH Ph,CH m-ClC,H, o-IC~H,
A2
(BM)
Ref.
1.16 0.93 1.16 1.20 1.20 1.24 1.27 1.25 1.02
1 4 1 4 1 4 4 4
-
1.30 1.18 1.33 1.25
4
5
5 1 1 I 3 3
1. V. T. Kalinnikov, V. V. Zelentsov, 0. N. Kuz'micheva and T. G. Aminov, Russ. 1.Inorg. Chem. (Engl. Trawl.), 1970,15,341. 2. A. T.Casey and J. R. Thackeray, Aust. 3. Chern., 1969,22, 2549. 3. A. T. Casey, B. S. Moms, E. Sinn and 3. R. Thackeray, Aust. 1. Chem., 1972,25, 1195. 4. J . P. Walter, D. Dartiguenave and Y. Dartiguenave, 1. Inorg. Nucl. Chem., 1973, 35, 3207. 5. S. Prasad and K. N. Upadhyaya, 1.Indinn Chem. Soc., 1961,38, 163.
.v 0
0
@
C
Figure 24 Schematic re resentation of the molecular structure of (a) the anion of K2[VO(HC0,),J showing the polymeric structure59yand (b) the [(VO),(CO,),(OH),]'~ anion.6w Some internuclear distances are indicated
515
Vanadium
. - V=O - - * interactions (see Figure 30), has small ferromagnetic interaction in contrast V-0 to that in polymeric [VO(MeC02)2]. ESR studies on [VO(MeC02)2] and its adducts with ethanol, acetic acid, phen, bipy and 2-picoline have been reported:600,60' Different monocarboxylate compounds have been prepared (Table 25). Table 25 Monocarboxylate Compounds of Vanadium(1V) with Molecular Structures Determined by X-Ray Diffraction Studies
Internuclear distances (A) VanadyI oxygen
Basic units K~[VO(HCOZ)~I (PlYmCric) [V303(PhCO2)6(m)1 (oxo-centred)
Oxygen
(tram)
1.598 2.212 1.626 2.186 (TIP) 1.582 2.344 1.568 2.452 1.951 2.290_(ITHF) (v(V=O) = 981 crn )
Na4[(V0)2(CF3C02)~(THF~~(H20)~~
(cyclic centrosymmetric units)
Equatorial monocarboxylate oxygem 2.018 (av) 1.978 (av) 2.002 (av) 1.980 (av) 2.004 (av)
Ref. 1 2 3
1. T. R. Gilson, 1. M. Thorn-Postlethwaite and M. Webster, J . Chem. Suc., DuIron Trans., 1986, 895. 2. F. A. Cotton, G. E. Lewis and G. N. Mott, Inorg. Chem., 1982, 21,3127. 3. F. A. Cotton, G. E. Lewis and G . N. Molt, Inorg. Chcm., 1983.22, 1825.
Revision of early workm on carbonato complexes formulated the violet NH: salt obtained at pH -3.5 as V12028H14(C03)8(NH4)10-23Hz0 and the purple K+ salt obtained at pH -8.5 as V1202sH14(C03)8Kln.16Hz0.bm v(V=O) occurs at 950 cm-' in both and v(V--0-V) at 525 and 515 cm-' for the NH: and K+ salts.602Deep violet crystals of the NH: salt made by reacting VOC12 with NH&C03 under COz had the formula (NH&[(VO)6(C03)4(OH)9]-10H20.M)4 The anion (Figure 24b) consists of crown-shaped hexanuclear aggregates consolidated by bridging hydroxo and carbonato groups, the latter functioning in a p6 mode as well as in the more common p2 mode. The six V atoms, each octahedral, are within 0.07A of their mean plane in a pseudo-hexagonal arrangement, (ii) Polycarboxylutes
Oxovanadium(1V) forms stable complexes with polycarboxylic acids and many formation constants have been determined, including mixed complexes (Section 33.5.10). Potentiometry and NMR led to analysis of the paramagnetic contribution to relaxation as a function of [V02+], [VO(oxal)] and [VO(oxal)z]2- for the oxalate system and [V02+], [VO(Hmalon)]+, [VO(malon)] and [ V O ( m a l ~ n ) ~for ] ~ -the malonate system.584Table 26 summarizes their rate constants. Proton exchange rate constants are higher than those for the free ion ( k = 1.7 x 16s-1,584 6.2 x 10-4s-1,4598 x lo4 s-l 605 and 1.8 x lo5s-' "). Similar faster exchange was found in other systems and may be explained assuming that the coordination of the ligand weakens the VO-OHz bond, increasing the exchange rate of the water molecule, reflected also in the proton exchange. The surprising proton exchange rate for [V0L2l2- was explained by an intramolecular rearrangement (53 54) .584
-
Table 26 First-order Rate Constants of the Proton Exchange Between the Bulk Water and the Different Paramagnetic Species ( I = 1 M NaClO, and T = 25 "C)
Species
k x 1 0 P (s-l)
[VO(oxalate)] [VO(oxalate),]'[VO(malonate)] [VO(malonate),Jz[VO(Hrnalonate)]+
1.85 2.30 1.85 1.55 1.43
I. Nagypgl and I. Fabyan, Inorg. Chim. Ana, 1982,61, 109
Vunadium
516
(54)
(53)
Che and Kustin studied corn l e ~ a t i o nresults ; ~ ~ ~ for oxalic and malonic acids are in Table 27. From previous relaxation data' and their own results, they concluded that the rate constants are more consistent with a 'normal' dissociative pathway if VOL formation from [VO(OH)]' is assumed. Table Z7 Forward Rate Constants for Vanadyl Complexation (I=0.5MKN03and T=25"C) Ligand Malonate Oxalate
k , x IO-' (mol-' s-') -20 2.18" -40 -1"
Ref. 1 2 1
2
Including [VO(OH)*] in the overall reaction mechanism. 1. H. Hoffmann and W. Ulbricht, Ber. Bumenges. Phys. Chem., 1972, 76,
a
1052. 2.
T.H.Che and K, Kustin, lnorg. Chem., 1980, 19, 2275.
Many formation constants involve polycarboxylates; Table 28 summarizes the data. NAgypiil and Fgbiin's report on the oxalic and malonic systems seems the most complete as hydrolysis of both metal ion and complexes has been included.584 A concentration distribution of the complexes in the malonic system is shown in Figure 25. The order of basicities is succinic > citraconic > itaconic > maleic > malonic acid and log BllD should follow the same order. However, from Table 28, the order of stabilities is citraconic > malonic > maleic > itaconic > succinic acid.60g
'I: t
110
l20/ I
PH
Figure 25 Speciation in oxovanadiurn(1V) -tmalonic acid solutions calculated assuming NBgyp6l et af.584formatior constants ( I = 1.0 M NaCIO, and 25 "C) for C,, = 0.02 M and C,,/C,, = 2.0
Many solids have been isolated and Table 29 summarizes most. Solid oxalate complexes have been prepared from vanadiurn(1V) salts and from V20,. They form readily and are mainly oi two types: (M)2[VO(oxal)2j.xH20 and (M)2[(VO)z(o~al)3&H20. VO(malon)-4H20 and VO(maleate).2H20 were prepared from vanadyl hydroxide, and bis malonate and maleate complexes have been synthesized from vanadium(1V) arid vanadium(V) salts. Several mixed-ligand complexes containing polycarboxylates as one ligand have also been prepared (see Section 33.5.10). Complexes with succinate, p t a r a t e and adipate have subnormal magnetic properties thar indicate V. * . V interactions. 58 Oxalato, malonato and maleato 1:1 complexes apperu monomeric as indicated by v(V=O) and pi;, and ease of formation, stability constants anc decrease in v(V=O) follow the order: oxalate > malonate > maleate. For the 1:2 complexer
Vanadium
517
Table 28 Formation Constants j3x,,z of Oxovanadium(1V) Polycarboxylate Complexes Ligand
(H,L )
LogB11,
Malonic acid
Oxalic acid
5.59 5.23 6.10 3.80 6.45 4.65 f
-
I
Maleic acid Diglycollic acid
Dithiodiacetic acid
Glutaric acid Succinic acid Adipic acid Itaconic acid Citraconic acid Thiodiacetic acid Ethylenedithioacetic acid" Phthalic acid
5.19 4.41 4.05 3.94 3.84 4.06 5.01 3.96 3.82 3.66 3.98 3.18 3.65 3.66 3.48 3.91 6.33 3.14 2.68
-
LogB,,,
9.48 8.85 10.60 11.78 9.35 12.0 9.76 9.76
-
6.68 6.43 6.26 6.70 6.78 6.48 6.28 6.81
-
-
-
Logs,,,
6.20
-
-
-
-
LogB11.1
0.52
-
,.
1.07 1.75
-
-0.33
-
-
-
-
-
-
-
L
-
-
-
-
-
-
-
-
I
-
-
-
LogB12-1
2.56 -
-
I
-
I
-
-
-
Ref 1" 2b 3= 4d 2b
'5 6 78 gh
3" 5"
9' 9' P 91
-
10" 9' 9' 9k 9' 1lrn 3'
e
5e
-
-
-
I
-
-
-
5" 3' 3'
lom 11" 12"
'Z=1MNaC104 and T=25"C. bZ=1MNaC104 and T=2O0C. CI=O.lMNaCIO, and ,,T=3OoC. dT=ZO°C. ' I = 0.1 M W O , and T = 30°C. Evidence for formation. Z = 0.5 M NaCIOak and T = 18 "C. Z = 0.05 M NaCIO, and T = 18°C. 'Z=,$1MNaC104 and T=25"$. 'Z=0.2MNaC104 and T=25"C. Z=0.3M and T=25"C. 'I=O.lMNaCIO, and Z ,=0.5 M and T = 25 T. Comparing the stabilities of the glutaric, thiodiacetic and ethylenedithiodiaceticacids, Napoli T = 35 "C. (ref. 11) referred to the basicity of the ligand "oyputing log~llo/log&,, and mnduded that sulfur is probably also involved in the coordination of the ethylenedithiodiicetate ion. Room temperature ESR spectra of aqueous VO*+-phthalic acid (HJ'ta) solutions at pH < 5 were interpreted by McBride (ref. 12) in terms of V02+ and [VO(Pta)] only. 1. I, Nagypal and I. FBbiAn, Znorg. Chine. Acta, 1982,61, 109. 2. A. A. Ivakin, E. M. Voronova and L.V. Chaschina, Russ. J . Inorg. Chem. (Engl. Transl.), 1970,15, 1839. 3. K. J. Narasimhulu and V. V. Seshaiah, Indian J . Chem., Sect. A, 1980,19, 1027. 4. P. K. Bhattacharya and S. N. Banerji, Curr. Sci., 1960,29, 147. 5. S. P. Singh and J. P.Tandon, A m Chim. Acad. Sci. Hung., 1974,80,425. 6. L. P. Ducret, Ann. Chim. (Paris), 1951, S12, 6, 705. 7. R. TrujillO and F.Torres, An. Fk. Quim.,1956, 52B, 157. 8. R. Trujillo, F. Torres and J . AscaNo, An. Fir. Qu'm., 1956, 52B,669. 9. R. K. Baweja, S. N. Dubey and D. M. Pun, J . lndian Chem. SOC., 1980,57, 244. 10. A. Napoli, J . Inorg. Nucl. Chem., 1973,35, 3360. 11. A. Napoli, Monatsh. Chern., 1981,1U, 1347. 12. M. B. McBride, Soil Sci. Soc. Am. J., 1980,44,495.
'
the stability and bond strength (as indicated by v(V=O)) follow the same The ease of dissociation follows the inverse order. (NH4)[VO(oxa1)2(H20)].HZ0 has been studied by X-ray diffraction and involves the equatorial coordination of one water molecule (see Figure 26a).612(Hpy)[VO(oxal)(F){€&O)~] is monomeric with a pseudo-octahedral geometry (see Figure 26b). l2 In (H2Tmen)[VO(malon)2(H20)1.2Hz0,the vanadium is octahedrally coordinated to four malonato oxygens, to vanadyl oxygens and to a water molecule (trans to V=O).613 Comparison with [VOClz(tmn)2] (tmn = N,N,N',N'-tetramethylurea) suggests that a weak extended exchange interaction of magnitude comparable to the nuclear hyperfine splitting is responsible €or the ESR.614
33.5.5.7 Hydroxy carboxylates (i) Tartrate Salts of oxovanadium(1V) tartrate anions have been prepared, e.g. by reduction of VzOs with an excess of either the optically active or racemic acid followed by addition of MOH.'=
Vanadium
518
Table 29 Oxovanadium(1V) Polycarboxylic Acid Complexes Complex
Na* or K+) (NH,)2[(VO)2(oxal),l.2H,O VO(rnalon).4Hz0 (Ct),[VO(malon),].nHzO (n = 0-5; C t = H + , NH:, Na"', K", Rb*, CS+, ~ 1 +iCt ; = Ca2+, Sr", Ba2', K+H+)
p$
(BM)
v(v=o)~(cm-")
1.71-1.73 1.74 1.71 -
986-988 986 980 973,988
1.83 1.82 1.82 1.82 1.91-1.99
[940-980]
1.23 1.14 1.25 1.65-1.70
[940-9801 [94&980] [940-980] 990-1000
8 8 8 9 3,lO-13
1.72 1.78 1.72-1.87
988,976 990 967-988
10,12 14 12.15-17
1.72 0.72 1.22 1.29 1.23
ID00 1003 -
(H,tmen)[VO(malon),(H,O)].H,O VO(maleate).2H2O (NH,),[VO(maleate),] .3H,O [VO(succinate)] [VO(ghtarate)] [VO(adipate)]
Ref.
-
18 14 16 15 15 15
-
N,N,N',N'-tetramethylethylendiamidium; Hmorph = morpholinium.
a Hztmen :
'When the line of the table corresponds to more than one complex, the values presented for p
z and v(V=O) are the lowest and highest reported in the original publications. For many compounds no data are available. 1. D. N. Sathyanarayana and C. C. Patel, J. Inorg. Nucl. Chem., 1965,27, 297. 2. K. C. Satapathy, R. Parrnar and B. S a h w , Indian J . Chem., 1963,1,402. 3. D.N. Sathyanarayana and C. C, Patel, J. lnvrg. Nucl. Chem., 1966,28, 2277. 4. J. Selbin and L. H.Holmes, Jr., 1. Inorg. Nucl. Chern., 1962,24, 1121. 5. Grnelins Handbuch der Anorganischen Chemie, 8. A d a g e , Vanadium, Teil B, Lieferung 1, 1967,p. 349. 6. A.J. Edwards, D. R. Slim, J. &la-Pala and J. E. Guerchais, Bull. Sac. Chim. Fr., 1975,2015. 7 . A. K. Sengupta, B. B. Bhaumik and R. K. Chattopadhyay, Indian J. Chem., Sect. A, 1980,19,914. 8. B. B. Bhaumik and R. K. Chattopadhyay, Indian J. Chem., Sect. A , 1981,u),417. 9. R. E.Oughtred, E. S. Raper and H. M. M. Shearer, Acta Crystallogr., Secr. B, 1976, 32, 82. 10. D.N. Sathyanarayana and C. C. Patel, J. Inorg. Nucl. Chem., 1965,27,2549. 11. J. Selbin, L. H. Holmes, Jr. and S. P. McGlynn, J. Inorg. Nucl. Chem., 1963,25, 1359. 12. C. G.Barraclough, J. Lewis and R. S . Nyholm, J. Chem. SOC.,1959,3552. 13. Gmelins Handbuch der Anorganischen Chemie, 8. A d a g e , Vanadium, Teil B,Lieferung 2, 1%7, pp. 408 and 441. 14, D. N. Sathyanarayana and C. C. Patel, Bull. Chem. SOC.Jpn., 1967,40,794. IS. V. T.Kalinnikov, V. V. Zelentsov, 0. N. Kuz'micheva and T. G. Aminov, Rws. J. Inorg. Chem. (Engl. Trunsl.), 1970,W, 341. 16. D.N. Sathyanarayana and C. C. Patcl, Indion 3. Chem., 1968,6, 523. 17. Ref. 13, pp. 408,441, 475,491,499,517,523,530, 551. 18. D. Collison, B. Gahan and F. E. Mabhs, J. Chem. Soc., Dalton Trans., 1983, 1705.
2 . 0 2 2 2 7 ,
I
O OHC\
$I o, C \\0
Ia) (b) and (b) [VO(F)(oxal)(H 0)2]-;612 Figure 26 Perspective diagrams of the complex anions (a) [V0(0xal),(H,O)]~interatomic distances are in A. I n (b) the H,O and F- are distributed in a 'disordered fashion' and the 1.75 distance may be considered an average of the distances expected for a V-F bond (1.9 A) and a V=O bond (1.6 A)
A
Vanadium
519
According to M (Na+, K+, NH:, NMet, Rb+, BaZ+, NEt:), M,[(V0)2{( +)-tart}{( -)tart}].xH20 or M,[(V0)2{( )- or ( - )-tart}2].xHz0 have been ~b t ai n ed . ~* ,615 Several methyl tartrate derivatives have been used as ligands and compounds isolated. Table 30 summarizes products. The ( , + ) [or ( - , - )] isomer requires a trans configuration about vanadyl and the f a cis configuration. Compounds include (N)i4)4[(V0)2{( + ) - t a 1 % } ~ ] - H ~ 0 , ~ ~ ~ Na4[(V0)2{( + )-tart}{( - )-tart}].12H20,616 (NEb)[(V0)2(( )-tart}{( - )-tart}]-8H20 (V-V distance = 3.895 A), Na4[(V0)2{( +)-dmt}{( - )-dm:])xH2O ( x = 6,617 12618) and Na4[(V0)2{( + )-tho-mmt}{( - )-threo-mmt}].14H20. The geometries of the dimeric anions are those of Figure 27 and only slight changes occur with the counterion.6m
+
+
+
Table 30 Some Oxovanadium(IV) Tartrate Compounds Compound a Na,[(VO),{ (+)-tart) { (-)-tart)].12H20 Na,[(VO),{ (+)-tart),].6H20 (NH4)2[(V0)2t (+>-tart),1.2H,O Na4[(VO)2{(+)-mmt}{( -)-mmt}].14H20 Na,[VO),{ (+)-mmt}{( -1-mmt}]-lOH,O
v(V=O) or ,u$ (300 K)
I
1.72BM 925 an"', 1.75 BM
1.33 1.10 1.20 1.39 1.40 1.10 1.20 1.26
924 cm-'
Na,[(VO),{(+)-mmt}*I.SH,O Na,[(VO),{(+)-dmt}(( -)-drnt}].12H20 Na,[(VO),{(+)-dmt}{( -)-dmt}]fiH,O Na4[(VO),{ ( i ~ ) - d m t } ~ ] - 1 4 H ~ O Na,[(VO),{ (+)-dmt},].12H,O Ba,[(VO),{ (f)-dmt}2].12Hz0
939 cm-' 937 cm-'
1.06 924 cm-'
1.10 1.13
Band maxima (pm)b II 1.88 1.70, 1.95 1.70, 1.96 1.90 1.90 1.65,1.87 1.75 1.88 1.72,1.89 1.72,1.90 1.71, 1.91
III 2.34 2.51 2.55 2.36 2.38 2.43 2.74
2.70 2.52 2.45 2.58
tart = tartrate4- anion, mmt = monomethyltartrate4- anion (rbreo isomer) and dmt = dimethyllartrate4- anion. Positions of band maxima in visible spectra of solid complex salts as Nujol mulls. 1. S. K. Hahs, R . B. Ortega, R. E. Tapscott, C. F. Campana and B. Morosin, Inorg. Chem., 1982,21,664.
a
(a)
(bl
Figure 27 Schematic representation of [(VO),(tart units in tartrate complexes. Internuclear distances are given for (a) ~NH4)d(V0)21(+)-tart),].H,0~~~nd (b) Na,{(VO),{ (+)-tart} { (-)-tart}].12Hz0616
ESR suggested that the methyl-substituted dimers exhibit a shortened V-V distance."l In Na4[(V0)2{( )-dmt}{( - )-dmt}].12H20 the V-V distance is 3.429 %, and the vanadium distance to the basal plane is 0.394%,,0.60 and 0.13 A shorter than for ( f ) - t a ~ T r a t e ~ - . ~ l ~ Tartaric acid is dibasic (H2tart), carboxyl protons being ionized by pH5. The hydroxyl groups are not ionized (pKa3= 14 and pKa4= 15.6).622The formation constants of ( + )-, ( - )-, ( f )- and meso-tartaric acids with V02+ confirm binuclear complexes of optically active and, to a lesser extent, meso acids with V02+.623 Electronic and CD spectra624of the solid and solutions of oxovanadium(1V) ( + )-tartrate are identical, as are their IR spectra,61s good evidence that the dimeric species in solution is the same as that found in the crystals. An unusually well-defined triplet state ESR has been recorded for liquid and frozen s o l ~ t i o n s . ~ ~ ~ ~ ~ ~ Racemic and optically active solutions behave differently, with large stereoselective
+
Vanadium
520
effects.627The reaction corres nding to equation (44) has K = 16623and a reaction enthalpy of (6.9 f 0.4) x lo3J mo1-1.628," The kinetics were also studied.629
go
$[(VO),{(
+ )-ta~-t}~]~+ t[(vo),{(- )-tart},14- e [(VO),{( + )-tart}{( - )-tart}14-
(44)
(ii) Other hydroxycarboxylate ligands Hydroxycarboxylate systems have been extensively studied. Solution spectra of most V0 2 + complexes contain, at most, three bands in the visible region that may be assigned to d-d transitions; however, alkaline solutions of most a-hydroxycarboxylates exhibit four bands365 ascribed to a reduced symmetry of the ligand field by tram coordination of two strongly bound ionized hydroxyl oxygen^^^' (perhaps accompanied by distortion towards a trigonal bipyramidal geometry).630Table 31 summarizes part of the reported spectra: in several cases, the four bands suggest that a-hydroxycarboxylato complexes have trans coordination. I n Na(Et4N)[VO(benzilato)z].2PrOH,vanadium is five-coordinate and distorted square pyramidal, and the bidentate ligands are mutually trans.@' The solution, single crystal and polycrystalline optical spectra exhibit four bands similar to those of the trans-vanadyl-( + )tartrate system (Table 31). Table 31 Spectral Bands Observed in Optical Spectra of Oxovanadium(1V) Hydroxycarboxylates in Aqueous Solution
(f)-Tartrate (+)-Tartrate Benzilate Mandelate Lactate Malate Citrate
-8 -8 8.5 8.5 >7 >7 >9
a
737 (25.4) 902 (22.6) 847 (31) 826 813 815 775 1052
536 (25.4) 590 (22.4) 533 (27.7) 599 (26) 541 (27) 599 535 599 529 608 530 67 1 555 649 571
421 (52.9) 399 (46.4) 417 (36) 413 407 410
1. Collected in R.M. Holland and R. E. TapScott, J . Coord. Chem., 1981,11,17. 2. E. M. Nikolova and G.S . Nikolov, J . Inorg. N w l . Chem., 1967, 29, 1013.
Complexes with (R)-lactate have been studied by ESRb31and CD.5807632 Lactate is near water in the spectrochemical series and its low pK, helps reduce error due to hydrolysis and oxidation. Table 32 summarizes the data. The pK, of the R-OH is quite high (especially for the aliphatic acids), Many have considered coordination in V02+ hydroxycarboxylate to involve the CO; and OH groups and assumed the dissociation of the OH. As structures (55) and (56) or (57) and (58) are not pH-metrically distinguishable, if the pKFH is not known, other techniques must establish the species present.
(55)
(57)
From ESR, for (R)-lactic acid (HJact), five species were required: [VO(Hlact)]+, [ V 0 ( H l a ~ t ) ~ [VO(lact)], ]~, [VO(lact)(Hlact)]- and [ V O ( l a ~ t ) ~ ] ~ for - ; glycolic acid (Hglyc): [VO(glyc)] and [ V O ( g l y ~ ) ~ ] ~Another - . ~ ~ ' ESR study assumed only [VO(g l y ~)~]~formation.With salicylic acid (H,SA) and 5-sulfosalicylic acid (H,SSA), titrations have been interpreted in terms of coordination with COzH and OH ionized, although the pK, values of the OH groups are high (pKFH=11.4 for HSSA- and Z13 for HSA).633ESR spectra of solutions containing salicylic acid (H2SA) at p H S 5 were interpreted in terms of V02+, [VO(SA)] and [VO(SA),]'- species.634A different model involves V02+, [VO(HSA)'] and
521
I I I I I I I I I I I Il;la
I I I1
a
s
522
Vanadium
[VO(SA)] (see Table 32).635Early reports include (Ct)2[VO(€A)z].3Hz0 (Ct = K', NH: and ,Ca") and T1z[VO(SA)z~.3~"'b DMF solutions of chelates of l-hydroxycyclohexanecarboxylateand mandelate exhibit the half field ESR at -1500 G due to Ah4 = 2 ( g = 4),358as does aqueous V02+ citrate at pH 6-9, indicating V - V interactions.636 [(VO),(citrat0)~].4H~O has pkk = 1.51 BM, like VO(trihydroxyglutarato)~0.5Hz0(p:& = 1.57 BM).637Dimeric 2 :2 complexes with citrate and malate explain proton relaxation in the pH range 3-10;638 comparative dialysis and spectrophotometry at pH 2-7 suggested the existence of 1:3, 1:2, 1:1 and polymeric 1:1 species.639Early reports include (Ct)4[(VO),(citrato)z].nH20 (Ct = Na+, Kf, NHt with n = 12, 6 and 0 respectively) .357h (NH4)Z[VO(benzilato)z],(NH4)2[VO(fsa)(OH),H20] and (NH&[VO(fsa),].2Hz0 (fsa = 3formylsalicylato) have pkh 1.67-1.73 3M,a0 as well as [VO(3,5-dinitr0)SA)~]-2H~O."~ Some of these have high antimicrobial activity.640 The effect of exchange of lactic, mandelic and sulfosalicylic acids on the relaxation of solvent protons gave rate constants (k) of exchange from 1.73 to 0.701 mol-' s-l.@' Kinetics of complex formation with mandelic (HMDA) and vanillomandelic acids (HVMDA) gave rate constants (1.09 x lo3 and 1.13 x lb rno1-l s-l for MDA- and VMDA-) consistent with a dissociative (Eigen) mechanism.438 As in the case of oxalic and malonic acids (Section 33.5.5.5.ii; Table 27), species with coordinated hydroxyl are labilized.
33.5.5.8 Other oxygen ligunrls
(i) Sulfoxides Complexes with DMSO as one of several ligands have been re orted frequently and early ones include [VO(X)2(DMS0)3] where X = $SO$-, C1-, Br-38.p,643 and [VO(L),(DMSO)] where L is a bidentate ligand (e.g. a ~ a c )[VOBr2(DPS0)3] .~~~ (DPSO = diphenyl sulfoxide) and [VO(DMSO)5]Br2 were prepared by the reaction of VOBr3 with the sulfoxides in c y c l ~ h e x e n e .[VOCl,(DMSO),] ~~~ and [VOC12(DPSO)3] were prepared by the method in equation (16);409two V - C l IR-active bands were observed for both compounds (353,321 cm-' and 312, 282 cm-l, respectively), suggesting a trigonal bipyramidal (13) structure. For adducts of VOX2 (X = C1, &SO$-)with aromatic and aliphatic sulfoxides, the sulfoxide groups are coordinated through the oxygen.644 In [VO(DMS0),](ClO4),, d(V=O) = 1.591, d(VO(equatoria1)) = 2.034 (av) and d(V-O(axia1)) = 2.18 A.3R7,h45 'Adducts' of VOBr2 and V012 with DMSO and DMF have been isolated; in [VO(DMS0)5]12the distances are slightly shorter (ca. 0.01 A) than in [VO(DMS0)5](C104)2.646
(ii) N-Oxides and related ligands [VO(C~H~NO)S](C~O [VOC12(Ph3P0)2], ~)~, [VO(ph3pO)~l, [VO(Ph&0)4](C104)2 and [VOCl,(DMSO)2] have been prepared by the reaction of VC13.6H20 with the ligand in ethan01."~ Perchlorates are obtained if LiC104 is added. In [VOC12(Ph3P0)2],vanadium is pentacoordinate with the PhJPO ligands trans; their 0 atoms and two C1 atoms comprise the base of a square pyramid completed by an apical multiply bonded 0 atom.647Internuclear distances are as follows: d(V=O) = 1.584, d(V-0) = 1.986 and 2.002, and d(V-C1) = 2.312 and 2.304 A. This and [VOC12(hmpa)2](hmpa = hexamethylphosphoramide) have also been prepared by equation (16) and have only one IR-active V-Cl band each at 384 and 385 cm-l. Seddon's suggestion would imply square pyramidal structure^.^^ However, the structure determined by X-ray is based on a distorted trigonal bipyramid."8 In [VOClz(dimp)2] (dimp = PriMePO), v(V-Cl) is at 431 From the single crystal ESR of [VOC12(Ph3P0)z], no magnetic exchange interactions exist; the s ectrum consists of eight lines from interaction of the electron with the nuclear spin of 51V (1 = .650 In poly(dimethylsi1oxane) mulls of [VOC12(Ph3P0)2]and [VOCl,(hmpa),], optical bands were assigned according to a crystal field orbital splitting different from that of Ballhausen and Gray (Section 33.5.1, Figure 16).3b4
5
Vanadium
523
(iii) Miscellaneous oxygen ligands Complexes with N,N,N’,N’-tetramethylurea (tmu) have been extensively studied. In [VOC12(tmu)2],the vanadium is pentacoordinate and square pyramidal (see Figure 28) .651 The single crystal ESR of [VOX2(tmu)2] (X = Br, Cl) gave unusual fine structure at 170-120 K, which was interpreted in terms of an electronic exchange interaction, within a two-dimensional (pftc =gerfluoropinacolate) and the deoxygenated [VCl,(pftc),] layer.614ESR for [ VO(pft~)~] obtained by its reaction with SOC122 suggests a trigonal prismatic structure for [VClZ(pftc)2]. The reaction between VC13.3THF and three equivalents of 2-hydroxy-6-methylpyridine (Hmhp; 59) in CHzC12 in air produces bright blue [Vz02C1,(p-Hmhp),] (60)characterized by crystallography, ESR (g = 1.976) and lR,6sz in the region 1000 k 50 cm-’. The structure consists of discrete dinuclear units.
Figure 28 Molecular structure of [VOCl,(tetramcthylurea),j. Some intcrnuclear distances (in /i) are indicated’“
Several complexes which contain halogen ligands (with and without the vanadyl bond) with oxygen donors such as diethyl ether, MeOCH2CW20Me,benzaldehyde, benzophenone, THF, dioxane and pyridine N-oxide derivatives are included in ‘Gmelins Handb~ch’.”’~[VOF2Ln] complexes428 (equation 22) include [VOF&] with L = urea and formamide (fmd) and [VOF2(H20)L]with L = DMF and dioxane.
33.5.6
Sulfur Ligands
33.5.6.1 ThiovanadyI complexes The first complexes of the thiovanadyl ion, VS2+, were made by the reaction of [VO(salen)] or [VO(acen)] (H2salen = N,N‘-ethylenebis(salicylideneamine), H2acen = N,N‘ethylenebis(acetylacetony1ideneamine)) with the powerful oxophile B2S3 in CH2C12 with rigorous exclusion of oxygen and moisture (equations 45 and 46).6s3~654 The magenta [VS(salen)] is relatively stable to oxidation or hydrolysis and shows no colour change in air for months, although it then smells of H2S and IR shows the presence of V=O. In sharp contrast, on swirling in air a solution in CH2CI2, it quickly changed to green, forming [VO(salen)] quantitatively with the liberation of H2S.6s4The magenta [VS(acen)] is also relatively stable but -in contrast to [VS(salen)] -solutions in organic solvents are decomposed to IVO(acen)] only after a few hours of exposure to the atmosphere. IR of (61) and (62)showed no V=O stretch, and new medium-to-strong bands at 543 (61)and 556cm-’ (62), were assigned to v(V=S). (61) and (62) exhibit eight-line isotropic solution ESR; g and A decrease in magnitude on substitution of oxygen by sulfur, consistent with greater covalency in the VS bond.653[VS(acen)] has square pyramidal geometry with the sulfur atom at the apex and the coordinated atoms of acen comprising the basal V=S was 2.061 A, shorter than V-S single bonds (Section 33.5.6.2 and 33.5.6.3).The vanadyl analogue (Figure 32, p. 536) also has
Vanadium
524
square pyramidal geometry around the V atom; the difference between V=S and V=O is 0.47 A. VO(sa1en) + B2S3
CHzClz
hexane
VS(salen)
(45)
(61)
By allowing a slurry of (PPh)Na[VO(edt),].2EtOH (edt2- = ethane-132-dithiolate)in MeCN to react with (Me3Si)zS, orange-brown crystals were obtained.656They contain [VS(edt)2]2-, whose structure is essentially identical with [VO(edt)2]2- (see Section 33.5.6.3). The difference between V=O and V=S is 0.462 A. In (NMe4)Na[V0(edt),].2EtOH, v(V=O) was at 930 cm-’ and the V=O distance (1.625 A) is rather long but is similar to that in [Vo(~dt)~]’-(pdt = propane-1,3-dithiolate) (1.628A).656 On O/S substitution, the band at 930 cm- disappears and a strong, sharp band at 502 cm-’ appears, v(V=S). This change (428 cm-l) is similar to that between VE(acen) and VE(sa1en) (E = 0,S), paralleling changes in V=E bond length. Electrochemical oxidations of [VS(acen)] and [VO(acen)] are reversible at 0.45 and 0.66 V us. SCE, with one electron In contrast to [VO(acen)]’, which is quite stable, the [VS(acen)]+ product readily reacts with residual water and is converted to [VO(acen)]+ and H2S. Vanadyl complexes with O/N-based ligands have oxidation potentials in the range 0.6- 1 1V The potential for [VO(edt)$- is more negative by -1V.656 On OlS substitution, the potential becomes even more negative by 0.27 V, behaviour also present in VE(acen) (E = 0, SI (-0.26V). Again the basal ligands govern the absolute magnitude of a property whereas changes in O/S substitution are essentially constant.656 By treatment with (Me3S$S the conversion of [VO(edt)2]z- to [VS(edt)z]2- gave an intermediate [V(OSiMe3)(edt)2]- as the PPht salt.658It contains a discrete, square pyramidal angle of 140.8’. VN with d(V-0) = 1.761 A, mean d(V-S) = 2.322 A and a V-&Si Changes in ESR and electronic absorption from [VO(edt 4’- to [VS(edt)z]2- are similar tc those in the series [VE(acen>l and [VE(salen)] (E = 0, S).63, The spectroscopic results for thc [VE(edt)z]z- complexes have been interpreted assuming a ~ , Z - ~ Z ground
. .
33.5.6.2 CS2-based and related ligands
(i) Dithiocarboxylates The interesting reaction of oxovanadium(1V) with dithiocarboxylates RCS; cleaves tht vanadyl bond, in water or ethanol, with formation of stable [V(RCSS),]. When pure, they art not air-sensitive.%’ They are insoluble in polar solvents and generally soluble in nonpolar ones however, the solutions decompose rapidly. Various stereochemistries are possible bu molecular weights, IR, electronic and ESR spectra are consistent with eight-coordinatr geometry,661-664with d&z lowest-lying (Dzd symmetry). Several [V(dithiocarboxylato)4 complexes have vanadium coordinated to eight sulfur atoms with a dodecahedral geo r n e t ~ y . ~ ~The , ~V-S ’ , ~ distances are of two kinds and for [V(MeCS,),], nonequivalent atom have (i) a regular or (ii) a distorted dodecahedral configuration. Some data are in Table 33.
( i i ) Dithiocarbamates
-
Dithiocarbamate (dtc) complexes are prepared by mixing VOSO, and the ligand ir water-ethanol solutions. Some of the isolated complexes are in Table 34; they are susceptiblt to air oxidation, especially in solution, and are generally monomeric. [VO(Et2NCS2)2] has discrete molecules with five-coordinate square pyramids, with thr oxygen at the apex with d(V=O) = 1.59 The vanadium atom lies approximately 0.75 h above the ‘plane’ of the four sulfur atoms and the average V - S distance is 2.402& shorte than for most vanadium dithiolate c~mplexes;~’ IR, electronic and ESR spectra are consisten
Vanadium
525
Table 33 Some Properties and X-Ray Data of [V(dithiocarboxylato),1 Complexes
V--S aueruge internuclear
1 2 3 4 5
,4$
R in [V(RCW,l
Colour
(BM)"
Me Ph PhCH, p-MeC,H, C,H,N"
Red-brown Red Red Red Red-brown
1.74b 1.70-1.79 1.71 1.69- 1.75
dirtances V S A
(A)
2.50 2.53 2.56 -
-
Y(V+
v-&i
(cm-')
Ref.
-
1-2 3,4 3
460
5
2.46 2.47 2.45
-
-
1
"The magnetic behaviour of compounds 1-4 in the temperature range 77-300K was studied (ref. Z), but the authors considered that ferromagnetic impurities, undetectable by chemical analysis, are the otigin of part of the anomalous behaviour of some of the complexes. Determined by the Faraday method. Hacda, 2-amino-1-cyclopentenedithiocarboxyfate. 1. 0.Piovesana and C. Furlani, Chem. Commun., 1971,256. 2. 0.Piovesana and G. Cappuccilki, Inorg. Chem., 1972,11, 1543. 3. N. Bonarnico, G. Dessy, V. Fares and L. Scararnuzza, J. Chem. SOC., Dalton Trans., 1974, 1258. 4. M.Bonamico, G. Dessy, V. Fares, P. Porta and L. Scaramuzza, Chem. Commun., 1971, 365. 5 . R.D. Bereman and J. R. Dorfman, Polyhedron, 1983,2,1013.
Table 34 Oxovanadium(1V) Dithiocarbamate Complexes [VO(dtc),] ..
R in RCSS-, or R ' , R" in R'R"NCSS
Me Et
1 2
3
v(V=O) (cm-')
Ref.
982 984
1.69 1.73
992
1.74
4
Pr'
995
1.77
5
0-
985
[1.56]
985
-
A
945
[1.751
A
965
-
6
A
OWN-
7 8
9 10 11 12 13
MeN
N-
u Et, Ph
970
Me, CY CY, CY
14
-
-
. -
968 940 942
1.70- 1.72 1.70-1.72
-
-985
-
-985
-
-985
-
I 15
I 16
I 1. B. J. McCormick, Inorg. Nucl. Chem. Leit., 1967,3, 293. 2. B. J. McCormick, Inorg. Chem., 19a,7 , 1965. 3. G.Vigee and 5. Selbin, J. Inorg. Nucl. Chem., 1%9,31, 3187. 4. D. C. Bradley, I. F.Rendall and K. D. Sales, J . Chem. Soc.. Dalton Trans., 1973,2228. 5 . R.D . %ereman and D. Nalewajek, J. Inorg. N u l . ckem., 1978,40,1313. 6. J. Selbin, Cmrd. Chem., Proc. John C . Bailar, Jr., Symp., 1969,248. 7. F.Forghieri, G. Graziosi, C. Petri and G. Tosi, Transitton Mer. Chem. (Weinheim, Ger.), 1983, 8, 372.
Vanadium
526
with a square pyramidal structure for other [ V o ( d t ~ ) ~complexes. ] The structures are undoubtedly similar to [VO(a~ac)~] (Figure 22) and so it is not surprising that adducts form in s o l ~ t i o n ~and ~ * in , ~the ~ ~solid state.670 The tendency of [VO(dtc)z] to bind a sixth ligand is lower than for [VO(a~ac)~] and no correlation between v(V=O) and the ligands seems to exist.669The studies are more difficult than with [VO(acac)2] owing to the sensitivity to oxygen. Although some [V(dtc),] complexes have been isolated: oxovanadium(1V) compounds are usually obtained by the direct reaction of oxovanadium(1V) salts with dtc bases. Several [V(dtc),] complexes have also been prepared by insertion of CS2 into V-N bonds of vanadium tetraamides (equation 47) .419,672 V(PdR&
+ 4Csz
cyclohexane b
(47)
(oxygcn-free conditions)
Table 35 shows some of the [V(dtc),] complexes prepared. Compounds 2-7 in Table 35 were prepared from VOSo, and in some cases in the presence of water. This therefore is one more example where the vanadyl bond is broken; others include SOCl, (and SOBrz), catechols and dithiocarboxylates (these forming eight-coordinate complexes). However, [V(dtc),] complexes are not stable towards conversion back to vanadyl. Thus, these ligands seem to be intermediate in behaviour between dithiocarboxylates and other dithiocarbamates; this probably reflects n-bonding capabilities.671 ESR spectra of compounds 4-7 (Table 35) show well-resolved hyperfine structure and the rather low A,, and A, values (-117 and -37 X lo-, cm-') indicate both a significantly covalent system and an eight-coordinate geometry.671The ESR spectra of [VO(dtc),] complexes with the same ligands show much higher A,, and A l values (-151 and -49 X lop4 In addition to the reaction of [V(Et,NCS,>,] with oxygen producing vanadium(V) (equation 48), two other reactions may occur upon heating (equations 49 and 50) .672 2V(Et,NC&),
+ O2
-
2VQ(Et,NCS,), -t(EtNCS,),
(48)
+
V(Et,NCS,),
(49)
V(Et,NCS2)3 $(Et2NCS2)2
VO(Et2NCS2)3
5 VO(Et2NCS,), + i(EtzNCS2)2
(50)
Table 35 Some [V(dithiocarbamato),] Complexes
R in RCSS 1 2 3 4
5
6
CN-
Brown
1.72
'
CHCI,/C,H,/H,O
1 233 4
-
Red-brown
1.68
1.973,a 1.976'
Red-brown
-
MeCN/H,O
5
1.979", 1.976b
Red-brown
-
MeCN/H,O
5
1.976,', 1.979"
Red-brown
-
MeCN/H,O
5
Red-brown
-
MeCN/H,O
5
1.975"
a < 07; / ,
7
-
Me,N Et,N (PhCHAZN
ca,
N'
I
-.N,,'
0-0
1.975,a1.980b
\
l a In
1. 2. 3. 4. 5.
toluene solution.
In CH,Cl, solution.
CS, insertion reaction into [V(NMe).,].
E. C. Alyea and D. C. Bradley, J . Chem. SOC. ( A ) , 1969, 2330. D. C. Bradley, I. F. Rendell and K. D. Sales, J . Chern. SOC.,Dalton Tmnr., 1973,2228. M. B. Dehkordy, B. Crociani, M. Nicolii and R.L. Richards, 1. Orgunornet. Chem., 1979, 181, 69. G. Soundararajan and M. Subbaiyan, Indian I . Chem., Sect. A , 1983,22,454. R. D.Bereman and D. Nalewajek, 3. Inorg. Nucl. Chem., 1978,40,1309.
Treatment of tris(tricaprylylmethylammonium)tetrathiovanad~te(V) solution with solid tetraisobutylthiuram causes an immediate change from violet to brown. Yellow-brown crystals
Vanadium
527
were obtained and characterization showed that a dimer (63) formed via induced internal redox (equation 51).674
(63) consists of discrete dimers where the V atoms are bridged by two symmetry-related p-q2-Szligands, forming an Mz(p-q2-S2), core, the octahedral coordination of vanadium being
completed by two bidentate dithiocarbamate^.^^^ d(V-S) = 2.405 (av) and d(V-V) = 2.851 A;674the V-V distance and the observed diamagnetism are consistent with a V-V bond. (iii) Dithiophosphae and related ligands
ESR spectra of complexes of dithiophosphate (dtpo, P(OR),SS-), dithiophosphinate (dtpi, PR2SS-), PR‘(0R)SS- and dithioarsinate are of particular interest since they exhibit appreciable phosphorus31 superhyperfine splitting which arises from V-P interactions over distances of about 3 8, or more. For example, in solutions containing (a), phosphorus causes the vanadium hyperfine lines to be further split into three components, intensitities 1:2 :1. A total of 24 lines are predicted; however, owing to overlap, only 17 were o b ~ e r v e d . ~With ~~’~’~ the cyclohexanol (R’O) ethyl (R) derivative, 23 of the expected lines are resolved whereas only 17 are for the corresponding cyclohexanol phenyl complex (65).677 The superhyperfine splittings have been attributed to direct transannular V-P interactions involving mainly the 3dX2-,,2vanadium orbital and 3s (and 3p) phosphorus orbital^.^",^^^ Wasson and co-worker~~~’ studied [VO(S2PR(OR’)}2] and discussed [VO(dtpo)2] and [VO(dtpi)2].678,679The general trend of the phosphorus superh erfine splitting constants is S2P(OR)2> (S2PR(0Rf)> SzPR2. The ESR of [VO(S2PCy2)2] [VO(dtc),], [VO(dtpi)z] and [VO(dtc)(dtpo)] have been reported,680.681as has coordination of py, DMF and hexamethylphosphoramide to fivecoordinate [VO(S2PMe2)], [VO(S2PPh2)]and [VO{S2P(OEt),>j,682 using hyperfine interactions between 51V and 31P.
:’‘
Most solid compounds (see Table 36) were prepared with VOSO, as starting material (VOC13 for the fluoro complex) and can be kept indefinitely under nitrogen or vacuum. The ethoxy, fluoro and trifluoromethyl complexes are rapidly decomposed by air. [VO(S2PMe2)2] consists of monomeric square pyramidal chelate complexes. 679 However, the clearly different properties of the F- and Me-substituted complexes, with anomalously high p:; (-2.0 BM) and v(V=O) at 860-870 cm-l, suggest V - - - V interactions. In solution, the 860-870 crn-l bands disappear and && are close to 1.75 BM, indicating absence of magnetic exchange. Tfie inverse dependence of susceptibility on field strength (H), and the slight increase of pee with decrease in temperature suggest ferromagnetic interaction.358However, molecular weights suggest a monomeric nature and no half-field ESR spectrum was observed. The compound [ V O ( d t a ~ ) ~pre ] , ~ared ~ ~ by the reaction of V0C12 with dimethyldithioarsinate (dtas), has v(V=O)=986cmand the electronic spectrum is similar to those for [VO(dtc),] and [VO(dtpi),j complexes. Its isotropic ESR spectrum shows at least 27 discernible lines, from interaction with at least one 75Asatom. [VO(dpd),f (Hdpd = (EtO),P(O)NHCSSH) has been r e p ~ r t e d . ~ ,
P
(iv) Xanthates
Xanthato compounds have been prepared by the addition of aqueous VOSO, to a solution of the ligand at 0-5 0C;a5 green products precipitate and were recrystallized from CH2C12,CHC13 or n-hexane (or mixtures). Some properties are in Table 37. They were studied by electronic
528
Vanadium Table 36 Some IVO(dithiophosphinate),j and [VO(dithIophosphate),1Complexes
R in [V~(S2PR,),I
(BM)"
B(K) from Curie-Weiss lawa
Et0 Me Ph F CF3
1.66 1.72 1.76 2.25 2.13
0.7 -1.5 -5.1 17 10
P:;
-
CY
Y (V=O)h
-
(cm-')
Ref.
960 s 991 s 998 s 1025 w, br, 860 s 1020 w, br, 870 ms YIV-S) = 325 (?)
1 1 1 1 1 2
With no TIP corrections. v(V=O) is indicated unless otherwise stated. R. G. Cavell, E. D. Day, W.Byers and P. M. Watkins, Inorg. Chern.. 1972. 11, 1591. 2. H. J. Stoklosa, G. L. Seebach and J. R. Wasson, J . Phys. Chem., 1974, 78, 962. 1.
spectroscopy, thermogravimetry and differential thermogravimetry.68s [VO(PriOCSS)2] with pyridine and piperidine produces the last compounds in Table 37, which are diamagnetic and have no visible band. These vanadium(V) compounds result from oxidation during the long reflux times used to prepare them. In fact, in most reactions with long reflux times, oxidation can be avoided on1 by taking extreme care to avoid O2 participation, or in the presence of reducing agents. 6 8 6 , 2 7 Table 37 Some [VO(xanthate),] Compounds v( v - 0 )
Complex [VO(EtOCSS),] [VO(Pr'OCSS)J [VO(Bu'OCSS),] [VO(i-C,H,,OCSS)-J [V0(GH70CSS),l ~ ~ ~ ~ ~ ~ ~ ~ ~ [VO(Pr'OCSS)(pip)J-5H20
P:;
v(T.'--s)
(cm-l) 1020,990 sh 1010,980 sh 970,930 sh 1030,970 sh 980 > ~ 950 P Y ~
lo00
(BM)
(cm-')
2
370 385 370 390 340 l 380 s H 380
,
1.62 1.68 1.66 1.67 1.63 ~Diamagnetic Diamagnetic
pip = piperidine. A. D. ViUarejo, A. Doadrio, R. Lozano and V, Ragel, An. Quim., 1981, n,50
33.5.6.3 Dithiolates By mixing stoichiometric amounts of VOS04.5H20 and NaS2CCN.3DMF at 20-50 "C for -1.5 h an olive green maleonitrile dithiolate (mnt) (Ph,PMe),[VO(mnt),] was obtained.688 If heated for 3-5 h at about 80 "C, deoxygenation occurs and [V(mnt)3]2- forms. The vanadyl complex is monomeric and v(V=O) = 963 cm-l. In M(l,2-dithi0late)~,the more oxidized species usually have a trigonal prismatic coordination, whereas highly reduced species possess near octahedral ~ t r u c t u r e s .A~ ~detailed ESR study of magnetically dilute oriented crystals (with diamagnetic hosts, (PhP)2[M~(mnt)3] for [V(~n n t } ~]~and (MePPh3)2[TiO(Cz04)2]for [VO(mnt)2]2-) led for the vanadyl complex to a scheme d,z+ < d,,, dyr < dry< 4 2 (~d;2-~2 ground state).689Kwik and Stiefe149also studied the ESR spectrum of [V(mnt)3]2- doped in crystals of (Ph&)2[Mo(mnt)3]. While the experimental results agree, the interpretations differ. A third assignment differs again.@' (Ct>2[V(mnt)3] (Ct = Me&+, Et@+, Bu4N+, Ph4N+, Ph3PMe+, Ph4As+) have been prepared from VOC13,52,588,691and in (Me4N),[V mnt)3] the six sulfur atoms are around the vanadium at an average distance of 2.36 f0.01 (Figure 29a), in distorted D3hsymmetry.52 Reaction of [VO(a~ac)~] ethanolic solutions containing an excess of sodium ethane-1,Zdithiolate (Na2edt) under inert atmosphere yields deep green solutions. NMe4Cl slowly deposits the vanadium is square green prisms of (NMe4)Na[VO(edt),]-2EtOH (Figure 29b);656,692 pyramidal with d(V=O) = 1.625 A and d(V-S) = 2.378 8, (av). The V atom lies 0.668 8, above the sulfurs. (NMe4)Na[VO(edt)2]in DMSO exhibits rather similar g,, and gl (1.981 and 1.984) and relatively small values of A,, and A, (133 and 42 x cm-') when compared with [ V O ( a ~ a c )for ~ l example. This suggests significant z interactions between the sulfur lone pairs __ and .the . metal d orbitals.s92 [VO(pdi)2]2- (pdt = propane-1,3-dithiolate) is similar to [VO(edt)2]2- .656
d
529
Vanadium
(a ) Figure 29 Molecular structure of (a) the [V(mnt)]$- anion5' (the g a n o groups are not inciuded) and (b) the [VO(SCH,CH,S):-] anion
33.564 Other S-concaining ligands
Early publications include the vapor-phase reaction of VC14 with thiophene to yield VC14C4H4S.693Reduction of VOC13 by sulfides has long been known and fairly pure [VOCl,b] (L = MezS and Et2S) complexes have been obtained.3w [VOCl,(tmtu),] (tmtu = tetramethylthiourea) had v(V=O) = 990 cm-1.369 In W F 2 ( 9 2 1 (tu = thiourea), prepared by the method in equation (22), the IR indicates a V-S bond in contrast with [VO(NCS)2b] (L = tu or N-substituted thioureas), with nitrogen bonding (see also Table 12). 33.5.7
Selenium Ligands
Early reports include 1: 1 [VC14] adducts with MeSe(CH&SeMe and Pr'SeMe, purple-grey decomposing at 114 and 130 0C.694 From iterative extended Huckel molecular orbital (EHMO) calculations, [VX]"' (X= S, Se; n = 0, 1,2) ions and their tetrachloro complexes [VXCl4Im- (rn = 4,3,2) decrease in stability O > S > S e and with increasing charge of the anion.695 The [VXCl4I2- and [VXCLI4complexes are expected to be S = $ systems while [VXC14]3- species are expected to be diamagnetic.695
33.58 Halogen Ligands Only complexes [VOXn]2-nand [VXm]4-mwill be included. Gmelins Handbuch discusses work in this area up to 1967.357a 33.5.8.1 Oxovanadium(1V) Complexes
Stability of the V02+ complexes will decrease F- >> C1- > Br- > I-, with complexes formed by the three heavier halides being weak. Fluoride and chloride have been studied in aqueous For fluoride, the following stability canstants have been determined at 20 "C and Z = 1 M NaC104): PI= 2.0 x lo3, 8, = 3.7 x lo5, B3 = 1.3 x lo7 and P4= 6 x lo7, 98 much stronger than chloride (& = 1M-'). The fluoride Complexes are more stable than those of Cu*+, Zn2+ and Ni2+ but are weaker than the UOp+ complexes.697VOF, is known as well as some hydrated forms V0F2.xH2O Fluoro complexes have been mainly studied with alkali and ammonium ions356and stoichiometries [VOF3J-, [V0F4]'-, [VOF5I3-, [V2U2F7I3and [v2o2F6(H20)~]have been r e p ~ r t e d . ~ ~The . ' ~ basic structural units are [V0F5l3octahedra, found isolated in (Ct)3[VOF5]701,7m and [MJNH3)6][VOF5],6wconnected in infinite chains linked by cis-bridgin F atoms in K2[V0F4l7 and (NH4)2[VOF4]704 or to complex chains in (CS[VOF~]}Z~&O!~~ where two VOF5 actahedra are linked via an F-F edge; the V-F distances s an the range 1.881-2.205 %, and V=O (1.583 and 1.595 A) are shorter than in [VOF&€zO)]'- (d(V=O) = 1.602 and d(V-O(tram)) = 2.268 In Cs3[V202F7], the anion consists of two octahedra sharing a face.7mIn (NMe4)2[V20zF6(H20)2], dimeric units of [V202F6(H20)~]2-are linked in chains by short hydrogen bonds, V-F distances ranging from 1.894 to 2.173 d(V-V) = 3.292 and d(V-OH,) = 2.074 A. Blue crystalline
5
.3557428
Vanadium
530
(N2H5)[VOF3]has been synthesized from V205 and an excess of hydrazine hydrate in aqueous HF.m The hydrazine hydrate not only acts as a reducing agent but also provides the N2Hf cation. The Ct(Na+, K+ and NH;) salts were prepared by metathesis between (N2H5)[VOF3] and CtF. Their &.& are in the range of 1.51-1.53 BM.700Spin-Hamiltonian parameters are in Table 38. Table 38 Spin-HamiltonianParameters for Vanadyl Complexes All
(x
Complex 1.993 1.937 1.932 1.937 1.948 1,945 1.959
1.981 1.978 1.973 1.977 1.979 1.985 1.984
A,
cm-')
182.8 177.2 182.0 178.5 168.8 173.O 172.0
Ref.
72 0
1
68.5 66.7 64.0
2
62.8 63.8
5
70.7
3 4
6 7
Aqueous solutions of (N2H,)[VOF,] at 100 K. [VOFJ- in single crystals of (NH,),SbF,. [VOF,] - in single crystals of (NH,),AlF,. [VOCI,],': in single crystals of $NH4),SbC1,. [VOCI,] (or [VO(Cl,)(H20)] - ?) in single crystals of (NH4)2[ln(Q)(H,0)]. Ethanenitrilesolution of (Ph,As),[VOCl,] at 138 K. 1. C. J . Ballhausen and €I. B. Gray, Inorg. Chern., 1Y62, 1, 111. 2. M. K. Chandhuri, 5 . K.Ghosh and J. Subramanian,Inorg. C k m . , 1984,23, 4439. 3. K. K . Sunil and M. T. Rogcrs, lnorg. Chem., 1981, 20, 3283. 4. P. T. Manoharan and M. T. Rogers, J . Chem. Phys., 1968,49, 3912. 5 . J. M. Flowers, J. C. Ilempel, W. E. Hatfield and D. H. Dearman, J . Chem. Phys., 1973,58, 1479. 6 . K. DeArmond, B. B. Garrett and H. S. Gutowsky, J . Chern. Phys., 1%5,42, 1019. 7. K. R. Seddon. Report 1980, EOARD-TR-80-13(from Gov. Rep. Announce. Index (U.S.), 1980, 80(26), 5628;Chern. Absfr., 1981, 94, 112126t).
a
Low v(V=O) values are expected for fluoride complexes. In fact, in (NMe4)z[Vz02F6(H20)2] the v(V-0) was at 968cm-' (980cm-I Raman); in (NH&[VOF5] it was at 937 and 947 in Ni[VOF4].7H20 at 950 ~ m - ' , ' in ~ ~(Ct)[VOF3] (Ct = N,H:, NH:, Na+, Kf)at 9 7 0 - 9 8 0 ~ r n - ~ , while '~ in (M(NH3)6)[VOFs] (M=Cr, Co or Rh) v(V=O) was at 907913 ~ 3 n - l v(V=O) . ~ ~ ~ in hexamminemetal(II1) pentafluorooxovanadates(1V) reflected multiple hydrogen bonds.699 Simple (Ct),[VOCl,] are green (hydrates may have other colours) and are extremely air and moisture sensitive, due to the ease with which V-C1 bonds are hydrolyzed. In many (Ct),[VOCl,], [V0Cl4l2- probably has a square pyramidal structure as they present only one strong IR-active band ( v ( V 4 1 ) = 340-350 (Sections 33.5.3.1 and 33.5.3.2) and the ESR spectra of [VOC14]2- and [VOCl,f- have been interpreted according to this (Table 38). (Ct)[VOC13] have also been reported.71 Oxobromovanadates(1V) were first characterized in 1973, of the tyges (Ct)[VOBr3], (Ct)2[VOBr4] and (Ct)3[VOBr5] as well as several [VOBr2] adducts.400,O1 Most [VOBr2j adducts were included in a special report .711 Normally, the oxobromovanadates(1V) are' prepared by reduction of VOBr3 by bromides in acetonitrile and/or nitromethane. When Vv is reduced by Br-, free bromine is liberated and unless conditions are carefully regulated, perbromides form, giving re arative difficulties.") Generally, v(V-F) have been assigned in the region 460-520 cm-1,6 ' 7oR v(V-Cl) bands in the region 340-400 cm-' 409 and v(V-Br) in the region 320-360 cmL1.400'711 The adducts reveal the trend for v(V=O): anionic >fivecoordinate > six-coordinate > bidentate.'ll The far IR of (NEt4)2[VOBr4]shows a complete and reversible transformation within the temperature range -30 to 35"C, due to a reversible equilibrium (equation 52) between two forms of [VOBr4I2-, a C4" form being stable at room temperature with v(V-Br) at 296cm-l, a C2,, form being stable at lower temperatures with two v(V-Br) bands at 274 and 325 ~ m - ~ .A~ similar " transformation has been suggested for [VOF4]- '13 as well as for [VOCl$ (Section 33.5.3.2).
8,x
CZL, (-30 "C)
c,
(35 "C)
531
Vanadium
Anhydrous VOIz has not yet been prepared and most reports of iodides of VIv are not firmly based. I- can reduce vanadium(1V).
33.5.8.2 Vanadium(1V) complexes Potassium hexafluorovanadate(1V) can be prepared from [VF4] and KF in selenium tetrafl~oride(II1)~'~ or by fluorination of K z W 5 for example. K+, Rb+ and Csf salts obey the Curie-Weiss law with high values of 8. [VF4]reacts with py and NH3 forming [VF4L], probably fluorine-bridged p01ymers.~'~ Chloro complexes of VIv have been reported ([VCl$, [VC16]2-, [VC17]3- and [VCls]4-),714 but the last two at least need to be confirmed. In the reaction of PCIS with VOC& in hot POC13 or CHzCl2 and of [V0Clz] with PC& in AsC13 or P0Cl3, (PC1,+)[VClS]-may be precipitatedq714 The anion is trigonal bipyramidal and exhibits weak, presumably d-d transitions at 6200, 8100 and 16 000 cm-'.715 By the reaction of [VC14]or its ethyl cyanide adduct with alkylammonium chloride, (Ct),[VCl,] (Ct+ = Et3NH+ and Et2NH2+) may be isolated.'16 Other salts were prepared by the reaction of SOClz with (Ct)2[VOC14].717[VCI,] may form adducts with oxygen, sulfur, selenium, nitrogen, phosphorus and arsenic donors.6*718~720 With unidentate ligands, 1:2 com lexes are normal (e.g. with S(CN)2 or Se(CN)2),719 Thermal decomposition often results in V?*I compounds.6 With bidentate ligands, 1:1 complexes normally form6 Besides additions, complexes react with S0Cl2, PCls and SOBr2 another method has been used, [VO(~helate)~] producing [VX,(chelate),] (X = Cl or Br) hexamordinate VIv c ~ r n p l e x e s ~where ~ ~ ~ the "~~~~ two halide ligands may add either c i or ~ trans (cf. Section 33.5.5.4.ii, where the chelate is acac, or to other sections according to the chelate ligand involved, e.g. salen, oxine, etc.).
33.5.9 Mixed Donor Atom Ligands 33.5.9.1 Schi$ base ligands No general review has been published since 1967,357bReviews by SyamaP5' and others722 cover only part of the published work on oxovanadium(1V) Schiff base complexes.
(i) Vunadium(W) complexes with Schiff bases derived from the reaction of diamines and salicylaldehyde and derivatives The complexes with Schiff bases (SBs) derived from diamines and salicylaldehydes (66) or /3-diketones (67) normally have a 1:1 stoichiometry. R'
\
0
c-qllp-y,
/v\ f \ I
HC(
P' IJH
C=N N=C
HrC-CH
\R2
\
[VO(salnpn)] (66;R' = Rz= R = H; n = 3), prepared simply by mixing vanadyl salts and the SB, even in the presence of DMSO or py, as sparingly soluble yellow-orange needles, consists of molecules packed so that the vanadyl oxygen of one occupies the sixth position of the V in a n e i g h b o ~ rThe . ~ ~result ~ is an infinite chain of molecules about a two-fold screw axis (Figure 30). d(V=O) (1.633 A) is one of the longest reported to date, consistent with v(V=O) = 854 cm-l, and the bridging character of the vanadyl oxygen. The magnetic moment was 1.78BM at 295 K and it obeys the Curie-Weiss law over the range 95-295 K with 0 = -7 K.5Y9bOther authors conclude that not only do magnetic spin-spin COC3-R
Vanadium
532
(a) Figure 30 (a) A view of [VO(salnpn)] molecule with some internuclear distances (in representation of the chain structure
(b) schematic
interactions exist in this compound but also that a small ferromagnetic interaction is indeed found. Because the vanadyl ion is lopsided, its chelates have the possibility of forming a greater number of isomers than monatomic ions. For example, for an N,O-coordinating chelate (ems. glycine), two enantiomers, (&) and (69a), may form (S = solvent). S 0
/v.,I b\s
S
+
(aa)
S
Yli/s
\Il/
(69a)
L
/\
S
i
N
j"\,d" / 0
0 (ab)
\s
(69b)
In the case of dissymmetric SB (a), V 0 2 + has the possibility of forming twice the number of isomers for each R or S configuration of the diamine chain (Scheme 18). For example, in the case of [VO(sal(S)pn)], where sal(S)pn is the SI3 derived from Hsal and (S)-( t )-propane-1,2diamine, the preferred conformation does not always depend on the absolute configuration of the diamine; structure (70) represents the favoured form since it involves the least steric interaction between the vanadyl oxygen and the (S)-pn methyl g r o ~ p . ~ ~The , ' * actual ~ situation existing in solution is probably an equilibrium of the isomers with a predominance of the A conformation since the barrier for ring interconversion from I to 6 is small.'" In some complexes, as in [VO(sal(SS)chxn)] ((SS)chxn = (S,S)-( )-cyclohexane-l,2-diamine), the ligand is stereospecifically coordinated so that the chelate ring is locked in a 6 conformation.
+
Vanadium
533
Dissymmetry may occur if the ligand adopts a preferred flattened tetrahedral geometry about the metal ion. CD spectra of [VO(sal(S)pn)] and [VO(sal(SS)chxn)] exhibit evidence of A (71) and A (72) configurations.* From CD spectra, for complexes with tetradentate S B S , the ~~~ geometry around V02+ has a slight pseudotetrahedral distortion.
Many investigations have involved vanadyl complexes of type (66),5wa,72s726 particularly with 1,2-diamines. In the solid state p:; values are normally near the spin-only v a l ~ e . ~ ~ ~ ~ ’ Complexes of SB (73) are five-coordinate in solution and normally show no tendency to coordinate a basic ligand to form six-coordinate compound^.^^ The formation of adducts depends on the chelate ring, as in (70). With electronegative groups in the aromatic ring, e.g. 5-N02salen (73; R’= R2= H, R = 5-N02), electronic and CD spectra show hexamordinate adducts, and solid adducts were obtained with py and DMSO.725 Expansion of the ‘central chelate ring’ in tetradentate (56) from n = 2 to n = 3 favours six-coordinate species, as [VO(salnpn)] (Figure 30).m Complexes (66;R = H and n = 2) were five-coordinate square pyramidal monomers; in py, six-coordinate species form.m Compound (66;R’= R2= R = H), polymeric in the solid state (Figure 30), was considered to break down to five-coordinate monomers in CHCb and six-coordinate in py. Changes in the spectrum of (66;R’= R2= R = H, n = 9) in CHC13 on dilution indicate that the complexes with n 2 4 are not monomers. Compounds (66; R = 3-Me0) were five-coordinate monomers except when R1= R2 and n = 3. 557 Compounds (66;R = 5-CI) tend to be olymeric solids. Complexes with (74) appear monomeric in the solid state except when n = 3.5 8 R R
e r=c\Ha H/“N\
H
CH-CH
I r R’ RZ
&OH -
HC=N
\
R,,~(CH2)n
R2 (74)
(73)
\
(75)
SR
(76)
For (66;R’= R2= H, R = 3-Me0), the changes in the ordering of d orbitals as n varies are schematicall represented in Figure 31. Complexes with (75; R ’=M e or Ph) have been reported. 729,x0 Alkylation of the thiocarbonyl sulfur of thiosemicarbazone derivatives induces not only complexation through the terminal amino group but also enough acidic character for it to
Vanadium
534
Figure 31 Changes in the ordering of the oxovanadium(1V) d orbitals proposed for complexes with Schiff bases (66 with R' = R2= H and R = 5-MeOSs' (not to scale)
function as a monoacidic ligand.731 In the presence of V02+ (and Ni2+, Cu2+) salts, thesr ligands are capable of condensing at the terminal amino nitrogen atom through anothe aldehyde or ketone producing quadridentate ligands and forming (76).732In (76) and simila Ni2+ and Cu2+ complexes ( R = H , NH4, Na, K), the set of coordinating atoms and thi geometry appear similar to those with (73).735735 The V02+ compounds react with alky iodides producing complexes with R = Me, Et, Pr. Their electronic spectra in pyridine diffe from the spectra in CHC13;736a decrease in intensity with parallel decrease of the ESR show that V02+ is oxidized to vanadium(V) ,736 surprising compared with examples of Vv complexe being reduced to V'v385'686-687 and even to V"' compound^.^^ Other oxidations in the presem of py are in Table 37. When [VO(salen)], only slightly soluble in THF, acetone and acetonitrile, is added tc NaBPh4 in these solvents, a green solution is produced from which the sodium cation comple. (77) crystallizes as a light green solid, stable in air (equation 53).737[VO(saloph)] (saloph = N,N'-(0-phenylene)bis(salicylidenimine)) gives ([(VO(sal~ph)]~Na)BPh~.~~~ Compound (77 contains Na+ trapped in a coordination cage provided by six oxygen atoms. I ([VO(~alen)]~Na)BPh~, all the oxygens of [VO(salen)] are involved in coordination to the Nai giving a cationic polymeric structure. The geometry around Na' is pseudo-octahedral; the si oxygens are provided by four different [VO(salen)] units: two vanadyl 0 at an average o 2.368A and two pairs of 0 from salen units at an average of 2.405A. Vanadium i five-coordinate square pyramidal and is 0.606 A above the plane of the OzN2 atoms of the sale unit. The mean V=O is 1.584 A. Na+BPh;
+ 2VO(salen)
-
[ {VO(salen)},Na]BPh,
(53
(77)
Treatment of a basic aqueous solution of EHPG (EHPG = ethylenebis(0-hydroxyphen! glycine), protonated and deprotonated forms will not be specified) with VOS04 afforded l i d blue (NH4)[VO(EHPG)]vH20.EtOH;738 the V02+ is bonded to five atoms of the EHPG3 ligand with one amine nitrogen, one phenolate oxygen and two carboxylate oxygel; coordinated equatorial and an amine nitrogen coordinated trum to the vanadyl oxygen. I contrast, V o s o 4 and EHPG in aqueous ethanol yield a neutral dark blue product charat terized as the Vv complex [VVO(EHPG)],73s741stable as a solid. On allowing warm solution to stand, green [VO(salen)] forms.338,339 This corresponds to an oxidative decarboxylation i which two molecules of COz are lost. The reaction, rapid in DMF, proceeds slowly in alcoho! with [VVO(EHGS)] (EHGS = N,N'-ethylene((0-hydroxypheny1)glycine)) as monodecarboxj lated intermediate, isolated as the sodium salt Na[VO(EHGS)].1.5H20-MeOH, which may b prepared by exhaustive electrolysis of the Vv analogue. It is six-coordinate with the tw nitrogens and phenolic oxygen binding equatorial and an oxygen of carboxylate trans to th vanadyl oxygen .738,739 The final product of decarboxylation is formed by discrete molecules ( ~ a l e n ) ] ; ~all~ ~ aspects ~ ' ~ ~of the structure agree with those for ([VO(~alen)]~Na)BPl The electrochemistry of [V'"O(EHPG)]- and meso-[VVO(EHPG)] has been studied b cyclic v ~ l t a m m e t r y . ~ ~ ~ In [VO(EHPG)]-, [VO(EHGS)]- and [VO(salen)], v(V=O) correlates well with axi; (V=O) and equatorial bond lengths.738
RJs7
535
Vanadium
Complexes with SBs of general formula (78) form by adding a VOC1, solution to a solution containing the appropriate derivative and substituted ethylenediamine, followed by the addition of NaMeCOz and heating under reflux for 10-60 mi^^.'^^ According to the nature of R and R’, two types may be obtained: (i) complexes [V0(X~alenN(R)R’)~l in which the vanadium is linked to two SBs, and (ii) complexes [VO(Xsal)(XsalenN(R)Rr)]in which the vanadium is linked to only one SB and to a salicylaldehyde molecule. When the R and R’ groups have an electron-releasing effect, the P nitrogen becomes a strong donor, the SB acts as a tridentate ligand, and six-coordinate mixed-ligand complexes of type (ii) form. However, while with Ni2+,the bidentate ligand is an SB, with V02+ it is a sal- ion; this occurs even if the reaction is carried out using stoichiometric amounts of preformed SB (78) and the vanadyl salt. Type (i) complexes are ye en, and monomeric, with structure (79); the pyridine readily adds to vanadium in ~olution.’~In type (ii) complexes, the vanadium is coordinatively saturated and the electronic spectra in inert solvents and in py are very similar; this and other observations led to a proposed structure (80).742
( i i ) Vunudium(N) complexes with Schiff bases derived from the reaction of diamines and P-diketones
Several complexes with Schiff bases derived from the reaction of P-diketones and diamines (67) have been prepared and many publications on them have appeared.523,725,743=148 Almost all are monomeric (no V - . V interactions) and five-coordinate square pyramidal, as in (67). This structure has been confirmed in [VO(acen)] (acen = 67; R1= R2 = Me, R = X = H). Complexes (67)are often prepared by chelate exchange (equation 54), using [VO(acac),] as the starting product. In the case of H2tfen (tfen = 67; R’= CF3, R2 = R = X = H) this yields instead of [ V O ( a ~ a c ) , ] . ~ ~ ~ [VO(acen)], and [VO(tfen)] was obtained starting with [VO(tfa~ac)~] The complexes may be recrystallized from benzene, CHC13, etc., but, as in the case of (M), solvates are often obtained.
-
VO(acac),
+ P-diketonesdiamine
VO(B-diketone.diamine)
+ 2acac
(54)
The separation of the four possible isomers of [VO(acpn)] (67; R’ = R2 = R = Me, X = H) The 02Nz chromophore is not planar but has a slight tetrahedral distortion and the CD spectra suggest that the corres onding chirality has A absolute configuration for diamines of S or S , S absolute c~nfiguration.~ ? In [VO(acen>], the geometry around the vanadium is square pyramidal (see Figure 32),746,747 but P-ketimine V02+ complexes do not readily add a ligand to form six-coordinate complexes, except if electron-withdrawing groups are attached to the acac portion of the SB. For example, Cotton effects appear in the d-d transition of [VO(tfen)] in the presence of ( - )-l-phenyl-laminoethane in acet0nitrile,7~~ and, also, though a solid [VO(tfen)]-DMSO adduct forms, the sulfoxide is lost on standing overnight in the air. In (67; R1=R2=Me, R = H ) with X = B r green or yellow complexes were obtained, according to the solvent used, with v(V=O) at ca. 980 and 900cm-’, r e s p e ~ t i v e l yWhen .~~ X = C1, only the green-type complex may be obtained. The authors assume five-coordinate square pyramidal geometry for the green complexes and a chain structure .V=O. .V==O* similar to [VO(salnpn)], for the yellow complex. Solid adducts were isolated with l-methylimidazole. which is soluble in Allowing [VO(acen>] to react with S0Cl2, [VC12(acen)]was acetone, DMSO and CH2C12. This is less stable as a solid than [VC12(salen)] but its decomposition product from reaction with air is not [VO(acen)].
SA, S6, R I , R6 (compare with Scheme 18) was claimed.
a
-
1,
433744
536
Vanadium
Figure 32 The molecular structure of [VO(a~en)].’~’Some internuclear distances are indicated. The ‘en’ portion of the ligand assumes a configuration approximately halfway between gauche and eclipsed. The V atom is 0.58 8, above the plane of the SB coordinating atoms
(iii) Bidentate Schif bases
Most of the compounds with SBs expected to have structure of type (8lJ or (82) are in Table 39. Compounds 1-6 of this table were included in Gmelins H a n d b ~ c h .Preparation ~~ consists simply in adding an aqueous-alcoholic (EtOH or MeOH) solution of the corresponding amine and NaMeCOz to an alcoholic solution of VOClz (or VOS04) and the salicylaldehyde derivative. The complexes are often purified by recrystallization from ethanol. Compounds 7-13 are brownish grey-green and complexes 14-21 are they are soluble in CHC13 and CH2C12 but less soluble in methanol, ethanol and benzene. These complexes show electronic spectra in pyridine which are different from the spectra of the same complexes in the solid state and in CHC1,; this indicates, in pyridine, six-coordinate solvates.750
For several SB complexes ranging from yellow to green and maroon (22-30 in Table 39), poor solubility with failure to obtain six-coordinate species initiaHy suggested a polymer.721 However, they are soluble in hydrocarbons, and cryoscopy in benzene suggests a monomeric nature. For yellow bis(N-(Cchlorophenyl)salicylideneiminato)oxovanadium(IV) (22 in Table 39, v(V=O) =885 cm-I), the unit cell comprises monomers of C2 symmetry and the coordination was distorted trigonal bipyramidal (13). The three oxygens and vanadium are in the equatorial plane and the two nitrogen donors axial. V=O 1.615 A) is longer than normal. Syamal and Ka1e7s1-7s3reinvestigated previous report^^'^,"^,^ where a dimeric structure was proposed for compounds 33-36 in Table 39 and concluded that all exhibit moments in the range 1.73-1.75 BM at 295 K and obey the Curie-Weiss law with f3 = 0-4 K, indicating the absence of magnetic ordering in the temperature range 83-296 K. Osmometry indicates a monomeric nature. ESR spectra in CHC13 give the ‘normal‘ eight-line spectra with g = 1.98 and no triplet state spectra (AMs = It2 transition) were detected around 1600 G. On the basis of this evidence, complexes with SBs (81) and (82) with R = Ph are monomers and have square pyramidal structures. Although the compounds are not strictly the same, this conclusion disagrees in part with the work of Pasquali et aL7’l where a distorted trigonal bipyramidal structure was established for compound 22 in Table 39. Oxovanadium(1V) complexes with SBs derived from Hsal or substituted Hsal and 2-aminoethanethiol (83) and 3-aminothiophenol (84) were reported by Syama175b75Rand some properties are summarized in Table 40. The corresponding ligands with oxygen atoms instead of sulfur form tridentate complexes with antiferromagnetic properties (see Section 33 S . 9 . l . i ~).
I
Vanadium
537
Table 39 Oxovanadium(1V) Complexes VO(SB)2 with Schiff Bases of Type (81) P:$
X
R
1 2 3 4 5 6
5-C1 3,5-C1, 5-Br H H 5-Br
H H H Me Ph Ph
7 8 9 10 11 12 13
H H H H H €4 H
Et Pr. Pr' Bu
14 15 16 17 18 19 20 21
3-Me0 3-Me0 3-Me0 3-Me0 3-Me0 3-Me0 3-Me0 3-Me0
22 23 24 25 26 27 28 29 30
H
31 32 33
H
H H H H H H H H
H
H
Buf B u' CY Me Et Pr Pr' BU
Bus Bu'
CY C&Cl-4 C&Cl-3 C&N02-4 C&NO,-3 C6H40Me-4 PhCH, C6H,0Me-2 C,A, C16H33
C6H4Cl-4 C$I,OMe-4 C,H,SO,NH,-4
Colour
Grey brown Green Green Green Green Green Green
CGH4Cl-4 C,H40Me-4 C6H4Cl-4 C,H40Me-4
Greenish yellow Greenish yeliow Greenish yellow
40
5,6-Benzo
C6H4Cl-4
41 42 43
H
H
C6&Me-2 C&Me-3 C6HdMe-4 C6H,MeZ-2,3 C&Me,-2,4 W2Me3-2,4,6 C6H40Me-3 C6H4OH-4 CJ14Br-4
H H H
975,895"
1.76
1.54 1.70 1.75 1.54 1.69 1.54 1.72 1.68 1.70 1.71
Cinnamon Brown Cinnamon Cinnamon Cinnamon Greenish yellow
Green Green Yellowish green
6,7 6 6 6 6 6 6 6 6 6 6 6
1.67
5,6-Benzo 5,6-Benzo 4-OH 4-OH
H
988 986
1 1 2 3 3-5 2-5
6
1.77
Greenish yellow Greenish yellow Greenish yellow
36 37 38 39
H H
Ref.
6 6
1.81 1.76 1.66
Greenish yeIlow Greenish yellow
4s 46 47 48 49
1.75 1.74
Golden yellow Yellow green Light green Light green Yellow green Green Yellow green Beige maroon Beige maroon
Ph Ph
H
(cm-')
Green Green Green Green Green Green Green Green
5-c1 5,6-Benzo
44
r(V=O)
1.78 Brown Greenish yellow
35
34
(BM)
885 940 875 980 970 992 970 970,985' 975,985"
7 7 7 7 7 7
1028(?)
8 8
7 7 7
5
984 978
990 994
9 f,4,5,8 8,10 9 8 8 8 8 9
11 11 11 11 11 11 11 11 11
In cs,. 1. A. S. Pesis and E. V. Sokolova, Zh. Neorg. Khim., 1963,8,2518. 2. V. V. Zelentsov, I. A. Savich and V. I. Spitsyn, Proc. Acad. Sci. USSR, Chem. Sect., 1958,118-123,649. 3. L. Sacconi and U. Campigli, Inorg. Chem., 1966, 5, 606. 4. A. Syamal and K. S. Kale,Cum. Sci., 1977,46, 258. 5 . A. Syamal and K. S. Kale, J. Mol. Siruct., 1977, 38, 195. 6. S. Yamada and Y. Kuge, Bull. Chem. SOC. Jpn., 1969,42, 152. 7. M. Pasquali, F. Marchetti, C. Floriai and S. Merlino, J . Chem. SOC., Dalton Tram., 1977, 139. 8. R. L. Dutta and G. P.Sengupta,1. Indian Chem. SOC., 1971,48, 34. 9. A. Syamal, Bull. G e m . Soc. Jpn., 1977, SO, 557. 10. S. I. Gusev, V. I . Kumov and E. V. Sokolova, J . Anal. Chem. USSR (Engl. Transl.), 1960, 15,205. 11. V. A. Kogan, L. E. Lempert, 0. A. Osipov, G. V. Nemirov and A. N. Nyrkova, Zh. Neorg. Khim., 1967, U , 1400 (Chem. Ahsir,, 1967,67,60501g).
a
Vanadium
538
However, (83) and (84) behave as bidentate monobasic ligands and form complexes [VO(SB)2]; a square pyramidal structure was suggested with coordination through the oxygen atom and nitrogen of the azomethine group. The [VO(SB),] complex with (83; R = H) was also studied by IR and cyclic v~ltammetry.’~~ Although the presence of the protonated SH group could not be confirmed, the coordination probably involves N and 0 donors. Schiff bases (85) may behave as tridentate ONS ligands forming V O L - n H 2 0complexes (Section 33.5.9.1.i~); however, if heated, these SBs are capable of cyclization and may then form [VO(SB),] with (86) bidentate.760
/
H’
(W
Table 40 Some Oxovanadium(IV) Complexes Derived from Salicylaldehyde or Substituted Salicylalde3-Aminothiophenol (84) or 2-Aminothiophmol (86) (on reflux) hyde with 2-Aminoethanethiol (a),
R
H 5-c1 5-Br 5-Me0 4-Me0 3-Me0 5-NO2 2-OH
SchN base (83)’ v(V=O) (%$ (cm-’)
Schiff base p:; v(V=O) (3M) (cm-’)
1.71 1.71 1.72 1.75 1.72 1.75 1.72 1.72
1.73 1.72 1.71
970 980 980
R, R’, R”= H R = CI; R‘, R”= H R = NO,; R’, R” = H
1.74
980
R’ =NO,; R, R = H R” = C1; R, R’ = H
1.71 1.73
940 980
980 980 980 975 970 970 950 985
Schiff base p$: v(V=O) (BM) (cm-’)
1.86 1.86 1.76 1.69 1.78
914 900 910 (900) 920 895
I. A. Syamal, Transition Met. Chem. (Weinheim, Ger.), 1978, 3, 259. 2. A. Syamal, Transition Met. Chem. (Weinheim, Ger.), 1978, 3, 297. 3. U. C. Lee, A . Syarnal and L. J . Theriot, Inorg. Chem., 1971, 10, 1669
(iu)Tridentate Schiff bases With bidentate SBs, monomeric [VO(SB),] complexes are normally formed. On the contrary, with tridentate dibasic SBs, dimeric species often form. It appears that the dibasic character of the ligands forces the VOZc ion to dimerize leading to anomalous magnetic properties. Proposed structures have not been verified owing to their non-crystalline nature. Insolubility precluded molecular weight determinations and ESR and electronic spectral studies have been carried out in the solid state only. Solutions containing VOz+ and dinegative, tridentate ligands of the type -0-N-0are easily oxidized to the corresponding Vv complexes and possibly VTvcomplexes are stable only if insoluble. Evidence for dimeric structures based on single-temperature subnormal magnetic moments should be treated with caution, as small amounts of a VIv contaminant in a Vv complex may be the cause of the observed magnetic susceptibility. Complexes with the tridentate dibasic SBs (87)-(90) have subnormal magnetic pro erties. Part of the published work on these type of complexes was reviewed by S y a m a b and Casellato et aL7” and many of the reported compounds are included in Table 41. They are normally obtained by mixing alcoholic (MeOH or EtOH) solutions of the appropriate SB, often prepared in situ, and an alcoholic solution of a vanadyl salt. Zelentsov first characterized such comple xe ~,’~~ prepared from SB (87) derived from Hsal or substituted Hsal and o-aminophenol. Ginsberg et al. prepared these as well as some others and studied the magnetic The moments of the complexes (with the exception of properties in detail from 1.4to 300 R = NOz, R’ = NO*) decrease considerably as the temperature is lowered and the temperature dependence of the susceptibility is characteristic of intramolecular antiferromagnetic exchange. The authors suggested a dimeric structure 91 and calculated the exchange integral, 1, according to the Bleaney-Bowers equationJM ’concluding that substituents at R and R’
Vanadium
539
positions influence I in a different manner in oxovanadium(1V) and copper(I1) complexes of the same ligands.764This was attributed to a difference in the exchange mechanism operating in V02’ complexes resulting from the orbitals containing the unpaired electron not having the same symmetry. A tetrametallic structure (92) with V=O. - .V interaction was proposed for complex 5 in Table 41 to explain its higher I value and IR spectra.
’e e H
5
a
;
I--”
R\ @HOCHdy
N-/
H 0 O R
R’
/
\
?=N-CHz H
\
H
a
R
/*”
(89)
R (91)
(93)
The complex with R = NO2, R‘=H (7 in Table 41) has only a small TIP (temperature independent paramagnetism) term and does not follow the single-triplet susceptibility curve; the spins are completely coupled and only the damagnetic single state (S= 0) is populated. The IR spectrum provides no evidence for bridging through the vanadyl oxygen and the complex probably has a different structure.763The complex with R = R’ = 5-N02 (9) obeys the Curie-Weiss law with 8 = 1.4 K and p = 1.62 BM and the authors suggest a five-coordinate monomer in which a molecule of solvent occupies a coordination position.763 On treatment of (91; R = R‘ = H) with a strong chelating agent, such as phen, the dimer is broken, with the formation of a mononuclear rnixed-ligand complex [VO(SB)(phen)].765The dimeric structure is also broken on treatment of complex (91; R = H, R’ = 5-Cl) with pyridine; a monop ridine adduct forms which obeys the Curie-Weiss law with 0 = 2 K and p:; = 1.75 BM.Y63 Syamal reported the ESR spectrum of complex (91;R = R’= H) in polycrystalline solids; it exhibits a single-line spectrum with g,, = 1.99 at room temperature and 77 K, with no half-field band or hyperfine splitting3’’ Subnormal V02+ complexes (93) of SBs derived from 2-hydroxyna hthaldehyde and o-aminophenol and some substituted derivatives have also been obtained.76 The ESR spectra of the complexes (93; R = R’= R’ = H; R = C1, R‘ = R“ = H) in polycrystalline solids at room temperature exhibit two parallel lines, two perpendicular lines and a broad .line around 1600 G due to the AMs = 2 transition. At 77 K, both high- and low-field spectra exhibit hyperfine splittings. Several complexes of type (93) were also studied by Carlisle et al. (Table 41, 27-34).766*767v(V=O) values of complexes (93) lie in the range 955-1000cm-1 and the compounds exhibit only one d-d band in the region 620-700nm. As in the case of (91; R = R’ = H), on treatment of compound (93; R = R’= R = H) with phen the dimer is broken and a mixed-ligand complex is obtained.765
P
COC3-R’
Vanadium
540
Table 41 Oxovanadium(1V) Complexes with Tridentate Schiff Bases (87)-(90)
R'
R Schifl bases (87) 1 H 2 H 3 It 4 H 5 H 6 5x1 7 5-NO7 8 5-CI 9 5-NO," 10 H 11 5-NO,
1.48 1.46 1.54 1.so 1.30 1.47
H
-
118 90 115 218 120
5-C1 5-N02" S-Clmonopyridinate 5-Br
1.52
132
H 5-Me 5-c1 5-Br 5-NOZ H
125
1.75
H 5-c1 5-Br 3-Me0 3-Et0
1.52 1.18 1.43 1.57 1.42
126
Schiff buses (89) 17 H 18 H 19 H
H 5-CI 5-Br
4-Mc0 3,5-C12 hydrox'
1.11 1.19 1.18 1.23 1.11 1.10
298 266 264 241 307 274
5-Clb 5-Br 5-NO2 hydrox'
1.29 1.34 1.27 1.52
H H H
Schiff bases (90) 23 H 24 H 25 . H 26 H
V'v Schiff base complexes (93) R R' 27 H H 28
CI
29 30 31 32 33 34
NO2
H H NO, H Ph
H H
H H H NO2 H
R" H H H NO2
c1
NO2 H
H
983,993 999 978
1,2 2 1,2 1,2 2 2 2 2
2 2
1
Schiff bases (88) 12 H 13 H 14 H 15 H 16 H
20 21 22
990 993 992 991 900 (1010) 1000,989
1.54' 1.84,1.78 1.44,1.51 1.34,1.60' 1.71 1.48 1.70,d 1.65d 1.49, 1.44 1.46,1.47
910-985 121 20 1
3,4 3,4 3,4 3,4 3,4
910 900 910 910 9M 910
5,6
1005,995 900 914,904 960
7 7 7 7
151
1000
182 201
990 955 989 990 992
8 9,10 8,10
S, 6 5,6 5,6
5, 6 5,6
8,10 9
9 9,10 9,10 9,10
Unless otherwise indicated, complexes are ofbanhydrous[VOLI, type. Formubated [VOL].H,O.
a
8s
[VOL]4.5H20-EtOH.
Formulated as [VO(L)(&O)].
Formulated as
[VOL].EtOH.
Formulated as
hydrox = 2-hydroxy-1-naphthaldehyde. 1 . V . V. Zelentsov, Dokl. Akad. A'auk SSSR, 1961, 139, 1110. 2. A. P. Ginsberg, E. Koubek and H. J . Williams, Inorg. Chem., 1966, 5 , 1656. 3. A . Syamal and K. S. Kalc, Indian J . Chem. Sect. A , 1980,19,225. 1. A . Syamal, K. S. Kalc and S. Banerjeee, J. Indian Chem. SOC., 1979, 56, 320. 5. A. Syamal and K. S. Kale, J . Indian Chem. SOC., 1978, 55, 606. 6. A. Syamal and K. S. Kale, lnorg. Chem., 1979. 18, 992. 7. C. C. Lee, A. Syamal and L. J. Theriot, Inorg. Chem., 1971,10, 1669. 8. A. Syamal and L. J . Theriot, J . Coord. Chem., 1973, 2, 193. 9. G. 0. Carlisle, D. A. Crutchfield and M. D . McKnight, J . Chem. Soc., Dalton Tram.. 1973, 1703. 10. G. 0. Carlisle and D. A. Crutchfield,Inorg. Nucl. Chem. Lett., 1972, 8, 443.
With (88) and (89), several V02+ complexes were prepared (Table 41).768-771High melting or decomposition temperatures (>250 "C) and insolubility in common noncoordinating solvents suggest a dimeric or polymeric nature. v(V-0) occurs at 910-985 cm-' for complexes of (a) and at 900-910cm-' for those of (89); this argues against the presence of a . . -V=O. . -V=O. . . chain structure. IR suggests a phenolic oxygen bridge in (89) complexes76' and enolic oxygen bridges in (88) complexes.771p:; values of the complexes are remarkably less than the spin-only value (Table 41) and the magnetic moments decrease significantly as the temperature is lowered, suggesting antiferromagnetic exchange interaction
Vanadium
54 1
of neighbouring V02+ions. The ground state is S = 0 (singlet state) and the complexes exhibit no dependence on the ma netic field strength. The J values of the .OK complexes with (89) are higher than those with (88); this suggests that in complexes with (88) and (89), bridging does not involve the phenolic 0 atoms of Hsal. If this was the case one should expect similar J values in the two series of c o m p l e ~ e ssince ~~~,~~~ the electronic environment in the chelate ring would be the same. Complexes with (90) involving ONS coordination apparently similar to the O N 0 coordination in (87) were reported (Table 41).760v(V=O) lies in the range 900-1005 cm-' and some exhibit splitting of the V=O band. &&. values are in the range 1.27-1.52BM and the temperature dependence indicates exchange-coupled antiferromagnetism. A dimeric oxygenbridged structure, similar to (91), which provides an appropriate symmetry for the 3d,, orbitals to overlap and form a strong u V-V bond, was suggested.760 The synthesis and characterization of VOz+ complexes of hydrazones (94) and (95) derived from benzoylhydrazine and o -hydroxy aromatic aldehydes and ketones were also reported77z and molecular weight determinations, IR, electronic spectra and magnetic properties were explained assuming a dimeric structure (96) with each unit having a five-coordinate square pyramidal geometry. RI
?I
Oxovanadium(1V) complexes with (97)and (98) derived from alkyl aminoalcohols and Hsal or o-hydroxyacetophenone (and substituted derivatives), respectively, have been prepared as well as V02+ complexes with (99) derived from 2-hydrazinoethanol and Hsal and substituted Hsal. Most of these and some properties are summarized in Table 42. Generally the complexes were prepared by adding an alcoholic solution of the SB, either previously obtained as a solid or prepared in situ, to alcoholic vanadyl acetate. The mixture is normally refluxed for several hours and the precipitates are collected by filtration; as they are not soluble they are not recrystallized. Compounds obtained with SBs (97)-(99) have been formulated as [V0Ll2 compounds with They have structure (100). This is based on magnetic properties and IR subnormal p:; and the dependence of x? on temperature is characteristic of antiferromagnetic exchange. For at least some (15-23 in Table 42), it was that the complexes exhibit no dependence of xg'" on magnetic field strength, indicating absence of ferromagnetic interaction. ESR studies on polycrystalline samples at room temperature normally give g,, -1.99 and the half-field spectrum corresponding to the AM, = 2 is often observed. At 77 K , the high-field spectra often exhibit hyperfine splittings similar to those observed in VOzf tartrates (Section 33.5.5.7.9. The spectra of complexes with the tridentate SBs (97)-(99) generally exhibit three ligand field bands: (i) ca. 715-770, (ii) ca. 425 and (iii) ca. 500-550nm, assigned to d-d transitions according to the Vanquickenborne and McGlynn MO which corresponds to an ordering of levels as in the Ballhausen and Gray model (Figure 16a): dxy< d,,, dyz< ~ , L ~ z
-(99)"
R
R'
(BM)
1
H
2
H
1.42 1.06 1.41 1.36 1.53 1.17 1.26 1.27 1.19 0.94 0.89 1.52 1.48 1.34 1.54 1.58 1.54 0.87
3
5-Me0
4
5-CI 5-Br 5-NO2
5
6 7 8 9 10 11 12 13 14
3-NO2 3-Me0 5-C1 5-Br 5-NO2 3-NOZ 5-Me0 3-Me0
H
15 16
5-CI 3 ,S-Cl, 5-Br 3-Me0 hydroxb
17 18 19 20
21% 22 * 23*
H H
H
IJI
v( V=O)
(cm-')
(cm-')
215
971
141 398 330 321 339 486 519 153 166 294 147 117 137
970
25 t
26t 27 'r 7.w
S-NOZ 3-Et0 hydroxb
995
291
916 995 9'76,995 980 968 880 963 963 974,986
}
965-995
1.40 1.50 1.40
%} 380
965-985
1.39
1 2 3 2 3 3
Brown Reddish brown
987
470 450 350
1.35 1.30
Brown
Yellow green Brown
998
1.01 0.88 0.81 0.98 0.83 1.27
5-Ct 5-Br
Ref.
Green
Green Grey Green Green Yellow Green Green Green
3
3 3 3 3 3 3 3 3 3 3 2 4
385 -
4 4 4 4 4
I
H
24t
Colour
245-336
5
5 5 6 6 6 6
970 980 970 970 960
6
6
910
" C o m p l e x e s with Schiff bases of type (98) are indicated with an asterisk (*), thosc with Schiff bases of type (99) by a dagger hydrox = 2-hydroxynaphthaldefde. 1. S. N. Poddar, K. D e y , J. Haldar and S. C. Nathsarkar, J . Indian Chem. SOC., 1970, 47,743. 2. Y . Kuge and S. Yamada, Bull. Chem. SOC.Jpn., 1970,43,3972. 3. A. Syamal, E. F. Carey and L. J. Theriot, Inorg. Chem., 1973, 12, 245. 4. A. Syamal and K. S. Kale, Indian J . Chem., Secf. A , 1977,15, 431. 5. A. Syamal and K . S. Kale, Indian J . Chem., Sect. A , 1980,19, 486. 6. A. Syamal, S. Ahmed and 0. P. Singhal, Tramition Met. Chem. (Weinheim, Ger.), 1983, 8, 156.
(t).
OH R',
OH
I
R
n
d , ~ . ~Some @ of these bands are not well developed and this gave rise to ambiguity in their assignment. No band characteristic of V02+-V02+ interaction has been observed in complexes with these SBs. In Table 42, in complexes with SBs (97)-(99), the IJI values of compounds with n = 2 ( n in R = (CH,),) are larger than with n = 3. This suggests that the antiferromagnetic exchange interaction is stronger with n = 2, and this has been attributed to the difference in the chelate ring (five- or six-membered).TT5For n = 2, the J values also depend appreciably on the nature and position of the Hsal substituent; for n '3, they are relatively insensitive to ring ~ubstituents.'~~
Vanadium
543
In an early publication, Zelentsov reported the preparation of a complex with subnormal magnetic properties (&k = 0.87 BM) with the SB (101).777The compound was prepared from dibenzoylmethane and o -aminophenol. The V02+ complex with (102) has also been shown to be involved in antiferromagnetic exchange with J = - 139 cm-l. Revenko and Gerbeleu prepared several V02+ complexes of (103) derived from Hsal and thiosemicarbazide which were reported to involve ONS co~rdination.~~' The measured && were 1.27,1.21 and 1.33 BM for R' = H, C1 and Br, respectively. The complexes are not anhydrous and a dimeric structure (104) was proposed.
Several complexes were prepared with (105; R' = H or Me). All decompose above 300 "C and are insoluble in common nonpolar solvents. p:& values, in the range 1.32-1.S8BM, suggest exchange-coupled antiferromagnetism. The authors formulated the complexes as [V0(SB)(Cl)l2 and proposed a dimeric structure (106) but report no x$* temperature dependence results. In the IR, v(V=O) and v(V-CI) were in the range 975-985 and 360-385 ~ m - ' . ~Electronic ' spectra in Nujol mulls had three bands: (i) -395-412, (ii) 640-670 and (iii) 745-800nm. Structure (106) is probably not correct.
c1
Several V02+ complexes with (107) and (108) derived from dibenzoylmethane or pyrrole-2carboxyaldehyde and several amines (taurine, anthranilic acids, p-alanine) were formulated as [ V O ( S B ) ( H ~ O ) Z ] ; ~it~was " ~ proposed that these SBs behave as tridentate ligands and assume a monomeric structure on the basis of ufC and ebulliometric measurements in dioxane. A pyridine adduct [VO(PE)(py),] was ais;-obtained where PE is the SB (108; R - 0 CH2CHzS07).782 B&oylhydrazine and [VO(a~ac)~] react in methanol under dry Nz to produce violet [V(abh)2] where abh is the dianion of acetylacetone benzoylhydrazone (109;R = H).784 In air, a smaller yield of [V(abh)2] is obtained, with Vv complexes containing abh. The violet complex has pEk = 1.87 BM and the ESR shows the 'normal' eight peaks with g = 1.972; the structure has trigonal prismatic coordination in which the tridentate abh ligand forms both five- and six-membered chelate rings (110). Depending on the relative amounts of the reactants, chelate exchange between [VO(acac),] and Schiff bases (109) provides (i) complexes with subnormal ykk, probably dimeric, if a 1:l stoichiometry is used, and (ii) vanadium bis complexes, in which abstraction of vanadyl oxygen occurs, if 1:2 [VO(acac),] :SB stoichiometry is used.785 Complexes with SBs from Hsal and n-amino acids (glycine, L- and DL-alanine, L-methionine, L-valine, L-leucine, L- and Dbphenylalanine) were prepared and characterized .786 The complexes are bluish-grey and there is no appreciable interaction between V atoms. The compounds were [VO(SB)(H20)] ( l l l ) , confirmed in an X-ray study of (111;R =Me, L-Ala deri~ative).~"v ( V 4 ) was at -1000 cm-l; the compounds are not soluble in noncoordinating solvents, but when dissolved in py v(V=O) shifts to -970cm-l; several orange py adducts (112) were isolated.686 Complexes (111) have electronic spectra that resemble those of
Vanadium
544
(110)
[ V O ( a ~ a c )and ~ ] were considered as having a square pyramidal structure, although a trigonal bipyramidal structure was not Detailed IR studies of [VO(sal-Gly)(H,O)] were reported. In complexes (111), one water molecule occupies a coordination position and ‘prevents’ the dimerization of the compounds. Dimers were obtained on dissohing the monomer (111) in absolute methanol, when equilibrium (ii) in Scheme 19 takes place. The dimeric nature of the isolated complexes790is supported by the marked differences from the mononuclear complexes, by the low ,ut2 obtained and by molecular weights; v(V=O) was around 980cm-’. From dry methanol the orange products are the vanadium(V) complexes (114) (probably dimeric) ; the oxidizing agent seems to be dissolved 0 2 . 6 8 6 In wet methanol, complexes (115) were obtained as red needles. Further evidence for Vv arises from the fact that on treating (115) or (114) with a reducing agent (e.g. NaSH-xH20),bluish-grey complexes (111) are regenerated. Reaction of (115) with py ives (llZ), corroborating other repcrts of the reducing action of pyridine on vanadi~m(V).~ Dimeric oxovanadium(1V) complexes ( N B U ~ ) [ ( V O ) ( S B ) ] ~ have - ~ H ~also ~ been prepared with the SB from salicylaldehyde and L, D,L-serine and L-~ysteine.~” Magnetic moments are 1.1-1.4BM, higher than for the dimeric SB derived from the L-amino acids: Ala, Val, Leu, Met or Phe (0.5-0.8 BM) which are probably best described as Vv complexes (114).” Green complexes [VO(SB)(H20)], where H S B = 2-hydroxynaphthylidene amino acids, react with py or hen to form [VO(SB)(py)z] or [VO(SB)(phen)], which exhibit pki+ near the spin-only value.7? In methanol, these gave brown solutions from which the monomeric brown Vv complexes [VO(OMe)(SB)(MeOH)] were isolated. The treatment of these brown complexes with CH2C1, followed by n-hexane gave green precipitates of [VO(OH)(SB)],;765in CH2C12,the complexes are dimers. 788778y
F
( v ) Other polydentate Schif bases
Several polydentate SBs may act as binucleating ligands; see Section 33.5.12.1. Only a few miscellaneous polydentate SBs will be included here. A complex with the SB obtained in the reaction of two moles of benzoyl hydrazide with acetylacetone had v(V=O) = 995 cm-’, p:& = 1.7 BM and was monomeric square pyramidal (117).792A few complexes with the The ethylenediamine derivatives of 2,2‘-dihydroxychalcones (118) were prepared in stoichiometry is 1:l and the atoms involved in the coordination were two deprotonated oxygens and two nitrogen atoms. v(V=O) (970-985 cm-’) and ,ugk (1.73-1.83 BM) suggest a monomer; however, molecular weights in nitrobenzene suggest a dimer.
335.9.2 Amino acidr
The e uilibria in solutions containing amino acids have been studied by differenl method^.' ^*^^'^^^^ Relatively high pH is necessary to achieve an appropriate free liganc
Vanadium
545
Vanadium
546
concentration for complex formation. At pH 2 4, however, hydrolysis of V02+ takes place and oxidation to V" may also be significant. Hydrolysis of V02+ in the pH range 5-12 is not well understood (see Section 33.5.5.l.ii). For the glycine system, models I and I1 in Table 43 have been But (cf. Section ~~~ the, ~ ~ ~ 33.5.5.l.ii), for pH 2 5 other V02+ hydrolysis products should be i n c l ~ d e d . For L-alanine system (certainly similar to glycine), results were explained according to model 111 included in Table 43. [VO(HAlaO)'+] (119) must be included to explain the CD spectra. The inclusion of (124)gives a better fit of the data. Table 43 Species Included in Equilibrium Models proposed for the Oxovanadium(IV)-Glycine and -L-Alanine Systems at 25 "C with High Ligand/Metal Ratios Glycine (1 M NaClO, sohtions) Model I Model I1 Tomiyasu and Gordon' FdbyCin and N a g y p d HzGlyOf, HGlyO, GlyO[VO"],
[VO(OH)]', !(VO)Z(OH),]"
L-Alanine (2.2 M NaNO, solution^)^ Model 111 Gillard,Pessoa and V ila s - Bo a ~~' ~
HzGlyO+, HGlyO, GlyO[VOz'], [VO(OH)]', [(VO)z(OH)z]2'
a A value of n of 1 :as 'arbitrarily' assumed. The presence of these species was assumed in order to get an acceptable fit of the experimental data. The existence of these species cannot be definitely precluded but their inclusion is not necessary to explain the expcrimental results for pH < 8.U. 1. H. Tomiyasu and G. Gordon, J , Coord. Chem., 1973, 3, 47. 2. I. FgbyAn and I. NagypA1, Inorg. Ckm. Acto, 1982, 62, 193. 3. K.D. Gillard, J . C. Pessoa and L. Vilas-Boas, ComunicasZo C31.9, 5" Encontro Nacional de Quimica, Porio 1982. 4 . R. D. Gillard, J. C . Pcssoa and L. Vilas-Boas, ComunicaSio PAS4, 6" Encontro Nacional de Ouimica, Avciro 1983
(123)
(W
(125)
The kinetics of processes involving glycinato-VO'+ are summarized in Table 44. Only one species is formed from the addition of F- ion to a IVO(GlyO),] solution (equation 55; calculated K = 17 mol-'), with structure (125).800From 9F NMR, relaxation is controlled by the exchange reaction (56), which is first order in F-. Cysteine reduces v a n a d i ~ r n ( V ) the ; ~ ~ kinetics have been studied.804 VOZf-cysteine compounds form in the reduction of vanadium(V) with an excess of cysteineso5 and a Chinese publication reports K[VO(HCysO)(H20US04 and cis- and tran~-Kz[VO(CysO)~] (H2Cys0 = cysteine), and ESR as a function of pH. A purple complex from a 5: 1 mixture of cysteine
Vanadium
547
Table 44 Some Kinetic Data for the Oxovanadium(IV)-System at 25 "C Rates of reaction for VO' substitution reactions' Reaction
[VO(HGlyO)]:~ formation [VO(HGlyO)] dissociation HGlyO exchange on [VO(HGlyO)]'+ HGlyO exchange on [VO(GlyO),] Ring closure of [VO(HGIy0)]2t
k at 25 "C (s-')
Method used
1.3 x io3 a 4.6 X 10' (2-aminobenzoato)]~nHz0 (n = 0 or 1) were prepared with rather low && (1.49 and 1.34BM for n = 1 and O).861 In CHCL, the ESR spectrum shows splitting of A l into A, and A, components ascribed to asymmetry of the ligands.861 In [VO(oxine)(2aminobenzoato)]*nH~O (n = 0 or l),a pki value of 1.33 BM was reported for n = LS6* 33.5.11 Multidentate Macrocyclic Nitrogen Ligands
33.5.11.1 Pophyrins Much interest in environmental vanadium stems from its presence in fossil fuels, largely as porphyrins and related compounds which pose problems in refining and burning of highvanadium-content fuels. The metal porphyrins in crude oils and oil shales have been extensively studied since their discovery by Treibs, who postulated that the principal compound in geological samples is vanadyl deoxophylloerythroetioporphyrin (DPEP) and developed a Based on detailed scheme to show that this compound was formed from Treibs' observations, metal porphyrins in crude oils show their biological origin. The literature is extensive and will be very briefly covered. Although the geoporphyrin system is more complex than Treibs' original scheme,865vanadyl porphyrins found in crude oils and bitumens are believed to be the end product of the diagenesis of chlorophyll. The vanadyl analogue of chlorophyll, [VO(DPEP)J (158), provides some insight into some of chlorophyll's properties, including its resistance to oxidation.866XANESlEXAFS ex eriments on pulverized coal gave arguments discounting the presence of vanadyl porphyrins.' ?! More recently, the identification, including X-ray structure, of a vanadyl porphyrin isolqted from a geological sample unlikely to have been chemically altered by the extraction procedure was reported.869 Further, using asphaltenic fractions, from EXAFS, X A N S , ESR and UV-vis spectroscopy vanadium is predominantly porphyrini~.*~~ Several chemical and structural studies of vanadium porphyrins emphasize low valent metalloporphyrins and related systems with sulfur and selenium.lB Two general methods can be used to prepare vanadyl porphyrins: (1) using VO(acac)t in phenol at reflux temperatures, or (2) using VCL or VC13 in various solvents. In boiling quinoline, 100% incorporation is obtained in 2 h for meso-tetraphenylporphyrin (TPP) and octaethylporphyrin (OEP).871,872 ESR has been used to characterize vanadium compounds in crude oi1.354,873 In ESR spectra of [VO(lTP)], extrahyperhe structure from interactions with N atoms was detected in CS2 and CHC4 glasses and the splitting constants A:, A? were calculated (2.9f0.05and 2.8 f0.05 G).517 In [VO(DPEP)] (W), the four nitrogens are coplanar and the vanadium lies 0.48A out of (OEP = 2,3,7,8,12,13,17,18-octaethylporphyrinato), the four plane.874 In [VO(OEP)] , the same as for [VO(DPEP)] (excepting V-N(C)).~~The V-N distances average vanadium is 0.5438, from the mean lane of the four N atoms, in the range 0.48-0.728, predicted by EHMO calculations.8 For [VO(TTP)] ( l a ) , [VO(ETP)] (161) and [VO(ETP)z](H2Q) (162)(ETP = etioporphyrin and H2Q = 1,Cdihydroxybenzene) (Figure 38), the vanadium is five-c~ordinate.~~~ One interesting feature in (162) is the hydrogen bond between the quinol (H2Q) and the VO group. The 0. - 0 distance is 2.82 8, and the V=O- . - 0 and C-0. - -0angles are 159,3 and 118.9". 77868
P
Me
Et
Et
E
Et
H (158)
(159)
EXAFS with [VO(OEP)] and [VO(DPEP)] gave V=O and V-N
bond lengths almost equal
558
Vanadium
1160)
161 1
(a 1
(b)
(C)
F w e 38 Schematic representation of the molecular structures of (a) [VO(TPP)], (b) [VO(ETP)] and (c)
[VO(ETP)],(H,Q) (side view).877 The V=O bond distances are 1.625. 1.599 and 1.614& respectively, and the vanadium ‘N, plane’ distances are 0.53, 0.44 and 0.51 A, respectively. The V-N bond lengths are all in the range 2.04-2.09 8,
to the X-ray showing that this is a useful tool for investigating the coordination sphere of metalloporphyrins; it is often difficult to obtain good quality crystals. In solution, vanadyl porphyrins [VOP] can form adducts [VOPX] in the presence of a large excess of X (X = py or piperidine) but the equilibrium constants are rather small.87~ss1 For vanadyl uropo hyrin’, spectra in alkaline medium are different from those in organic solvents or crystalline?’ If a six-coordinate dihydroxide species [V(OH)2UroP] really forms, it would be the first example of a deoxygenated VIv complex bound to two OH- groups. However, the arguments presented leave the possibility of [VO(OH)UroP]- in alkaline solution.882 The action of SOX, or COXz (X=C1 or Br) on several V02+ porphyrins gives the dihalogenovanadium(1V’) porphyrin^.^^ These are extremely reactive and act as precursors in the synthesis of low-valent porphyrins. [VBr,(OEP)] was studied by EXAFS and the two Br atoms were coordinated trans. In reactions of porphyrins with halides MX,, (TiCb, SnCL, AlC13, A1Br3 and also SbCIS and SOClp), deoxygenation of the vanadyl occurs, octahedral [VX,P] being formed.8m In the intermediate stage, the authors suggest the formation of a bimetallic com lex of vanadium porphyrin with halide. In air, the products are dihalogen complexes of VPv with mono- and di-cation porphyrin ligands. All are entirely converted to the original [VOP] when small amounts of water are added. Reduction by zinc amalgam of a dry and oxygen-free THF solution containing [VC12(TPTP)] (TPTP = meso-tetra-p-tolylporphyrinato) and addition of elemental sulfur yields [VS(TPTP)].6s9 Similar thiovanadyl complexes were prepared with OEP, TPP and TMTP (Th4TP = meso-tetra-m-tolylporphyrinato). Analytical results and mass spectra agree with the formulation [VS(por)], and v(V=S) was at 550-565 cm-I (see Section 33.5.6.1). FT-EXAFS of [VO(OEP)] and [VS(OEP)] were recorded and the difference FT spectrum yielded
559
Vanadium
approximate values for the interatomic distances: V=O == 1.6 and V=S 2.06 f 0.02 A, which agrees with 2.061 8, for [ V S ( a ~ e n ) ] . ~ ~ ~ A convenient procedure for the isolation of selenovanadium(1V) porphyrins has been reported and involves the use of Cp2TiSe5(instead of elemental selenium) (Scheme 21).885,886 The IR spectra exhibit a medium-to-strong band at 434-447 cm-' assigned to v(V=Se). EXAFS with VSe(0EP) suggests that the V-Se bond is -2.19A and that the V atom is -0.47 8, above the plane of the four nitrogen ligands.Iz8 2:
Z" nmalpnm
VCL(Tm)
y V S ( r n P ) V"(Tl"JJ?)(TW)2 CptTiSe,
==+VSe(TPTP) Scheme 21
33.5.112 Phthalocyanines
The V02+ phthalocyanine [VO(pc)] has been the subject of patents as it is useful in photoelectrophoretic and xerographic imaging.gs7It can exist in at least three solid phasesm8 and the structure of so-called phase I1 was determined.889 It is composed of sheets of parallel and overlapping [VO(pc)] molecules and no discrete dimer pair exists. The vanadium is five-coordinate and square pyramidal, and is 0.575A above the 'plane' of the four nitrogens. The V-N distances do not differ significantly with a mean of 2.026A; the V-0 distance is 1 1.580A. ESR measurements of aqueous solutions of the V02+ complex of 4,4'4,",4'"tetrasulfophthalocyanine (tspc) show that it is polymeric.890 In dilute solutions (lW5 M), the dimeric form predominates and higher polymers increase with c~ncentration.'~~ The formation of adducts prevents polymerization and in solutions containing DMF most of the chelate is dimeric with a V-V distance of -4.5 A. 33.5.113 Tetraaza[14]annulenes and other macrocycles Complexes with the tetradentate tetraaza[l4]annulenes (163) and (164) ( = H2L), studied by spectroscopy,89zwere formulated as [VOL], probably similar to the complexes with porphyrins. v(V=O) was at 967 and 940cm-' for (163) and (164). In the ESCA spectra, the nitrogen donors are equivalent.892 n
(163)
(164)
The tetramethyltetraaza[14]annulene (165) reacts with vanadyl acetate to yield (166) similar to complexes of (163) and (164).%' It was the starting material for several reactions shown in Scheme 22, and exhibits a reversible one-electron oxidation wave at +0.245V (us. SCE) but reacts irreversibly with mild oxidants (02,Ag+, Ce4' or 12) to yield (167). With Et3N, (167) yielded a complex with properties similar to (166) but which mass spectral data indicate is a dimer. Structure (170) was proposed. Reaction of (166) with anhydrous HCl deoxygenates the V02+ compound, forming (168).660 Its ESR parameters are those expected for the substitution by two chloride ligands. (168) with H2S and base yielded the thiovanadyl analogue of (166) for which v(V=S) was at 545 cm-'. Hydrolysis of (169) regenerates (166). On allowing V(acac), to react with di(am)sar (172) for 3 days under N2,a deep red-brown developed.R93After separation (by cation exchange chromatography) of the red-brown product, its crystalline chloride was converted into the dithionate [Vrv(di(amH)sar-
Vanadium
560
h
0
m *
H+
v
0
>
Vanadium
541
2H)](S206) .2H20 whose geometry is close to trigonal prismatic with V-N in the range 2.06-2.102. It has pki = 1.80 BM and is stable in the pH range 1-10. Spectroscopic techniques have been used to study the V’” di(am)sar complex943 and other macrocyclic complexes.944
(172)
33.5.12
di(arn)sar
Polynuclear Complexes
33.5.321 Binucleating and compartmental ligands Mononuclear V02+ and heterodinuclear V02+ complexes (with Cu2+, Ni2+, Mn2+, Zn2+, Pd2+ as the second metal ion) have been reviewed.894In this section, we shall mainly discuss more recent work. Many complexes were with Schiff bases from the reaction of P-triketones or /?-ketophenols with ethylenediamine. One important aim has been to compare the magnetic, spectral and chemical properties for the metal ion Ma in [M,(diken)] and MI,in [M,(diket~nato)~], with the corresponding properties in the binuclear complexes formed (see equation 66).
MJdiken)
M,(diketonato),
(173)
In mononuclear Complexes with ligands of type (173),V 0 2 + has a strong preference for the OZOzenvironment as clearly demonstrated by the single crystal molecular structure determinations with [VO(H2baaen)] (174; R = (CH2)2,R’ = Me, R” = Ph),893,895,896 [VO(Hzdaaen)J (174; R = (CH2)2, R’= R”= Me)897 and [VO(Hzdaanpn)l.CHCb (174; R = (CH,),, R’ = R” = Me).897In the case of ligands (178;R = M e , Et), the outside 0202 donor set shows a low tendency to coordinate and V02+ coordinates in the N202 site as was found for the Ni2’ complex.898
.
(175)
.Ph
..
A
(176)
To prepare pure heterobinuclear complexes, it is important to minimize the possibility of producing homobinuclear chelates and to avoid the problem of the metal ions changing positions, i. e. positional isomers. [Ni(H2baaen)] (175) was the starting material for binuclear complexes including [NiVO(baeen)] (176) ,896 As Ni2+ in (175) has a relatively strong selectivity
562
Vanadium
NZ02,it can be easily prepared as a pure mononuclear N202positional isomer and used for the synthesis of binuclear complexes (equation 67). In (176),the NiZ+,coordinated in a s uare planar geometry, is diamagnetic with the entire paramagnetism arising from V02+;8g9peff values at 297 and 77K are 1.71 and 1.73BM.896The vanadium is 0.616A above the 0202 ‘plane’ in a square pyramidal geometry; the Ni-V distance is relatively short (2.991 A). [CuVO(daaen)].HzO (179) and [NiVO(daaen)].HzO came from the parent Cu and Ni complexes by a reaction similar to equation (67), and [(VO)2(baaen)] from [VO(H2baaen)]-MezC0.m The crystal structures of (179) and of the mononuclear V02+ complexes [VO(H2daaen)] and [VO(H2daanpn)] were determined (Figure 39) .897 Cu-V distance in (179) is 2.985A and [VO(H2daaen)] crystallizes in two forms (monoclinic and triclinic), but the corresponding molecular structures are indistinguishable. The molecular structure of [(VO)2(daaen)], only a small proportion of the molecules in the structure of [VO(H2daaen)], was proposed and is shown in Figure 39(d);8w one important feature is that both vanadyl bonds point in the same direction and thus the vanadium atoms are probably in good positions for relatively strong V. * . V interaction. to
Figure 39 The molecular structure of (a) [VO(H,daaen)], (b) [VO(H,daanpn)], ( c ) [CuVO(daaen)].H,O (179) and (d) [(VO),(daaen)]. In all four molecules, the vanadium atom is coordinated in a square pyramidal geometry and i s displaced above the O,O, ‘plane’ by cu. 0.56 8, (for (d), no data are given)897
As mentioned above, in the heterobinuclear preparations the presence of both mono- and homobi-nuclear V02+ complexes was d e t e ~ t e d ; ”two ~ alternative explanations were suggested: (i) any HC1 present in the CHC13 used for recrystallizing could strip the metals from the ligand and allow subsequent incorporation of V0 2 + into the outer compartment; (ii) during the synthesis, the mechanism depicted in Scheme 23 could take place. These observations suggest that in solutions of the heterobinuclear complexes, several species are probably present and the product isolated is a function of the insolubility of the individual species. When VOC12 was reacted with the p, d-triketones 1,5-bis(p-methoxyphenyl)l,3,5pentanetrione (H2dmba), 1,5-diphenyl-l,3,5-pentanetrione (Hzdba), 2,4,6-heptanetrione (Hzdaa) or the P-diketophenols 2-acetoacetylphenol (H2aap) and 2-benzoylacetylpheno1 (H2bap) in the presence of 2 moles of LiOH, mononuclear complexes were normally obtained.w1 With cations other than V02+, e.g. NiZ+and Co2+,binuclear complexes were the usual result. With ketophenols, mononuclear chelates can be obtained but are only stable at
Vanadium
(E'.:"
nn 0
563
i-ErOH
v metal lenchange
Scheme 23
low temperatures. With the exception of [VO(Hbap),(OH)], which is yellow and diamagnetic (it is a Vv compound), p:& values of the complexes obtained (in the range 1.65-1.73 BM) are those expected for magnetically dilute V02+ complexes. While attempts to prepare heterobinuclear complexes starting from the mononuclear chelates were unsuccessfui, green or red oxovanadium(1V) homobinuclear complexes may be obtained (depending on the reaction conditions) with ligands dha and dmba, while only one type of complex is obtained with all other ligands. The main difference in the IR spectra is that the green compounds show only one v(V=O) at 985-990cm-l, while the red ones have an additional strong band at 890-900 cm-' suggesting V 4 - - -V=O interactions.902The pL$ of the binuclear complexes are lower than the spin-only values except for [(VO)2(bap)20H]; &.& = 1.98 BM, suggesting the presence of a VO" and a VOW in the molecule.901The authors also report results of polymerization tests with these V02+ complexes as Ziegler-Natta catalysts. The features of the copolymer obtained are similar to those from widely used industrial catalysts but while the mononuclear complexes showed catalytic activities comparable to those of [V(acac),] and [V0Cl3], dinuclear compounds show much lower activities. The complex [(VO)z(dana)2] (dana = 1,5-bis(p-methoxypheny1)-1,3,5-pentanetrionato)was prepared and temperature dependent xM measurements show antiferromagnetic behaviour with 1 = -80 cm-1.902 The IJI value is much lower than with [M2(dana)z(py)2](M = Co, Cu) and this probably results from a different spatial orientation of the exchanging electrons: if the unpaired electron is considered to be initially in a dxyorbital a direct metal-metal interaction may be possible; as the V-V distance is large (ca. 3.0-3.2 V atoms are probably 0.5-0.6 out of the plane and possibly one above and one below the ligand planew2), one would expect a weak exchange for the direct V-V interaction. A V02+ complex with the compartmental ligand (177) was obtained by the dropwise addition of a VOS04 solution to an aqueous acetone solution containing LiOH and the ligand.9o3The complex is monomeric and the coordination involves the four 0 atoms. With Ni2+ and Cu2+ both mononuclear ositional isomers N202and 0202may be prepared according to conditions, but with VO' only the 0202 positional isomer is obtained. From the reaction between isophthalaldehyde (Hfsal)and substituted derivatives and aminophenol in boiling methanol, the ligand H3fsal(ap)z (180) may be obtained, which in its trianionic form may act as a binucleating agent. Several V02+ complexes with SBs (180; R' =M e and R variable) were studied by Okawa et al. and were formulated as
A;
coc3-s
564
Vanadium
CH*
/
Me
Me
Me (181)
[(V0)2{fsal(ap)2}(OMe)] with a structure (181).904v(V=O) of the complexes is at about 990cm-' except for (181; R = N 0 2 ) for which v(V=O) =900cm-'. All the ESR spectra are comparable and show bands at about 3200 and 1600 G. From magnetic susceptibility data, the IJ I values were obtained and increase with the increasing order of electron-withdrawing abilities of the substituent R. Since direct coupling between dxy orbitals seems to be the determining factor for the spin-exchange interaction in the binuclear V02' complexes, one may expect that the overlapping of these orbitals will increase in the order R ' = M e > H > C l ; in fact the ]Jl values increase in the same order.'= Oxovanadium(1V) complexes with SBs (182;n = 1 , 2 , 4 ) were prepared and formulated as [(V0)2(SB)(H20)2 having coordinating sites at the azomethine nitrogen and enolic and phenolic 0 atoms& pa.; values are in the range 1.62-1.70BM, close to the spin-only value, but xMmeasurements as a function of temperature were not performed. H\ /OH
/
'C=N-NHC(
II
0
H
CH,), CNH-N=C
II
Selbin and Ganguly obtained complex (183) with R' = R2= R3= H by reacting [VO(salen)] and a Cu" halide.906 The p:i of 1.69-1.90BM per complex molecule was considered as indicating the presence of strong antiferromagnetic interaction between the Cu2+ and V02+ ions. However, Okawa and Kida" argue that the large g value determined from ESR measurements is not compatible with the low pi; and prepared Selbin's compounds and four other new complexes (R1, R2 or R3= Me, X = Br or Cl). They obtained much lower pk& than the values reported by Selbin and from xM measurements at several temperatures" concluded that most of these compounds are diamagnetic, the paramagnetism being attributed to a paramagnetic impurity such as monomeric Cu2+ and V02+ complexes. In order to explain the diamagnetism of the present complexes in terms of the antiferromagnetic spin-exchange interaction between V02+ and Cuz+, one would have to consider a strong spin pairing between the two metal ions and this seems unlikely since their ground state configurations, (d,)' for V02+ and (dxz+)' for Cu2+, are not appropriate €or strong interaction (see also Section 33.5.12.2). A differem explanation, suggested by Okawa and Kida for the diamagnetism of
Vanadium
565
complexes (183) is that they are composed of Vv and CUI, and thus the metal ions have no unpaired electrons at all. R’
(183) X = C1, Br
Selbin and Ganguly also reported the preparation of the heterotrinuclear complex (184) with au ,& : value of 3.93BM,906and the authors infer the absence of magnetic exchange in the complex although one would expect a value of only 3.OBM if this were the case. Orbital contribution, especially from the Cu atom, may be a partial explanation, but magnetic exchange is also probably involved.3s8 Polynuclear complexes with more than two metal ions have been prepared with polyketone ligands but no such complexes have been reported with V 0 2 + . With the tetraketone 1,1’-(2,6-pyridyl)-bis-1,3-butanedione(H2L) , several polynuclear complexes have been prepared including [(VO)2(L)2] for which p:& = 1.21 BM was obtained.908
33.5.12.2 Heteropolynuclear complexes Studies involving heteropolynuclear complexes containing VIv have been discussed throughout this review. Magnetic interactions (V02+-VOz+ and V02+-Cu2+) have been mentioned. A study comparing the interaction in Cu2+-Cu2+, V02+-V02+ and Cu2+-V02+ pairs through the same oxalato bridging ligand was reported.909 Two new bimetallic complexes were synthesized, namely [(acac)VO(C204)VO(acac)]-4H20and [(trnen)Cu(C204)VO(C~04)].3H20 (tmen = N , N , N ’ ,N’-tetramethylethylenediamine) and the crystal and molecular structure of [(tmen>(H20)Cu(C204)Cu(H20)(tmen)](C104)2 was determined (Figure 40). The energy gaps arising from the intramolecular interaction, determined from magnetic data, are 385.4, 5 -75 and 4 c m - l in the CuCu, VOVO and CuVO compounds, respectively. The authors rationalized the results according to an orbital model in which the ‘magnetic orbitals’ are and a&), defined as the singly occupied molecular orbitals for each monomeric fragment (avo made up by the metal centre surrounded by the nearest neighbour ligands. The ‘magnetic orbital’ avoaround V02+ in the VO05 chromophore, schematized in Figure 40(c), transforms as a’ (referring to C, site symmetry) and is delocalized towards the oxygen atoms of the bridge with n metal-oxygen overlaps. Since the JG overlaps are weaker than the 0 ones, the delocalization towards the bridge is less pronounced in avothan in QCu. In the VOVO compound the interaction between the two ‘magnetic orbitals’ is weaker than in the CuCu compound since the delocalization of Qv0 towards the nearest neighbour oxygen atom is less pronounced. In the CuVO compound the two magnetic orbitals are strictly orthogonal but as they do not give any region of strong overlap density this explains why the triplet and singlet states are found to be almost degenerate. A rather different situation is found in the complex [CuVO{(f~a)~en)(MeOH)] (185)where ( f ~ a ) ~ e n denotes ~the binucleating ligand derived from the SB N,N’-(Zhydroxy-3carboxybenzylidene)-1,2-diaminoethane. The crystal structure is made up of heterobinuclear The thermal units (Figure 41a) with the MeOH molecule weakly bound to the Cu atom.910*911 variation of X M shows that upon cooling from 300K to around 50K,x M T increases and this clearly demonstrates that the triplet state (S = 1) is actually the lowest in energy. Thus, the intramolecular coupling is ferromagnetic and the separation between the triplet and singlet states was found to be 118 cm-l. The nature of the intramolecular interaction was explained by the orthogonality of the ‘magnetic orbitals’ Qcu and avo,centred on the Cu2+ and V02+ ions (see Figure 41b). The overlap integral of Qc,, and Qvo is identically zero and this means that
Vanadium
566
v
(a)
-
U
(b) (C)
proposed for (a) [(acac)Vo(~0,)VO(acac)]~4H20 and (b [(tmen)(H20)Cu(C,04)VO(Cz04)]~3H,0909 (only the structure of [(tmen)(HzO)Cu(CzO,)Cu(tmen)(H,O)]Z* wa: determined by X-ray diffraction); (c) schematic representation of the ‘magnetic orbital Qv0’ in the VOO, chromophorem’
Figure
40 Schematic
structures
there is no interaction between the orbitals favouring the pairing of electrons on a molecula: orbital of low energy. In order to explain why J is so large, the authors emphasize that thr entire molecular symmetry is important in deciding the nature of the coupling and not only thc local symmetry for each A-0-B linkage.”’ In the Cu2+-V02+ pair, the strict orthogonalit! which leads to ferromagnetic interaction occurs only if the phases at both bridging oxyger atoms are taken into account.
FEgure 41 (a) schematic representation of the molecular structure of [CuVO( (fsa),en)(MeOH)] (185);911(b) relativ symmetries of the ‘magnetic orbitals’;g11 (c) schematic representation of the geometry around V02+ in the trinucle; cation [VO(CuHAPen),12+ (186)9’4
Copper complexes can act as bidentate ligands towards transition metal ions.912.913 On mixin a solution of CuHAPen in CHC13 (CuHAPen = N,N’-ethylenebis(o-hydroxyacetophenont iminato)copper(II)) with an ethanol solution of VO(C104), a precipitate formed. Afte recrystallization from nitroethane the compound [V~(CUHAP~~)~](C~O~),.~H (186)was obtained and its crystal structure determined.914The two CuHAPen complex ligand bind to a central hexacuordinate V 0 2 + occupying nonequivalent positions in a distorte octahedron forming a dipositive trinuclear cation (see Figure 41c). The V atom lies 0.38 1 above the ‘equatorial plane’. The Cu(1)-V, Cu(2)-V and C u ( l W u ( 2 ) distances are 3.13: 3.015 and 4.558 A, respectively. The temperature dependence of xMof (186) was studied in th range 4.2-300 K;914the ground state of the trinuclear cation is a spin doublet, with the quartt separated by -3 cm-’, and a second doublet at -85 cm-’ ( =JI2). The large ferromagneti constant J12is associated with the coupling between Cu-2 and V,based (i) on comparisons wit other similar copper complexes and with (185) and (ii) on the geometry of the Cu-V bridge and the symmetry of the ‘magnetic orbitals’ of Cu-1, Cu-2 and V. Oxovanadium(1V) complexes were also prepared by the reaction of the phosphonate mnior (187)and (189) with [VO(acac),] (equations 68 and 69), and the products were characterize by elemental analysis and mass spectra.”’ In contrast to the reaction of (187) with [VO(acac) which gives (188) with p g = 2.03 BM, in the reaction of (189)only one acac ligand is displace by the organometallic chelate. Allowing TiC13(THF)3 and [VO(acen)] to react in THF at room temperature, a reducth deoxygenation of the V02+ occurs and a red solid precipitates, characterized as [V”
Vanadium vleo\
/OMe P-0
4-
2CSHsNi ; - +VO(acac), \p-O
567
-
/\ Med
'OMe
Me0
(187)
I
\'..
Me0
O Me
(188)
Me0 OMe \ /
/$C5H,Pd -
OMe M e 0
+VO(acac),
OMe
(189)
-
/
C,H,Pd
Me0
\I//v OMe
-C'
--.\ ,: ,CH \
Me
(190)
(acen)(Cl)(THF)] (4) (Section 33.4.8.1).916However, if the reaction is carried out at low temperature (ca. -7O"C), a red crystalline solid precipitates which was shown to be the This is bimetallic vanadium(1V)-titaniurn(1V) p-oxo complex [(Cl)(a~en)V0Ti(Cl)~(T€€F)~]. paramagnetic, with one unpaired electron and is probably an intermediate in the reductive deoxygenation of [VO(acen)] to yield (4). X-Ray data show that the p - 0 atom bridges the [V(Cl)(acen)] and [Ti(Cl),(THF),] units with a rather long V-0 bridging distance: 1.937 A (or 1.973A?); the C1- is coordinated trans to the p - 0 and the V atom lies approximately in the plane of the N202 core of the acen ligand, its displacement from it being 0.042A towards chlorine.916
33.5.13
Naturally Occurring Ligands
This subject covers a wide range of compounds and many of them have been included in previous sections. Most investigations concerning complex formation between V02+ and proteins have been carried out with VO" acting as a physicochemical marker of the binding sites, In recent years, there has been growing interest in employing V02+ in protein studies because of its chemical and spectroscopic properties. Vanadyl forms hundreds of stable complexes with almost all kinds of ligands; thus, it may coordinate with the functional groups encountered in proteins. Since vanadium(1V) complexes are dl systems, the interpretation of optical spectra is relatively simple. Further, oxovanadium(1V) has one position occupied by the vanadyl oxygen and the IR stretching frequency of the VO bond can be used as an additional probe of metal-binding sites in crystalline proteins where the V02+ concentration is sufficiently large.917 As vanadyl complexes consistently give sharp ESR spectra both in frozen and room temperature solutions and 5'V is nearly 100% abundant, ESR is particularly useful and there has been a growing interest in employing V02+ as an ESR spin probe. ENDOR studies have also been performed and future work using this technique and electron spin echo (ESE) will possibly be of great value in giving an insight into V02+ binding site structure. The use of V02+ as an ESR spin probe was reviewed by C h a ~ t e e nA . ~few ~ ~ studies have appeared since, e.g. ESR studies of the V 0 2 + apoferritin complex918and the V02+-S-adenosylmethionine synthetase i n t e r a c t i ~ n , ~ a 'CD, ~ ESR and acid-base study of V02+ binding to bleomycinYw0 and redox kinetics and complexation of VIv and Vv with serum proteins.945Some equilibria in solution, likely coordination geometries and ligand contributions to the observed hyperfine splitting An were also Since the discovery by Cantley et al. that vanadate is a powerful and allosteric inhibitor of Na+,K+-ATPase,921vanadium began to interest biochemists; and Chasteen has reviewed work published up to 1983.471 Particularly interesting new studies involve vanadium peroxidase^'^ and nitrogenase^.^^' Waltermann et al. studied the nonenzymatic hydrolysis of ATP by V02+ and V02+ coupled
Vunadium
5 68
with Hz02 and concluded that oxidation of VIv to Vv is an important factor in enhancement of ATP hydrolysis.”’ The authors also observed that ATP protects the VOZf from oxidation by H202and from ESR spectra they concluded that binding occurs between V02+ and ATP in aqueous solution. Sakurai et al. studied the formation of complexes between ATP and V02+ by potentiometric titrations, optical and ESR spectra and by 31Pand 13CNMR s p e c t r ~ m e t r y . ~ ~ ~ They concluded that as pH increases three different complexes form and proposed detailed coordination geometries for each of them. At acidic and neutral pH, two different 1:1 species predominate and in alkaline solutions a green 2 : 1 complex forms. The authors argue that 31P and 13CNMR are very effective for determining coordination sites in compounds containing organic phos hate and that the detailed structures for the ATP-VO*+ complexes can thus be determined.9Y4 Complexes formulated as [VOC12(HL)] [HL = purine (191), adenine (192) and guanine (193)] were prepared by refluxing the ligand and VOClz mixtures in ethanol-triethyl orthoformate. 25,926 Characterization was based on elemental analysis and spectral and magnetic studies. In the visible spectra (Nujol mulls) the d-d transitions were at -695, -500 and -430nm, and in the IR, v(V=O) was assigned at -1005cm-’; p:;=l1.7BM. These V0Clz adducts are said to be polymeric with a linear, chain-like structure (194) involving single bridges of bidentate ligands between adjacent V 0 2 + ions, the bridging probably being through N-7 and N-9 of i m i d a z ~ l e .This ~ ~ ~suggestion , ~ ~ ~ is based on IR evidence and by comparison with the molecular structure of [CU(HL)(H~O)~](SO~)-~H~O (HL = 191) .9’7
(191)
(194
(193)
(194)
Several V 0 2 + complexes with nucleotides have been prepared by the addition of aqueous VOS04 to an aqueous solution of the nucleotide.928 The compounds were formulated as [VOL].nHzO with a suggested polymeric structure. As in the case of the complexes with the purines (191)-(193), further work is needed to sort out the molecular structure of these compounds. Amavadin, a natural compound which occurs in the toadstool Amanita muscuriu has been said to be a 1:2 V 0 2 + complex of Hidpa (131; R = OH).378,816,92sY34,y48~949 Fra6sto da Silva and co-workers studied the formation of complexes of VO” with ligands of type (132) by pH potentiometric methods and concluded that with a ligand-to-metal ratio of 2, only 1:1 complexes form (see Table 45) except for the N-hydroxy derivatives (132; R = OH). With these ligands (e.g. Hidpa), the pK, associated with the N group drops -10000 times and at pH -7 the 2 : 1 complexes predominate. However, the formation constants ~ a l c u l a t e d ~ ~ ~ * seem low to explain the existence of amavadin in the toadstool and its ‘intact’ isolation after several extraction and chromatographic procedures, and further studies are needed to establish the detailed structure of amavadin and understand why V02+ binds N-hydroxyiminodiacetate ligands so strongly. Different formation constants have recently been estimated.948 Several studies have concerned interaction of V 0 2 +with soil humic and fulvic acids. Strongly corn lexed VOZ+occurs naturally in the organic fractions isolated both from lignite and ball clayg5 and from soi1.936,y37Since the ESR spectra of VOz+ provide information on the environment of the metal, much can be learned about the interactions between vanadium and fulvic and humic acids using V02+ as a paramagnetic probe, and several studies have appeared with V 0 2 + synthetically introduced to probe the metal-binding sites of these materials.93s94i The ESR parameters indicate that in most cases the ligands are oxygen donors, probably from carboxylate and phenolate groups. Nitrogen coordination has also been suggested938but, as noted by C h a ~ t e e n , ~catechol ’~ coordination complexes have V02+ ESR parameters which are comparable to those obtained with nitrogen chelates. M ~ B r i d e ~ studied ~’ the influence of pH and metal ion content on VO2+-fukic acid interactions and concluded that very low pH values prevent V02+ from bonding with fulvic acid and that insignificant quantities of free V02+ remain in solution at pH values of 4 or higher. Very high pH caused all the V02+ associated with fulvic acid to be oxidized except for a very small quantity apparently strongly bound to four nitrogen ligands, possibly porphyrin. V 0 2 + bound to wet humic acids exhibits a rigid limit ESR spectrum indicative of a highly
Vanadium
569
immobilized probe ion.939The ESR parameters suggest that bonding is mainly ionic. Room temperature solution ESR spectra of V 0 2 + complexes with podzol fulvic acid exhibit reduced intensity due to VOZc-VO2+ magnetic dipole interactions within the ‘same fulvic acid molecule940and V02+ binding facilitates aggregation of fulvic acid to form a soluble polymeric system.
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570
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571
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Vanadium
583
R. Chandra and R. N. Kapoor, Ann. Chim. (Rome), 1982, 72,309. J. Selbin and L.Ganguly, Inurg. Nucl. Chem. Lett., 1969, 5 , 815. H. Okawa and S. Kida, Inorg. Chim. Acta, 1977,23,253. D. E. Fenton, J. R. Tate, U. Casellato, S. Tamburini, P. A. Vigato and M. Vidali, Inurg. Chim. Acta, 1984,83, 23. 909. M. Julve, M. Verdaguer, M. F. Charlot, 0. Khan and R. CIaude, Inurg. Chim. Acta, 1984, 82, 5 . 910. 0. Khan, P. Tola, J. GaIy and H. Condanne, J. Am. Chem. Soc., 1978,100, 3931. 911. 0. Khan, J. Galy, Y. Journaux, J. Jaud and Y. Morgenstern-Badarau, J. Am. Chem. Soc., 1982, 104, 2165. 912. E. Sinn and C. H. Harris, Cuord. Chem. Rev., 1969,4, 391. 913. K. A. Leslie, R. S. Drago, G. D. Stucky, D. J. Kitto and J. A. Broee, Inorg. Chem., 1979, 18, 1885. 914. A. Bencini, C. Benelli, A. Dei and D. Gatteschi, Inorg. Chem., 1985, 24, 695. 915. H. Werner and T. N. Khac, Znorg. Chim. Acta, 1978, 30,L347,. 916. M. Mazzanti, C. Floriani, A. Chiesi-Villa and C. Guastini, Irzorg. Chtm., 1986, 25, 4158. 917. N. D. Chasteen, R. J. Dekoch, B. L. Rogers and M. W. Hanna, J. Am. Chem. SOC., 1973.95, 1301. 918. N. D. Chasteen and E. C. Theil, J, Biol. Chem., 1982, 257,7672. 919. G. D. Markham, Biochmistry, 1984,23,470. 920. L. Ban& A. Dei and D. Gatteschi, Inorg. Chim. Acta, 1982, 67, L53. 921. L. C. Cantley, L. Josephson, R. Warner, M. Yanagisawa, C. Lechene and G. Guidotti, J. Biol. Chem., 1977, 352,7421. 922. G. M. Wolterman, R. A. Scott and G. P. Haight, Jr., J . Am. Chem. Suc., 1974,%, 7569. 923. H. Sakurai, T. Goda, S. Shimomura and T. Yoshimura, Biochem. Biuphys. Res. Commun., 1982, 104, 1421. 924. H. Sakurai, T. Goda and S. Shirnomura, Biochem. Biophys. Res. Cummun., 1982,108,474. 925. C . M. Mikulski, S . C m o , N. Franc0 and N. M. Karayannis, Inorg. Chim. Acta, 1983, 78, 125. 926. C. M. Mikulski, S. Mattucci, L. Weiss and N. M. Karayannis, Inorg. Chim. Acta, 1984, 92, 275. 927. P. I. Vesnes and E. Sletten, Inorg. Chim. Acta, 1981, 52, 269. 928. N. Katsaros, Transition Met. Chern. (Weinheim, Ger.), 1982, 7 , 72. 929, E. Bayer and H. Kneifel, 2. Naturforsch., Teil B, 1972, 27, 207. 930. H, Kneifel and E. Bayer, Angew. Chem., lnt. Ed. Engl., 1973, U,508. 931. R. D. Gillard and R. J. Lancashire, Phytochemistry, 1984,23, 179. 932. G. Bemski, J. Felcman, J. J. R. Frahsto da Silva, I. Moura, J. J. Moura, M. Candida Vaz and L. F. Vilas-Boas, Rev. Port. Q u h . , 1985, 27, 418. 933. H. Kneifel and E. Bayer, J . Am. Chem. Soc., 1986,108, 3075. 934. E. Koch, H. Kneifel and E. Bayer, Z. Naturforsch., Teii B, 1986, 41, 359. 935. P. L. Hall, B. R. Angel and J. Braven, Chem. Geol., 1974, W, 97. 936. M. V. Cheshire, M. L. Berrow, B. A. Goodman and C. M. Mundie, Geochim. Cusmochim. Acta, 1977,41,1131. 937. A. L. Abdul-Hahm, J. C. Evans, C. C. Rowlands and J. H. Thomas, Geochim. Cosmochim. Acta, 1981,45,481. 938. B. A. Goodman and M. V. Cheshire, Geochim. Cusmochim. Acta., 1975,39, 1711. 939. M. B. McBride, Soil Sci., 1978,126,209. 940. G. D. Templeton and N. D. Chasteen, Geochim. Cosmochim. Acta, 1980, 44,741. 941, M. B. McBride, Can. J . SoiISci., 1980, 60, 145. 942. T.K. Myser and R. E. Shepherd, Inorg. Chem., 1987,26, 1544. 943. P. Comba and A. M. Sargenson, Aust. J. Chem., 1986, 39, 1029. 944. H. D. S. Yadava, S. K.Sengupta and S. C. Tripathi, Inorg. Chim. Actu, 1987, 128, 1. 945. N. D. Chasteen, J. K. Grady and C. E. Holloway, Inorg. Ckem., 2986, 25, 2754. 946. E. de Boer, Y.van Kooyk, M. G. M. Tromp, H. Plat and R. Wever, Biuchim. Biophys. Acta, 1986,869,48; E. de Boer, M. G. M. Tromp, H. Plat, B. E. Krenn and R. Wever, Biochirn. Biuphys. Acta, 1986, 872, 104; H. Vilter and D. Rehder, Inorg. Chim. Acta, 1987,136, L7. 947. R. L. Robson, R. R. Eady, T. H. Richardson, R. W. Miller, M. Hawkins and J. R. Postgate, Nurure (London), 1986, 322, 388; J. M. Arber, B. R. Dobson, R. R. Eady, P. Stevens, S. S. Hasnain, C. D. Gamer and B. E. Smith, Nature (London), 1987, 325, 372. 948. G. Anderegg, E. Koch and E. Bayer, Inorg. Chim. Acta, 1987,l27, 183. 949. M. A. Nawi and T. L. Rieckel, Inorg. Chim. Acta, 1987,136, 33.
905. 906. 907. 908.
34
Niobium and Tantalum LlLlANE G. HUBERT-PFALZGRAF, MICHELE POSTEL and JEAN G. RIESS Universite de Nice, France 34.1
586
SURVEY
34.2 OXIDATION STATE +V 34.2.1 Pentahalides 34.2.1.1 Pentafuorida 34.2.1.2 Pentachlorides 34.2 1.3 Pentabromides 34.2.1.4 Pentaiodides 34.2.1.5 Mixedpentahalides 34.2.1.6 Applications of the pentahalides 34.2.2 Halo Adducts 34.2.2.1 Anionic halo compiexes 34.2.2.2 Neutral adducts of the pentahalides 34.2,3 Solvolysis Products of the Pentahalides 34.2.3.1 Alkoxides 34.2.3.2 Halide alkaxides and related compounds 34.2.3.3 Thiolato and dithiocarbamato derivatives 34.2.3.4 Amido derivatives 34.2.3.5 Nitrenes 34.2.3.6 Dinitrogen acrivation 34.2.3.7 Azides 34.2.3.8 Nitrides 34.2 3.9 Miscellaneous 34.2.4 Pseudohalo Derivatives 34.2.4.1 Cyanides and isocyanides 34.2.4.2 Thio- and selenoxyanates 34.2.5 Oxo, Thio and SeIeno Halides 34.2.5.1 Oxo halides 34.2.5.2 Thio and seleno halides and their complexes 34.2.6 Complexes of the Oxo Halides 34.2.6.1 Anionic oxohalo complexes 34.2.6.2 Neutral adducrs of the oxo halides and related compounh 34.2.6.3 Miscellaneous oxygen abstraction reaction products 34.2.7 Solvolysis Products of the Oxo Trihalides 34.2.7.1 From oxygen compounds 34.2.7.2 From sulfur compounds 34.2.7.3 From nitrogen compounds: oxo amides and oxo isothiocyanates 34.2.8 Oxide- and Sulfidederived Compounds 34.2.8.1 Oxides and peroxides 34.2.8.2 Suljides, selenides and related compounds 34.2.8.3 Other oxygen- or sulfur-substituted metal compounds 34.2.9 Miscellaneous Metal-Element Derivatives
34.3 OXIDATION STATE +IV 34.3.1 Tetrahulides 34.3.2 Halo Complexes 34.3.2.1 Haiometallates (IV) 34.3.2.2 Neutral adducts of tetrahalides 34.3.3 Pseudohalo Complexes 34.3.4 Oxo, Thio and Seleno Halides 34.3.4.1 Oxo halides and oxo compounds 34.3.4.2 Thio, seleno and telluro halides 34.3.4.3 Adducts of the thio and seleno halides 34.3.5 Solvolysis of the Tetrahalides 34.3.5.1 Alkoxides 34.3.5.2 i3-Diketones,carboxylates and related chelates 34.3.5.3 Thiolato and related derivatives 34.3.5.4 Alkylamides and related compounds
585
589 5 89 589 590 590 590 591 591 591 591 594 600 MXI 603
607
w
613 617 619 620 620 623 623 624 625 425 626
626 626 628 63 1 632 632
634 634 635 635 638 638 639 639 639
640 640 640 645 646 646 646 647 649 649 650
651 652
Niobium and Tantalum
586 31.3.6 Chalcogenides 34.3.7 Activation of Srndl Molecules
654 654
34.4 OXIDATION STATE +I11 34.4.1 Survey 34.4.2 Halo Adducts 34.4.2.1 Anionic halo complexes 34.4.2,2 Neutral adducts of the trihalides 34.4.3 Activation Reactions Inuolving Niobiurn(ll1)and Tantalum(lll) 34.4.3.1 Activation of small molecules 34.43.2 Reactions with unsatvrated substrates 34.4.4 Solvolysis Products of the Trihalides 34.4.4.1 Alkoxides and related compounds 34.4.4.2 @-Diketonatesand carboxylates 34.4.4.3 Thiolates and dithiocarbamates 34.4.4.4 Dialkylphosphides 34.4.4.5 Nitrenes 34.4.4.6 Organometallics 34.4.5 Pseudohalo Derivatives 34.4.6 Miscellaneous 34.4.6.1 Chemistry in aqueous solution: suljato, oxalato and related compounds 34.4.6.2 Other products
655 655 655 655 656 660 660
34.5 NON-INTEGRAL OXIDATION STATES 34.5.1 Survey 34.5.2 Triangular Clusters 34.5.2.1 Binary halides 34.5.2.2 Adducts of the neutral halidm 34.5.2.3 Solvolysisproducts 34.5.2.4 Clusters with the hexamethylbenzene ligand 34.5.3 Tetranuclear Clusters 34.5.4 Octahedral Clusters 34.5.4.1 Clusters based on [he (M6X,2)q' core 34.5.5 Clusters Based on the ( M 6 X J + (q = 2, 3) core
667 667
34.6 OXIDATION STATE +I1 34.6.1 Dihalide Adducts 34.6.1.1 Synthesis and structure 34.6.1.2 Reactivity of the dihalo adducts 34.6.2 Miscellaneous
677 677
34.7 OXIDATION STATE +I 34.7.1 Phosphine Adducts 34.7.2 Carbonyl Derivatives 34.7.2.1 Non-cyclopentadienylcarbonyls 34.7.2.2 Cyclopentadienyl carbonyls 34. Z 3 Alkene and Diene Adducts
679 679 680 680 682 682
34.8 OXIDATION STATE 0
683
34.9 OXIDATION STATES LOWER THAN 0 34.9.1 Oxidation State -I 34.9.1 1 [M (C0)J 34.9.1.2 Other derivatiues 34.9.2 Oxidation State -III 34.9.2.1 [M(CO)J3-and [HM(CU)J'34.9.2.2 [M (CO),(Cp)]'-
684
34.10 MIXED VALENCE COMPOUNDS
685
34.11 REFERENCES
686
a
661 663 664
664 m14
665 665 665 666
666 666 667
667 647 668 668 669 670 670 671 677
677 67 8 679
684
6M 684 684 684
685
34.1 SURVEY Niobium (formerly called columbium) and tantalum, when compared with the neighboring early transition metals, are seen to share with titanium and zirconium an affinity for dioxyger
Niobium and Tantalum
587
and a reluctance to be reduced, but to differ from them in their much stronger tendency to form metal-metal bonds and in their rich cluster chemistry. With molybdenum and tungsten they share a propensity to yield oxo species and an ability to give multiple bonds with non-metals, as in carbenes, alkylidenes, nitrenes, etc., and to form M6-based clusters, although these are mainly of the [M&.]*+ type for molybdenum and of the [Md(12]*+ type for niobium or tantalum. Carbonyls are generally much less stable for the group V than for the group VI metals. Roughly, it may be said that niobium and tantalum have chemistries more comparable to those of titanium and zirconium in their higher oxidation states, but closer to those of molybdenum and tungsten in their lower ones. Compared to vanadium, the higher oxidation states are much more frequently and the lower ones much less frequently encountered in niobium and tantalum. Niobium and tantalum are also much more prone to extended metal-metal bonding in their lower oxidation states; V6-based clusters, for example, remain undiscovered. Little resemblance is found to the group VB elements phosphorus and arsenic. The chemistry of niobium and tantalum ranges from oxidation state +V to -111, but with no species of oxidation state -11 presently known (Table 1). The largest number of molecular compounds, by far, is found for oxidation state V. The very reactive pentahalides provide the most convenient entry to the molecular chemistry of these metals. Table 1 Oxidation States and Common Stereochemistries of Nb and Ta Compounds Oxidation state
Coordination
number
idealized geometry
Examples
588
Niobium and Tantalum
Until recently little attention was paid to oxidation states lower than 111, with the exceptior of the octahedral clusters of type [M+, which are predominant in aqueous solutions Preparation of molecular compounds in these oxidation states was for a long time more or lee accidental unless cyclopentadienyl groups were present. This lack of development is mainly duc to the paucity until recently of suitable precursors (halides or carbonyls), as well as to the hig1 sensitivity to dioxygen and to moisture of products in such intermediate oxidation states ai +III. Chemical reduction of pentavalent or tetravalent derivatives under strictly controllet conditions has since proven able to provide convenient starting materials for developing thr chemistry of niobium and tantalum in their lower oxidation states. Powerful reductants such ai sodium amalgam or naphthdenide are required for access to oxidation states lower than I11 The least -documented chemistry of the formally positive oxidation states of these metals is tha of oxidation state +II. Among the most valuable starting materials now available for access to oxidation states IV tc I1 are the tetrahalides and some of their more soluble molecular adducts such a [NbC14(THF)2], [NbCL(MeCN)2].MeCN and [MC14(dmpe)z], adducts of the trihalides, ir particular [M2C16(PR3),], and their derived hydrides, [M2C16L3] (L = SMe2, THT) an( [MClz(dmpe),]. Pentavalent alkylidenes also offer attractive entries into the chemistry o oxidation states +I11 and + I . Oxidation state -I is mainly represented by [M(CO),]- and [M(PF3)6]-. The firs compounds isolated with a metal in the -111 oxidation state were Cs,[M(CO),] ani CS~[M(CO)~(C~)] with M = Nb or Ta. Niobium and tantalum compounds form adducts with virtually all types of neutral ani anionic donors. The coordination chemistry of the higher halides is widely developed, and thei activity as Friedel-Crafts catalysts is another manifestation of their Lewis acidity. The stron acceptor capacity of the high valent metal compounds tends to favor the formation of dimers and sometimes of higher condensation products, which competes with coordination with othe donor molecules. Numerous simple anionic or heteropolyanionic species, but little cationi chemistry, and no simple metal salts, are known. Specific to the lower oxidation states is the tendency to metal bonding found in variou arrangements, including isolated dimers (with single or double metal-metal bonds) , polymer with M-M - * M-M . . - bond alternation, triangular M3 units, and, the most abundanl octahedral M6-derived clusters. These clusters, with non-integral oxidation states ranging fror +2.67 for [Md(12]4+to +1.67 for [Nb6I8I4+exhibit complex redox chemistry that may be usel in photocatalysis. Being among the largest early transition metals, niobium and tantalum can easil accommodate high coordination numbers. Although coordination number six is the moz frequent, stable compounds with coordination numbers of seven (especially in the lowe oxidation states), eight, nine and even ten are found; in contrast, coordination numbers beloi five are extremely rare. The rate relatively large metal radii also account for the less tigf coordination sphere, and hence for the stereodynamic character often encountered in th complexes. Reactions other than Lewis acid-base associations/dissociations are frequently observed wit donor molecules, leading notably to solvolysis, oxygen or sulfur abstraction, insertion reaction and carbon-carbon coupling reactions. The tendency to form metal-element multiple bonds 1 remarkable; in this respect the activation of dinitrogen by tantalum or niobium is unique. Th formation and chemistry of constrained reactive me tallacycles open another promisin fast-developing area, on the frontier with organometallic chemistry. Owing to lanthanide contraction, niobium and tantalum have virtually identical atomic rad (1.47 A) and close ionization energies (Nb 6.67, Ta 7.3 eV), and usually display very simil: chemical behavior. Some definite differences can however be noted; these can usually be trace to the lower sensitivity of tantalum to reduction and to its higher affinity for dioxygen. Th tantalum-element multiple bonds are usually stabler, while M A arrangements are so fi known only for niobium. The 93Nb nucleus has 100% abundance, Z = 912, and the third highest sensitivity (after 'I and 19F and before "Al) in NMR; the use of this technique is therefore likely to expand. ?I (99.9877%), Z = 7/2, is less promising in view of its much lower sensitivity and its tenfold largr quadrupolar coupling. The magnetic moment in the NbIV derivatives usually ranges betwee 1.6 and 1BM the spin only value expected for d1 compounds being 1.73 BM). The magnet ,are usually lower than 1BM as a result, at least in part, of a larger spin-orb moments of Tai coupling (A = 750 cm-' for Nb4+ and 1400 cm-' for Ta4+). The magnetic susceptibilities usual
Niobium and Tantalum
589
exhibit a Curie law dependence upon reciprocal temperature, which strongly supports an orbital singlet ground state well separated from the next excited state. Typical isotropic ESR spectra display 10 lines for niobium and eight lines for tantalum derivatives, the line width being larger for the latter, probably owing to non-resolved quadrupolar coupling. The g values of the niobium compounds are closer to 2 than are those of tantalum, which is no surprise as the latter has a higher spin-orbit coupling constant. Magnetic data on the other oxidation states are still scarce. This review intends to cover essentially the molecular non-organometallic chemistry of niobium and tantalum. When organometallic compounds are mentioned, it is in principle only in so far as they are involved in some coordination process. The literature has been covered to the end of 1983; thanks to their authors, a few more recent preprints were able to be analyzed. Emphasis is given to the more recent material. Some of the presently most active areas such as those involving multiple bonding, dinitrogen activation and nitrenes are highlighted by subtitles. Earlier reviews on the subject are those of Fairbrother (1967),' Kepert (1972): Walton (1972),3 Gmelin (1973); Brown (1973)5 and Mehrotra (1976).6 The first review of the chemistry of the d1 metal compounds of early transition metals was published by Bereman in 1972.' For the organometallic chemistry of these elements see Labinger (1982).8 The following abbreviations and conventions will be used throughout this chapter: A: cation; Z: anion, M = Nb, Ta; M' # Nb, Ta; X: halogen; X and Y': halogens when mixed; Y = S, Se, Te; E = N, P, As, Sb or Bi; LL = bidentate; ax: axial; eq: equatorial; b: bridging; t: terminal. Ligands: bzac: benzoylacetone; bipy02: bipyridyl N , N'-dioxide; cat: catechol; Cp': q5-GR5; dbrnH: dibenzoylmethane; DEF: diethylformamide; DMA: dimethylacetamide; dpac: 1,2bis(dipheny1arsino)ethane; dmpe: 172-bis(dimethylphosphino)ethane; dpmH: dipivaloylmethane; dth: 2,5-dithiahexane; edta: ethylenediaminetetraacetate; oxH: 8-quinolinol (8hydroxyquinoline); pic: picoline; py0: pyridine N-oxide; R2dtc: dialkyldithiocarbamate; SB: Schiff's base; T: tropolone; THP: tetrahydropyran; THT: tetrahydrothiophene; TPO: triphenylphosphine oxide; triars: triarsine; TU: thiourea.
34.2 OXIDATION STATE +V
34.2.1
Pentahalides
The pentahalides of niobium and tantalum are predominantly covalent. Their volatility decreases from the fluorides to the iodides. Nb and Ta belong to the few elements which form stable pentaiodides. Selected physical and thermodynamic properties for the halides are listed in Table 2. All are sensitive to moist air, water or hydroxylic solvents. Table 2 Selected Thermodynamic Properties for the Pentahalidesasb M.p. B.p. ("C) ("C) 79 97 [NbCIJ 203.4 215.9 NbBr,] 254
234 229 247.4 232.9
[TaBr5] WI5I
344 543
[NbF,]
{TaF,]
p]256 [TaIs]
-
496
365
Density' (gcm-7 2.6995 3.8800 2.75
3.68 -
-
91.41 70.31 0.921 1.003 -
1.63 X 1.56 x
io-,
-1810
-1902 2 . 2 ~ ~-796 ' 3.0 X lo-' -857 -556
-
-
-598d -270 -293
60 54 63.5
55 93.5 61.5
75.5
94.5
-
89e 85.4' 116 106 82
12.2 397.15 - 222 - 378 - 38 245 101.5 180' 41.5 248 ias 1 n . 6 ~ - 305 132 198 45.5 305 99.5 184 343 - - 6.7 -
"From D. Brown, 'Comprehensive Inorganic Chemistry', ed. J. C. Bailar, Jr., H. J. Emeleus, R. Nyholm, and A. F. Trotman-Dickenson, Pergamon, Oxford, 1973, p. 553, unless specified. AH values: kJ mol-'; A S values: J mol-' K-'; all values measured at 298 K. At m.p. *A. D. Westland, Can. 3. Chem., 1979, 51, 2665. 'A. D. Westland and D. Lal, Can. 1. Chem., 1972,50,1604.
3421.1 Pentajuodes The pentafluorides are best prepared by direct fluorination of the metals at 250"C.1,2 Alternative methods include reactions of the metals with HF, ClF3 or BrF3, or SnF2. They have
590
Niobium and Tantalum
also been prepared from MC15 and HF or ZnF2. The synthesis of high purity NbF, by competitive Lewis acid-base reactions between [SbF5] and [MF& in HF has also been reported. The MF5 molecules crystallize as tetramers in which the metal atoms are approximately octahedrally surrounded by fluorine atoms, and are linked together by linear bridging fluorine atoms (M-Fb: 2.06 A; M-F,: 1.77 A)." It was suggested, on the basis of the high viscosity of the melts and the high Trouton constants, that this polymerization state is retained in the liquids2 Molecular weight data in the vapor, obtained through mass spectrometry and vapor density measurements, were interpreted to imply the simultaneous presence of monomeric, dimeric and trimeric species.11-13 However, a different model, which assumes mixtures of monomers and tetramers, has since been reported to fit the vapor density data better.14 Electron diffraction investigations also showed that the gaseous MF5 molecules are associated at lower temperatures;' thus, at the nozzle temperature of about 45 "C, TaF5 was found to be mostly trimeric (Ta-Fb: 2.062(2) to 2.076(3) A; Ta-Ft: 1.81(6) to 1.894(6) A).15 The molten pentafluorides ossess measurable electrical conductivities that were attributed to slight ( MBr5 for the associative one.97 NMR studies of [TaC14(dppe)(Cp')] showed exchange between the free and monodentate dppe. The rates of intramolecular exchange between equatorial and axial fluorine atoms in [MF5L]95 have been evaluated by "NMR for L=OMe2 and OP(NMe2)3-,(OMe),, where n = 0-3. NQR spectra have been measured for [MC&L], where L = nitriles, ethers and phosphine oxides.98 The electron transfer spectra of MX5 adducts of several series of ligands (nitriles, dialkyl chalcogenides, phosphoryl and thiophosphoryl ligands, phosphines)w have been reported. Intermolecular charge transfer spectra indicate that MCls interacts with aromatic hydrocarbons;lm the equilibrium constants showed these interactions, however, to be very weak. The donor capacity of the solvent towards NbC& was reported to decrease in the order: MezNCOH > MeCN > RCOOR' > RCOR' > R 2 0= C4H802> C6H6.101 [MCI,L] + L* I [MCl,L*] + L L = R C N , OPR3, Me,Y (Y = S,Se, Te)
Niobium and Tanralum
595
[MX&] has also been isolated for many 0, S, N, P and As donors. The MF5 derivatives consist of ionic [MF4L4][MF6]in solution, while molecular structures with seven-coordinate metals appear to be equally likely in the solid state.'02-'" The analogous chlorides and bromides seem to be essentially molecular.
(i) Oxygen donors MF5 reacts with aliphatic ethers to yield volatile adducts of the [MF5(OR2)]type; DMSO forms the bis adducts [MF5(DMS0)2].MC15 and MBr5 form numerous [MX5(ether)] adducts. Abstraction of oxygen from the ethers occurs at around 100°C to produce oxo trihalides and alkyl halides.' The ability of crown ethers to coordinate and reduce MC15 has been explored;'04 [(MCl,),L] and [MC15L] were isolated with L = dibenzo-18-crown-16, 18-crown-6 and 15crown-5. NMR data indicate that these compounds disproportionate. MC15, MBr5 and acetophenone or benzophenone yield 1:1 co rn p l e~es . '~~ TaCL,, when allowed to react with ethyl methyl ketone or diethyl ketone in a 1:l molar ratio, gave 1:l adducts, whereas oxygen abstraction occurred in the case of NbC15, resulting in the formation of [(NbC14),0(R2CO)2];106with excess ketone, [MOC13(R,C0)2] was obtained in both cases. Ketone complexes [NbCI,Lj were, however, isolated from benzene solutions.'07 The reactions of [MC15-,Me,] (x = 1,2) with bulky ketones yielded [MC15-,Me,(RR'CO)] for R = R' = cyclohexyl; R = Me, R' = But; R = Me, R' = neopentyl, while addition of the methylmetal group to the carbonyl was found to occur with less bulky ketones and with benzaldehyde.1U8 Dialkyl esters RCOOR' were allowed to react with NbCl5lWto yield 1:1 and 2: 1 complexes; the former, partially ionic in solution, were formulated as [NbC14(RCOOR')2][NbCl& In the [MX5L,] adducts formed with amides (X= CI, n = 1, 3, 4; X = B r , n = 1, 3; L = RCONH,, R = H, Me or Ph), the amide is bonded to the metal through its oxygen only.'" Urea complexes [MX,L]"' (X = C1, Br) have been prepared from MX5 and tetramethylurea in CH2C1,; coordination was shown by IR to be through oxygen only. On the other hand, oxathiane and oxaselenane gave [NbC15(C4H80Y)](Y = S, Se) in which, on the basis of IR and NMR data, they were proposed to be S- or Se-bonded, and not 0-bonded, to the metal.
(ii) Adducts of phosphine or arsine oxides, sulfides and selenides MF, forms [MF5L] and [M2F1&] complexes with phos horyl ligands.93,11zJ13 In the dimeric compounds, the ligand is cis to the bridging fluorine.' P With PSX3 and PSeX3, molecular pseudooctahedral [MF5L]complexes were obtained in which the ligands are bound to the metal through S or Se.'14 Atmospheric oxygen reacts with [MF5(SPX3)] or [hF5(SePX3)] to give [MF5(OPX3)] along with sulfur or selenium precipitate^."^ Oxygen coordination took place when MF5 was allowed to react with RzP(0)SR to give [MF5LJand [MzFloL].113A thione-thiol rearrangement was observed when thiophosphoric esters (Et0)2P(S)OMe were added to a solution of TaF5,'15 resulting in the formation of [TaF5{OP(OEt),SMe}]; in the presence of traces of oxygen [TaF5{OP(OEt)20Me}] was also detected. The same rearrangement occurred in a solution of TaF5 and Ph,P(S)OEt, but at slower rates, so that both [TaF5{SP(OEt)Ph2}] and [TaF5{OP(SEt)Ph,}] complexes could be identified along with (in the presence of air) ITaF5{OP(OEt)Ph2}]. Solutions of TaF5 and PBu3 yielded [TaF5(OPBu3)] upon aging.@ 9FNMR showed that equimolar solutions of MF5 and of R3AsY (Y =0, S; R = B u , Ph) contain molecular [MF5L] together with ionic [MF4L][MF6] compounds.'16 The following donor capacities towards TaF5 were derived from '% NMR chemical shifts: R3As0 > R3P0 > R3AsS z R3PS. Phosphine oxide complexes [MC15(R3PO)$(R = Ph, Me, NMe2)'17-1'9 and [MC15(R2R'PO)] (R = Ph; R' = Bz) have been prepared. Nb forms an oxochloro adduct in the presence of excess OPPh3.1,120When conducted in a non-basic solvent in the absence of MeCN, the action of HMPA on NbzCllo led to oxygen abstraction and formation of [NbOC13(HMPA)2],117 while the 1: 1 adduct [NbCl,(HMF'A)] formed exclusively when a mixture of MeCN and CH2C12was used as solvent, indicating the initial formation of a stable monomeric six-coordinate adduct [NbC15(MeCN)]. The 0-coordinated [MC15(OPCl,)] adducts are monomeric in benzene but dissociate in nitromethane.'*' The existence of the niobium adduct in the gas phase has also been established.'" The enthalpy of formation of the crystalline adduct from its components COC3-T
Niobium and Tantalum
596
was evaluated. ‘ 2 3 TaC& and NbC& were found to react readily with triphenylphosphole oxide to yield 1:1 MX5L (X = C1, Br; L = SPPh3 and SePPh3) was obtained; the formation of 2: 1 complexes was not observed with these ligands. The pentabromides, but not the pentachlorides, gave 1:1 complexes with Ph2P(S)CHzP(S)Phz .125
(iii) Sulfur, selenium and tellurium donors [MX5L] complexes (X = F, L = SMe2, SEt,; X = C1, L = SMe2, SEt,, SPr2, SC4Hs, SC4HIo, X = Br, L = SMez, SEt2, SC4Hs, SC5HI0, SC&) were easily prepared; they could all be either distilled or sublimed unchanged, except for [NbBr5(SEtz)], which decomposed on heating. Alternatively, the ether ligands of [MC15(OEt2)] and [MCl5(OPr2)] were readily replaced by their sulfide analog, while [MCls(OMe2)] remained unaffected by SMe2. IR, Raman and NMR studies of the complexes formed with ambidentate 0,s ligands showed bonding to occur via sulfur exclusively and the com lexes to be mononuclear. With SMe2 and SC4Hs evidence was found for [NbX&] adducts. 12? In [NbCl,(dth)], the dth = MeSCH2CH2SMe ligand was shown by IR to be in the gauche chelating form. Bidentate ligands having bulky end groups, e.g. BufSCH2CH2But and PhSCH2CHzSPh,adopt a tram conformation and bridge the two metal atoms in the [(MC1s)2L] adducts isolated. lZ6Reactions of MCI, with cyclic polysulfides yield dimeric complexes in which the two MC15 units are bridged by the ligand; such [(MC15),L] complexes were isolated with L = 1,4-dithiane1” and various S4 and S6 cyclic polysulfides.’2YA 1 :1 complex [TaC15L] was, however, obtained with 1,3,5-trithiane.127 The structure of [(NbCl5),(CloHzoS4)](Figure 1) shows that the ligand exists with the most unusual ‘inside out’ c a n € ~ r m a t i o n MC15 . ~ ~ ~yielded -~~~ S-coordinated 1: 1 complexes when allowed to react with tetramethylthiourea.lll A sulfur abstraction reaction occurred however when the reaction was followed by exposure to an intense light source, leading to [(Me2N),CC1][NbSCb]. SC&tRO, SeC4H,0;
Figure 1 Molecular structure of [(NbCl,),(C,,H,S,)]
(reproduced from ref. 128 with permission)
NbCl, and TaC15 were found to form 1: 1 adducts with MeNCS and MeSCN;131,132the presence of both S- and N-bonded isomers was assumed on the basis of IR. Sulfur abstraction reactions occurred when TaCl, was allowed to react with PhNCS, to yield [TaSCl3(PhNCClz)J. Reactions of [MC15-,Mer] (x = 1,2) with MeSCN afforded [MC14Me(MeSCN)] and [MC13Me2(MeSCN)],133which consist of mixtures of N- and S-bonded isomers.
(iv) Nitrogen donors Niobium and tantalum halides form adducts with various nitrogen donor ligands including aliphatic and aromatic amines; nitriles, Schiff‘s bases and imidazoles (Table 5 ) . The reactions of MX5 with pyridine and related ligands such as bipy or phen depend critically on the reaction conditions. With py at low temperature MX5 (X = C1, Br) yielded 1:1 adducts that are rapidly reduced to [M&(py),] on increasing the temperature, with formation of 1-(4pyridy1)pyridinium halide. Similarly, bipy and phen reduced the metal in MeCN to oxidation state +IV and formed monoadducts of type [M&(bipy)] at room temperature, while at 0°C the same reactions yielded [NbCl,(bipy)(MeCN)] and [TaX5(bipy)(MeCN)] (X= C1 or Br). NbBrS and TaIS formed [MX5(bipy)2], which were formulated as the eight-coordinate [MX4(bipy)2]X.1Reduction of the metal can however be prevented, even at room temperature,
Niobium and Tantalum Table 5 Complexes Formed by Ligand Amines NH,R (R = H, Et) NHEtz NR, (R = Me, Et)
597
with Nitrogen Donors
Compounds and comments [NbFsbl [NbF,L], [MCl,L]; from C,H6 tNbFJ-1 [NbCl,(NEt,)(MeCN)]; from MeCN
Re$ 1,2 w , 3 1,2
4
Aromatic amines
PY biPY terpy phen Nitriles RCN
Dinitriles CN(CH,), CN
[NbX,(py)]; rapidly reduced to fNbX,(py)J [TaX,(py)l, [TaX,(p~)~l; X = CI, Br [MX,(bipy)] M = Nb;X = F, C1, Br; M = Ta; X = Cl, Br [M&(bipy),]X; X = Br, I [TaCl,Me,(bipy)]; X-ray struaure [MX,(terpy)]; X = C1, M = Nb; X = Br; M = Nb, Ta [TazCl,o(terpY)I. [TaCl,(phen)]; ionic
1 5 1 6
7 8
[m(MeCN)I, [MF,(MeCN)4IIMF,I [MCl,(RCN)]; R = Me, CD,,Et R = H; X-ray structure rMBr,(MeCNN [MX,{CH,=dHCN)]; X = Br, CI [MCl,(PhCN)], [MCl,Me(PhCN)], [TaCl,Me(CH+HCN)] [NbC13Me2(CH,ICN)], [NbCl,Me,(CH,ClCN)], [NbCl,Me,(CH,-CHCN)] IMCI,(RCN)]; R = Me, But, CH,F, CH,Cl, CHJ, Ph, C,H,Cl (relative stabilities; L exchange)
9,10
[MX,L]; n = 1-4, X = C1, Br [(ms)&]
18 19
11,12 13 14 15 16
17
Schifs bases 20
X = H, Me, Br, I, NO,
21
Niobium and Tantalum
598
Table 5 (Conrinucd)
Ligand
Compounds und comments
@kGR
Ref.
R = H,4-Me, 4-OMe, 4-C1, 4-Br, 4-N02, 3-Me, 3-N02, 3-OMe, 2-C1, 2-OMe, 2,4,6-Br, 2,4,6-Me
22
[MX,(oxH),]; n = 2, 4,6, 8 or 10; X = C1, Br
23,24
\ I OH
oxH Imidazoles 3,5-Dimethylpyrazole Benzimidazole Benzothiazole Benzazole R-benzimidazoles
[NbCI,L,] n = 1, 2, 3
25
w35L4I
R = 2-Me, 2-Et, 2-Ph, 2-(o-OHC6H,) R = 2-(o-aminophenyl), 2-(B-pyridyl)
26 27
1. D, L. Kcpert, ‘The Early Transition Metals’, Academic, London, 1972. 2. E. G. Win, M. M. En h w a , M. A. Glushkova and Yu. A. Buslaev, Dokl. Akad. Nuuk SSSR, 1978, 243, 1459. 3. Yu. A. Buslaev, M. M .Ershova and M. A. Glushkova, Koord. Khkn,, 1979,5,532. 4. Yu. A. Buslacv, M. A. Glushkova, N. A. Chumaevslii, M. M. Ershova and L. V. Kumelevskaya, Koord. Chim., 1982,
8, 457.
5. 6. 7. 8. 9.
C. Djordjevic and V. Katovic, J . Chem. SOC.( A ) , 1970, 3382. M. G. B. Drew and .I. D. Wilkins. J. Chem. Soc., Dalton Tram., 1973, 1830. B. Begolti, V. Valjak, V. Allegretti and V. Katovic, J . Inorg. Nucl. Chem., 1981, 43, 2785. L. A. Ugalava, N. I. Pirtskhalava, V. A. Kogan, A. S. Egorov and A. 0. Osipov, Chem. Absrr., 1975, 83, 184471. K. C. Moss, J. Chem. Soc. ( A ) , 1970, 1224. 10. J. A. S. Howell and K. C. Moss, J. Chem. SOC. ( A ) , 1971, 2483. 11. D. L. Kepert and R. S. Nyholm, J. Chem. Soc., 1965, 2871. 12. G. A. Ozin and R. Walton, I . Chem. SOC. (A), 1970, 2236. 13. C. Chavant, G. Constant, Y. Jeannin and R. Morancho, Acta Crysrallogr., Secf. B, 1975,31,1823. 14. D. Brown, G. W. A. Fowles and R, A. Walton, Inorg. Synrh., 1970,12,225. 15. G . W. A. Fowles and K. F. Gadd, J. Chem. SOC. (A), 1970, 2232. 16. J. D. Wilkins, 1. Orgammer. Chem., 1975, 92, 27. 17. A. Merbach and J. C. Biinzli, HeZu. Chirn. Acta, 1971, $4, 2543. 18. M, S. Gill, H. S. Ahuja and G. S. Rao, Inorg. Chim. Acra, 1973, 7 , 359. 19. M. S. Gill, H. S. Ahuja and G . S. Rao, J . Inorg. Nucl. Chem., 1974,36,3731. 20. L. V. Surpina, 0. A. Osipov and V. A. Kogan, Zh. Neorg. Khim., 1971,16, 685. 21. L. V. Surpina, G. F. Litovchencko, S. M. Artamonova, A. D. Garnovskii and 0. A. Osipov, Zh. Obshch. Khim., 1978, 48, 1830. 22. L. A. Ugulava, N. I. Pirtskhalava, V. A. Kogan, A. S. Egorov and 0. A. Osipov, Zh. Obshch. Khim., 1975,45, 1575. 23. M. J. Frazer, B. G. Gillespie, M.Goldstein and L. I. B. Haines, J. Chem. SOC. ( A ) , 1970, 703. 24. A. V. Leshchenko and 0. A. Osipov, Zh. Obshch. Khim.,1977,47,2581. 25. L. V. Surpina, A. D . Garnovskii, Yu. A. Kolodyazhnyi and 0. A. Osipov, Zh. Obshch. Khim., 1971,41, 2279. 26. N. S. Biradar, T. R. Goudar and V. H. Kulkarni, J. Inorg. N u d Chem., 1974,36,1181. 27. N. S. Biradar and T. R. Goudar, J. Inorg. Nucl. Chem., 1977, 39, 358.
when the reaction is conducted in dry benzene;134a series of isomorphous heptacoordinatd compounds of type [MXs(bipy)] have thus been obtained. The structure of [TaCl~Me3(bipy)]’~~ shows tantalum to have distorted capped trigona prismatic environment, with one chlorine atom in the capping position (Ta-Cl: 2.54(1) A); thr nitrogen atoms (Ta-N: 2.29(2)A), one chlorine and one methyl group form the cappec quadrilateral face, with two methyl groups ( T a x : 2.24(4) and 2.16(6)& occupying t h d remaining edge. Reactions of MX5 (X = C1, Br) with 2,2’,2’’-terpyrid~l’~~ yielded the poorly soluble anc moisture sensitive [MBr5(terpy)], [NbCls(terpy)] and [Ta2Cllo(terpy)].IR data suggest that a1 three pyridyl groups are coordinated. Alkanenitriles readily gave 1: 1 and 2 : 1 adducts with MF5;1373103the 2: 1 adducts are ionil and should be formulated as [MF,L,][MF,], while the 1:l adducts are neutral [MF5LJ.Thi penta-chlorides, -bromides and -iodides afforded the monomolecular [MXs(RCN)] (Table 5). l3 In [NbC15(HCN)],’39the niobium atom is octahedrally surrounded by five chlorines and b: the nitrogen atom of HCN; the N M 1 bond trans to nitrogen is the shortest (2.243(1) a
Niobium and Tantalum
599
compared to an average 2.313(1)A for the other N b - C l distances); the N S distance of 1.090(4) A is shorter than in free HCN. The reactions of [MCl,-,Me,] (x = 1-3) with nitriles yield simple 1 : l adducts (Table 5), except for the formation of a small quantity of insertion product in the reaction of [TaC14Me]with CC13CN.140 Complexes of MX5 with dinitriles CN(CH2),CN (n = 1-4) have been Both [Mx5L] and [MX&] have been isolated in the case of malonitrile, succinonitrile and glutaronitrile, whereas only [(MX5)zL]formed with adiponitrile. In the case of succinonitrile and glutaronitrile, tram conformers have been assigned to the 2 :1 adducts, while a gauche conformation was assumed in 1:1 complexes; the more stable gauche-trans-gauche conformation for adiponitrile was proposed in its 1:2 adducts. The reactions of 8-quinolinol (oxH) with NbX5 (X = C1, Br) produced [MXs(oxH),] (n = 2, 4, 5, 6 or 10).143,144 The interaction of NbC15 with Schiffs bases led to isolation of crystalline, sparingly soluble compounds having 1:1, 2 :1 and 3 : 1 stoichiometries (Table 5); the ligands act as monodentates through the azomethine nitrogen atom. NbC15 was reported to form 4: 1 adducts with substituted imidazole^,^^^^^' which are 1:1 electrolytes in DMF. IR data indicate that coordination takes place through the unsaturated nitrogen of the imidazole ring in an eight-coordinated [NbCW4]+ cation. TaC15 and S a 4 were reported to form [TaC15(Sfi4)J,148,149 the S4N4group being bonded to the metal through one of its nitrogen atoms (Figure 2).
Figure 2 Molecular structure of [TaCl,(S,N,)J (reproduced from ref. 149 with permission)
Complexes [NbCLLICl, where L = RNHCSNHpy, were obtained;',' their IR spectra indicate bidentate behavior of L via sulfur and the heterocyclic nitrogen; they exhibit semiconductor properties. Similar behavior and properties were reported for the adducts formed with N, N'-diarytsubstituted formamidino-N"-aryl-substituted carbamides and thiocarbamides.
(u) Phosphorus and arsenic donors The group Vb triphenyls form [NbC15(EPh3)] ( E = N , P, As, Sb or Si) in carbon tetrachloride, n-hexane or cyclohexane, while [(NbC15)z(EPh3)]was obtained in benzene.' Table 6 illustrates the tendency toward increasing coordination numbers in the series PPh3< PPhzMe < P(OPh)3 < PMe2Ph = dppe < dmpe, which follows the decrease in the ligand's cone angle Addition of diars to M a 5 immediately precipitated the isomorphous red [NbCl,(diars)] and yellow [TaC15(diars)];'S1being diamagnetic, they were reported to be seven-coordinate. X-Ray structure determinations,152however, indicated the presence of [TaCl,(diars),][TaCl,(OEt)], [NbC14(diars)z][NbOC14] and [NbCl4(diar~)~][NbCl~O~], with an eight-coordinate dodecahedral cation. Similarly the compound first formulated as paBr5(diar~)]151was shown to be [TaBr4(diars)z][TaBr6];80 the cation is a crystallographically imposed dodecahedron, (Ta-Br: 2.583(10); Ta-As: 2.765(1)A). Fluorinated ditertiary arsines afforded [MX5(diarsine)]153 (X= C1, Br), for which the exact formulation could not be established.
Niobium and Tantalum
600
Table 6 Complexes with Phosphorus and Arsenic donors
Comments
Compounds
Ref.
Obtained from CCl,, n-hexane; cyclohexane
E = P or As; obtained from benzene Unstable at room temperature Stable only at -60 "C in solution Obtained from CC14 [MCI,(PMe,Ph)] also present in solution x = 1-3
Sevencoordinate compound Seven-coordinate compound Seven-coordinate compound x=3 x z= 1-3
Monodentate phosphine i X-Ray structure X-Ray structure Either seven-coordinate, or mixed six-eight-coordinate ionic formulation
5,677 5 8 10 11 12
1. D. L. Kepert, 'The Early Transition Metals', Academic, London, 1972. 2. M. A. Glushkova, M. M. Ershova, N. A. Ovchinnikova and Yu.A. Buslaev, 2%. Neorg. Khim., 1972, 17, 147. 3. M. Valloton and A . E. Mexhach, Helv. Chim. Actu, 1975, 58,2272. 4. G. Jamicson and W. E. Lindsell, inorg. Chim. Acta, 1978, 28, 113. 5. C . Santini-Scampucd and J. G . Riess, J. Chem. Soc., Dalton T m . , 1973, 2436. 6. G.W. A. Fowles, D. A, Rice and J. D. Wilkins, J . Chem. SOC., Dalron Tram., 1972, 2313. 7. G . W. A. Fowles, D. A. Rice and J. D. Wilkins, J . Chem. Soc., Dolton Tmm., 1974, 1080. 8. R. D. Sanner, S. T. Carter and W. J. Bruton, Jr., J . Organomet. Chem., 19U, 240, 157. 9. J . D. Wilkins, J . Inorg. Nuel. Chem., 1975, 37, 2095. 10. M. G. B. Drew, A. P. Wolters and J. D. Wilkins, Acta Crystallogr., Secr. B, 1975, 31,324. 11. J. C. Dewan, D. L. Kepert, C. L. Raston and A. H. White, J . Chem. Soc., Dafton Trans., 1975, 2031. 12. D. L. Kepert and K. R. Trigwell, A m . 1. Chem., 1976,29, 433.
34.2.3
Solvolysis Products of the Pentahalides
342.3.1 Alkoxides (i) Synthesis Equation (3) represents the reactions of MC15 with alcohols and phenols. They can be driven to completion by usin ammonia as proton acceptor in the case of primary and secondary alcohols (Table 7).15"' [T~(OBU')~] was obtained in the presence of pyridine, while under the same conditions NbC15 yielded [N~O(OBU')~]. [Nb(OBu'),] could, however, be prepared from [Nb(NR&].'57 MC15 + 3ROH MC1, + SPhOH
--
+ 3HC1 + 5HCl
[MCl2(OR),] [M(OPh),]
(3)
Transesterification reactions have been extensively used for the preparation of further a l k o x i d e ~ . 'Mixed ~ ~ ~ ~alkoxides ~ [M(OR),-,(OR'),] were obtained from the same alcoholysis reaction^.^^^.^^^ Exchange reactions between [Nb(OEt)5] or [Nb(OPr'),] and organic acetates have also been exploited for the preparation of higher alkoxides. Trialkylsiloxo derivatives were prepared by the reaction of the pentaethoxides with
Niobium and Tantalum
601
Table 7 Pentaalkoxides M(OR)5
R Me
Et, Pr, Bu, n-pentyl But, PI’ CH,CF, SiR, MeCH=CHCH, C H + X C ( Me)2CH+31CHrz MeCH=CHCH(Me) CH+HCH,(Me) MeCH=CHCH(Et) (Ph)$H=CHCH* Ph Naphthyl
R@.
Comments Dimeric both in the solid and in solution ‘H NMR: dynamic intramolecular exchange Solvation studies X-Ray structure Dimeric in benzene; monomeric in alcohol
172
,,
3-47 5
6
7 1,2,3 8,9 10 s, 11 12 13 14
Dimeric in benzene Dimeric in benzene Monomeric in benzene Dimeric in benzene Average association in benzene: 1.5 Monomeric in benzene Monomeric in benzene Monomeric in benzene Dimeric in benzene Dimeric in benzene
15
.
16 17 18 19 19
1. 2. 3. 4. 5. 6. 7.
D. C. Bradlcy, E . N . Chakravarty and W. Wardlaw, J . Chem. SOC.,1956, 2381. D. C. Bradley, W. Wardlaw and A. Whitley, J . Chem. Soc., 1955, 726. D. C. Bradleyand C. E. HoIloway, J. Chem. SOC. ( A ) , 1968,219. J. G. Ricss and L. G. Pfalzgraf, Bull. Soc. Chim. Fr., 1968, 2401. L. G. Pfalzgtaf and J. G. Riess, Bull. SOC. Chim. Fr., 1968, 4348. J. G. Riess and L. G . Hubert-Pfalzgraf,J . Chim. Phys. Phys. Chim. Biol., 1973, 70, 646. A. A. Pinkerton, D. Schwamnbach, L. G. Hubert-Pfahgraf and J. G. Riess, Inorg. Chem., 1976, 15, 11%. 8. 1. M. Thomas, Can. J . Chem., 1961,39, 1386. 9. R. C. Mehrotra and P. N. Kapoor, J . Less-Common Met., 1964, 7,98. 10. P. N. Kapoor and R. C. Mehrotra, Chem. Znd. (London), 1966, 1034. 11. D. C. Bradley and I. M. Thomas, Chem. Ind. (London), 1958, 1231. 12. S. C. GoeI and R. C. Mehrotra, Z . Anorg. Allg. Chem., 1978, 440, 281. 13. S. C. Goel, V. K.Sigh and R. C. Mehrotra, Z . Anurg. AIlg. Chem., 1978,447, 253. 14. P. N. Kapoor, S. K. Mehrotra, R. C. Mehrotra, R. B. King and K. C. Nainan, Inorg. Chim. Acta, 1975, U ,
273. 15. S. C. Goel, V. K.Singh and R. C. Mehrotra, Synth. React. Inorg. Metd-org. Chem., 1979, 9, 459. 16. S. C. Goel and R. C. Mehrotra, Synth. React. Znorg. Metal-Org. Ckem., 1981,ll, 35. 17. S. C. Goel, Synth. React. Inorg. Metal-Org. Chem.. 1983,l3, 725. 18. S. C. Goel, S. K. Mehrotra and R. C. Mehrotra, Synth. Reucr. Inorg. Metal-Org. Chem., 1977,7,519. 19. K. C. Malhotra and R. C . Martin, J . Orgunornet. Chem., 1982,239, 159.
trialkyl~ilanols,~~~ or trialkylsilyl acetates, or by that of dialkylamides with triethylsiianol.157 Mixed alkoxo-siloxo derivatives were obtained when M(OEt)5 was allowed to react with Me3SiOAc in various molar ratios.lm Double alkoxides with alkali metals A[M(OR)6] (Table 8) were formed from the reaction of the pentaalkoxides with alkali metal alkoxides.6 The double alkoxides of Mg, Ca, Sr and Ba with Nb and Ta have been synthesized in the presence of MgC12 as a catalyst.’@ Refluxing M(OPr‘)5 and A(OPi)3 (A = A1 or Ga) in isopropyl alcohol afforded double isopropoxides of the type [MA(OP&] and [MA2(OPr’)ll]170.[NbTa(OMe)lo] appears to be the first mixed transition metal dkoxide is01ated.l’~NMR showed it to be in dynamic equilibrium with the symmetrical MZ(OMe),, dimers in solution, with close to random distribution of the three species.
(ii) Structural aspects Nb(OMe)5 appears to be the only one of these pentaalkoxides whose structure has been ~olved.~’’It revealed the presence of two crystallographically different dimers with distinct conformations in the unit cell. Both are centrosymmetric and consist of two approximately octahedral units with a shared edge, but with either a cis or a trans arrangement of the terminal methoxo groups on each metal atom with respect to the equatorial plane (Figure 3). The volatility of the alkoxides increases, and their molecular complexity decreases, with increased ramification of the alkyl chains (Tables 7 and 8). Variations observed in the degree of association in different solvents were assigned to the donor capacity of the solvent rather than to differences in dielectric constant^.'^^^'^^ The mean dissociation energies of the M - O R bonds
Niobium and Tantalum
602
Table 8 Double and Mixed Alkoxides Compounds
[NbTa(OMe),,l A[M(OR),I . Mg[Nba(qPr')lZI [MA(OPfl,l [MA,(OPr'),,l [M(OEO(~R)'iI
Comments
Dynamic equilibrium with [Nb,(OMe),,] and [Ta,(OMe),,] in solution A = Li, Na, K A=AlorGa A=AlorGa Molecular complexity: R = Pr:2.05 (Ta) R = :B ! 1.96
Ref. 1
2 3 4 5
R = Pr': 1.93 [M(OBu')(OR),l
[M(OEt),-x(OSiMeJ,l
R=pentyl: 1.15 (Ta); 1.13 (Nb) Molecular complexity: R = Me, 2.05 (Nb); 2.07 (Ta) R = Et; 2.02 (Nb); 2.03 (Ta) R=Pr': 1.15 (Nb); 1.21 (Ta) x = 1-4
5
6
1. L. G. Hubert-Pfalzgraf and J. G. Riess, Inorg. Chem., 1975,14,2854. 2. R . C. Mehrotra, A. K. Rai, P. N. Kapoor and R. Bohra, Inorg. Chim. Actn, 1976, 16,237. 3. S. G o d , P. N.Kapoor and R. C. Mehrotra, J . Inorg. Nucl. Chem., 1976,38,172. 4. S. G o d , P. N. Kapoor and R. C. Mehrotra, Inorg. Chim. Acta, 1975, 15, 43. 5. R. C. Mehrotra and P. N . Kapoor, J . Less-Common Met., 1966, 10,354. 6. K. C.Mchrotra and P. N.Kapmr, J . Indian Chem. Soc., 1967,44, 345.
were estimated to be 429f 10 and 440f 12 kJmol-I for the Nb and Ta alkoxides re~pectively.'~~ Proton NMR studies have shown that M(OR)5 species (R= Me, Et, Bu, Pr)67174178have dimeric bioctahedral structures in solution, and that the bridging and the two distinct types of terminal groups rapidly exchange positions. When R = Me the terminal-terminal alkoxide exchange was found to occur at a significantly faster rate than the terminal-bridging alkoxide exchange; hence these proceed through distinct mechanisms. The activation energy (E,) for the intramolecular scrambling of terminal and bridging alkoxo groups was found to be fairly constant (42 to 50 kJ mol-'), while the activation free energy AGb increased by small amounts as the degree of chain-branching in the alkoxo ligand increased.177 The presence of a monomer-dimer equilibrium was proven in the case of the pentaisopropoxides and the enthalpy of dissociation of the dimer was evaluated.178 The monomeric t-butoxides showed a rapid intramolecular exchange down to -90 "C. Alcohol interchange was slower with dimeric species than with monomeric ones.
Figure 3 Molecular structure of [Nb,(OMe),,] (reproduced from ref. 172 with permission)
Low temperature 'H NMR of [Nbz(OMe)lo] in a variety of solvents has revealed significant variations of the chemical shifts, different for the various non-equivalent methoxo sites, particularly where bulky polar solvents are concerned; these were interpreted in terms of preferential s01vation.l~~
(iii) Chemical properties M(OR), species do not readily form coqrdination complexes but prefer to autoassociate into dimers, trimers, etc. The coordination chemistry of M(OMe)5 has been investigated by low
Niobium and Tantalum
603
temperature NMR, and rationalized in terms of frontier orbitals and charge distribution on both the ligand and a l k o ~ i d e . ' ~ ~ , ' ~ ~ ~ * ~ The electro-attractive alkoxo groups render metal alkoxides very sensitive to nucleophilic attack.6 They are extremely susceptible to hydrolysis, which under controlled conditions yields oxo alkoxides; they require strictly anhydrous conditions of handling. M(OR)5 reacts with p-diketones, thio-@-diketones, p-keto esters and @-hydroxy esters (Table 9). Only three of the five OR groups could be replaced by these bidentate monoanionic O H ligands. Su risingly, the reaction of tropolone (HT) with Nb(OEt), was reported to yield [Nb(OEt)T4]F whose structure remains unknown. A more versatile behavior was observed with dianionic H m H ligands (Table 9): diols act as bidentates,'86'187 while whydroxycarboxylic acids behave as monodentate monoanionic ligands, e.g. in cinnamates,lR8or as bidentate dianionic ligands1893190 in alkoxycarboxylates. In the case of catechol, ionic s ecies were shown to exist in solution for both metals, but were isolated for tantalum only." The bidentate monoanionic diphenylphosphinate ligand can replace from one to three alkoxo groups in Ta(OMe)5 to yield low molecular weight polymers (Table 9) The alkoxo groups on NbV and TaV are not readily replaced by nitrogen ligands. Thus, with one equivalent of ethanolamine only the hydroxylic group was s ~ b s t i t u t e d . ' ~Substitution ~ , ~ ~ ~ by the amino group could however be forced by using a two- or three-fold excess of ethanolamine. Diethanolamine behaved as a tridentate trianionic ligand, while triethanolamine yielded metallatranes. The oximates and oxyarninates of Nb and Ta have been i s ~ l a t e d . ~The ~~,~~~'~~ reactions of M(OR)s with both monofunctional bidentate and bifunctional tridentate Schiff's bases have been studied (Table 10).19%203 Insertion of unsaturated isocyanates into the M-O bond of M(OR)s has been r e p ~ r t e d ~ , ~to~ give , ' " ~mono- to penta-substituted products (equation 4). No insertion into the Nb-0 bond was observed with phenyl isothiocyanate, [M(OR),)
+xPhNCO
-
[M(OR),,(N(Ph)COOR},]
(4)
M(OMe)5 undergoes peroxidation2Mon treatment with H202. The use of labelled H20: indicated the fixation of the labeled oxygen between M and OMe. M(OEt),, when allowed to react with MeCOX (X = C1, Br)207in 1:1, 1:2 and 1:3 molar ratios, formed [MX(OEt),J, [MXzOEt),] and [MX3(OEt)2(MeCOOEt)], respectively; molar ratios of 1:4 or higher yielded [M&(OEt)(MeCOOEt)], while the reaction with acetyl bromide afforded [MOBr3(MeCOOEt)]. Nb(OPh),, when refluxed with excess acetyl bromide, gave [NbBr5(MeCOOPh)].*g3Redistribution reactions were shown to occur between TaIs and [Ta(OPh)5] in MeCN, and [TaI,(OPh)(MeCN)] crystallized out. Attempts to synthesize mixed [NbIS-,(OR),] derivatives from similar reactions have so far failed.20s
34.23.2 Halide alkoxides and related compounds
(i) Halide alkoxides Direct reaction of MC15 with ROH yielded the mixed halide alkoxides [MCl,(OR),-,] (R = Me or Et, x = 2 or 3;6 R = MeOCH2CH2,x = 2;m R = Ph, x = 1; R = @-naphthyl,x = 2; R = anthranyl, x = 2;210R = C€€F2CF2CH2, x = 36). Stepwise alcoholysis of MX5 in CCh afforded [MX,(OR),-,] ( x = 0-4 when X = C1, R = Ph or naphthyl;211x = 1 or 2 for X = Br and R=Ph*''). The reactions of M(OR), with acetyl halides afforded easy access to [MX,(OEt)5-x] (X = C1, Br; x = 1 or Z2O7) and to [TaI(OPh)4].20sSubstitution of chlorine for a third or a fourth alkoxo group proceeded with coordination of the corresponding ester to the metal, while substitution by a fourth bromine resulted in the formation of oxo derivatives. Treatment of TaC15 with an excess of LiOAr (OAr = 2,6-di-t-butylphenoxide) in benzene led to [TaC13(OAr)2].213TaC15, when allowed to react with the chelating ether MeOCH2CH20Me, gave [TaC14(0CH2CH20Me)],through cleavage of an alkyl-oxygen bond.214Neopentylidene complexes of the type [TaX3(CHCMe3)(PR3)2Jreacted with [WO(OBU')~] to yield [TaX3(OB~f)2]2.215 Alkoxo fluorides were synthesized from the appropriate chloro methoxide with KF, or by the reaction of M(OEt)5 with MF5.6 Treatment of the halide alkoxides with (NEt4)Cl afforded crystalline (NEt4)[MCl,(OR)6-,].216 With 1:2 molar ratios, the oxo derivative (NEt4)@bOClS] was CC€3-T*
Niobium and Tantalum
604
Table 9 Reactions of the Alkoxides with 0-containing Reagents
Compounds and comments
Reagent
8-Diketones : R IC( O)CH,C( 0 ) R ” Monomeric species in boiling benzene [M(OR),-,(acac),]; x = 1-3; R = Et, Bur acacH [M(OR),-,(bzac),]; x = 1-3; R = Et, Bur bzacH [MIOR),-,(Clbzac),]; x = 1-3; R = Et, But R’=Me, R” =p-ClC,H, ClbzacH R = Me, R” = p-FC6H4 [M(OR),-,(Fbzac),]; x = 1-3; R = Et, But FbtacH M[(OR),-,L,]; x = 1-3; R = Et, But R‘= CF,, R” = thenoyl Thio-8-diketones: R’C(S)CH,C(O)R”(HL) R‘ = Me, Ph and R”=Ph, Me M[(OR),L]; R = Me, Et, Pr‘; R‘ = R” = Me, Ph;
Ref.
1 2 3,4 3,4 394
5
8-Keto esters : MeC( O)CH, COOR ’(HL) R‘ = Me, Et 8-Hydroxy esters ( H L )
[Ta(OR),-,L,],
x = 1-3;
R = Et, But;
M[(OR),-,L,];
x = 1-3;
R = Et, Bu‘
O ? & C O 2 E t
~CH(OH)CH2C02Et
nC(OH)(Me)CH,CO,Et
v Tropolone (HT)
Diols (H2L) Propane-l,2-diol Propane-1 $diol Butane-2,3-diol Hexane-l,6-diol Pinacol
[Nb(OWT41 [NbT,]+; X-ray structure
9 10
[M(OEt),-,L,]; x = 1, 2; dimeric compounds [MLJHL)]: monomeric and dimeric species in solution
11,12
[M2L51
Catechol (H,L)
[Ta(OR),L]; K[Ta(OR),Ll; K[Ta(OR),LI; K2ITa(OR)L,I CY-Hydroxycarboxylic acids (H2L) Lactic, mandelic or [M(OR),-,L,]; x = 1-2, monomeric; [ML,(HL)] salicyclic acids Benzylic acid [M(OR)S-&Lx],x = 1-2, dimeric cram-Cinnamic acid or [M(OR)s-,L,]; x = 1-3; dihydrocinnamic acid monodentate cinnamate [OPPhlOl[Ta(OMe),-,(OPPh,O),]; x = 1-3
2
13 14 15 16
1. P. N. Kapoor and R. C. Mehrotra, J. Less-Common Met., 1965, 8, 339. 2. R. Gut, H. Buser and E. Sciunid, Heiv. Chim. Acta, 1965,48, 878. 3. A. K. Narula, R. R. Goyal and R. N. Kapoor, Synth. React. Inorg. Metd-0%. Chem., 1983,W, 1. 4. A. K. Narula, B. Singh, P. N.Kapoor and R. N. Kapoor, Synth. React. Inorg. Metal-Org. Chem., 1983, W, 887. 5. R.K. Kanjolia and V. D. Gupta, Inorg. N w . Chem. Left., 1980,16, 449. 6. R. C. Mehrotra and P. N. Kapwr, J. Less-Common Met., 1964, 7 , 453. 7. R. C. Mehrotra and P. N. Kapoor, J. Less-Common Met., 1964, 7 , 176. 8. A. K. Narula, B. Singh, P. N. -or and R. N. Kapoor. Transition Met. C h m . , 1982, 7 , 325. 9. S. K. Mehrotra, R. N. Kapoor, J. Uttamchandani and A. M. Bhandri, Inorg. Chim. Acta, 1977,22, 15. 10. A. R. Davis and F. W.B. Einstein, Inorg. Chem., 197S, 14, 3030. 11. R. C. Mehrotra and P. N. Kapoor. J . Less-Common Met., 1966, IO, 237. 12. R. C. Mehrotra and P. N. Kapoor, J. Lms-Common Mer., 1965,8,414. 13. S. Prabash and R. N. Kapoor, Inorg. Chim. Acta, 1971, 5, 372. 14. A. K. Narula, B. Singh, P. N. K a w r and R. N. Kapoor, J. Indiun Chem. Soc., 1962, 59, 195. 15. A. K. Narula, B. Singh, P. N. K a p r and R. N. Kapwr, Transition Met. Chem., 1983,8, 195. 16. H. D. Gillman, 1. Inorg. Nucl. Chem., 1975, 37, 1909.
Niobium and Tantalum
605
Table 10 Reactions of the Alkoxides with Schitrs Bases
Compounds and comments
S c h i r s base
Ref.
HZL [M(OPr'), -zr L, 1
x = 1: hexacoordinated monomer x = 2: solution equilibrium between hepta-
1,2,3,4
coordinated, monomer and octacoordinated dimer (OPr'bridges)
(R = H,Me; R' = CH,CH,OH, CH,CH(OH)Me) GcR=NUHC(zpiH2
(R=H, Me; Z = 0,S)
aoH CH=NNC( Z)NHz
[MLJHL)] octacoordinate monomer
(Z = 0 , S )
[M(OR),-,L,], [Mb(WI
0
~CR~=N(CH~)~N=CR
HO
R = Pr', But, x = 1, 2
5
[M(OH),L] heptacoordinated monomer
/
(R = Me, Ph) RR'C=NN=C,
/SH SCH2Ph
[Nb(OW)i] solution equilibrium between monomer and dimer [NbZL,I
(R= PhOH, R' = H; R = MeCOCH,, R' = Me)
HL [M(OPr'),L,] x = 1-5 Monomeric hexacoordinated (x = 1) or heptacoordinated
tYR aoH
(R = o-toluidine, aniline or Z-pyridyl) CH=NR
8.9
(x = 2) species in boiling benzene
PhCN=NR (R=CHzCHzOH, CH,CH(Me)OH, PhOH) 1. P. Rashar and J. P.Tandon, 2.Norurforsch., Teil B, 1971,36,11. 2. P. Prashar and J . P. Tandon, Bull. Chem. SOC.Jpn., 1970, 43, 1244. 3, M.N.Mookerjee, R. V. Sin@ and J. P. Tandon, Ann. SOC. Sci. ErureZh, Ser. 1, 1980,94,207. 4. M. N. Mookerjee, R. V. Sigh and J . P. Tandon, Indian J . Chem., 1981,20,246. 5 . M.N. Mookerjee, R. V. Sigh and J . P. Tandon, Gazz. Chim. lral., 1981,111,109. 6. J. P. Tandon, S. R. Gupta and R. N. Prasad, Acta Chim. Acad. Sci. Hung.,1975,86,33 (Chem. Abstr., 1975,81, 171 946). 7 . M.N. Mookerjee, R. V. Singh and J. P. Tandon, Synrh. React. Inorg. M d - O r g . Chem., 1982,l2, 621. 8. S. R. Gupta and I. P. Tandon, Ann. Soc. Sci. Bruxelles, Ser. I , 1974, 88, 251. 9. J. Uttamchandani,S. K. Mehrotra, A. M. Bhandari and R. N. Kapoor, Tranririon Met. Chem., 1976,1,249.
Niobium and Tantalum
606
formed. The reaction between (NEt4)[MC16]and methanol yielded (NEt4)[MC14(0Me)2]? while (NEt4)[TaCl,(OCHzCHzOMe)6--x] (x = 4, 5 ) was obtained with 2-methoxyethano1.m NaOEt and pyHC1, when allowed to react with [NbC14(OEt)] and [NbC1(OEt)4], yielded Na[NbCIS(OEt)],Na[NbCl,(OEt),j, (pyH)[NbOC14] and Na[Nb(OEt)6].6The thermal decomposition of [NbC1z(OMe)3]z17in an argon or oxygen atmosphere gave NbzO5 and ethylene as the main products. The 19F and 93NbNMR spectra of MFS/M(OEt), or M(0Et)dHF mixtures established the formation of neutral, anionic and cationic com lexes.21g"1 The structure of [TaC13(OCH2CH20Me)2]zzF showed the metal atom to have an octahedral environment, one ligand bein bidentate and the other monodentate, with three different Ta-0 bond lengths (1.80(3) f or the monodentate; 1.91(3) and 2.23A for the chelating ligand).
1
(iij Adducts of the halide alkoxides and related compounds The halide alkoxides behave as Lewis acids; they react with 0, S, N and P donor ligands to give the adducts listed in Table 11. Haloalkoxo adducts of the type MX3(OR),L were obtained in anhydrous a l ~ o h o l Unexpectedly, . ~ ~ ~ ~ ~ [NbC13(OR)z(HMPA)]was also obtained from the reaction of [Nb(OR)3C12]2with HMPA in CHzC12or Etz0.225Mixing of M(OR)s and MX, in MeCN gave [NbCl(OEtj4(MeCN)],' [NbBr4(OR)(MeCN)]:12 where R = Me, Et, Pr, Bu: CH2Me3, CMe3, [Nb15-,(OR),(MeCN)j, where x=O-4 and R = M e , Et, Pr, Ph, and [Ta14(OR)(MeCN)], where R = Et, Ph.*' [NbC13(0Et)z(MeCOOEt)]was isolated by allowing MeCOCl to react with [NbC12(0Et)3]2and further afforded [NbCls(OEt)~(py)]by addition oi pyridine.m7 These adducts are moisture sensitive, monomeric and predominantly non-ionic. The crystal of [NbCl,(OPr'),(HMPA)] is built up of the meridional isomer.22sThe same isomei was always predominant in fresh solutions, while oxo species were detected in aged solutions. MCls when allowed to react with N-arylsalicylaldimines [(salR)H] in methanol or ethanolz2' afforded [MClz(OR),(salR)H], in which the neutral Schiff's base behaves as a unidentate liganc bound through the oxygen atom. When chlorophenoxo derivatives were allowed to react witt (salR)H, [MC14-,(OPh)x(salR)] (x = 1-4) was obtained.=' When MClS was treated in CCL with a slight excess of monoanionic HL or dianionic HzL' Schiff's bases derived from salicylaldehyde, vanillin, p-phenylazoaniline or p-aminodiphenylamine, [NbCl&] , [NbC13L'. and [NbC1L$IUs were obtained, in which Nb is he tacoordinated. The mass spectra anc fragmentation paths of [MC1(OR)2L]were examined.' Halide-8-quinolinol adducts [MXroxH),] lose HX upon heating: 143 [MX,(OX)~-,]complexe: (x = 1-4) were prepared for all combinations of M, X and x except for [NbBr2(ox)3] [MBr(oxk] complexes were found to be 1: 1 electrolytes, while conductance measurements a n c IR indicated the others to be non-electrolytes, with metal coordination numbers of nine, eight seven and six for x = 1 to 4. Complexation of NbC1, by 1,6-dihydroxyphenazine yielded i brown insoluble linear coordination polymer formulated as [M~C~~(C~ZH&OZ),].Wit€ bidentate hydroxy esters (HL), MC15 gave [MClS-,L,] (x = 1, 2).231 Complexes of the type [MXx(OR)4-,(/I-diket)], where X = C l or Br, R = M e or Et. /I-diket = acac, bzac (Table l l ) , have been obtained from Mxs and the /I-diketones in the parent alcohol as solvent. Chlorophenoxo niobium and tantalum derivatives react with benzoin (HL) to yield [MCI(OPII)~L]and [MC12(OPh)2L]in which L acts as a chelating ligand,232while MC15 were found to form [MC14L]and [MC13L] with benzoin.u3 The preparation of relativelj stable NbV complexes containing N ,N-dialkyldithiocarbamate ligands (R2dtc) has beex reported; the structure of [NbC1(OMe)2(Etzdtc)z]z34 shows the metal's environment to be s pentagonal bipyramid with the halide and four sulfur atoms forming a plane containing the metal; the two OMe groups occupy the apical sites.
P'
(iii) Halide carboxylates and related compounds
Chlorocarboxylato complexes were obtained from the reaction of TaC15 with RCOOH;23! dimeric [MC14(RCOO)], cornpounds were isolated for R = H, Me, Et, Pr, Pr', But, CF3, whilc [TaC13(BufC00)2]was the only compound obtained with two carboxylato groups. Both type! of compound were proposed to have sevenfold coordination. The reaction of NbC15 witt gave [NbC13{X(C00)z}], where X = (CH2), (n = 0-4), CH(OH)CH(OH: dicarboxylic
Niobium and Tantalum
607
Table 11 Adducts of the Halide Alkoxides
Commenrs
Compounds R = Me, Et, Pr, Bu
X = C1, Br; R = Me, Et; L = OPPh,, OAsPh,, HMPA, OSPh, X-Ray structure L = OPCl,, OPPh,, HMPA, p y 0 R =Me, Et; R' = Ph, p-MeC& L = py, E-pic L = OPCl,, OPPh,, HMPA, py0 L = CY-, b- and y-pic0, HCONHMe, DMF, MeCONHMe, MeCONMe, L = MePhCO, Ph,CO
X = C1, Br; X-ray structure HL = benzoin salH = salicylaldehyde X = C1, Br; R = Me, Et HL = salicylaldehyde, acetoacetanilide, benzoylacetanilide, 2-hydroxyacetophenone, 0-hydroxybenzophenone, benzoylacetophenone HL = benzoin
R = Me, Et; HL = 2,4-pentaline, salicyaldehyde
Ref. 1 2 3 4 5 5 6 7
8 6 9 10 11 12
17 13 14 14
15 17 13 16 13
HL = benzoin K. M. Sharma and S. K . Anand, J . Insf. Chem. (India), 1979, 51,208 (Chem. Abstr., 1980, 92, 156979). M. A. Glushkova, M. M. Ershova, N. A. Ovchinnikova and Yu. A. Buslaev, Zh. Neorg. Khim., 1972,17, 147. K. Behzadi and A. Thompson, 1.Less-Common Mer., 1977,56, 9. L. G. Hubert-Pfalzgraf,A. A. P i e r t o n and J. G. Riess, Inorg. Chem., 1978,17,663. R . C. Mehrotra and P. N. Kapmr, I . Less-Common Met., 1966,10, 348. K. C. Malhotra, U. K. Bamrjee and S. C. Chaudhry, Trunsition Met. Chem., 1982,7, 14. K. Yamanouchi and S. Yamadn, Inorg. Chim. Acra, 1976,18,201. 8. K. C. Malhotra, U. K. Banerjee and S. C. Chaudhry, J . lndian Chem. SOC., 1980,57,868. 9. K. C. Malhotra, U. K. Banerjee and S. C. Chaudhry, Nutl. Acad. Sci. Len. (India), 1979,2, 175 (Chem. Abstr., 1980,92,68793). 10. K. C. Malhotra, U. K Banerjee and S. C. Chaudhry, Acta Cienc. hdica (Ser. Chem.), 1980, 6, 236 (Chem. Abstr., 1981, 95, 53 985). 11. R. Gut, H. Buser and E. Schmid, Helv. Chim. Acta, 1965,94,878. 12. J. W. Moncrief, D. C. Pantaleo and N. E. Smith, Inorg. Nucl. Chem. Len., 1971,7, 255. 13. K. C. Malhotra, U. K . Banerjee and S. C. Chaudhry, J . Indian Chem. SOC., 1979,56,754. 14. C. Djordjevic and V. Katovic, J . Inorg. Nucl. Chem., 1963,25, 1099. 15. A. Syamal, D. H. Fricks, D. C. Pantaleo, P. G. King and R. C. Johnson, 1.Less-Common Mer., 1969,l9, 141. 16. D. Stefanovic, J. Stambolija and V. Katovic, Org. Mass Spectrom., 1973, 7 , 1357. 17. K. C. Malhotra, U. K. Banerjee and S. C. Chaudhry, Indian J . Chem., Sect. A , 1978,16,987. 1. 2. 3. 4. 5. 6. 7.
and OX&. I9FNMR data on solutions of [MF5 and (CF3COO)- indicated that [TaF,(CF,COO)]- and [TaF4(CF3C00)2]-had formedu7]with monodentate trifluoroacetate; a cis structure was assigned to the dicarboxylato derivative. Reactions between MC15 and hydrogen phosphonates (HL = (RO),P(O)H; R = Et, P t , Bu, Ph) afforded [MClS-,L,] ( x = 1-3), which were reported to be polymeric through -0-P-0 bridges.238NbX5 when allowed to react with Na[S2P(0R),] (R = Me, Et, cyclohexyl) gave ;239 recrystallization from the appropropriate alcohol afforded [Nb(OMe),Cl{ S2P(OR)2}2] complexes having other alkoxo groups. The reaction of dithiophosphates with Tax, did not afford the analogous tantalum derivatives.
342.3.3 Thiolato and difhiocarbamato derivatives Dithioalkoxo- and dithiophenoxo-niobiurn(V) trichlorides [MCl,(SR),] were obtained from NbC15 and thiols and benzenethiol respectively.240The synthesis of tris(benzenedithio1ato) complexes was accomplished by reaction of the appropriate metal amide with a mixture of the dithiol and its sodium salt in THF. The structures of (AsP~)[M(S~C&)~]"""""~showed the
Niobium and Tantalum
608
metal to be surrounded by six sulfur atoms in a close to trigonal prismatic coordination, with the dithiolene ligands radiating from the metal in the usual 'paddle-wheel' arrangement (Figure 4). The coordination sphere of [Nb(SCH2CH2S),]- is midway between trigonal prismatic and ~ctahedral.",~
I
Figure 4 Structure of [Nb(S,C,H,),]-
(reproduced from ref. 243 with permission)
The rrinuclear [Ta(Cp)2(y-SMe)2Pt(y-SMe)2Ta(Cp)2]2anion was obtained by reaction of [Ta(SMe)2(Cp)2]and [PtC12(PhCN)2],244 The X-ray diffraction analysis245showed the arrangement around the Pt atom to be an almost regular tetrahedron, a situation hitherto unknown for The compound was therefore assumed to formally contain Ta" and Pto, although the short Pt-Ta contacts of 2.788(7) and 2.809(8) A are indicative of metal-metal bonds. [Ta(R2dtc)5],where (R2dtc) = (S2CNR2)-, were reported to result from insertion of CS2 into Ta(NR2)5; in the case of niobium, the final product was [Nb(R2dtc)4].246 [Ta(R2dt~)5]247 and [MXX(R2dtc),-,] ( x = 1-3) have been obtained in the reaction between MC15 and Na(R2dtc) in oxygen free solvents, while [MX(OR)2(R2dtc)2]was formed in [Nb2Br3(R2dt~)5] was isolated from NbBr5 in benzene.250Niobium, in acidic media in the presence of C10; ions, was found to form [Nb(R2dtc)4](C104) with pyrrolidine- and hexamethylene-dithiocarbamates.251 [Nb(R2dtc)Me(Cp)21(R2dtc) was obtained when [ N ~ M ~ ~ C was P ) ~allowed ] to react with R2NC(S)SC(S)NR2. s2 In all these air sensitive complexes the dithiocarbamate ligand is bidentate through the sulfur atoms. The [MX,(Rzdtc)5-,] adducts were found to be ionic, hence must be formulated as [M(R2dtc)4]X, [{Nb(R2dtc)3}2C13]C1 and [M(Rzdtc)4][M&].249 The structures of [Ta(SzCNMe2),]+ in the C1- 253 and [TaClJ 254 salts showed the eight-coordinate [Ta(R2dtc)4]+cation to be at the center of a 4ddodecahedron, the bidentate ligand spanning the AB edges. Surprisingly, this cation is stereochemically rigid on the NMR time scale and appears to be the first eight-coordinate chelate for which this is observed.
3423.4 Amido deriuatiues
Several reviews have dealt with the chemistry of transition metal-nitrogen derivati~es,2~*=~ including compounds with metal-nitrogen multiple bond^,^^,^^^ especially those encountered in high oxidation states. Solvolysis of MC15 by ammonia was studied as early as 1924, but the products were not conclusively characterized.m,261 Halo monoalkylamides262and dialkyl amides were prepared. The monoalkylamides undergo facile a-H abstraction reactions to nitrenes, especially when labile substitutents are present (Section 34.2.3.5); dialkylamides (Table 12) have been extensively studied by Bradley.26s265
(i) Synthesis and structure
Homoleptic NbV and TaV dialkylamides have been obtained (equation 5).263,2&9 Extensive reduction, increasing with the length of the alkyl chain, was observed with niobium, the predominant products then being [Nb(NR2)4](R = Et, Pr",Bun; Section 34.3.5.4). Oxidation
Niobium and Tantalum
609
Table 12 Niobium(V) and Tantalum(V) Dialkylamides
Compound
Commenfs Insoluble Isolated for L = py, y-pic, unstable for L = MeCN, OEt, Impure, contaminated by [TaCl,(NMe,)3]z X-Ray structure for [TaCl,(NMe,),(NflMe,)] X-Ray structure, unstable towards redistribution Isolated for L = Me,NH, observed in solution for L = py, y-pic X-Ray structure, M = Nb tetragonal pyramid; PES M = Ta: isomorphous with the niobium analog R = Et, trigonal bipyramid Observed in solution X-Ray structure, tetragonal pyramid Unstable, evolution to nitrene at room temperature
1. J. C. Fuggle, D. W, A. Sharp and J. M. Winfield, J. Chem. SOL, Dolion Trans., 1972, 1766. 2. P. J. H. Camel1 and G. W. A. Fowles, J . Less-Common Met., 1962,4,4(J, G . W. A. Fowles and C. M. Plcass, J . Chem. SOC., 1957,2078. 3. P. J. H. Camel1 and G. W.A. Fowles, J . Chem. SOC. (A), 1959,4113. 4. M. H. Chisholm,J . C. Huthnan and L. S. Tan, lnorg. Chem., 1981,20,1859. 5. D. C. Bradley and I. M.Thomas, Proc. Chem. Sac., 1959, 225; Can. J. Chem., 1962,40,449, 1355; D.C. Bradley and M. H.Gitlitz, J. Chem. SOC. (A), 1969, 980. 6. C. E. Heath and M. B. Hursthouse, J. Chem. Soc., Chem. Commwz., 1971, 143. 7. M. H. Chisholm, A. €3. Cowley and M. Lattman,J . Am. Chem. Soc., 1980, 102,46. 8. R.J. Smallwod, Ph.D. Thesis, London, 1975. 9. M. H. Chisholm and M. W. Extine, J. Am. Chem. SOC., 1977,99,792. 10. J. M. Mayer, C. J. Curtis and J. E.Bercaw, 1. Am. Chem. Soc., 1983,105, 2651. 11. C. Santini-Scampucciand G. Wilkinson, J . Chem. SOC., Dalton Trams., 1976, 807.
state +V was retained for tantalum, and either the amides [Ta(NR&] or the mixed nitrene amides [Ta(NR)(NR2l3J(R = Et, Pr"; Section 34.2.3.5) were isolated. MCl,
+ 5LiNR2
-
[M(NRZ)S]+ SLiCl
R = Me, Et, PI",Bu"
The coordination polyhedron of the dialkylamides is largely determined by the steric requirements of the NRz groups and by their ability to form 0 and n bonds.256Even the NMez group is sterically demanding, and the homoleptic Nb and Ta amides are all monomers. Dimeric halogen-bridged structures were observed for mixed dialkylamido compounds, as, for example, in [TaC12(NMe2)3]2.For pentacoordinated do compounds, a trigonal bipyramidal geometry is expected to be the most stable.266[Ta(NEt&] indeed exhibits this geometry,25"but [Nb(NMe&] (Figure 5 ) and [Nb(NC5Hlo)5] approach a square pyramidal geometry.z67 Although all nitrogen atoms have approximately planar geometry, only the axial Nb-N bond appears to have some K character on the basis of its shorter length (single Nb-N bond lengths are expected to be in the 2.04 to 2.088, range); the planarity found for nitrogen has been interpreted to result from the rather tight packing of the ligands around the metal. [M(NMe,),] species were found by photoelectron spectrometry to be isomorphous in the vapor phase, but their exact geometry has not been established.268 Mixed halodialkylarnido derivatives are accessible by several methods .262,269 The chlorodimethylamidotantalum(V) derivatives have received special attention from Chisholm and coworkers.2m Reaction of the pentachloride with an excess of dimethylamine provided [TaC13(NHMe2)]as the major product (85%), accompanied by [TaC12(NMe2)3(NHMe2)](6%) and by a dinuclear oxo derivative [{TaClz(NMe2)z(NHMe2)}20] (2%), whose origin is undetermined. A common feature of these products is their fairly strong Ta-N bond. Metathesis reactions between [Ta(NMe&] and Me3SiC1 led to close to random distributions of the halodialkylamido complexes, from which [TaC12(NMe2)3]2was easily isolated from pentane
610
Niobium and Tantalum Me
Me
Figure 5 Coordination polyhedron of [Nb(NMe,),] (reproduced from ref. 267 with permission)
and a mixture of [TaC12(NMe2)3]2and [TaC13(NMe2)2]2from toluene solutions. The dimeric [TaClz(NMe2)3]2is readily cleaved by nitrogen donors to form [TaC12(NMe2)3L]adducts (L =,Me2NH, y-pic). Attempts to isolate [TaCl(NMe&] were unsuccessful. The structures of [TaC13(NMe&(NHMe2)] and [TaC12(NMe2)3]2have been solved.270 Tantalum adopts a distorted octahedral geometry in all structures. The central TaC13N3core of the trichloro adduct has a meridional arrangement of the chlorine atoms with cis dimethylamido ligands. The two Ta-NMe2 bonds are comparable in length (1.958 8, av.), and much shorter than the Ta-N€iMe2 distance (2.370(6) A). The centrosymmetric dimeric structure of [TaC12(NMe2)& dis lays two fac-TaN2C13 arrangements with bridging chlorines (Ta-Cl,,: 2.586(1) and 2.635(2) ). The amido TaNCz units are all planar. This, together with the short Ta-N distances and the close to 90’ dihedral planes between these units, provides evidence for strong Ta-N d,-p, bonding. The Me2N ligand in each of these molecules was considered as a four-electron donor ( 2 n’), leading to 18- or 16-electron valence shells in [TaC12(NMe2)3]2and [TaC13(NMe2)2(NHMe2)],respectively. A fac-TaN3 arrangement is a common feature of all these structures, as ligands having a strong trans influence tend to prefer a mutual cis configuration, irrespective of their n acceptor or n donor properties.”’ A cis arrangement of the dimethylamido groups was therefore assumed for [TaC13(NMe2)2]2and [TaClz(NMe2)3(NHMez)]. Pentavalent silylamido derivatives are limited to Ta, as reduction occurs with Nb.272*273 The bulkiness of the [N(SiMe,),]- ligand precluded complete chlorine substitution as well as formation of dimers. [TaCl~{N(SiMe2)2}z]has a trigonal bipyramidal structure with short equatorial Ta-N bonds (1.930 %, av.). The first hydrazide(-1) compounds, [M2C18(H“RR’)2] (R = Ph, R’ = H or Me) and [TaC13(HNNHPh H2NNHPh)], were obtained from the reaction of the pentachlorides with the hydra~ines?~’S(pectroscopicdata on [TaCl,(HNNHPh),(H,NNHPh)] are consistent with an equilibrium between neutral and ionic structures, as also observed for molybdenum hydrazido derivatives.
R
+
(ii) Reactivity Niobium and tantalum dialkylamides can easily undergo substitution or insertion reactions, as well as C-H activation. Substitution reactions have been used to obtain dialkylamidesz2 or dithiolenes.”’ Aminolysis reactions, for instance, were assumed to occur via a simple associative mechanism (equation 6).276 M-NRZ+NHRl
[
RlN-H M-NRZ
]
M-NR;+HNRZ
(6)
The M-N bond is susceptible to insertion reactions, the Ta-N being even more reactive than the Ta-C bond; thus in [Ta(NMez)zMe3],insertion of CS2 was found to occur on1 in the former bond.”7 Insertion of COz, CS2 and COS in [M(NR&] has been o b ~ e r v e d . ~1’In ~ . the ~~ case of niobium, the insertion of CSz was accompanied by reduction, giving [NB(S2CNR2)4] (Section 34.3.5.3.i).279 The N,N-dialkylcarbamato ligand, 02CNRz, is much less prone to oxidation and the pentavalent state was retained for both niobium and tantalum. The synthesis of monothiocarbamates is limited to [Ta(OSCNMez)5]2”
Niobium and Tantalum
611
The mechanism of the COZexchange and insertion reactions has been extensively studied on [M(NMe2)5] using kinetic and labeling experiments. The insertion is catalyzed by amines (equations 7 and 8). It is rapid and quantitative, and proceeds through [M(NMe&-,L,] intermediates, the Ta derivatives being more stable than those of Nb.272None of the mixed species is favored. [M(NMe2)2(02CNMe2)3] formed for both metals, but could be isolated only for tantalum. The formation of [M(NMe2)3(02CNMe2)2] was not observed. HNMez+CO,
H02CNMq
(7)
MNMe,+LH
MLi-HNMe, L = O,CNMe,
The coordination spheres of [M(OzCNMe2)5]and [Ta(NMez)z(0zCNMez)3]contain both mono- and bi-dentate dialkylcarbamato ligands. The magnitudes of the Av("C-~C) and A Y ( ' ~ O - ' ~ O(A180) ) shifts in the IR were rationalized by assuming that v(02CN)is mainly e N in character for a bidentate ligand (A180 < 5 cm-l), while a significant contribution from the ( S O moiety is observed for a monodentate ligand ( A l s o cu. 20 cm-I). X-Ray diffraction data for [Nb(02CNMe2)5]278 show that niobium is eight coordinated, with three bidentate and two monodentate cis (Me2NCO2)- ligands; The Nb08 moiety does not correspond to any idealized M x 8 polyhedron. The Nb-0 bond distances are much shorter for the monodentate (av. Nb-0 = 1.91(2) A) than for the bidentate ligands, the latter bein asymmetrically bonded (av. Nb-0 = 2.08(4) and 2.19(2) A). In [Ta(NMe2)z(02CNMe2)3] 2 4 the metal's coordination polyhedron can be described as a distorted pentagonal bipyramid with two bidentate carbamato ligands in the equatorial plane and one monodentate ligand in an axial site. This geometry is particularly propitious to Ta-N n bonding (Ta-N: 1.970A av.), to attain a saturated valence shell configuration, which accounts for the kinetic and thermodynamic stability of the compound. It should be noted that the bite of the carbamato ligand (2.15(5) A) compares with that of NO, in [Ti(N03)4]and is considerably smaller than that of the dithiocarbamato ligand (2.84(3) A), which may explain why oxidation state +V is maintained for niobium. In solution these compounds are highly fluxional. Whereas the methyl groups are all stereochemically inequivalent in solid [Nb(02CN?vle2)5],they appear equivalent in solution, even at -80 "C, on the 13CNMR time scale. The mono- and bi-dentate carbamato substituents can, however, be distinguished in solution by IR. A facile exchange between coordinated and uncoordinated oxygens of the monodentate carbamato ligand was also observed in [Ta(NMez)z(OzCNMe2)3].Hindered rotation about the Ta-N linkage was detected at -95 "C by I3CNMRSz8' Although cyclometallation reactions have seldom been observed with do metal compounds, intramolecular met ailation involving early transition metal dialkylamides with formation of azametallacyclopropane, which occurs by H abstraction, has been reported.281 On distillation, [Ta(NEt2),] undergoes substantial decomposition (equation 9) to give ethyliminoeth l(C,N)tris(diethylarnido)tantalum (1) and ethylimidotris(dimethy1amido)tantalum (2).282, The IR and NMR data of the methine carbon of (1) [13C: 6 = 64p.p.m., J(C-H) = 147 Hz (d)] indicate a notable C-N double bond character. Compound (I) reacts easily with methyl isocyanate, according to equation (10). The metallacycle (1)was shown to be an intermediate in the formation, above 100 "C, of the ethylimido complex (2)and ethylene. The reaction is first order with respect to the azametallacycle; the activation parameters were evaluated. The formation of ethylimino ( C , N ) groups (3) appears to be a general feature in the thermolysis of 'overcrowded' early transition metal diethylamido derivatives. It has also been observed for niobium, together with [Nb(NEt2),] as the major product. A similar threemembered metallacycle could not be isolated in the case of [Ta(NMe&], but was presumed to form as an intermediate in the facile and reversible incorporation of deuterium from 'HNh4e2 into the methyl groups (equation 11). The intermediate azametallacyclopropane (4) can apparently be trapped by terminal alkenes, resulting in the catalytic aminomethylation of the alkenes (Scheme 1). [Ta(NMe2)5] was recovered unchanged after the reaction, while the niobium analog was reduced to an as yet unidentified active species with evolution of methane. Hydrogen abstraction reactions from the (NR& ligand have been observed even at room temperature (equation 12).284 With diisopropylamidolithum, the reaction is more complex; the only product isolated (=10%) was the unusual methylene- and hydride-bridged compound (7; equation 13). Labeling experiments gave evidence that the hydride originates from the
2
Niobium and Tantalum
612 [Ta(NEt,),] 6
[
EtN, MeA,.Ta(NEtz)3]
+ [Ta(NEt)(NEt2),] + Et2N H
'
(9)
(1) 22%
M 'CHMe
(3) (CD42ND
[M(NMe2)5]14&180 "C
'
(Me)(CH,D)NH
Me
Scheme 1
diisopropylamide. It was suggested that the formation of a Ta-N bond is prevented by the steric bulk of the NPr' group; hydride donation then becomes competitive. These spontaneous or thermally induced reactions may all be viewed as unimolecuiar hydrogen abstraction or elimination reactions. With monoalkylamides, LY hydrogen transfer processes appear to be more facile than /3 hydrogen migrations, and open a route to nitrenes (equation 14). The rate of the hydrogen transfer process depends on the nature of the group (OR, NR2, CR3) bonded to tantalum; caution should be used in extrapolating the observed rate order, since it is based on compounds with the TaMe,(Cp') moiety. (5) R'NL $-Cp' = C,Me,;
R = Me; R' = Me; or Pr"
I
cQ
'
TaMe,(Cp')
34.2.3.5 Nimms Coordinated nitrenes W- have been thoroughly investigated in recent yearszp5Ya6owing to their formal similarity to carbenes and to their potential importance, e.g. as catalysts in the Haber ammonia process. Theoretical studies have suggested that imido ligands may also prove effective in promoting alkene m e t a t h e ~ i s . ~ ~
(i) Synthesis and structure The first tantalum nitrene was obtained in 1959 by thermolysis of [Ta(NEt2)I5.%This class of compounds is presently accessible by several routes, including hydrogen abstraction from the mono- or di-alkylamides, reaction of metallacarbenes with organic imines, oxidation of low valent species by organic azides, or reductive coupling of nitriles (Table 13). The tantalum derivatives are usually stabler than those of niobium. As stated above, IY H abstraction is favored with respect to /3 H transfer, and the unstable monoalkylamides are converted to nitrenes. [Ta(NBut)(NMe2)3](8) has been prepared by two
Table U Niobium(V) and Tantalum(V) Nitrene Derivatives (without Cyclopentadienyl Ligands) Compounds
Cornmentr
Re&
~-
[M(NR)X&I (R = Me, But, Ph; X = Cl, Br; L = THF R = Ph, NSiMe,; L = THF, PR,) [Ta(NMe)Cl,] [M(N~~)C~~(SM%)IZ [Ta(NPh)Np,(THF)I [Ta(NBu')CIzX(NHzBuc)1, (X = NHBu', OEt)
15NNMR; X-Ray structure for mer-[Ta(NPh)CI,(PEt,)(THF)]
1,2,3
Analytical data only X-Ray structure, M=M and N=N; metathesis Stable with respect to neopentylidene formation X-Ray structures; for related compounds see Scheme 2
4 5 1 6
[Ta(NSiMe,)Cl(CHCMe3)(PMe,),l [M(NR)(NR231 (M = Nb,Ta; R = But; R = Me; R = Bu; R i = BuMe; M = Ta: R = R ' =Et. Pr) [Ta(NB$j(NMe,)(OCPh&e&] [M(NBu )(OzCNM%)31 [M(NBu')(R@),l M = Ta; R = But; R' =Me, Et [Nb(NPh),Cl(dmpe)]
X-Ray structure for Pa(NBu')(NMe,),], Ta=NC required by symmetry; : alternate synthesis
linear
1 7,8,9
No further insertion of Ph$O
7 I
X-Ray structure for [Nb(Np-tolyl)(Et2dtc),1;
10
M = Nb, various R and R', see equation (17) 5
1. S. M. Rocklage and R. R . Shrock, J . Am. Chem. SOC.,1980,102,7808;1982,104,307'7. 2. M.R. Churchill and H. J. Wassennan, Inorg. Chem., 1982,21,223. 3. J. R. Dilworth, S.J. Banison, R.A. Henderson and D. R. M. Walton, 3. Chem. SOC., Chem. Commun., 1984,176. 4. P. J. H. Carnell and G. W. A. Fowles, 3. Less-Common Mer., 1962,4,40. 5. L. G.Hubert-Pfalzgraf and G. Aharonian, Inorg. Chim. Acta, 1985,100, L22;F. A. Cotton, S. A. Duraj and W. I. Roth,J . Am.
Chem. Soc., 1984,106,4749. 6. T. C. Jones, A. J. Nielson and C. E. F. Rickard, 3. Chem. Soc., Chem. Commm., 1984,205. 7. W.A. Nugent end R. L. Harlow, J . Chem. Soc., Chem. Conunun., 1978,579;W. A. Nugent, Inorg. Chem., 1983, ZX,965. 8. D.C.Bradley and I. M. Thomas,Proc. Chem. SOC.,1959,225;Can. 3. Chem., 1%2, 40,449, 1355. 9. D.C. Bradley and M. H. Gitlitz, J . C k m . Soc. (A), 1969,980. 10. L. S.Tan, G. V. Goeden and B. L. Haymore, Inorg. Chem., 1983,22, 1744
Niobium and Tantalum
614
routes (equations 15 and 16).=' Base- romoted deprotonation reactions of primary amido groups have also been used (Scheme 2).Fw [Ta(NMe,),] TaCl,
+ Bu'NH,
+ LiNHBu' + 4LiNMe,
-
Me,SWHBu'
M2ClKI -
-
+ 2Me2NH
[Ta(NBut)(NM&] (8)
[Ta(NBu')(NM%)),] + Me,NH (8)
+ 5LiCl
[M(NBu')C13(PMe3),]
+ PMe,.HCl
[M(NBu')Cl,(NH,Bu')
Jn
(15)
(16)
+ 2Me3SiC1
BuUH,
[M(NBU')C~,(NHBU')(NH~BU'>], (9) M = Ta Scheme 2
The structure of [TaCl(NBut)(NHBut)(NH2But)(p - Cl)lz (9) was shown to be dimeric with nearly symmetrical, long chloro bridges (Figure 6). Compound (9) is remarkable as imido, amido and amino ligands are present simultaneously on the same metal. Also of interest is the very short Ta-N(amido) bond and the large corresponding TaNC bond angle. The lengthening of the Ta-Cl bond trans to it has been considered as a confirmation of significant JC contribution to the Ta-N(amido) bond. A close intermolecular approach to the metal 2.0 A), resulting in pseudo seven-coordination, was observed for amido (Ta * - H-N: protons; a two-electron three-centered bond has been suggested.
f i p w 6 Molecular structure of [TaCI(NBut)(NHBu')(NH,But)]; (9) (reproduced from ref. 290 with permission)
More complicated sequences involving successive substitutions of the chlorine on the metal have also been used successfully to obtain nitrenes (equation 17).291A comparable approach led to the first Nb hydrazide( -2) compounds: [Nb(NNR,)(Rldtc),] (R,= Mez; PhMe; R' = Me or Et) and [N~(NN(CH,),}(E~,~~C)~].
+
+
NbC15+ 3RNH2 3Me,Si(Ridtc) --+ [Nb(NR)(Ridtc),] 3Me3SiC1+ 2RNH3Cl R = Me, Pf,Bu', But, Ph,p-tolyl, p - N C a O M e , R' = E t ;
R = Bu',p-tolyl,p-C,H,OMe,
(17)
R' = M e
Metatheses between neopentylidene complexes and or anic imines are an attractive simple general and selective entry to alkylimido compounds.2 T h e experimental results can be rationalized by the formation of an intermediate with an MGN ring (equation 18). They also provide a straightforward route to p-dinitrogen compounds, using PhCH=NN=CHPh (Section 34.2.3.6). Alkenylimides were prepared by nucleophilic attack of nitriles on the (Y
Q
Niobium and Tantalum
615
carbon of alkylidenes (equation 19). A similar reaction with acetonitrile was used to trap the unstable [TaC1(CHCMe3)(CH2CMe,)2] at -78 M=CHCMe3
R‘CH=NR
>
CMe, M-R’ N ’‘
[M(=CHCMe3)(CH,CMe,),] M=Ta; R=Me, Ph M=Nb;R=Me
--Mcf32H==CHR’
M=NR
(18)
R
a
(Me3CCH2)3M=N ‘CkCHCMe,
R/
]
(19)
Oxidative addition of organic azides to low valent species, mainly in oxidation state SIII, was found to be a general method for obtaining pentavalent mono- or di-nuclear nitrene derivatives.294Applied to Nb’ compounds, it led to the first dinitrene compound. Niobium(II1) and tantalum(II1) adducts with sulfur or nitrile ligands undergo remarkable reactions involving reductive coupling of nitriles and oxidation of the metal to pentavalent nitrenes.295*296’297 [Ta2C16(THT)3](lo) (THT = C,H8S) can serve as a convenient source of Ta”’; its reaction (equation 20) with nitriles is rapid and quantitative. As the reaction was not observed with pivalonitrile or benzonitrile, it appears to be sensitive to steric hindrance. [Ta2C16(THF),(p-C&&)] (11; Figure 7)297and the [NbzClB(MeCN)2(p-C~H~NZ)]2anions (12)295are centrosymmetric with octahedral arrangements around the metal atoms. Most remarkable is the new bridging 1,2-dimethyl-1,2-diimodoethylene ligand, which derives from two MeCN molecules by a process formally represented in equation (21). The Ta-N and Nb-N bond distances (1.75(1) A for both 11 and 12) imply a close to triple bond character (a distance of 1.70 A is expected for an MEN bond). The MNC arrangements are essentially linear, the organic skeleton being planar with a trm configuration. The central C--C distances are slightly longer than the standard C=C distance of 1.335 A. A notable lengthening, of ca. 0.20 %i, is seen in (11) for the Ta-0 distance trans to the multiple Ta-N bond.
[(THT)Cl,Ta( p-Cl)& -THT)TaCl,(THT)I
KC’
(10)
[
/R L&L,Ta=NC R>CN=TaC13L]
L = RCN; R = Me, Et, Pr”, Bur
Me N
I
Ill
c+c
-
Ill Me I
N
Me
:v
/NE ‘c=C ‘Me
n O
figure f
X-Ray structure of
[Ta,CI,(THF),(~-C~,N,)](11) (reproduced from ref. 297 with permission)
(20)
Niobium and Tantalum
616
X-Ray data are also available for monomeric nitrene derivatives such as [Ta(NBut)(NMe2)3] and [Nb(p-NC6H&e)(Et2dtc)3] (14).291M-N bonds compare well with those reported above, and the MNC angles are close to 180", in all but (14) in which the metal is heptacoordinated. The metal-nitrene bond was therefore described as an MENR triple bond, and the imido ligand as a four-electron donor. The oxo and organoimido ligands are isoelectronic. The N k N R bond in (14) is indeed just 0.04 A longer-the expected difference in radii between N and 0-than the oxo bond (1.74(1) A) in [NbO(Et2dtc),L These observations are consistent with those made for tantalum on [TaO(NPr2),] 85 and [ T ~ ( N B u ' ) ( N M ~.290 ~)~] All these compounds have high thermal stability, and are more soluble than their oxo analogs. They show a vibration around 135Ocm-l in the IR, whch shifts to lower frequency upan 15N labeling (Av(I4N-l5N) = 22 cm-1).299,292 This fairly high energy absorption, whose frequency is only slightly affected by the ancillary ligands, has been interpreted as a combination of the M-N and N-C stretching modes.291"NNMR data have been reported for both Ta" and Ta"' nitrenes, the latter appearing at higher fields.29zIn the 'H NMR, the key feature is a downfield shift of ca. 1p.p.m. of the hydrogen on the CY carbon of the NR The 13C chemical shift difference, A = SC, - SC,, was used as a probe for evaluating the electron density on nitrogen.300 (8),290 [Ta(NPh)Cl3(PEt3)(THF)I
(ii) Reactivity Information on the chemical reactivity of these nitrenes is limited; the NR groups are tightly bound to the pentavalent metal and not readily displaced. The Tav nitrenes formed by reductive coupling of acetonitrile appear to be relatively resistant to protic attack.'% [ M ( N R ) ( R ~ ~ ~undergoes C)~] reversible protonation by strong acids, but the site of attack, as well as the reaction products, could not be determined.291It is of interest that hydrogenation of [Ta(NR)Me2(Cp')] afforded the imido hydrides [TaH2(NR)L(Cp')] (E) and not the amide complexes, even in the presence of an excess of phosphine (equation 22).284 Compounds (15) appear to be, with the Re" derivatives [ReH(NR)C12(PPh3)2]and [ReH(NR)C1(OR')(PPh3)2],the only mononuclear transition metal imide hydride complexes described so far. Their hydrogens are 'hydridic'; they reduce acetone to the isopropoxo group (equation 23). This reaction and the metathesis of imidochloride derivatives by alkaline reagents292constitute alternatives for producing imido alkoxides. The action of alcohols removes the imido ligand (equation 24). Ta(OR)5 was obtained in high yields from [Ta(NPr)(NPr&], although the NR group is more resistant to alcohols than the NRz group.
+L
[Ta(NR)Me(Cp')]
H2(-4 atm)
[TaH,(NR)L(Cp')]
+ 2CH,
(22)
W)
-
R = But, CH,CMe3; L = PMe3, PPhMe, [TaH,(NR)L(Cp')] + 2 0 = C M e ,
(W
-
R = But, CH,CMe,; [TaH,(NR)L(Cp')]
+ MeOH
(W)
[Ta(NR)(OCHCM%)(Cp')]+ L
(23)
L = PMe,, PPhMez [Ta(OMe),(Cp')]
+ FWH, + L
(24)
L=PMe3
The various Nb and Ta nitrenes were converted to oxo derivatives by an excess of carboxylic c ~ r n p o u n d sThis . ~ ~seems ~ ~ ~characteristic ~~ of the nucleophilic reactivity of nitrenes, but water is sometimes required (equation 25). [M(NBu')(Me,dtc),]
+ PhCHO
[M(O)(Me,dtc),]
+ Bu'N=CHPh
dry
Apart from the reactivity involving the NR group, the above compounds display the
Niobium and Tantalum
617
reactivity of their ancillary ligands. Compound (8) undergoes the typical reactions of the NR2 ligands with electrophiles (ROH,R2C0, C02, CSz or RI). The insertion observed with ketones (equation 26 is noteworthy, both for its reversibility and for its sensitivity to steric restrictions.286 Alkoxo substituents allow the reactions of an alkylidene ligand to give metathesis products with alkenes.301 An imidoalkylidene analog [Ta(NSiMt@Cl(CHCMe3)(PMe3)2]reacted with ethylene to give rearrangement but no metathesis products.292
[Bu‘N=Ta(NMe,)
Ph2C=O(excass)
3]
(26)
B u ‘ N = T a/ O 4 CCPPhh 2 N mM e 2e ,
09
\ NMe2
I C 0 2
[ButN=Ta( O,CNMe,) 4
342.3.6 Dinitrogen activation The reaction of niobium and tantalum derivatives in low oxidation states with N2 led to [{N~BU(C~)Z}Z(NZ)(O~)] (equation 27) ,302 [{NbCl(dmpe)2}z(p-N2)]303 (equation 28) and [NbzCl,(Bu:P)3(N2)] (equation 29),304which were only poorly characterized owing to their low stability. IR investigations of the interaction between the terminal Nz ligand of brans[ReCl(N2)(PhPM&),] and a number of acceptor transition, metal halides showed that NbC1, produces the greatest shift of the Nz stretchin frequency (292crn-l), but the nature of the polynuclear species formed was not reported?” The development of this exciting area is mainly due to Schrock and coworkers, who were able to obtain stable tantdum p-N2 compounds in high yields, by reducing neopentylidene complexes in the presence of N2 at normal pressure (equation 30).292,306
2NbC15+ 3PBu:
-t Mg (excess)
CHlQl
Nz(lsm)
[Nb,C&(PBu:),(N,)]
+ MgC1,
(29)
The structure of [{Ta(CHCMe3)(CH2CMe3)(PMe3)z}z(p-Nz)] (17) revealed an unusually high activation of the Nz ligand, with formation of a close to linear Ta(p-N2)Ta bridge, ~ystern.~” This ‘diimido’ character is confirmed by the fact that formulated as a Ta=N-N=Ta similar compounds are accessible in high yields by reaction of tantalum or niobium alkylidenes with a diimine reagent, PhCH=NN=CHPh (equation 31) .292,308 The molecular structure of the centrosymmetric [{TaC13(PBz3)(THF)}z(p-N2)] (19) is depicted in Figure 8. The p-N2ligand is the sole bridge between the two metals, which present distorted octahedral environments with a meridional arrangement of the chlorine atoms.3w The Ta-N bonds (1.796(5) A) are much
Niobium and Tantalum
618
shorter than the usual TaV-N single bond (ca. 1.978,), and close to the Ta=N distance of 1.765(5) 8, found in It is also notable that in the alkylidene (17), the Ta-N than the Ta=C bonds (av. 1.935(9)A). These data support the description of the Ta-N linkages in (17)and (19)as double bonds. The N-N bond length of 1.282(6)A in (19) is significantly longer than in free dinitrogen (N=N: 1.0976 A), suggesting a bond order between one (N-N= 1.45 %.)and two (N=N = 1.24 A). The production of such p-dinitrogen compounds with M=N-N=M units could become a common feature in niobium and tantalum chemistry (Table 14).
PBz3
FEgure 8 Molecular structure of [(TaC13(PBz,)(THF)},(B-N,)1 (19) (reproduced from ref. 309 with permission)
These p-Nz complexes contain the longest most 'activated' N2 groups reported to date for simple M2N2 systems (longer N-N bonds, ca. 1.35& are known for complexes such as [{(PhLi)3Ni}2(N2](Etz0)2]2 and related species, but in which NZ interacts with more than two metallic centers ). They contrast sharply with the p-Nz complexes reported for group 1VA metals, where these bridges are almost linear and relatively short (N-N bonds ranging from 1.15 to 1.188, for [{M'(N2)(Cp')z}2(p-N2)] (M' = Ti311aor Z13lIb), the corresponding metalnitrogen bonds being relatively long (2.00 to 2.09 A). If one considers the metal to be TaV, the Ta=N bond then implies a combination of ligand to metal (T and n donation from a formal Ni- ligand, with unit N-N bond order. The experimental N-N bond lengths suggest bond orders between 1 and 2, consistent with donation of electrons from the ~d"2p orbitals of Ni- into the empty d orbitals of TaV.320This description also accounts for the eclipsed conformation of the equatorial ligands in (19), although the staggered one should be the more favored sterically.309 Compounds (17) and (19) are formally 1Celectron species. By contrast, the p-dinitrogen complexes [{M'(N~)(CP')~}~( q-Nz)](M' = Ti, Zr) are respectiveiy 16-electron and 18-electron species, and the low oxidation state of the metal is preserved. The spectroscopic data also illustrate the differences between the tantalum and these Til' or Zr" p-dinitrogen compounds. ~ shift comparable to that The I5N spectrum of (16) shows a singlet at 4 1 4 ~ . p . m . a, ~chemical Table 14 Niobium and Tantalum p-Dinitrogen Compounds Compounds ~
~~~~~~
~~
~
[{MC~,(THF)Z}~(PL-N~I [{TaC&}z(PL-N2)I (L = PEt,, PWMe,, PhPEt,, P w ) [{Tac13(pBZ3)(~)}~(p-N2)1 [ { M ( C H 2 ? ' @ 3 ) 3 ( m }) 2(P-N2)1 {Ta(OBu ) 3 ( m ) 1z(P-N.2)1 [{M(Etzdtc)3}zNz1 [{Ta(CHCMe3)C1~}Z(~-N,)1
(L = PMe,)
Ref.
Comments ~~
Insoluble for M = Ta v(Ta-N,) = 855 cm-'; tram-mer isomer
u 3 1,2
X-Ray structure, linear Ta=NN=Ta
2,4
linkage
Fluxional "N NMR = 414 p.p.m. (NH,)
[{Ta(CHzCMe3)(CWCMe,)~},(p-Nz)] X-Ray structure linear, Ta==NN=Ta linkage, trigonal bipyramidal coordination (L = PMe,) 1. S. M. Rocklage and R. R. Schrock, J. Am. Chem. Soc., 1980, lot, 7809. 2. S. M. Rocklage and R. R. Schmck, J . Am. Chem. Soc., 1982, 104,3077. 3. J. R. Dilworth, S. J. Harrison, R. A. Henderson and D. R. M. Walton, Cham. Commun., 1984, p. 176. 4. M. R. Churchill and H. J. Wasseman, Inorg. Chem., 1982, 21, 218. 5. S. M. Rocklage, H. W. Turner, J. D. Fellmann and R. R. Schrock, Oganometuhs, 1982,1, 703.
2 2 3
1,5 196
Niobium and Tantalum
419
found in imido compounds such as [Ta(NPh)C13(THF>2](S = 350-370 p.p.m.), but different from the 679 p.p.m. observed for the bridging dinitrogen in [{Zr(Nz)(Cp')}z(p-Nz)].311b lSN labeling experiments have established that the Ta=N-N=Ta group is characterized by an bridge is also absorption around 845 cm-I in the IR. This description of the Ta=N-N=Ta supported by the reactivity of these p-NZ adducts, which behave as imido complexes. Thus the addition of an excess of acetone produced dimethylketazine, Me2(kN-N=CMe2. Several p-NZ complexes also reacted readily with excess HCl to give hydrazine (as Nz€&.2HCl) only; no ammonia was d e t e ~ t e d . ~ ~ ~ ~ ~ , ~ ~ ~
36.2.3.7 Azides (Table 15) Both neutral and ionic niobium and tantalum chloride azides are known. The neutral azides [MN3c14]2 were obtained from MC15 and C1N3.312They both explode spontaneously, although the tantalum derivative is somewhat stabler. The more stable ionic [MN3X5]- (X=C1, Br) species were synthesized from the hexahalometallates(V) and iodoazide or from the pentahalides and phosphonium or arsonium azide^.^^,"^ Table 15 Azido and Phosphineiminato compounds
Compounds
Ref.
CO?W&WtS
(a) Azido cornpun& [MCI&I, (PPh4)[MXSN31 (X = C1, Br) (AsPh4)[TaClSN3] (Et4N)[NbCkN,I [Ta"iR3)212(N,),I
MCl, + ClN, or NaN,; explosive; X-ray structure for M = Ta MX, N3PPh3; non-explosive; X-ray structure for M = Nb and X = C1 (disorder of the azido group)
+
NbCl, + Et,",; non-explosive; analysis only [TaCl, {N(SiMe3),} 2] + Me,SiN3
1
2,3
2 4
5
(b) Phosphaneirnilaato cornpoundF X-Ray structure for both M = Nb and Ta [Mc14(Npphdlz (PPh4)[MXsW"h3) I (X = CI, Br) (AsPh.,)[NbCI,(NAsPh,)] Photochemical synthesis PPh,; X-ray structure, i~rrr?sisomer ( p p h ~ ) [ T a c l ~ ( N p ~ h ~ ~ TaC&N3 ~~ [(NHPPh,)TaC14(NPPh3)] Proton abstraction from the solvent (C#,Cl,) by [TaCl,][TaCl,(NPPh,)J
+
6 7 3,2 3 6 7
1. J. Strtihle, 2.Anorg. Allg. Chem., 1974,405, 139. 2. R . Dubgen, U. Muller, F. WeUer and K. Dehnicke, 2.Anorg. Allg. Chem., 1980,471, 89. 3. U. Muller, R. Dubgen and K. Dehnicke, Z. Anorg. Allg. Chem., 1981,473, 115. 4. M. Kasper, R. D. Bereman, Inorg. Nucl. Chem. Lett., 1974,10, 443. 5 . R. A. Andersen, Inorg. Nucl. Chem. Len., 1980, 16, 31. 6. H. Bezler and J. Striihle, Z . Naturforsch., Teil B, 1979, 34, 1199. 7. H. Bezler and J. Str2hle, 2.Nmrforsch., Ted B, 1983, 38, 317.
The structures of [TaN3Cl& '12 and (PPh)[NbN3C15]73 have been determined. Two independent dimeric molecules, of (2% and D2h symmetry, were found for the former with azido groups bridging the two metals in a slightly asymmetric manner, in a dimer of C, symmetry (Figure 9). The (Ta-p-N) rings are planar, and the Ta-N linkages are all longer than expected for single bonds (2.08 The azido compounds are characterized by stretching frequencies around 2100 [v,,(N3)], 1350 [v,(N3)] and 420 [v(M-N)] cm-' in the 1R. Both the neutral and the ionic azides undergo reactions analogous to the Staudinger reaction, giving compounds containing the phosphineiminato M=N=P group (equation 32)313,314The reaction between [TaN3Cl4I2 and PPh3 provided [TaC4(NPPh3)I2 and [TaC14(NPPh3)(NHPPh3)].314 The latter was suggested to result from the reaction of the initially formed [TaCb][TaCi4(NPPh3)2]salt with the solvent C2H4C12. As a result of the higher stability of the tantalum azides, the reactions with [TaN3X,]- must be activated photochemically. Attem ts to extend the Staudinger reaction to AsPh3 were successful only with [NbN,Cl,]-. 7 9
A).
[NbN,Cl&
+ 2PPh3
-
[NbCL(NPPh,)],
+ 2N2
(32)
620
Niobium and Tantalum
N
Figure 9 X-Ray structure of the [TaCl,N,lZ dimer of C,, symmetry (reproduced from ref. 312 with permission)
Centrosymmetric dimers with terminal phosphineiminato groups were found in the solid for [MCL,(NPPh3)I2 (Figure 10).313-314 The asymmetry of the bridge was interpreted as resulting from the strong trans effect (0.34 A for M = Ta) of the phosphineiminato ligand, which also leads to a very long Ta-C1 bridge. The PNM sequence is linear, and the distances consistent with P - N and M=N double bonds. The Ta-N bond is longer (1.97A) in trans[TaCL(NPPh3),]-, while the N-P distance is only 1.56 8,and the TaNP arrangement is slightly bent (162"). Thus the Ta-N linkage exhibits almost single bond character, probably as a result of the unfavorable trans position of the phosphineiminato Although no X-ray data are presently available for the arsineiminato [NbC15(NAsPh3)]- compound, the IR data suggest a lower n character of the As=N bond with respect to the P=N bond.
Figure 10 Structure of the phosphineiminato complex [TaCI,(NPPh,)], (reproduced from ref. 313 with permission)
X23.8 Nitrides (Table 16)
The obtention of nitrides by thermal decomposition of the azides was only successful for the synthesis of [MNBr4I2- from [MN3Br5]-. Thermolysis of MX5 with ammonium salts afforded a more eneral access to nitride halides in oxidation state V, or IV if higher temperatures were used.3 6217 Oxo nitrides were obtained if moisture was admitted during the pro~ess.~" The only well-characterized nitrides are [TazNBrld3- 316 and the oxo nitrides MON.3'8 The structure of the former is similar to that of [TazOFlo] - (Section 34.2.6.1.i), with a symmetrical linear TaNTa arrangement and a Ta-N bond length (1.849(2) A) consistent with double bond character. The ligands cis to the bridge are eclipsed; the lengthening of the axial Ta-Br bond (0.18w) reflects a strong trans effect. A polymeric ribbon type structure with nitrido and halo bridges has been assumed for all nitride halides [MNX,], (X= F, Cl, Br).319,3" Their stability decreases from the fluoride to the bromide, as does the bond order of the M=N linkage, as reflected for instance by the diminution of the Nb==N frequency in the IR from 800 for X = F to 720 cm-' for X = Br.
8
3423.9 Miscellaneous ( i ) Porphyrin and related complexes
Porphyrin derivatives of NbV and Tav were first reported in 1972 and have since aroused increasing interest. Fluoride complexes MF3L were obtained from MC15 and the octaethylporphyrin in the presence of HF (L = OEP).321*322 Niobium chloride and bromide
Niobium and Tantalum
621
Table 16 Pentavalent Niobium and Tantalum Nitrides Compounds
Commenrs
ReJ
~
(NH4)3[M2NBriol (PPh&[mBr4] [(TaNCI,),I [(NbNX,),] (X = F, Ci, Br) [MONI [Tam
NH,Br + NH,MBr, at 400 "C; X-ray structure Decomposition of [MBr,N,]-, v(M=N) = 900cm-' TaC!, NH,CI at 350 "C; polymeric (ribbons) NbX, NH4X at 210 "C (X = C1, Br); NbNCl, + F2 at room temperature (NH,)[MCI,] heated in the presence of water or MOCI, + NH,; X-ray structure, heptacoordination TaCI, NH,CI at 850 "C, X-ray structure
1
2 3
+ +
4
+
6
5
1. K. P. Frank, J. Strahle and J. Weidlein, 2. Naturforsch., Teil B, 1980, 35, 300, M. Harner, K. P. Frank and J. Strahle, 2. Naturforsch., Teil B, 1986, 41,423. 2. R.Dubgen, U. Muller, F. Weller and K.Dehnicke, 2.Anorg. Allg. Chem., 1980,471,89. 3. M. A. Glushkova, M. M. Ersbova and Y. A. Buslaev, Rws. .I. Inorg. Chem. (Engl. Transl.), 1965,10, 1290. 4. E. M. Shustorovich, 1.StrUa. Chem. (Engl. Trams[.),1962,3,204; S . M. Simtsyna, Rws. J . lnorg. Chem. (Engl. TransL), 1y17, 22, 402.
5. W. Weishaupt and J. Strtihle, 2.Anorg. A&. Chem., 1977, 429, 261
6. J. StrBhle, 2.Anorg. Allg. Chem., 1413,402,41.
complexes have been prepared by treatment of [NB203L]with dry gaseous HX for L = TPP (meso-tetraphenylporphyrininato),TMTP (meso-tetra-m-tolylporphyrinato)and TPTP (mesotetra-p-tolylporphyrinato) The kinetics of the solvo rotolytic dissociation of [NbCl3(TPP)-] in MeCOOH/H2S04 mixtures have been studied.3;h: The reaction of porphyrins with NbC15 afforded after hydrolysis the [NbzO&] dimers,326 with L = OEP, TPP, TPTP and OMP (octamethylporphyrinato). An unusual geometry was The Nb atoms found for these dimers, as illustrated in Figure 11 in the case of L = TPP.327,330 are heptacoordinated with the two Nb-TPP units linked together through three p-oxo bridges; Nb atoms lie 1.008, from the 4N planes of the porphyrinato ligands. The geometry of this do-do [(NbL),03] compound was rationalized in terms of optimal Nb-0 ~t interaction^.^^' m323332.1
Figure 11 Coordination polyhedron of [Nb2(TPP),0,] (reproduced from ref. 330 with permission)
5
Bubbling HF through solutions of the NbZO&] dimer in anhydrous benzene afforded [NbOFL] (L = OEP, TPP, OMP, TPTP). 21,332 The structure with L = OEP showed a cis geometry on the six-coordinate niobium. The metal-macrocycle distance of 0.905 A is shorter than in seven-coordinated complexes, probably as a result of decreased steric hindrance ( N W : 1.7620(6) A; Nb-F: 1.888 A). Addition of HI to [Nb ~0 3 (0 E P )~] in CH&h solutions yielded [NbOI,(OEP)], which on heating under vacuum liberated iodine and [NbOI(OEP)] .322 [NbO(TPP)(acac)] was prepared by reaction of [Nb203(TPP),] with excess a ~ a c H . ~ % Recrystallization of [NbZO&J from acetic acid gave [NbOL(MeCOO)(MeCOOH)] {L = OEP, TPP, TPTP and The structure of the L = T P P d e r i ~ a t i v e ~ ~showed ' , ~ ~ ~the niobium atom to be seven-coordinated by the four nitrogen atoms of the porphyrin and by the three oxygen atoms, which are cis to each other; the metal is displaced by 0.99(2) A from the porphyrinato plane towards the oxygens. A high resolution NMR study of [NbOL(MeCOO)(MeCOOH)] and [Nb203(0EP),] revealed the anisochrony of the methylenic protons of the porphyrin, induced by the out-of-plane position of the metal atom. The redox chemistry of [NbO(TPP)(MeCOO)] was investigated by means of cyclic voltammetry and controlled potential electrolysis; the reduction of NbV to Nb'" was found to occur prior to the reduction of the ligand. The redox potentials were measured for the [NbVO(TPP)(MeC00)]/[Nb'VO(TPP)~/[Nbn'(TPP)]+/~Nb1 (TPP)] systems and found to be -0.94, 1.1 and -1.48Vrespectively.
622
Niobium and Tantalum
. Irreversible oxygenation of the low valent [NbX2L] porphyrins by O2 yielded [Nb(Q)X,L] which, from ESR and IR data, was described as a superoxide.323 Phthalonitrile and phthalocyaninedilithium react with MX5 to yield [MX3(PC)].335*336 The reaction between [TaC1,Me3] and Nasalen yielded [Ta(salen)Me3].277 (ii) Pyrazolylborate derivatives
Tantalum pyrazolylborate derivatives [TaC1Me3{HB(pz)3}] (20) and [TaC1Me3{H2B(3,5Me,pz),}J (21) (pz = pyrazolyl) have been reported and characterized by X-ray and NMR studies.33 Complex (21)adopts a capped octahedral structure in the solid with a close contact between tantalum and one hydrogen, which was described as a three-centered B-H-Ta bond, filling the seventh coordination site (Figure 12). The Ta-B distance is short (2.90(1) A; usual M' - - - B distances are ca. 3.8 A) probably as a consequence of extreme puckering of the six-membered ring. A methyl group is found to cap the face formed by C(5), C(6) and Cl(4); there are two geometrical isomers (one chiral) in the crystal. The existence of the Ta-H-B interaction was also shown by IR (v(B-H) = 2013, 2070 cm-')and by "B NMR in solution, where it is preserved up to at least 110 "C. Dynamic behavior, which equilibrates the three isomers present in solution, was observed.
Figure 12 Molecular structure of one isomer of [TaCIMe3{H,B(3,S-Me~pz),)l (21) (reproduced from ref. 337 with permission)
Reactions of K[HB (Me2pz)3] with MCls in equimolar proportions yielded [MC14{HB(Me2pz)3}] and [HB(Me2pz),BH][MC16]." The presence of the novel dibora cation was confirmed by X-ray diffraction in the case ot tantalum. Acid-catalyzed cleavage of €3-N(pyrazoly1) bonds also takes place and generates, for instance, [NbC1{HB(Me2pz)3}(Me2pz)3]with pyrazolate ligands. Reactions between NbCL and K [ H B ( ~ z ) ~and ] K H,B(pz),] have led to ready reduction to Nb'" products (Section 34.3.5.4.iii); the Nb salts K[NbC15{HB(~)3}]and K[NbC15{H2B(pz)2}]could however be isolated from low temperature reactions.33
I
(iii) Nitrogen-containing insertion products (Table 17)
Reactions of [MCIS-,Mex] (x = 1, 2 or 3) with heterocumulenes have led to insertions into the M-C bond.* N , N'-Dialkylacetamidinato derivatives339were obtained (equation 33). N-Acetamides or N-thioacetamidesW were formed with isocyanates or isothiocyanates, respectively (equation 34). The rate of insertion varied in the order [MC14Me]> [MC13Me2]> [MC12Me3],following the established order of acceptor properties of the methylmetal chlorides, the isocyanates being more reactive than the isothiocyanates. Hexa- or hepta-coordinated species were obtained.
623
Niobium and Tantalum
Table 17 Niobium and Tantalum Nitrogen-containingInsertion Products (Acetamidinato, Acetamido, Thioacetamido and NitrosohydroxylaminatoDerivativea)
Cornmentr
Compounds
[MCl3X'{NRC(Me)NR}] (R = Pr', Cy, p-tolyl; X' = Cl, Me) [MCI,Me2{ NRC(Me)NR) ] (R = Pf, Cy; M = Ta; R =p-tolyl) [TaCI,X' {NRC(Me)NR}2] (R = Pr', Cy,X' = C1, Me; R =p-tolyl, X' =Me) [MCLzX'{NRC(O)Me},] (R = Me, Ph; X'= Cl,Me) [TaCl,{ NMeC(0)Me}3] [MC14{NRC(S)Me}] [TaCI3Me{NRC(S)Me} ] (R = Me, Ph) [NbCl,{NMeC(S)Me),] [MCl,X' {ON(Me)NO},] (X' = C1, Me; X' = OMe, M = Ta) [NbMe,{ ON(Me)NO},] [TaMe3{ON(Me)NO)~1, [ W O ) {ON(Me)NO)(cP)2ln
Ref.
Six coordinated
1
Six coordinated, Me probably cis
1
X-Ray structures: pentagonal bipyramid for X' = C1, R = Pr' (orthorhombic and triclinic)
1,293
Bidentate acetamide, X' = Me unstable for M = Nb
4
Probably seven coordinated M = N b , R = M e ; M=Ta, R = P h Disproportionate rapidly for M = Nb
4
X-Ray structure: pentagonal bipyramid M = Ta, X' = Me; X-ray structure: pentagonal bipyramid
5,6
Eight coordinated
8 8 8
Final product from [NbMe,(Cp),]
+ NO
5 5
7
1. 5. D. Wilkins, J. Orgonomet Chem., 1975, SO, 349. 2. M. G. B. Drew and J. D. Wilkins, J. Chem. Soc., Dalton Trans., 1974, 1579, 1973; 1975, 2611. 3. M. 0.B. Drew and 3. D. Wilkins, Acta Crystallogr., Sect. E . 1975, 31, 177, 2642. 4. J. D. Wilkins, J . Organotnet. Chem., 1974,61, 269. 5. J. D. Wilkins, J . Organomt. Chem., 1974, 65, 383. 6. M. 0.B. Drew and J. D.Wilkims, J. Chem. Soc., Dalton Trans., 1974, 198. 7 . J. D. Willcins and M. G. B . Drew, 1. Organornet. Chem., 1974,69,111. 8. A. R. Middleton and G. Wilkinson, J. Chem. SOC., Dalton Trans., 1980, 1888.
The coordination polyhedron of the metal in its N ,N-dialkylacetamidinato derivatives is generally a distorted pentagonal bipyramid with two chlorines in the apical position~.[TaCl~Me{CyNC(Me)NCy}~] adopts a different geometry, with one bidentate ligand in equatorial sites, while the other spans axial and equatiorial positions, thus maximizing the intramolecular distances between the cyclohexyl group .411 Comparable insertions occurred when methylmetal derivatives were exposed to NO, yielding N-alkyl-N-nitrosohydroxylaminatocompounds (equation 35).342 Nitric oxide may however also behave as an oxidant, and the reactions are sometimes more complicated (Section 34.2.6.3).343
34.2.4
Pseudohalo Derivatives
34.241 Cyanides and isocyanides
NbCls and HCN yielded [NbCl,(CN)(Et20)] in EtzO and [NbCl,(HCN)] in CCL. The addition of NEt3 to the latter afforded (HNEt3)[NbC15(CN)]; the corresponding bromo complex was also obtained.344 No isocyanide complex of NbV or Tav a ears to have been reported. The insertion of isocyanides into M-C bonds of [TaCl,Me$' and into the M-X bonds of MX5345-347 has however been described. [Iv&{CX(NR)}(RNC)] was isolated (R = Me, But). In the case of R = Me,345 further insertion occurred, yielding [MC13{CC1(NMe)}12-(MeNC).
Niobium and Tantalum
624
[Ta3r2{Br(NMe)}3(MeNC)] could even be obtained by the reaction of TazBrlo with excess MeNC in CH2C12; in EtzO, the same reactants afforded an oxo species, [TaOBrz{CBr(NMe)}(MeNC)] .347 The monoinserted compounds [M&{ CX(NMe)}(MeNC)] reacted with PPh3 to form, through a second insertion, [MX3{CX(NMe)},(PPh3)].345 [TaBr3{Cbr(NMe)}(dppe)]Br was prepared directly from [TaBr5(dppe)] and MeNC. Attempts to induce further insertion under the action of phosphines when R = B u t instead of Me resulted in the displacement of the terminal isocyanide ligand to yield [W{CX(NBu')}L], where L = PPh3, PMe,Ph, PMePhz, dppe .347 34242 Thio- and seleno-cymates (Table 18)
The synthesis of M(NCS)5 complexes was first effected by allowing KSCN to react with TaC15 in MeOH.348They were isolated from MeCN solutions as A[M(NCS)6(MeCN)]. Complexes of the type [MC1x(NCS)5-xL](x = 0-4) were obtained on varying the KSCN/MC15 stiochiometry in MeCN or Et,O. [TaX5L] (X = NCS or NCSe) formed with L = bipy or phen. [NbzCllo{p&H4(SCN)Z}] was obtained from Nb2Cllo and the organic thiocyanate in CC14.34993NbNMR data on MeCN solutions of NbCl, in the presence of KSCN350were taken to indicate the presence of [Nb(NCS),&-,](X = C1 or Br) and [Nb(NCS)#-. Tsble 1% Isothio- and Isoseleno-cyanides Compounds
A[M(NCS),I A[M(NCSe),l [MClx(NCS)s-xSl [M(NCS)sIz [Ta(NCS),L], [Ta(NCSe)(bipy)] [M(NCS)z(OR)dJiFY)l [M(NCS)2(OR)*(dbm)l tM(NCS)(OR)3(dbm)l
Comments
A = Li, Na, K, NH,, AsPh, A = K, AsPh, x = 1-4; S = MeCN, Et,O
L = py, bipy R = Me, Et; octacoordinated dimer R = Me, Et, Pr, Pr', Bu, Bu' X-Ray structure (R = Et): hexamordination R = Me, Et, Pr' X-Ray structure (R = P i ) : hexamordination
Ref:
1-4 5 1,6,7 5 8 9 10 11 10 12
1. H. BBhland and E. Trede, J. Less-Common Met., 1967,13,224. 2. T. M.Brown and G. F. b o x , 3. Am. Chem. SOC., 1967,89, 52%. 3. G. F. Knox and T. M. Brown, Inorg. Chem., 1969, 8, 1401. 4. G. F. Knox and T. M. Brown, Inorg. Synrh., 1971, U, 226. 5. T. M. Brown and E. Zenker, J. Less-Common Met., 1971,25, 397. 6 . H. Bohland and E. Zenker, 3- Less-Common Mer., 1968,14,397. 7. H. Bohland, E. Tiede and E. Zenker,J. Less-Common Met., 1968,15,89. 8. J. N. Smith and T. M. Brown, Inorg. Chem., 1972 11, 2697. 9. N. Vuletic and C. Djordjevic,J . Chem. SOC., Dalton Trans., 1972, 2322. 10. R. Kergoat, M.-C. Senechal-Tocquer,J. E. Guerchais and F. Dahan, BUZZ. SOC. Chim. Fr., 1976, 1203. 11. F. Dahan, R. Kergoat, M.-C. T q u e r and J. E. Guerchais, Acra Ci-ysrallogr., Sea. B, 1976,32, 1038. 12. F. Dahan, R. Kergoat, M.-C. Senechal-Tocquer and J. E. Guerchais, J. Chem. Soc., Dalton Tram., 1976, 2202.
Alkoxoisothiocyanato complexes [M(NCS)*(OR),(dbm)] or [M(NCS)(OR)3(dbm)] have been synthesized from NbC15, KSCN and dibenzoylmethane (dbmH) in the appropriate alcohol.351The [M(NCS),(Ok)3(bipy)] adducts were obtained from [MC12(0R)3].352 [Mz(NCS)lo]reacts with ammonia and with primary and secondary aliphatic amines to yield [M(NCS)(NH,),(NH)],, [M(NCS)3(NHBu)z(H2NBu)]or [M(NCS)3(NEt,)z(HNEt,)1.353Similar reactions with [M(NCS)5(MeCN)] afforded, through aminoIytic cleavage of the M-NCS bonds, [M(NCS){NC(NH2)Me},{ MeC(NH)NH,}], and [M(NCS)3{NC(NHBu)Me} { MeC(NH)NHBu}] complexes .353 The isothiocyanate groups are N bonded, in agreement with the hard and soft acid-base model. The structures of [Nb(NCS)z(OEt)2(dbm)]355 and [Nb(NCS)(OPri)3(dbm)]356showed the niobium atom to be located at the center of a distorted octahedron (Nb-N: 2.08&(5), 2.101(5) and 2.176(7) 8, respectively). Bonding of the thiocyanate ligand through its sulfur atom has however been proposed357on the basis of 93NbNMR investigations of acetonitrile solutions containing [Nb(NCS),(SCN),C&-~,,,~]-; [Nb(SCN),]-, the first example of a homoleptic complex in
Niobium and Tarataluh
625
which the thiocyanate ion is S bonded to a class a metal ion, was shown to be present in these solutions at concentrations comparable to those of [Nb(NCS)&. 34.2.5
Oxo, Thio and Seleno Halides
34.25.1 Oxo halides The synthetic routes available for the preparation of the known oxyhalides, listed in Table 19, range from the halogenation of the oxides to the carefully controlled reaction of the halides with dioxygen. Table 19 Anhydrous Oxo Halides of Niobium(V) and Tantalum(V) Fluorides
Chlorides
Bromides
Iodides
(i) Oxo fluorides MOF3, in contrast to the other oxo trihalides, has not been fully characterized but merely described as an intermediate formed during the hydrolysis of MF,, or the thermal decomposition at 600 "C or above of M02F.358The IR on the vapor above Nb02F at 600-800 "C shows a strong band at 1030 cm-', which was assigned to the v(Nb0) stretching vibration of NbOF3;on cooling this species was reported to decompose into NbOzF and NbF5.359 TheY Nb02F and TaO2F were obtained by allowing the oxides to react with HF or F2.3ho,361 show an Reo3 type structure in which the fluorine and oxygen atoms are randomly distributed in octahedral positions around the metal atoms.360The thermal decomposition of TaOzF was reported to result in the formation of gaseous TaF5 and TaOF3 and in the release of solid Ta307Fand Ta205.358 Nb307F and Nb5OI2Fwere prepared by heating mixtures of NbzO5 and NbOzF in sealed platinum tubes at 800°C. Nb307F crystallized in the orthorhombic system359with the U308 structure type.362
(ii) Oxo chlorides, bromides and iodides Of the known oxo halides (Table 19), NbOC13 is the most extensively studied, partly because it forms during the preparation of NbClS from which, owing to their similar volatilities, it is extremely difficult to separate. NbOC13 can ho&ever be conveniently prepared by the reaction of oxygen on NbCl5 at 300-500 "C, or of anhydrous NbzOs with an excess of NbCls vapor.3 The activation energy of 93.5 kJ mol-' found for the formation of NbOCl3 from solid Nh O5 and molten NbC15 is relatively low, and this redistribution has been shown to be mainly diffusion controlled.363Partial hydrolysis of NbC15 by H2*O has been utilized to prepare Nb'80f13.364 NbOC13 was also formed when NbC15-R20 adducts were heated, as well as from the reaction of NbC15 with oxygen-containing ligands such as sulfoxides, phosphine oxides or arsine oxide^.^ Similar reactions were observed with substituted halides. The structure of NbOC13 consists of chains of dmeric Nb2C16 units linked together by bridging oxygen atoms, the coordination around each niobium being approximately ~ c t a hedr al.~ A strong band at 770 cm-I in the IR was attributed to the Nb-0-Nb stretching vibration. The Raman spectra of gaseous N b O Q was shown to be consistent with a monomeric C4umolecule, and the vibration found at 997 cm-' is characteristic of an Nb=O double bond.
Niobium and Tantalum
626
TaOC13 has been much less extensively studied. Unlike NbOC13 it can apparently not be prepared by partial hydrolysis of TaCL3 Disporportionation of MOC13 at 300-400 “C yielded MO2C1 and MC15.3 Nb307C1 has been prepared from Nb205 and NbOC13 at 600 “C; its structure shows octahedrally coordinated metal atoms. A tantalum analog has been mentioned. NbOBr3 was obtained by methods analogous to those used for NbOC13.365A broad complex absorption around 74Ocm-’ in the IR, characteristic of Nb-4-Nb chains, and in the NQR two groups of lines with markedly different frequencies, indicating the presence of both terminal and bridging bromine atoms, attest that the structure of NbOBrS is analogous to that of NbOC13. TaOBr3 and TaOzBr have also been ~ b t a i n e d . ~ NbO13 was conveniently prepared by the reaction of Nb205 with NbIs, or from a mixture of niobium and i ~ d i n e It . ~ decomposes to NbO12. M021 has been obtained as red needles by heating a 1:2 :6 molar ratio of M : Mz05:I2 at cu. 500 0C.3By contrast with the oxo triiodides, they are air stable. 34.25.2 Thio and seleno halihs and their complaes
The thio and seleno derivatives reported so far for NbV and Tav appear to be limited to the [M&7]4-,366 (Y = S or Se),367 [NbOS3I3-368 and [NbO2S2I3-369 anions (Section 34.2.8.2.i), to [MYX3] species and to some of their adducts and derivatives. The thio and seleno halides [MYX,]were prepared by the reaction of Sb2Y3with MCIs at room temperature in CS2.370MSC13 was formed when BzS3was allowed to react with MCl5?’l Myx,, with Y = S, Se, and X = C1, Br, were obtained through thermal decomposition of MYZXZ. 372 [NbSX3(THT)2]was formed from NbSX3 and THT in solution; a crystal structure (X = Br)373 showed the Nb atom to be six-coordinated (Nb=S: 2.09 A). The reaction between NbSC13 and Ph3PS afforded a 1:1 a d d ~ c twhose ~ ~ ~unit , ~ cell ~ ~contains two identical five-coordinate monomers and a centrosymmetric six-coordinate dimer with halogen bridges, the coexistence of which is rather unusual. The niobium atom is displaced from the plane formed by the three chlorine and the ligand’s sulfur atoms toward the sulfido sulfur atom (0.533A monomer, 0.397 A dimer); this displacement is larger than in NboCl3 adducts. [TaSC13(PhNCC12)]was obtained, as the result of sulfur abstraction, when Ta2Cllowas allowed to react with PhNCS.13’ It was reported to react with alkali metal thiocyanates to give [TaSC1x(NCS)3-,(PhNCC12)] ( x = 1 or 2) and [TaS(NCS)4(PhNCC12)]-.376 [MS(Et2dtc)3]”9was isolated from the reaction of MC1, and NaS2CNEt2in non-aqueous solvents. The Ta compound377is monomeric with seven sulfur atoms bound to Ta as the corners of a distorted pentagonal bipyramid (Ta=S: 2.181 A). 34.2.6
Complexes of the 0 x 0 Halides
342.6.1 Anionic oxohalo complexes
(i) Oxofluoro anions Oxofluoro complexes are formed much more readily for niobium than for tantalum. This difference, indeed, is the basis of the classical method of separation of the two metals. When, for example, a 2-3% H F solution of the oxides is concentrated, K2TaF7precipitates first and K2(NbOF5).H20 only later.’ (a) [MOF6I3-. Guanidinium oxohexafluoroniobate was synthesized by adding CN3H6F to a solution of Nb205 in The crystals consist of [Nb0F6l3- and guanidinium ions linked together into chains through NH F hydrogen bonds. The structure determination for t ~ - N a ~ [ N b o F ~shows ] ~ ~ ’niobium to be located at the center of a slightly distorted pentagonal bipyramid and displaced from the equatorial plane toward the oxygen atom ( N b F e q : 2.016-2.050(3) A; Nb-F=: 2.084(3) A; Nb=O: (1.738(3) A). (b) [MOfil2-. Oxofluoroniobates A[MOFs] (A = Mn,Co, Ni, Cu,Zn, Cd) were obtained from NbzOs, HF and the carbonates of the bivalent metals; in the same conditions TazOs gave fluorotantalates.48 The strength of the M=O bond depends on the nature of the counterion, as illustrated by the Nb=O bond strength constants shown in Table 20. In the hydrazinium salt the [NbOF5I2- anion has a distorted octahedral configuration (Nb=O: 1.75 A).380 ( c ) [M20F10]2-. The formation of [Ta20Flo]2- from [TaF6]- and Et4NOH was proposed to
-
Niobium and Tantalum
627
Tnble 20 Calculated N b O bond strength in AINbOF,]’
A
k x lo8(N A-’)
Mn
Co
Ni
Cu
Zn
Cd
Rb
7.24
7.31
6.87
6.78
7.36
7.17
6.50’
1. R. L. Davidovich, T. F. Levchishina, T. A. Kaidalova and V. I. Sergienko, J . Less-Common Met., 1972, 27, 35. 2. L. Surendra, D. N. Sathyanarayanaand G . V. Jete, 1. FIuorine Chern., 1983,23, 115.
proceed through an intermediate hydroxo compound [TaFSOH]- followed by a condensation reaction.381Its X-ray analysis382showed the two tantalum atoms to be bridged by an oxygen atom, the overall symmetry being approximately D4*. The linearity of the M-(r-M bridge (M-0 = 1.875 A) may be interpreted as resulting from n-bonding interactions between tantalum and oxygen. This is further supported by the high value of Y,, = 880 cm-I and low in the IR and Raman spectra.381 value of Y,= 270 cm-’ found for v(Ta-0-Ta) ( d ) [MOfiI-. [NbOF4]- was first obtained in 1866 by C. Marignac, by adding N H a to an bridges.383 HF solution of NbO5.’ It contains Nb-0-Nb ( e ) Miscellaneous. (NEf4)4[Ta406F12]was obtained by hydrolysis of (NEt4)2[(TaF5)z0].384 The anion has an adamantane type Ta406skeleton comparable to that of P406, each metal atom being further coordinated by three fluorine atoms in a fac octahedral arrangement (Figure 13). i9FNMR data indicated the presence of the [Nbz02F9r-, [NbOF5I2-, [Nb0F6l3-, [TaOF6I3- and [Ta203F,J3- anions in aqueous s o h t i o n ~ . ~ * The ~ stepwise formation constants of the [NbOF,]‘ complexes with 2 < x < 6, which exist in solution for a range of concentrations in acid, metal and fluoride ion, were also
’*
Figure 13 The Ta,O, skeleton of [Ta4F,,0,] (reproduced from ref. 384 with permission)
( i i ) Oxo anions of the higher halides
( a ) [MOX5]’- ( X = Cl, Br). NbC15, NbOC1, and Nb205dissolve in concentrated HC1, from which A2[MOClsj salts (A = NH:) were obtained by addition of an excess of AC1.318,389-392 AZ[NbOCl5] salts also formed as intermediates during the oxygenation of A[NbCb] (A = K, Rb, C S ) . A2[TaOC15] ~~~ was prepared by heating the appropriate hexachlorometallate(V) with ~ methods for a stoichiometric amount of SbzO3 in chlorine or under v a ~ u u r n .Analytical extracting niobium(V) from HC1 solutions involve the use of A = amines,394salicylhydroxamic acid395or benzohydroxamic acid;396the substances entering the organic phase were shown to be (AH)2[NbOC1~].(AsPh4)@bOC15]was formed in the reaction of NbOC13 with ( A S P ~ ~ ) C ~ . ~ ~ The structure shows an octahedral arrangement for [NbOCl5I2- with a very long N b - 0 distance of 1.97 A. Nb2OSforms oxobromo complexes Az[NbOBr5] (A = Rb, Cs) in concentrated HBr solutions of the alkali metal bromide^.^ (b) [Ta2Cll0O]*- and related species. The compound (PMe3Ph)2[TazClloO]was obtained ‘serendipitously’ by passing oxy en through a toluene suspension of TaClS in the presence of unit (Ta-0: PMezPh and Na/Hg amalgam.3 The structure revealed a linear TaV-O-TaV 1.880(1) A) comparable to that found in the [F5Ta-0-TaF5]*ion (Section 34.2.6.1.i)?= (PPh4)2[Ta2ClloO] was prepared from NO[TaOC4] and PPh4C1.3w [{TaC12(NMe2)z(HNMe2)}zO], where three chlorine atoms on each tantalum of the parent [Ta2Cllo0]2-ion are replaced by nitrogen atoms, was obtained as a minor product (2%) from the reaction of TaC15 and HNMe2.”’ The central Ta-0-Ta bonds are almost equal (T a-0 : 1.928(6) and 1.917(6) A) and significantly longer than those found in [Cl5Ta-0-C15J2- 398 and
8
cocs-u
628
Niobium and Tantalum
[F5Ta-0-TaF5l2(1.880 and 1.875(1) A r e s p e ~ t i v e l y ) .The ~ ~ Ta-Cl bonds trans to the NMez groups are longer than in the decachloro anions, which can be accounted for by the stronger tendency of these groups, as compared to bridging oxygen, to be involved in n bonding to the metal. (c) [M2CZgO]-.[{TaC12(Cpf)>2(p-C1)3]2[Ta2ClgO)2]400 was isolated as a by-product in the reaction of [SnBu3(C5Me5)]with TaC15. (C4ClPh4)2[Nb2C190]2 was obtained from NbC& and PhCkzCPh in the presence of water. The M-0-M bridges are asymmetrical in both [M2Cl90]gPanions, as are the two central chlorine bridges that link the two [M2C190]units together (Figure 14). CI CI
Figure 14 The [M2QO]:- anions (reproduced from refs. 400 and 401 with permission)
(d) [MOC14]- and [M02CZ3]2-. A[MOC4] (A = alkali metal) salts were obtained by heating the appropriate hexachlorometallate(V) with the stoichiometric amount of SbzO3 in chlorine or under vacuum.42o(NOANbOCL] was prepared from NOCl and NbOC13. [NbOC14](pyH)403*404 and (R3NH)[NbOCL] were precipitated from HCl solutions of NbC1, or NbOC13 and the appropriate amine; IR indicated the presence of -NbONbONb-- chains. The addition of diars to NbC15 in dry ccl4 caused the precipitation of NbC15(diars);attempts to recrystallize it yielded [NbC14(diars))2][NbOC14]and [NbCL(diars)2]2[Nb02Cl3]. [NbOCIJ is basically a square pyramid with the oxygen at the apex ( N k O : 1.70(2)A); one of the N b - C l bonds is outstandingly long (2.438(4) A) with the chlorine atom pointing to the sixth potential coordination site of the metal of a neighboring [NbOCLj- anion, with a very short contact of 3.011(6) A.152 (e) [M02CZ2]-. Compounds of composition A[Ta02C12(H20)], where A = K, Rb, Cs or NH4, were isolated by saturating solutions of TaC15 and of the alkali or ammonium chloride with HC1.406The IR absoi-pptions found in the 830-875 cm-I region were assigened to tantanyl groups joined by asymmetrical Ta=O-Ta bridges. Under the same conditions, NbC15 led to A2[NbOC15](see above).
34.26.2 Neutral adducts of the oxo halides and related compounds Numerous 1 :1 and 1:2 adducts are known for MOX3, but few for NbOF3, and none yet for NbO13. The adducts of the oxo trihalides of NbV and to a lesser extent of Tav were ofter? obtained from the pentahalides through oxygen abstraction from the ligand or the solvent. Others were prepared directly from the oxo trihalides.
(i) Oxygen, sulfur and selenium donors Oxygen was abstracted from phosphine oxides, phosphonates>Marsine oxides and sulfoxides by MX5 as well as by [MKMe] (X = C1, Br),“* and more easily by the Nb than by the Ta derivatives, to yield MOX, adducts (equation 36; Table 21). IR indicated the following sequence for the metal’s acceptor character: MC15 > MOC1, > MOC12Me. NMR showed that in
Niobium and Tantalum
429
the presence of an excess of oxo ligand [NbClyleIz is 6rst and rapidly converted to the 1;1 adduct [NbCLMeL]; the oxygen us. chlorine exchange then occurs at a slower rate, which depends, in the case of phosphoryl ligands, on the n character of the phosphoryl bond (Table 22) .@*
-
MX5 + 3QE [MOX,(O=E),] + ECl, E = PR,, AsR,, SR,; X = C1, Br
(36)
Table 21 Typical Adducts of MOX, with Oxygen Donors ..
~
L
Compounds
Ref.
Comments X-Ray structure (M = Nb) Monomeric X = H, p-NMe,, p-OMe, p-Me,p-Cl, p-Br, m-C1, rn-NO, A v ( P - O ) , = 1168 7.4~;emX=F,M=Nb;X=Cl, Br,M=Nb,Ta v ( N b - 0 ) : 967cm-' (X = Cl); 951 em-' (X = Br) v(Nb-0-Nb): 765 cm-l X = F , C1; R = M e , Ph
+
OSR,
1-4 2,3 5-7
4,6,7 8
7,9,10 11
P h a P h
1. L. G . Hubert-Pfalzgraf and A. A. Pinkerton, Inorg. Chem., 1977,16, 1895. 2, R. J. Dorschner, I . Imrg. N u l . Chern., 1972, 34, 2665. 3. L. G. Hubert-Halzgraf,R. C. Muller, M. Postel and J. G . Riess, Inorg. Chem., 1976,15,40. 4. D.Brown, J. F. Easey and J. G. H. D u Preez, J . Chem. SOC. (A), 1966,258. 5. E. G.Amarskii, A. A. Shvets and 0. A. Osipov, Zh. Obshch. Khim., 1975,45, 898. 6. D.B. Copley, F. Fairbrother and A. Thompson, J . Less-Common Met., 1965,8, 256. 7. J. Sala-Pala, J. Y. Calves and J. E. Guerchais, J . Inorg. Nucl. Chem., 1975, 37, 1294. 8. R.J. H.Clark, D. L. Kepert and R. S. Nyholm, J. Chem. Soc., 1%5,2877. 9. D.B. Copley, F. Fairbrother, K. H. Grundy and A. Thompson, 1. Less-Common Mef., 1964,6,407. 10. J. G. Riess, R. C. Muller and M. Postel, Inorg. Chem., 1974, 8, 1802. 11. D. Budd, R. Churchman, D. G . Holah, A. N. Hughes and B. C. Hd,Cm. 1. Chem., 1972,50,1008.
Table 22 Dependence of the Oxygen us. Chlorine Exchange Reaction Rate upon the n Character of the p--O Bond"
L
OPMe, HMPA O[OPINMe2)212
Pnn' character 0 0.2
-
v(P=O) (em-')
Reaction time in CH,CI, (min)
1182 1210 1230 1195
30 30 30 180 1fO
OPPh3 WOWtNM%),
0.370 0.390
OP(OMe),{NMe,)
0.77
1220 1260
OP(OMe),
1.079
1278
a
60
From C Santim-Scampucciand J. G. Riess, J . Chem.Soc., Dalton Trans., 1974,1433. No apparent reachon after 6 months in CH2CI,.
Although the F'=O bond is weaker in the phosphole oxides (TPPO) than in Ph,PO, only traces of [NbOC13(TPPO)] were obtained from NbCl, and excess phosphole.'" Furthermore, oxygen was picked up at phosphorus and not at the metal, to form [NbX,(TPPO)], when NbX5 was allowed to react with TPPSe in a non-rigorously-oxygen-freeatmosphere.
(ii) Nitrogen donors (Table 23) Niobium and tantalum oxo halides react with amines in stoichiometric amounts to form polymeric 1/1 materials containing MOM0 chains. Monomolecular [M0X3(NR3),J derivatives were obtained in the presence of excess amine.365,4w11
Niobium and Tantalum
630
Table 23 Nitrogen Donor Adducts of MOX,
L
/""\
H
Compounds
CommenLs
Ref.
R = Me, Et
1
R = €3; R' = H, p-Br, p-N02, rn-NO,, o-OH R = o-OH; R' = H, p-Br, p-N02, p-NMe2 X-Ray structure (M = Nb; R = Me) X = C1, Br
2
R
RC6H4N=CHC6H,R'
RCN biPY
3 4-7 4 4
X = C1, Br; R = Me, Et, Bu, octyl R = Me, Et, Bu
7 8-11 10
12
R = Me, Ph
12
NHEt,; NH,Bu
11
1. K. Yamanouchi and S. Yamada, Inorg. Chim. Acta, 1976,18,201. 2. A. V. Leshchenko,L. V. Orlova, A. D. Garnovskii and 0. A. Osipov, Zfi. Obshch. Khim., 1969,39, 1843. 3 . C. Chavant, J. C. Daran, Y. Jeannin, G . Constant and R . Morancho, Acta CrystaZlogr., Sect. B, 1975,31, 1828. 4. C. Djordjevic and V. Katovic, Chem. Commun., 1966, 224. 5. D. Djordjevic and V. Katovic, J . Chem. SOC. (A), 1970, 3382. 6. V. Katovic and C. Djordjevic, Inorg. Chem.,1970, 9, 1720. 7. N. Brnicevic and C. Djordjevic. f. Less-Common Mer., 1967, W, 470. 8. S . M. Sinitsyna, T. M. Gorlova, V. F. Chistyakov,Zh. Neorg. Khim., 1973, 18, 2114. 9. S . M. Sinitsyna, V. G. Khleboddrov and M. A. Bukhtereva, 2. Neorg. Khim.,1974, 19, 1532. 10. S. M. Sinitsyna, V. I. Sinyagin and Yu.A. Buslaev, Izv. Akud. Nuuk SSSR,Neorg. Muter., 1969, 5, 605. 11. Yu. A. Buslaev, S. M. Sinitsyna, V. I. Sinyagin and M. A. Polikarpova, Zh. Neorg. Khim., 1970, 15,2324. 12. A. V. Leshchenko,V. T. Panyushkin, A. D. Garnovskii and 0. A. Qsipov. Zh. Neorg. Khim., 1966,ll, 2156.
Reactions of bipy with MX5 and NbOC13 yielded [MXs(bipy)] and [NbOC13(bipy)] in benzene, while mixtures formulated as [NbOC13(bipy)][NbC15(bipy)]and [TaOC13(bipy)][TaClS(bipy)13 were isolated in ether. Solutions of NbC15 and bipy in alcohols containing a small, controlled amount of water yielded [NbOCl,(OR)(bipy)]. A series of monomeric, six-coordinate niobium(V) adducts with neutral Schiff s bases was synthesized from NbOC13410 or NbOC13(OPPh3)2.207 134347412414
(iii) Structures
In spite of the large number of oxo trihalide adducts reported, there is still little structural information available on them. X-Ra diffraction studies are limited to those of [NbOC13(MeCN)2]$19[N%OC13(HMPA)2]X6 and [NbOC13(OPC13)]4?35[NbOC13(HMPA)2] is monomeric in the solid (Figure 15) and in solution. In the solid the metal is octahedrally surrounded; the two HMPA ligands are cis to each other, one of them being trans to the oxo bond; the same isomer always predominates in solution. The N k - 0 distance is comparable to that found in [NbOC13(MeCN)2](1.68(2) A); the lesser steric requirement of the MeCN ligands allows for a larger distortion of the coordination polyhedron and for shorter Nb-Cl bonds. A considerable trans effect was found for the metal-ligand bonds in both the HMPA and the MeCN adducts. In solution [NbOC13(HMPA)2]undergoes drastic changes and gives complex solvent dependent mixtures, as depicted in Scheme 3. All the species in solution are in dynamic equilibrium on the NMR time-scale. By evaporation the initial crystalline puns isomer of [NbOCb(HMPA)*] was integrally recovered. Similar solution behavior was observed for [NbOC13(DMS0)2],[NbOC13(OMF'A)], [NbOC12(OEt)(HMPA)2]and [Nb0Cl2Me(HMPA)~]P"
Niobium and Tantalum
631
0
Figure 15 Coordination polyhedron of [NbOCl,(HMPA),] (reproduced from ref. 410 with permission)
NbOC13.2L (72%) c1
NbOC1,-L(MeCN) (19%)
2 isomers in equilibrium + free ligand
c1
clL*E1}
ClV''kL L+C1-
1 NbOC13.3L 8%
0 60%
(7% L = HMPA
+
5%)
Scheme 3
The oxygen-bridged tetrameric Nb404unit in [NbOC13(OPCI,)], 235 is almost planar with two different values for the Nb-0 distances in agreement with a (-Nb=O-), structure.
34.263 Miscellaneous oxygen abstraction reaction products Oxygen abstraction reactions from alcohols, ketones, acyl halides and other oxygencontaining compounds by MC15 or related derivatives also afforded a route to relatively simple complexes of the oxo halides (Table 24). Thus Nb2Cll0 was found to abstract oxygen from higher ketones to yield, after hydrolysis, alkanes, chloroalkanes or arylalkanes.414 Table 24 Typical Oxygen Abstraction Reactions 0 species RCOX (X = Q, Br)
Et,O Me,CO RR'CO(R = R' =Et; R = M e , R'=Et)
Reactions Nb(OR'), + RCOX (excess) NbC1, NbCl,(Et,O) + P i p y 3TaC1, TaCl,(Et,O) 4bipy NbCl,Me, 2Me,CO MCI, RR'CO MCI, RR'CO (excess)
+ + + + +
+
Compounds
[NbOX,(RCOOR')] [Nb,0Cl,(bipy)2] [Ta4OClI8(bipy),] [NbOCl,(OCMe,)l {MCldRR'CO)),Ol [MOCl,(RR'CO),]
Ref.
1 2 2 3 4
4
1. R. C. Mehrotra and P. N. Kapoor, J . Less-Common M e t , 1966,10,348. 2. C. Djordjevic and V. Katovic, J . Chem. SOC. (A), 1970, 3382. 3. J. D. WiIkins, J. Organomel. Chem., 1974, EO, 357. 4. M.S. Gill, H. S. Ahuja and G . S.Rao, J . Indian Chem. SOC., 1978,55,875.
Oxygen abstraction from SOC12.H20 by [NbCl,(RCp)(R'Cp)] led to a series of new [NbOCl(Cp)(Cp')] compounds.417 This reaction, which allowed the synthesis of diastereoisomeric pairs of optically active species, could be reversed upon heating (Scheme 4). The unex ected formation of [NbOCl(Cp),] as a by-product in the synthesis of [NbC13(Cp)2f1' can probably be attributed to the presence of traces of oxygen or moisture. When [NbClz(Cpl2] was allowed to react with oxygen, [NbOC1(Cp)z]302,419,420 or [{NbC12(Cp)2)2Oj4 was obtained. The latter compound shows anticancer activity, lower than I
-
Niobium and Tantalum
632
[Nb'VC1,(Cp),]
t
SOCI2.H20
[NbVOC1(Cp)2]
SOCI,
Scheme 4
that of [NbCl,(Cp),] but paralleled by a significant reduction of the toxic side-effects. The ionic ~(NbCl(Cp)z}zO][BF4]4zz derivative was obtained from [NbClz(Cp)z] by oxygen abstraction from water and addition of €€3F4;its N b O - N b bridge is significantly non-linear (169") but the Nb-0 bonds (1.88(1) A) are relatively short, indicating a certain degree of n bonding.423 The reaction between [NbMe,(Cp),] and an excess of NO proceeded via a nitrosyl adduct, probably with a bent MNO arrangement (v(N0): 1670 cm-') and gave a methyloxo derivative which further inserted NO (equation 37).343
(37) [NbO(ONMeNO)(Cp)&
Catalytic oxidation cycles involving the niobium oxide cations NbO+ and NbO; in the gas phase have been reported.424 34.2.7
Solvolysis Products of the Oxo Trihalides
34.2.ZI From Oxygen Compounds (i) Oxoalkoxo compounds (Table 25) Synthetic routes to well-characterized NbV and Tav oxoalkoxides remain surprisingly scarce. The direct alkoxylatiors of MOX3 is far from providing a general route to these compounds. The alternative exchange of oxygen us. chlorine between [NbC12(OR)3]2and an oxo ligand often results in redistribution products. The controlled hydrolysis of MC15 or [Ta(OR)5]2 ( R = Et, %Me3) led to polymeric oxo alkoxides. On the other hand the hydrolysis of NbC12 OR),], in the presence of bipyridyl led to [NbOCl,(OR)(bipy)] (R = Et, Pr') adducts.13 ,364*412 An X-ray structure for R = Et showed a distorted coordination octahedron, with the two chlorine atoms tram to each other ( N W : 1.71(3)A). The Nb-OR bond is only 0.16A longer than the Nb==O oxo bond, and was presumed to have considerable double bond character, which is also consistent with the very open NbOC bond angle of 149". MC15 when allowed to react with acidic aqueous solutions of tropolone (HT) gave the tetrakis(tr0 olonate) cations [MT,] . Hydrolysis of the niobium derivative afforded [Nb0T3];42pan oxygen-labeling experiment established that the oxo atom is abstracted from the displaced tropolone molecule. N-Benzoylphenylhydroxylamine (BPHA) reacts with NbzOs in bioling acidic a ueous solutions to yield a monomeric seven-coordinated niobium compound [NbO(BPHA)3].4 The hydroxylamine ligand could be displaced by tropolone or 8-quinolinol, and the rateOxoniobium and tantalum(V) determining step was shown to be associative (equation 38) 8-quinolinolates have also been reported. The optimal conditions for the formation and extraction of the [NbO(C6H402)3]3-ion formed in the niobium(V)-pyrocatechol system, have been investigated.428
I
(
+
9
[NbO(BPHA),J
[NbO(BPHA),(HL)] L=HT,O X H
-
products
(38)
(ii) Oxo-/3-diketonutoderivatives The action of acetylacetone on MOX, (X=Cl, Br) was reported to yield [NbOX2(acac)],413,429 while the analogous TaV compounds were not formed. Ionic /3diketonates [NbOC13(RCOCHCOR')]- were obtained from oxotetrachloroaquaniobates.4M
Niobium and Tanfulum
633
Table W Oxoalkoxo Derivatives Compounds
Comments R = Et, Pr; monomeric Solutions consist of complex mixtures of monomeric species in dynamic equilibrium Dimeric ( N S N b ) compound Monomeric; NMR: various monomeric isomers in dynamic equilibrium Monomeric; NMR: various monomeric isomers in dynamic equilibrium M+M bridges M&M bridges Dimeric; NMR: dynamic eqdibrium between p-alkoxo and p-oxo isomers Dimeric, pentacoordinated Nb Nb-0-Nb NMR: various monomeric hexacoordinated species in dynamic equilibrium R = But Polymeric compounds (Ta--O-Ta) HT = tropolone BPHA = [PhC(O)N(O)Ph]-; monomer oxH = 8-hydroxyquinoline cat = catechol
~MO(Ox),(oxHj] [NbO(cat),P-, H[NbO(cat),)
Re5 1,4 5
6 7
7 8 8 6
6 6 6 9 10
11 12
13 14
1. C. Djordjevic and V. Katovic, Chon. Commun., 1966, 224. 2. C. Djordjevic and Y. Katovic, J. Chem. SOC. (A), 1970, 3382. 3. V. Katovic and C. Djordjevic, Inorg. Chem., 1970,9, 1720. 4. B. Kamenar and C. K. Rout, J . Chem. SOC.(A), 1970,2379. 5. L. G . Hubert-F'falzgcaf,R. Muller, M. Postel and J. G . Riess, Inorg. Chem., 1976, U, 40. 6. L. G. Huben-F'falzgrafandJ. G. Riess, Inorg. Chim. Acta, 1980, 41, 111. 7. L. G . Hubert-Pfalzgraf and J. G . Riess, Inorg. Chim. Acta, 1981,47,7. 8. C. Djordjevic and V. Katovic, I. Less-Common Mer., 1 9 7 O , t l , 325. 9. D. C. Bradley, B. N. Cbarkravartyand W. Wardlaw, J . Chem. SOC., 1956, 4439. 10. D. C. Bradley and C. Prevedoroudemas, J . Chem. SOC. (A), 1956,1139. 11. E. L. Muetterties and C. W. Alegranti, J. Am. Chem. Soc., 1969,9l, 4420. 12. F. I. Lobanov, V.M. Fes'kova and I. M. Gibalo, Zh. Neorg. Khim., 1971, 16,776. 13. H. A. Szymanski and J. H.Archibald, J . Am. Chem. Soc., 1958, So, 1811. 14. F. I. Lobanov, 0.D. Savrova and I. M. Gibalo, Zh. Neorg. Khim., 1973., 18, 408.
Their solutions consisted of mixtures of the three possible monomeric octahedral isomers; these are rapidly interconverting on the NMR time-scale through an intramolecular mechanism.431 The crystal structure of the [NBOC13{C4H3SC(0)CHC(O)CF3}]- ion established that the isomer present in the solid state has the thenoyl groups trans to the N b - 0 bond.432The niobium atom dis lays a distorted octahedral arrangement, and is displaced toward the oxo oxygen b 0.295 ( N M : 1.70(3)A). Of the two chelate Nb-0 distances, 2.044(34) and the longer is trans to the oxo group. 2.285(3)
1, R
(ii) Oxocurboxylato Oxocarboxylato derivatives were shown to form by treatment of NbX5 or Nb(OR)5 with an excess of carboxylic acids235,433,434 or anhydrides435(equations 39a and 39b). [MCl,],
-
2[MOCl,(RCOO)] + 4HC1+ 2RCOCl M = Ta; R = H, Me, Et, PI',But M = Nb; R =Me, Et, Pr', But, CH,Cl, CH,F, Ph
+ 4RCOOH
[M(OEt),] + 4RCOOH
-
[MO(RCOO)J
+ RCOOEt -4- 4EtOH
(39a)
(39b)
M = Nb, Ta; R =Me, Et, Pr
The reaction of niobioporphyrin [Nb203(TPP)2]with acetic acid gave [NbO(TPP)(MeCOO)] in which the metal is seven-coordinated and lies within a polyhedron of symmetry close to C,,
Niobium and Tantalum
634
with a square based defined by the four nitrogen atoms of the pyrrole rings and a nearly
parallel triangular base formed by the three oxygen atoms (Figure 16).33”
Figure 16 Structure of [NbO(TPP)(MeCOO)] (reproduced from ref. 339 with permission)
Ascorbic acid with niobium(V) in acid solution forms an intensely colored complex [Nbo(OH)2(C&O6)], which was used for the spectrophotometric determination of Nb.436 Malato complexes of niobium(V) were obtained from aqueous racemic malic acid solutions of Nb205.437Niobic and tantalic acids dissolve in solutions of oxalic acid,’ and mono-, bis- or tris-oxalatooxo niobium(V) com ounds were reported to exist in these solutions, depending on pH and ~ o n c e n t r a t i o n s . ~ X-Ray ’~ diffraction studies on [NbO(&04)3]3-,4413442 [Nb0(Cz04)2(OH)(Hz0)]2-443 and [NbO(C204)2(H20)2]-444 have shown niobium to be seven-coordinated. The oxo atom is at one of the apices of a pentagonal bipyramid and the metal is displaced toward the oxo atom by 0.18, 0.21 and 0.2458, ( N k O : 1.71, 1.64 and 1.69 8, respectively). The water molecules in [NbO(G04)2(HI,0)2]-exchange with oxygen donors; mixed ligand complexes with sulfoxides,44sphosphine and arsine oxidesM6have been reported. Mixed complexes with the 4-(2-pyridylazo)resorcinol (PAR) N donor ligand are used in analytical hemi is try.^^,^ Oxohalooxalato niobium anions [NbO(C204)2F2]3-449 and [NbO(&04)3Br]4- 450 have been obtained from the parent tris(oxotrioxa1ato) anion. Complexation of the oxalato ligand as a bis(bidentate) bridge in [Nb2O2JZ6(GO4)I2-(X = F, C1, Br) has been established on the basis of 1 ~ . 4 5 1 The solubility of Ta205 in aqueous oxalic acid solutions is much lower than that of Nb205 and the resulting solutions are more complex owing to the presence of polymeric species.452
34.2.7.2 From sulfur compounth [NbO(R2dtc)3]has been prepared for R = cyclopentyl and ~ycloheptyl.~’~ All are monomers, with sulfur-bonded dithiocarbamates. The structure of the N-diethyl derivative4s6 showed the niobium atom at the center of a pentagonal bipyramid with the oxygen at one of the apices. Two of the chelating ligands lie approximately in the equatorial lane, while the third spans an axial and an equatorial site. The axial M-S distance (2.753(4) ) is longer than the equatorial ones (2.47-2.598 A). The metal atom lies above the best plane passing through the equatorial sulfur atoms on the side of the oxygen (-0: 1.74(1) A).
BI
3427.3 From nitrogen compounds: oxo amides and oxo isothiocyanates The aminolysis of NbOC13410or NbOBr336s by an excess of primary amine was shown to affect only one of the halogen atoms, to give [NbOX2(NHR)]. Excess of secondary amines with short alkyl chains displaces two bromine atoms. With increasing chain length, the tendency of a second halide atom to be substituted decreases and the second molecule of amine rather coordinates to the initial aminolysis product to yield [NbOX2(NR2)(NHR2)].Trialkylamines gave the addition products [NbOX3(NR3)].Monomeric [NbO{N(SiMe,),}] was obtained from NbCL(THF)2.4s8 [NbO(NCS)]’- salts were precipitated from acidic solutions of NbVwith excess thi~cyanate.~” of its (Ph4As)’ salt showed the metal to be at the center of The a distorted octahedron (-0: 1.70 A), and to be displaced by 0.21 A out of the nitrogen’s plane toward the oxygen. The N b N bond trans to the oxo ligand is longer (2.27 A) than the others (2.03 to 2.16 A).
635
Niobium and Tantalum
Addition of heterocyclic nitrogen bases to acid solutions of Nb and KSCN gave the mixed monomeric complexes listed in Table 26. [TaOC13(MeCN>2]and [TaSC13(PhNCC12)]react with alkali metal thiocyanates in MeCN to yield [TaOCl,-,(NCS), (MeCN)2], A[TaO(NCS)+ [TaSC13-,(NCS),(PhNCC12)]or A[TaS(NCS),(PhNCC12)J ( x = 1-3; A = NI-L,, K, Na).3 [NbO(NCS)3L], where L is C6H100or C9H140, was isolated from NbC15 and KNCS solutions in acetone.460 Table 26 Mwed Oxo-Isothiocyanato complexes'
coF?lpounds
a
Medium
r ( ~ b = O ) (cm-')
A = phen. DMP = 2,3-&methylphen. CoIl = (S)-collidine.
1. From F. N. Lobanov, V. M. Zatonskaya and I. M. Gibdo, Zh. Neorg. Khim., 1980, 25,3003.
34.2.8
Oxide- and Sulfide-derived Compounds
34.2.8.1 Oxides and peroxides The ultimate products of the oxidation of the metals are the pentoxides. Reduction of M20s by Hz yields M 0 2 and MO. These are beyond the scope of this chapter. Numerous orthometallates [M04j3-, metametallates [MO,]- and poiymetallates are known; their chemistry is reviewed in Chapter 38.
(i) Peroxo acids and s a h Orthoperoxometdlates A3[MJO2),] precepitate from alkaline solutions of niobates or tantalates on additon of H202. The four O2 groups are linked side-on to the metal in [M(0&I3- , which releases oxygen progressively in mild conditions, and explosively at 80 OC.The [Nb(02)4]3- ion has nearly DZds mmetry; the M-O distances range from 1.99 to 2.07 A, and the 0-0 mean distance is 1.50 On treatment with H2S04,the orthoperoxo salts gave the corresponding acids HM04.461,46w72 The niobium derivative was formulated as [Nbz02(02)2(OH)2].xH20on the basis of IR data.&' Metaperoxo salts AMOS (A=Na, K, Rb, Cs) have also been Oxidation of the coordinated 03- moiety by CeIV or OH radicals yielded com lexes of Nb" of type [Nb(02)(0H)2] containing the 0;radical ion, as evidenced by EPR.84,475
w.466
(ii) Peroxo complexes The di- and tri-peroxo compounds (Table 27), in which the metal is bound to the or edta ligands, were prepared from alkali metal niobates or tantalates(V) in hydrogen peroxide solutions. [Nb(O&phen]- is to date the only example of a triperoxo complex authenticated by X-ray crystallography (Figure 17). The metal, at the center of a distorted dodecahedron, is located 0.34A above the mean plane containing two peroxo groups and one nitrogen atom, on the same side as the third peroxo group. In [Nb(02)2(c20&]3-,441 the two peroxo groups are cis, which is unusual (0-0 mean distance 1.48 A); the coordination polyhedron is a distorted dodecahedron. coc3-U'
Niobium and Tantalum
636
Table 27 Di- and Tri-peroxo Complexes
Compounds
Comments
Ref.
1 . R. N. Shchelokov, E. N. Traggeim, M. A. Michnik and K. I. Petrov, Russ. J . Inorg. Chern. (Engl. Trans[.), 1972,17,W O . 2. J. E. Gucrchais and 8 . Spinncr, E d . SOC. Chim. Fr., 1965, 1122. 3. G. Mathern and R. Wciss, A m Ciytullogr., Sect. B, 1971, 27, 1572. 4. N.Vuletic, E. Pruc and C . Djordjevic, Z . Anorg. A&. Chem., 1979,450, 67. 5. G. Mathern and R. Weiss, Acta Crptallogr., Sect. B , 1971, 27, 1582.
Figure 17 Structure of the [Nb(O,),(phen)]- anion (reproduced from G. Mathern and R. Weiss, Acta Crystullogr., Sect. B, 1971,27, 1582 with permission)
(iii) Peroxohalo derivan'ves (Table 28) Peroxofluoro complex anions [Nb(OZ)F6l3-, [M(02)F5;I2-, [M(Oz)F4L]-, [M(02)2F4I3-, [M(OZ)zF3]2-and [Ta~(Oz)30F,3(H20)]6were reported. Their structures often proved difficult to solve, due to disorder in the crystal and to facile decomposition of the peroxo moiety. The structure of the [M(02)F#- anion has been repeatedly investigated; pentagonal bipyramidd arrangements have been found with the peroxo group in the equatorial plane (0-0: 1.45-1.47 8, for M = Nb; 1.389-1.443 8, for M = Ta at 170 K). The [Ta(O2)F4(C6H7NO)]-anion also exhibits a seven-coordinate tantalum atom at the center of a pentagonal bipyramid, the two apical positions being occupied by fluorine (0-0: 1.43-1.67 A). Salts of [M(Oz)zF4]3- were isolated and characterized by IR; a disordered structure has been reported for [Ta(Oz)zF4]3-.[Ta3(02)30F13]6-consists of [Ta(0z)2F5]2-and of p-oxo-bridged [ T a ~ 0 ( 0 ~ ) ~ Fanions ~ ] ~ -in the same unit cell, with very different -0 distances in the various peroxo groups. The Ta-CLTa bridge in the dimeric anion is asymmetrical (Figure 18). A series of [M(OZ)F3(diket)]-complexes has been prepared by the reaction of B-diketones with [M(Oz)F4(HzO)]-. 'H and I9FNMR spectra indicate that the products exist as a mixture of geometrical isomers in MeCN. The reaction of oxalic acid with [Nb(Oz)F4(H20)]- yielded [ N & ( O Z ) Z F ~ ( ~ ~A~ )bridging ] ~ - . oxalato group and a coordination number of seven for the metal were proposed. [M(02)F3(phen)], [M20(02)F4(bipy)2]and [M(0z)F3(0A~Ph3)z] were obtained when the neutral ligands were allowed to react with [M(O2)F4(HZO)]-. A few peroxochloro derivatives of the [M(Oz)CI5l2-391 and [M(02)C14(H20)]-431 anions are known. Addition of p-diketones to solutions of the latter afforded [Nb(02)C13(diket)]-; the stereorigidity of these heptacoordinated species in solution was assigned to the bidentate peroxo moiety.431 [NbC1z(RCp)2](R = H, Me) complexes react with Hz02,affording [Nb(O,)Cl(RCp)z]. In the complex with R = H the two oxygen atoms ( ( L O : 1.47(1) A) and the chlorine atom are in the plane which bisects the Nb(Cp)2 bent sandwich system; the coordination around niobium is thus pseudotetrahedral. In the presence of H2OZr[Nb(Oz)Cl(Cp)~]catalytically converts cyclohexene to its epoxide.
Niobium and Tantalum
637
Table 28 Peroxo Halo Derivatives
Comments
Compounds
Az[M(02)F,I A3tM(O2)2F41 A[M(02)F&] KS[T~~(O~)~OFI~(~@)~ A,[M(Oz)zF3]
(NEt4)[M(02)F,(RCOCHCOR')] (NEt4)[{Nb(02)F,>,(~0~) AZ[WO2)C',I (NEt4)[M(02)a4(H20)1
(NEt4)[M(02)C13(RCOCHCOR')] [M(%)F&zI [M20(02)ZF4(bipy)21 tNb(O2)C1(CP)Zl
A = Na, K, NH,, oxHz X-Ray structure A = Na, K, NH4 X-Ray structure L = H,O, 2-MepyO; NMR X-Ray structure (A = NEt,) X-Ray structure: pa(Oz)Fs]- and [{Ta(0,)F4}20]4- in the same cell R = R ' = M e ; R = R = P h ; R=Ph,R'=CF, R = C,H,S, R'= CF3;R = R' = CF3 Heptacoordinated Nb;bridging oxalato A = K , Rb, Cs,NH, NMR: stereorigidity Heptacoordinated L = OAsPh,; L,= phen M-0-M bridges X-Ray structure
Ref. 1 2-6 1 7 8-9 10
12 11 8-9
9 13,14 15 15 16 16 17
1. N. Vuletic and C. Djordjevic,J. Less-Common Mer., 1976, 45, 85. 2. R. Stomberg,Acta Chem. Scand., Ser. A , 1981, 35, 489. 3. R. Stomberg, Acta Chem. S c a d . , Sei. A , 1981, 35, 389. 4. R. Stomberg, Acto Ckem. Scand., Sei. A , 1982, 36, 423. 5. R. Stomberg, Acra C h m . Scand., Sei. A , 1982, 36, 101. 6. R. Stomberg, Acia Chem. Scand., Ser. A , 1983, 37, 523. 7. R. Schmidt, G. Pausewang and W. Massa, 2.Anorg. AUg. Chem.. 1982,488, 108. 8. J. Y. Calves and J. E. Guerchais, J. Fluorine Chem., 1974, 4, 47. 9. J. Y.Calves and J. E. Guerchais, J. Less-Common Met., "3,32, 155. 10, J. C. Dewan, A- J. Edwards, J. Y. Calves and J. E. Guerchais, J. Chem. SOC., Dalton Trans., 1977, 981. 11. Yu.A. Buslaev, E. G. IIyin, V. D. Kopanev and V. P.Tarasov, Zh. Stmk Khim., 1972, l3,930. 12. W. Massa and G. Pausewang, 2.Anorg. Allg. Chem., 1979,456, 169. 13. J. E. Guerchais, B. Spinner and R. Rohmer, Bull. Soc. Chim. Fr., 1965,55. 14. J. Dehand, J. E. Guerchais and R. Rohmer, Bull. SOC. Chim. Fr., 1966,346. 15. J. Y.Calves, J. E. Guerchais, R.Kergoat and N. Kheddar, Znorg. Clrim. Acta, 1979, 33, 95. 16. J. Y.Calves, J. Ma-Pala, J. E. Guerchais, A. J. Edwards and D. R.Slim, Bull. SOC. Chim. Fr., 1975, $17. 17. J. Sala-Pala, J. Roue and J . E. Guerchais, J. Mol. Cam/., 1980, 7, 141. 18. I. Bkouche-Waksman, C. Bois,J. Sala-Pala and J. E. Guerchais, 1. Orgunornet. Chem., 1Y80, 195, 307.
F
figurn 18 Structure [Ta3(0,)30F,,(H20)]"- showing the [Ta(O,)F#- and [Ta20(02)FJ4- anions (reproduced from W. Massa and G. Pausewang, 2.Anorg. Allg. Chem., 1979, 456, 169 with permission)
638
Niobium and Tantalum
N212 SuFdes, selenides and d a t e d compounrls [MY4I3- (Y = S or Se)367and [wb0S3l3- 368 are known only as insoluble salts synthesized from the elements at high temperatures; [NbOZS2l3-369 was obtained from strongly basic sensitive to O2 and HzO, solutions of Nb2O5 saturated with H2S. The salts (NEt4)4[M6S17],366 were prepared from [M(OEt)5], (Me3Si)2Sand Et4NC1 (1: 6: 3) in MeCN. Single crystal X-ray analysis (Figure 19) revealed 10 non-planar M2S2 rhombs in convex fusion forming a crown-shaped cage. Each metal atom is additionally bonded to a terminal S atom and to the bridging S atom inside the cage, thus achieving pentagonal seven-coordination for top M(1), and tetragonal six-coordination for the other metal atoms.
Figure 19 Structures of [M,S1,I4'" (reproduced from ref. 366 with permission)
NbV and TaV complexes with the S2 and Sez ligands, formally similar to 02,appear to be limited to bis(cyclopentadieny1) complexes. The reaction of [NbC12(OH)(Cp)2]with HzS in methanol, in the presence of ionic halides or pseudohalides (KCl, KBr, NH41, KSCN), afforded the monomeric [Nb(S2)X(Cp)z](X = C1, Br, I, SCN) complexes.418When X = C1 or Br, a second compound of the same formula was obtained and assumed to be polymeric with disulfide bridging groups. Surprisingly, when X = SCN, a cyano complex, [Nb(S2)CN(Cp)2J, was formed. [NbMe2(Cp)2] was reported to be oxidized by s8 to yield [Nb(S2)Me(Cp)2].4 ) ~ ] [TaMe2(Cp),l gave Similar procedures with [Nb(q '-Cp)2(Cp)2], [ N ~ H ( C H F C H E ~ ) ( C ~and the [M(S2)R(Cp),] complexes, where R = ql-Cp, Bu and Me, respectively. The reaction of [NbMe2(Cp)2] with red selenium afforded [Nb(Se2)Me(Cp)2].480[Nb(S2){SP(S)(OR)2}(Cp)z] complexes (R = Me, Et, PI!, But) were obtained from [NbC12(Cp),] and P4S10.481The R = Et and Pf derivatives were reported to form from s8 and ~b{SP(S)(OR)2}z(Cp)2].252 The structures of [Nb(S2)X(Cp),] were solved for X = Cl,482CN and Me483(S-S: 2.03(1) A). [Nb(S2)Me(Cp)z] was shown to transfer sulfur to a phosphine.&*
342.83 Other oxygen- or sulfur-substituted metal compounds
(i) Susfates The interest in sulfates ari'ses mainly from the development of processes using sulfuric acid in the treatment of mineral concentrates. MCl, reacts with SO3 in S02C12to form the insoluble
[Nbz0(S04)4] and [Ta2(S04)5], respectively. Whereas [Nb20(S04)4], [Nb202(S04)3], wz03( S04)2] and [Nb204(S04)]could be isolated from the m 0 5 - S 0 3 - H 2 0 system, only [Taz(S04),] and [TaZ04(S04)(H20)]were obtained in the case of TaV.l The hydrated oxo (n = 3, 4 or 5) were also re orted. In H2S04, Mz05 and MOC13 sulfates [Ta203(S04)2(H20)n] form H[MO(HS04)4], while MX, gives H[M(HS04),].a)" The more soluble N b , ' Na,&, Cs486,487 and Rb488double sulfates have also been described.
(ii) Nitrutes, phosphates, arsenates and related compoundr
The reaction between M a 5 and liquid N2Os resulted in the moisture sensitive tris(nitrato)oxo compounds [MO(NO&] .l The (NMe4)[NbO(N03),] complexes were prepared by addition of liquid N20s to the appropriate hexachloro complexes with covalently bonded
Niobium and Tantalum
639
nitrato groups. Solvated polymeric nitratodioxides, [M02N03], were obtained from MC15 and Nz04in ionizing solvents. Metaphosphates, [MOP04] or [M205(P205)], have been reported. Single crystals of NbOP04 were gepared by heating together H3P04 and Nb2O5 in a gold capsule at 600°C under 1800 bar. A com und of composition [Nb9025P]was obtained by a similar procedure at 750 "C under 2000 bar.' [TaOP04(H20)], [TaOP04], [TaOAs04] and [Ta9AsOZ51are also known.491The orthophosphates [M3(P04),] were prepared by reaction of excess H3P04 with MX, in the molten state.' NbOP04 has tetragonal symmetry, and its structure is made up of chains of corner-sharing Nb06 octahedra with non-equivalent N b - 0 bonds (1.78 and 2.32 A), linked together by PO4 tetrahedra. In the hydrated crystalline niobium phosphates, the individual Nb06 octahedra are no longer connected to each other, and display a layer structure capable of hosting various molecules such as Hz0492*493 or H3P04.494 NbC15 and TaC15 abstract oxygen from diisopropylmethylphosphonate to yield polymeric complexes [Mo(IMP)~], (IMP = isopropoxymethylphosphonate),4M which contain M=O groups and both chelating and bridging phosphonato groups. The methylphosphinates [M20(MeP03)4]precipitate when methylphosphinic acid and the pentachlorides are heated together.' [Ta(OR)5-,(Ph2P02)x] (x = 1-3) were prepared from Ta(OR)5 and Ph2P(0)OH as low molecular weight polymers. The polymeric oxides [Ta02(Ph2PO)],, [Ta203(Ph2PO)2]nand [TaO(Ph2PO)3], have been obtained by suspending the methoxide or ethoxide in boiling water. The sodium salts of dialkyldithiophosphate (ddtp) react with NbX5 (X= C1 or 3 r) in methanol to yield [MX(OMe)2(ddtp)2];recrystallization from the appropriate alcohol afforded [MX(OR)2(ddtp)2].UgThe P-S stretching vibrations in the IR around 600-750 cm-l suggest that the ligands coordinate through both sulfur atoms. The surprising formation of the diamagnetic [Nb(S2){SP(S)(OR)2}(Cp)2] from P4S10and [NbC12(Cp)2]in ROH illustrates the ease with which NbIV is oxidized in such complexes;481the dialkyldithiophosphate ligand appears to be monodentate; the origin of the S2 group is as yet not understood. The reactions of F2P&H with MC15 gave the chloro(difluorophosphinato) complexes [MC13(S2PF2)2]$95 in which both S2PFF ligands are bidentate. 34.2.9 lMiscellaneoas Metal-Element Derivatives
Zwitterionic four-membered PN2Nb heterocycles were obtained in the reaction of q5phosphorus-nitrogen ylides with NbC15 according to equation (40) .496 2"/
RN=P
+NbCI,
-
\NR
NR* I
X-P-N-R
I+ I
R-NzNbCl, R = SiMe,
In the course of an investigation of the Ta/P/S system, formation of a compound of overall composition TaPS6 was noted.497Its structure showed each tantalum atom to have eight nearest sulfur neighbors arranged in a bicapped triangular prism. Two such prisms, sharing a common face, form Ta& units; these are linked together by PS, tetrahedra into endless chains. The com ound can be formally considered as containing two cations, Ta5+ and P5+,and two anions, (S$ and S2-, and can be formulated as [Ta(PS&]. Compounds containing Nb-Zn or Ta-Zn bonds have been obtained by the reaction of [MH3(C P ) ~with ] \Zn( CP)~].The structures of [NbH2(Cp)2Zn(Cp)]and [TaH(Cp') {Zn(Cp)}21 were determined. The reaction of [N~H,(CP)~] with HgX2 afforded the first complexes with Nb-Hg bonds;500[Nbfp),(HgS2CNEt2),1 has been characterized by X-ray diffraction analysis (Nb-Hg: mean 2.790 ). 34.3 OXIDATION STATE +W
34.3.1
Tetrahalides
The niobium and tantalum tetrahalides are all known, except TaF,, whose nonexistence is consistent with the fact that [TaF,], should be the least reducible halide of the group VA
640
Niobium and Tantalum
element^^^'^'^'^ They are dark solids, obtainable by reduction of the pentahalides by H2, Al, Nb or Ta at high temperatures. The most popular preparation involves the thermal gradient method, with a metal as reductant. Nb14 is easily obtained on heating Nb15 to 300°C.501The reduction of TaIs is slower, and an easier way to prepare Ta14is to allow Ta15 to react with an excess of pyridine, giving [Tah(py),] which, upon heating, loses the ligand.5m Nb14 and Ta14 are trimorphic and dimorphic, depending on the synthetic procedure. All tetrahalides except NbF4 disproportionate thermally. These reactions may be suppressed by maintaining a sufficiently high partial pressure of the pentahalides during the high temperature synthesis. NbF4 is a paramagnetic solid in yhich the Nb atoms lie at the center of octahedra that share four corners in an infinite sheet structure.' The other tetrahalides are essentially diamagnetic, and often orthorhombic. NbC14503 and Nb14'04 form one-dimensional polymers consisting of trans edge-sharing polyoctahedra, with very weak interchain coupling. These [Nb&(p-X),], polymers exhibit bond alternation (Figure 20), with metal-metal pairs. Powder data suggest that NbBr4 and TaX, (X=Cl, Br, I) are isomorphous with NbC14 and U-NbI4. NbBr4 is extremely sensitive to moist air, while NbC14 is relatively air stable.
F i e 20 Section of the linear chain of (Nb,Cl,), showing the bond alternation (reproduced from ref. 503 with permission)
Recent interest in low dimensional conducting materials has led to various experimental and theoretical studies on NbX+5057506Their structure, with two types of metal-bridging halide bonds on unequal lengths alternating along the chain, is responsible for their semiconducting properties. Their electrical conductivities increase sharply with temperature or pressure, and NbI, was shown to exhibit metallic behavior under high pressure.5WThe activation energy for conduction decreases from NbCl, to Nb14, despite the increase in metal-metal distance. This trend has been accounted for by a diminution of the valence band and band gap energies (-0.1 and 1.0 eV in NbC1,; -0.3 and 0.8 eV in Nb14). 34.3.2 Halo Complexes
34.3.2.1 Halometallates (1V) The hexahalometallates obtained by electroreduction of Nb&- or from Mx, have been isolated as their alkali or tetraalkylammonium salts A,[M&] (M = Nb, X = C1, A = K, Rb, Cs, N€&; X = B r , Et4N, A = Cs and M = T a , X=C1).7*50s Their thermal stability, higher for tantalum, is in marked contrast with that of the corresponding neutral tetrahalides (AH;% in the -1614 to -1719kJmol-' range, compared to -694kJmol-' for NbC1&05w K3[NbF,] is isostructural with K3[NbOF6]and (NH4)3[ZrF7]; it approaches D5hsymmetry.
343.22 Neutral adducts of tetrahalides (i) Synthesis Numerous adducts of MX, with nitrogen, phosphorus, arsenic, oxygen and sulfur donors have been obtained, mainly for niobium with X = C1 or Br' (Tables 29-33). The absence of MF4 adducts, except for [NbF4(py)z],S11may be due less to their instability than to a lack of investigations. An ill-defined fluoro trichloride complex, [NbCl#(MeCN)2.65], has also been reported .51'
Niobium and Tantalum
64 1
These adducts were synthesized by extraction of solid [Ma8], by the ligand, or by reduction of MXs (X = Cl, Br, I) with an excess of ligand. Such reductions occur most readily with monodentate, or, even better, with bidentate or heterocyclic nitrogen donors (equation 41),s02,s1salthough it was also observed with macrocyclic ethers, phosphinesSl3and d i a r ~ i n e s . ~ ~ ~ Longer reaction times or higher temperatures are required with the tantalum pentahalides. As a result, the adducts of NbIV and TaIV with nitrogen donors (nitriles and heterocycles) are predominant (Table 29). Partial solvolysis of Nb& was often observed with aliphatic amines (Et,NH, en, etc.) and their adducts are confined to [NbC14(NEt3)],. Adducts with nitriles are limited to [NbC14(RCN),] (R = Me, Ph) and acrylonitrile [M&(C2H3N)2] (X = C1, Br) adducts.s16 Adducts with various phosphines have been isolated (Table 30), while the adducts with arsenic donors are limited to those of the bidentate o-phenylenebisdimethylarsine (diars) and related ligands (Table 31). A few mixed ligand adducts have also been reported.
The complexes with group VIB donors are collected in Table 32. No Ta& adducts with monodentate oxygen donors have so far been isolated, probably as a consequence of the strong 0 abstraction pro erties of Talv; the reaction between TaCL and dioxane led, for instance, to [TaOCls(diox)z]?R Such reactions are less favored for NbIV and require powerful oxo ligands such as they are not observed with dry DMF.'" [NbC14(THF),] is of interest because of the lability of the THF molecules, which makes it a useful starting material in N b I V So far, only crown ethers have been able to achieve 0' complexation of TaCl,, providing [(TaCL),(lS-CRW-6)] .513 Sulfide adducts are listed in Table 33.
Table 29 Adducts of Tetrahalides with Amines ~
Compounds
Comments
Ref.
Trans isomer (IR) R = H isolated, powder X-ray data; R H not isolated, ESR evidence, trans isomer X = Cl, p = 1.37 or 1.53 BM
1 2,3,4
Powder X-ray data; X = Br interconvertible green and red isomers; X = I, unstable in solution (I; + Nb"'?) X-Ray powder data, X = C1 trans isomer; X = I loses py on heating X = C1, M = Nb p = 1.18 BM; M = Ta y = 0.77 BM
2-4
+
M = Nb, X = Cl, LL = bipy: ionic in MeCN; LL = o-phen, X = Cl,M = Nb y = 1.38 BM; M=Tap=00.67BM Seven coordinate species Tridentate terpy, free py; reduction to Nb"' (E" = -0.51 V) by cyclic voltammetry Electrolyte in Me,CO Diagrnagnetic Partial reduction
m,
Rpy = 3-RC,H4N (R = Me, Et, Br); 4-RC,H4N (R = Me, Et, Fh, NI3.J. C,H,,N, = N,N,N',N'-tetramethylethylenediamine. 1. F. E. Dickson, R. A . Hayden and W. G. Fateley, Spectrochim. Acto, Port A, 1%9,25,1875. 2. R. E. McCarley, B. G. Hughes,J. C. Boatman and B. A. Top, Adw. Chem. Sei., 1%3,37,243; R. E. McCarley and B. A. Torp. Inorg. Chem., 1963,2,540. 3. D. P. Johnson, J . Wilkinski and R. D. Bereman, .I. Inorg. NwI. ulem., 1973, 35, 2365. 4. D. J. Machin and J. F. Sullivan, J . Less-Common Met., 1969, 19, 405. 5. R. E. McCarley and J. C. Boatman, Inorg. Chem., 1963, 2,547. 6. M. AUbutt, K. Feenan and G. W. A. Fowles, J . Less-Common Met., 1964. 6, 299. 7. G. W. A. Fowles, D. J. Tiidmarsh and R. A. Walton, Inorg. Chem., 1%9,8,631. 8. B. Begolli, V. Valjak, V. Allegretti and V. Katovic, J . Inorg. NucZ. Chem., 1981,47, 2785. 9. M. f.Brown and G. S. Newton, Inorg. Chem., 1966, 5, 1117. a
Niobium and Tantalum
642
Table 30 Adducts of Niobium and Tantalum Tetrahalides with Phosphorus Donors I _
Compounds
Comments
Br)
Vi = Ta, X-ray structure: trans isomer; M = Nb unstable at room temperature X-Ray structure: cir isomer, X = Br, C-H
Ref.
L2,3
activation of P--CH,
4,2,13
X-Ray structure: trans isomer Not isolated, ESR data (frumisomer)
5
X-Ray structure, capped octahedron ( C , symmetry) X-Ray structures, monomer-dimer equilibrium in solution
2 1,6
Diamagnetic, soluble species, unstable with respect to Nb"'
7
Diamagnetic, insoluble
8
Analysis only Not isolated, ESR, data (?runs geometry) Analysis only M = Ta; X-ray structure, approximate square antiprism
9 10 9 3 11,12
1
1. F. A. Cotton, S. A. Duraj and W. J. Roth, Inorg. Chem., 1984, 23, 3592. 2. F. A. Cotton, S. A. Dura) and W. J. Roth, Inorg. Chem., 1984, 23, 4046 3. L. E. Manzer, Inorg. Chem., 197?,l6, 525. 4. L. G. Hubcrt-Pfalzgraf,M. Tsunoda and J. G.Riess, Inorg. Chim. Acta, 1981,52,231. 5 . G. Labauze, E. Samuel and J. Livage, Inorg. Chem., 1Y8O,l9, 1384. 6 . F. A. Cotton and W. J. Roth, Inorg. Chem., 1984, 23, 945. 7. J . L. Moranpis and L. G. Hubert-Pfalzgraf, Transition Mer. Chem., 1984,9, 130.
8. D. J. Machin and J. F. Sullivan, J. Less-Common Met., 1969,l9, 405. 9. R. Gut and W. Perron, J . Less-Common Met., 19n, 26,369. 10. E. Samuel, G. Labauze and J. Livage, Nouu. J . Chim., 1977,1,93. 11. S. Datta and S. S. Wreford, Inorg. Chem., 1977, 16, 1134. 12. F. A. Cotton, L. R. Falvello and R. C. Najjar, Inorg. Chem., 1983, 22, 770. 13. N. Hovnanian, Ph.D. Thesis. University of Nicr, 1986.
Table 31 Tetrahalide Adducts with Bidentate Arsines
Compounds [NbX4(diars),] (X = C1, Br, I) [TaC14(diars),] [TaC14(diars)], [NbX,(Etdiars),] (X = C1, Br, I) [TaCl,(Etdiars)], [NbCl,(Rdiars),] [NbCl,(naar~)~r a
Ref.
Comments
X-Ray structure for X = C1 (dodecahedron), iodide not isomorphous (p = 1.8BM; X = Cl, I) Elusive, impure material, X-ray powder data Low magnetic moment (p = 0.4 BM) X = Br or I, partially dissociated in MeCN and completely in py Diamagnetic R =Me, F Dodecahedron (UV)
1,2,3 2 2 2
2 273 4
naars = 1,8-bis(dimethylarsino)naphthalene
1. J . H. Clark, D. L. Kepert, J . Lewis and R. S. Nyholm,J . Chern. Soc. (A), 1%5,2865. 2. R . L. Deutscher and D. L. Kepert,Inorg. Chem., 1970, 9,2305. 3. D. L. Kepert, B. W. Skeltnn and A. H. White, J . Chem. Soc., Ddton Trans., 1981, 652.
4. D. L. Kepert and K. R. Trignell, Aust. J . Chem., 1975,28, 1245.
(ii) Structure The solubility of the Mx, adducts is generally poor, but increases from the chlorides to the iodides. In contrast to the tetrahalides themselves, most of their adducts are monomeric and paramagnetic. Characterization by ESR was sometimes achieved. The absence of notable paramagnetism was usually taken to support the existence of oligomeric or polymeric structures. X-Ray data remain scarce, other than for a number of phosphine adducts. With monodentate ligands, paramagnetic pseudooctahedral diadducts [MX&] of D4hor C , symmetry are most frequently formed. X-Ray structure determinations are available on trans-[MC14(PEt3)& ~is-[TaC4(PMeJ-'h)~];~~ ~ i s - f N b B r ~ ( M e C N ) ~ ]and ~ ~ l cis-[NbC14compounds has however been (MeCN)&MeCN. * The stereochemistry of most [ W L ]
Niobium and Tantalum
643
Table 32 Tetrahalide Adducts with Oxygen Donors
Compoundr [Nb&6(02C4H8)23 (X = C1, Br) [Nb&d(TmZI (X = Cl, Br) [NbX4(THP)21 (X = C1, Br) [NbC14(DME)] [Nb&(DMF)ZI (X = C1, Br) [Nb14(DMF)81 [NbCl4(DEF)zl [NbC14(DMA)*I [NbCl,(HMPA)] [NbCI,(HMPA),] [(NbC14),(18-CRW-6),] [(NbC14),(15-CRW-5),] [(NbC14),(15-CRW-5)] [(TaC14)2(18-CRW-6)]
Ref.
Comments
Dioxane monodentate, cis stereochemistry in the solid (IR), truns in solution (ESR) THF very labile, cis stereochemistry in the solid (IR), trans in solution (ESR) Cis stereochemistry in the solid (IR), X = C1; p = 1.54 BM Not isolated, ESR evidence Trans isomer, X = Br ionization to [NbBr,-,(DhXF),]"' in solution Extensive ionization to [Nb14-, (DMF)Jm+ even in the solid Not isolated, ESR evidence (wuns) Not isolated, ESR evidence (frum) Not isolated, ESR evidence { g ) = 1.8910, ( A ) = 183 G Not isolated, ESR evidence (trans) ( g ) = l.8869,a {A) = 199.9 Ga" Diamagnetic Diamagnetic Diamagnetic Diamagnetic
1,2 3,1 1
2 4,2 4
2
2 2 2 5 5
5 5
"Estimated assuming A, and g- are the same for the mono and bis adducts. 1. G. W. A. Fowles, D.J. Tidmarsh and R. A. Walton, Inorg. Ckem.. 1969,8, 631. 2. D. P. Johnson and R.D. Bereman, J . Inorg. Nucl. Chem., 1972,34, 2957. 3, L. E. Manzer, Inorg. Chem., 1977,16,525. 4. K. Kirkscy and J. B. Hamilton, b o r g . Chem., 1992, 11, 1945. 5. L. G . Hubert-Pfalzgraf and M. Tsunoda, Inorg. Chim. Acta, 1980,38,43.
Table 33 Tetrahalide Adducts with Sulfur Donors Compounds
[~&(sRz)I (X = Cl, Br, I; R = Me) [Nb&(SR2)12 (X = C1, Br, I; R = Me, Et)
Comments
Ref.
Cis isomer probable, unstable with respect
1,2
to the monoadducts ' Dimer-monomer equilibrium in C&
L2
Cis isomer (IR)
1
Cis isomer (IR) Two products (aand and structures Cis isomer (IR)
p ) of different solubilities
ESR for X = Cl, Br (dodecahedron) [NbCI,] + [NbC14(dth)J (very low yield) six-coordinated metal S-bonded, soluble and extremely air sensitive S-bonded, as [Nb(TU),I,]I in MeCN
1,2 1
1 3,4
3 5
5
1. G. W. A. Fowles, D. J. Tidmarsh and R. A. Walton, J . Inorg. N u d Chem., 1969, 31, 2373. 2. J. B. Hamilton and R. E. McCarley, Inorg. Chem., 1970,9, 1333. 3. J. B. Hamilton and R. E. McCarley, Inorg. Chem., 1970,9, 1339. 4. B. L.Wflson and J. B . Harmlton, Inorg. Nucl. Chem. L e a , 1976, l2,59. 5. D. J. Machin and 3. F. Sullivan, J . Less-Common Met., 1%9,l9, 405.
deduced from IR or ESR data. Distorted d1 octahedral MX& complexes generally exhibit g,,>gl except for [NbC&J2-,where all ligands have similar electron-donating properties. The hyperfine coupling parameters decrease with increasing ligand basicity (Table 34). A few monoadducts have been reported for NbIV. These are favored by bulky ligands; thus tetrahydrothiophene formed diadducts with NbC14, while the bulkier methyl and ethyl sulfides provided monoadducts and unstable diadducts.523 Monoadducts were also obtained with HMPA, NEt, and PBu:; they are only slightly paramagnetic, and halogen-bridged bioctahedral structures with M-M bonds have therefore been postulated. The monoadducts are stabler for the chlorides than for the iodides, illustrating the importance of M-M bonding.
Niobium and Tantalum
644
Table 34 ESR Data for Nb'" and Ta'" Derivatives (Frozen Solutions) _____~
~
Compouflh
wc1612-
[NbOC14(H20)]2ENbO(acac)*l [NbC&(n)21 [NbC14(y-pic),] INbCl,(PEt,),l ITaCL(PEt,),l [fiCl,(dth),l [NbBr,(dth) 1
[NWN),~~' [Nb(mb)4l a
(g>
1.892 1.894 1.906 1.892 1.945 1.927 1.740 1.997 2.008 1.984
811
1.919 1.916 1.903 1.913 1.924 1.959 1,831 1.931 1.990 1.969 1.984 2.002 1.952 1.923
8,
A,
alld
Comments
-
-
Silent at room temperature C , symmetry
1.976 99 1.966 107
62 201
131 108 122 139 103 175 112 101 76 117
-
2.018 131 2.991 101
277 249 270 269 218 285 187 187 151
1.951 1.883 1.964 1.894 1.955 1.912 1.695
(A)M
177 179 159 177 183 141
211 2.029 138
All
60
-
Cisisomer Trans isomer 23.6 Trans isomer 25 Trans isomer - Dodecahedron (D,) - Dodecahedron (Dzd) - Solid: dodecahedron (&) Solution: square antiprism (&) - Du distortion of the tetrahedron
-
Re$ 132 3 4 3 5 6 6 7 7 8 8 9
Hyperhe splittings in gauss.
1. S. Maniv, W. Low and A. Gabay, Phys. Lett. A . , 1%9,29,536. 2. M. Lardon and H. H. Gunthard, J . C h m . Phys., 1966, 44,2010. 3. D. P. Johnson and R. D. Bereman, 1. Inorg. Nucl. Chem., 1912,34,2951 and 679. 4. I. F.Gainullin, N. S. Garifyanov and B. M. Kozyrev, Dokl. A h d . Nuuk SSSR, 1968,1%0,858 (Chem. Abstr., 1968, 72714). 5. D. P. Johnson, J. Wilinski and R. D. Bereman, J . Inorg. N u l . Chem., 1973,35,2365. 6. E. Samuel, G. Labauze and J. Livage, Nouu. 1. Chim., 1977,1, 93. 7. B. L. Wilson and J. B. Hamilton, Inorg. Nucl. Chem. Lett., 1976, l2, 59. 8. P. M. Kiernan and W. P. Griffith, J . Chem. Soc., Dalton Truns., 1975, 2439. 9. D. C. Bradley and M. H. Chisholm, J. Chem. Soc. (A), 1971, 1511.
The systematic study of the phosphine adducts of MC1, offered a series of [MCl,(PR,),: compounds. Of particular interest are the dinuclear [Nb2Cb(PR3)4](R3= Me3, Me2Ph), whicl display two square antiprismatic [NbC14(PR3)2]units sharing a square C14 face (Figure 21).5B Strongly basic phosphines such as PBu: or PCy, also favor metal-metal interactions to givc soluble dinuclear [NbzC18(F3u:)2] and [Nb2C18(PR3)3](R = But, Cy) complexes.304The lattei stoichiometry is rather unusual, and triply-chloride-bridged structures, comparable to thost found in [Nb2Cl9I3- or [Nb2C13(C0)8)-,526 have been assumed. Tantalum is apparently les! prone to adopt dinuclear structures, and the seven-coordinate [TaC14(PMe&] (a cappec octahedron of C% symmetry) and ck-[TaCl,(PhPMe&] have been obtained.
Figure 21 Drawing of the central core of the [Nb2C1,(PPhMe2)4]molecule (reproduced from ref. 524 with permissior
Multidentate ligands tend to stabilize higher coordination numbers? generally givinl monomeric paramagnetic structures, although distorted octahedral complexes were obtainet with some bidentate ligands such as bipy and phen. Heptacoordination is seldom encounteret with bidentates, but occurs in [MCl,(terpy)]. Octacoordination is more common, as illustratet by the formation of [ M C L ( d r n ~ e ) ~[Nb&(dth)2]527 ],~~~ and [Nb&(diar~)~] (X= C1, Br, I):= it which the coordination polyhedra were assumed to be dodecahedral on the basis of UV or ESE data. It has been found that usually g,, > g l for an archimedian antiprism, while gll < gl for : triangular dodecahedron (Table 34). The latter geometry was established by X-ray dBractioi for [NbCL(diar~)~],5~~ while [TaCL(dmpe)2]s29was found to be a square antiprism. Thd bidentate o-phenylenebisdimethylarsinedisplays a different behavior, since only the monoad duct [TaC&(diars)] was found to be stable, but its structure remains unknown. Thl [ M a ( d m ~ e ) (22) ~ ] complexes have been widely used as precursors to lower oxidation states. The tetrahalide adducts are stable to disproportionation, except for 'Ta&(MeCN)2' (X = C1 Br), which gives TaV and Ta"' species, the latter undergoing reductive coupling with MeCN These observations corroborate the results obtained through polarography in MeCN; TaC15 i reduced directly to Ta111.531 Spontaneous reduction to niobium(II1) was observed for the crowded dinuclear PCy3 and
Niobium and Tantalum
645
PBu: adducts. Reduction to undefined products was also noted for [ N b L , ( ~ y ) ~ ]and ~l~ [Nb&(LL)], where LL = N,N,N‘,N‘-tetramethylethylenediamine.532 A poorly characterized borohydride derivative [Nb(BH&C12(MeCN)2] has been re~orted.~”
34.3.3
Pseudohalo Complexes (Table 35)
A number of stable niobium(1V) isothiocyanates and less stable isoselenocyanates are known. Rigorous exclusion of water is required to avoid the formation of selenium during the synthesis of the latter.534 Table 35 Niobiumw Pseudohalide Derivatives Compounds
Ref.
Comments
( a ) Thiocyanates and selenocyanates -42[WNCS),I N-bonded in the solid (IR) (A = K, AsPh4) Kz[WNCSe),l N-bonded in the solid, unstable thermally, dissociation in dilute MeCN solution Trans isomer (IR) [Nb(NCS),@Y )21 X-Ray structure for LL = bipy (square antiprism), [Nb(NCS),(LL),I (LL = bipy, dmbipy)” air stable ’ [m(NCSe)&L)2] N-bonded in the solid (IR), air stable (LL = bipy, dmbipy)a [NbCWCS),In Not fully characterized, probably dinuclear via NCS bridges Isolated, ESR [Nb(SCN)z(CP),l Not isolated, ESR evidence [N~C~(SCN)(CP)ZI
1 2
3 3,4 3 5 6 6
(b) Cyanides A4[m(CN)d ‘xH20 (A = Na, x = 4; A = K, Cs, x = 2; A = Ti, PrNH,, x = 0) [NbC13(CN)(MeCN)*1, [Nb(CN)z(Cp)zl [NbcNcN)(C~)zl
X-Ray structure for A = K, (DUdodecahedron); DM in solution (ESR); stable in aqueous solution only in the dark
7,a, 9
Not fully characterized Isolated, ESR Not isolated, ESR evidence
5
( c ) Cyanates [Nb(CNO),C11, [Nb(CNO)ACP)zl
Not fully characterized, N-bonded (IR) Isolated, ESR
5 6
6 6
dmbipy = 4,4’dmethyI-Z,Z’-bipyridyl. 1. T. M. Brown and G. F. Knox, 1. Am. Chem. SOC., 1967,89,52%; G . F. Knox and T. M. Brown, Inorg. Chem., 1969, 8, 1401. 2. T. M. Brown and B. L. Bush,J . Lers-Common Met., 1!371,25,397. 3. J. N. Smith and T. M.Brown, Inorg. Chem., 1972,11,2691. 4. E. J . Peterson, R.B.Von Dreele and T. M. Brown, Inorg. Chem., 1976,15,309. 5. D. J. Machin and J. F. Sullivan, J . Lars-Common Met., 1969,19,413. 6. C. P. Stewart and A. L. Porte, J . Chem., Dalton Trans., 1973,722. 7. P. M. Kiennan, J. F. Gibson and W. P. Oriffith, J . Chem. SOC., Chem. Commun. 1973, 816. 8. P. M. Kierman and W. P.GriWth, J . Chem. SOC.,Dalton Trans., 1975, 2489. 9. M. Laing, G. Gafner, W. P.Griffith and P. M. Kierman, Inorg. Chim.Acra, 1979,33, L119. a
Neutral six- or eight-coordinated adducts were obtained by allowing [Nb(NCS)6]2- or [Nb(NCSe)6]2- to react with pyridine or bipyridyl, respectively. An alternative route to the paramagnetic pyridine adducts is the reduction of [Nb(NCS),]- by the ligand. [Nb(NCS)4(bipy)2] adopts a nearly perfect DM square antiprismatic structure, with the bidentate ligand bridging the square faces.535N coordination has generally been established for the ambidentate thiocyanate ligand in the solid. Paramagnetic MIv octacyanide anions have been isolated as alkali metal or tetraalkylammonium salts. &[Nb(CN)*] has been obtained in low yield by air- or hydrogen-peroxideoxidation of K5[Nb(CN)8] prepared by electrolytic reduction of NbCls in an MeOH-KCN medium.536&[Nb(CN)8]*2H20 disproportionated photolytically to restore the Nb”’derivative. The [Nb(CN)8I4- anion is dodecahedral in the solid; all Nb-C distances are equal
Niobium and Tantalum
646
(2.255 A),537ESR, IR and Raman spectroscopies support a configurational change from symmetry in the solid to D4d(antiprism) in solution. The known cyanato compounds are summarized in Table 35.
ad
34.3.4 Oxo, Thio and Seleno Halides 343.4.1 Oxo halides and oxo compounds
Apart from the fluorides, all six MOX2 compounds have been reported.'^^^^ Attempts to prepare NbOFz yielded a single phase between the stoichiometric limits Nb02F and [Nb01.25F1.75] (average metal oxidation state of 4.25), with an Reo3type structure, and it was concluded that NbOF2 does not exist. The oxo halides are generally obtained through high temperature reduction of MzOSby the metal in the presence of the halogen. NbO12 may also be prepared by decomposition of NbO13. They are diamagnetic, stable in air as well as thermally, and the structure of NbOC12 is closely related to that of NbC14. Some Nb'" oxohalo anions have been identified (Table 36), but only the slightly paramagnetic dioxofluoroniobate salts A[Nb02F] (A = Li, Na, K) were isolated. [NbOCl4(L)I2-(L = HzO or EtOH) and [NbOCl5I3- were prepared by reduction of MX5 by Zn in aqueous HCI; they result from the hydrolysis of [NbC@-, and have been characterized in situ by ESR. Extraction with acetylacetone yielded [NbO(a~ac)~], whose structure is related to that of [ V O ( a ~ a c ) ~ ] . ~ ~ ' Table 36 Nb'" Oxo Halide Derivatives Compounds A[NbO,F] (A = Li, Na, K) [NbOF4I2[Nboc14L]2(L= H 0,EtOH) [NbOCl$[NbO(acac),] [NbOCl,(dPm)l" INb20Cl,(oEt),(THF),I [NbOCl,(MeCN),(NO)]
Ref.
Comments NbO,
+ excess AF (600-800 "C)
1
Not isolated, ESR data Not isolated, ESR data
2 2,3
Not isolated, unstable [Cl-] < 12 N, ESR Not isolated, NbCI, Zn HCl, acacH, ESR Isolated, Nb(dpm), Clz in oxygen free CCl,, not fully characterized X-Ray structure, metathesis, from Nb"' Isolated, hydrolysis of [NbCI,(MeCN),(NO)], not fully characterized
3 2
+ + +
4
5
6
1. W.Ruedorff and D. Krug, 2.Anorg. Allg. Chem., 1964,329,211. 2. J. F. Gainullin, N. S. Garifyanov and B. M. Kozyrev, Dokl. Akud. Nu&. SSSR, 1968,180,858 (Chem. Absrr., 1968,69,72 714). 3. D. P. Johnson and R. D. Bereman, J. Inorg. Nucl. Chem., 1972, 34,679 and 2957. 4. G. Podolsky, Dirs. Abs. B, 1973, 33, 5199 (Chem. Absrr., 1973, 79, 487 845). 5. F. A. Cotton, M. P. Diebold and W.J. Roth, Inorg. Chem., 1985, 24, 3.W. 6. Yu.A. Buslaev, M. A. Glushkova and N. A. Ovchinnikova, Koord. Khim., 1975,1,314 (Chem. Absrr., 1975, 83,52 565).
Neutral adducts are limited to the oxo porphyrin derivatives (Section 34.3.5.4.i) and to the uncharacterized [NbOCI,(NO)(MeCN),] obtained by hydrolysis of [NbC14(NO)(MeCN)2].539 This high stabilit of the MOX3 (X = C1, Br) adducts toward reduction probably hinders easy access to such M adducts. The reduction of NbSX3 (X = C1, Br), on the contrary, allowed the development of sub-thiohalide coordination chemistry.
x
34.3.4.2
Thio, seleno and telIUr0 halides
The sulfido, seleno and telluto halides of the transition metals have been reviewed Numerous Nbw chalcogenide halides have bean reported. For tantalum , only [Ta(&)C12] and [Ta(§,),Cl,] (m= 1-3, n = 1-4), of unknown structure, obtained from TaClS and S8, have so far been reported.543 [Nb(Y)ZXZ] (Y= S or Se, X = C1, Br, I) was prepared from the elements at 400-500 "C by chemical transport reactions. The obtention of pure [Nb(Y)212]required an excess of iodine,
Niobium and Tantalum
647
and it must be stored under a pressure of Nbl, even at room temperature. The chlorine derivatives are also available (equations 42-43). A similar reaction between tantalum and SzClz led to impure TaS3. [N&(Y)zBr6] (Y = Se, Te) and [Nb2(Te),16] were obtained by heating the elements to 780 “C. 3Nb
+ 1OY + 2NbC15
500-480“c
5NbYzClZ
(42)
Y=S,Se Nb+&Cl,
~
NbSCI,
(43)
The isomorphous [Nb(Y,)X,] compounds are monoc1inic544~”sat room temperature and triclinic at low temperature. The structure of [Nb(S2)CH2]2(Figure 22) exhibits the pairwise connection of two niobium atemti by metal-metal bonds specific of most of the Nbm halides. The molecules contain cage-shaped Nb2(Y& units of approximate D2hsymmetry consisting of a pair of Nb atoms lying perpendicular to the plane of the two Y2 groups. The Y-Y distances in lNb(Yz)Cl2] (Y = S, Se) correspond to single bonds, and the structures may be described as ,[Nb2(Y2)2X8/2).The occurrence of metal pairs explains their diamagnetic and semiconducting properties. The IR and Raman spectra of [Nb(S2)X2] (X= C1, Br, I) have been extensively The vib tions of the S2 groups, at 588cm-’ for [Nb(S2)C12], decrease from the chloride to the iodi e; they are higher than those of the thioniobyl group.
r
Figure 22 Drawing showing the 2[Nb,(SZ)Cl,,] structure of NbS,Cl, (reproduced from ref. 545 with permission)
[Nbz(Yz)&] is best described as chains of type k[NbZ(Y2)X&2] with short Nb-Nb and Y-Y distances. The Nb2 and Y2 pairs ( N b N b : 2.832(4); Se-Se: 2.305 A for [Nb2(Sez)Brs]) interact to form a quasitetrahedral cluster. The structural and magnetic properties support the formal oxidation states Nb4+and Y1-.547 Isotypic compounds of stoichiometry [Nb3Se5X7](X = C1, Br) have been obtained from the corresponding [Nb(Sez)Xz] derivatives and NbC14 in the case of the chlorine compounds, or in the case of bromine by decomposition of [Nb(Se2)Br2] in the presence of [NbSeBr3].548The structure of [Nb3Se5C17](Figure 23) shows two types of niobium atoms; Nb(1) which form pairs, while Nb(2) is isolated. Similarly, the atoms Se(1) and Se(2) form an Sez group (2.30 A) corresponding to a single bond, while Se(3) is coordinated to one metal atom only, at a remarkably short distance, which suggests the existence of a selenoniobyl group. X-Ray and ESCA data established that [Nb3Se5Cl7] is a mixed valence compound that should be formulated as [Nb~vNbV(Se2)2SeC171, Nb(2) being in oxidation state +V. The overall structure consists of chains of composition [Nb:v(Se2)2Cls] to which side chains [NbvSeClz] are attached. Semiconducting behavior was found for this compound. 343.4.3 Aa’ducts of the thio and seleno halides
A synthesis of [NbSCl,] from a solid state reaction between NbCL and Sb2S3at 200°C has been claimed,549but attempts to reproduce it were un~uccessful.~”By contrast, the reaction between Nb& (X = Cl, Br) and Sb& in MeCN at 50 “C easily provided adducts of type [NbSXZ(MeCN)2]2as highly air sensitive solids. The chlorine adducts were obtained as brown or green materials, depending on the number of MeCN groups. They are the only adducts of NbSX2 that have been obtained directly from tetravalent metal derivatives. The dimers have Dut symmetry; the niobium atoms are bridged by two sulfur atoms (Nb-S: 2.338(8)-
Niobium and Tantalum
648
Figure 23 Section of the structure of the mixed valence [Nb$"Nb"(Se),SeCI,] (reproduced from ref. 548 with permission)
2.349(7) A), and bear two chlorine atoms in apical and two acetonitrile ligands in equatorial positions. The octahedral environment of the metal is distorted as a result of niobium-niobium interactions (2.862(2)-2.872(3) A). [N~zS&(THT)~](X = (3, Br) results from a complex disproportionation in neat THT (equations 44 and 4 9 , two S2- ions being formal1 oxidized to (SZ)'-. Sulfur abstraction reactions from [NbSBa] have also been ~uggested.~' Y Reactions between [NbSC13] and THT, and those between [NbSX,] (X = C1, Br) and MezS follow similar patterns. Analog studies with [TaSX3] and THT gave no evidence for Tal" compounds. Crystal structure determination led to the formulation [Nb2&(THT)4(p-S)(p--Sz)]of approximate C , symmetry (Figure 24), the most salient feature being the Nb(p-S)Nb(p-Sz) ring. The Nb-Nb distances suggest a single bond; the three sulfur atoms form isosceles triangles (3.78(1), 3.76(1) and 2.01(1)A). The S-S distances are slightly shorter than in ionic (S2)2-.No significant tram effect was observed.
-
[NbSBr,] [NbSBr,(THT),I
Figure 24 Molecular structure of
+ 2THT
[NbSBr,(THT),]
[ N b 2 S 3 B r 4 ( T W 4+ J [NbBr5(THT),1
(44) (45)
[(THT),BI,N~(~-$)(~-S,)N~B~~(THT)~] (reproduced from ref. 373 with permission)
Attempts to synthesize selenium analogs of [NbSX2(MeCN)2]2from [NbSeBr'] led to reduction of the metal and formation of a tetranuclear cluster (equation 46). Formally two Se2ions form an (Se2)'- ion with concomitant release of two electrons, which reduce two NbIV to Nbnl atoms, and formation of the mixed valence tetramer.550 The latter (Figure 25) is best described as consisting of a central (MeCN)zBrzNb(p-Sez)(p-Se)NbBrz(MeCN)z fragment linked by long bromide bridges to two fac-NbBr3 units. Se(3) is bound to all three metal atoms. No metal-metal interaction is found between the two Nb"' centers, as also observed for [NbCl3(Ph=Ph)l4 (Section 34.4.2.2.ii). The sample was ESR silent down to -196"C, and diamagnetic, which was considered to be consistent with the two Nbnl atoms' being in an approximately C% environment and having a low-lying doubly occupied dz2 orbital, while the two NbIV atoms exhibit a metal-metal interaction. 4[NbBr4(MeCN),]
+ Sb,Se3 ' 5p$d* [Nb4Br10(Se2)(Se)(MeCN)4] + 2SbBr3 (23)
(46)
Niobium and Tmtalum
649
1'
Br Br
Figore 25 Molecular structure of [Nb,Br,,(Se,)(Se)(MeCN),]
34.3.5
(23) (reproduced from ref. 550 with permission)
Solvolysis of the Tetrahalides
343.5.1 Alkoxides (Table 37) Several NbIV alkoxo compounds have been obtained, by miscellaneous methods, but were often only poorly characterized. A series of salts containing the [NbCl5(0R)l2- anion (R=Me, Et, Pr') has been prepared by electrolytic reduction of NbC15 in saturated HC1-ROH solutions.' A different experimental treatment of the initial electrolytic solution led to the diamagnetic [NbC1(OEt)3py]p,which was further converted to the extremely moistureand oxidation-sensitive diamagnetic [Nb(OEt),], (Scheme 5 ) . Tnble 37 Niobium(1V) and Tantalum(IV) Alkoxides Comments
Compounds"
Ref.
Electrolytic reduction of ROH-HCl-NbCl,, addition of ACI, A = Me,N alternate synthesis: solvolysis of NbCL,(MeCN),
+
[NbCl,(MeCN),, MeCN] KOMe, analysis only Sublimable, diamagnetic Alcoholysis of [Nb(NEh)J, monomeric; R = OMe adduct O.SEt&H, diamagnetic in the solid M = Nb electrolytic reduction, diamagnetic; X-ray structure for L = EtOH diamagnetic X-Ray structure, metathesis from Nb"' Electrolytic reduction [Nb,CI,(OR),] t Libipy; unidentified Nb"' species if excess Libipy [NbCI,(MeCN),, MeCNJ+ MeOH, analysis only Diamagnetic Diamagnetic
3 4
5 4.6
7 8
9 3 6 6
Qui = quinuclidine;ado = adamanto 1. R. A. D. Wentworth and C. H. Brubaker, Inorg. Chem., 1963,2,551. 2. R. A. D. Wentworth and C. H. Brubaker, Inorg. Chem., 1962,1,971. 3. R. Gut and W. Perron, J . Less-Common Met., 1972,26,369. 4. R.A. D. Wentworth and C. H. Brubaker, Inorg. Chem., 1964,3,47. 5. M.Bochmann, G . Wilkinson, G.B. Young, M. B. Hursthouse and K,M.A. Malik, X Chem. SOC., Dulton Tram., 1980,901 6. L.G.Hubert-Ffdzgraf and P. Laurent, to be published. 7. F. A, Cotton, M. P. Diebold and W. J. Roth, Inorg. Chem., 1985, U,3.509. 8. C. Djordjevic and V. Katovic, J. Chem. SOC. ( A ) , 1970,3382. 9. N. Vuletic and C. Djordjevic, J . Chem. SOC., Dalton Trans., 1973,550. a
650
Niobium and Tantalum
The synthesis of M(OEt), shows that, contrary to previous observations, tetravalent alkoxides are stable in the presence of alcohol, at least for a short time. The alcoholysis of [Nb(NR2)4]usually led to [Nb(OR')5],263but oxidation state SIV was retaine alcohols, e.g. for tetra(1-adamanto)niobium and tetra( 1-adamantylmethoxo)niobiurn,illustrating the close dependence of the metal's oxidation state on steric requirements. [ N b ( l - a d ~ ) ~ ] was reported to be monomeric, but the absence of ESR signals down to -1.50 "C, as well as its reluctance to form complexes (Me2NH, PMe3, THT) is puzzling.551Dinuclear and tetranuclear diamagnetic niobium chlorocatecholates have also been obtained. The TalV alkoxides are limited to some diamagnetic chloro alkoxide adducts.
343.5.2 JP-Diketonates,carboxy lutes and related chelates NbC14 reacts with a variety of B-ketoenols or oximes in the presence of a base to give moderately soluble eight-coordinate tetrakischelates (Table 38). These have one unpaired electron and are not isostructural with the related Zr and Hf compounds. Dodecahedral structures have generally been proposed on the basis of UV or ESR data.s52However, the larger bite of the dipivaloylmethane ligand (b = 1.28 A) allows the stabilization of a square antiprism. [Nb(dpm),] corresponds to an idealized D4(lllf) isomer, and was the first example of this geometry to be reported.553 Evidence for nine-coordination was found in m b ( a ~ a c ) ~ ( d i o x a n e )TaCL ] . ~ ~ ~generally reacts by abstraction of oxygen, and tetrakischelated compounds are limited to [Ta(dbm)4]. Some chloro-p-diketonato compounds, which result from reactions conducted in the absence of base, are also described. [Ta,C14(dpm)4] was derived from the tantalum(II1) adduct [Ta2C16(y-pic)4].544 Table 3% Tetravalent B-Diketonates and Related Chelates Compounds
[Nb(aCac)d [Nb(B-diket),dioxane] [Nb(dpm),l [M(dbm),l [NbC1,(/3-diket)2] [TaCl2(dpm)21* [Nb(LLhI [NbTd, [NbC1T31, [W8-hdq),l
Comments
ESR (dodecahedron), no complexation with THF P-Diket = acac, bzac, monodentate dioxane in the solid (nine coordination), for bzac free dioxane in solution X-Ray structure, square antiprism M = Nb, ESR (dodecahedron); M = Ta, not isomorphous, low p (0.53BM) B-Diket = acac, dbm; analysis only; dpm: X-ray Diamagnetic, soluble, obtained from Tax" LL = benzoyltrifluoroacetone (Ma), tetrathenoyltrifluoroacetone (tta) Polymeric ( p = 0.74 BM) Polymeric, ionic species in MeCN
Ref.
3,4 1
1. D. L. Kepert and R. L. Deutscher, Znorg. Chim. A m , 1970, 4, 645. 2. R. L. Deutscher and D. L. Kepert, Chem. Commun., 1969, 121. 3. T. J. Pinnavaia, G. Podolski and P. W. Godding, J. Chem. Soc., Chem. Commun., 1973, 242. 4. T. J. Pinnavaia, B. L. Baroett, G . Podolski and A. Tulinsky,1. Am. Chem. Soc., 1975, 97, 2712. 5. R. Gut and W. Perron,3. Less-Common Met., 1972,26,369. 6. F. A. Cotton, M. P. Diebold and W. J. Roth, Polyhedron, 1985, 4, 1485. 7. J. L. Moranpis, L. G. Hubert-Pfalzgraf and P. Laurent, Inorg. Chim. Acta, 1983,?1,119.
The 0 dealkylation reactions often observed between carboxylic acids and group VA halides could be controlled by the use of bulky substituted acids. Stepwise substitution and oxidation of [Nb(B&)(Cp)d via [Nb(O,CBu')(Cp),] and [Nb(02CBut),(Cp)z] thus provided [Nb(OzCBu'>4] in good yield. The latter is aramagnetic, the metal being octacoordinated. It is a rare example of a monomeric Nb' compound soluble in non-polar hydrocarbons, and may therefore constitute an interesting starting material for NbrVchemistry, but its reactivity does
'
Niobium and Tantalum
651
i"
k Figure 26 Molecular geometry of [ I(Cp)Nb(y-0,CH)}3(~-OH~z(y3-O)l. The nature of y, groups is not known with certainty (reproduced from ref. 556 with permission)
not seem to have been investigated. [Nb(02CCR3)z(Cp)2](R = F, Ph) was formed in similar reactions. The only Talv carboxylate known is the diamagnetic I(Ta2C14(0zCBut)4}2ButC02H]. A trinuclear compound [{Nb(OzCH)(Cp)}3(OH)Z(0)2~ was isolated from the reaction between [Nb(CO),PPh,)(Cp)] and HCOOH, which involves decarboxylation of the formate It has an almost regular triangular structure (Figure 26) with Nb - - - Nb distances close to the usual single Nb-Nb bond length, but the magnetic properties (one unpaired electron) and the large angles around the bridging oxygen argue against direct metal-metal bonding.
343.5.3 Thiolato and related derivatives The dialkylthiocarbamates (Table 39) were initially made by insertion of CS2 into the M-N bond of [M(NR2)5], which, in the case of niobium, proceeds with reduction to Nb'" and Table 39 Tetravalent Dialkyldithiocarbarnates and Related Compounds
Compounds
Ref-
Comments
(a) Dithiocarbamates Insertion of CS, into the NbV-NR, bond (R = Me, Et) or [~(SzCNRzhI metathesis of NbCI, (R = Et); octacoordinated (IR) (R = Me, Et) [Nb{szcN(cHz),} 41 Metathesis of NbCl,, octacoordinated (IR) [mzBr3(SzCNEtz)d Electrolyte [Nb,Br,(S,CNEt&]Br in MeNO, [N~C~(S~CNE~&{C(C~)NBU'}~]~ [Nb(s,CNEtz)(CP),lz.H,O Metathesis of [Nbcl,(C~)~], not fully characterized (Z = PF,, BPh,)
1,2,3 4
3
5 6
( b ) Dithiocarboxylacesand ditkiolates
[Nb(SZCR),I (R = Me, Ph [Nb(sZ%H4)3#[M(SzCd%I [N~C~Z(SZC~H~)IZ
Single crystal ESR studies, octacoordinated
7
Not isolated, ESR (trigonal prism) Polarographic evidence (M = Nb, Eln = -0.38 V; M = Ta, E,,, = -0.71 V) Isolated, diamagnetic
8 9
1. D. C. Bradley and M. H. Gitlitz, J . Chem. SOC.(A), 1969, 1152. 2. J. N. Smith and T. M. Brown, lnorg. Nucl. Chem. Len., 1970, 6, 44. 3. D. J. Machin and J. F. Sullivan, 1. Less-Common Met., 1%9,19,413. 4. T. M. Brown and J. N. Smith, J. Chem. Soc., Dalton Tram., 1972,1614. 5. M. Behnam-Dehkordy. B. Cnxiani, M. Nicolini and R. L. Richards, 1, Organornet. Chem., 1979, Wl,69. 6. J. Amaudrut, J. E. Guerchais and J. Sala-Pala,J. Orgunomer. C h m . , 1978, l57,C10. 7. D. Attanasio, C. Bellitto and A. Ramini, Inorg. Chem., 1980,19,3419. 8. J. Stach, R. Kirmse, W.Dietzsch, 1. N. Markov and V. K. Belyaeva, 2. Anorg. Allg. Chem., 1980,466,36. 9. J. L. Martin and J. Takats, Znorg. Chew., 1975,14, 73. 10. L. G.Hubert-Pfalzgraf,to be published.
10
Niobium and Tantalum
652
oxidation of the organic moiety to tetraalkylthiuran In the case of tantalum, the pentavalent state is retained, and no Taw dithiocarbamates are presently known. IR has been used as a criterion for establishing the coordination mode of the dithiocarbamato ligand, which appears to act as a bidentate in all these compounds. NbTVdithiocarboxylates have been prepared, and were studied by single crystal ESR. Their geometries are intermediate between a triangular dodecahedron and a square antipri~m.~~’ Tris(maleonitriledithio1ato)niobate [Nb(mnt),I2- (mnt = S&H.$-) has been obtained in solution by reduction of NbCl, by Zn in an HCl-MeOH solution followed by in situ complexation of the Nb4+ species.55BThe ESR data ((g) = 1.988, { A } = 94.4 G) are consistent with a nearly trigonal prismatic arrangement. They exclude the formation of an oxo species such as [Nb(mnt)2]2- whose ESR parameters should compare with those of [NbO(aca~)~] ((g) = 1902, ( A ) = 142 G). Polarographic results indicated that tris(dithio1ene) NbV and Tav undergo one-electron transfers. [NbC12(S2C6H4)]2appears to be the only dithiolene adduct isolated so far. Compounds with thiolato groups are unstable, especially in solution, but may be stabilized as [M(SR),(Cp),] derivatives (M = Nb; R = Me, Pr, Ph;87559M = Ta, R = Me). Selenium analogs are known for niobium, and have been used as ligands toward Fe(NO)(CO) and CO(CO)~ moieties.560 Tetrakis 0,0‘-dialkylthiophosphates have been obtained for niobium starting from oxidation states +V or +IV (Table 40). These chelates display a distorted dodecahedral geometry in the solid (X-ray) as well as in solution (ESR).s61An unusual cation, [{Nb(Cp)2}2(PS,)]+,in which a PS:- moiety is doubly bridging the terminal NbCp, units, has been obtained from [NbCl,(Cp),] and P4Sl0. The metal has a distorted tetrahedral environment of approximate & symmetry, while symmetry at phosphorus is reduced to CZv. Table 40 Niobium(1V) Thiophosphate Derivatives Compounds
Comments
Ref.
1. R. N. McGinnis and J . B . Hamilton*Inorg. Nucl. Chem. Len., 1972, 8,2.45. 2. V. V. Tkachev, S. Shchepinov and L.0. Atovmyan, J . Snucr. Chem. (Engl. Transl.), 1977,18, 823. 3. R. G. Cavell and A. R. Sanger, Inorg. Chem., 1972,11, 2016. 4. B. Viard, J. Sala-Pala, J. Amaudnrt, J. E. Guerchais, C. Sanchez and J. Livage, Inorg. Chim. Acta, 1980, 39,99. 5. C. Sanchez, D. Vivien, J. Livage, J. Sala-Pala, B. Viard and J. E. Guerchais, J. Chem. Soc., Dalton Trans., 1981, 64. 6. J. Sala-Pala, J. L. Migot, J. E. Guerchais, L. Le Gall and F. Grmjean, J . Orgunomei. Chem., 1983, 248, 299. 7. B. Viard, J. Sala-Pala, J. E. Guerchais, R. Mercier and B. Douglade, to be published.
343.5.4 Alkylamides and related compounds
(i) Dialkylamides and dialkylphosphides A series of homoleptic Nbw dialkylamido compounds has been obtained as highly air sensitive liquids by spontaneous reduction of [Nb(NR2)5] (R= Et, Pr”, Bu”; Table 41). Aminolysis of either NbV or Nbw dialkylamides provided mixed ~pecies.”~ The existence of quadrivalent tantalum dialkylamides is still questionable. [Ta(NEt&] was reported to have been detected during the thermolysis of [Ta(NEt&] ,263 which led predominantly to the Tav nitrene (2; Section 34.2.3.4.ii), but other workers could not find any evidence for its formation in a similar experiment.=’ [Ta(NMeBu&,] has been reported to be formed in the decomposition of the corresponding pentavaient dialkylamide. The monomeric, paramagnetic alkylamides constitute rare examples of magnetically dilute
Niobium and Tantalum
653
Table 41 Niobium(1V) Alkylamides and Silyiamides Compoundr
[Nb(NR*),I fR =Et. PI".Bun',
[hi(meBun)j
'
tNb(NC5H,o)4I [Nb(NEtz),,(NC,H,,),] ( x = 1, 2) [Nb(NMe,),(CP~ln
Comments
Monomeric, ESR (gll(CNBU~)]~, [NbC12{C(Cl)NBut}(ox)]2 and [NbCl{C(Cl)NBu'}(SzCNEt2)]2 are dinuclear, but only the last two are diamagneti~.~~' Diamagnetic polymeric nitrides, [TaNBr], 324 and [NbNI],,325 have been obtained by high temperature thermolysis of MX5 with ammonium salts. 34.3.6 Chalcogenides The chalcogenides MY3 and M Y 2 (Y = S, Se) can be prepared by various methods, including heating the elements together, thermal decomposition of the [Nb(Y2)X2]halides, and action of CS2 on the oxides at about 900-1300 0C.17542 The structures of [NbS,(py)] and NbSe3"O show trigonal prismatic chain arrangements in which the chalcogenide atoms form approximate isosceles triangles. It was suggested that they should formally be considered as [Nb'V(Y2-){(Yz)2-}] (Y= S or Se), an idea which is supported by studies on the electronic structure of NbSe3.571The presence of Nb-Nb pairs (3.04 A) in NbS3 is responsible for the observed diamagnetism and semiconducting properties; by contrast no Nb-Nb pairs were found in NbSe3, which indeed exhibits metallic pro erties. The remarkable electrical properties of TaS3 may also be explained by Ta-Ta pairing. 972 A number of nearly two-dimensional intercalates of formula [MY2(L),] (Y = S, Se; L = N donors; x = 0.5 for L = py, Et2NH, Et3N, piperdine, etc.) with promising superconducting properties have been Niobium disulfide is an intercalation electrode in lithium batteries, whose effectiveness is close to that of titanium d i s ~l fi d e. ~'~
34.3.7 Activation of small molecules The short-lived [MH,(CP)~] and [TaH4(dmpe)2]have been obtained from the Mv hydrides using photogenerated t-butoxy radicals, and were characterized by low temperature ESR.575 On the other hand, thermally stable, well-defined dinuclear or mononuclear MIv hydrides have been prepared by oxidative addition of H2 to dinuclear Mu' or mononuclear M" halide phosphine adducts, respectively. They constitute attractive entries to lower oxidation state compounds, and will be reviewed in Sections 34.4.3.1.i and 34.6.1.2.i. [TaHC12(Cp")12 (24) (Cp" = q5-C5Me4Et) reacts easily with CO (equation 47) to give [ {Ta(C12(Cp")}2(p-H)(p-CHO)] (25).5'6 Crossover experiments have established that the dimer (24) is not split by CO. X-Ray diffraction data show that the hydride and formyl ligands are bridging two skewed TaC12(Cp") fragments; the Ta-Ta distance was considered too long for a full single bond, but consistent with some interaction (Figure 27).577The C-0 distance is unusually long; this activation is in agreement with its being cleft by PMe3 (equation 48) to give (26) for which a Ta-Ta bonding has been proposed (2.992(1) A)."' The formyl ligand of (25) is apparently stabilized by side-on bonding between the two metals, and (25) does not react readily with CO or H2. It is, however, susceptible to electrophilic or nucleophilic attack; methanol is produced when excess aqueous HCl is added to (25) in protic solvents. [TaHC1,(Cp")124- CO
-78"c
I {T*CL(CP")
}z(CO)I
Figure 27 Drawing of the structure of [{TaCl~(Cp")},(~-H)(~-CHO)J (25) (reproduced from ref. 574 with permission)
Niobium and Tantalum
655
[TaHClZ(Cp")l2(24) reacts rapid1 with one and only one equivalent of acetonitrile to give [{TaC12(Cp")(p -H)(p-C1)( p -q -N, q Y-C, N-NCHMe) }TaC1(Cp")] (27) ,579 whose molecular structure resembles those of (25) and (26). The TaCls(Cp") and TaCl(Cp") fragments are asymmetrically triply bridged by a chloride ligand, a hydride ligand and the NCHMe ligand derived from acetonitrile by hydrogenation. Multiple bond character has been suggested for Ta(1)-N (1.901(8) A), but not for Ta(2)-N (2.059(8) A). The paramagnetic [{N~(BU")(C~)~}~(N~)(O~)] (equation 27) is the first example of simultaneous fixation of Nz and O2 by a niobium complex (v(N2) = 1740 cm-l). The reaction between NO and [NbC14(MeCN)2].MeCN led to [NbC14(NO)(MeCN)2].539 Ligand exchange reactions provided [NbCl,(NO)(MeCN)(LL)] (LL = bipy or phen), [NbC14(NO)(MeCN)(DMF)3]and [NbCl4(NO)(MeCN)(Ph2P0)],this last resulting from the oxidation of the PPh3 ligand by NO. The diamagnetism of these products suggests polymeric structures in the solid. The phenanthroline and dimethylformamide adducts behave as electrolytes in solution. For the reaction between [Nl ~M e~(C pand )~l excess NO, see Section 34.2.6.3.
34.4 OXIDATION STATE +I11 34.4.1
Survey
The chemistry of niobium and tantalum in their lower oxidation states is expanding rapidly. The first structurally characterized molecular Nb"' derivative was reported in 1970,525while Nb"' and Ta"' halide adducts were described in 1973580and 1978, respectively.s81 That this now fertile ground had long remained untilled is essentially attributable to the lack of convenient starting materials (in particular those without cyclopentadienyl ligands). Indeed, the stoichiometric trihalides MX3 (X = C1, Br) are only incidental compositions in homogeneous They can be obtained by reduction of the pentahalides by the metal, or, in the case of tantalum, by disproportionation of the tetrahalides. Reduction to the trichloride can be used as a means of separating tantalum from niobium. Stoichiometric Nb13 appears as a residue in the thermal decomposition of the higher iodides; it is also the main product of the reaction of the metal with iodine at about 500°C. Nb13 is, however, unstable and disproportionates at 513 "C bo 1y-Nbb and Nb318.501 The existence of trifluorides has been claimed, but the products isolated invariably contained oxygen, and the existence of pure MF3 remains d o ~ b t f u l . ~ , ~ ~ The 'trihalides' are thermally unstable, dark-colored, polymeric, unreactive solids, and their chemical properties have received little attention.
34.4.2
Halo Adducts
34.421 Anionic halo complexes The niobium(I1I) salts Cs3[Nb2X9](X = C1, Br, I) and Rb3pbzBr9]have been prepared by disproportionation reactions (equation 49). Other alkali metals gave rise to different reaction products.525There is as yet no evidence for the corresponding tantalum anions [Ta2&I3-. These salts were reported to be isotypic with Cs3[Cr2X9]. The magnetic susceptibility of Cs3[Nb2X9]corresponds to two unpaired electrons (p = 2.68 BM, 8 = -210 "C, for X = Br), which was considered to be in agreement with a formal Nb-Nb double bond and a &h symmetry. 3Nba8 + 12AX
+
4A3[m2)(9] + Nb
(49)
A = Cs, Rb X = C1, Br,I
The reaction between [Nb2C16(THT)3]and Et4NCI in various stoichiometries provided the ionic (Et4N)2[Nb~Cls(THT)]and (Et4N)3[Nb2Clg].5g0
Niobium and Tantalum
656
3A4.2.2 Neutral adducts of the trihalides
(i) Synthesis
Activity in this area has stemmed from the discovery that reduction of MX5 or their complexes to yield molecular MI1' trihalide adducts can be achieved rather easily by using reductants such as sodium amalgam, sodium-potassium alloy and magnesium in the presence of ligands. The first report on this approach was on the synthesis of [Nb&(THTk] (X= C1, Br, I) by McCarley and Maas, using a one-electron procedure (equation 50)., The soluble Nb" adducts formed were easily separated from the sodium derivative and isolated in high yields (60-70%). The tantalum analogues [Ta&,(THT),] (10;X = C1, Br) were similarly prepared, but from
Similar reducing procedures-the commonest using Na/Hg in toluene, or Mg in methylene chloride-were applied to the pentahalides in the presence of a large variety of monodentate ligands (Table 42). Dinuclear diamagnetic compounds of different stoichiometries, such as [M&l,@Me2)~1 (a) ,58*585 [M2&(PhPMe2)4] $29; M = Nb, X = C1; M = Ta, X = C1),586 [MzC16(PMe3)4]587358x and [Ta2C16(y-pic),] (30) s4 were thus obtained in variable yields. [NbC16(diox)z]was obtained by reduction of NbCh with Mg in the presence of dioxane.sM Side-reactions were often observed when oxygen donors were used. Reduction of NbCI, by Mg also occurred in the presence of THF, but no pure Nb"' trihalide adduct could be isolated, as a result of THF degradation reactions.589 The nature of the adducts formed with phosphines depends on the experimental procedure. The diamagnetic [Ta2Cb(PMe3)4](31) was obtained through reduction of TaC1, by sodium amalgam in toluene,587while the paramagnetic [TaC13(PMe3)3]was isolated when the reduction was achieved in diethyl ether in the presence of an excess of ligand, or by an exchange reaction from [Ta2Cls(THT)3]at 60 *C.% Alkylidene derivatives may also be converted to trivalent phosphine adducts (equations 51 and 52).s90[TaCl3(PMe&j converts readily in solution to the dimer (31 . Puzzling results were obtained when TaBr5 was reduced by Mg in the presence of P ~ I F ' M ~'~A. ~very clean one-step reduction of NbC15 was observed with diphenylacetylene, which acts both as the reductant and as the ligand (equation 53).592
b
4NbC1, + 8 P h G S P h
+
[NbCl,(PhC%CF%)]4 4 t - P h C l M C l P h 75%
(53
Although some adducts with monodentate ligands, including [NbC13L3] (L = py, y-pic 3,5-Me2py)593and [TaC13(PMe3),] have been obtained from [MzCl&3] (L = SMe2 or THT through ligand exchange reactions, this procedure was mainIy employed to prepare adduct bipy, dto (dto = dithiaoctane)] with bidentate donors, yielding [M2C16(LL)2][LL = [Nb&16(LL)z] [LL = diars, triars (triars = 1,l,l-tris(dimethylarsinomethyl)ethane)] ant [Ta2Cl,j(dmpe)z].95 The latter compound could not be prepared directly from TaC1, with Mg since the reduction stopped at [TaC14(dmpe)z],5s7 while the use of Na/Hg offeret [TaC12(dmpe)2],596 without evidence for a Ta' I intermediate. Mixed ligand adducts such a [T~zC~(SM~~){P(NM~~)~}~],~~~ [Nb4Cl,2(SMe2)4(dppm)J598 and [Nb4Cl1~(PhPMe&{MeN ( P F z ) ~ }were ] ~ ~ also obtained by the ligand exchange procedure.
Niobium and Tuntatuna
657
Table 42 Neutral Adducts of the Niobium and Tantalum Trihalides with Monodentate Donors
comments
Compounds
Ref.
X-Ray structure for M = Nb, X = Br and M = Ta, X = C1, Br; NQR X-Ray structure for M = Ta (Ta-Ta = 2.691(1) A) X-Ray structure (Ta-Ta = 2.704(1) A) ESR at room temperature: (g) = 2.04, { A ) = 179.1G for L = y-pic Diamagnetic, dissociation in solution for L = y-pic
4,5,6,7
ESR active in frozen solution only: (g) = 1.92, (A) =BOG; unstable towards dimerization, oxidative addition of H2 X-Ray structure for M = Ta; oxidative addition of Hz
11,12
X-Ray structure Activation of N,(1 atm) during the reductive synthesis with Mg; [M,Cl,(PCy,),] in solution by addition of PCy,
16
8
9 9.10
13,14,15 17
11,18 L = L' = PhPMe,; L = PMe3, L' = PhPMe, not isolated, identified by 31PNMR - - ..
18
.
1. R. E. McCarley and E. T. Maas, Inorg. Chem., 1973,12,1096. 2. J. L. Templeton, W. C. Dorman, J. C. Clardy and R. E. McCarley, Ztwrg. Chem., 1978,17, 1263. 3. J. L. Templeton and R. E. McCarley, Znorg. Chem., 1978,17,2293. 4. A.D.Allen and S. Naito, Cdn. J . Chem., 1976,54,2948. 5 . L. G.Hubert-Pfalzgraf, M. Tmoda and J. G. Riess, Inorg. Chim.Acra, 1980,41,283. 6. M. Tsunoda and L. G. Hubert-Pfalzgraf,Inorg. Synrh., 1983,21,16. 7. F. A. Cotton and R. C. Najjar, Inorg. Chem., 1981,20,2716. 8. F. A. Cotton, L. R. Falvello and R. C. Najjar, Inorg. Chim. Acta, 1982, 63, 107. 9. M.E. Clay and M. T. Brown,Znorg. Chim. Acta, 1982,58, 1. 10. J. L.Moranpais, L. G . Hubert-F'falzgraf and P. Laurent, Znorg. Chim. Acta, 1983,71,119. 11. S. M. Rocklage, H. W. Turner, I. D. Eellmann and R. R. Schrock, Orgammetallics, 1982,1,703;H. W. Turner, R. R. Schrock, J. D. Fellmann and S. J. Holmes, 3. Am. Chem. Soc., 1983,105,4942. 12. J. R. Wilson, A. P. Sattehrger and J . C. Huffman, J. Am. Chem. Soc., 1982,104,858. 13. A. P. Sattelberger, R.B. Wilson and J . C. Huffman, Znorg. Chew., 1982, 22, 2392, 4179, 14. L.G.Hubert-Pfalzgraf and I. G. Riess, Inorg. Chim. Acta, 1978,29, L251. 15. L. G. Hubert-Pfalzgraf, M. Tsunoda and J. G. Riess, I n o g . Chim. Acta, 1981,52,231;N. Hovnanian and L. G. Hubert-Pfalzgraf, to be published. 16. F. A. Cotton,M. P. Diehold and W. J. Roth, Znorg. Chim. Acta, 1985,105,41. 17. J. L. Moranpis and L. G. Hubert-Pfalzgraf,Tramition Met. Chem., 1984,9, 130. 18. S. M.Rocklage, J. D. FeHmann, G . A. Rupprecht, L. W. Messerle and R. R. Schrock,J . Am. Chem. Soc., 1981, 103, 1440.
( i i ) Structure and characterization
The isolated monomeric trivalent adducts are limited to [NbCl,L3] (L = py, 7-pic), [TaX3(PMe3)3](X = C1, Br, I) and [NbBr3(PhPMe&]. The existence of magnetically dilute d2 compounds was confirmed by magnetic susceptibility (p = 2.03 BM for [NbCl,(y-pi~)~]at 20°C), and ESR data.593The ESR signal of [TaC13(PMe3)3]could be detected only in frozen solutions, and 'HNMR indicates a meridional arrangement of the PMe3 ligand^.^" [NbBr3(PhPMe2)3]is the only monomeric structurally characterized adduct.591 The formation of diamagnetic compounds involving metal-metal bonding is more frequent. Four different geometries-for dinuclear and tetranuclear compounds-have so far been found by X-ray crystal studies. The metal center is generally heptacoordinated. The adducts with monodentate ligands were characterized by 'H or 31PNMR techniques. Usually no ligand dissociation reactions were observed. The adducts formed with bidentate ligands are often of poor solubility. 'H NMR data on the diamagnetic [M2&L3] (L = SMe2 or THTs83,s80)show one bridging neutral ligand for two terminal ones. Their crystal structures display confacial bioctahedra of C , symmetry with one bridging sulfur atom tram to both terminal ones (Figure 28). The short
658
Niobium and Tantalum
M-M distances (Nb-Nb: 2.728(5); Ta-Ta: 2.710(2) 9.) found for [ {MBr2(THT)}(p-Br)Z(pTHT){MBr2(THT)}] were considered to reflect strong M-M bonding,m and to be in agreement with a formal double bond. The M-Sb bonds are markedly shorter than the M - S , bonds; the St atoms appear to be hindered from closer approach to the metal by the repulsion exerted by the four cis halogens in the plane (short St-3r contact of 3.51A in [Ta2Br6(THT)3]).An unusually close approach (3.30 A) of the two bridging bromines has been observed, and was explained, on the basis of MO considerations, by the stereochemical activity of the metal-metal n-bonding electrons. The low C% symmetry of these [M&L3] adducts was considered to be responsible for their diamagnetic behavior, compared to the paramagnetic behavior of [Nb2X9I3-.
U Figure 28 Molecular structure of [(THT)TaBr,(p-Br)2(p--THT)TaBr,(THT)] (10) (reproduced from ref. 580 with permission)
A thorough NQR study r N b , 35Cl,81Br, 1271) reflects the environmental difference between bridging and terminal halides. The latter resonate at lower frequencies, which was considered to mean that the relative amount of n bonding from halide to metal is much larger for X, than for X b . A single crystal X-ray study of [Ta2C14{P(NMe2)3}2(p-Cl)2(p-SMe2)], the only one available on a mixed ligand adduct in oxidation state +III, showed the structure to be related to that of [Tad&(SMe&( p-Cl)~(P -SMe?)]-5w The isolation of [M2C&(PR&] (Table 42) shows that adducts of that stoichiometry can also be stabilized by crowded phosphines;m their structure is probably related to that of [NbzCl9l3with triple chlorine bridges. The [M2Cl&4] stoichiometry is the most commonly encountered with phosphines, although it has also been observed for M = Ta and L = py, y-pic. X-Ray data are limited to (31), which consists of two somewhat distorted octahedra sharing a common edge (Figure 29). The short Ta-Ta distance and acute Ta-Clb-Ta angle indicate strong metal-metal bonding. Considerable distortions involving the axial substituents and non-bonded H - - - C1 interactions are observed. The structure with axial and equatorial phosphines was reported to be favored for electronic reason^.'^' According to the MO calculations of Hoffmann et al. for bridged M2LIO complexes,601the ordering of the low-lying d-orbitals is alag) < n(bl,) < a*(u,) < S(b3& < n*(bzg) o * ( h ) This . means a formal bond order of 2 for the d2-dZ system. The 31PNMR of (31) offered a rare example of P-P coupling across a metal-metal bonded dimer ( J ( P P') = f2.46 Hz).
Figure 29 Coordination polyhedron of: [Ta2Cl4(PMe,),(p-C1),] (31) (reproduced from ref. 587 with permission)
Niobium and Tantalum
659
The adducts involving bidentate ligands are all of type [M2C16(LL),]. The structures of those with bidentate phosphines (M = Nb, LL = dppe;602M = Ta, LL = dmpeS9') are related to that of (31), but with the chelate rings spanning two equatorial positions. The metd-metal distances are consistent with either a double or a single bond (Nb-Nb: 2.712(2)A; Ta-Ta: 2.710(1) A). In contrast to the diamagnetism-with small temperature independent paramagnetic contribution-f the adducts formed with monodentate ligands, those with bidentate ligands exhibit temperature dependent paramagnetism and produce easily observable ESR spectra even at room temperature.594 Although the magnetic moments (0.54 to 0.87BM at room temperature for the niobium adducts) are lower than those generally observed for the tetravalent adducts, it has been suggested that the metals are only singly bonded, the magnetism being presumed to be lowered by spin-orbit coupling and/or antiferromagnetic interactions. A complex hyperfine structure was observed for the bipyridyl adducts. The IR data suggest that the bipy ligand is negatively charged with electron delocalization into the ligand ring system. 'HNMR data on the niobium I11 adducts with triphos and triars established that these ligands behave as bidentates only. 83,603 [NbCl3(Ph=Ph)],, the only structurally characterized tetranuclear compound (Figure 30),592can be regarded as a centrosymmetric distorted double hexahedron with two missing corners. The Nb atoms are heptacoordinated, linked by two triply bridging chlorines, but do not exhibit metal-metal bonding. The alkyne C-C distances and C 4 - C angle are consistent with its formulation as a metallacyclopropene (Section 34.4.3.2.i).
0
Ph-
Pi
figure 30 Molecular structure of [NbCl,(PhC%CPh)],
(reproduced from ref. 592 with permission)
The trivalent adducts display a wide range of colors, but electronic absorption spectra have been reported only for [M2&(THT)3] (X = C1, Br), [Nb216(THT)3],(Et4N)2[Nb2C18(THT)J and the magnitude of the extinction coefficients supports strong metal-metal (Et4N)~[Nb~C19];580*5w interactions. The similarity between the spectra of [Nl&l&'HT)3] and (Et4N),[NbzC1,(THT)] incidates that the bridging THT ligand is retained.
(iii) Reactivity Investigations on the reactivity of the MI1' trihalide derivatives have so far mainly been limited to that of [Ta2C16(PMe3)4],[M2C16(PhPMe2)4],[Ta2C16(y-pic)4]and [M2C16L3](L = SMe2 or THT). Oxidative additions occur easily, and these compounds are therefore suitable precursors of M V and dinuclear M'" derivatives. Metathesis reactions may lead to other trivalent derivatives. The dinuclear unit is generally retained, although condensation to tetranuclear species is sometimes observed. The only monomeric substitution products '
coc3-v
Niobium and Tantalum
660
reported so far, [Nb(NCSf3(py)3] and [Nb(Etzdtc)3], derive from [NbC13(py)3]. Chemical reduction of the chloro adducts appears difficult, and seems favored for monomeric M"' adducts (Section 34.6). Electrochemical reduction offers possible alternatives.604
34.4.3
Activation Reactions Involving Niobium(1II) and Tantalum(iI1) Adducts
344.3.1 Activation of small molecules (1)
Dihydrogen and related molecules
The reactivity of the red [Ta2CL(PMe3)4(p-C1)z](31) is of interest, because it leads to TalV and Ta' compounds (Scheme 6). Complex (31) reacts cleanly with Hz under mild conditions to produce the air stable emerald green diamagnetic [Ta2C~(PMe3)4(p-H)2(y-Cl)z] (32), one of the very few compounds with a metal-metal bond bridged by four ligands (Ta-Ta: 2.621(1) A). It is also the first example of direct interaction of H2 with a metal-metal multiple The terminal and bridging groups are in a mutually staggered arrangement, and the coordination polyhedron about each tantalum is roughly a square antiprism. The equatorial phosphines, but not the axial ones, are chemically equivalent. The presence and location of the hydrides were established by spectroscopic techniques (6, = 8.5(m) p.p.m.; v(Ta-H-Ta) = 1280cm-'). The Ta-Ta separation (2.621(1)A) is shorter than the Ta-Ta bond in the original Ta"' adduct. Compound (32) is the precursor of the air sensitive [Ta2CL,(PMe3)4(p-H)2](33), characterized by X-ray diffraction at -160 "C. The Ta2C14(PMe3)4substructure resembles that of the quadruply bonded dimers [MiC14(PMe3)4] (M '=M o or W) with the ligands in eclipsed conformation and retention of the Ta=Ta bond (2.545(1) A). Rapid rotation of the end groups and/or bridging hydrides is required to account for the apparent magnetic equivalences in 'H and 31PNMR. Complex (33)offers a convenient entry into dinuclear TaIVchemistry. Its ether solutions react readily with H2, HC1 or C12to give the quadruply bridged TaIVcompounds (M), (35) or (32) (Scheme 6). Alternative syntheses of [Ta2C14L4(p-H)4](34)are the hydrogenation of the [TaHC12(CHCMe3)L3]alkylidene or the thermolysis of [TaH2C12(PMe3)4(Section 34.6.1.2.i, Scheme 9).
lL
Na/Hg
Scheme 6 (L = PMe,)
Compound (34) has an imposed DZd symmetry and a very short Ta-Ta length (2.511(2) A)-hte shortest reported to date; the bridging hydrogens were located in the solid (Ta-H: 1.81(2) A). Unlike its tantalum(II1) precursor (33),it is static on the NMR time-scale. According to the predictions of Hoffmann,60 all the metal p type orbitals are engaged in the Ta-H-Ta bonding, and a substantial rotational barrier arises from the overlap of orbitals of n symmetry. The oxidative addition of H2, HC1 and C12 to the multiply bonded metal-metal complex (31) occurs without gross structural rearrangements, and has no equivalent in binuclear chemistry. Oxidative addition of H2 under mild conditions with,formation of diamagnetic MIv hydrides appears to be a general feature for the dinuclear MI" adducts with monodentate phosphines. Bridging or terminal hydrides are [Ta2C16(dmpe)z]and [Nb2C16(dppe)2],however, are inert toward Hz even under severe conditions.s95 Monomeric diamagnetic [TaH2CI(PMe&]
Niobium and Tantalum
6-61
(36), was obtained by reduction of TaIV monomeric hydride, resulting from the oxidative addition of Ta" derivatives (Section 34.6.1.2.i). \ (ii) Dioxygen and dinitrogen Admission of oxygen into a solution obtained by reduction of TaC15 by two equivalents of Na/Hg in the presence of PMe2Ph led to the linear oxo derivative (PMe3Ph)2[C15TaOTaC15] (Section 34.2.6.1A). Although [{TaC13(PMe&}2(p-N2)]could be obtained via the 'azine route' (Section 34.2.3.6), this product did not form when [TaC13(PMe3)3]was exposed to dinitrogen, even under 68 atm. The dimer (31)396robably forms too rapidly from [TaC13(PMe&], and does not react readily with dinitrogen. Activation of N2 under mild conditions (25 "C, 1atm) was observed during the reduction of the MC15 by magnesium (equation 29).304Unfortunately, the products were unstable and only a compound of empirical formula [Nb2CI6(PBu:),N2] could be isolated. IR data (v(N2) = 1720 cm-') suggest a mode of bonding different from those found in Schrock's compounds. X4.3.2 Reactions with unsaturated substrates
(i) Alkynes-three-membered
metallacycles
The tetrahydrothiophene adducts [MzC16(THT)3] (10) react with a variety or alkynes
R W R ' , but the result depends considerably on R and R' (Scheme 7). With diphenyl-
'
acetylene the initial product, presumed to be [TaC13(THT)2(PhC+CPh)], converted to (pyH)[TaCL(py)(Ph-Ph)] by recrystallization in the presence of pyridine.608The most remarkable feature of this seven-coordinate anion is the very strong symmetrical binding of tolan to Tav. The T a x and C-C distances were interpreted to mean that the alkyne acts as a four-electron donor, or as a two-electron donor in a simple bent bond mode.609 A comparable metalIacycIopropane-TaV moiety was obtained when t-butylmethylacetylene was used, giving [Ta~C1~(THT)2(ButC%CMe)2(p-Clz)].6'0 Evidence for strong interaction with the alkyne is given by the substantial asymmetry of the chlorine bridges, the Ta-Cl(1) bond trans to the alkyne being the longest. The Ta-Ta distance of 4.144(1) 8, is too long to support any bonding interaction. Di-t-butylacetylene afforded [Ta2C14(THT)2(p-Cl)2(p-But~But)],611 whose structure derives from that of (10) with THF molecules in place of the termind THT ligands and a bridging alkyne substituted for the THT bridge. The short metal-metal &stance indicates the retention of the double bond. Adducts (10) catalyze the polymerization of substituted alkynes: into cyclic trimers for the primary alkynes R-H ( R = P h , Et, Pr'), into high polymers for the internal alkynes '~ complexes might be R W R ' (R = Ph, R' = Pr; R = Me, R' = B u ~ ) . ~Metdlacyclopropene the first step in cyclotrimerization. Quite different results were obtained by allowing the unusual alkyne P h 2 P m P P h 2 to react with [Ta2C16(SMe2)3] .613 Two products were (37) and [(M%S)Cl3obtained: [C14?a-CH( PPh2)C(bPh2)=C( PPh,) CH(PPh,)-TaCl,] Ta==C(PPh2)C(PPh+C(PPh2)C1(PPh2)=~aC13(SMe2)] (38; Scheme 8). In these two centrosymmetric compounds, the starting alkyne has dimerized to form a four-carbon chain, which acts as a bridging ligand (equation 54), while the tantalum atom's oxidation state has increased to +V. Each metal atom belongs to an unusual three-membered metallacycle, T k 4 - P for (37) and Ta=C-P' for (38). In (37), one C1- ligand is replaced by Me2S in a heptacoordinated ring reported environment. Complex (37) appears to contain the first example of an" for an early transition metal, while the .ring with the T a=C double bond of (38) is entirely unprecendented. PhZPC,
2Ph,Pc--CPPhz
PPh2
c=c/
+
P$P/
(54)
'CPPh,
Comparable three-membered metallacycles were also obtained by oxidative addition of C-H bonds. Reduction of TaC15 by sodium sand in PMe3 as a solvent afforded [Ta(PMe&($CH2PMe2)(q2-CHPMe2)](39; 7% yield), whose structure (Figure 31) shows that C(l) is bonded
662
Niobium and Tantalum
Scheme 7
Ph Schrmr 8
I
(38)
Ph
Niobium and Tantalum
-
663
to only one hydrogen atom, which is coplanar with the T a x - P triangle, the whole group affording the first example of an M(q-CHPMeJ moiety. Complex (39) is an example of a seven-coordinate, pentagonal bipyramidal, 18-electron Ta"' P
P
Figure 31 Crystal structure of [Ta(PMe3),(q2-CH,PMe2)(q2-CHPMe2)] (39) (reproduced from ref. 614 with permission)
(ii) Reactions with other unsaturated substrates Reductive coupling reactions with C--C bond formation of unsaturated ligands such as nitriles (Section 34.2.3.5.i), alkynes and isocyanides were often observed for the [M2C&] (L = THT,SMeJ adducts. Thus the reaction between (28) and an excess of t-butyl isocyanide led to binuclear and trinuclear products (Section 34.5.2.2; equation 55).615 In the dinuclear species (40), the bridging ligand arises from the cis coupling of two isocyanides (Figure 32). The average C-N distance (1.35(2) A), the C - C distances and the planarity at the nitrogen atoms suggest a delocalized JG system with C-C and C-N bond orders higher than one. The molecule is fluxional. Oxidation state +IV for each niobium, or +I11 for one and +V for the other, may account for the observed diamagnetism.
BU'
\
I
BU'
Figure 32 [Nb,C&(Bu'NC),(y-Bu'NCCNBu')]
34.4.4
(40) (reproduced from ref. 615 with permission)
Solvolysis Products of the Trihalides
Products that formally derive from the trihalides by solvolysis have been obtained (i) by metathesis of M"' trichloride adducts; (ii) from reactions of [TaC12(dmpe)z](23); and (iii) by
664
Niobium and Tantalum
reductive synthesis starting from M Vcompounds. The second and third procedures usually give mononuclear species, while di- or poly-nuclear compounds are generally obtained by solvolysis of M"' adducts.
34,441 A Ikoxides and related compounds Diamagnetic [Ta2Cl2(0R),], [Ta2C12(OR)4( y - ~ i c ) ~(42; ] R = Pi, But) and \Ta2C12(OBu')4]2 ~ ~ , ~ (42) have been obtained from [TazCb(y-pic)4](30) and the lithium a l k ~ x i d e s . ~Complex may coordinate an additional molecule of y-picoline to give the paramagnetic [TaCl(OR)*(ypic)3]. Reaction of (42) with excess acetone generated various pinacolates by reductive coupling of acetone. Carbon-carbon coupling was also observed with MeCO2C&CCOzMe. [Nb2C15(OPr')(Pr'OH)] has been characterized by X-ray diffraction.616bMonomeric chloro butoxides [T~C~,(OBU')(PM~~)~(C~H~)] and [MC~(OBU'),(PM~,)~(C~H,)] were also shown to form by reduction of alkylidenes in ethylene metathesis experiments (equation 56) .301
-
[T~C~,(OBU')(CHCM~,)(PM~~)~] + C&
[TaClz(OBut)(PMe)2(G&)]
+ Me,CCH==CHMe + Me3CCHzCH=CHz 24%
(56)
65%
The reaction between (28) and O-C6H4(OSiMe3)2 offered tetranuclear chloro catecholates [Nb&l~(cat)z(SMez)~]or [Nb4CLt(~at)~(SMe&]depending on the atmosphereAr or N O used. A diamagnetic complex [Nb(SQ),] (SQ = semiquinone of 3,5-di-t-butyl-1,2benzoquinone) of unknown structure has been synthesized from NbC15.617Displacement of the alcoholic ligands from im ure 'NbC13(C4H80)3' by salicylaldehyde provided [NbZCL(O C ~ ~ C H O ) Z ( T ?' ~ ) ~ ] [Ta2CL,(dmpe),(p-0)(p-SMe2)], whose formula was established by X-ray diffraction, was isolated as a minor product in the preparation of [Tazcl6(dmpe)2]. By contrast with [Ta2C14(dmpe)2(p-C1)2], the chelating ligand spans axial and equatorial positions. The Ta-Ta distance (2.726(1) A) has been taken to support the assignment of oxidation state +I11 to the metal.'18 3444.2 b-Diketonates and Clsrboxylates
M"' trihalide adducts display different types of behavior towards p-diketones. With nobium, a careful control of the reaction conditions allowed the isolation of the diamagnetic, dinuclear, soluble Nb'" chlorodiketonato complexes INbzC13(dpm)3], [NbzC15(dpm)(PhPMe2)2]and [Nb2C14(acac)2(acacH)2(PhPMe2)2] .619 No Ta" chloro diketonates have so far been stabilized, even with the bulky dpmH ligand; oxidation to [TazC4(dprn)4]was observed with (30), while oxygen abstraction reactions, with formation of [TaOC12(dpm)12are favored for (29). A paramagnetic ( p = 1.39 BM) compound formulated as [Ta(dbm),] has been obtained by addition of Et,N and dbmH to the solution which results from refluxing TaC1, in MeCN.6WIts formulation is however questionable, because in these conditions [T a2 C 1 6 (M eC N)2 (p -c~2 )] would be expected to form instead of [TaC13(MeCN)3]. Adducts [Nb302(0zCR)6(THF)3]' (R = Ph, But) have been prepared by allowing (28) to react with sodium carboxylates (Section 34.5.2.3). [Nb2C12(OAc)5(THF)]was obtained with NMe40A~.621 The reaction between (30) and excess pivalic acid afforded [Ta2C14(ButC0z)4]-2Bu'C02H as a diamagnetic soluble species. The reaction between Ta2C16(SMe2)3and lithium pivolate gave Ta2C15(02CBu')(SMe2)(THF)2.621 A niobium tdactate has been isolated by adding lactic acid to the Nb"' species generated electrochemically from NbC15 in DMF-HC1.62z [Nb(OzCBu')(Cp)2] with an anomalously high v(CO2) coordination shift (347 cm-') has also been described, but chelating behavior was found by X-ray diffraction.623 36.4.4.3
ThioIates and di$hiocnrbamates
The arenethiolates [M(SR)3] (R= phenyl or naphthyl) were easily obtained as monomeric soluble products by reductive elimination of organic disulfides from the pentathiolates
Niobium and Tantalum
665
[M(SR),] .624 The selenophenolates were formed by a similar procedure. Monomeric [Nb(Et2dtc)3] has been prepared (equation 57); dimerization was observed in non-polar solvents in the presence of dithiocarbamate^.'^^ Poorly characterized pyrrolidinedithiocarbamate, [Nb(S2NC5H&],has also been reported.625 [NbCl,(py),]
+ 3NaEt2dtc
[Nb(Et,dtc),]
MeCN
+ 3 N a C l + 3py
(57)
3444.4 Dialkylpbosphides
Attempts to obtain M"' amido derivatives have apparently so far been restricted to the reactions between [Nb2C16(SMe2)3]or Nb2C16(PhPMe2)4and LiN(SiMe3)2,but these reactions are of poor selectivity. Related MI" dialkylphosphides are better known. A diamagnetic phosphido compound, [TaH(PPh2)z(dmpe)2],was formed from [TaC12(dmpe)2],apparently by hydrogen abstraction from the solvent (equation 58). It was the first structurally characterized example of a terminal phosphide, and consists of a pentagonal bipyramid, with apical PPhz moieties.626Salient features are the monodentate diphenylphosphido ligands and the tilting of two of their phenyl rings toward the hydride ion. The geometry about the diphenylphosphido centers is nearly trigonal planar, as often observed in the structurally related dialkylamides. The metal achieves a closed shell electronic configuration if the Ta-P(PPh2) bond orders are assumed to be 1.5. An ionic homoleptic Nb"' dialkylphosphide, [Li(DME)z][Nb(PCy2)4],has been obtained directly from NbC15.565Terminal phosphido groups, stabilized by interaction of two of the phosphorus atoms with lithium, have been postulated.
+
[TaCl,(d~nge)~] 2KPPh2(dioxane),
THF
+
[ T a H ( P P l ~ J ~ ( d r n p e ) ~ 2] K C l +
2 dioxane
(58)
56%
344.4.5 Nitrenes
Ta"' nitrenes have been prepared by reduction of Tav nitrenes under argon (equation 59). The lability of the PMe3 ligands allowed the production of [Ta(NPh)C1L3(alkene)](alkene = ethylene or styrene). [Ta(NR)Cl,(THF),] R = M e , But, Ph
2NaRlg
tramr-[Ta(NR)ClL,]
(59)
L = PMe,, 0.5 dmpe
A diimido Tam complex (43) analogous to the Tav derivatives (Section 34.2.3.6) was formed by reduction of [TaC13(PMe&(GH4)] (equation 60). Dissociation of the PMe3 ligand trans to the p-N2 unit was observed by NMR.
(43)
344.4.6 Organometallics
A few salient results that have appeared since the publication of 'Comprehensive Organometallic Chemistry' have been collected here. Low-valent tantalum-bromide-PhBMe2 adducts undergo facile P-Me bond cleavage reactions spontaneously or in the presence of strong bases, giving tantalum a-methyl and tantalum methylphosphido derivative^.^^' Smooth substitution of chlorine in [MzCl@hPMe2)a] using Me3SnCp easily yielded [Ta2CL(PhPMe2)2(Cp)2],while the poorly soluble [Nb,C1,(PhPMe2)3(Cp)], is favored for niobium, although the disubstituted compound may be obtained after long reaction times. The reaction of [TaC12(dmpe)2]with [NaCp(DME)] apparenty proceeds through disproportionation, giving [Ta(dmpe)(Cp);?lCIand [TaCl(dmpe)2(Cp)]C1.28
666
Niobium and Tantalum
[MX2(CO)3(Cp)]has been obtained by several routes.' Carbonylative reduction at atmospheric pressure of [MCl,(Cp)] was achieved (equation 61) .629 The tantalum derivative was obtained only as an impure material in poor yield ( Nb and F > C1> Br > I, and Nb6111is the lowest niobium subhalide reported so far. All are based on octahedral (Ma1#+ (X = Cl, Br; 4 = 2, 3, 4) or (q = 2, 3) units (Figure 36) as for NbdIll (Section 34.5.4.2). Although their chemistry has long been investigated-mainly in aqueous solution-their formulation and structure, and those of their derivatives, have been
Niobium and Tantalum
671
established only recently, or, in many cases, remain uncertain. The first evidence for the existence of the [Mdil2I2* group is due to Vaughan et al., who in 1950 studied ethanolic solutions of some of these complexes by X-ray scattering.656
i
(b) L
Figure 36
Octahedral clusters: (a) the (M6X12)4f core with additional centrifugal ligands L; (b) the (MJJ4+core
Conflicting data, due to studies performed on mixtures of compounds in which different oxidation states were present (oxido-reduction processes occur easily, especially in the are reported. presence of The metals of the octahedral clusters can readily add a variety of ligands in 'centrifugal' positions, resulting in compounds of type [(M&12)YnI+n](q-n) (X = C1, Br; Y = F, Cl, Br, I, OH; L = neutral donor; n = oxidations state). In the description of such species, the central core containing the metal atoms will be represented in parentheses. 345.4.1 Clusten based OR the (MSX,)S+ core
( i ) Neutral binary halides The first metal-cluster fluoride known, [(Nb,iFIZ)F3],was prepared by disproportionation of NbF4 or by reduction of NbF5 by Nb. The compounds of composition M a l 4 have been obtained by reduction of the higher halides by various metals, and Nb6Cl14also by heating Nb3Cls. Phase diagram studies established that the only tantalum cluster iodide formed was Ta,J14, whereas in the case of bromine both T%Br14and Ta&r15 formed. The compounds are dark, air stable solids; Ta&l14 and Nb&r14 dissolve in water and ethanol, and Nb& and T~ Cl15are unaffected by dilute mineral acids. Regular octahedra of metal atoms were found with rather short metal-metal distances (Table 44) for Nb& and Ta6Cl15. Nb6C114 and Tad14 are isotypic and contain flattened octahedra, hence two types of centrifugal ligands. The strong equatorial Ta-Ta bonds of TaJ14 are accompanied by a greater distortion of the Ta6 octahedron as compared to that of Nb6 in Nb6C114. The regular Oh symmetry usually given for isolated clusters is obviously an oversimplified view here. For the Ma14 class of compounds, the three-dimensional network can be described as ~ [ ( N b 6 c 1 1 & 1 ~ ) c 1 ~ .646 c 1 ~For ] Nb& and ThCl15 additional intercluster bridging halogen atoms are bonded to each type of metal atom.
(ii) Ionic halides [(hf6x12)&]4- and hydrates with an
(hf6x12)2+core
(a) Synthesis and structure. The hydrates M&4-8HZO (X = C1, Br), being more soluble than the anhydrous lower halides, are useful starting materials for preparing subsequent
derivative^.^^' The reparation of [M&4].8H20 generally involves high temperature reactions, the (Mal2$+ cation-containing products being extracted with boiling water and crystallized from aqueous HX, When the extraction was achieved with warm water, [M&lI4]*9H20 was obtained.660The first of these hydrates, [Ta6CIl4]43H20,was prepared as early as 1907, but at that time it was formulated as [TaC12].2H20. Harned's method, which has long been used to obtain the ternary halides required for the
672
Niobium and Tantalum Table 44 Structural Data for Various Niobium and Tantalum Clusters of Type
Compound
M-M
SYmetrV
[W6FI2)F3l [(Nb6CI,,)C12]
Cubic Orthorhombic
[(Nb&2)c16]4[(Nb6c1&16]3[(N~I,CI,,)C&]~-
Monoclinic Trigonal rhombohedral Trigonal Orthorhombic
[(T%IiJIJ
M-X bridging
2.80 2.05 2.96(basal) 2.40 2.89(apex) (2.36-2.39) 2.92 2.48 2.97 2.43 3.02 2.42 2.80(basal) 2.75-2.83 3.08(apex)
Bond lengths M-X cenirifugal 2.11 2.58 3.04 2.61
2.52 2,46
(A) Observations
Ref.
Regular octahedron Tetragonal distortion
1 2
Regular octahedron Regular octahedron Regular octahedron Tetragonal distortion
3 4 5
6
Von Schnering, K. J. Niehues and H. G. Niedervarenholz,J . Less-Common Met., 1966,9,95. A . Simon, H. G. Von Schnering, H. Wohrle and H. Schafer, Z. Anorg. Allg. Chem., 1965, 339, 155.
1. H. SchSer, H. G.
2. 3. 4. 5. 6.
A. Simon, H. G. Von Schnering and H. Schiifer, Z. Anorg. ANg. Chem., 1968,361,235. F. W. Koknat and R. E. McCarley, Imrg. Chem., 1974, l3,295. F. W. Koknat and R. E. McCarley, borg. Chem., 1972,11,812. D. Bauer, H. G. Von Schnering and H. Schafer,J. Less-Common Met., 1965,8, 388.
preparation of the hydrates, involves the reduction of the pentahalide by cadmium and formation of [Cdz(M&lz)&] ( Nb, C1> Br, and with increasing oxidation states. Evidence has been reported for partial replacement of the chlorines by hydroxo groups in [Nb&115]-7H20.672 Irradiation of (TaJ3rI2)’+ in deaerated HCl solution led to (TaJ3rlZ)’+ and evolution of H2. It was assumed that the key step involves a two-electron transfer from (TaJ3rz2)’+ to a water molecule in the solvent cage, yielding (Ta&12)4+ and H-, which is scavenged by H+, generating H2.673 The mixed metal octahedral clusters ( T ~ ~ M O Cand ~ ~ (Ta4M02C112)4C ~)~+ were obtained by reduction of TaC15-MoC15 mixtures with aluminum in NaA1Cl4-A1Cl3 Further reduction of ( T a ~ M o c l ~ by ~ ) ~zinc + offered the +2 cluster. Spectroscopic and magnetic data indicate that [(TasMOCllz)C16]3+ and [(Ta4MOzCl1~)~6~are analogous to (16 M-M bonding electrons), and [(Ta~MoC1~~)Cl~l - to [(Ta6cl1z)c16]~- (15 bonding electrons). (c) Structure. The influence of the oxidation state on the structure of (Md(&+ or (M&18)(q-6) can be evaluated from the comparison of X-ray data available on &[(Nb&112)C&], (Me4N)~[(Nb6C112)Cl6]and (PYH)2[(Nb6C11*)C16]675, and, in the case of Table 47 Niobium and Tantalum Cluster Adducts Containing the (wX12)4t(q= 3, 4) Core
Ref.
Comments
Compound
X-Ray powder data
1,2,3
M = Nb, n = 3, analysis; M = Ta, n = 0, IR characterization only M = Nb, R = Me4N, X-ray structure and ESCA; M = Nb, R = E t a , polarography and ESR
4 5,6,7,8
2
Analysis p = 1.46BM
[(Nb6C1iz)X4(EtoH)zI (X = C1, Br) [(Ta~X,2)X4(H20)Z~‘nHZ0 (X = Cl n = 7; X = Br n = 3) A~[(M~X~~)%I (M = Nb, X = X’, A = PrNH,; M = Ta X = X‘ = C1 A = Ph.,As; X = Br, X‘ = CI, A = Et,N) A2I(MsC112)X61 (X = C1, A = Et4N, PYH; X = Br A = BbN) H~[(T~&z)C~~I*~HZO
2 1 5 5 9
[(T~C1,,)C1~(0H),]is obtained by room temperature vacuum dehydration
2,4 2
X-Ray structure for M = Nb and A = pyH NQR for M = Nb and A = Et4N
10,11 12,6
X-Ray structure; regular Ta6 octahedron
13
1. B. Spreckelmeyer,Z.Anorg. ANg. Chem., 1968,358,148. 2. B. G.Hughes, J. L. Meyer, P. B. Fleming and R. E. McCarley, Inorg. Chem., 1970,9,1343. 3. B. Spreckelmeyer and H.ScWer, J . Less-Common Mer., 1%7,19,122. 4. P.B. Fleming, J. L. Meyer, W. K. Grindstaff and R. E. McCarley, Inorg. Chem., 1970,9,1769. 5 . P. B. Fleming, T.A. Dougherty and R. E. McCarley, J . Am. Chem. SOC., 1%7,89,159. 6 . S, A. Best and R. A. Walton, lnorg. Chem., 1919,18,484. 7. F.W.Koknat and R. E. McCadey, Inorg. Chem., 1974,W , 295. 8. R.A. Mackay and R. F. Schneider,Inorg. Chem., 1967,6,549. 9. N.Brnicevic and B. Kojic-Prodic,2. Anorg. A&. Chem., 1982,489,235. 10. B. Spreckelmeyer,2. Anorg. A&. Chem., 1969,368,18. 11. B. Spreckelmeyer and H. G.Von Schnering, 2.Anorg. Allg. Chern., 1911,386,27. 12. P. A. Edwards, R. E. McCarley and D. R. Torgeson, Inorg. Chem., 1972, U, 1185. 13. C.B. Thaxton and R. A. Jacobson, Inorg. Chem., 1971,10, 1460.
676
Niobium and Tantalum
In an M6Cl18 unit tantalum, on H2[(Ta6C112)C16].6Hz0676and ~[(T~c112)(CN)6]-12H20.6n each metal is surrounded by an approximately square planar set of four bridging chlorine atoms cib. The metal is located slightly below this plane. One- and two-electron oxidations of, for instance, [Nb6Cll8I4-result in an increase of the Nb-Nb distances in two equal steps, from 2.92 to 2.97 and to 3.02A with retention of the octahedral symmetry (Table 44), which has been interpreted as meaning that the electrons are removed from bonding orbitals centered mainly on the M6 cluster. Since (Nb6C112)3+and (Nb6C112)4+are diamagnetic, and since (Nb6C112)3+shows a paramagnetism corresponding to one electron, these electrons appear to be removed from the same orbital. As anticipated, a significant shortening of the Nb-C1 bond occurs after oxidation. However, the terminal N M 1 , distances remain longer than that of N&Clb or the covalent bond (2.35-2.40 A); even the strongest N u l t bonds therefore appear relatively weak, which was attributed to the geometry of the Nb6Cl18unit: upon oxidation, the niobium atom moves up below the plane of its four Cl, atoms, and becomes more accessible to Cl,. Apparently only the Nb-Cl, bonds benefit from the lowering in Nb-Nb bonding strength upon oxidation. [(Ta&112)(CN)4]4- consists of a regular Ta6 octahedron with terminal cyanide ligands. The T a - C and C-N distances are comparable to those found in normal cyanide complexes. Each nitrogen atom is hydrogen-bonded to two oxygen atoms. (d) Other characterizations. The (Iv&X~~)~+ clusters have been the subject of far-IR studies and normal coordinate a n a l y ~ i s . A ~ ~uniform ~ ~ ~ high ’ ~ ~shift (ca. 10 cm-l) per unit increase in the: charge of the cluster has been observed for the strong bands assigned to the M-X stretching modes. The M-M stretching vibration has been located around 140 cm-’ or in the 59 to 109 cm-l range, i.e. at much lower frequencies than for (Mo6C18)Q+ (230 cm-I). The force constant for the M-M bonds in the niobium or tantalum cluster halides was estimated to be J A-1. less than 3 x ESCA spectra have been recorded on several ionic or neutral niobium and tantalum clusters.681The C12p1n,3n spectra generally exhibit three doublets, which were assigned to intracluster and intercluster bridging clb and to terminally bonded C1, halogens. The binding energy order was found to be Clb> C1, with AE(Clb - Clt) between 1.4 and 2.1 eV, the lower values being characteristic of the hydrates. No discernible variations in the metal core electron-bonding energies of [(M&2)&](q-6) clusters were observed upon oxidation. By contrast, an increase in the oxidation state in [(M6C11,)C12(PPr;)4](PF6), (n = 0-2) is paralleled by an increase in the metal core electron-bonding energies Nb3& or Ta4fin These results in conjunction with the ESR spectra and the fact that the cyclic voltammetry data are quite unaffected by changes of the phosphines671point to the HOMO of (M6C112)4+ions’ being almost exclusively metal-based in character. NQR studies on (Me4N)2[(Nb6C112)C16] revealed only the Ch, resonance. It was suggested that the C1, resonance may have occurred outside the observed range (3 to 33 MHz) as a result of a high ionic character of the Nb-C1, bonds.
(iu) Bonding, electronic spectra and magnetism A bonding scheme for the (h&X1#+ clusters should account for the magnetic properties and observed electronic transitions. Several semiempirical MO calculations, for instance those of Cotton and Haasm2based on the & group, or those of Robin and Kueblerm3on the (M6X12)’+ group, are available. A valence bond description has also been proposed.’ The effects of both X and L on the bonding have also been c o n ~ i d e r e d the ; ~ ~energy level scheme closely resembles that of Cotton et al. Another model including both metal-metal and metal-ligand interactions, with spin-orbit coupling introduced as a final perturbation, is available.685 The observation that both the (W12)’+ (16 electrons) and (M&12)4+ (14 electrons) clusters are diamagnetic, and the magnetic moments found for the (Md(12)3+(15 electrons) cluster (1.62 BM), show that the fifteenth and sixteenth electrons occupy a non-degenerate MO. The ESR spectra obtained on (Nb6C112)3+(g = 1.95) display a complex splitting, with 49 detectable components. The isotropic hyperfine structure accounts for a single electron uniformly delocalized over six equivalent Nb atoms (55 components expected) and suggests that the HOMO is an M-M orbital, of type A symmetry. The absorption spectra of the (Ma12)9+ (X= C1, Br; q = 2-4) clusters, as well as those of (Nb&2)3’ and (TaJ12)2+, have been measured over the range 4-50 kK;6637678,683 they are fairly
Niobium and Tantalum
677
complex. The correspondence between the spectra of the niobium and tantalum complexes (E(Nb)/E(Ta) = 0.5-0.8) supports the absence of notable spin-orbit coupling influence. A compilation of detailed visible spectra on the solid state or in solution is available.686Several tentative assignments have been p r e ~ e n t e d . ~ ~ ~ , ~ ~ Magnetic circular dichroism spectram5 and magnetic s u s c e p t i b i l i t i e ~ ~have ~ , ~ ~been ions are paramagnetic, and [(M,&12)&]34 anions generally display measured. The (Md(12)3+ simple Curie law behavior. The magnetic moments at room temperature are significantly larger than expected for one unpaired electron. Deviations from Curie law behavior were observed for M a l 5 ( M = Nb, X = F ; M=Ta, X = C1, Br) and for [(Ta&112)C13(H20)3]-3H20. The existence for the anhydrous halides of bridging halogens between cluster units could cause Curie-Weiss behavior via superexchange interactions. The origin of the same behavior for the hydrate remains unexplained. Large temperature independent paramagnetic susceptibilities were found for the diamagnetic clusters. The higher values for niobium than for the analogous tantalum compounds indicate a decrease in energy separation between the excited and the ground states for niobium. 34.5.5 Clusters Based on the
(M&)q+
(q = 2,3) Core
The group VA clusters containing the unit are limited to the niobium subiodides. Nb6111 is formed by thermal decomposition of Nb31s or by its reduction with niobium.',643 Nb3Br8 is not reduced under similar conditions. In contrast to the ( M a # + (M' = Mo, W) clusters, which possess a formal oxidation state of 2 and a closed shell electronic configuration, (Nb618)3+has a non-integral oxidation state and an open shell electronic configuration. A salient feature of the structure of Nb6111689,690 is the pronounced orthorhombic distortion of the Nb6 core with Nb-Nb distances ranging from 2.72 to 2.94A, comparable in average to those found in the metal (2.86A). In contrast to the (Nb+ clusters, there are eight bridgin halogen atoms, one above each face of the distorted octahedron (Figure 3%). The (Nb618f+ clusters are joined together by sharing centrifugal iodine atoms, forming [(Nbd8)16/2]. Nineteen valence electrons are available for the Nb6 framework. The magnetic susceptibility of Nbgl1, studied over the 5-863 K temperatdre range, suggests the presence of one and three unpaired electrons per cluster at low and high temperatures, respectively. Heating NbsIll with H2 at 300 "C under normal pressure yielded the remarkable hydride [Nb&UII]. Neutron diffraction investigations on [Nb@11] and [Nb6DI11] permitted the location of the hydrogen atom at the center of the cluster. The compound is diamagnetic below 200K, but approaches the magnetic susceptibility of Nb& between 400 and 800K. A vibration was found at 1120 cm-' for the hydride in the cluster. A more reduced, moderately air stable cluster, [CsNbJl1], containing the (Nb618)2+unit, was obtained by reaction of Nbdll or of its precursor Nb31s with CsI in the presence of the metal (equations 73 and 74).69' It has D3d symmetry and Nb-Nb distances of 2.835 A, comparable to those in Nbdl1. The distortions of the (Nb61#+ (q =2, 3) octahedra were attributed to different modes of packing and to strains at the bridging iodine atoms. Hydrogenation of [CSNb&] at 400 "C (1atm) gave the isostructural hydride fCsNb&Ill]. No hydrogenation was clusters. observed in comparable conditions for the (I&3$+
34.6 OXIDATION STATE +II Molecular compounds in which niobium or tantalum has a d3 configuration are stili very rare, and are mainly limited to adducts of monomeric dihalides with phosphorus donors [MC12(dmpe)2]and [MC12(PMe3)4].Niobium aryloxo analogs have recently been de~cribed.7~~" 34.6.1 Dihalide Adducts
34.6.1.1
Synthesis and structure
The first member of this series, [TaClz(dmpe)2],596was reported in 1977; all the other compounds have been reported, since 1983, by Sattelberger.692,588 TaCl, is reduced smoothly
Niobium and Tantalum
678
by sodium amalgam in ether in the presence of PMe3 to give the brown, air sensitive [TaCi2(PMe3)4](49) in a typical yleld of 60% (equation 75). Excess phosphine is necessary to counter the tendency towards dimerization of the intermediate mer-[TaCl3(PMe&]. Once (31) is formed, it is no longer reduced. The reduction of NbC& [NbC14(PMe3)2]in similar conditions gave poor, but reproducible, yields of [NbC12(PMe3)4], the Nb"' dimer [Nb2C14(PMe3)4(p-C1)2] being the major product. TaCls
+ 3Na/Hg + PMe, (excess)
Et20
t
[TaC1,(PMe3),] + 3NaC1
(75)
(49)
The dmpe adducts have been obtained in high yields as brown sublimable thermally stable solids, soluble in hexane and other non-polar solvents (equation 76). Reduction of TaC& by one equivalent of sodium amalgam in the presence of dmpe in THF and benzene solvent gave [TaC14(dmpe)z], together with [TaC1z(dmpe)z] when the T€€F:C6H6 ratio was higher than 15%.596 The TaII and Nb'' dmpe complexes were also obtained quantitatively from [MC12(PMe3)4];there was no evidence for dinuclear products. [TaBr2(dmpe)2]was identified by mass spectrometry as the major product of the reaction between [TaC14(dmpe)2]and an excess of LiMe containing LiBr. [MCl,(dmpe),]
+ 2Na/Hg
(24
THF
[MCl,(dmpe),]
+ 2NaCl
(9)
The products are paramagnetic ( p = 1.45 BM, for instance, for 49). Solutions of (49) in toluene are ESR silent at room temperature, and 'H NMR data suggest a trans octahedral geometry. This was confirmed by X-iay crystallography (DMsymrnet-;); Ta-Cl: 2.464(3) A, Ta-P: 2.543(2) A).692 ~. Poorly characterized hexamethylbenzene adducts [Nb2C4(C&le6)2] and [TaBr2(C6Me6)], formed in 9% and 51% yields, respectively, on a plication of the reducing Friedel-Crafts procedure to the p e n t a h a l i d e ~ .Short-lived ~~ MI P species have also been generated by electrochemical reduction of [NbzC16(PhPMe2)4]604 or electrochemical oxidation of sevencoordinated M' complexes of types [MX(CO)z(dmpe)z] (M=Ta, X=C1, Br, I, Me, H; M = Nb, X = Cl) or [TaC1(q4-C4Hs)(dmpe)2].693 ~
\
I
I
34.61.2 Reuctiviry of the dihalo adducts The paramagnetic 15-electron MU species react readily with HZor D2 (equations 77 and 78) to give parama netic, thermally stable MIv 17-electron hydrides, except for [NbC12(PMe3)4].588, 92 These inorganic radicals are the first isolated M'" monomeric h drides, as [TaH4(dmpe)2] and [NbH,Cp,] were characterized only by low temperature ESR?' Ether solutions of [NbC12(PMe,),] react with H2, but [Nb2C&(PMe3)4(p-H)4] was the only product isolated, even in the presence of excess ligand, and may arise from the rapid decomposition of transient [NbH2C12(PMe3)4].Preliminary results show the potential of (51) as a starting material for preparing tantalum compounds in low oxidation states (Scheme 9), especially [TaHCll(PMe3)4].
Q
Strong terminal M-H stretching modes are found in these adducts. The magnetic moments are indicative of d' complexes with orbitally non-degenerate ground states. ESR spectra are observed at room temperature ([NbH2C12(dmpe)2]:( g ) = 1.96, (A)m = 109 G, = 25.5 G, ( a ) H= 11.1G).
Niobium and Tantalum
N [{TaH,CIL,} (p-NZ)]A [TaH,CIL,]
679
UBH4
80°C. I h
.I. [T~zC~~L~(P-H)~]
(34) Scheme 9 (L= PMe,)
The tantalum(IV) hydrides [TaH2C12(PMe&] (51) and [TaH2C12(dmpe)2](50) were characterized by low temperature X-ray c r y s t a l l ~ g r a p h y . ~Co ~ mplex ~ , ~ ~ (51) adopts a distorted dodecahedral geometry in the solid state, while (50) is better described as a distorted square antiprismatic complex. The hydrogen atoms have been located. Reductive carbonylation of [NbC12(PMe3):] or reduction of [TaCl,(dmpe)2] (50) by vitride offered [NbC1(C0)3(PMe3)3]and a Ta aluminohydride adduct, respectively (Section 34.7). No divalent product derived from solvolysis of the dihalide adducts is yet known. The reactions of (50) with Cp- and PPh; show that replacement of chloride by ligands having stronger fields leads to species resembling organic radicals in reactivity, which rapidly disproportionate or abstract hydrogen atoms from the solvent. The Ta"' derivatives [Ta(dmpe)(Cp)2]Cl, [Ta (drn~e)~Cl( Cp)]C1 and [TaH(PPh2)2(d m ~ e ) ~(Section ] 34.4.4) were obtained. No evidence for the [Ta(PPh2)2(dmpe)2]intermediate was found by ESR.
34.6.2 Miscellaneous Air stable materials of formula MPc (Pc = phthalocyanine) have been obtained from MC15, phthalonitrile and quinoline at 220 0C.694Nb" alanates [LiNb(AlH&] andJLiNb(Al&),] were obtained from the low temperature reaction of NbC1, with excess LiAlH4. Reduction of (44)yielded a singly bonded Nb-Nb dimer (equation 79).568
34.7 OXIDATION STATE +I The number of authenticated niobium and tantalum coordination compounds in oxidation state +I is very limited. They were generally obtained through reduction of [MCL(~fmpe)~], [TaC12(dmpe)2]or Nb(OCJ33Me2-3,5)2(dmpe)2,731a
34.7.1 Phosphine Adducts Treatment of [TaC12(dmpe)2]with two equivalents of vitride offered [TaC1(CO)2(dmpe)~] (53) under CO, while a coordinatively unsaturated complex, [Ta{H2AI(OC2H40Me)2}(dmpe)2]
Niobium and Tantalum
680
(H), was obtained under an argon or ethylene atmosphere. It is the only structurally characterized non-carbonyl MI compound.69s Its molecular structure (Figure 37) consists of a centrosymmetric dimer with a bridging H2A1(OR)(p-OR)2A1(OR)H2entity. The Ta atoms are approximately square pyramidal, with the four phosphorus atoms forming the basal plane (Ta lies 0.64A out of it). The relatively short Ta-A1 distances are com arable to those found in other transition metal aluminum complexes (Ta-Al: 2.79-3.13 ). The hydrogen atoms have not been located, but were evidenced by chemical and spectroscopic techniques (IR: 1605, 1540 cml'; 6'H NMR: 16.30 p.p.rn.). The Ta-(p-H2)A1 unit is relatively stable, and (54) is inert to carbon monoxide or trimethylamine. It is a poor catalyst in the isomerization of 1-pentene. Formation of complexes analogous to (54) may explain the low yields often obtained from alkoxoaluminohydrides and metal halides.
f
Me
I
0
\
C
\
0
I
Me :
Figure 37 Molecular structure of [Ta{A,AI(O~H40Me),}(dmpe),] (54) (the dotted lines indicate the less abundant (20%) structural form resulting from a different orientation of the AI,O, bridge) (reproduced from ref. 695 with permission)
[{NbCl(dmpe)2}(p-N2)] was formed through reduction of [NbCl,(dmpe),] (equation 28). Electrochemical reduction of [NbC16(PhPMe2)4]yielded a dimeric Nb' derivative. Compounds of empirical formula jTa(py)](ClO,) and [Ta(Et2NH)](C104),generated by anodic dissolution, were also reported.69
34.7.2
34.7.2. I
Carbonyl Derivatives
Nonqclopentadienyl carbonyls
The monomeric carbonyl complexes of d4 MI isolated so far are mainly the sevencoordinated [MX(C0)2(dmpe)2]compounds (M = Nb, X = C1, Br, N3;697M = Ta, X = H, C1, Br, CN, Me, Et, P?); (53) has been obtained by reduction of [TaC12(dmpe)2]with one equivalent of sodium naphthalenide under CO; the other tantalum derivatives were generally prepared by oxidative additions to [Ta(CO)2(dmpe)2]- formed in situ by reduction of (53; Scheme 10). The niobium analogs were formed by reductive carbonylation of [NbCL(dm~e)~].~~~ The hydride ( 5 9 , also obtained through carbonylation of [TaH5(dmpe)2],has a distorted capped octahedral structure (A) established by X-ray diffraction (Figure 38).6w Spectroscopic data show that bulkier ligands (X#H) all favor a capped trigonal prismatic geometry (B). Stable seven-coordinated monomeric carbonyls [MCl(C0)3(PMe3)3] (56) could also be synthesized.'" The smooth reaction of (36)with CO offered [TaCI(CO),(PMe,),] (equation So). The niobium analogue of (36)is still unknown and [NbC1(C0)3(PMe3)3]was obtained by
Niobium and Tantalum
681
reductive carbonylation of [NbC12(PMe3)4](equation 81). Attempts to prepare (56) directly from NbC15/PMe3 mixtures by reductive carbonylation have been unsuccessful. These new carbonyls (sharp CO stretchin modes at 1962, 1862 and 1845cm-l for M =T a) are diamagnetic, 18-electron monomers. They are stereochemically rigid in solution, as are also the [I~€X(CO)~(dmpe)~l adducts. Complex (56) (M=Ta) has an overall C1 symmetry and is best described as a capped trigonal prism with the chlorine atom in the apical position, three phosphines and one carbonyl ligand constituting the capped quadrilateral face.
i
c
0
(a)
Fcgure 38 Coordination polyhedron found for the heptacoordinated species of type [MX(CO),(dmpe),]: X = H (structure a); X # H (structure b)
(56) 40%
The reaction of [TaH2(BH4)(PMe3)4] in the presence of CO and PMe3 led quantitatively to a salt [Ta(C0)3(PMe3)4][Ta(CO)5(PMe3)J. The cation has a capped octahedral geometry, one PMe3 capping a trigonal Ta(C0)3 face. 98 Dinuclear carbonyl anions [M2(C0)8X3]- were obtained through a two-electron transfer to protons by treatment of [M(C0)6]- with HX (X = Ci, OAc, OMe) (equation 82).526The anion has CZ, symmetry. The niobium atoms are heptacuordinated with four terminal carbonyl groups each and three bridging chlorides located at the vertices of an approximately equatorial triangle perpendicular to the metal-metal axis (non-bonded Nb - - - Nb distance: 3.43 A). The carbonyl stretching vibrations are in agreement with the local C4usymmetry of the M(CO), moiety. 2[M(CO),]-
+ 4HC1
[M2CI3(CO),]-+ 2H2+ 2c1- + 4CO
(82)
Niobium and Tantalum
682
The [M2(C0)&13]- anions are the first halocarbonyl complexes (without other ligands in the coordination sphere) of group VA metals to be reported. They probably originate from the unstable HM(C0)6 carbonyls.
347.22 Cyclopentadienyl carbonyls The recent development of the synthesis of [Nb(CO)4(Cp)] by reductive carbonylation of [NbC14(Cp)] (57) (i) by Zn in THF under CO (200 atm)630with 27% yield; or (ii) by Na sand, in the presence of a Cu/A1 alloy as a halogen acceptor, in benzene under CO (135 "C, 330 atm, 90% yield)701has spawned a renaissance in its chemistry. Extension of the latter procedure to [TaCb(Cp)] gave [Ta(CO)4(Cp)J in 7% yield, the main products being chlorocarbonyl derivatives, isolated as q 4-oxabutadieneTaJ compounds (equation 83). [TaCL(Cp)l
*
Mef2===CHCOhic
(83)
[TaC12(CO)y(CP)l
Photolysis of (57) in the presence of phosphines yielded [Nb(CO),(LL)(Cp)] with L L = 2PEt3, PhzP(CH,),PPhz (n = 1-5), Ph2PCH=CHPPh2, Ph2AsCH2CH2PPh2(arphos), and C & ( A S P ~ & P P ~ Z .The ~ ~ air sensitive tetrahaptocycloheptatriene and cyclooctatetraene complexes [(Cp)Nb(CO)2(q4-G&)] and [(Cp)Nb(CO),( q4-C&bJ] have also been obtained. The CsHs ring is fluxional. Photolysis of (57) in the absence of additional ligands provided an unusual cluster, [Nb,(CO),(Cp)s], the sole carbonyl cluster of niobium reported to date (equation 84)."9 Its quantitative declusterification to the parent carbonyl is also uncommon. X-Ray diffraction techniques revealed that one of the CO ligands acts as an q 2 ( p 3 - C , p 2 - 0 bridge ) symmetrically spanning a nearly equilateral triangle, and was proposed to act as a six-electron donor towards the three metal atoms. Its length (1.303(14) A) as compared to that of the terminal carbonyls (1.08-1.17A) is also reflected by a long-wave shift of its v ( C 0 ) absorption in the IR (1330 an-'). This q2-(p3-C,p2-0) ligand is mobile; a migratory process as well as its conversion from a six-electron to a four- or two-electron donor bridging N k N b double bonds were observed.703 hv, hexane (-SCO) [Nb(CO)4(CP)l
'
CO (20ann), T H F
(84)
[Nb3(COWP)31
(47)
(57)
Photolysis of (57) in the presence of H2S or MeSH produced [{Nb(CO)2(Cp)}z(p-S)2](58), [{Nb(CO)z(Cp)}2(p-S)3] and [ { N ~ ( C O ) ~ ( C ~ ) } ~ ( ~ - S(59). M~Z)~] isostructural with comparable Nb-Nb bond lengths (3.143(1) and angles (=75"), but the introduction of a third sulfur atom leads to the opening of the bridge angle (up to 85') and loss of the metal-metal bonding (3.555(1) A).704Complex (57) afforded [Nb\C0)312(Cp)] by oxidative addition of iodine. It may therefore provide a useful entry to Nb" chemistry.630The reaction between (57) and [{Cr(SBu')(Cp)},S] provided the tetra.705. nuclear heterometallic cluster [NbCr3(p-S)4(Cp)4]
34.7.3
Alkene and Diene Adducts
TaI adducts with ethylene have been obtained as highly air sensitive solids by reduction of the corresponding TalIr compounds under argon (equation 85),'= or by reductive elimination of H2 from [TaHzClL4](Scheme 9). A similar procedure, but under dinitrogen, gave Ta" nitrenes (Section 34.2.3.6). The same Ta"' precursor (60) provided 0 alkyl derivatives (equation 86). Complex (63) catalyzes the selective dimerization of ethylene to l-butene. h u n s , m e r - [ T a C l , ~ ( ~ H 4 ) ]2Na/Hg +
[TaClL4(C&)]
L = PMe3, 0.5 dmpe
L I
L
C ,l-f,lI
/?\ c1
+3EtMgBr
c1 L
El 0
L=PMe3
II
1I ,
";'i".,,
2 Et-TaII 1'' \ L
+ 2NaCl
(61)
(85)
Niobium and Tanlulum
683
Butadiene d4 complexes were obtained (i) from [NbCL(dm~e)~] and magnesium butadiene (equation 87);706(ii) by dimerization of ethylene using alkylidenes (equation or (iii) by metal vapor techniques (equation 89), which yielded sublimable methylallyl derivative^.^^ Compound (62) could not be prepared by Na/Hg reduction of (22) in the presence of butadiene. Compound (63)is also accessible from [TaH2ClL4](Scheme 9). [NbCl4(dmpe),1 f [Mg(THF),(C4H6)] (22)
L
CHCMe,
I/ CI-Ta I \ CHCMe, L
[NbCl(dmPe)&H6)1+ (62) 24% L
I /
MgCb
(87)
c1
L = PMe,
Treatment of [TaC14(dmpe)2]with sodium naphthalenide afforded [TaCl(dm~e)~( v4-C10H8)] was reduced by further addition of sodium naphthalenide to unisolated Na[Ta(dmpe)2(~4-CloH8)]. Protonation or methylation of this anion afforded the Ta' adduct [TaX(dm~e)~(q -clo?i&)] (X = H, Me). Related procedures gave the analogous 1,3cyclohexadiene adduct [TaCl(dmpe)2(r14-CsH8)]in poor yields .709 The molecular structure of (64) may be considered as a pentagonal bipyramid with the naphthalene, the chlorine and two phosphorus atoms in the pentagonal plane. It is structurally related to (53) by a 45" rotation of the naphthalene unit about the T a x 1 vector. The temperature dependent '€3 and 31PNMR spectra indicate that such a process is operative in solution. The structural parameters suggest that the n-accepting interaction is substantial; this is consistent with the inertness of the Ta-naphthalene unit toward substitution. An allyltetrakistrifluorophosphine Ta' compound, unstable at room temperature, has also been obtained (equation 90). Attempts to substitute all the allyl groups under higher pressure led to metallic tantalum and
(a), which
34.8 OXIDATION STATE 0 Examples of derivatives of zerovalent niobium and tantalum are very rare. Cocondensation of vapors of the metals, generated by an electron gun furnace at 3000"C, operating at a positive potential with dmpe, gave excellent yields > 60%) of stable crystalline [MCdm~e)~], which displays a distorted octahedral geometry."' Attempts to obtain the corresponding niobium compound by reduction of NbC& with sodium naphthalenide at room temperature in the presence of an excess of dmpe failed and led to metal depo~ition.~~' By contrast, reduction of MX5 by sodium amalgam in the presence of bipy or o-phen (LL) afforded the paramagnetic M(LL)3 adducts in ca. 80% yield.713 Bis(arene) [ M ( T ~ - C J ~ ~niobium R ~ ) ~ ] (R = HJ, H2Me or 1,3,5-Me3),and less stable tantalum derivatives (R = H3) were prepared in a similar way.714They are extremely sensitive to oxygen, but relatively stable to water. The photoelectron spectra of the volatile bis(q6-arene) niobium display low values for the first ionization potentials (5.18-5.57 eV), indicating that the compounds are electron rich. The IR spectra of [Nb(C&i3R&] (R = H, Me) in an argon matrix at 80 K are in agreement with a symmetrical sandwich structure. The ESR spectra of all zerovalent derivatives are consistent with the paramagnetism ex ected for 17-electron d5 compounds with one unpaired electron ( [ N b ( ~ f - c ~ H ~ M e ) ~ ] : (87 = 1.992, ( A ) = 45.9 G). Cocondensation of Nb atoms with Nz in an Ar matrix gave
Niobium and Tantalum
684
unusually numerous absorptions, some of which were assigned to Nb(N2) and to an Nb(N2)4 complex of D4,, symmetry. [Nb(CU),(N2),] species were similarly produced.715
34.9 OXIDATION STATES LOWER THAN 0 Compounds containing niobium or tantalum in negative formal oxidation states -I and -111 are mainly metal carbonyl anions. Although these are organometallic derivatives, the report of efficient procedures for the synthesis of [M(C0)6]- since the review of Labinger* merits mention, as it can be anticipated that these highly reduced and reactive species will be important precursors of a large variety of new coordination compounds and metal clusters. 34.9.1 Oxidation State -I
34.9.1.1 [M(CO),lUntil recently, the synthesis of the yellow [M(CO),]- required the reduction of Nb2C€10by Na or Na-K alloy in diglyme as solvent under a high pressure of CO (>360atm), and only poor and non-reproducible yields ( 4 4 % ) of the anions were obtained.716Several mild and efficient carbonylation procedures have now become available. The reductive carbonylation of MCIJ using the Mg/Zn couple as reductant and pyridine as reactive solvent occurs at room temperature under one atmosphere of CO (yields=48% and 35% for M = N b and Ta respectively). The reduction is largely due to magnesium.717 Treatment of MX5 with alkali metal naphthalenide in DME at -60°C provides thermally unstable brown intermediates, assumed to be [M(Cl0H&]- or [M(1-MeCl&)2], which react With CO (1 atm) to give [M(C0)6]- in 30-54% yields.71SCarbonylation, using sodium in the presence of cyclooctatetraene in THF under CO (latm, 40”C),has also been reported, although its efficiency is low in the case of Ta (12%).719Cyclooctatetraene complexes are thought to form as intermediates, but the mild carbonylation conditions compared to those of [Nb(COTJ,]- (50atm, 100°C) suggest that a compound of different stoichiometry may be involved. 2o The [M(CO),]- anions were isolated with solvated sodium or organic cations as counterions. [Ni(phen)3][Nb(C0)6]2has also been obtained.721The unsolvated Na[Ta(C0)6] is pyrophoric, and the Nb analog is light- and air-sensitive. The crystal structures of both (PPN)[M(CO)6] derivatives have been determined.717The coordination polyhedron is octahedral (Nb-C: 2.098(5) A; CNbC: 89.2(2)”). The PPN moiety is constrained to be centrosymmetric, and thus linear. These compounds correspond to the lowest oxidation state of niobium and tantalum for which structural data are available. A single v ( C 0 ) is found in the IR (1854 and 1852cm-l for Nb and Ta respectively). Comparable spectra are observed for Na[M(C0)6] in pyridine, but in solvents of lower dielectric constants such as tetrahydrofuran, additional bands attributed to distortion of the anion by the countercation are observed. [Nb(C0)6]- appears to be the most labile carbonyl of the group VA analogs.
3d 9.1.2 Other derivatives [Nb(PF&]- has been pre ared by photolysis of [Nb(CO)& under PF, and studied by multinuclear NMR, The Nb chemical shifts suggest weaker Nb-PF3 than Nb-CO interactions.722 A diamagnetic compound whose analysis corresponds to Li[Nb(bipy),]-3.5THF was reported to form by reduction of NbC1, with Li2bipy. As bipy can behave as bipy;, and in the absence of any spectroscopic data, the metal’s oxidation state may still be q u e s t i ~ n e d . ” ~ * ~ Na[Ta(dmpe)2(CO)2]was characterized by its derivatives only (Scheme 10).
P
34.9.2
Oxidation State -HI
349.21 [M(CO)d 3- and [HM(Ci?)S] ’The pentacarbonylmetallate trianions of Nb and Ta are the first compounds to contain these elements in a formal oxidation state of -111; they appear to be the first transition metal
Niobium and Tantalum
685
trianions isolated as analytically pure substances (equation 91)."' The unsolvated Na,[M(CO),] are thermally unstable, but isolation was achieved as Cs,[M(CO),] in 40-50% yields. These salts are shock sensitive, and their reactivity was mainly explored in solution. Addition of Ph3SnC1 and N b C l provided [M(C0)5(Ph3Sn)]2- and [M(CO)S(NH~)]-,which were isolated as their Et& and P h A s salts, respectively. The amine complexes are very labile and react readily with a variety of sr acceptor ligands L, such as PR3, P(OR), and RNC, to provide [M(C0)5L]- in 5 0 4 0 % yields. Addition of Et4NBH4 to M(CO)$- provided (E~~N)~[HM(CO)S]. 525b Na[M(CO),]
+ 3Na
N H 3 'lq
--780c
Na,[M(CO),]
+0.5Na2G02
(91)
34.9.22 [M(CO)~(Cp)l'-
Treatment of [M(C0)4(Cp)] with sodium in liquid ammonia at -78 "C generates the corresponding pyrophoric Na2[M(C0)3(Cp)] salts.'% Exchange reactions offered the thermally very stable Cs2[M(CO)3(Cp)] (dec. >250 "C). These dianions are the only established examples of transition metal complexes of formula [M(CO),(Cp)]'-. Reactions of [M(CO),(Cp)]'- with weak Bronsted acids such as MeCN, Et&+ gave [MH(CO),(Cp)]-, for which 93NbNMR data are available.'*' The [M(C0)3(Cp)]2- dianions were also characterized by their triphenylstannyl derivatives (EW)[M(CO),(SnPh3)(Cp)]. 34.10 MIXED VALENCE COMPOUNDS The only well-established, structurally characterized mixed valence compounds are the niobium selenide derivatives Nb3Se5Clynv and [(MeCN)zBrzNb(p-Se)2(p-Se)NbBr2(MeCN)2]'Vn" (Section 34.3.4). The niobyl phosphoniobate [NbVONb"'(P04)2]~6H20has been described in aqueous solution (Section 34.4.6.1). The reaction between NbCls and LiA1H4 in ether offered highly reactive, generally pyrophoric, lower valent niobium alanates [Nb(A1H.&J (n < 5).642 The mean oxidation state n decreases with increasing temperatures (equation 92). In the presence of an excess of L i d & , ionic derivatives were isolated: [LiNb2(A1€&)7]at -70 "C, [LiNb2(AlH4)5]at +25 "C and [LiNb(A1H4)3] at +25 "C. No structural data are however available on these compounds, of formal oxidation states 3.5 or 2.5, and their re-examination might prove rewarding. Unstable mixed valence M1'-M"' compounds have also been formed by pulsed radiolysis of dinuclear M"' add~cts.''~ Et20 5-n NbCI,
+ 5LiiW-L
-
[Nb(AlH4),]
+ (5 - n)AlH3+ (1
) H 2 + SLiCl
(92)
Dicyclopentadienyl derivatives in a formal oxidation state of 3.5 appear to be relatively common. A purple, paramagnetic compound with one unpaired electron (1.32 BM) for two Ta atoms was isolated and formulated as a dimer (65; equation 93).729A similar reaction did not yield the niobium analog, but violet or red-brown compounds were observed as intermediates in the reduction of [ M x , ( c ~ ) ~to ] [MX(CP)~](X= C1, Br). They could be isolated in good yields (-60%, Scheme ll).' TaC1,
+ NaCp + LiPPh, e [T~,C.I,(CP)~]+ P,Ph4 .f *
*
(93)
(65) 20%
Structural studies are as yet limited by the lack of stability of these MIV-MT" compounds, especially in polar solvents. Their formation was considered a consequence of the acidic character of the MX(CP)~entity, which achieves 18 electrons, and an alternative structure with [MX&P)Zl
NaRig
' tMX(CP)21
J/
IMXz(CP)d
0 5 NaNph
[M,X,(Cp)J
(65)
M = Nb; X = CI, Br M = T a ; X=C1
Weme 11
686
Niobium and Tantalum
only one bridging chlorine has been proposed. Reaction with AgC104 led to cationic complexes. Note: Since the chapter was completed, relevant papers concerning the chemistry of niobium and tantalum in various oxidation states with bulky anionic oxygen donors [2,6d i a l k y l a r y l ~ x oor ~ ~silox ~ ( B u ~ S ~ O ) ’have ~ ’ ~ appeared. Reviews concerning the analysis and classification of X-ray data of niobium 33 and are also now available.
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Niobium and Tantalum 721. 722. 723. 724.
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I
35 Chromium LESLIE F. LARKWORTHY and KEVIN €3. NOLAN University of Surrey, Guildford,UK
and PAUL O'BRIEN Queen Mary College, London, UK
35.1 INTRODUCTION TO CHROMIUM COMPLEXES
701
35.2 CHROMIUM(0) AND CHROMIUM(1) 35.2.1 Group N Ligads 35.2.1.1 Cyanides 35.2.1.2 Aryl and alkyl isocyanides 35.2.1.3 Si-, Ge-, Sn- and Pbi.ontaining ligands 35.2.2 Nitrogen Ligandr 35.2.2.1 Complexes of bi- or tridentate N heterocyclic liganh 35.2.2.2 Nitrosyls nnd dilaitrogen complexes 332.3 Phosphorus Ligands 35.2.3.1 Mixed complexes of dinitrogen and tertiary p h o s p h h 35.2.3.2 Fluorophosphine complexes 35.2,4 Miscellaneous Chromium(0) Complexes
702 703 703 704
35.3 CHROMIUM(I1) 35.3.1 General Synthetic Methods 35.3.1.1 Chromhm(II) solutiom 35.3.1.2 Chromiwn(II) salts 35.3.2 Group W Ligands 35.3.3 Nitrogen Ligands 35.3.3.1 Ammines 35.3.3.2 Bidentate saturated amines, ethylenediamine,propylenfdiamine, etc. 35.3.3.3 Polydentate amines, diethylenetriamine,piethylmetetramine and facultative ligan& 35.3.3.4 N heterocyclic ligands 35.3.3.5 Nitrosyls and hydrazine 35.3.3.6 Dialkylamtdes and didylarnides 35.3.3.7 Thiocyanates 35.3.3.8 Polypyrazolylboratesand carboranes 35.3.3.9 Nitriles 35.3.4 Phosphorus and Arsenic Ligands 35.3.4.1 Mono- and bi-dentate P donor ligands 35.3.4.2 o-Phenylenebb(dimethy1diarsine) 35.3.4.3 Tetradentale 2ripod'ligands 35.3.5 Oxygen Ligands 35.3.5.1 Aqua complexes 35.3.5.2 Alcohols, &oxides and aryloxides 35,3.5.3 ~-Ketoenolates,tetrahydrofuran, ureas and biuret 35.3.5.4 Oxo anions 35.3.5.5 Carboxylato and other complexes containing C r - C r quadruple bonds 35.3.5.6 Aliphatic and aromatic hydroxy acids 35.3,5.7 Dimethyl sulfoxide, phosphine oxides and arsine oxides 35.3.6 Sulfur Ligands 35.3.6.1 Dithiocarbamates 35.3.6.2 Ethane-l,Zdithiol 35.3.6.3 Thioureas 35.3.7 Halogens as Ligan& 35.3.7.1 Anhydrous halides 35.3.7.2 Complexfluorides 35.3.7.3 Complex chlorides, bromides and iodides 35.3.8 Hydrogen and Hydndes as Ligands 35.3.9 Mixed Donor Atom Ligan& 35.3.9.1 Complexes of P-ketoamines and Schiffs bases
716 716 716 717 718 718 718
coc3-W'
699
709 709 709 713 713 713 714 716
720 721 722 729 729 729 731 732 732 732 734 134
735 735 137 738
740 740
753 753 754 754 755 755
755 755 756 758 766
766 766
700
Chromium
35.3.9.2 Amino acids 35.3.9.3 Complexones 35.3.9.4 Bidentate mixed donor atom ligands 35.3.10 Multidentate Macrocyclic Ligands 35.3.10.1 Planar (closed)macrocycles; porphyrins, corrins and phthalocyanines 35.3.10.2 Other polyazamacrocycles 35.3.11 Stereochemistry of Bivalent Chromium 35.4 CHROMIUM(II1) 35.4.1 Group N Ligands 35.4.1.1 Cyanides 35.4.1.2 Isocyanides 35.4.1.3 Alkyls and aryls 35.4.2 Nitrogen Ligands 3 5 4 . 2 1 Ammonia 35.4.2.2 Monodentate amines 35.4.2 3 Ethylenediamine and related bidentate amines 35.4.2.4 Open-chain polyamines 35.4.2.5 N heterocyclic ligands 35.4.2.6 Nitrosyl, thionitrosyl, azo, hydrazine, hydroxylamine and related Iigands 35.4.2.7 Amido ligands 35.4.2.8 N-Thiocyanato, N-cyanato, N-selenocyanato, azido and related ligands 35.4.2.9 Polypyrazolylborates 35.4.2.10 Nitriles 3 5 4.2.11 Oximes, biguanides, guanidines, N-coordinated ureas and amides 35.4.3 Phosphorus, Arsenic and Antimony Ligands 35.4.4 Oxygen Ligands 35.4.4.1 Aqua complexes 35.4.4.2 Polymeric hydroxy complexes 35.4.4.3 Oxides and oxide hydroxides 35.4.4.4 Other oxide system 35.4.4.5 Peroxides 35.4.4.6 Alcohol and alkoxide complexes 35.4.4.7 f3-Ketoenolates and related Iigands 35.4.4.8 Catecholates, quinones, tetrahydrofuran and 0-bonded urea 35.4.4.9 Non-C oxo anions 35.4.4.10 Carbon-containing oxo anions 35.4.4.11 Hydroxy acidr 35.4.4.12 Sulfoxides, N+xides and P+xides 35.A 5 Sulfur Ligands 35.4.5.1 Thiolate, disulfide and thioeiher ligands 35.4.5.2 Bidentate S donor ligands: diihiocarbamates, dithiophosphates, I, 1- and 1,2-dithiolates,etc. 35.4.5.3 Thiourea 35.4.6 Selenium and Tellurium Ligands 35.4.7 Halogens as Ligands 35.4.7.1 Simple halides CrX, 35.4.7.2 [CrXJ and related anions 35.4.7.3 [crx6l3-anions 35.4.7.4 Dimeric structures 35.4.7.5 Solution chemisrry 354.8 Mixed Donor Atom Ligands 35.4.8.1 Schiffs bases, p-ketoamines and related ligands 35.4.8.2 Miscellaneous mixed donor atom ligands 35.4.8.3 Amino acids 35.4.8.4 Complexones:edta, pdta, nta and related ligands 35.4.9 Multidentate Macrocyclic Ligands 35.4.9.1 Porphyrins and corrins: oxidation states 11 to V 35.4.9.2 Polyazamacrocycles: cyclam, tet a, tet b and related ligands 35.4.9.3 Phthalocyanines
35.5 CHROMIUM(IV) 35.5.1 Anhydrous Halides and Complex Fluorides 35.5.2 Chloro and Cyano Complexes 35.5.3 Chromates(W) and 00, 35.5.4 Alkyls, Alkoxides and Amides 35.5.4.1 Alkyls 35.5.4.2 Alkoxides 35.5.4.3 Amides 355.5 Porphyrins
768 768 769 770 770 770 772 772 773 773 779 779 779 780 787 789 806 815 823 835 837 845
846 849 852 856 856
857 859 859 859 860
861 865 867 869 873
874 f-36
876
883 888 889 889 889 889 889 890 891 892
892 897 902 908 911 911 918 924
927 927 927 928 92x 928 928
930 931
Chromium
701
35.6 CHROMIUM(V) 35.6.1 Halogen Compounds 35.6.1.1 Chromium(V) fluoride 35.6.1.2 Chromyl(V) halides 35.6.1.3 Oxohalochromtes(V) and other derivatives of chromyl(V) halides 35.6.2 Complexes of Oxide and Peroxide 35.6.3 Complexes of Tertiary ru-Hydroxy Carboxylates 35.6.4 Porphyrim and Schifs Bases
931 932 932 933 935 936 936 938
35.7 CHROMIUM(V1) 35.7.1 Hexafluoride and ChromyI(V1) Halides 35.7.2 Other Chromyl(TI) Compounds 35.7.3 Chromium(VI) Oxy Compoundr 35.7.3.1 Chrornium(V1)oxide 35.7.3.2 Chrornates, dichronrptes and polychrornates 35.7.4 Anionic Oxo Halides and Other Substituted Chromates 35.7.5 Organoimidochrorniurn(V1) Complexes 35.7.6 Mixed Oxidation State (III and VI) Oxo Compounds 35.7.7 Peroxo Complexes of Chromium(N), (V) and (VI) 357.8 Biological Effecrr of Chromate(V1)
938 938 940 941 941 941 944 945 945 945 947
35.8 REFERENCES
948
35.1 I N T R O D U a O N TO CHROMIUM COMPLEXES Chromium, molybdenum and tungsten are the transition metal members of group VI of the periodic table. Chromium has the outer electronic configuration 3d54s' and forms compounds in oxidation states -11 to +VI (Table 1). It differs in its chemistry from molybdenum and tungsten, which are alike because of their similar atomic and ionic radii. Most resemblances are in the 0 and I oxidation states, which are stabilized by n-acceptor C,N heterocyclic and P donor ligands. The extensive organometallic chemistry of Cr, Mo and W is described in the companion series 'Comprehensive Organometallic Chemistry'. Chromium(I1) has extensive coordination chemistry provided the strongly reducing complexes are protected against aerial oxidation. Coordination numbers from three to seven are exhibited by chromiurn(II), although it is commonly six-coordinate, forming complexes which are high-spin (&e: configuration) or, with strong-field ligands, low-spin (t$ configuration). Marked Jahn-Teller distortion of the octahedron (4 2 coordination), arisin from the uneven filling of the d-orbitals, has been found for the d9 (Cr"') and high-spin df (Cr"') ions, and complexes of these ions are often isomorphous. The distortion leads to weak ferromagnetism in the halide-bridged structures of [NRH3],[Cr&]. Examples of planar chromium(I1) complexes (the limit of 4 + 2 coordination) are becoming more common, but none containing tetrahedral chromium(I1) has been firmly established except the atypical complex [Cr(NO){N(SiMe3)2}3]. There are similarities to molybdenum(I1) in dinuclear complexes like the carboxylates, but the ionic nature of CrX, is very different from the covalent clusters of MoX2. Complexes of chromium(III), the most stable and important oxidation state, continue to be discovered as the complexing ability of new ligands with the first transition series is investigated. There are now a few examples of coordination number greater than six. The coordination chemistry of chromium(II1) far surpasses that of molybdenum(II1) and tungsten(II1). Octahedral chromium(II1) complexes are kinetically inert and this, combined with ease of synthesis, is why so many have been isolated. Their magnetic moments are close to the spin-only value (& configuration, 3.87 BM), except in the numerous hydroxo-bridged and other polynuclear species where antiferromagnetism produces lower, temperature-variable values. Chromium(II1) complexes have well-characterized electronic spectra and extensive photochemistry. Chromium(1V) and chromium(V), previously encountered as the oxides and halides and as unstable intermediates in solution, are now represented by complexes of ligands stabilized by heavy substitution against oxidation by the metal ion, and oxo and nitrido derivatives. These remain unimportant oxidation states compared with molybdenum and tungsten. Chromium(V1) is powerfully oxidizing and is found in comparatively few but important
+
Chromium
702
Table 1 Oxidation States and Stereochemistry of Chromiuma Oxidation state
Cr', d6 Cr', ds Cr", d4
Coordination number
4
Octahedral Octahedral Distorted 'T' Planar
5
Distorted tetrahedral Trigonal bipyramidal
6 6 3
6
7 Cr"', d3
3
4 5 6 7 GIV,
d2
CrV, d'
4 5 6 7 8 4 5
6
8 Cr"', do
Stereochemistry
4 5
h 7
Examples
Square pyramidal High-spin (distorted octahedral) Low-spin (octahedral) (~r=Cr)'+ units Tetragonal base: trigonal cap (4:3) Pentagonal bipyramidal Trigonal planar Tetrahedral Trigonal bipyramidal Octahedral Pentagonal bipyramidal Tetrahedral Square pyramidal Octahedral Pentagonal bipyramidal Dodecahedral Tetrahedral Square pyramidal Trigonal bipyrarnidal Octahedral Dodecahedral Tetrahedral Pentagonal pyramidal Octahedral Pentagonal bipyramidal
CrtNH3)3(02)* CrH (PMe&HZCH,PMe2), 00:[Crock,]-, CrN(salen), [G O (O,COCMeEt),]CrF, [CrOC1,]2-
"There are many examples of organometallic compounds of chromium in oxidation states -11 to 111.'
compounds. These are usually oxo or oxo halide species. Besides chromates and dichromates, tri- and tetra-chromates are known, but compounds analogous to the isopoly- and heteropolymolybdates and -tungstates, and the molybdenum and tungsten bronzes, are not formed by chromium. There are weak similarities to sulfur(V1). The industrial and biochemical importance of coordination compounds of Chromium is described in Volume 6, and aspects of theoretical, mechanistic and solution chemistry are discussed in Volume 1 of this series. Earlier work on the coordination and general chemist of chromium is described in several treatises:24 there are reviews for the years 1979-1983?*6 and a list of crystal structures to mid-1976 is available.' Summaries of chromium chemistry have been provided in two recent te~tbooks.',~ 35.2 CHROMIUM(0) AND CHROMIUM(1) The extensive organometallic chemistry of chromium, i.e. the hexacarbonyl and its derivatives, organochromium compounds without carbonyl Iigands, cyanide and isocyanide complexes, alkene, allyl, diene, cyclopentadiene and arene derivatives, and complexes of a-donor carbon ligands, has been recorded in Chapters 26.1 and 26.2 of Volume 3 of 'Comprehensive Organometallic Chemistry'. In the present section, chromium complexes
Chromium
703
mostly in oxidation states 0 and I are considered, although where the ligands are such that redox reactions lead to easy interconversion between oxidation states, complexes of other oxidation states have been included.
35.2.1 Group IV Ligands 3521.1 Cyanides Homoleptic cyanochromium complexes are known in oxidation states 0 to 111, examples being &[Cr(CN)6], K,[Cr(CN),], &[Cr(CN)6] and K3[Cr(CN)6]. In addition, there are the cyanonitrosyls &[Cr(CN)5NO] and K3[Cr(CN)5NO], the chromium(1V) peroxo derivative K,[Cr(02)2(CN)3] and mixed cyanoamine derivatives such as ci~-[Cr(CN)~(en)~]l. All except the last, for which see Section 35.4.2.3, have been discussed in the companion series (Volume 3, Section 26.1.3.2).l More recent research on chromium(II1) cyano and cyanonitrosyl complexes is covered in Sections 35.4.1.1 and 35.4.2.6 and the present section is restricted to some recent work on chromium(I1) and chromium(0) complexes. Attempts to isolate pure cyanochromium(I1) complexes from the deep red solutions containing [Cr(CN)6]4- ions formed when chromium(I1) acetate is added to aqueous potassium cyanide in excess, and in other ways, have not been very successful. Now, through careful control of concentration and of pH, the known but poorly characterized green crystalline hexacyanochromates(I1) &[cr(cN)6]*2Hzo (where M = Na or K), the new dark green simple cyanide Cr(CN)z,2Hz0, and their anhydrous forms have been prepared pure (Scheme 1 and Table 2).l" K.+(Na)[Cr(CN),]+2H206
Crz'(aq) ",~~',",",',"'
(pH = 7)
KCN -+
Cr(CN)2.2H20J
5C I ( C N ) ~
Scheme I
Table 2 Magnetic and Spectroscopic Data for Cyanochromium(I1) and Cyanochromiurn(0) Complexes
~
2.73 2.79 3.14
3.42 3.30
3.48 diam.
2182 2062 2181 2122 2020 2122 2022 2132 2020 2132 2020 2018,1920
1800
diam.
2020 1920
520
-
37.4
8.5
528
439
37.6
8.7
45 1
345
38.0
25.0
14.5
45 1
345
458
358
37.0 9.1 37.0
458
357
48.4 14.5 46.0 14.6 47.6
26.0 7.4 29.6 7.7 26.0 7.8
605 535 535
450
36.5 9.1
455
The low-spin t& configuration of these complexes gives rise to a 3qgground term. and ,AZgterms are possible, but no assignments Spin-allowed transitions to the 3E8, ,TZs, 3A1g have been made because of the breadth of the reflectance bands. The CN stretching frequencies of Cr(CN), and Cr(CN)2-2H20suggest that the former contains bridging and the latter bridging and terminal cyano groups. The decahydrate Na4[Cr(CN)6].10H20 has been prepared by reduction of Na3[Cr(CN)6] with amalgamated aluminum sheet and its crystal
704
Chromium
structure determined. l1 Apparently, the decahydrate crystallizes but the dihydrate is the stable phase. In liquid ammonia, hexacyanochromates(I1) can be reduced to the diamagnetic chromium(0) complexes M~[cr(cN)6]by the appropriate alkali metal. The K salt was earlier obtained by reduction of K,[Cr(CN),]. Greater metal-ligand A bondigg is presumably responsible for the lower CN stretching frequencies of M6[Cr(CN)6]. The potassium salt is oxidized to K,[CT(CN)~] by ammonium cyanide in liquid ammonia (equation 1)'' 2K,[Cr(CN)6]
+6 W C N
-
&[Cr(CN),]
+ 6KCN + 6NH3 + 3H2
(1)
35.21.2 Aryl and alkyl isocyanides
Only more recent investigations of organic isocyanide complexes of chromium are in this section, because general synthetic methods for metal isocyanide complexes, including mixed carbonyl isocyanides, and the nature of the metal-isocyanide bond are discussed in Sections 27.1.3.2.2 and 26.1.2.5.2.ii of Volume 3 of 'Comprehensive Organometallic Chemistry'.' Organic isocyanides are n-bonding ligands like CO but are stronger u donors and weaker A acceptors. Chromium(0) complexes such as Cr(CNPh)6 are formally isoelectronic with Cr(CO)6, but differ markedly in being sequentially oxidizable to Cr', Cr" and Cr"' derivatives. These oxidation states were first observed in electrochemical studies (equations 2 to 4)13,14but solids of all four oxidation states are known, so, unlike carbon monoxide, isocyanides, owing to their greater u-donor ability, form homoleptic complexes in normal as well as low oxidation states (Table 3 and Schemes 2-4). The earlier assignment of the Eln value of equation (2) (ca. -0.3 V) to the half reaction Cr(CNPh)$!/Cr(CNPh); has been revised15 since attempts to reduce Cr(CNPh)6 were unsuccessful, and Cr(CNR); would be one electron in excess of the 18-electron configuration. Cr(CNAr), Cr(CNAr);
e e
- -0.3 V (versus SCE) Cr(CNAr)i+ + e-, EIn - 0.2 V Cr(CNAr); +e-, Eln
Cr(CNAr)y 6 Cr(CNAr)?
+ e-, E,,2- 1 V
(2) (3) (4)
Table 3 Complexes of Alkyl and Aryl Isocyanides
Complex
Comments
Chromium(0) Alkyl isocyanides c r (CNR), Red-brown cr(cNc6H1d6 Yellow-orange Aryl isocyanides Cr(CNC&R), Red or orange
R = But, diamagnetic, 'H NMR, S 1.40p.p.m., v(CN) = 21OOw, 1960s, 1870s cm-'; R = Bu" Diamagnetic, v(CN) = 1871cm-'
1-3
R = H, 4Me, 4-OMe, 4-C1,4-Br, 3-C1, 3-C1-2Me, 4-Cl-2-Me, 2,4-Me2, 2,5-C12, R = 2-Me, 3-Me, 4-NMe2,4-F, 3-OMe, 3-CF3; from Cr,(O,CMe),(H,O),/EtOH or MeOH, and ArNC; diamagnetic; v(CN) = 1935-1993 cm-' (most intense peak); stable except R = 4NMe,; R = H, octahedral, C r - C , 1.938 A; C-N, 1.176 A; Cr-4-N, 173.7'
4 5
R = H, 2-Me, 4-Me, 4-OMe, 4-Cl; from Cr(CNC,H;R), + AgPF,; R = 4-Me also from air + Cr(CNR), and NHJF ' ,; andzgsK[Cr(CNPh),]BPh, from Cr(CNPh)6, I2 and NaBPh,, pes - 2.042.18 BM, v(CN) = 2060 cm-' (most intease peak), R = 4-Me also from Cr2*/EtOH, PhNC and KPF, Prepared as above; v(CN) = 2051 cm-'
7
1,3
6
Chrornium(I) Aryl bocyanides [Cr(CNC6H4R)61PF6 Yellow to red
From Cr(CNPh)6+ AgCF3S03, octahedral, C r - C (av), 1.975 8;C-N, 1.159A,CrX-N, 178.7"
8
9
10
Chromium
705
Table 3 (conn'nued) Complex
Comments
Ref.
Chromium(II) Alkyl isocyanides
[Cr(CNR)61(pF6)2 Pale yellow [Cr(CNR)71(PF6)2 Yellow
R = But: v(CN) = 2180 cm-'; p:'. - 2.9 (Gouy), 2.87 BM (CH,CI,),, R = C,H,,: v(CN) = 2185 = 2.75 BM (CH2C12);R = Pr' R = But: v(CN) = 2160 cm-'; diamagnetic (Gouy); 'H NMR, 6 1.644.1p.p.m. (dec., CDCI,); C r 4 , 1.W-2.016, C-N, 1.142-1.155A R = C6Hll: v(CN) = 2150 cm-I; diamagnetic, 'H NMR, 6 1.50, 4.35-1.4, 3.25, 1.4p.p.m. (dec., CDC1,)
11 12 11 13
R = H, 2-Me, 4-Me, 4-OMe (maroon 4C1; from Cro(CNC6H4R)6t
7
ern-';;:;
11
Aryl isocyanides
[Cr(CNC6H4R)61(PF6)2 Yellow
2AgPF6; v(CN)=2150cm-'; p,'~3.05-3.14BM, R = H , octahedral, Cr--C(av), 2.014; C-N(av), 1.158 A; angular distortion of octahedron From [Cr(CNC6H3Me,-2,6)6](BF4)3 and air; v(CN) = 2147 cm-'
'[ Cr(CNC6H3Me,2,6)6](BF4)2.8H,O.HBF,' Orange Mixed-ligand complexes (alkyl isocyanides) R = But, C6Hll; R' = Et, Pr, Bun: pen = 2.6-2.9 BM (CH,Cl,); trans-[Cr"(CNR),(PR;),](PF,), v(CN) = 2128-2150 cm-' (one band) R = But: pew= 2.69 BM, v(CN) = 2190, 2154 cm-'; R = C,Hll: pen = cis-[Cr"(CNR),(dppe)](PF,), 2.78 BM, d C N I = 2197,2162 cm-' R = Bu'; v(CN) ='2175,2i30,2105, 2045shcm-'; R = C6Hl,: v(CN) = cls-[Cr"(CNR),(dppe)](PF6), 2186,2131 cm-' Mixed-ligand complexes (aryl isocyanides) Dec. to [Cr(CNPh),](PF,), on recrystalkation; v(CN), 2129s, 2178w, [CrCl(CNPh),]PF, 2194w cm-'; pee = 2.68 BM (CH,CI,) Yellow green R = Et, Pr, Bun; R, = Et,Ph; v(CN) 2112 cm-'; p,, = 2.73-2.98 BM tram-[CrCI(CNPh),( PR3)]PF6 Orange to yellow v(CN) = 2144m, 2097s cm-I; peE= 2.59 BM (CH2C12) [CrCl(CNPh),(dppe)]PF, Orange red [ C ~ ( C N P ~ ) S ( ~ P P ~ ) ] ( P F ~ ) Zv(CN) = 2170m, 2121s, 2080s cm-I; diamagnetic, stable in solution Orange Chromium(III)
10 14
11 11
11
i=
Aryl isocyanides [Cr(CNPh)dSbC16)3 Purple
From Cro(CNPh), and SbCI,; v(CN) = 2208 cm-' Cr3+/C3+= 0.73 V" CH2C12solvate, octahedral, Cr-C(av), 2.068; C-N, 1.141 A From Cr(CNC6H3Pr',-2,6), and SbC1,; v(CN) = 2180 cm-' Cr3'/C?' = 1.27 Va X = SbCI, (deep purple), prep. as above; v(CN) = 2188 cm-' X = BF, (maroon); from Cr' CNC6H@e,-2,6),1BF4 and NOBF,, v(CN) = 2214 cm-'; p:i7' = i.89 BM, C3'/C? = 0.82 V"
"Formalpotentials versus corrected AgCI/Ag in CH,Cl, with 0.1 M NBu,PF,. 1. E. P. Kiindig and P. L. Timms,J . Chem. SOC.,Dalton Trans., 1980, 991. 2. J. Mtiller and W. Holzinger, Z. Narurforsch., Teil E , 1978,33,1309. 3. K. W. Chiu, C. G. Howard, G. Wilkinson, A. M. R. Galas and M. B. Hunthouse, Polyhedron, 1982,1,803 4. L. Malatesta,Prog. Inorg. Chem., 1959.1,283. 5 . G . J. Esscnmacher and P. M. Treichel, Inorg. Chem., 1977,16,800. 6 . E. Ljungstrom,Acta Chem S e d . , Ser. A , 1978,32,47. 7. P. M. Treichel and G. I. Essenmacher, Inorg. Chem., 1976,15,146. 8. F. R. Lemke, D. E. Wigley and R. A. Walton, 1. Organomet. Chem., 1983,248,321. 9. D. A. Bohling, J. F. Evans and K. R. Mann, Inorg. Chem., 1982,21,3546. 10. D. A. Bohling and K. R. Maan, Inorg. Chem., 1984,23,1426. 11. W. S. Mialki, D. E. Wigley, T. E. Wood and R. A. Walton, Inorg. Chem., 1982,21,480. 12. D. E. Wigley and R. A. Walton, Orgunometullics, 1982, 1, 1322. 13. J. C. Dewan, W. S . Mialki, R. A. Walton and S . J. Lippard, J . Am. Chem. Soc., 1982,104,133. 14. D. A. Bohling and K. R. Mann, Inorg. Chern., 1983,22,1561.
14 9 10 14 9 14
9
Chromium
c
n
c
v
N
h N
0
Chromium
707
Some recent advances are the isolation of aryl isocyanide complexes of chromium(111) [Cr(CNAr)&, and six- and seven-coordinate alkyl isocyanide complexes of chromium(I1) [Cr(CNR)6,7]Xz.All but the chromium(0) and the seven-coordinate chromium(I1) complexes have less than 18 electrons in the valency shell.
(i) Syntheses: Oxidation state 0 Hexakis(ary1 isocyanide)chromium(O) complexes have been known since 1952 when they were described by Malatesta and co-workers.l6 Alkyl isocyanide chromium(0) complexes were thought to be less stable than their aryl analogues, but this appears to be due mainly to a lack of suitable preparative methods because the complexes Cr(CNR)6 (R = Bu, But or C6H11) have now been obtained by the action of RNC in excess on Cr(C10H8)2,Crz(GH8)37'7the chromium(1V) compound CrPri l8 or, more conveniently, anhydrous chromium(I1) acetate in THF in the presence of Na/Hg.19 The cleavage of the quadruple metal-metal bond in [Cr2(02CMe)4]or the dihydrate by an excess of aryl isocyanide has long been standard13for the synthesis of the aryl derivatives cr(CN!~r)~. This reaction (equation 5 ) invokes disproportionation, although no Cr3+complexes have been isolated from the reaction mixture. It may be that the use of Na/Hg as for the alkyl isocyanide complexes would improve the overall yield by reducing the C?' species. The same reducing agent produces trans-[Cr(CNB~')~(drnpe)~] from [CrC12(dmpe)2](Scheme 8). Many zerovalent aryl isocyanide complexes are now known (Table 3). 3[Cr:'(0,CMe),(H20)z] f 12CNAr 2Cru(CNAr),+ 4Cr" + 12MeCO; + 6H20 (5)
-
(ii) Syntheses: oxidation states I , 11 and III Air oxidation of chromium(0) aryl isocyanides will produce chromium(1) cations [Cr(CNAr)6]+,but oxidation by stoichiometric amounts of AgPF6 to give first the chromium(1) complexes [CI-(CNA~)~]PF~ and then the chromium(II) complexes [Cr(cNAr)6](PF& (Scheme 2 and Table 3) is far more e f f i ~ i e n t . ' ~Chromium(1) ,~~ alkyl isocyanide complexes have not been reported except for a comment17 that Cr(CNBUt)6 oxidizes in air, presumably to [Cr(CNBu')6]+. In addition to the series [Cr(CNAr)6]PF6, [Cr(CNPh)6]SPh, has been isolatedm so that the species thought to be [Cr(CNPh)5]BPh was probably mischaracterized. Solid complexes in oxidation states I and I1 are often stable, although decomposition occurs in solution. Irradiation of Cr(CNAr)6 in well-degassed CHC13 at 436 nm causes oxidation to [Cr(CNAr),]+ (Ar = 2,6-P&H3), whereas on irradiation in pyridine with Ar = Ph photosubstitution produces Cr(CNPh)5py.21 Alkyl isocyanide chromium(I1) complexes could not be prepared from [Cr2(02CMe)4(H20)2]or [Cr,(mhp),] (mh H = 2-hydro~y-6-methylpyridine)~ but by the addition of alkyl isocyanides and KPF6 to C& solutions from the reduction of CrC13.6H20 in ethanol with Zn/Hg, the homoleptic complexes [Cr(CNR)6](PF6)z (R = But or WI1) are obtained (Scheme 4).22 However, with phenyl isocyanide, the mixed-ligand complex [CrC1(CNPh)#'F6 forms, apparently through its kinetic stability in the Cl--containing medium since it decomposes to [Cr(CNPh)6](PF6)2in CH2C12. All are paramagnetic, low-spin complexes (tig configuration). It seems that the reaction product is substituent- and mediumdependent because the same procedure with the more reducing 4-methylphenyl isocyanide yields the chromium(1) s d t [Cr(CNC6H4Me-4)6]PF6,and in the presence of Bu'NH2 phenyl isocyanide produces the chromium(0) complex Cr(CNPh)6.Z3Also, the use of CrXz (X= C1, Br) instead of the acetate in equation ( 5 ) produces crystalline orange-red [CrClZ(CNPh),] or olive brown [CrBr2(CNPh)4].These low-spin chromium(I1) complexes are stable in air but structures have not been determined." Partial replacement of isocyanide in [Cr(CNR)6](PF6)zor [CrC1(CNPh)5]PF6by phosphines PR3, bis(dipheny1phosphino)ethane (dppe) or bis(diphenylphosphino methane (dppm) provides two series of mixed-ligand complexes (Scheme 4)&923 The complexes [Cr(CNR)4(dppe)](PF6)2,which must be cis, have more complicated spectra in the v(CN) region than the complexes [Cr(CNR),(PR;),](PF&, which are therefore trans (Table 3). Nitric oxide reacts with [Cr(C")6](PF6)2 to give [Cr(NO)(CNR)5](PF6)2from which many mixed nitrosyl isocyanide complexes of Cr' and Cro can be derived (Section 35.4.2.6).
Chromium
708
The low-spin, diamagnetic complexes [Cr(CNR),](PF&, which contain seven-coordinate cations (1; see also Table 95), result when neat RNC is added to [Cr(CNR)6](PF6)2.The reducing ability of the ligands and perhaps steric effects prevent the formation of sevencoordinate cations from aryl isocyanides.
Chromium(II1) complexes, of aryl isocyanides only, e.g. [Cr(CNPh),](SbC1,),, have recently been prepared in good yield by oxidation of Cro or Cr' isocyanides with an excess of SbC15 or NOBF4 (Schemes 2 and 3). Strong oxidizing agents are necessTIbecause of the highly positive formal reduction potentials (equation 4 and Table 3). The Cr complexes are stable in the is slowly absence of water. On exposure to the atmosphere solid [Cr(CNC6H3Me2-2,6)6](BF4)3 reduced, presumably by water vapour, to an orange Cr'I complex (Table 3). Unlike typical Cr"' complexes, the isocyanide ligands are labile in polar non-aqueous
(iii) Spectroscopic and crystallographic results
The general increase in the values of v(CN) as the metal's oxidation state increases from 0 to 111 (Table 3) reflects the decrease in number of d electrons and the diminution in n donation to the ligand; the chromium(II1) complexes have higher CN stretching frequencies even than the unbonded isocyanides (ca. 2130 cm-l), suggesting a balance of a-donor and n-acceptor behaviour by the ligands. There is also crystallographic evidence2k28for d,-p, back-bonding, in that the C r - C bonds shorten with decreasing oxidation state in the series [Cr(CNPh)$+ (n = 0, 1, 2, 3) and in Cr(CNPh)g, compared with [CP(CNBU'),](PF~)~, where the difference in formal charge alone should have the opposite effect. There are parallel increases in e N bond lengths (Table 4). Table 4 Bond Length Variation with Oxidation State in Hexaorganoisocyanide and Hexacyano Complexes Complex Cr(CNPh)b Cr( CNPh); Cr(CNPh)? Cr(CNF'h)i+ Mn( CNPh)$ Cr(CN)dCr(CN)aCr(CNBu');+
Configuration
LS d6 LS d5 LS d4 d3
LS d6 LS d4 Cd3
LS d4
M-c
(au) (A)
1.938 1.975 2.014 2.068 1.901 2.053 2.071 1.995
b N (av) (A) 1.176 1.159 1.158 1.141 1.182 1.156 1.156 1.149
N-C
(A)
Ref.
1.388 1.397 1.414 1.396 1.411
-
1.457
1
1 1 1 1 1 1 2
1. D. A. Bohling and K. R. Mann, Inorg. Chem., 1984,23, 1426. 2. J. C. Dewan, W. S. Mialki, R . A. Walton and S . J. Lippard, 1.Am. Chem. Soc., 1982, 104, 133.
The Cr 2p3,2 binding energies (XPES) increase from Cr(CNPh), to [2Cr(CNPh)$ to The shake-up [Cr(CNR)$+, the values being 574.5, 575.3 and 576.7eV satellite structure associated with the N 1s and C 1s binding energies in these spectra most probably arises from M (d)+ n* {CNAr(R)} excitations accompanying the primary photoemission.
Chromium
704
(iv) Electrochemistry
Electrochemical studies have established the three reversible one-electron transfers in equations (2) to (4).1sis,23 The electrochemical behaviour of a series of complexes [Cr(CNAr)6](PF6), in which Ar is Ph, 4-Me2NCd4, 4-MeOC6H4, 4-MeC6€&, 4-CIC6H4, 2-MeCsH4, 2,6-Me2C& or 2,6-h$&, has been studied by cyclic voltammetry in CH2C12 and MeCN.14 With pura substituents, and some metu substituent^,'^ there is a linear relation between the formal potentials of equations (2) to (4) and the Hammett a, parameter, as expected from the usual operation of electronic effects. With ortho substituents this was true for the 0 .r) 1+ couple (equation 2) and the a, parameter, but not for the 1+t)2+ and 2+ -3+ couples (equations 3 and 4), particularly the latter. There were significant anodic potential shifts in the formal potentials beyond those expected from the uo values indicating destabilization of the higher oxidation state of a couple, particularly Cr3+ in the 2+ -3+ couple. The anodic shifts also increased with the number of ortho substituents. These effects are attributed to weakening of the CrI’I-C bonds due to interligand repulsion among the substituents in the ortho-substituted complexes, a weakening also- manifested through the appearance of new waves in certain media arising from substitution of ortho-substituted ligands by MeCN or ClO; .14 The photochemical behaviour of the ortho- and para-substituted complexes also differs Comparkofb between the half-wave potentials (equations 2 to 4) of [Cr(CNR)6](PF6)2,e.g. for R = But, -1.04, -0.28 and 0.84 V (versus SCE),” with those for [Cr(CNPh),](PF&, i.e. -0.35, 0.25 and 1.00V,20shows that alkyl and aryl isocyanides favour respectively the higher and the lower oxidation states as expected from the greater a-donor and weaker n-acceptor capabilities of the alkg over the aryl isocyanides. Similarly, the phosphines in the mixed ligand relative to isocyanide ligands, stabilize the CrIII oxidation state. The complexes (Table 3), great difference in the relative stabilities of C r - C bonds in the cyano and phenyl isocyanide complexes is indicated by the magnitude of the shift (cu. 2.0V) between the Cr(CN);-/Cr(CN):- (-1.130 V) and the Cr(CNPh);+/Cr(CNPh):+ reduction potentials.28 vZ3
(v) Tricyanomeihide The pale blue tricyanomethide complex Cr[C(CN)3J2has a magnetic moment of ca. 4.7 BM and is thought to be polymerkM
35.2.1.3 Si-, Ge-, Sn- and Pb-containing ligands
I[
The yellow complex [NPr4 6 Cr GeCl,),] is formed in poor yield if Cr(C6H6)2is refluxed in acetone with [NPr4][GeC13].3 (
35.2.2
Nitrogen Ligands
35.2.2.1 Complexes of bi- or tridentate N heterocyclic ligands Since complexes of 2,2‘-bipyridyl and 1,lO-phenanthroline with chromium in oxidation states I and 0 can be obtained by reduction (Scheme 64) of the chromium(I1) complexes, these oxidation states will be considered together. Oxidation, as shown in Schemes 65 and 68 of Section 35.4.2.5, gives chromium(II1) complexes, which are often best prepared in this way. Earlier work has been extensively reviewed, and few complexes of 2,2’:6’,2’’-terpyridyI are known.32A chromium(1) phthalocyanine derivative is mentioned in Section 35.4.9.3.
(i) Syntheses Various methods to prepare tris( a-diimine) complexes of many metals in low oxidation states have been devised, mainly by Herzog and ~ o - w o r k e t s ,Those ~ ~ used for chromium complexes are illustrated in Schemes 5 and 6. Chromium(I1) salts, anhydrous or hydrated, are the commonest starting materials. To these are added stoichiometric amounts of the diimine,
Chromium
710
and the Cr" complex is then reduced. When bipy is added to the acetate suspended in water, disproportionation into [Cr(bipy)3]" and [ C r ( b i ~ y ) ~occurs. ] ~ + The use of the anhydrous acetate in THF with bipy or phen and Na/Hg seems to provide a more efficient route to the zerovalent The complexes LiCr(bipy),.4THF, Na2Cr(bipy),.7THF, Na3Cr(bipy)3.7THF and Ca3Cr(bipy)3.7NH3, which are prepared from Cr(bipy), and as appropriate Li,bipy , Li or Na metal in THF, or Ca in liquid ammonia,34are not included in Schemes 5 and 6.
bipy,MeOH
C r C Uaq )
riclo,
[Cr"(bipy),1(C1O4), black-violet
3 [Cr'(bip~)~lClOd
I
Cr(bipy)X2
bipy 01 phen
CrX2hydrate
indigo blue
[Cr" (bipy)W 2
3 hipy or phen
7 [Cr(bipy),]X,
or
(ii) Magnetic properties The mono(amine)chromium(II) complexes, CrCl,(bipy) etc., are high-spin (&e:), but show antiferromagnetic interaction presumably through halide-bridged s t r ~ c t u r e sThe . ~ ~ bis- and the tis-(amine) chromium(I1) complexes are low-spin (t$), as are all the terpy complexes and those of chromium in oxidation states below I1 (Table That spin pairing takes place in chrornium(I1) on the coordination of a second bipy molecule is in agreement with the observation that the second stepwise stability constant is greater than the first: log K1== 4, log Kz= 6.4, log K3 = 3.5 ( € 3 2 0 , ionic strength 0.3 M) (Table 39).37 The ground term of low-spin C P is 3T1gand incomplete quenching of the orbital contribution is expected to produce magnetic moments in excess of the spin-only value (2.83 BM) which are temperature-dependent. This is not generally found with the low-spin bipy, phen or terpy complexes (Table 9, and attempts to account for the small or zero temperature dependence theoretically in terms of spin-orbit coupling, distortion from octahedral symmetry, and electron delocalization have not been very successful especially at low temperatures (-50 K) where the experimental values do not drop as the theory predicts. This may be because there is greater deviation from cubic symmetry than the theory can accommodate.36 From further magnetochemical studies3* and NMR rnea~urements~~ it has been concluded that there is an appreciable trigonal component in the ligand field and relatively little delocalization of the unpaired electrons. The low-spin complexes [CrXz(diars)z] also have essentially temperatureindependent moments.40 .5).35736
Chromium
711
Table 5 Complexes of bipy, phen and terpy with Chrornium(O), (I) and (11) (see Section 35.4.2.5 for Chromium(II1) Complexes) Complex
Comments
Rd.
Chromium(0) complexesa
CWPY13 Black Cr(phen13 Black Cr(terpy), Chromium(I) complexes
Diamagnetic Cr-N, 2.08 A, N-Cr-N, Diamagnetic Diamagnetic
L3,4
[Cr(bip~)~lX Indigo blue [Cr(phen)3lX Chromium(l1) complexes
X = I , Clod, pe,=2.07BM
132
x =I, c104
1,2
[Cr(biPY),lXz Deep violet
X = Cl, Br, I, Br.4H2O (and below), pee = 2.9-3.4 BM with little temperature variation (300-90 K), similarly X = Br.2H2O, C10, (300-20 K), X = CI-H,O, Br.H,O: pew= 2.94 and 2.73 BM (MeOH) X = C1, Br, I, CIO,, as above; x = c l , pee = 2.62 BM (MeOH) ~ ~ = 3.35, ~ K = 3.32 BM .5 .uZK X-: OAc p = 3.01 BM; X = CI, p,, =2.95 BM; also phen compound p:rK = 2:91:'pgK = 2.86 BM, v(CN) = 2082,2070; Y M NCS 362,347cm-' pead 295 s . 0 7 , = 3.06 BIVI,~V(CN) = 2078,2062: v(M-NCS) = 363, 335 cmR2=4,4'-Me,, pee -2.76BM (MeOH); R2= 5,5'-Me, R2 = 4,7-Me, x = 2, peff= 2.90 BM (MeOH) R, = 5,6-Me,, x = 1, pen= 2.87 BM (MeOH) R, = H,4-Me; H,5-Me; 3,4-MeZ;4,5-Me,; or 4,6-MeZ;all x = 1 X = C l , p%5K=4.19, pgK=3.60BM, B ~ 4 5 " X = Br, pfg"" = 4.37, p$K = 3.75 BM, 0 = 45" X = C1, ~ 2 = 4.57, % ~p g K ~ = 3.90 BM,8 = 60" X = Br, p$?K = 4.54, p g K= 3.90BM, @ ~ 6 0 " pm = 2.83 BM, also phen compound X = I.H,O, CIO,, p c B = 2.9 BM, almost temperature-invariant
1-4 74.7"
11
LiK
5
1,3,4
2,6 7 8
6-8 9
1 9 9
8, 10
a
11 9,12 9
1 7 , 13
Some complexes of formal oxidation states lower than 0 have been reported: LiCr(bipy),.4THF, Na&r(bipy),.iT€€F, Na,Cr(bipy),.WHF and Ca3Cr(bipy),.7NH,; thcir magnctic moments are respcclivety cu. 1 .S3, 2.85, 3.8s and 2.46 BM.' 1. W. R. McWhinnie and J. D. Miller, Adv. Inorg. Chem. Radiochem., 1969, 12, 135. 2. F. Hein and S. H e m g , in 'Handbook of Preparative Inorganic Chemistry', ed. G. Brauer, Academic, New York, 2nd edn., 1965, vol. 2, sect. 24. 3. J. Quirk and G. Willcison, Polyhedron, 1982,1, 209. 4. H. Behrens and A. Miiller, 2.Anorg. Allg. Chem., 1965, 341, 124. 5. G. Albrecht, Z . Chem., 1963, 3 , 182. 6. A. Earnshaw, L. F. Larkworthy, K. C. Patel, K. S. Patel, R.L. Carlin and E. G. Terezakis, J . Chem. Soc. (A), 1966, 511. 7. P. M. Lutz, G. J. Long and W. A, Baker, Jr., Inorg. Chem., 1969, 8, 2529. 8. G. N. La Mar and G . R. Van Hecke, 1.A m . Chem. Soc., 1969,9l, 3442. Y. A. Earnshaw, L. F. Larkworthy, K. C. Patel and B. J. Tucker, J. Chem. Soc., Dulion Tram., 1977, 2209. 10. I. Fujita, T.Yazaki, Y. Toni and H. Kobayashi, Bull. Chem. SUC. Jpn., 1972,45,2156. 11. G. N. La Mar and G. R. Van Hecke, Inorg. Chem., 1970, 9, 1546. 12. H. Lux, L. Eberle and D. Same, Chern. Ber., 1964,97, 503. 13. S. H e m g and H. A d , Z. Chem., 1966,6,382. a
(iii) Proton NMR studies of tris(bipy) and trisbhen) chelates Proton NMR spectroscopy has been used to investigate paramagnetic complexes of low-valent transition metal ions because the contact shifts provide information on the degree of covalency of the metal-ligand bond. The low-spin complexes [Cr(phen),]Cl, and [Cr(bipy)3]Clz, and their methyl-substituted derivatives (Table S ) , have been extensively well-resolved 'H NMR spectra have been obtained from i n v e ~ t i g a t e d . ~Na ~ ,mow, ~~,~~ [Cr(Rzphen)3]2+(R = H; R2= 4,7- and 5,6-dimethyl) and [Cr(R2bipy)3]2+(R = H; R2= 4,4'dimethyl) and the shifts result from both L+ M (Iand M-, L n charge transfer. The x bonding is not obvious in the tris chelates with symmetric ligands owing to near cancellation of two contributions of opposite sign.4' The relative x-acceptor abilities of the differently substituted
Chromium
712
ligands, and the orbital ground states of the complexes were elucidated only after some mixed-ligand chelates (obtained in solution only) had been investigated. The M+ L delocalized n spin density is centred predominantly at the 4,7 positions of phen or 4,4' positions in bipy. This facilitates the reducing ability of these chelates dnd is believed4' to be consistent with the stereoselective outer-sphere reduction of [C~(phen)~],+ by [Cr(phen)#+: it is found that (+)-[Co(phen)d3+ yields (-)-[Cr(phen),],+ as the main product on reduction with racemic [ Cr ( ~ hen) ~]' +.~ Anomalously broadened methyl resonances are ascribed to solvent (water) complex interactions at the 4,7 positions which involve hydrogen bonding to pockets of electron density on the ligand assisted by the n bonding. From spectral changes it was suggested35that bipy separated from [Cr(bipy)3]X2(X = C1, Br, I) in ethanol, but there are no NMR signals of the free ligand from solutions of these salts in d4-MeOH unless some is added.
(iv) Electronic, ESR and LR spectra The electronic spectrum of C r ( b i ~ y )resembles ~ that of bipy- in the lower wavenumber region, but formulation of the complex in terms of integral oxidation states is unreal is ti^.^,^' According to an X-ray study, the molecule is trigonally distorted, the N-Cr-N angle being 74.7 f 1.5". The Cr-N distance is 2.08 A.46 The ion [Cr(bipy),]+ in solution exhibits an ESR spectrum from which g = 1.9971 and the cm-'. There is further splitting of the h erfine splitting constant of 53Cr,A'', is 20.4 x P Cr hyperfine lines due to the I4N nuclei of the ligands, which indicates strong u b~nding.~' Its electronic spectrum consists of charge-transfer and intraligand bands.45 Ligand-field theory gives a reasonable account of the energies but not the intensities of the bands between 12000 and 32000cm-' in the spectrum of [Cr(bipy),I2+, which are due to spin-allowed transitions of the low-spin d4 configuration. There are also many charge-transfer or internal-ligand bands which are weakly perturbed by methyl substitution in bipy.48 The assignment of formal oxidation states (I1 and I11 respectively) to the metal in [C r(b i ~y )~f+ and [Cr(bipy),I3+ (Section 35.4.2.5) is valid but not in complexes of lower overall ~ h a r g e .From ~ spectral changes in aqueous solution the mixed complex [Cr(bipy)*(4,4'-bi yH)(H20)I3+ has been identified in rapid equilibrium with [Cr(bipy),]'+ and the free ligands.4 r In the series [ C r ( b i ~ y ) ~ ] X[Cr(bipy)3]Xz, ~, [Cr(bi~y)~]X, [C r(b i ~y )~] the Cr-N stretching frequency varies little with oxidation state from I11 to 0. Since this implies small changes in bond strength with decreasing oxidation state and greater occupation of the tZg orbitals, it appears that increasing M + L n donation is offset by lower L a M D donation as the oxidation to aid in the assignment of the state is lowered. Some complexes were labelled with "Cr or two Cr-N stretching bands (Table 6).5" There are discrepancies with assignments for unlabelled c ~ m p l e x e s .The ~ ~ splitting of the v(CN) band in the IR spectra of the complexes [Cr(NCS)z(bipy)z]and [Cr(NCS)2(phen)z]indicates cis configurations but this has not been confirmed crystallographically.35
Table 6 Far IR Spectra of the [Cr(bipy),](ClO,),, Series (cm-I)
[WQY 385m (Hidden) 349m 226w 193vw
351s 359m
343m 225m 205w 18%
177w
175w
[Cr(bipy)3]0
Assignment
371m 352m 343m 228w
382m 357vw
v( Cr-N) Ligand v(Cr-N) Ligand
195w 17%
203s
)31+
308s 227w 178w
'
Ligand Ligand
The absorption spectrum and first formation constant of [Cr( bipy)]'+ have been determined spectrophotometrically in hexamethylphosphoramide (HMPA) (see also Table 39). Since log K1 (4.61) is slightly larger in HMPA than in water, it is said that [Cr(bipy)(HI~lPA)~]~' is tetrahedral, although from the general chemistry of chromium(II) the ion would be expected to be in planar or distorted octahedral coordination."
Chromium
713
(v) Electrochemistry Voltammetric investigations of chromium complexes of bipy, phen and terpy in MeCN and other solvents, as well as magnetic and other data above, show that the metal forms series of tris-(bipy) and -(phen) and bis(terpy) complexes related by reversible one-electron transfers in which the metal has formal oxidation states 111, 11, I, 0, -I (see also Section 35.4.2.5). Some half-wave otentials are given in Table 7.53In aqueous solution complications occur because [C r( bip~) ~ fundergoes + catalyzed ligand exchange at the dropping mercury electrode to form [Cr(bipy)2(H20)2J3+,which reduces more cathodically than the tris corn lex, to give [ C r ( b i p ~ ) ~ ( H ~ O )The ~ ] ~phen + . complexes and [CrClz(bipy)z]+behave similarly Table 7 Half-wave Potentials
DC polarography (VY
AC polarography
-0.110 -0.516 -1.030 -1.95
-0.120 -0.525 -1.035 -1.95
11-1 I+ 0
-0.209 -0.715 -1.28 -1.91
-0.200 -0.710 -1.28 - 1.89
[Cr(phen),l I11+I1 II+ I I+ 0 o+ -I
-0.237 -0.732 -1.29 -1.79
-0.228 -0.838 -1.28 -1.79
Complex couple
o+ -I
a
(V
Versus AglAgCl saturated NaCl electrode.
35.222 Nitrosyls and dinitrogen complexes Nitrosyls are included in Section 35.4.2.6 and dinitrogen complexes with the phosphorus donors immediately below. 35.2.3
Phosphorus Ligands
35.2.3.1 Mixed complexes of dlnitrogen and tertiary phosphines There are few examples of complexes of chromium with molecular nitrogen, althou& molybdenum and tungsten form many such complexes.1 Unstable species containing Cr-I% bonds produced by matrix isolation techniques have not been in~luded.’~ The chromium(0) complex [Cr(N2)2(PMe3)4] is not well established because it decomposes at room temperature with liberation of PMe3 and N2. Since there are two IR absorption bands at 1990 and 1918 cm-’[v(N=N)] it has been assigned a cis octahedral structure. The dark brown complex can be prepared from anhydrous CrCL by Scheme 7 or directly from the reactants. There is no spectroscopic or structural information on the intermediates [Cr“’C13(PMe&] and [Cr11C12(PMe&].57 The systems CrCl,-Mg-THF and CrC12-Mg-dppe-THF absorb nitrogen and some NH3 and N2& are produced on hydrolysis. The paramagnetic solids Cr2N2Mg4C14(THF)5 (black) and [ C r ( d ~ p e ) ~ ] ~(brown), N, which are difficult to obtain reproducibly, have been i~olated.’~ The chromium(0) complex trum-[Cr(N,),(dmpe),] (2) has been prepared together with some incompletely characterized cis isomer from [CrC12(dmpe)2]according to Scheme 8, which also shows that its reactions all involve displacement of dinitrogen. The trans dinitrogen ligands are linear, and the N-N bond distance (1.122 A) is normal. The IR spectrum of the trans isomer
Chromium
714 CrC13
FMc) THF
'
[Cr1'1C13(PMe3)31 brown violet m.p. 144-147 "C
M PMcj
(&ccab)
'
ICr'[CI,(PMe3)31
from deep blue solution m.p. 51 "C
I-.
PMe, N2 Mg
[Cr(CO),(PMeAl
f
-:&
[Cr(CO)dPMe&l
4
-%,
ICr(W2(PMe&l
dark brown dec. at r.t. Scheme 7
has one band (N=N stretch) at 1932 cm-' and the cis isomer bands at 1920 and 1895 cm-l. Since the Cr-P distance (2.296A) is shorter than in CrM&(dmpe)2] (2.345& Section 35.3.4.1) there is significant metal-ligand n back-bonding.s
5
1.122 A
I
I
N
Ill
N
trans-[Cr ( C NB ~ ') , ( d mp e )~] Scheme 8
The cocondensation of chromium vapour and the phosphorus ligand is used to prepare [Cr(dmpe),] and the trimethyl phosphite complex [Cr{P(OMe)3}6](Table 8). The former has a distorted octahedral structure, and the latter forms a seven-coordinate hydride CrH2{P(OMe)3}5which, with other chromium(I1) complexes of P donor ligands, is considered in Section 35.3.4.
35.23.2 Fluorophosphine complexes The reaction of tris(n-al1yl)chromium with the x-acceptor ligand PF3 produces stable crystals of [Cr(PF&] via [(C3H&Cr(PF3)] (Scheme 9). At hi h pressure, only [Cr(PF&] is isolated;6o it can also be obtained by metal vapour synthesis! and its He(1) PE spectrum has been assigned.62 Complexes of chelating aminodifluorophosphines Cr{(PF2)2NR}3(R = Ph (m.p. 233 OC)" ox Me (m.p. 182 "C)@)can be prepared by UV irradiation of Cr(C0)6 with an excess of the ligand
Chromium
715
Table 8 Tertiary Phosphine and Arsine Complexes of Chromium(0) Including Mixed Complexes with Dinitrogen
Complex tram-[Cr(N,),(dmpe)d Red-orange
[Wdmpe),l Yellow [Cr{P(0Me)3j61 Yellow Cr(L2)3
Comments
M.p. > 350 "C, stable in solution to 90"C, diamagnetic NMR: 'H, 6 1.29 (PMe, and PCH,, s); 31P[1H], 6 69.3 (PhH-d,, 25 "C), v(N,), 1932 cm-I (hexane), 2001 cm-' (Raman Cr-N, 1.874, N-N, 1.122, Cr-P (av), 2.296 , Cr-N-N, 178.2" (CrCI,(THF),], dmpe in THF + NaC,,H, or LiAIH,; cocondensation of Cr and dmpe vapours, airsensitive; distorted octahedron, Cr-P, 2.317 A, P-C-P (chelate ring), 76.5" M.p. > 100°C (dec.), cocondensation of Cr and P(OMe), vapours Lz = dppm, dppe or Ph As(CH,),AsPh,; from &[Cr(CN),] and L2 tn NH3(k) at -60 "C
1
Ref. 1
2
3
4
5
1. J. E.Salt, G. S. GiroIami, G. Wilkinson, M. Motevalli, M. Thomton-Pettand M. B. Hursthouse, J. Chem. SOC.,Dalton Trans., 1985,685. 2. J. Chatt and H. R. Watson, J . Chem. SOC., 1962,2545. 3. F. G.N. Cloke, P. J. Fyne, M. L. H. Green, M. J. Ledoux, A. Gourdon and C. K. Prout, 1. Organomet. Chem., 1980,198,C69. 4. S. D. Ittel, F.A. Van-Catledge and C. A. Tolman, lnorg. Chem., 1985,2A, 62. 5. H. Behrens and A . Miiller, Z. Anorg. Allg. Chem., 1965,341,124.
in diethyl ether. Cocondensation of chromium vapour and MeN(PF2)2 also affords [Cr{(PFz)zNMe}3].65,66Although the ligands have a small bite and must form four-membered chelate rings, the colourless complexes (3) are volatile and stable. In accordance with the formulation as zerovalent tris(bidentate) chelates, the 'N and 13C NMR spectra each exhibit a single resonance from the three equivalent methyl groups, and there are intense molecular ions in the mass spectra. The strong P-F stretching absorptions are at 831cm-I (R=Me) and 865 cm-' (R = Ph).
The cocondensation of chromium vapour with a 4:1 mixture of Me2NPF2and MeN(PF& forms the homoleptic chromium(0) derivative [Cr(PF2NMe&] and the mixed ligand derivative [Cr(PFzNMe2)4{(PF2)2NMe}].The former slowly decomposes when left in air, especially if in solution. This instability, compared with [Cr(CO),] or [Cr(PF&], is due to the steric hindrance of the six bulky monodentate ligands. The latter, with only four monodentate ligands and one bidentate ligand, can be sublimed and is considerably more stable. When prepared, it was contaminated with a little [ C T ( P F ~ N M ~(PF2)2NMe}2]. ~)~{ An X-ray determination of the
Chromium
716
structure of [Cr(PF2NMe&( (PF&NMe}] shows the expected octahedral configuration distorted by the small bite of bidentate (PF&NMe (P-Cr-P = W), which relieves steric strain compared with Cr(PF2NMe&; no bond distances were supplied. The stable zerovalent complexes [Cr(PF20Pr)6] and [Cr{ (PF(OMe)2}6] can be obtained by U V irradiation of [Cr(CO),j in pentane in the presence of a slight excess of the ligand.67
35.2.4 Miscellaneous Chromium(0) Complexes Several chromium nitrosyls, and complexes of quinones which are members of redox series! can be considered to contain chromium(0) or chromium(1) (see Sections 35.4.2.6 and 35.4.4.8).
35.3 CHROMIUM(I1) Chromium(I1) complexes, especially in solution, are very rapidly oxidized by air, and thi: restricted synthetic and other investigations until comparatively recently when efficient inen atmosphere boxes had been developed, and the use of Schlenk and similar techniques hac become general.68,69Solids show varying behaviour towards aerial oxidation; some arc pyrophoric but a few are stable for da s or longer. The need to use oxygen-free condition: should be assumed throughout the Cr'Y section. Few chromium(I1) compounds, possibly tht acetate and some anhydrous halides, are commercially available. Consequently, those wishin! to investigate the coordination chemistry of chromium(lI) must first prepare their startin! materials. Chromium(I1) has been extensively used in investigations of inner-sphere electron transfe: because inert chromium(lI1) captures the ligands attacked by chromium(I1).70
35.3.1 General Synthetic Methods Methods for the preparation of chromium(I1) solutions and the isolation of solids synthetic work are set out below.
foi
35.3.11 Chromium@I) solutions
(i) Dissolution of chromium in acids The impure metal dissolves easily in mineral acids and in fluoroboric, sulfamic ant trifluoromethylsulfonic acids to give C? solutions, but oxidation of CP' by hydrogen ion (equation 6), Eo(Cr3+, Cr2+)= -0.41 V) even in an inert atmosphere is catalyzed by t h impurities and various ions.71 Indefinitely stable chromium(I1) solutions can be obtained fron the pure (electrolytic) metal (995% or better), although the reaction with acid may need to bl initiated by heat and the inclusion of some metal previously attacked by acid. The use of ai excess of metal, which can be filtered off, ensures that little acid remains. In near neutra solution the hydrogen potential is lowered and the Cf+ ion is stable. In alkaline condition brown Cr(OH)2, which slowly reduces water, precipitate^.",^^
Cr2++H+
-
Cr3++ f H 2
(6
(ii) Reduction of chrorniurn(i1I) solutions The reduction of aqueous chromium(II1) solutions can be carried out electrolytically o chemically with zinc amalgam, zinc and acid or a Jones reductor.2,24Electrolytic procedures ca be cumbersome, and with chemical reductants contamination with other products can OCCUI Chromium metal and acid can be used to reduce chromium(II1) salts, and this requires less c the metal than in the method described in Section 35.3.1.1.i.
717
Chromium (iii) Non-aqueous solutions
The hydrated chloride, bromide and iodide (Table 9) are soluble in ethanol, butanol and other organic solvents, but in many systems traces of water cause oxidation, hydrolysis or failure to complex with weak donor ligands. Water can be avoided by dissolving the metal in THF, ethanol or diethyl ether through which hydrogen chloride is b ~ b b l e d . ~ , ~It~is, ”also possible to dissolve or suspend in organic solvents the anhydrous acetate or the halides CrX2 (Table 9), and dehydration of the hydrated halides with 2,3-dimethoxypropane in ethanol, followed by vacuum removal of the liquid, produces mixed alcoholates suitable for use in water-free condition^.'^ Triethyl orthoformate may be used similarly. Table 9 Preparations of Some Chromium(I1) Compounds Used as Starting Materials Preparatim
Compound [Cr,(O,CMeMH2O)Zl Dark red Cr,(0,CMe),.2MeC02H CrC1,.4H20 Blue CrBr2.6H,0 Blue CrI,.6H20 Blue Cr(C10,),.6H20 Blue
C3’(aq) + Na0,CMe; brown [Cr,(O,CMe),] on pumping at 100 “C Cr powder, anhydrous MeCO& with some MeCOCl Cr + HC1, conc. solution, add Me,CO; “CrCl, by thermal dehydration at 140°C Cr HBr, conc. solution, add Me&O; “CrBr, by thermal dehydration at 120°C Cr HI, conc. solution; soluble org. solvents, ‘CrIi on pumping at r.t.” Cr HClO,, C O ~ C .solution; soluble org. solvents, solid decomposes under N,, internal oxidation/reduction [Cr(bipy)3](C10,), known but probably dangerous to prepare CIO, salts of Cr’I cations generally Cr + H,SO,, add acetone; forms CrSO,.H,O and then CrSO, on heating, stable in air when dry Cr,(O,CMe),(H,O), acacH, H,O, reacts slowly with H,O Anhydrous” CrC12 (above) in EtOH + MeCN
+ + +
+
CrC1, extracted with THF Cr
+ HCl(g) in THE:
Crystallized from CrBr, in THF ~~~
~
Ref 1,2 3 4-6
4-7 4-6 4-6 234
2,4-6 1
6 8
9 10
~~~~~~~~~~~
The anhydrous halidesare also preparedby the methodsin Section 35.3 7. 1. L.R. Ocone and B.P. Block,Inorg. Synrh., 1966,8,125,130. 2. D. N. H u m and H. W. Stone, J . Am. Chem. Soc., 1941,63, 1197. 3. H.-D. Hardt and G. Sireit, 2.Anorg. Allg. Chem., 1970,373, 97. 4. A. Earnshaw, L. F. Larkworthy and K. S. Patel, 1. Chem. Soc., 1965,3267. 5. H. Lux and G. &ann, Chem. Ber., 1958,91,2143. 6. D. G.Holah and J. P. Fackler, Jr., Inorg. Synth., 1%7,10, 26. 7. M.A. Babar, L. F. Larkworthy and A. Yavari, J . Chem. Soc., Dalton Trans., 1981,27. 8. R.J. Kern, J . Inorg. Nucl. Chem., 1962,24,1105. 9. L.F.Larkworthy and M. H. 0. Nelson-Richardson,Chem. lnd. (London), 1974,164. 10. D.E.Scaife, AWL J. Chon., 1967,M ,845.
a
A method which gives a chromium(I1)-substituted akoxide ([Cr(oCBuj),-LiCl(THF),], Section 35.3.5.2) directly from CrCL is to add a solution of the alcohol previously treated with BuLi to a slurry of the halide in THF.77How the reduction to chromium(I1) takes place has not been clarified.
35.3.12 Chromium(11) salts The poorly soluble acetate [Cr2(0,CMe)4(H20)2] can be crystallized from aqueous C? solutions by the addition of sodium acetate. When dry it reacts slowly with oxygen and can be handled briefly in air, but the anhydrous acetate chars on exposure. The hydrates CrS04.5Ht0, CrC12.4H20, CrBrzm6Hz0 and Cr12.6H20, and the anhydrous halides, all most easily isolated as in Table 9, are useful starting materials. Thermal
718
Chromium
dehydration of halides can lead to hydrolysis, but this is not significant for preparative use o the anhydrous chromium(I1) compounds. The iodide and perchlorate are difficult to obtail hydrated to a definite degree. On pumping Cr12.6H20loses its blue d o u r as water is removed but the solid remains a chromium(I1) species. The perchlorate should be used with extremd caution, if at all, because the juxtaposition of C? as part of a complex cation containing iil organic ligand and C10; is intrinsically dangerous and not only impact but the accidenta admission of air to the solid could cause an explosion. The use of BPh4, BF4, NH2S03, PF6 anc CF3S03salts has not yet been much exploited. Aqueous chromium(I1) solutions have long been available as standard reducing agents ii volumetric analy~is.'~More recently, the use of chromium(I1) salts as reducing agents ii organic chemistry has been reviewed. Chromium(I1) chloride, sulfate and perchlorate haw been classified as powerful reducing agents whereas the acetate, a relatively insoluble salt, act as a mild reducing agent in near neutral conditions. Chromium(11) ethylenediamine complexe have been used in dimethylformamide," and transition metal chelates have been reduced b chromium(I1) acetylacetonate in toluene and THF.80 Chromium(I1) compounds also readit reduce various iron oxides.81These uses of chromium(I1) compounds are likely to increase. Metals can be used as sacrificial anodes in the electrochemical synthesis of anhydrous salt and complexes. This method frequently produces chromium(II1) complexes, but electrochemi cal oxidation of chromium in a mixture of concentrated aqueous HBF, and acetonitrile yield [Cr(MeCN)&BF4)2.
35.3.2
Group IV Ligands
Cyanide and organic isocyanide complexes of chromium(1I) have been included with th lower oxidation states in Sections 35.2.1.1 and 35.2.1.2.
35.3.3
Nitrogen Ligands
353.3.1 Ammines
(i) Syntheses A deep blue colour appears when concentrated solutions of ammonia and aqueou chromium(I1) are mixed, but brown chromium(I1) hydroxide separates before any ammine ca be Gaseous ammonia and solid CrC12 form CrCI2-nNH3(n = 2, 3, 5 and 6) an when ammonia is bubbled through ethanolic solutions of the hydrated halides pale blue, violc and then greyish precipitates successively appear corresponding to the formation of diammine: pentaammines and hexaammines (Table The hexaammines lose ammonia so readily (an reversibly) to form pentaammines that they must be sealed in ammonia gas. The easy loss E ammonia appears to be a 'chemical' effect of the Jahn-Teller distortion (see later). Th tetraammine [ C T ( N H ~ ) ~ ( H ~ O J ~ is] Sobtained O~ by bubbling ammonia through an ethanol] suspension of CrS04.5H20 and the diammines by heating the tetraammine or th pentaammines.
(ii) Magnetic and spectroscopic properties and structures Chromium(I1) complexes may be hi h- or low-spin (Figure l), and the hexaammines an pentaammines are typical, high-spin C$ compounds with effective magnetic moments close t the spin-only value (t&ek configuration, pso = 4.90 BM). With these magnetically dilul complexes the experimental moments are temperature-independent (Table 10) so the Curie 1% is obeyed ( 6 = 0 "C). The diammines show weak antiferromagnetic interaction since t h moments decrease as the temperature is lowered and the Curie-Weiss law is followe (6== 35 "C). The 0 antibonding effect, arising from the uneven occupancy of the d orbitals in high-spj chromium(I1) complexes, has a marked effect (Jahn-Teller distortion) on the electronic specQ and structures. Chromium(I1) has a 5Dfree ion ground term and this is split in an octahedr (Oh) ligand field as shown in Figure 2 to give a 'Eg ground term. The Jahn-Teller theorei
Chromium
719
Table 10 Magnetic and Diffuse Reflectance Data of Ammines Reflecrance spectra" (cm-')
e Complex
295K
90K
(")
4.57
4.05
40
4.77
4.74
0
4.85
4.78
0
4.33
4.09
23
4.76
4.80
0
4.87
4.90
0
4.87
4.87
0
4.88
4.91
0
4.69
4.26
32
4.89
4.74
8
VI
v2
~
a Spectra at
17Ooosh 17 OOOsh 17 800svb 18200s 16 OOOsvb 17 300svb 17 500sh 17 m h 17 5Wsvb l8zooS 15 4oosb 15 7OOsb 17 600s 18 100s 15 200s 15 400s 14 OOOvb 14 OQOS 18 OOOvb 18 400vb
14 OOOsvb 14 l00svb
11300sh 11OOOsh
14 500sh
11400sb
13 200s 13 700s
8500rn 8500m 9500sh 9500sh
14 500sh
14 OOOsh
11 500sh
11300sh 7500m 7700m 12 8OOsh 112OOsh 7600m 7800m
9800sh 15 OOOsh
room and liquid nitrogen (second line) temperatures for each complex
high-spin
low-spin
p s 0 =4.90BM
p s 0 = 2 . 8 3BM
Figure 1 Occupancy of d orbitals in octahedrd chromium(II) complexes
requires that in an orbitally degenerate, non-linear system, distortion will occur to remove the degeneracy, and this can take place in high-spin d4 (and d9) systems by elongation or compression along an octahedral four-fold axis (Figure 2, tetragonal distortion, Lhh).In solids, the Jahn-Teller distortion may be local to the coordination spheres of the individual C? ions or there may be interactions between neighbouring ions affecting the whole lattice, and this cooperative Jahn-Teller ef€ect has been reviewed (Section 35.3.7).86
04,
(elongated)
0,
o, (compressed)
Figure 2 Energy levels of high-spin chromium(I1) in ligand fields of Oh and D4h symmetry
Chromium
720
From Figure 2, one spin-allowed d-d transition is expected for a regular octahedral complex, and three such transitions for a tetragonally distorted complex. Experimentally, most high-spin chromium(I1) complexes exhibit one absorption band (vZ, the main band) with a weaker band or shoulder (vl, the distortion band) to lower wavenumber; vL is usually assigned to the 5B1g+5Alg transition and v2 to superimposed ’B1,-, 5&, ’Eg transitions. There are also examples where there is one very broad band due to the overlap of all three transitions and others where they are resolved. There are few single crystal spectroscopic studies of chromium(I1) complexes. The spectra of the ammines follow the general patterns outlined above. From the magnetic and spectroscopic data, and X-ray powder comparisons with copper(I1) complexes, the hexaammines and pentaammines have been assigned tetragonal octahedral and square pyramidal structures respectively, and the diammines anion-bridged six-coordinate structures (4). The tetraammine sulfate is square pyramidal, with coordinated water; other fivecoordinate chromium(I1) complexes are listed in Table 42. The acetates [Cr2(0zCMe)4(NH3)z] and Cr(NH3)4(0zCMe)2are discussed in Section 35.3.5.5.
35.3.3.2 Bidentate saturated amines, ethylenediamine, propylenediamine, etc.
Although earlier attempts to isolate chromium(I1) complexes of various bidentate amines from aqueous solutions produced chromium(II1) complexes and hydrogen, the predominantly non-aqueous methods outlined in Scheme 10 provide complexes of ethylenediamine (en), 1,2-diaminopropane (pn) , Iy3-diaminopropane (tmd), 1,2-diamino-2-methylpropane(dmp), N,N-dimethylethylenediamine (NNdmn) and N,N‘-dirnethylethylenediamine (NN’dmn) (Table 11). In general, ethanol is a suitable solvent but with some amines it is necessary to dehydrate the halide with 2,2-dimethoxypropane (DMP) and dry the ethanol carefully to prevent hydrolysis and oxidation.
[CrCl,(NN’dmn),] [CrX,(NNdmn),] X = Br, I
‘CrX,wMeOH’
[CrCl,L]
[Cr(en),lSO,, [crso4(en)21
DMP, A
CrX,.nH,O
[Cr(en),]X, [CrX,(en)2]
J
[CrX,LI X = C1, Br
Crl,
[C&L,I
[Cr(pn)3lX2 X = Cl, Br, I
L = tmd OF dmp Scheme 10
,
X = C1, Br X = Br, I
721
Chromium Table 11 Bidentate Saturated Amines
Complex Blue Blue Light blue Violet Violet Violet Light blue Blue Blue Violet Violet Mauve Light blue Violet Purple Mauve Light blue Mauve Violet Purple
4.84 4.75 4.87 4.74 4.73 4.80 4.80 4.90 4.83 4.80 4.87 4.80 3.87 4.75 4.78 4.79 4.02 4.81 4.82 4.93
4.85 4.75 4.85 4.71 4.73 4.56 4.75 4.83 4.78 4.75 4.89 4.83 2.14 4.55 4.52 4.45 2.61 4.80 4.81 4.77
0 0
0 0
0 10 0 0 0
0 0 0
10 12 19
0 0 8
16 OOOm 15 700m 16 800m 17 900m 18 200m 17 700m 16 OOOs 15 800s 16 Ooos 17 600s 18 OOOs 18 300s 18 600sh 17 500s 17 400s 18 200s 18 600sh 16 900s 17 700s 16 500s
8300m 8300m 7800m 13 2OOw 14 500sh 11 6oow 7700s 7600s 7600s 12 600sh 14 200sh 15 300sh 14 100s
13 400s 11 400sh
The [Cr(en),]'+ and [Cr(pn),]" salts have reflectance spectra (Table 11)resembling those of the hexaammines, and the six N donor atoms are assumed to complete tetragonally distorted octahedra around the metal. Stability constant measurements (Table 39) have shown that the ions [Cr(en)(aq)12+ (Vmm = 18 300 cm-', E = 25 dm3mol-' an-') and [Cr(en)2(aq)]2+(Vmm = 17 500 cm-l, E = 17 dm3mol-' cm-')exist in aqueous solution, but that, as in the copper(I1) system, the third ethylenediamine molecule is only weakly bound, and care is needed to prevent loss of en from tris(amine) complexes in the preparations. Several bis(amine) complexes, e.g. [CrBr2(en)2], have been isolated, and these are assigned trans structures because of IR spectral resemblances to the corresponding copper(I1) complexes. Since the spectrum of [Cr(S04)(en)2]also shows the presence of bidentate sulfate, this is assigned a l r ~ n r octahedral structure with bridging anions. The pale blue tris(amine) complexes [Cr(pr~)~]X, separate readily from solutions containing an excess of the amine. Violet solutions form when 2: 1 ratios of pn and metal halide are used, but pure bis(amine) complexes have not been obtained. On the other hand, the other bidentate amines give only the bis(amine) complexes [CrX2(tmd)2],[CrXz(dmp)2],[CrX2(NNdmn)2]and [CrC12(NN'dmn)2] (Table ll), even from solutions containing an excess of amine. These are also given trans structures. The antiferromagnetic monoamine complexes [CrC12(tmd)] and [CrC12(dmp)] are believed to be chloride-bridged polymers. No crystallographic evidence is available. The bands v2 and vl, particularly the latter, move to higher wavenumber with weaker field axial ligands: I > Br > SO4 >NH2. This behaviour is characteristic of distorted
35.3.3.3 Polydentute amines, diethylenetriamine, triethylenetetramine and facultative ligands ( i ) Diethytenetriamine
The ions [Cr(dien)2]2+and [Cr(dien)12+have been identified in aqueous solution by stability constant (Table 39) and other measurements: &g3 = 4.9 BM, = 17 OOO cm-l, E = 39 dm3 VmX = 16 300 cm-', E = 32 dm3 mol-' cm-' [Cr(dien)(aq)12+,but the mol-' cm-' [Cr(die~~)~]*+; solutions decompose on standing, and the solids [Cr(dien)z]Xz and [CrX2(dien)] have been crystallized from alcoholic media. The bis(dien) complexes have similar magnetic moments and electronic spectra to the tris(en) complexes and so contain tetragonally distorted six-coordinate Cr2+. Unlike [Cr(en),]' ions, which lose en, the [Cr(dien)2]2+ions maintain their integrity in solution in organic solvents. Halide-bridged binuclear structures have been suggested for the antiferromagnetic mono(dien) complexes (Table 12).
Chromium
722
Table U Diethylenetriamine Complexes
Complex" [Cr(dien),]Cl,.H,O [Cr(dien)z]Brz [Cr(dien)J12 [CrCl,(dien)] [CrBr2(dien)] [CrIz(dien)]
(:XI)
300K
90K
("1
4.82 4.88 4.84
4.80 4.82 4.84
0
4.38 4.26 4.28
R@ctance spectra
e
3.39
3.30 3.74
2
0 d
d c
(cm-') V1
v2
b
16100 16000
8600
16100
b
8800 ( E
16 800 ( E = 19)c 14 5Wvb 14 5OOvb 14 900vb
= l0)c
All compbxes are blue. Not measured bclw 10 033 cm-'. DMF, in cip3mol-' cm-'. dJca. 1 0 ~ ,-g = 2 . a
'Jca. 4cm-',g=2.
( i i ) Triethylenetetramine
From stability constant measurements (Table 39) the [Cr(trien)12+ ion (pee= 4.9 BM, = 16 900 cm-', E = 46 dm3 mol-' cm-') exists in aqueous solution, but only incompletely characterized solids have been i~olated.'~
f,,
(iii) Facultative ligands Chromium(I1) forms the trigonal bipyramidal complex [CrBr(Me6tren)]Brwith the 'tripod' ligand tris(2-dimethylaminoethy1)amine (Me6tren) (Section 35.3.4.3), and pyrazolyl-substituted ligands also form five-coordinate complex cations (Section 35.3.3.4.v; see also Table 42). 35.3.3.4 N heterocyclic ligands
(i) Pyridine and substituted pyridines Like other bivalent metals of the first transition series, chromium(I1) forms many complexes with pyridine and substituted pyridines. These are mostly of the types [CrX2L4],in which X = Cl, Br and I, and [CrX2L2], where X = C1, Br and occasionally I, but a few of the general formulae [CrX2(H20)2&] and [ C ~ ( p y ) ~ ]are x ~ known (Table 13). The complexes [CrXzL] invariably obey the Curie-Weiss law, with large 8 values, and this antiferromagnetic interaction is evidence for the halide-bridged structure (5), which is confirmed by the isomorphism of [CrX~(py)~] and [C~X,(py)~l (X = C1, Br). Table l3 Complexes of Pyridine and Substituted Pyridines
4.84 (r.t.1 4.62 (298 K) 4.81 (1.t.)
-
4.94 (r.t.)
-
40
-
5.03 (r.t.)
0
322 v ( C r 4 1 ) 281 v(Cr-N) 272 v(Cr-Br) v(Cr-N)
1 -
15 050
10 800
1-4
14 300
10 300
2,4
17 000
11 600
2
17 OOO
12 750
4.98(298K) 5.03 (r.t.)
-
a-
4.79(292K) 5.03 (298K)
-
-
0
I
2 3
12 100
2 3 3
Chromium
723
Table 13 (continued)
5.01(298K)
-
0
300K 4.79
86K 4.74
2
4.90
4.79
4.86
4.87
4.79
4.89
4.93
4.86
4.90
I
3
-
9800
4
17 OOOsh
10 800
4
292 v( Cr-N)
17 500sh
11800
4
298 v(Cr-N)
17 OOOsh
9700
4
3
2% v(Cr-N)
17 OOO
10 800
4
4.80
5
298 v(Cr-N)
17 500sh
12OOO
4
4.88
4.80
4
16 600
11 400
4
4.87
4.79
5
283sh v(Cr-N) 278 v(Cr-N) 275 v(Cr-N)
16 500
11800
4
4.62
4.01
46
14 500
11300sh
4
4.76
4.16
42
14 200
11OOOm
4
4.70
4.09
45
14 600
11700sh
4
4.60
4.03
41
320 v (Cr-Cl) 282 Y ( 0-N) 282 v(Cr--N) 272 v(Cr-Br) 332 v ( C r 4 ) 280 v(Cr-N) 296 v (Cr-3r) 271 v(Cr-N)
14 200
11500sh
4
295K 4.08
90K 3.65
37
14 300
10700
5
4.51
3.93
48
13 800
10 100
5
4.80
4.85
310 v ( C r 4 ) 272 v(Cr-N) 274 ~(CI-N) 260 v(Cr-3r) 270 v(Cr-N)
10 700sh
5
4.59
4.10
37
10 4Oosh
5
4.59
4.11
37
16 500 12 OOO 14 200 12 150sh 13 900
10 400
5
4.83
4.78
2
10 700sh
5
4.61
4.13
36
16 300 12 OOO 13500
10 600
5
4.64
4.30
22
10 900
5
4.69
4.30
27
10 400sh
5
4.60
4.27
22
11OOOsh
5
4.71
4.06
53
14 500 12 600sh 14 100 12 OOOsh 13 600 12 lOOsh 13 750
4.67
4.00
57
4.69
4.33
24
4.83
4.63
11
5
295 v(0-N) 280 286 v(Cr-N)
0 -4
-3
324 v(Cr-41) 268 v(Cr-N) 272 v(Cr-N) 252 v(Cr-Br) 265 v(Cr-N) 264 v( Cr-N) 238 v(Cr-I) 324 v(Cr-Cl) 260 v(Cr-N) 282 v(Cr-Br) 258 v(Cr-N) 272 v(CI+-N) 234 v(Cr-I) 324 v ( C r 4 1 ) 272 r(Cr-N) 272 v(Cr-N) v(Cr-Br) 262 v(Cr-N) 250 v(Cr-I) 270 v (Cr-N)
}
9600
5
13 600
9300
5
13 200
8800
5
15 200s.sh
7000
5
Reflectance spectra at liquid nitrogen temperature. H. Lux, L.Eberle and D. S a m , Chem. Ber., 1964,97,503. D.G . Holah and J . P. FacUer, Jr., horg. Chem., 1965,4, 1112. A. Earnshaw, L. F. Larkworthy and K. S. Patel, Chem. Ind. (London), 1965, 1521. M. M. Khamar, L. F. Larkworthy and M. H. 0. Nelson-Richardson, Znorg. Chim. Acra, 1978, 28, 245. 5. M.M.Khamar, L. F. Larkworthy and D.J . Phillips, Inorg. Chim.Acta. 1979, 36, 223.
a
1. 2. 3. 4.
coc3-x
724
Chromium
The complexes [CrX2L4] and [CrX2(H20)2Lz]are high-spin and magnetically dilute. The former are believed to have trans octahedral structures and there is some evidence that the latter have too.85,9c93 Some pyridine complexes, e.g. [CrClz(py)z], have been prepared from the hydrated chromium(I1) halide in aqueous solution, but it is more common to use solvents such as acetone, which is particularly useful for iodo complexes, and ethanol. In some cases it is necessary to recrystallize from dimethylformamide to remove water arising from the use of the hydrated halide. It is also sometimes necessary to control the amount of pyridine fairly closely, use cold solutions and wash with solvents containing small amounts of pyridine if the desired complex is to be obtained. Some complexes, e.g. [ C r ( p ~ ) ~ ] B and r ~ [Cr(py)&, have been prepared by dissolving the bis- or tetrakis-amine complex in an excess of pyridine. These two complexes, which are isomorphousg1with [ C ~ ( p y ) ~ ] Bare r ~ ,the only known hexakispyridine complexes of chromium(II), although more could probably be obtained with [BPh4]- and similar anions. The iodo complex [Cr12(3,5-Clpy)2]was prepared by warming the tetrakis(amine) in acetone so thap, it lost two chloropyridine molecules, and [Cr12(3-Brpy)2] was obtained by the use of 2,2-dimethoxypropane in acetone. Attempts to isolate complex halides containing 2-methylpyridine produced blue substances, which could not be ~ha ra cteriz ed.~~ Bridging iodide is less common than bridging chloride or bromide and the few chromium(I1) iodo complexes, [Cr12(3-Brpy),], [Cr12(3-Ipy)2] and [Crl2(3,5-Clpy),], might be expected to be tetrahedral. However, because of their antiferromagnetic behaviour, they are apparently linear polymers; tetrahedral monomers would be expected to have magnetic moments greater than 4.90 BM. The reflectance spectra of the iodo complexes are also similar to those of the corresponding bromides and chlorides and the molar absorbances are low (E 2= 30 dm3 mol-' cm-' in e t h a n ~ l ) . ~ ' Bands in the far IR spectrum of [CrClZ(py),] at 328 and 303cm-' were assignedg4 to v(Cr-Cl), and a band at 219cm-' to v(Cr-N), but in a reinvestigationg3 rather different frequencies (322vs, 281s and 215s cm-l) were found. The band at 322 cm-l has been assigned to v(Cr-C1) since it was absent from the spectrum of CrBr,(py),]. Instead, the bromide has an intense, broad and asymmetric band at 272cm- , and strong absorption rising to a maximum at 200 cm-l. It has therefore been suggested that v(Cr-N) is at 281 cm-' in the and chloride, the band at 272cm-' for the bromide contains superimposed v(Cr-N) v(Cr-Br) absorptions, and the band near 200 cm-' corresponds to G(N-Cr-N). These spectra were recorded only to 200 ern-', but the assignments are confirmed by investigations of [CrX2(Mepy)2](X= Cl, Br) and [CrXz(halo-py)z] (X = CI, Br, I), which show that v(Cr-Cl) is near 320 cm-l; v(Cr-Br) and v(Cr-N) sometimes overlap in the range 250-300 cm-', and, from a few examples, v(Cr-I) is near 240cm-l. The v(Cr-N) vibrations are generally at lower frequencies in the halopyridine complexes because of the greater mass of the amines (Table 13).92,93 The far IR spectra of [CrX2(Mepy)4](X = C1, Br, I) do not differ significantly over the range 400-70 cm-I except that the chlorides have two bands in the region of 136 cm-' which may be due to 'long-bonded' v(Cr-C1) vibrations, i.e. in these complexes the halide ions lie along the elongated tetragonal axis. The reflectance spectra show the usual two .band pattern of distorted six-coordinate, high-spin chromium(I1). The complexes [CrC12(py)z] and [ C T ( C F ~ S O & ( ~have ~ ) ~ ]been used in the preparation of macrocyclic complexes of Cr" and Cr(N0) (Section 35.3.10.2 and Table 64). The chromium(I1) ion is in an uncommon trans planar environment (6) in bis(2,6-dimethylpyridine)bis(trifluoroacetato)chromium(II), which is prepared by the reaction sequence in
E
Chromium
725
Scheme 11. Each ligand is monodentate and the methyl grou s of the 2,6-Mepy molecules above and below the Cr02N2plane block further coordination. 9 P
I CF3
(6) CrC13
NaBH4
[(THF)Cr(BH,),]
CF,CO,H
[(THF)Cr(O2CCF3)212
Scheme 11
--
2,6-Mepy
[Cr(O,CCF,),(2,6-Mepy),l
Thermal decomposition of [CrBrz(H20)2pyz]takes place96according to reactions (7) to (9). [ C ~ B ~ ~ ~ & O(cryst.) )~PY)~I
[CrBr2(py),1 (cryst.) + 2H20 (g)
(7)
[CrBrz(PY)21(cryst.) [CrBr2(py)l (cryst.) + PY (8) (8) [crBr2(py)l(cryst.) CrBr, (cryst.) + py (g) (9) [Cr12(py)4] was found to lose two pyridine molecules to give [Cr12(py)2],with the loss of further pyridine not beginning until 237 "C, but it was not possible to distinguish the stepwise loss of the remaining two ligands. The heat of reaction for the removal of 2H20 from [CrBr2(H20)2(py)2] ( A H = 117.5kJ mol-') is close to that for the removal of 2py from [Cr12(py)4],suggesting H 2 0 and pyridine form bonds of similar strength. Values of A H for the loss of two pyridine molecules from [CrXz(py)2] are 148.4, 132.1 and 103.7 kJ mol-', for X = C1, Br and I respectively. The value for [CrC12(py)2]is greater than those for [MC12(py)z] where M = M n through Cu. The chloride decomposes according to equations (8), (10) and (11).
(ii) Nicotinic acid and saccharine Nicotinic acid (nic) forms a series of complexes [ M " ( n i ~ ) ~ ( H ~ in 0 ) which ~] it is N-bonded. The yellow, crystalline chromium(I1) complex is prepared by running an aqueous C? solution into a nicotinic acid solution under hydrogen. It is air-stable and the analyses were consistent with [Cr1"(nic)2(H~0)30H]or [Cr"(nic)~(H20)~], but the molecular structure (7) agrees with the chromium(I1) formulation. The complex has a magnetic moment (3.1 BM) corresponding to low-spin chromium(II), which is unexpected since the complexes [crX, (p ~)~] are high-spin (Section 35.3.3.4.i above), and two bipy or phen molecules are necessary to cause spin pairing (Section 35.2.2.1).* This complex was prepared in an investigation of chromium(II1) nicotinic acid complexes related to the glucose tolerance factor (Section 35.4.8.3). The X-ray powder patterns and IR spectra of the complexes [MI'(r1ic),(H~0)~]in general are closely similar to those of the Cr*' complex.w Saccharine forms a similar series of complexes [M1'((c,H4N03S)z(H20),1.2H20. The chromium(I1) complex is yellow and has a trans structure (8) analogous to that of * Found later to be contaminated with isomorphous Zn" complex; Cr-N = 2.128, C r 4 , two at 2,039, two at 2.471 8, (W. E,Broderick, M. R. Pressprich, U. Geiser, R. D. Willet and 1. I. Legg, Inorg. Chem., 1986,25,3372).
726
Chromium
(7)
[ C r ( n i ~ ) ~ ( H ~ (7), 0 ) ~ ]but with the effect of Jahn-Teller distortion apparent in the Cr-0 distances (Table 34).98 (iii) Pyrazoles and imidazoles Complexes of imidazole (iz),w,loo N-methylimidazole (Nmiz), pyrazole (pz) , 3,5-dimethylpyrazole (dmpz) 3(5)-methylpyrazole (5Mepz) , l-methylpyrazole (NMepz), 2methylimidazole (2Meiz) and l-ethylimidazole (NEtiz)'" (9) have been prepared from ethanol or butanol by methods resembling those in Scheme 10. As with the analogous pyridine complexes, those of the general formula [CrX2L43 are high-spin, tetragonally distorted, six-coordinate complexes, and those of the formula [CrX2&] are also six-coordinate, but are antiferromagnetic halide-bridged polymers; 2-methylimidazole, however, forms cations which are probably square pyramidal: [CrX(2Mei~)~lX (X = C1, Br), [Cr(2Mei~)~](BPh&and [Cr(NCS)2(2Meiz)2](Table 14). In the last complex five-coordination (Table 42) is believed to be achieved through NCS bridges.The reduction of chromium(II1)-imidazole complexes at the dropping mercury electrode has been studied (Section 35.4.2.5.iii).
c"y
H (9) X = N, Y = CH, imidazole X = CH, Y = N , pyrazole
A pale blue-green complex [CrC12(PMT)2] (pzp = 4.55 BM) of pentamethylenetetrazole (PMT) has been obtained by the addition of solid PMT in excess to a solution of hydrated chromium(I1) chloride in a 50% DMPIMeOH mixture."" Pyrazines and Cr2' give intensely coloured 'pyrazine green' products (Section 35.4.2.5.i~).
(iu) Bidentate ligands: bipyridy ls, phenanthrolines, 2-arninomethylpyridine, 8-aminoquinoline and adenine Chromium(I1) complexes of bipyridyls, terpyridyl and the phenanthrolines have been discussed in Section 35.2.2.1. Complexes of the ligands 2-aminomethylpyridine (pic, 2-picolylamine) and 8-aminoquinoline (arnq) , which have one heterocyclic and one amino nitrogen donor atom, have been prepared by methods similar to those in Scheme 10. The bis(amine) complexes are typical high-spin, distorted octahedral complexes, and the mono(amine) complexes, from their antiferromagnetic behaviour and reflectance spectra, are six-coordinate, halide-bridged polymers (Table 15).lo3 No tris(amine) complexes could be prepared so the attempt to find spin isomeric systems in octahedral chromium(I1) systems was unsuccessful ([Cr(en)3]Xz are high-spin and [ C r ( b i ~ y ) ~ ]and X ~ [CrX2(bipy),] low-spin). Complexes of adenine (10; Ade) have been isolated by mixing the ligand in 2-methoxyethanol with chromium(I1) halides in butanol. The low magnetic moments (Table 16)lO4 indicate dinuclear species [Cr2Xz(Ade)$+] (Section 35.3.54, or polynuclear structures with bidentate adenine.
(v) Bi-, tri- and tetra-dentate pymzolyl-substituted ligands Chromium(I1) complexes of the ligands 1,l'-methylenedipyrazole (11;H2Cpz2), 1,l'-methylenebis(3,5-dimethylpyrazole) (12,H2Cdmpzz), tris( l-pyrazoly1)methane (13; HCpz3),lo5t i s ( 1pyrazolylethy1)amine (14; R = H, and tris(3,5-dimethyl-l-pyrazolylethyl)amine
727
Chromium Tabb 14 Complexes of lmidazoies and Pyrazoles
13300sh
1
13OOOsh 16 600sh
2 1
4.87 4.77
17 500 14000(~=40)a 17 500 19200 17 400 (E = 30)" 19 300vb 19 OOOvb
16400sh
2
4.77
4.75
17200
12400sh
2
4.93
4.90
18 200
12500sh
1
13 300sh
1
15 400sh 13 300sh 14 OOOsh 15 500sh
1 1 1 1
4.87
4.86
4.87 4.85
4.86
4.84 4.78
I
12 500 ( E = 49)a
4.93
-
4.93 4.90 4.90 4.90 4.69 4.48 4.95 4.62 4.81 4.82 4.84 4.80
4.88
4.80
-
[CrIz(2Meiz),]
4.84
-
[Cr(NCS),(2Mei&]
4.48
[Cr(2Meiz),1(BPh4),
4.78
4.06 (84 K) -
[CrCl,( NEtiz) 3] [CrBrz(NEtiz),] [CrCIz(NMePz),l [CrBr,(NMepz),] .H,O
4.22 4.80 4.74 4.90
a
2
-
4.30 3.65 4.95 4.06
-
-
3.84
-
4.46 4.90
18 500 13 300 ( E = 20)a 18 300 17700 17 700 17 700 13 950 15 m 15 000 14 700 17 400 17 550 17 850 16950 16 OOO ( E = 40)" 17 850 17 400 ( E = 50>a 17 850 17 550 ( E = 40)" 18870 18 200 18OOO [E = 50)' 16 700 17400 13 409 12 900
1 83SOsh
1 1 1 3 3 3 3
11 100
12 100 12900sh 15 050
3 3 3
3 12 100 12 750sh
3 3
3 3
dm3mol-' cm-' in DMF.
1. F. Mani and G. Scapacci, Znorg. Chim. Acta, 1976,16, 163. 2. L. F. Larkworthy and J. M. Tabatabai, Znorg. Chim. Acta, 1977,21,265. 3. P. Dapporto and F. Mani, J . Chem. Res. ( S ) , 1979,374.
Table 15 Magnetic and Reflectance Data for Complexes of 2-Aminomethylpyridine and 8-Aminoquinoline Reflectance spectra (cm-')
Pefl (BM)
Complex
300K
90K
y2
y1 _
Orange Orange Orange
Light green Green Dark red Red Red Greenish yellow
4.86 4.90 4.90 4.45 4.40 4.86 4.91 4.85 4.28
4.82 4.82 4.85
3.70 3.46 4.82 4.87 4.83 3.59
2 4 2 J=6cm-l I = 9 cm-' 2 2 2
J=7cm-'
_
_
22 400 22 ooo 21 800
~
~
~
13 OOOsh 13 600 13 800 14 Ooovb 13 6oovb
CT CT CT
cr
13 500sh 13 7ooSh 13 BoDsh 13 5 a h
~
~
Chromium
728
,
(14) R = H, TPyEA R = Me, MeTPyEA Table 16 Complexes of Adenine
Pef(BM)
Complex
1.63(295K) 1.49(286K) 2.44(296K)
CrCl,(Ade),~C,H,O, CrBr,(Ade),.C,H,O, CrI,(Ade),~0.5C,W,02
1.11(103K) 0.75(85 K) 0.64(85 K)
Repectance spectra (cm-')
19 800 19 050 19 200
14 300sh 14 800sh 14 300sh
10 500sh 10 OOOsh 9500sh
8000
7800
(1%R = Me, MeTPyEA) have been isolated from non-aqueous solvents (Table 17).105108 From their electronic spectra, simple paramagnetic behaviour and stoichiometry [CrC1(H2Cdmpz2)2]BF4, [CrX(H2Cdmpz~)2]BPh4and [CrX(H2Cdmpz2)2]X (X = Br, I) are thought to be five-coordinate. The spectra differ from those of distorted six-coordinate or square pyramidal chromium(I1) complexes and, except for intensity differences, resemble those of other trigonal bipyramidal chromium(I1) complexes. Conductance and solution spectra suggest that the trigonal bipyramidaI cations are present in solution too. From similar evidence [CrX(MeTPyEA)]BPh4 (X= Br, NCS) contain square pyramidal cations."' The blue complex [CrNCS(TPyEA)]BPh4, dissolved in acetone or acetonitrile, is oxidized very rapidly by air to give a green solution from which the p-oxo chrornium(I11) complex [{ Cr (NCS)(TPyEA)}20](BPh4)2 crystallizes (Section 35.4.2.5.iv).*07 The remaining complexes are mostly high-spin six-coordinate complexes with temperature-independent magnetic moments, but in a few, e.g. [CrC12(H2Cpz2)]and [CrBr(TPyEA)]BPh4, the halide bridges needed to produce six-coordination lead to weak antiferromagnetism. Table 17 Complexes of Pyrazolyl-substituted Ligands
Pe
(2986 (84K)
Complex
[Cr(HCPz,)zI(BPh,)z [CrC~z(HzCPzz)l iCrBr2(HzCPzz)zl [CrIz(HzCPzz)zl [CrC1(HzCezz)zIBPh, [CrBr(HzCP~)zlBPh, [CrBr(H,Cdmpz,),]Br [CrI(HzCdmPzz)zlI [CrC1(H,Cdmpz,),]BF4 [CrBr(HzCdmpzz)z]BPh4 ICrI(HzCdmPz2)21BPh4 [CrBr(MeTF'yEA)]BPh,.EtOH [Cr(NCS)(MeTPyE A)]BPh, [CrBr(TPyEA)]BPh, [Cr(NCS)(TPyEA)]BPh4 a
In acetone 14 300
(E
= 40 dm-3
(W
Greenish grey Greenish blue Green Lilac-grey Green Greenish grey Light blue Light blue Blue Blue Blue
mol-' cm-I). 11400sh cm-'.
4.82 4.45 4.80 4.80 4.42 4.14 4.93 4.83 4.80 4.85 4.88 4.83 4.88 4.77 4.74
3.60 3.80 3.15 4.85
4.85 4.80 4.80 4.92 4.54 4.42
Reflectance spectra (cm-')
vZ
VI
18 900 13200 14 100 17400 14700 17200 15 400 15 300 17000 16000 14700 14900
10 2oow 10 500sh 82OOw
13200sh 8600w 11600sh 11300 11 100 12 500 12 500 11 100 10 4wa
16 400 16100 10 150 v(CN) = 2085 cm-'
Chromium
729
353.3.5 Nitrosyls and hydratine Nitrosyls are discussed in Section 35.4.2.6, and hydrazine is the only ligand of its type from which chromium(I1) complexes have been prepared.
(i) Hydrazine The pale blue complexes [ C T X ~ ( N ~ H(X~ = ) ~C1, ] Br or I), which are reasonably stable to dry air, separate rapidly when an excess of hydrazine is added to an aqueous solution of the appropriate halide. '09 The chloride has also been prepared by the addition of hydrazine to a solution of [CrC12(H20)4]Q reduced with Na/Hg.lLDThe complexes have been assigned the polymeric structure (W) because the IR spectra are almost identical to those of analogous complexes with crystallographically determined structures. The reflectance spectra (absorption bands at ca. 17 500s and 11 OOOm cm-') are independent of the halide ion suggesting that these lie along the distortion axis, and the magnetic moments show little temperature variation, consistent with weak antiferromagnetic interaction arising from hydrazine rather than halide bridges. The reaction of CrF, with N2& gives unstable and impure [CrF2(N2H4)2].111
35.3.3.6 Dialkylamides and disilylamides The disproportionation of chromium(II1) dialkylamides, e.g. [Cr(NEt2),] into [ C T ~ ( N E ~ ~ ) ~ ] and [Cr11(NEt2)2],has been used to prepare several chromium(1V) dialkylamides (Section 35.5.4.3) but pure chromium(I1) dialkylamides have not been obtained by this procedure. In fact, detailed work is restricted to silylamide derivatives. The first, [Cr{N(SiMe3),>,(THF),5, is trans planar (16), an uncommon stereochemistry (Table 41) for chromium(I1); and it has a magnetic moment of 4.93 BM at 298 K.l12 Other presumably planar complexes in which one or more THF molecules are replaced by other bases are exemplified in Scheme 12. Some solids, e.g. [Cr{N(SiMe3)2)z(Et20)2], decompose even in an inert atmosphere, but solutions in organic solvents are stable for several weeks so dietherate solutions especially were used in other reactions. The compounds were characterized by analyses and reflectance spectra, and it was necessary to use carefully prepared anhydrous CrClz in the preparations. The reflectance spectra were not assigned and no magnetic measurements were reported.
The nitrosyl [Cr(NO) {N(SiMe3)2}3]is diamagnetic and has a pseudotetrahedral structure (135). It can be formally considered to contain Cr" and NO+ (v(N0) = 1698 cm-') and is the only exam le of a crystallographically confirmed tetrahedral Cr" compound (Section 35.4.2.6).1lP
35.3.3.7 Thiocyanates The deep blue solutions produced on the addition of thiocyanates to aqueous Cr2+ decompose on standing or concentration. Nevertheless, thiocyanato complexes can be
730
Chromium PY
Cr[N(SiMe,),]2(Et2r))Z stock solution
,
.*
CrClz dcticicncy
4 C O N ( SiMe,),(THF), blue
~
Crcl,
I
THF
? RCrN(SiMe,),(THF)
ClCr(0R)
R = Me, Et, Pr, Pr', Bu' ALSOC T [ N ( S ~ M ~ ~ )L~=] ~Bu'CN, L ~ , 3,3-dimethoxyethane, etc Scheme l2
deep blue s o h .
NH,SCN l4:l)
I
I I
Crz+(aq)
7
[CatIBr
H2O
NaSCN (q)in excess, 0 "C
Na3[Cr(NCS),].9?H,0 deep lilac-blue
from CrC1,.4H20 or CrBr2.6H20 CrS04(aq)
Ba(SCNI,
'Cr(SCN)2'
+ BaSO4L
For Cat see Table 18 Scheme 13
crystallized if the cations are Na+ (but not NHt , K+, Cs+ or Ba2+) or various substituted ammonium ions (Scheme 13). The pentathiocyanato complex Na3[Cr(NCS)5]*9-11H20 is thought from its magnetically dilute behaviour and IR and electronic spectra to contain N-bonded NCS groups in a square pyramid around the metal ion with a water molecule possibly in the sixth position. With organic cations, tetrathiocyanatochromium(I1) salts crystallize, and several were isolated in two forms dependent upon the experimental procedure: NBua, brown or blue; and with or without ethanol hexH+, NEt; and NPrd. One of each pair of complexes, and all the complexes which were obtained in one form only, are antiferromagnetic since their magnetic moments are below the spin-only value and are temperature-dependent. Thiocyanato-bridged structures have been proposed and this is compatible with the complexity of the v(CN) absorptions and reflectance spectra. Dissolved in acetone, both NBu: salts and the NMe: salt give high-spin magnetic moments showing that the low moments of the solids arise from intermolecular antiferromagnetism. Tetrahedral anions are excluded by the low extinction coefficients. Those which are magnetically dilute i.e. [ ~ ~ x H ] , [ C ~ ( N C S )[NEt4]z[Cr(NCS)4].EtOH, ~], [NPr4I2[Cr(NCS)4]EtOH and brown [NBQ],[C~(NCS)~], probably have planar [Cr(NCS)4]2- anions with bridging prevented by the cations or coordinated EtOH molecules (Table 18).l14 The tetragonally distorted ion [Cr(NCS)6]4- has been identified in molten KSCN,'I4 and a few mixed ligand complexes containing thiocyanate are known (Sections 35.2.2.1, 35.3.3.4 and 35.3.10.2). Thiocyanate will displace H20 from [Cr2(02CMe)4(HzO)2](p. 746).
Chromium
731
Table 18 Thiocyanatochromates(I1)
Na3[Cr(NCS),].9- 1lHzO Deep lilac-blue
4.5
[PYHIz[Cr(NCS),l Green-brown [NMe4l,[Cr(NCS)41 Pale blue [NEt4IZP(NCS)41 Mauve [NEt,],[Cr(NCS),] .EtOH Blue [NPr,l,[WNCS),I Bright blue [NPr,],[Cr(NCS),].EtOH Mauve [NBu,],[Cr(NCS),] Brown Blue
0
2085
81Ow
16 800 "17 200
Na3[Cr(NCS)5].MezCO-6H,0 4.31 Pale blue [enHzI[Cr(NCS)41 Grev-blue [heiH]2[Cr(NCS),]b Blue [hexH],[Cr(NCS),].EtOHb Blue
4.5 3.55
77
4.24
3.36
80
4.75
4.75
0
4.74
3.92
70
810 724
-
4.29
3.27
90
3.78
3.36
35
3.81
2.98
90
Zll8sh 2085 2118 207Dsh 2090 205Osh 2080 2050sh 2130sh, 2115 2105,2075 2120sh 2050 2095
4.38
3.91
38
2080
18000
4.63
4.63
0
2080
17 OOO
4.47
3.47
82
4.69
4.66
0
2115 2075 2060
808 795sh 826
4.76 4.30
4.75 3.36
0 90
2075 2126 2090sh 2070
812 790sh
13 5OOsh
17 7U0 16 800
12 OO0 17Ooo
"17 400
16600sh
15 OOO 810
17 400
810 793
16700
15 800
19 200 18 OOO
15 OOOsh 14 500
'Liquid nitrogen temperature.
hex = hexamine (hexamethyleneretrammine). L = 5,7,7,12,14,14-hex~e~~1-1,4,~,1l-tetraazacyc1otetradeca-4,11-~ene.
35.3.3.8 Polypyrazolylborates and carboranes On the addition of anhydrous CrBr2 dissolved in warm ethanol to a warm solution of the [NE4]+ salt of the appropriate polypyrazolylborate (17) in the same solvent, crystals of the complexes [ C~ {H,B (PZ)~-~}~] (n = 0, 1 or 2 appear) (Table 19).l15 These are moderately air-stable, and the ethyl derivative [Cr{Et2B(pz)z}2]2,which was obtained from [Crz(OzCMe2)4] and Na{Et2B(pz)2) in THF on which hexane was layered to induce crystallization, is stable to air.'16 An indazole (a 4,5-substituted pyrazole) derivative [Cr(H2B{4,5-(5-N02benzo)pz}2)2] is also known (Table 19). The isolation of [Cr{H€3(p~)~)~] and [Cr{B(p~),}~lfrom aqueous solution117could not be repeated."'
The complexes [ C ~ ( H B ( ~ Z ) , and } ~ ] [C~{B(PZ)~}~] are high-spin, with similar reflectance and solution spectra indicative of distorted octahedral structures, which implies that B(pz); is ) ~ ] ~[ ~ C'~ ( E t ~ B ( p z ) ~ (18)ll6 } ~ l are planar, acting as a tridentate ligand. Both [ C T { H ~ B ( P Z ) ~ and the latter with the Et groups positioned protectiveIy over the C I ' atoms. This stereochemistry is unusual for Cr" (Table 41), and the extreme distortion is manifest in the reflectance spectrum of [ C ~ { H , B ( ~ Z ) ~ }which ~ ] , has a band at 20500cm-l, much further to high wavenumber than its less distorted six-coordinate analogues, and an iil-defined absorption at 11000 cm-'. coc3-X'
732
Chromium Table 19 Polypyrazolylborates Pe
f93 K
[Cr{B(pz),hl Yellow [Cr{HB(pz),),l Light green [Cr { H2B (PZM 21 Orange-red ICr{Et,B(Pz),I,l Bright orange [Cr(H,B {4,5-(5-N0,benzo)pz}J2]
4.90
a E
I
(3M)
Complex
Reflectance spectra (cm-')
4.80
11500sh 18 500 (33)a 11100sh 18 100(24)" 20500
-
-
4.85
13900
4.15
aoa
11 12 OOOsh 10 700 10 500
11 OO(Xv,br 11 700w,br
Comment
Ref.
Distorted octahedral Distorted octahedral Square planar Cr-N, 2.055, 2.069 A Square planar Cr-N, 2.058, 2.061 8, Distorted octahedral
I 1 1 2
3
(dm3mol-' cm-', in dichloroethane).
1. P. Dapporto, F. Mani and C. Mealli, Inorg. Chem., 1978,17,1323. 2. F. A. Cotton and G . N. Mott, Inorg. Chem., 1983,22, 1136. 3. 2. A. Siddiqi, S. Khan and S. A. A. Zaidi, Synth. React. Inorg. Metal-Org. Chem., 1982, 12, 433.
r-=
(18)
Magnetic data for the low-spin, air-stable chromium(I1) metallocarborane sandwichll' compound [NEt4]2[Cr(C&0H12)2]have been compared with those for chromocene.
35.3.3.9 Nitriles The acetonitrile complexes [CrX2(MeCN)2] (X = C1, Br or I) may be planar or tetrahedral monomers or halide-bridged polymers, and there is no clear evidence. The chloride and bromide are high-spin at room temperature, but variable temperature studies have not been carried out; the reflectance spectra are very similar, and typical of Cr" in six-coordination presumably achieved through bridging halide. The much higher wavenumber of the main band in the spectrum of [Cr12(MeCN),] must be due to greater distortion suggestive of a planar structure (Table 20).lL9 The addition of tetraalkylammonium halides to solutions of the chloride and bromide in acetonitrile and ethanol causes shoulders at ca. 1000Ocm-l to be replaced by moderately intense bands ( E = 45 dm3 mol-' cm-I), which were thought to be due to the formation of distorted tetrahedral anions [Cr&]'-. In view of the results in Section 35.3.7.3 it is likely that the changes are due to the formation of t r a n ~ - ~ C r ~ ( s o l v e n tspecies. )~]~The complexes [CT(M~CN)~] (BF& and [Cr(MeCN)6](BF4)2have been prepared electrolytically (Table 20).82 35.3.4
Phosphorus and Arsenic Ligands
35.3.41 Mono- and bidenrate P donor ligands Strongly basic aliphatic tertiary phosphines form blue solutions with CrXz (X = C1 or Br) in benzene or xylene and it is believed that the equlibrium (12) is set up. [CrCl,(PR,),] blue
[CrC1,(PR3)] + PR3 (R= Et, Pr, Bu) white
Chromium
733
Table 20 Complexes of Acetonitrile and Chromiurn(I1) Halides Complex
Comments
+
CrCl, in EtOH MeCN; &iSK = 4.81 BM Reflectance: 13 300,9800sh (cm-') MeCN soh: 13 200 (35),a 10OOOsh CrBr, extracted with hot MeCN; pzgK= 4.85 BM Reflectance: 13 200, 9600sh (cm-') MeCN soln: 11 800 (SO),a 10 OOOsh CrI, in MeCN reduced with Zn/Hg Reflectance: 16 200, 10 ooosh MeCN soln: 15 400 (4)," LO OOO (1) Electrolysis with (sacrificial) Cr anode in MeCN-HBF,(aq); bridging BF;? Electrolysis with (sacrificial) Cr anode in MeCN-HBF,-Et,O
The only solid isolated was white [CrC12(PEt3)],, and the system is difficult to handle.'" A related complex [CrC12(PMe3)3]has been suggested as an intermediate (Sectiorn 35.2.3.1). Greater success has been achieved with PMe2Ph in hexane since the complexes [CrX2(PMe2Ph)2] have been prepared with good analyses and magnetic susceptibilities approximating to high-spin chromium(II).121 Polymers containing dppe and tetramethyldiphosphine have been prepared from [CrC12(MeCN),] or CrClz (Scheme 14); these phosphines also combine with coordinatively unsaturated Crl' in a silica surface. 122 Chromium(I1) surface compounds have catalytic activity and can be used for gas purification etc.lZ3The complex [ C ~ C ~ , { O - C ~ & ( P M ~has ~ ) ~been } ~ ] formed electrochemically in MeCN but not isolated, and the C?'/C?' potential determined. 124
MeCN
CrCl,
Me2PBie2
[CrCI,(Me,PPMe,)], dark brown Scheme 14
The reactions of [CrClz(dmpe)z], from which [Cr(Nz)z(dmpe)z] is prepared, are given in Scheme 8, Section 35.2.3.1. Included in Scheme 8 are the dimethyl compound, trans[CrMe2(dmpe)2] (19), which is an unusual Cr" alkyl (others are the diamagnetic, quadruply bonded species in Section 35.3.5.5), and the hydride [CrJ&(dmpe),], which is the first example of eight-coordinate CrIV (Section 35.5). The tetrah dride is diamagnetic, and the coordinated hydrogen appears as a binomial quintet in the 7H NMR spectrum, in agreement with a dodecahedral structure.59 D
Chromium
734
The chromium(I1) hydride [CrH2{P(OMe>,},] is fluxional and the seven-coordinate structure (20) is likely from an analysis of the 31Pand 'H NMR spectra (Table 21).'" Some mixed ligand complexes with organic isocyanides are given in Table 3. Table 21 Complexes of Chromium(I1) and (IV) with P Donor Ligands" Complex
Re&
Comments
Chromium(I I ) [CrXz(PMe,Ph),l Blue green [CrMeAdmpe),l Red-orange [CrCl,(dmPe)zl Yellow-green [CrCl,(dPPe)l [CrH,{P(OMe),M Off-white Chromium(N) [CrH.i(dmpe),l Yellow
From CrX, (X = C1, Br) and PMe,Ph in hexane, high-spin
1
M.p. 195°C (dec.), p>k = 2.7 BM, vcI3(CrMe),2780 crn..', NMR, 'H: 6 0.9 (PCH2, s, W,, = 260 Hz).-28.6 (PMe,, s, W,,= 310 Hz) (PhH-d,, 25 "C), Cr--C,2.168; Cr-P, 2.342 and 2.349 A, isomorphous with [Cr(N,),(dmpe) ] M.p. 270°C (dec.), p:: = 2.76BM (soh.) NMR, 'H: 6 -13.1 (PCH,, s, W,, = 240 Hz), -33.5 (PMe,, s, W,, = 310 Hz) (PhH-d,, 25 "C) Also [CrCl2(dppe),.,(Me,CO)1, [CrCl,(dp~e),.,(MeCN)~l and [CrCI,(Me,PPMe,)], Scheme 14 [Cr(P(OMe),},] in C,H,, or toluene and H, at 1 atm, sevencoordinate, (20)
2
M.p. 130°C (dec.), diamagnetic, NMR, 'H: S 1.23 (PMe and PCH,, s), -6.91 (Cr-€I quintet, JFAH= 56.1 Hz); "Pf'H}: 6 78.8 (PhH-d,, 25 "C), v(Cr-H), 1757m,1725s,1701m cm-', Dzd dodecahedral geometry: Cr-P (av), 2.255, Cr-H (av),
2
2 3 4
1.57 A
a
For chromium(II1)complexes see Section 35.4.3.
1. W. Seidel and P. Schol, Z. Chem., 1978,18,106. 2. G. S. Girolami, J. E. Salt, G. Willrimon, M. Thornton-Pett and M. B. Hursthouse,J. Am. Chem. SOC., 1983,105, 5954;J. E.Salt, G. S. Girolami, G. Wilkinson, M. Motevalli, M. Thornton-Pett and M. B. Hursthouse,J. Chem. SOC.,Dalton Tram., 1985,685;G. S.Girolani, G. Wilkinson,A. M. R. Galas,M. Thornton-Pett and M. B. Hursthouse,1. Chem. SOC., Dalton Trans., 1985, 1339. 3. 5. Ellermann, K.Hagen and H.L. Gauss, 2. Anorg. Allg. Chem., 1982,487,130. 4. S. D.Ittel, F. A. Van-Catlcdge and C. A. Tolman, Inorg. Chem., 1985,24,62.
353.4.2 o-Phenylenebisrdirnethylamine)
Diarsine forms the low-spin, trans octahedral chromium(I1) complexes [CrX2(diars),] (X = C1, Br, I); the magnetic moments are in the range 2.85 to 2.99 BM and independent of temperature (84-298 K). Reflectance spectra have been recorded, but no assignments made.40 35.3.4.3 Tetradentate 'tripod* ligands A few five-coordinate complex cations, apparently of trigonal bipyramidal stereochemistry, are known. These contain coordinated halide and a tripod-like quadridentate ligand, which can be a P, mixed P,N or N donor: [CrX{P(CH,CH,PPh,),)]+, [crx{P(2-c6H,PPh2),}]~, [CrBr {N(CH2CH2PPh2)[CrX{N( CH2CH2PPh2 3 [C~X(N{CH~CH~P(C&I I)&)] , (CHzCH2NEt2)2}] and [CrBr { N( CH2CH2NMe2)3}]+. 40~1 'To prepare the complexes, the anhydrous chromium(II) halide in butanol or ethanol is added to the ligand in the same solvent or in l,Zdichloromethane, and the cations isolated as BPh4 or PF6 salts (Table 22). Attempts to prepare complexes of N(CH2CH2AsPh2)3127and the linear quadridentate ligand (-CH2PPhCH2CH2PPh2)2 have not succeeded. The complexes are high-spin. As [CrBr{N(CH2CH2NMe2)3}]Bris isomorphous with its trigonal bipyramidal cobalt(I1) analogue , this stereochemistry is suggested for the chromium(I1) complex too."* In the cobalt(I1) complex the bromide lies along the three-fold axis and the symmetry is Chr rather than All complexes are 1:l electrolytes, and there are close resemblances between the solution (1,2-C6H4C12)and reflectance spectra, which consist of a broad intense band with a shoulder or less intense band to higher frequency; thus the five-coordinate structures are retained in solution. The electronic bands are assigned to the 5A;+ 5E'and 5Ai+ 'E" transitions. The bands are to higher wavenumber in the spectra of the phosphine complexes, which also exhibit extinction coefficients many times greater than those +
+
'"
Chromium
735
Table 22 Complexes of Tetradentate P, PN and N Donor Ligands Complex
[CrX(P(CH2CH2PPh2),}]BPh4 Blue
[CrX(P(2-C&4PPh2)3}]BPh4 Green
[CrX{N(CHzCHzPPh2),)]BPh4
c1
4.49
Br
4.70
I
4.70
c1
4.85
Br
4.82
I
4.89
c1
4.84
Br
4.88
c1
4.62
14.7 15.1 (560) 14.3 14.8 (585) 14.3 14.4 (440) 13.4 14.1 (762) 13.6 13.7 (360) 13.3 13.4 (830) . , 13.3 13.3 (546) 13.2 13.2 (415) 13.5 13.8 (510) 13.0 13.3 (530) 12.7 13.15 (520) 11.6 .
[CrBr{L}]BPh, (L = N(CH,CH,PPh-J( CH,CH,NEt& [CrBr{N(CH2CHzNMe,)d]Br
Br
4.w
I
4.95
-
4.97 4.85
r11.0 0.8 (84)
r
19.0sh
1
18.lsh 18.lsh 18.7 18.7sh
2
16.4 16.4 (370) 15.6 15.8 (340) 16.7 17.0(470) 16.1 16.0 (415) 15.4 15.6 (380) 14.1
2
3
=13.O
14.0 (32)
Reflectance spectra and solution spectra ( E , dm3mol-' cm-', in parentheses). Independent of temperature (77-293K). 1. F. Mani, P. Stoppioni and L. Sacconi, J . Chem. Soc., Dalton Trans., 1975, 461. 2. F. Mani and P. Stoppioni, horg. Chim. Acta, 1976,16, 177. 3. F. Mani and L. Sacconi, Znorg. Chim. Acta, 1970, 4, 365. 4. M. Ciampolini, Chem, Commun., 1%6,47. 5. M. Ciampolini and N. Nardi, Inorg. Chem., 1966, 5, 1150. a
of [CrBr{N(CH,CH,NMe&}]Br, presumably because of the greater covalent character of the Cr-P bonds. The electronic spectrum of this last compound is quite different from those containing CrN6 or CrX2N4chromophores (Section 35.3.3.2), and square pyramidal structures would be expected to produce electronic spectra resembling those of the CrX2N4chromophore; thus the trigonal bipyramidal structures are well established, although not yet confirmed in detail crystallographically. Other five-coordinate species are given in Table 42.
35.3.5
Oxygen Ligands
35.3.5.1 Aqua complexes
(i) Hydrated chromium(I1) salts Hydrated chromium(I1) salts can be prepared by the methods in Section 35.3.1.2. They are high-spin and magnetically dilute except for the monohydrogen phosphate, which is weakly antiferromagnetic (Table 23).130,131 The fluoride CrF2.2H20 has been little studied.85 The reflectance spectra are typical of Cr" in distorted six-coordination. Only one crystal structure determination has been reported, that of CrCl2.4H20; there are four normal length Cr-0 bonds to water molecules in a square plane, and two long trans Cr-C1 bonds (Table 34).'32 Chromium(I1) sulfate pentahydrate is isomorphous with CuS04.5Hz0, and the parameters D =2.24cm-l, E=O.lOcm-l, g, =g, =2.00, g, = 1.96 gave the best fit to its ESR spectqm. 133 Aqueous Ct' solutions have a broad asymmetric absorption at 14000cm-I and a weak shoulder at 9500 cm-' due to tetragonally distorted [Cr(HzO)6]2+ions.'31
736
Chromium Table 23 Magnetic and Spectroscopic Properties of Some Chromium(I1) Salts Reflectance spectra (an-')
Complex CrS0,.5H20 Blue CrC12.4H20 Blue CrBr,.6H,O Blue CrI,.6H,O Blue Cr(C1O,),.6H20 Blue CK(HPO,) .4H20 Blue CrXOH.H,BO, (X = C1, Br, I>" a
v2
V1
14 300
10 300sh
14 700
10 OOOsh
14 900
io 500
Pef7 (BM)
95K
295K
4.98
4.98
4.92 4.92 4.94 4.94 4.98 4.90 4.96 4.98 5.02
4.13
4.91
4.98 4.94 4.99 =14 300vb
Ref.
1 2
10 OOOsh
14 100
-
-
( e = 660) ca. 4.5
Said to be air-stable, but chromium(I1) borate is pyrophoric.
1 . A. Earnshaw, L. F. Larkworthy and K. S. Patel, J . Chem. Soc., 1965, 3267. 2. J. P. Fackler, Jr. and D. G. Holah, Inorg. Chem., 1965,4, 954. 3. A. J. Deymp, Inorg. Chem., 1964, 3, 1645.
(ii) Hydrated double sulfates Chromium(I1) double sulfates can be crystallized by the addition of ethanol to concentrated aqueous solutions containing equimolar quantities of the components. The hexahydrates A2S04.CrS04.6H20 (A=NH4, Rb or Cs) are high-spin and have reflectance s ectra compatible with the presence of tetragonally distorted [Cr(H20),I2+ ions (Table 24).lP4The potassium and sodium salts crystallize as pale blue dihydrates with similar properties, and from the splittings of the SO:- absorption bands in their IR spectra it seems that coordinated sulfate anions are present. Table 24 Double Sulfates
(BM) 3mf ~ O K Pe
Compound (NH4),SO,CrSO4-6H2O Rb2S0,.CrS0,.6H20 Cs2SO4CrS0,.6H20 Na2SO,.CrSO,*2H,O K2SO4CrSO4.2H,O Cs,SO,CrSO4.2H,O Cs,SO,.CrSO, (N,W,)2S04~CrS04~H,0
Biue Blue Blue Blue Blue Violet Dark blue Blue
4.88 4.95 4.93 4.89
4.77 0.88 4.50 4.89
4.85 4.94 4.90 4.86 4.69 0.48 4.20
sp) 0 0 0 4 4 -
16 5
Reflectance spectra (cm-') v2 V1 14000 14500 14100 14300 14900 18000 13200
8500 8800 9100 10500sh 11500sh 12400sh
The cesium salt is unusual; it loses water readily at room temperature in vacuum or over P4Ol0to give a violet dihydrate. This dihydrate has a very low magnetic moment and its IR spectrum contains bands exactly as expected for bidentate sulfato groups. Consequently, it is proposed that the violet dihydrate has a sulfato-bridged dinuclear structure analogous to that of [Cr2(02CMe)4(H20)2] (Section 35.3.5.5). Complete dehydration affords dark blue CsZS04-CrS04. Hydrated and anhydrous chromium(I1) hydrazinium sulfates (N2H5)2Cr(S04)2are known. They are air-stable, and in the latter (from isomorphism with the Zn" complex) each metal ion is coordinated to the nitrogen atoms of two hydrazinium ions and four 0 atoms of bridging sulfato anions.13*
Chromium
737
35.3.5.2 Alcohols, alkoxides and aryloxides (i) Alcohols Anhydrous and hydrated chromium(I1) halides are soluble in alcohols, and these are useful readtion media. The hydrated halides can be dehydrated by heating with 2,2-dimethoxypropane or triethyl orthoformate and alcoholates remain on removal of the solvent, but none have been characterized. The spectra of CrC12 in EtOH and MeOH have been r e ~ 0 r d e d . l ~ ~
(ii) Alkoxides and aryloxides Many alkoxides have been prepared by alcoholysis of [Cr{N(SiMe3)2}2(THF),Iand similar compounds (Section 35.3.3.6). The general reaction is represented by equation (13), in which L and L' are Et,O, THF, py or Bu'CN or various combinations of them, and ROH is an alcohol or a phenol [R =Me, Et, Pr, Pri, Bu", Bus, But, CsHI1, CHMePr, CHEt2, l-adamantyl, CEt,, CPh3, Ph, 2,6-Ph(B~')~, 2,4,6-Ph(B~')~ and 2,4,6-Ph(Ph3)1. The solvent can be a hydrocarbon, Et20, THF or a mixture of these. Some complexes of the type [Cr(OR)2(ROH)]were obtained from the phenols and the alcohols with bulky aliphatic groups. It is necessary to dry the alkoxides carefully to obtain materials free from solvent. Alkoxides containing branched alkyl chains and aromatic groups show reasonable solubility in organic solvents, but the alkoxides of primary alcohols are insoluble.'37
+ 2ROH
Cr{N(SiMe,),},(L)(L')
,zz,.Cr(OR), +
2HN(SiMe,), f L -I-L'
(13)
Many alkoxides have been prepared from chromocene in THF or hydrocarbons (equation 14). Only partial replacement of cyclopentpdienyl by R occurs with Me3COH and silanols (equation (15)).
-
[Cr(C5Hs)2]+ 2ROH R =Me, Et, Pr, Pri, Bun,
[Cr(OR)2], + 2C5H6 R = Me, Et, Pri, Me3CCH213'
(14)
[Cr(CSH5)2] + ROH ~[C~(GHS>(O+ R )C5H6 I~ (15) R = But, Me3Si: Ph3Si139 The product from 3u'OH is an antiferromagnetic, butoxy-bridged dimer (21) with a non-planar central (Cr-p-O)2 moiety and a C r - C r separation of 2.65A. The magnetic properties (pea= 1.8s BM at 333.5 K) suggest that there is one unpaired electron per metal atom. It is thought that the compound may contain a C r 4 r single bond. The dimer structure is preserved by reaction with CO, NO and CF3C%CCF3 but with C 0 2 in THF, [Cr(CSH=J2] and [Cr2(02CBu')4(THF)z] (CsCr-bonded, Section 35.3.5.5) are formed. Antiferromagnetic interaction has been found in Cr(OMe)2 (prepared from CrC12 and LiOMe in MeOH as well as from chromocene) and other alkoxides, and polymeric O-bridged structures have been proposed.lm Me
Me \ /Me
I
c I
I Me/ 1 'Me Me C
When the more sterically hindered alkoxides dissolve in donor solvents such as TKF, adducts [Cr(0R)2(THF)2] form and these are probably tram planar like [Cr(N(SiMe,),},(THF)J. There are considerable colour variations among the alkoxides and their adducts, so there must
Chromium
738
be considerable structural differences imposed by the different R groups. The magnetic and spectroscopic properties have not been investigated in any detail.137 The complex [C~(OCBU:)~.L~CI(THF)~], which was obtained according to Scheme 15,77 has a unique structure. The metal ion is three-coordinate, and the donor atoms outline a distorted T (22). It is not obvious how the reduction occurs because the BuLi is added in stoichiometric quantity to the alcohol and this solution then added to 'the CrC13 suspension. Since LiCl is eliminated on the addition of hexane to give [Cr(OCBu:),(THF),], Scheme 15 may provide a simpler route to chromium(I1) alkoxides than that from the very air- and moisture-sensitive complex [Cr{N(SiMe3)z}Z(THF>2]. BuiCOH (Et@)
+
BuLi (hexane)
CrC13. 1 HF T , ~ ~ ~
Cr(OCBu\),.LiQ(THF), ~ , emerald green
1
hcxanc
[C~(OCBU;),(THF)~] + LiCl m.p. 196-200 "C, blue green Scheme 15
On addition to halogenated solvents the alkoxides give intensely coloured solutions, stable to oxygen and believed to contain Cr"'-C species.'37 The p-oxoalkoxide [Cr{OAl(OPr')2}z] dissolves in benzene and cyclohexane to give blue solutions in which it is
35.3.5.3
b-Ketoenolates, tetrahydrofuan, ureas and biuret
(i) /3-Ketoenolutes Complexes of acetylacetone (acacH), benzoylacetone (bzacH) and dipivaloylmethane (dpmH) have been reported. The acetylacetonate [Cr(acac),] has been prepared from chromium(I1) acetate and a c e t y l a ~ e t o n e . ' ~ ~It, ' ~ can ~ also be obtained by the addition of aqueous sodium acetylacetonate to an aqueous solution of chromium(I1) chloride, but in any preparation the yellow solid must be filtered off and dried as rapidly as possible, otherwise the chromium(II1) compound is obtained. Its magnetic moment is 4.99 BM at room temperature consistent with a high-spin d4 configuration.'41 The powerful reducing ability of [Cr(acac),] has been used to prepare iron(I1) and chromium(I1) complexesmof porphyrins and related ligands. On slow sublimationlU of [Cr(acac>,] a mixture of separate purple and light brown crystals was obtained. The cell constants of the former showed them to be [Cr(acac>,], and the latter were [Cr(acac),]. In the chromium(I1) complex the metal atom lies on an inversion centre and has rigorously planar coordination to four oxygen atoms at a mean Cr-0 distance of 1.98A (Table 41). Methine carbon atoms of adjacent molecules lie at 3.05 A above and below the C r 0 4 plane so that there is severely tetragonally distorted octahedral coordination of the chromium atom (23). At this distance there cannot be much Cr-C +bonding. Chromium(II), copper(I1) and palladium(I1) acetylacetonates are isomorphous. It is presumably the weak axial interactions that lead to the poor solubility of the essentially planar [Cr(acac),], but the structure is very different from that found in other metal(I1) acetylacetonates, which are trimers (Ni, Zn) or tetramers (Co)with bridging anions coordinated by oxygen atoms.
C (23)
The benzoylacetonato complex is known only as the black bis(pyridine) adduct [Cr(bzac)z(py)z], which crystallizes when a mixture of the ligands in acetone is added to
Chromium
739
aqueous chrornium(1I) acetate.145It is a low-spin 3d4 compound: pCff= 3.2 BM independent of temperature over the range 295 to 90 K. The IR spectra of the Cr", Ni" and Zn" complexes are almost identical and they are considered to have trans octahedral structures. [Cr(d~m)~ is] prepared from anhydrous chromium(I1) acetate in strictly air- and water-free conditions according to Scheme Attempts to prepare the monothio analogue were unsuccessful. There is no absorption band below 23OOOcm-' in the mull spectrum of [C r(d ~m ) ~] although , the spectrum of a toluene solution has a weak shoulder at 16500 cm-I (E = 12 dm3mol-' cm-') as well as a band at 23 OOO cm-l superimposed on a more intense band in the UV region. It is high-spin (p;gzK=4.84BM in toluene), but the spectrum is quite different from high-spin complexes such as [CrX2(en)2] (Section 35.3.3.2) in which there is moderate tetragonal distortion. This is because [Cr(~lprn)~] is planar, it is isomorphous with planar [Ni(dpm)z], and the extreme distortion has caused the d-d transitions to move to high frequency where metal-to-ligand charge transfer transitions occur. There appears to be no theoretical study of the spectra to be expected for high-spin, planar chromium(II), dpmH + BuLi (THF) (hexane)
-
Lidpm
' [Cr(dpm),] + Li0,CMe
[Cr2(02CW41
-~~~~
71 I. 11.
111.
warm to r t remove solvent extract pentane
[Cr(dpm)J, m.p. 193 "C golden yellow Scheme 16
(ii) Tetrahydrofuran Adducts of chromium(I1) halides with tetrahydrofuran [CrX2(THF)2]can be prepared by extraction of CrClz with the hot solvent,147and by crystallization from solutions of CrBr, in THF14' (Table 25). The latter method did not give solvates of reproducible composition with CrIZ or CrClZ. When dry hydrogen chloride is bubbled through THF over an excess of chromium, the metal dissolves with evolution of hydrogen to give a malxve solution (Cr"' spectrum). When the passage of HC1 is stopped and the mixture heated to 65-70 "C, evolution of hydro en from residual metal and dissolved HC1 continues and the solution becomes bright blue (C spectrum). The light blue crystals which separate after filtration and the addition of more THF could not be characterized because on drying in vacuum white [CrCIZ(THF)], suitable as a source of Cr" for non-aqueous preparations, is f ~ r r n e d . ~ ~ , ~ ~
$
Table 25 Tetrahydrofuran and Substituted Ureas Reflectance spectra (an-') Y2
Y1
11 600 12 700 13 300 13 OOO 14 400
la OOOsh 10 800sh 9400 12 800
~~
Pale green White Pale blue-green Pale blue Blue Pale blue Pale blue Pale blue
Pale blue
4.71
-
4.86 4.39 4.78
4.85 4.79 4.87 4.85
-
-
3.50 4.83 4.90 4.79 4.90 4.86
15
-
-3 -2
13 OOO 13 500
0
-2 0
8400
15 OOO
11 OOOsh
(iii) Ureas and biuret The complexes of urea (ur) and biuret (bi) in Table 25 crystallized when a solution of the ligand in ethanol was added to a solution of the hydrated chromium(I1) halide in the same solvent (an ethanol/2,2dimethoxypropane mixture in the case of [CrC12(ur),]). Complexes of methylurea would not crystallize from ethanol; they were obtained from the halide dissolved in a mixi we of acetone and 2,2-dimethoxypropane, and methylurea in acetone. The reflectance spectra indicate trans octahedral complexes, achieved through chloride bridges in [CrC12(ur)z], which is antiferromagnetic. The positions of the reflectance bands indicate 0 coordination as do the IR spectra. X-Ray powder data show that [CrC12(bi)z]is isomorphous with [CuClZ@i)2],
Chromium
740
which is known to have a trans structure with 'long' Cu-41 bonds and 0 donor biuret. Copper(I1) forms complexes in which anionic biuret is coordinated via nitrogen, but when biuret in alkaline solution was added to aqueous chromium(1I) chloride, immediate oxidation
35.3.5.4 Oxo anions
( i ) Sulfato complexes Sulfato complexes of chromiurn(I1) are known, and the insolubility of the monohydrogen phosphate Cr(HP04).4H20suggests anion coordination (Section 35.3.5.1). The high-spin phosphites CrHP03.H20 (p:;'" = 4.82 BM, 0 = 10") and CrH4P20e-H20 (pzFK= 4.91 BM) have reflectance bands at cu. 13500 cm-' typical of hexacoordinate Cr", and their Raman and IR spectra have been assi ned The loss of water on heating under nitrogen is followed by internal oxidationlreduction. 1 4 1 . The chlorosulfate Cr(SO,Cl), has been prepared from the acid and [Cr2(02CMe)4].It forms adducts Cr(S03C1),L4 (L = MeCN, py, pyNO, acridine, = bipy).'% The formation of chromium(I1) complexes of many oxo anions, e.g. NO;, is precluded by the reducing ability of the metal ion.
(ii) Phosphinato complexes A few chromium(I1) phosphinates [Cr(OPR,O),] (R = Me, Ph, C&Hl7; Rz= Me, Ph) have been prepared from chromium(I1) acetatel'l or aqueous CrC121'2,153and the potassium phosphinate. They have been little investigated because their preparation was incidental to that of the green, phosphinato-bridged and linear chromium(1II) polymers to which they are easily oxidized by air. The n-octyl derivative [Cr{OP(C8H17)20}2]is blue and anhydrous. Since there is a typical Cr" band at 15 400 m-' and the symmetric and antisymmetric POz stretching bands at 1055 and 1133 cm-l are broad, it is believed that the C P is six coordinate through dif€erent types of phosphinate bridge. When heated above 185 "C, there is a colour change from blue to pink, which is reversed on cooling. The diphenyl derivative is pink and a monohydrate, and insoluble in common solvents.'53 The colour suggests a chromium(I1) acetate type structure, but the magnetic moment has not been reported.
35.3.5.5 Carboxylato and other complexes containing C d r quadruple bon& Chromium(I1) acetate monohydrate, discovered by Peligot in 1844, is the best known chromium(I1) compound because it can be precipitated readily from aqueous Cr" solutions by sodium acetate, is stable in air for short periods, and is used as a reducing agent and a source of other chromium(I1) compounds (Table 9). Nonetheless, in its deep red colour, feeble paramagnetism and acetate-bridged, binuclear structure (24) (determined in 1953 and refined in 1971),154,155which are still not completely understood, it differs sharply from other chromiurn(I1) salts (Section 35.3.5.1). Extensive investigations by Cotton and his co-workers have shown that structure (24) is a prototype for a great variety of corn lexes of chromium(I1) (and Mo" and W") in which there are quadruple metal-metal bonds.' Besides carboxylates (24) to (35), the anionic bridging ligands in these complexes are zwitterionic glycinate (X), diethylcarbamate (37), carbonate (a), various 2-oxopyridines (39) to (41), 2,4-dimethyI-6-oxopyrimidine (42) and 2-methyl-6iminopyridine (43), substituted acetanilid0 groups (44)to (51), carbanilido (52), 173-diphenyltriazinate (53) and N , N'-dimethylbenzamidinate (54). In addition , there are organochromium dimers containing 2-substituted phenyl (55) to (58), Me2P(CH& (59) and Me3SiCH2 (60) bridges, the methyl (61) and allyl (62) complexes which are binuclear but do not contain bridging ligands, and the solvated lithium dimers (63)and (64). There are also a few examples with mixed bridging ligands (65) and (a), complexes containing Cr$+ and CP+ units in the same lattice (32), and the mixed metal complex (67). In general only one bridge is illustrated in the structural formulae, and as indicated by the Cr-Cr and Cr-axial ligand distances, these structures have been determined by single crystal methods. Furthermore, there are the tetramers (68)and (69)which contain Cri+ units.
P
Chromium
74 1
Me
L = H20155.185 Cr-Cr = 2.362 8, (2.3532,90 K) Cr-OH, = 2.272 (2.2598.80 Kf L = MeC02H15' Cr-Cr = 2.300 C r - O C ( O H ) M e = 2.306
L = pyridine'60 Cr-Cr = 2.369 Cr-N = 2.335
Me
I
Me
\?
C /-% 0
L =~ . ~ ( p y r a z i n e ) ' ~
I
Cr-Cr i ~ 2 . 2 9 5 Cr-N = 2.314, pyrazine bridges Cr4,' units
L =piperidine''' Cr--Cr
Cr-N
= 1.342 = 2.338
0 . .cri;-+_ Cr.. -0
L =4-cyan~pyridine'~~
C
~
Cr-Cr = 2.288A Cr- --0= 2.327 (intermolecular) (26) [Cr,(O,CMe),]'h'
Cr-Cr = 2.317 Cr-N(py) = 2.322 (25) [Cr,(O,CMe),LI
I
Me (24)
see also ref. 167b
But
I
CF,
Cr-Cr = 2.388 8, Cr-0 = 2.44 and 2.47 (intermolecular) (28) [Crz(02CBut)41'"
Et,0-Cr---&-OEt2 Cr-Cr = 2.541 A Cr-OEt2 = 2.244 (27) [Cr,( 02CCF,),(EtZ0)2]
PhC( 0 H ) O - C d r - O C ( O H ) P h Cr-Cr = 2.352 8, Cr-0 = 2.295 (benzoic) (29) [Crz(0,CPh),(PhC02H)2]'"
H I
I
I
I
*Cr -scr-O Cr-Cr = 2.283 A Cr-0 = 2.283 [CH,(OMe)CH,OMe]
(30) [Cr2(9-anthracenecarboyxlate).,(l,2-DME)]1w
I
L-, CrEEESCr t L L = HZO Cr-Cr = 2.373 A, Cr-OH, = 2.268 C r 4 r = 2.360, Cr-OH2 = 2.210 Two independent dimers
[Cr,(0,CH),(H20),],~10H~0165 L = pyridinelW Cr-Cr
= 2.408, Cr-N
= 2.308
(31) [Cr2(02CH)4(L)21
Chromium
742 G-CH
\
0
I
I
Cr-Cr = 2.451 8, Cr-0 = 2.224 (32) [Cr3(02(33)4H@)2]1u
Cr-0 - + C r m C r Cr-Cr = 2.348 & , = 2.309
O+Cr (dimer of
(33) [Cr,( 2-phen ylhenzoato),] .2PhMe
Et
I
I
I
C,H80-Cr~Cr--OC4H8 Cr-Cr = 2.316 Cr-O(THF) = 2.275'"
(34) [Cr2(2-phenylbenzoate),(THF),]
(35)
I
NEt,
C
o,;+-+o I
!
C r - C I = 2.581 Cr-Br = 2.736
Cr-Cr Cr-N
= 2.384 A = 2.452163
(36) [Cr,(0,CCH2NH3),X,].~H20168 (37) [Cr2(diethylcarbamato)4(NEt,H)2]
H,&Crw~-%r-OH, Cr-Cr = 2.214 A Cr-0 = 2.300158
(38) (NH4)4[Cr,(C03)4(H20)2]-1-2H20
Chromium
743
A II 0 Cr - s C r
I
= 1.889 A (CH,Cl, ~olvate)"~ = 1.879 (unsolvated, 74 K)"'
Cr-Cr Cr-Cr
I
CrzzzCr
6-methyl-2-oxopyridine (39) [Cr,(mhp),l
Cr-Cr = 1.955 A 6-chloro-2-oxopyridine (40) [ C r d ~ h p ) , l ' ~ ~
~r-~r'=2.150A Cr-O(THF) = 2.266 6-fluoro-2-oxopyridine (41)
[Cr,(fhp),(THF)].THFIR6
Me
1
Cr=Cr
' 1 CreCr
Cr-Cr = 1.898 A, form I Cr-Cr = 1.907, form I1 2,4-dimethyl-6-oxopyrimidine (42) [Cr2(dmhp)41173
h
M
e
Cr-Cr = 1.870 A 2-methyl-6-iminopyridine (43) [Cr2(ma~)4117b
Cr-Cr
= 1.873 A, w = 48"
acetanilid0 (44) [Cr2(C,HsNC(0)Me},J'83
Me I
Cr-Cr = 1.937 A, w = 89.7" 2,6-dimethyla~etanilido'~ (45)
I
C m C r
Cr-Cr = 1.949 A, w = 93.3" C r - C l = 3.354 and 3.58Iw
[Cr2{2,6-Me,C,H,NC(0)Me},]-l.5C,H,Me(46) [Cr,{2,6-Me,C,H,NC(0)Me},].2CH2C12
I
(47)
Cr-Cr = 1.961 8, Cr-Br = 3.554 and 3.335Iw [Cr,{2,6-Me2C,H,NC(0)Me),l.2CH2Br2
C,H,O-Cr=Cr Cr-Cr = 2.023 A, w = 88.3" Cr--O(THF) = 2.315lX3 (48)
[Cr,{2,6-Me2C,H,NC(0)Me),l(THF)].PhMe
C,H@-Cr~Cr-OC,H, Cr-cr = 2.221 A, w = 88" Cr-O(THF) = 2.32Ix3 (49) [Cr,{ 2 ,6-Me2C,H,NC(0)Me},(THF),].THF
I
C,H,O-CrsCr
Cr-Cr = 2.006 A, w = 81.7" Cr-O(THF) = 2.350'83 (51) [Cr2{4-NMe,C,H4NC( O)Me},(THF)]
C,H,CLCrmCr-OC,H, C r - C r = 2.246 A, w = 76.2' Cr-O(THF) = 2.350183 carbanilido (52) [Cr,{ PhNC(O)NHPh},(TIfF),].C,H,4
Chromium
744.
I
Me Me \ . &C\ / N N I I Cr =Cr C r 4 r = 1.843 8,
I
I
Cr-Cr Cr-Cr = 1.858 8, 1,3-diphenyltrizinato (53) [Cr,(PhNNNPh)4]'7X
Me0
I
CrSCr Cr-Cr = 1.849 8, 2,4,6-trimethoxyphenyl (56) [Cr2(TMP),1'
= 1 347 8,
2,6-dimethoxyphenyl (55) [Cr,(DMPhl'
Br-- .
CrECr Cr-Cr = 1.8288,
2-methoxy-5-methylphenyl
(57) [Cr,( 2-MeO-S-MeC,H3),]
. & ~ r---Brb ~
Cr-Cr = 1.830 8, Cr---Br = 3.266 (58)
Meq\o
M e 0 9 'OMe I
OMe
CrCCr
Cr-Cr
[Crz(MeNCPhNMe),]1R3
OMe
fi 1
N,N'-dimethylbenzamidinato (54)
2-oxophen yl Li,[Cr,(2-OC,H,),]Br24Et,0'
CrECr
Cr-Cr = 1.895 8, dirnethylphosphoniurndimethylido (59) [C~,((CH,>ZP(CH,),>~]'
SiMe,
AH,-
Me3SiH2C ,PMe, \ /' cr b s L&r / /' \ MeJ' CH, CH2SiMe,
Me
'\\
Cr-Cr
SiMe, = 2.1007 8,
(60) [Cr,(CH2SiMe3),(PMe7>?1'
Cr-Cr = 1.980 8, (63) [Li,CrMe4(THF),] i
Me Me Me Cr-Cr = 1.980 A (61) Li,[Cr,Me,]'
Chromium
745
\
Cr-Cr = 1.862 A bis(2-oxophenyi)di(acetate) (65) [Cr,(O2CMe),(2-Bu'OC,H,),]'
Me
I
C
?4''--b 0 -.Cr-Ma = 2.050 A (CrMo)-0 = 2.548 8, intermolecular infinite chain structure (67) [ C ~ M O ( O ~ C M ~ ) ~ ~ ' ~ ~
\ /
C
I
Cr(lFCr(2) = 2.544 A
Me
Cr-Cr Cr--N
Cr(3)-Cr(4) = 2.554 Cr-O(centre) = 2.005 (av) acetate and oxo bridges
= 2.531 8, = 2.444
(68) [Cr,(0,CMe),tOCH2CHtNMe,),]"6
(69) [Cr4(2-NMe2CH2C6H4)4(0,CMe),0]'7Y
(i) Syntheses The method used by Peligot can'be extended to carboxylates generally (equation 16). Since the complexes with n > 1 were dried in vacuum at 100 "C before analysis, their degree of hydration has not been established. All have low magnetic moments (pee = 0.7 BM) so it is likely that they have the acetate str~cture.'~' 2 C P + 4RCO;
+ 2H20
-
[Cr,(O,CR),(H,O),]~ R = CnH2,,+lrn = 5-7, 11, 17
Water can be replaced by ligands L through recrystallization from L if a liquid, or from an I
746
Chromium
organic or aqueous solvent containing L (L = MeCOZH, piperidine ,15' pyridine, pyrazinelm). Adducts [Cr2(02CR)4bf (R = CMe3, L = pyridine, 2- and 4-methylpyridine7 quinoline and 2-methylquinoline, R = Ph, C4H3S, L = THF; R = C4H30, L = THF, pyridine, 2-methylpyridine, 2-methylquinoline) have been prepared159 directly in THF by Scheme 11 in which RC02H and L replace CF3C02H and Me2C5H3N.95 When ammonia gas is bubbled through a suspension of the acetate in ethanol, ammonia molecules replace the water, but in liquid ammonia the dinuclear structure is destroyed to give the tetraammine (Scheme 17). Unless kept in an atmosphere of ammonia the tetraammine reverts to the dinuclear ammine adduct.161
The water molecules can be displaced by thiocyanate to give [NR&[Cr2(02CR)4(NC)2] (R' = Me, Et; R = Me, Et162)and a complicated reaction between [Cr(NEt2)4]and C 0 2 leads to a diethylcarbamato-bridged complex [Cr2(02CNEt2)4(NEt2H)2](Section 35.5.4.3, Scheme 119).163 Formate gives a complicated series of hydrates according to conditions (Scheme 18),1s7*164-165 and adducts of Cr(02CH)* with urea, THF etc. are known, but their structures have not been determined.15'
purple gel
A
[C~~OZCH)~PY)~]
[Cr3(02CH)6(M20)21 red (Cr;' and Cr2+) Scheme 18
Dimeric carbonato-bridged complexes A4[Cr2(C03)4(H20)2](A = NH4, Li, Na, K, Rb, Cs; A2= Mg) can be reci itatedlBBfrom aqueous suspensions of the acetate, and the ammonium salt (or CrC03)" sukended in ether will react with the stronger acids CF3COzH'64 and CF2HC02H167to give dinuclear carboxylates (equation 17). [Crz(C03)4(HzO)z]4+ 4CF3COZHE$o [Cr2(OZCCF,),(Et~O),]+ 4coz + 4HzO
(17)
Pyridine and 4-cyanopyridine react with [Cr2(02CCF2H)4(Et20)2]to give the mixed oxidation state, oxo-centred complexes [Cr"Cr$I'O( O~CCF2H),(py)3(Et20)] and [C~Cr:n0(02CCF2H)6(4-cNpy),].PhMe (see also p. 869).16' Dichromium(I1) Complexes with zwitterionic glycine bridges can be crystallizedlM from aqueous solutions (equation 18). 2CrX, + 4H3&CH2C0;
H20
[Cr,(02CCH2NH3)~X.,] purple (X = C1, Br)
Crystals of the anhydrous acetate169 and [Cr2(02CB~')4]164 for X-ray investigation were grown by vacuum sublimation. The anhydrous acetate (Table 9) is the source of Cr;+ for the preparation of many water-sensitive dinuclear complexes. In general, the lithium salt (not usually isolated) of the anionic ligand which is to replace the acetato bridges is first prepared from BuLi and the acid. The solid anhydrous acetate is then added to the lithium salt solution, the solution filtered after reaction to remove the lithium acetate, and the [Cr2Y4]crystallized from the filtrate, or, if poorly soluble, extracted from the LiOzCMe (Scheme 19). BuLi/hexane -t YH/THF or Et,O
0T
LiY
Scheme 19
ICrdOFMe)J
[CrzY4]+ Li0,CMe
Chromium
747
These reactions were used to prepare complexes of 2-oxopyridines (39) to (41), 2-substituted phenyl derivatives ( 5 5 4 7 ) and others. One of the first oxopyridine complexes was prepared slightly differently (Scheme 20) .'70
In another variation the dilithium salt was obtained from 2-bromophenol and 'contamination' with LiBr followed by Soxhlet extraction to solubilize the binuclear anionic salt and obtain crystals of Li6CrZ(2-OC&)4Br2-6Et20(Scheme 21) .17'
Scheme 21
The 2-methoxyphenyl complex, [Cr2(2-MeOC6H&], has been prepared from CrBrz(THF)2 (equation 19).17' The yellow, poorly soluble and pyrophoric product is diamagnetic and is likely to be a Cr4* dimer (equation 19). [CrBr,(THF),]
+ 2-MeOC6H4MgBr
1MF
[Cr2(2-MeOC6H4)41
(19)
Oxidation of the metal carbonyl is used in the preparations illustrated by equations (20) to (22),l7>l7' in the third of which a heteronuciear dimer is formed. No reaction occurred between Cr(CO)6 and 2-amino-6-methylpyridine.176 The binuclear nature of red [Cr(PhC(NPh)&] has not been established, and an impure sample of [CrM~(rnhp)~] has been prepared from [ C T M O ( O ~ C M ~ ) ~ ] . ~ ~ ~ Me Cr(CO),+
c]
[Cr,(dmhp),]
reflux
OF1 dimethylhydroxypyrimidine Me
x . ~ ,
(dmhpH) cr(co)6
+ PhC(NPh)(NHPh)
pet. ether
[Cr{PhC(NPh),},]
N, N'-diphenylbenzamidine Mo(C0)6
+ [Crz(OzCMe)41 in excess
(21)
red
Wewho cH2c12+ MeCO+
[C~MO(O,CM~)~I
(22)
pale yellow
In the p r e p a r a t i ~ n ' of ~ ~ the I ,3-diphenyltriazinato-bridgedcomplex an organochrornium dimer was the source of C T ~(equation + 23). [Li(THF)],[Cr,Me,]
+ 4PhNHNNPh mLF[Cr,(PhN,Ph),] + 4CH, f 4LiMe
(23)
Chromocene can also be used as a starting material (Scheme 22).lM Tetranuclear complexes, composed of two Cr;' subunits are made by1'6*179reactions (24) and (25) in which the acetate bridges were not all displaced.
[Cr2(02CMe)d]
+ 2Li(2-NMe,CH,C6H4)
[Cr,(2-NMe,CH2C6H4)4(02CMe)20] (25) orange
748
Chromium [cr*(02cB’Jt)41 red
PhCO211
Bu‘COZH bemcnr
[c4~s-c5H5)21
’ [Crz(O,CPh),(PhCO,)zj
(1.2-DME)
[Cr2(9-anthracenecarboxylate),(DME) J
[Cr2(2-phenylbenzoate),].2PhMe
red
red Scheme 22
Several organochromium(l1) dimers containing short C E C r bonds (55-64) have been included for completeness. For their preparations the companion series’ should be consulted. Complexes (65) and (66)I9O contain mixed carboxylato and organo bridges. (ii) Structures Crystallographic investigations demonstrate that the length of the Cr--Cr quadruple bond is primarily dependent upon whether axial ligands are present and, if so, upon their nature and number. This is apparent from a comparison of the Cr-Cr separations found for complexes (25) to (38), (41), (46)to (52) and (67), and, without axial ligands, (39), (a), (42) to (45) and (53) to (66).Those without axial ligands and r(Cr-43) < 1.9 A are consideredlS6 to contain ‘supershort’ bonds compared with those in the carboxylates (2.28-2.55 A). The Cr-Cr separations in the tetramers (68) and (69) are equal to the greatest found in the dinuclear carboxylates. There is considerable variation of r(Cr--Cr) within the tetracarboxylate series mainly dependent upon the donor ability of the axial ligands. In [Cr2(02CR)4(R’py)2](R = But, Me, H, CCLH, or CHF,; R’ = H, 4-NH,, 4-CN etc.) the Cr--Cr and Cr-N distances are inversely related,167band axial coordination of oxindole in [Cr2(oxindolate)4(oxindole)2] produces a long C E C r bond (2.495 A).167cNo carboxylates without axial ligands are known: in anhydrous compounds oxygen atoms of neighbouring dimers bond axially (26) to give polymers; and substitution in the bridge is too distant to be effective (30) or is nullified by the final configuration (33). In (33) the four 2-phenyl groups are directed towards the same end of a CrECr unit and block association [though not coordination of THF (34)], but dimers of dimers are formed through association at the other end. Carboxylates producing steric hindrance at both ends of the Cr2 units have not been devised. Careful choice of bridging ligand is necessary to prevent axial donation or association and produce very short Cr-Cr bonds. In the acetanilido complex (44) this is achieved: the unlike ligand atoms on each chromium atom are trans and with a small torsion angle w about the NPh bond the axial positions are blocked. Dichromium compounds (46)to (50), in which w has been increased to approximately 90” to open up the axial positions, have been obtained by introducing 2,6-dimethyl groups. Steric effects then force the rings to be perpendicular to the 0-Cr-N plane. Electron donation by a 4-NMe2 substituent acts similarly. Access by axial ligands then lengthens the C r X r bond in the series of complexes (48) to (51) compared with (44)without change in bridging ligand or overall geometry. Complexes (44)and (45) have no axial ligands and the smallest Cr-Cr separations, and the axial CH2X2molecules in (46)and (47) interact at the most very weakly with the metal. Axial donation modifies the C r - C r distance as in the carboxylates, but in the acetanilido compounds the bond separation without axial ligands is known. Metal-metal separations in dimolybdenum complexes are not sensitive to axial donation. There are no axial ligands in the Zoxopyridine, 2-iminopyridine and 2-phenyl derivatives which have the shortest Cr-Cr bonds. The bromide ions in (58) are off-axis and beyond bonding distance. The nature of the bridging ligand is also important; the quadru le bonds (the ranges are given) shorten with type of bridging bidentate ligand in the order: 1 2
.
2.55-2.21‘8,’ 2.35-2.01 8,
,
1.96-1.87
8, 1.87-1.848,
1.868,
1.85-1.83
8,
Chromium
749
The presence of axial ligands is signified by (ax), and the formally negatively charged atom is shown, but with N-C-0 bridges the formal charge is on N or 0 according to whether the anion is from an amide or an hydroxypyridine. In addition to the 2-phenyl derivatives (55) to (58) with CCO bridges, there are organodichromium species (59) to (64) with C r - C r separations in the range 2.1 to 1.89& two of which do not contain bridging ligands. In the mixed ligand complexes (65) and (66) (CCCN bridge) the Cr-Cr distances are very short presumably because the C donor dominates. The general trend is that the Cr-Cr bonds shorten as the donor strength (basicity) of the anions increases: 0- < N- < C-. In keeping with the so far perceived trends, the longest bond (2.541 A) is in [Cr2(02CCF3)4(OEt2)2](27) where there is axial ligation and the bridging ligand the least basic, and the shortest (1.828 A) in [Cr2(2-MeO-5-MeC6H3)4](57). The ranges of metal-metal and N-C-O(ax) series overlap so that dichromiurn compounds distances in the 0-C-O(ax) no longer fall into two distinct classes, i.e. carboxylates and others with much shorter Cr-43 bonds. In order to compare C r - C r bond lengths with other bond lengths a formal shortness ratio (FSR) for a bond A-B has been defined:
The FSR values for complexes (57) and (27) are 0.771 and 1.07, compared with 0.786 for NEN. 156 Chromium resembles molybdenum and tungsten in its quadruply bonded compounds but differs in not forming dimetal compounds of lower bond order. Although salts of the stable binuclear anions [Mo2CisI4- and [Moz(SO4)4I4- are well known, chromium(I1) forms very different chloro complexes (Section 35.3.7.3), and the violet, nearly diamagnetic sulfatobridged dichromium complex CS~[C~Z(SO&,(H~O)~J.~H~O, obtained only by heating CszSO4~Cr2SO4.6H20, re-forms [Cr(H20)6]2+with water.16z (iii) Theoretical description of C e C r bonds
The Cr-Cr bond lengths vary from supershort bonds of about 1.85A to long bonds approaching 2.6A. This is a wide range for what is formally considered a quadruple bond. Molecular orbital calculations of electron density in the formate [Cr2(02CH)4]as a longbonded example and in ‘CrZ(H2P(CH2)2}4’as a model for the supershort Cr-Cr complexes [Cr,{ (CH&P(CH&},] and [Cr,(mhp),] show agreement with the experimental electron densities obtained by high resolution low temperature single crystal X-ray investigations on [Cr2(02CMe)4(H20) and [Cr2(mhp)4].’87 The contraction of the Cr-Cr bond in (25; L=HzO) by 0.009 on cooling to 90K agrees with a shallow Cr-Cr potential and the sensitivity of the bond to axial donors. From the X-ray experiment, there is an electrondeficient area along the Cr-Cr bond, and a broad region of excess electron density off it, but the computed map for [Cr2(02CH)4]has a broad maximum around the centre of symmetry. The difference is tentatively related to weakening of the CJ bond by interaction of the axial water molecules present in the acetate. In the supershort cases, the electron density is at a maximum at the bond 1nidp0int.l~~ The pure quadruple bond configuration a2nz62(Figure 3) makes only a small contribution as a leading term in the calculations for the carboxylates, but is much more important in short in each case excited configurations must be inciuded in Cr-Cr species (as for the calculations. From comparisons of the results of calculations on the formate and the hypothetical tetrakis(formamidato) species [Cr,(NH(O)CH),], and the formate and that greater charge on the ligand donor atoms ‘[Cr2{H2P(CH2)2}4]’,it has been leads to better overlap of the d orbitals and a shorter quadruple bond. The importance of the inductive effects of the bridging ligands does not seem to have been clearly established experimentally: on the other hand the importance of the axial ligands on Cr-Cr bond lengths is clear from X-ray work, but not quantitatively explained theoretically. Other calculations of the C r - C r interaction are based on a model in which there is weak antiferromagnetic interaction between hip-spin Cr‘-+ atoms. 19’ This is not believed to be realistic, at least for the supershort bonds. ” A computational method which supposes multiple bonds to be single bonds intensified by screenin of the internuclear repulsion is said not to support the antiferromagnetic interaction mode.’ f
i
Chromium
750 U
*
8
figure 3 Qualitative order of energies of orbitals formed by metal d - d overlaps I
The standard enthalpies of formation of crystalline (25; L = H 2 0 ) , (26), (67), (39), (55), [ M o ~ ( O ~ C M and ~ ) ~ ][M~~(O,CMe)~(acac)~] have been derived from calorimetric measurements of oxidative hydrolysis in solution. It has not been gossible to distinguish between various theoretical treatments from the thermochemical data.' J~~
(iu) Photoelectron spectroscopy
Assignments by different workers of the photoelectron spectra of [Cr2(mhp)4]agree that the lowest energy peak A (6.7-6.8 eV) is due to ionization from the d metal-metal bonding level. The next feature is complex with an intense peak B at 7.75 eV and a shoulder C at 8.15 eV. It was considered that B and C are the 3t and (T ionizations, but it seems that B is due to ionization from n levels of the ligand and C is the n metal-metal i o n i z a t i ~ n . ' ~ The ~ , ~ CT~ , ~ ~ ~ ionization has been assigned to the second of three components at 9.8, 10.2 and 11.1 eV of a complex peak, and the other two components to ligand n ionization^.'^^ (When metal-ligand interactions are included, the molecular orbital diagram is of course more complicated than Figure 3.) Ionizations from the 6 and JG levels of dimolybdenum tetracarboxylates have been resolved, but as shown in Table 26,lWnot those from [Cr2(02CMe)$. The location of the ionization from the u metal-metal bonding levels remains u n ~ e r t a i n .Table ~ ~ ~24 ~ also ~ contains core electron ionization energies. Table 26 Experimental Corea and Valenceb Electron Ionization Energies (eV)
590.2 580.6 6.70 8.15 536.4 404.2 291.2sh 290.2
c r *In Cr *3,? M-M (6) M-M (x) 0 1s N 1s
c Is
Mo 3d30 MO 3d5n M-M(S) M-M (n) 0 1s NlS CLP
Mo2wPh 236.9 233.8 5.84 8.02 536.5 404.2 290.0
= Gas-phase X-ray PES. I,He' and He" PES. 'Formate data also in ref. 190
591.1
}:.:g. 537.8 -
294.3 291.1
Mo,(0,CMe); 237.7 234.7 6.92 8.77 537.9
-
295.7,293.4 291.1
Chromium
751
The oxygen core IE values of [ M Z ( O ~ C M ~(M ) ~=] Cr, Mo) are the same within experimental error, as are the oxygen and nitrogen IE values for [M2(mhp)4]. Also, the decrease in the binding energy of the Cr 2p levels from [Cr2(02CMe),] to [Cr&d~p)~]matches that of the Mo 3d levels. Thus, the charge distribution is the same within [Cr2(02CMe)4]and [ M O ~ ( O ~ C M ~ ) ~ ] , and within [Cr,(mh~)~] and [Mo2(mhp),]. Consequently, the significant lengthening (0.40 A) in going from [Cr2(mhp),] to [Crz(OzCMe)4]as compared with the Mo pair (0.03 A) is not due to differences in the bonding of the ligands, but to the different nature of the metal-metal interactions, notably the shallower Cr-Cr potential well, which makes the Cr-Cr separation more sensitive to changes in the electron density distribution.
(u) Electronic spectra The absorption spectra of [Mz(mhp)4] (M=Cr, Mo, W) in THF are similar. The lowest frequency band (22 500 cm-', M = Cr, Table 27) has been assigned to the metal-localized 6+ 6* transition (IAl+ 'B2, 4 d symmetry). The vibronic structure on this band in the low temperature spectra of the solids, with progressions of 320 (M=Cr), 400 (M=Mo) and 320 cm-l (M = W), is consistent with the assignment. Table 27 Electronic Absorption Spectra of Some Dichromium Species Complex
Electronic spectra (cm-l) 43700 (39000)
38000 (22000)
37 OOOb 40 OOOvb
34 800b 33 900sh 35 300b 39100sh 43100
33 800sh 36OOOb 33 OOOsh 35600
I1 =30 OOO 29 OOO 30 OOOb 28 Soob
28400sh 247ooW
22600 (480) 22 000 ( [Cr(CN),S]’-
0 2
A Cr(OH),
hv or A
S=H20
> [Cr(CN),-,S,]’-”
+ 6HCN Scheme 27
The slow addition of K3[Cr(CN)6] to a solution of FeSO4-7H20at 0°C gives a polymer of approximate composition Fe,[Cr(CN)&, which on the basis of IR (vCN at 2170cm-* and therefore indicative of a bridged chromicyanide, vMC= 488 cm-l) and Mossbauer (all the iron present as high-spin Fez+) spectroscopy seems to contain Cr3+-CkN-Fe2+ On heating at 50 “Cin an inert atmosphere half the Cr3+ ions are displaced by Fe2+ions from their carbon ‘sites’. In the presence of air further rearrangement and partial oxidation occurs to give a compound which on reduction with hydrazine affords an isomer of the starting material containing Cr3+-N=C-Fe2+ linkages (vcN= 2100 cm-l, vMC= 525 cm-’). In aqueous acid solution [Cr(CN),l3- aXuates via a series of stepwise stereospecific reactions to give [Cr(HzO),l3+ as the final product. O Some of the intermediate cyanoaqua complexes in this sequence have been isolated and details of their electronic spectra are presented in Table 44.These complexes aquate by both acid-independent and acid-dependent pathways, the latter involving protonation of the cyano ligands followed by aquation of the singly protonated species. Some of these reactions are found to be catalyzed by C9+ and kinetic data for the aquation of [CrCN(H20),J2+ are consistent with the transition state structure (85).307In the presence of Hg2+ and Ag’ cyanoaquachromiurn(II1) Complexes undergo strong association followed by ‘flipping’ of one or more of the cyano ligands, depending on the heavy metal to complex concentration ratio and the time elapsed since mixing the reactants (Scheme 28).321 Addition of C?’ to solutions of cyanocobalt(II1) complexes produces the metastable intermediate [CrNC(H20)5]2+(atLH’] = 0.9 M, A, = 535 nm, E = 19.6 dm3mol-’ cm-l; A, = 396 nm, E = 23 dm3mol-’ cm- ). This isomerizes to [CTCN(H~O)~]’+ (A, = 525 nm, E =
Chromium
775
25.2 dm3mol-1 cm-'; A, = 393 nm, E = 20.0 dm3mol-1 cm-') in a Crz+-catalyzedreactionwhich occurs by a ligand-bridge electron-exchange mechanism. In aqueous solution containing CNand OH-, [Cr(CN),I3- exists in equilibrium with [Cr(CN),OHI3- when [CN-] and [OH-] are both greater than 0.1 M and when [OH-]/[CN-] < 2.323The C9+-catalyzed aquation of [Cr(CN),I3- under these conditions seems to proceed by an outer-sphere redox mechanism represented by equation (27) although the formation of a short-lived bridged intermediate is not excluded by the kinetic data. The complex K3[Cr(CN)50H].H20 may be obtained as orange-yellow crystals by reaction of [CrC1(NH&]Cl2 and KCN in boiling water, followed by purification on a Sephadex [Cr(CN)J-
f
[Cr(CN),OH]'-
e [Cr(CN),]"- -+ [Cr(CN),OHI3"
(27)
Table 44 Details of the Electronic' and IRb Spectra of Cyanoaqua and Cyanoamrnine/Aminechromium(III) Complexes A,
Complex
(E)
(nm, dm3mol-' cm-')
kmin( E )
V(CN)(cm-') ~
391 (10.0) 385 (12.5) 379 (14.0) 4M)(10)h 400 (Wh 4% (21): 432 (25)' 406 (13)
~ir-[CrCN(en),H,0]~' cb-[Cr(CN)OH(en),]+
525 (26.0), 393 (20.5)' 535sh (30.2), 474 (45.3), 464 (45.5), 378 (25.8)d 467 (115), 360 (25.2)' 427 (-llO), 332 (-50)f 433 (-120), 365 (-75)' 451 (42.6), 347 (37.7) 440(42.6), 344 (41.5) 436 (49.0), 342 (37.6) 468 (48),354 (32) 468 (34), 355 (32) 494 (55), 381 (39) 491 (46),$37 (44) 483 (44),362 (36) 433 (50.1), 337 (42.7) 434 (70.0), 339 (62.3) 434 (69.5), 339 (62.2). 467 (53.4), 356 (49.1)' 487 (59.4), 386 (54.5), 345sh (35.9)k
[Cr(CN),(NH,),H,OI + 1 ,6-CN-2-WZO I ,Z-CNd-H,O [Cr(CN),edtd(H,0)]3
449 (39), 351 (41) 452 (53), 350 (34) SI2 (70.4), 386 (83.0)
395 (19)' 391 (17f 450 (45.0)
[crcN(w)*12+ c~Y-[c~(cN),(H,o),]+ f~c-[Cr(CN),(H,O),l [Cr(CN),H,Or[Cr(CN),OW] [CrCN(NH3),IZ+ mam-[Cr(cN)dNUl' ck-[Cr(CN),(NH,),]+ P~-[c~cN(NH,),H,o]Z C~-[C~CN(NH,)~H,O]~' mum-[CrCN(OH)(NW ) 1' ck-[CrCN(OH)(NH,),]3+4 C*-[C~CN(NH,),DMSO]~+ rram-[Cr(CN),en,] + cb-[Cr(CN),en,]+
All
I 1 1 2,3 3 4
-
214ow -2130~ -213oW
5
5 5 5 5 5 5
-
213%
-
-
6 7 6 7 7
-
8
213%
-
'
8 9
21ww
in aqueous solution. As Nujol mulls or KBr discs. 'In, 0.6 mol dm'.-3NaCIO,. d p H 2.0-4.0. pH 10.4, I = 2.0. 5 x mol d K 3 HCIO,. pH 7. pH 11. pW 3. pH 10. ' pH 2.9, I = 2.0. 1. D. K. Wakefield and W. B. Schaap, Inorg. Chem., 1969,8,512. 2. L. JeftiC and S. W. Fetdberg, J. Am. Chem. SOC., 1970, 92, 5272. 3. L. JeftiE and S. W. Feldberg, J. Phys. Chem., 1971, 75, 2381. 4. P. Riccieri and E. Zinato, Inorg. Chem., 1980, 19, 853. 5. P. Riccieri and E. Zinato, Inorg. Chem., 1981, u),3722. 6. A. D. Kirk and G. B. Porter, Inorg. Chem., 1980,19, 445. 7. A. P. Sattelberger, D . D. Darsow and W. P. Schaap, Inorg. Chem., 1976,15, 1412. 8. E. Zinato, P. Riccieri and M. Prelati, Inorg. Chem., 1981, 20, 1432. 9. 2.Chen, M. Cimilino and A. A. Adamson, Inorg. Chem., 1983,22, 3035. a
4.5.
Refi
~~~~~
e pH
3.0-
CN
I [(1420)4Cr-OH-Cr(HzO)5]3'
(85) fac-(H,O),Cr(CN),
+ M"'
[(H,O),(CN),Cr~N-MI"'
-
[(H,O),(CN),Cr-N=C--M]R+
Scheme 28
(ii) Cyanoammine- and cyanoamine-chromium(II1) complexes Although the range of reported acidoammine/arnine chromium(II1) complexes is' quite extensive, only a few such cyano species have so far been prepared, due mainly to the difficulties associated with the CN- anation of the appropriate aqua complexes. These difficulties stem largely from the ionization of coordinated water in the presence of CN-. This not only generates a leaving group of greater nucleophilicity but also makes complex
Chromium
776
decomposition competitive with anati~n . ~" Anation reactions of non-aqueous solvates circumvent these Hence the reaction between [Cr(NH&DMSO](C104)3 and excess NaCN in DMSO at 70°C affords the monocyano complex [Cr(CN)(NH3)5]2+,which was isolated as a perchlorate in 40% yield.326Details of the electronic and IR spectra of the complex are presented in Table 44. This complex phosphoresces in aqueous solution and its photoreactivity on LF band irradiation consists exclusively of NH3 aquation. Thermal aquation of the complex on the other hand involves loss of CN- and follows both acid-independent and acid-dependent pathways, the latter involving N protonation of the CN- ligand. The isomers cis- and ~ ~ U ~ - [ C T ( C N ) ~ ( NasHwell ~ ) ~as ] +c~~-[C~(CN)(NH,),DMSO](C~O have also been synthesized by the anation reactions shown in Scheme 29.325While anation in DMSO is accompanied by stereochemical change, the reaction in H 2 0 is stereoretentive. The dicyano complexes undergo H+-assisted thermal aquation, involving the successive loss of the CN- ligands. The trans complex is about ten-fold more reactive in the first step than the cis, an observation attributed to the trans labilizing effect of CN-. cis -[Cr(CN)(NH,),DMSO] (C10,)2
/
NaCN. LCIO, in DMSO or H,Oa
[Cr(NH,),(DMso),l(C1o,), cis or rransb (rcactant) NaCN, tiCl0,
cis (reactant)
in H,O, 35 Y:
in H20, 40 "C
trum-[Cr(CN),(NH,),]CIO,
I
J"'.. \
[Cr(CN)z(NK),]ClO,b
cis-[Cr(CN)(NH,),DMSO](CIO,),
HCIO,
~~U~~-{C~(CN)(NH,),H,O]~+
cationic resin, 0.02 M NaCIO,
in Fold H,O
trans-[Cr( CN),(NH3),]C10,
cis-[Cr(CN),(NH,),]ClO,
I
HCIO,
cis-[Cr(CN)(NH,),H,Ol*pX, = 5.6& 0 .p
cis-fCr(CN)(NH,),OH]+
I
1
HCIO,
cis -[Cr( NH3)4(H20)213' This reaction proceeds as shown with either reactant isomer in DMSO (15 min, 70 "C) but only with the c k isomer in H,O (2 h , 40 "C). Both isomers give a mixture containing 50% cis and 50% trans product but the cis isomer is the recommended starting material because of its ease of preparation. These are concentration pK, values at 298 K , I = 0.1 mol dm-3. Scheme 29
a
The synthesis of the isomers of [Cr(CN)2(en)2]' is discussed in Section 35.4.2.3. Like the previous1 mentioned cyanoammine complexes, cis-[Cr(CN),(en),]+ is quite labile in acid which This behaviour contrasts markedly with that of ~is-[Co(CN),(en)~],+ undergoes no apparent decomposition over a period of five days at 80°C in aqueous The addition of Ag+ to an aqueous solution of ~is-[Cr(CN)~(en)~]+ results in ligand isomerization, equation (28), and formation of adduct (86) which has an electronic spectrum
Chromium
777
characteristic of a CrN6 chromophore.jZ8 The 1R spectrum of the adduct, which has been isolated as its perchlorate salt, has a CN stretching band (2128cm-l) in a similar position to that of the parent complex (2130 cm-l) but of much greater intensity, consistent with extensive chromium cyanide n bonding. Addition of NaI to a solution of the adduct causes the precipitation of AgI and the regeneration of the original complex, a reaction which emphasizes the critical role played by Agf in stabilizing the isocyano isomer. The preparation of trans-[Cr(CN)2(cyclamm)lC104is described in Section 35.4.9.2. The photochemical behaviour of cyanoammine/amine chromium(II1) complexes differs from that of other acido chromium(II1) species. The higher ligand field strength of CN- relative to ammine/amine ligands causes the component of the first spin-allowed quartet excited state to lie at lower energy than the 'Eg component for D4*complexes.329 Since this state is associated with selective population of the d , 2 ~ ~ 2orbital, LF band irradiation causes photolabilization of the in-plane amine ligands. Hence the complexes tran~-[Cr(CN)~(NH~)~l+ and frans-[Cr(CN),(en),)+ behave 'antithermally' on photolysis, the former losing an ammine ligand (on 440nm excitation, GNH3= 0.24, GCN- = 0.005)329and the latter undergoing a reaction involving release and protonation of one end of an en ligand.330 In the case of trans-[Cr(CN)2(cyclam)]+, however , the presence of the macrocyclic ligand militates against cleavage of a chromium-amine bond and this complex shows no discernible photochemical reactivity.331 The complex Na3[Cr(CN)zedta(H20)].4H20 which contains tridentate (2N,O) edta, is referred to in Table 44.
(iii) Bi- and multi-nuclear cyanochrorniurn(2Ii) complexes A number of binuclear complexes containing Cr"'-NC-M bridges have been reported. The methods of preparation and spectroscopic properties of these complexes are presented in Table 45. The IR spectrum of each complex displays two CN stretching bands, the higher wavenumber one attributed to bridging CN and the other due to terminal CN ligand^.^" Magnetic susceptibilities down to liquid helium temperatures have been determined for C~-K[(N-N)~FC~-NC-C~(CN),] (N-N = en, tn) and ~k-K[(en)~Fcr-NCCrNO(CN)'] .333 Whereas the first two complexes show weak antiferromagnetic interaction between the chromium ions, the nitrosyl complex behaves as a Curie paramagnet. The absence of antiferromagnetic superexchange in this complex has been suggested to indicate that the NO ligand is trans to the cyano bridge. Reaction of Na2Cr(CO), with BrzCNNCBrz in THF affords the tetranuclear complex (THF)3Cr'11[NCCr(CO)5]3(87) .334 A pair of binuclear linkage isomers involving the CN- ligand has been prepared in solution according to Scheme 30.335
0 0
(C0)5CrCN, 1 ,NCCr(CO), 1 (CO),CrCN ,/cr\o-
co,
"ui
i
v(CN) = 2102 cm--', v(C0) = 2055, 1990, 1946 cm
'
(87)
[Cr(H,0),j3* + {Co(CN),]'-
-
(H,O),Cr-NC-Co(CN), A,,,=560(~=
[Co(CN),N,I3-
a [Co(CN),]'-
1 6 . 2 ) , 4 0 1 ( ~ . = 2 0 . 1 ) , 310 ( ~ = 2 1 8 )
ICI(H~O)~CNI~~
(H,O),Cr-CN-Co(CN),
A,,,
= 552 ( E = 18.3), 347 (E = 231)
Amax in nm, E in dm3mol-' cm-' Scheme 30
778
Chromium
Chromium
779
The thermal decomposition of a number of double complexes containing the [Cr(CN)$anion has been investigated and the results obtained under quasi-isothermal and -isobaric conditions are summarized in Scheme 31.336The 'CN-flipping' reactions involved in the scheme resemble similar reactions of Prussian blue analogues such as Co3[Cr(CN)& 337 and Fe3[Cr(CN)6]2.338 The unusually low magnetic moment of Cr(CN)6Cr indicates considerable metal-metal interaction via the CN bridges. [Cr(NHd&Cr(CN)6] yellow; 2130," 454,b 344'cm-'; 3.8BMZwd
-6NH3
Cr(NC),Cr dark brown; 2190, 527' cm-'; 2.4 BM2-, 2.5 BM290
205-31O'C
T
-n,o, NH,
[Cr(H,O) (NH,):] [Cr(CN),] red-yellow; 2135, 455, 345 cm- ; 3.8 BMIVb
- 4NH3 1 7 ~ 2 2 oc 0
~
220-30sc
(HzO)(NH3)Cr(NC),Cr(CN)2 light brown; 2175, 502 cm-'; 2.6 BM,
(a) [Cr(NH3)f1[Co(CN)h] yellow; 2135, 562, 415 cm- ; 3.8 BMZB8
[Co(NH3)61[Cr(CN)6] orange; 2135, 459, 344 cm-'; 3.8 BMzs9
-6NH3 205-290X
~
Cr(NC)hCo brown; 2200, 600, 512, 321 cm-'; 3 . 8 B M 2 9 9 , ~ ~
-6NH3. 'CN Ripping' l 5 W O -C
Co(CN),Cr
' grey-green; 2200, 596, 510, 317 cm-';
4.2 BMZ8, 4.0 B
T
-5NH,, 'CN-flpping' 120-330°C
-H20
[Co(H,O)(NH,),](Cr(CN),]
1 1 ~ ~ 1 "c 20
,(NH,),Co(NC)Cr(CN), yellow
red-yellow; 2130, 460, 345 cmIC)
Y(CN):
Y(MCN). s ( C MC ) . Magnetic moment at the temperature (K) subscripted. e v(MC) o r d(CMC) not adequately assignable in the bridged complexes.
a
Scheme 31
35.4.1.2 Isocyanides
Some aryl isocyanide complexes of the type [Cr(CNR),]X, (X = SbCl;, BF;) have recently been reported. DetaiIs of the preparations and properties of these complexes are given in Table 3.
3541.3 Alkyls and aryls The chemistry of alkyl and aryl chromium(II1) complexes is covered in the companion series306and has recently been the subject of a thorough review.305Some of these complexes are also referred to in Sections 35.4.2.5 and 35.4.8.l.i~. 35.4.2
Nitrogen Ligands
Complexes af chromium(II1) with ammonia and amines were among the first to be recognized by the early coordination chemists. Consequently, there is a vast literature, and this section is restricted to work since the comprehensive review by Garner and in which
780
Chromium
new solids have been characterized. In many cases the compounds are tabulated without comment. 35.4.2.1 Ammonia
Some cyanoammines are discussed in Section 35.4.1.1; other ammines which are new or have been prepared by improved methods are listed in Table 46 or mentioned below. Table 46 Ammines Complex
Ref. 6,15 26 5
15,16,21,22 7 7 9 11 23 12 25 25 8,14,16,20 20,28 20,28 28 30 31 32 1,3,6,13 1,13 172 2 1 1,13 13 1 1,13 13 192 29 1 192 1,3 27 6 19 19 6 4
4 4 17 17 5 5
17 17 8,14 8 14 8
10,14
Chromium
781
Table 46 (continued) Complex
Rej.
trans-[CrC1DMSO(NH3),1(C10,)2 cis-[Cr(NH3),(DMSO),](C10,), truns-[CrCI(DMF)(NH,),](CIO,), [ C r s o , ( H , o ) ( N H , ) , I [ C r ( ~ O ~ ) ~ ([CrSO,(H,O)(NH,),I,S,0, ~~~)~~, [CrCrO,(NH,),]X (X = I, Clod, &O,) tram-[Cr(CN),(NH,),]CIO, cis-[Cr(CN),(NH,),]CIO, cis-[Cr(CN)DMS0(NH3),1C10, cis- and trans-~Cr(nic-O),(NH,),]ClO, trurzr-[Cr(Hnic-O),(NH,),10,” K[Cw3),(NH,),I
10,14 10,14 14 11 23 24 24 24 30
30 33
single structure determination; structural determinations also on ICr(NH,),]X, (X = [HgC15],’4 [MnC15,H,0j3‘ and [Cr(ONO)(NH,)5]C12.36 1. J. Glerup and C. E. Schaffer, Inorg. Chem., 1976, 15, 1408. 2. D. W. Hoppenjans and J. B. Hunt, Inorg. Chem., 1969, 8, 505. 3. G. Wirth, C. Bifano, R . T. Walters and R. G. Linck, Inorg. Chem., 1973, 12, 1955. 4. D. W. Hoppenjans, G. Gordon and J. B. Hunt, Inorg. Chem., 1971, 10,754. 5. T. Ramasami, R. K. Wharton and A. G. Sykes, Inorg. Chem., 1975,14,359. 6. A. D. Kirk and C. F. C. Wong, Inorg. Chim. Acta, 1978, 27, 265. 7. A. Creix and M. Ferrer, Inorg. Chim. Acta, 1982, 59, 177. 8. J. Casab6, J. Ribas, V. Cubas, G . Rodriguez and F. J. Fernandez, Inorg. a i m . Acta, 1979, 36, 183. 9. J. M. Coronas, J. Casab6 and M. Ferrer, Inorg. Chim. Acta, 1977, 25, 109. 10. W. G. Jackson, P. D. Vowles and W. W. Fee, Inorg. Chim. Acta, 1977. 22, 111. 11. J . M. Coronas, J . Ribas, I. Casab6 and N. Cabayol, Inorg. Chim. A m , 1977, 21, 51. 12. 3. Casabb, J. M. Coronas and M. Fcrrer, Inorg. Chim. Acta, 1976, 16, 47. 13. W. G. Jackson, P. D. Vowles and W. W.Fee, Inorg. Chim. Acta, 1976, 19,221. 14. P. Riccieri and E. Zinato, 3. Inorg. Nucl. Chem., 1981, 43, 739. 15. E. Zinato, R. Lindholm and A. W. Adamson, J . Inorg. Nucl. Chem., 1969, 31,449. 16. N. Al-Shalti, T. Ramasami and A. G. Sykes, J . Chem. Soc., D a h n Trans., 1977. 94. 17. T. C. Matts and P. Moore, J. Chem. SOC. ( A ) , 1971, 1632. 18. T. C. Matts, P. Moore, D. M. W. Ogilvie and N. Winterton, J . Chem. Soc., Dnlron Tram., 1973, 992. 19. P. Riccieri and E. Zinato, J. Am. Chem. SOC.,1975, 97, 6071. 20. N. J. Curtis, G. A. Lawrance and A. M.Sargeson, A u t . J . Chem., 1983,36, 1495. 21. R. Davies and R. B. Jordan, Inorg. Chem., 1971,10,2432. 22. E. Zinato, C. Furlani, G. Lanna and P.Riccieri, Inorg. Chem., 1972, 11, 1746. 23. J . Casab6, J. Ribas and I. M. Coronas, J . Inorg. Nucl. Chem., 1976, 38, 886. 24. P. Riccieri and E. Zinato, horg. Chem., 1981, 20, 3722. 25. E. A. Hosegood and J. L. Burmeister, Synrh. Inorg. Metd-Org. Chem., 1971, 1, 21. 26. P. Riccieri and E. Zinato, h r g . Chem., 1980, 19, 853. 27. H. Ueno, A. Uchara and R. Tsuchija, Bull. Chem. Soc, Jpn., 1981,54, 1821. 28. N. E. Dixon, G. A. Lawwnce, P. A. Lay and A. M. Sargeson, Inorg. Chem., 1984, 23, 2940. 29. P. Aadersen, T. Damhusus? E. Pedersen and A. Petersen, Acta Chem. S C Q ~ .Ser. , A , 19X4,38, 359. 30. J. C. Chang, L. E. Gcrdom, N . C. Baenzigcr and H. M. GOB,h m g . Chem., 1983,22, 1739. 31. T. Schonherr and H. H. Schmidtkc, Inorg. Chem., 1979, Is, 2726. 32. N. V. Duffy and F. G. Kosel, Inorg. Nml. Chem. Len., 1969,5,519. 33. A. H. Norbury, Adv. Inorg. Chem. Radiochem., 1975,17,231. 34. W. Clegg, J. Chem. Soc., Dalton Tram., 1982, 593. 35. W. Clegg, Acta Crystallogr., Sect. B, 1978, 34, 3328. 36. E. Nordin, Acta Crysrallogr., Secl. B , 1978, 34, 2285. a X-Ray
(i) Diarnrnines and friamrnines
Of the five ossible isomers of the diammine [CrClZ(N€€3)Z(H20)2]C1 the one with all-trans ligand pair~?~’and that which is cis with respect to NH3 and trans with respect to C1340have been isolated; [CrBr2(NH3)2(H20)2]Bris also known.M1The triammines n~er-[CrX~(NH~>~l (X= C1, Br), mer-[CrBrC1,(NH3),] and rner-[CrBr2C1(NH3)3]have been obtained by thermal decomposition of the hexaammines etc. in a quasi-closed system which produces much purer intermediate compounds than the more usual open dynamic conditions;342f a ~ - [ c r C l ~ ( N H ~ > ~ ] and fa~-[Cr(NH~)~(H~0)~](C10~)~ have been isolated as in Scheme 34.343 (ii) Tetraammines
Schemes 32 and 33 illustrate the preparations of various members of the numerous trans-tetraammine Cr”’ starting from trans-[C~F~(py)~]I. Analogous cis complexes
Chromium
782 trans-[CrF,(py),]T
NHJ(I)
rram-[CrF,(NH,),]I
sruns-[CrFH2O(NH,),J2+
1 1-
solution N H 3 W
cis-[CrF2(NH3),]I
HCIOI
Hsz' HFO)In
trans - [CrCl, (NH3)J
Hg2- in
trans- [CrClF(NH,),] +
+
HFW
and trans-[CrC1H20(NH,)4]2+
I
Scheme 32
HBr HNOj
I$:/; I
in
2
tram-[CrBrF(NH,),]+ HF(I) ~ a n s - [ C r B r ~ ( N H ~ ) ~ ] + trans-[Cr(NH3)4(H20)2]3+ HzO 60°C
trans -[CrBr(H20)(NH3),I2+
trans -[Cr(NH3),( HZO)OH]'+
Scheme 33
Cr2+(aq)
NH$.'NH3. Nz charcoal. 0 ' C
W+/ammine oxidation, H2f
HC1(g), fi'rcr'O'C
'
faC-[CrCldNH3)31 blue-grey
1
HgZf/HC1O4, chromatography
Scheme 34
have long been known. The and ESR spectra3* of various trans ammines and [crXY(p~)~]"+ have been determined and ligand field parameters derived. C anide anation in DMSO has proved useful in the preparation of cyano complexes (Scheme 29)JZ5 Cyanide displaces coordinated DMSO cleanly but deprotonates rather than displaces coordinated water. The preparation of trans- and c~.T-[C~(CN)~(NH~)~]C~O~ starts from [Cr(DMS0)2(NH3)4]3+ or [CrC1(DMSO)(NH3)4]2f. The insolubility of tram[Cr(CN)2(NH3)4]C104 permitted its isolation from solution, but it was necessary to separate the ch isomer by ion exchange. Rapid trans to c k isomerization of trans-[Cr(DMSO)2(NH3)4]3+ occurs on the addition of CN- before anation produces the dicyano complexes stepwise, both apparently via c~~-[C~CN(DMSO)(NH~)~]~+, which in an unmfnmon rearrangement gives a mixture of trans- and cis-dicyano complexes. Substitution of DMSO by CN- in water is stereoretentive.
Chromium
783
(iii) Pentaarnmines Many new pentaammines have been isolated, mainly for photochemical and kinetic investigations. The absorption and luminescence spectra of single crystals and powders of [Cr(OH)(NH3)5](C104)2have been investigatedM7from 7 K to room temperature. The OHgroup produces a large tetragonal component, which causes a large splitting of the second but not the first spin-allowed band. From the fine structure in the luminescence spectrum the totally symmetric Cr-0 stretch is at 57Ocm-'. The far-IR, Raman and vibronic spectra of [Cr(NCO)(NH3)5](N03)2have also been There is some dispute over the ability of crystal field calculations to account for the detailed splittings. The ESR spectra of [CrX(NH3)5]"+ and other tetragonal Cr'" complexes have been analyzed.349Little magnetic ordering occurs at very low temperatures in complexes such as [cr(H20)(NH3)5][Cr(CN)6], e = 0.34 ~ . 3 5 0
(iv) Hexaammines As might be expected with the well-known hexaammines, there has been little additional work. Scheme 31 describes the thermal decomposition of some hexaammines and pentaammines; the borohydride [CT\NH~)~][BH~]~ has been prepared by a modified method and its thermal stability studied,351,52 and [Cr(NH3)6][C~(N3)6]has been isolated.353 The skeletal vibrational frequencies of [Cr(NH3)6](N03)3have been assigned,354and from low temperature single crystal investigations of [Cr(NH3)6](C104)2Cl-KCIit has been deduced3s5 that the Cr-NH3 bonds are lengthened by 0.12 A along two axes and shortened by 0.02 8, along a third in the excited 'Gg state compared with the 4A2 ground state. The zero field splitting in [Cr(NH3)6](C104)2Br-C~Br has been determined356 by single crystal magnetic susceptibility studies from 0.040 to 4.2 K, and the value is in agreement with ESR results. Even allowing for zero field splitting in [Cr(NH3),][CdC15] it appears that there is weak antiferromagnetic interaction between the cations (8= 1.4 K) although the Cr-Cr separation is 7.9 Ai.357 The study of e uilibria between Cr"' and ligands in aqueous solution is complicated by the robustness of CrI# complexes. Equilibria can be established quickly in Co"' systems if charcoal is added because this reduces a little Co"' to CoII, but in Cr"' systems Cr" must be added as well as charcoal. Oxidation by the medium could remove ail the Crrr before equilibrium is reached among the Cr"' species, but it has been that a constant concentration of Cr" can be maintained by electrolytic reduction until equilibrium is reached. Then the remaining Cr" can be allowed to oxidize and the equilibrium concentrations of the different Cr"' species determined. In this way log PS for complexation with ammonia has been found to be 13 (24 "C, 4.5 M NH4C1) and log f13 (en) = 19.5 (24 "C, 1M NaCl). The method works also with pn and edta, but fails with glycine and non-amine ligands like ox and SCN- where a steady state concentration of Crn cannot be built up. The photoinduced reactions of the cations [Cr(NH3)6--n(H20)n]3+have been and their visible spectra recorded during a studym of the formation of [ C T ( N H ~ ) ~by ] ~the + acid-catalyzed hydrolysis of [Cr(NCO)(NH3)5]2+.
( v ) Polynuclear complexes
Table 47 lists the polynuclear ammines which have been extensively investigated since the review by Garner and House.3M The interconversions between monobridged dinuclear Cr'" ammines with their traditional names are illustrated in Scheme 35 .302,361 Inter-relations exist between the mono-p-hydroxo and di-p-hydroxo species in acid solution, and these are outlined in Scheme 36. Acid hydrolysis of the mono-bridged dinuclear cations to mononuclear species is slow and salts of (94) and (95) as well as of the ion -established p-diols are known. In ammonia buffer [(OH)(NH3)4Cr(OH)Cr(NH3)4(OH) forms ?62 The basic rhodo complexes [(NH3)5CrOCr(NH3)5]X4(88) cannot be obtained free from the basic erythro salts [(NH3)5Cr(OH)Cr(NH3)40H]X,(90) unless cold solutions protected from light and containing concentrated ammonia are used.361 The pure samples exhibit reproducible antiferromagnetic behaviour; it seems that the linear Cr--O--Cr bridge with its short Cr-0 distances (4.&O A)363*364 permits the transmission of strong exchange interactions in which JZ
f'+
Chromium
784
Table 47 Oxo- and Hydroxo-bridged Ammines Complex
Comments
X = C1, Br, CIO,; X = C1, Br, singlet-triplet sepn. 450 cm-', linear Cr-0-0, C r 4 , ca 1.80, CrN (av), 2.12 A 1H20: singlet-triplet sepn. 31.5 cm-l, O a r , 165.6", Cr-0, 1.94, Cr-N, 2.07-2.15 A 2H,O: C r a r , 158.4", C r 4 , 1 974, Cr-N, 2.082.09 A X = C1, Br, CIO,, O.S&O,; X = CI, Br, 0.5S20,, singlettriplet scpn. 23 cm-'; X, = (S20&.3H,O, singlet-triplct scpn. 21 cm-', C r a r , 142.So,Cr-0 (br), 1.Y62, 1.989. C r 4 (tcrm), 1.915, Cr-N, 2.05-2.11 A X = C1, NO,; X - C1,,3H20, singlet-triplet IS sepn. 34.6 cm- , Cr--O--Cr, 155.lo,C r 4 (br), 1.983, 2.084, Cr-N (av), 2.070, disorder in mums ligands, Cr - O ( N ) (av), 2.071 A x = CI, c10, (E, A),,,=: (97.7, 519.5), (77.4, 385.5) (E,
A)max: (98.0,514.0), (62.7,380.0)
Ammonia buffer (E. A)max: (100.0, S34.0), (91.0,402.0)
Ref.
1
2-4
5
2,6
2 7
7 7
13
N' (98) Analogue of Werner's brown salt (Co"')
15
Rhodoso complex
13,14,15
(99)
X, = (S20,),*4H20:
X = CI, Br, C104, O.SS,O, (E, A),,,ax: (124.5, 536.0), (67.3, 386.5) Triclinic form, singlet-triplet sepn. 1.43 cm-', Cr+ Cr, 100.83", C r 4 (av), 1.973, Cr-N, 2.068-2.081, Cr--Cr, 3.041 A , 0 = 50" Monoclinic form, singlet-triplet sepn. 4.1 cm-', CrM r , 99.92". C r - 0 (av), 1.974, Cr-N, 2.074-2.089, Cr--Cr, 3.023A, e = 41" Singlet-triplet sepn. 5.8 CN-', C r - W r , 101.54', Cr0 (av), 1.965, Cr-N, 2.078-2.082, C r - C r , 3.045 A, 0 = 24"
8-10 7 11
9
9
Chromium
785
Table 47 (continued) Complex
Comments
Ref
12
12
12
E. Pedersen, Acta Chem. Scand., 1972,26, 333. D. W.Hoppenjans and J. 3 . Hunt, Inorg. Chem., 1969,8, 505. J. T. Veal, D. Y. Jeter, J. C. Hempel, R. P. Eckberg, W.E. Hatfield and D. J. Hodgson, Inorg. Chem., 1973, l2, 2928. P. Engel and H. U. Giidel, Inorg. Chem., 1977,16, 1589. D. J. Hodgson and E.Pedersen, Inorg. Chem., 1980,19, 3116. S. J. Cline, J. Glerup, D. J. Hodgson, G. S. Jensen and E. Pedersen, Inorg. Chem., 1981, 20, 2229. 7. F. Christensson and J. Springborg,Acta Chem. Scand., Ser. A , 1982, 36, 21. 8. J. Springbog and C. E. Schiffer, Inorg. Synth., 1978, 18, 75. 9. S. J. Cline, D. J. Hodgson, S. KalleMe, S . Larsen and E. Pedersen, IROQ. Chem., 1983, 22, 637. 10. S. Decurtins and H. U. Giidel, Inorg. Chem., 1982, 21, 3598. 11. D. J. Hodgson and E. Pedersen, Jnarg. Chem., 1984, 23, 2363. 12. P. Andersen, K * M.Nielsen and A. Petersen, Actu Chem. Scund., Ser. A , 1984. 38, 593. 13. P. Andersen, T. Damhus, E. Pedersen and A. Petersen, Acta Chem. Scand., Sw.A, 1984, 38,359. 14. H. U. Giidcl, U. Hauser and A. Furrer, Inorg. Chem., 1979, 18, 2730. 15. E. Bang, Acta Chem. Scand., Ser. A, 1984,38, 419. 1. 2. 3. 4. 5. 6.
Crz- + NH,OH
+ NH:
*
Ix-
6
[(NH~)SCrOCr(NH,),X]”+ ‘bare t(NH,),Cr(OH)Cr(NH,>,XI“’ acido rhodo ion Xido erythro ion (93) (92) X = F, C1, NCS, O N 0 Scheme 35
bonding is important. An almost linear bridge is also present in [{Cr(NCS)(TPyEA)}20](BPh4)2.1°7 The normal (acid) rhodo complex [(NH3)sCr(OH)Cr(NH3)5]C1s.H20 (89) has a bent bridge (Cr-0-Cr angle = 165.6’) and shows much weaker antiferromagnetic interaction (Table 47).365The Cr-N bonds trans to the bridge are a little longer (0.07A) than the other Cr-N bonds. This may relate to the formation of the basic elythro salts by the substitution of a terminal NH3 by OH. However, no bond lengthening is apparent in the structure of the the polarized electronic spectrum of which has dihydrate [(NHS~SCr(0H)Cr(NH3)51C15.2Hz0, been investigated in order to gain information an the exchange-induced intensity-gaining mechanisms for spin-forbidden transitions.366
786
Chromium
OH (NH3),Cr/
0 H
1 4 -
‘Cr(NH4,
\o/ H
+ H,O
k1=1.21~10-~
‘Cr(NH3),
(NH,),Cr’
k_1=3.80~10-~~-1
1 4 +
1
I
OH2
OH
(94) reddish
H‘
‘H
(95) Scheme 36
Structural, spectroscopic and magnetic investigations have been carried out on the basic erythro complex C~-[(NH~),C~(OH)C~(NH~)~(~H)](S~~~)~-~H~~ (W),the acid eryrhro complex trans-[(NH3),Cr(OH)Cr(NH~)4(HzO)]C1~~3Hz0368 (91), for which the bridging angles are respectively 142.8” and 155.1”, and the di-phydroxo complexes [(NH3)4Cr(OH)2Cr(NH3)4]X4.4H20 (X = Cl, Br or 0.5SzOe).-373 The dibridged ammines are examples of the general dinuclear complex (96), with L = NH3 and R = H. Much research has been aimed at relating antiferromagnetic behaviour and spectroscopic properties to structure. There does not seem to be a simple correlation between the magnitude of the exchange parameter J (singlet-triplet splitting) and bridge geometry, i.e. the Cr-0-Cr angle #, the Cr-0 bond length r, or the C r - C r separation. Evidence is accumulating that there is a correlation between the magnitude of J and the angular displacement of the R (or H) atom out of the Cr202plane (dihedral angle e). The implications are that superexchange through a pn orbital on 0 is dominant and this is at a maximum only if the H atom lies in the plane. Recently, a model has been proposed which relates the magnitude of the magnetic interaction to aII three parameters r, #,I and r3.372*374
The synthetic work has been extended to di-p-hydroxo-fac-triammine derivatives (97) (Table 47 and Scheme 34). The crystals of (97) contain two different centrosymmetric cations, the trans diaqua and trans dihydroxo species, linked alternately in infinite chains by short (2.45 A) hydrogen bonds. The bond distances of the diaqua cation differ slightly from those of the dihydroxo cation.37s The preparations of the trinuclear complex [Cr3(OH)4(NH3)10]Brs(98) and the tetranuclear complexes [Cr{(OH)zCr(N~3)4}3]Br6 and [Cr4(OH)6(NH3)12]C16 (99) (Table 47 and Scheme 34) start with aqueous Cr”-NH:/NH3 buffer solutions and The antiferromagnetic behaviour of (98) has been analyzed in terms of its triangular structure [.Il2 (7.90 or 18.7 cm-l) J1~=523(18.7 or 4.3 cm-I)], which resembles that of (99) (Table 47). The compler [Cr{ (OH)zCr(NH3)4}3]Br6also behaves antiferromagnetically, and is structurally an analogue of Werner’s brown salt /Co{(OH)2&(NH3)4}3]C16. The tetranuclear ‘rhodoso’ complex [Cr4(OH)6(NH3)12]C16n4Hz0 has the structure (99).376737 Its antiferromagnetic behaviour can be reproduced in terms of dominant exchange parameter! along the side and shorter diagonal of the planar rhombus formed by the four chromium atoms Some differences in parameters may be due to the use of slightly different crysta m ~ d i f i c a t i o n s . Neutron ~ ~ ~ ’ ~ ~inelastic ~ scattering has been used to observe the exchangt
787
Chromium
splittings of the ground electronic state and provide refined and additional exchange parameters. There is a sharp drop in magnetic moment below 10K which requires that the lowest level of the ground state be a spin singlet with a low-lying triplet. The triplet from the temperature dependence of the neutron inelastic scattering spectra lies less than 4 cm-I above the singlet. The exchange splittings of the ground state have also been observed through the temperature dependent intensities ( 4 1 K) of the transitions to the 'E and '7'' levels.378 35.4.2.2 Monodentate amines
A number of chromium(II1) complexes containing monodentate amine ligands are known and details of their electronic spectra are given in Table 48. Table QS Electronic Spectra of Monodentate Amine Chromium(II1) Complexes
A,,,
Complex [Cr(NH3)613+ [Cr (MeNH,),J' [Cr(EtNH,),I + [CrCl(MeNH,),]Z' {CrBr(MeNH,),]'+ [Cr(OSO,CF,) (MeNHJS]" [Cr(MeNH,),H,O]" [Cr(MeNH,)SNH,]3+ [Cr(EtNH,),NH,]3C [Cr(MeNH,),(en)$'" trans-[CrF,( MeNH,) 41 trans-ICrF,(Pr"NH,),]+
462 (44.0) 477 (71.8) 478 (71.5) 526 (-) 535 (46.3) 506 (49.1) 490 (55.6) 474 (52.5) 474 (61.7) 476 (65.2) 465 (84.1) 494 (22.4) 502 (26.1)
(E)(nm, dm3 mol-' m-l) 346 (37.2) 360 (62.5) 363 (61.5)
-
390 (54.7) 374 (45.8) 368 (49.2) 359 (45.5) 360 (52.9) 362 (56.0) 354 (71.6) 408 (14.5) 356 (16.3) 413 (16.1) 356 (17.5)
Medium DMSO/gIycerol DMSO/glyceroI DMSO/glycerol Glycerol/H,O HBr/H,O CF,SO,H H,O, PH 4 HBr/H,O DMSO/glycerol DMSO/gIycerol DMSO/glycerol O.1MHCI 0.1 M HC1
Ref. 1 1 1 2 3 4
5 3 1 1
1 6 6
Prepared from @am-[CrBr,(en),]Br and liquid MeNH, but product of unknown isomer composition. The pn and tn complexes were prepared similarly. Kupka, I. J . Phys. Chem., 1981,85, 665. 1. K. Kiihn, F. Wasgestian and € 2. H. L. Schlaefer, H. Gausmann and H. Witzke, Z . Phys. Chem. (Franwrt am Main),1967, 56, 55. 3. K. Kiihn and F. Wasgestian, Inorg. Nucl. Chem. Letts., 1976, 12, 803. 4. N. E. Dixon, G . A. Lawrance, P. A. Lay and A. M. Sargeson, Inorg. Chem., 1984,23, 2940. 5. Yu. N. Shevchenko, V. V. Sachok and N. K. Davidcnko, Rurs. 1.Intrrg. C k m . (Engl. Tranrl.), 1979,24,30. 6 . J . Glerup, J. Joscphscn, K. Michelsen, E. Pedersen and C. E. SchBiffcr, Acta Chem. Scand.. 1970, 24, 247. a
(i) Aliphatic hexatnines The reaction between CrBrJ and liquid alkylamines RNHz (R=Me, Et) at reflux temperatures affords the yellow-coloured hexamines [Cr(RNH&I3+, which have been isolated as nitrate (R = bromide (R = Me)380 and perchlorate (R = Me,380 Et3'') salts. Hydrolysis of [Cr(MeNH&I3+ in HC1O4 solution gives [Cr(MeNH2)5H20]3+but in solutions of other inorganic acids (HX) gives the acidopentarnine complexes [CrX(MeNH2),I2+.380,382 Reaction between [Cr(MeNH&]Br3 and (NH4),[B,&,] in aqueous solution leads to crystalline [Cr(MeNH2)63Br(BloHlo).2Hz0 which undergoes thermal decomposition to a monodentate decaborane complex (Scheme 37)."' The isomers c h - and h~ns-[Cr(MeNH~)~(en )2]Br(BloHIo) are stable up to 250°C but above this temperature both lose MeNHz and give the mmmon product tr~ns-[Cr(B,~H~~)Br(en),].~~~ [Cr(MeNW,),]Br(B,oH,o).2H20 70-80 '5 [Cr (MeNH,),]Br(B LrnT
Cr(BloH,o)Br(MeNH2)4
1ZG150 'C
[CrBr(MeNH,),IB
Scheme 37
(ii) Aliphatic pentarnines In contrast to CrBr3 the reaction of CrC13 with neat alkylamines stops at the pentamhe stage and gives the products [CrC1(RNH2)5]C12(R = Me, Et, Pr", Bu", Bu', C&).384,385 The COC3-z
788
Chromium
light-red-coloured complexes [CrBr(RNH2)5]Br2(R = Me, Et) may however be obtained by controlled addition of liquid amine to CrBr3.381i386 These complexes readily react with liquid NH3 to give [Cr(NH3)(RNH2)5]Br3.381 Owing to the general lability of the triflate ligand, the complex [Cr(OS02CF3)(MeNH2)5](CF3S03)2, obtained from a solution of [CrC1(MeNH2)5]C12 in CF3S03H, is a potential starting material for the synthesis of [CrX(MeNH&In+ complexes.387 An X-ray crystal structure determination of [CrCl(MeNH2)5]C12shows that the cation is severely distorted having C r - N K and N-Cr-N bond angles which deviate by up to 15 and 6.6" from normal tetrahedral and octahedral angles respectively.388 In accordance with the sterically crowded nature of its ground state, this complex undergoes base hydrolysis 225 times faster than [CTCI(NH~)~]~+ at 298 K which is in the expected direction for a Dcb Aquation of [CrC1(MeNH2)5]2+,however, is 38 times slower than that of the ammine complex . ~ ~reversal ~~~~ at 298 K, an order which is reversed in the analogous cobalt(II1) c o r n p l e ~ e sThis in reactivity is attributed to a change in mechanism from 1, in cobalt(II1) to I, in chromium(111).390 The orange-coloured complex [Cr(MeNH2)5H20](N03)3may be prepared by passing a solution of [CrBr(MeNH2)5]2+through an ion exchan e resin in the OH- form followed by acidification of the emuent with concentrated HN03.399
(iii) Aliphatic tetramines, triamines and diamines Reaction of tr~ns-[CrF,(py)~]+ with amines (RNH2) at 100 "Cprovides a convenient route to ~~~~S-[C~F~(R corn N Hlexes, ~ ) ~ a] +number of which have been isolated and characterized (R = Me, Et, Pr", C3H5).3y'Therrnal decomposition of [CrC1(EtNHZ)5]C1zin the solid state at 250 "C gives the green-coloured complex CrC13(EtNH2)3,while its reaction with KSCN in aqueous solution leads to the isomers 1,2,6- (red) and 1,2,3-Cr(NCS)3(EtNH2)3(lilac).3o2 The reaction between CrC13 and Me3N in the presence of zinc dust catalyst gives the five-coordinate purple-blue solid CrC13(Me3N)2.393This complex has a distorted trans trigonal bipyramidal structure and a magnetic moment of 3.88BM (298K), which is virtually independent of temperature .394 In benzene solution this com lex undergoes either ligand substitution or decomposition as outlined in Scheme 38!95,3q6 The hexacoordinated chromium(II1) dimer Cr2C16(Me3N)3has magnetic (peffat 297 K = 3.68 BM; obeys the CurieWeiss law over the temperature range 103-297 K) and spectroscopic (4A2g+'Gg at 13 300crn-'; 4 A z g 44Tgat 18 520 cm-') properties like those of [Cr2Cl9I3-, suggesting a similar confacial bioctahedral structure with no significant metal-metal interaction. A mechanism involving halogen bridging with synchronous Me3N expulsion and ligand reorientation is suggested for dimer formation. No reaction was observed between CrC13(Me3N)2and the potential ligands C02, CS2 and (Pr')20. In the reaction products, CrC13(tren) and [Cr(tren),]Cl,, tren behaves as a tridentate ligand, bonded to the metal through its primary amino groups in both cases. CrC1,L1
L (=PY, THF) 7
CrC13(Me3N)2
tren(l:l
CrCl,(tren) or [Cr(tren),]Cl,
C6H6
1
C6H6
Cr,C16(Me,N),
+ Me3N
Scheme 38
(iv) Anilines
A number of chromium(II1) complexes containing aniline PhNH2 and substituted aniline ligands are known. These include [CrC12(PhNH2)4]+302 and CrC13(PhNH& 397 although the configuration is not stated in either case. The latter complex was prepared by refluxing a solution of CrC13in aniline (the pyridine complex CrC13(py)3,prepared analogously, was shown to have a mer configuration). Reaction of CrC13(PhNH2)3and Br2 in glacial acetic acid proceeds at a similar rate and with identical substitution orientation (i. e. 2,4,6-Br3PhNHz product) as that of uncoordinated PhNH2.397Coordination to chromium(II1) therefore has a
Chromium
789
contrasting effect to protonation, which greatly deactivates aniline with respect to electrophilic substitution and causes it to occur at the metu position. Some Reinecke-type salts containing aniline ligands have been ~repared.~"The complexes Bi[Cr(NCS)4L]3 (L = PhNH2, morpholine, p-MePhNH2, p-anisidine, O.Sbipy), Bi&[Cr(NCS)4I.,J3 (X = thiourea, L = PhNH2, morpholine), CdX2[Cr(NCS)4(PhNH2)2]2(X= en, piperazine) and Cd[Cr(NCS)4(PhNH2)2]2may be used in the analysis of Bi and Cd by titrimetric, A number of double complex salts of the spectrophotometric or gravimetric methods.39type A[Cr(NCS)4(PhNH2)2], (A represents a range of cobalt(II1) and silver@)complexes) have been prepared and in the case of the Ag salts magnetic moments and positions of IR bands have been reported.&l Reaction of ~rt4ns-[Cr(C,O,)~(H~0)~]-and aniline gives [Cr(C204)2(PhNH2)J, which can exist in a rose-coloured or in an unstable green-coloured modification.3"z 35.4.2.3 Ethylenediamine and related bidentate amines
The chemistry of amine complexes of chromium(II1) up to 1969 has been extensively reviewed by Garner and House;302 consequently earlier work will not be covered here. Fluorodiamine complexes of chromium(II1) have also been carefully reviewed by Vaughn. 4023403
(i) General preparative methods The easy formation of hydroxo- or oxo-bridged Cl' polymers in basic aqueous solution, the comparative lability of the Cr-N bond, and the precautions needed to obtain chromium(I1) complexes compared with cobalt(I1) complexes have meant that the preparative chemistry of chromium(II1) is more difficult than that of cobalt(II1). A greater variety of non-aqueous solvents is now in use, and there is greater knowledge of chromium(I1) chemistry to be exploited in the preparation of chromium(II1) complexes generally, but few new methods of preparation of amine complexes have been devised since the early work. The general synthetic methods are: (a) reaction of anhydrous CrC13, CrBr3 or the sulfate with anhydrous amines alone or in an organic solvent, sometimes with the addition of zinc powder to provide a catalytic pathway via chromium( II) complexes-the anhydrous halides can be prepared from the hydrated salts by reaction with 2,2-dimethoxypropane, triethylorthoformate or thionyl chloride, or by distillation of water from solutions of the hydrates in DMF or DMSO; (b) reaction of hydrated chromium(II1) salts such as CrC13.6Hz0, [Cr(H20)4Br2]Brand Cr(N03)3-6Hz0in solutions buffered with an amine/amine-salt mixture to prevent the formation of oxo and hydroxo polymers followed by acidification-carbon, added as a catalyst, can change the course of the reaction; (c) reaction of [CrC13X3](X = py, THF, DMF, DMSO) with amines-the preliminary preparation of these compounds, even if only in situ, gives essentially anhydrous starting materials soluble in organic solvents, although aqueous solutions can be used in some cases; (d) reaction of salts such as K3[Cr(NCS)6], K3[Cr(o~)3]and [ C r ( ~ r e a ) ~ ] Cwith I ~ amines; (e) reaction of diperoxochromium(1V) amines with acids and the reduction of chromium(V1) compounds-these methods have not been applied recently; ( f ) oxidation of chromium(I1) complexes; and (g) substitution of aqua or acido ligands in amine complexes by ammonia (from liquid ammonia) or other amines to produce mixed ligand complexes. Complexes containing ethylenediamine and related bidentate N donor ligands which are new since 1969, or have been prepared by new methods, are listed in Tables 49 and 50. Water of hydration has not been specified. In general, the complexes have been prepared for kinetic and photochemical investigations. The list is restricted to solids which have been characterized at least by analyses. Cations obtained only in solution have not been included. Most complexes have also been characterized by conductance measurements, and IR, UV and visible spectroscopy. Spectroscopic methods have been used to distinguish between cis and trans isomers, and where cis isomers have been resolved this is indicated in the tables. ( a ) Recent syntheses. The most numerous complexes are those with the donor set X2N4(Xis anionic or neutral), especially with N4 = (en)2. The nitrosation of Cr-OH2 groups has been used to synthesize many trans compounds containing C r 4 N O groups (NO' addition reactions), e.g. rr~ns-[CrBr(ONO)(en)~]ClO~, but cis compounds could not be obtained because the O N 0 group labilizes cis ligands.404Destruction of Cr-ON0 groups with acids
Chromium
790
Table 49 Complexes of Ethylenediamine and Related Diamines (see also Table 50) Complex
[Cr(enj;j(NCS), racemic and (+)589 saltsa [Cr(tmd),lC13 [CT(Pn),lC13 [Cr(en),-,(trnd),]Br3 (n = 1, 2, 3 resolved) [CrL31C13 L = truns-l,2-~yclohexanediamine L = cis-l,2-cyclohexanediamine,mer- and Sac-A and -A isomers L = tranr-l,2-cyclopcntanediamine cU-and truns-[Cr(en),(NH2CH3),]Br(B,,H,,) [Cr(NH3),(en)lC104 For other CrN, species see [Cr(NH,),(en),]3'
Ref.
192 3,4,5,6 7 5 3 8 5,9,10
11 12 2 13
and [Cr(NCS),(en),]+ salts below and Table 50
CrX,N, (X is anionic or neutral)b trans-[CrF,(en),]X (X = Br, trans-[CrF(H,O)(en),]X, (X= Br, C10,)
ci.~-[CrF,(en),][CrF,(en)] &-[CrF,(en),]F, I, CIO, isomer has A configuration} cis-[CrF(H,O)(en),]X, (X = I, ClO,, BPh,) ((-)546 isomer has A configuration) cis-[CrClF(en),]X, (X = 1, C104) cis-[CrBrF(en),]I isomer has A configuration) cis-[CrF(NCS)(en),]X (X = I, SCN, C10,) tralls-[CrFX(en),]ClO, (X = CI, Br, SCN) cis-~CrF(NH,)(en),]X, (X = I, CIO, {(-)546-isomer},X, = CI, I) trans-[CrF(NH,) (en),] (CIO,), cis- and trans-[Cr(NH,),(en)~](ClO.,), cis-[CrCI(NH,)(en),]Br, cis-[CrBr(NH,)(en),]Br, tram-[CrC1(NH,)(en)z]Xz (X = Br, CIO,) trum-[Cr(NH,)(H,O)(en),]X3(X = CI, Br; X, = Br,CIO,) cis- and truns-[Cr(NH,Me),( en)t](CIO,), ; cis-[CrBr(NH,Me)(en),]Br, cis-[Cr(H,O)(NH,Me)(en),](CIO,), cis- and trans-[Cr(NCS)(NH3)(en)J (SCN ), trans-[Cr(NCS),(en),]X (X = C1, MO.) cis-[Cr(NCS),( en),] SCN cis-[CrCl,(en),]C1 cis-{CrBr,(en),]X (X = Br, CiO,) cis-~CrC1H,O(en),]X, [X = Br, I, M(CN),, (M = Pd, Pt)] cis-[CrBrH,O(en),]X, [X = Br, I, M(CN),, (M= Ni, Pd, Pt) cis-[CrCI(DMSO)(en),]X, (X = C1, ClO,) trans-[Cr(DMSO)(DMF)(en)2](CIO,),
c~T-[C~(DMSO)(DMF)(~~)~](CIO,),(NO~) trans-[Cr(DMSO)(H20)(en)*](ClO,), cis-[Cr(DMSO),(en),](C10,), cis-[CrBr(DMSO)(en)J (CIO,),
trans-fCrBr(DMSO)(en),](CIO,), rrans-~CrX(ONO)(en),]CIO,(X = CI, Br) cis-[CrCl(ONO)(en),]CIO, brans-[CrBr(H,O)(en),]X, (X = Br, Clod) tram-[CrCl(H,O)(en),]X, (X = Br, CIO,)
tram-[Cr(H,O)(DMF)(en),](CIO,), rrans-[CrX(NO,)(en),]I (X = C1, Br) rrarans-[CrBrI(en),]X (X = I, C104) trans-[CrCII(en),]I cis-[CrXI(en),]I (X = C1, Br; not isomerically pure) cis-[CrClBr(en),]X (X = Br, CIO,) ~aans-ICrClBr(en),]X(X = Br, C10,) traans-~CrX(NCS)(en),]CIO,(X = C1, Br) tram - [Cr (ONO)(H,O) (en)J( ClO,), trans-[Cr(OH)(ONO)(en),]CIO,
tra~-[Cr(ONO)(DMF)(en),](CI0,)2 trans-[Cr(ONO)(DMSO)(en),~(ClO,), cis-[Cr(ONO),(en),]ClO,, (+)4Ro isomer isolated cis- and ~rans-[CrBr(DMF)(en),](ClO,),
14,15,16 14 17 1,17,19 17,18,19 17,18 18 17,19
14,20,21 22,23 22 13 22 22 22 22 24 25 26,27 28,29 5 30 6,31 6,31 5 32 33 33 33,34 33,34 33,34 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
35 6
Chromium
791
Table 49 (continued) Complex cis- and tram-[CrCl(DMF)(en),](CIO,), cis- and trans-[CrBr(DMA)(en),]( CIO,), cis-[CrCI(DMA)(en),] (CIO,), tram-[CrCI(DMS0)(enh](CIO,),
cis-[CrCI(DMSO)(en),](C1O4)(NO3) trans-[CrCl(DMF)(en)z](C104)2 c~-[Cr(DMF),(en),](CIO,), and (CIO,),NO, rrum-[Cr(DMF),(en)2](CIO,), cis and trans-[Cr(DMA),( en)J( CIO,), w m -[Cr(DMSO),(en) ,] (C104)3
-
C~~-[C~(DMA)(DMSO>(~~)~](CIO,),NO, trans-[CrCl,(en),]X (X= CI, C10,) rrum-[CrBr,(en)JBr wum-[CrI,( en),]CIO,
tram-[Cr(OH)(H,O)(en),~X,(X = Br,CIO,)
cis-[Cr(OH)(H,O)(en),]S,O, trans-[Cr(H,O),(en),]Br, [CrI(H,O)(en),lI, ICrL(enLl1 h-[cr(C~),(en),]X (X = C1, I (resolved), CIO,) rrum-[Cr(CN),(en),]CIO, and cb-ICr(N,),(en)zlNO,, I+)jns-~~-[Cr(N,),(en),I[(+)~bOTl [Cr(SR)(en),]ClO, (R = 2-C6H,CO;, CH,CH,CO;) [Cr(SCH,CO,)(en),]ClO~ [Cr(SCH,CH,NH,)(en),]X, (X= CIO,, I) ICr(SCMe,CO,)(en),lCIOd -, . ,-_ ~Cr(SBzCH,NH,)(en),](CIO,), not fully characterized ?rum-[A][CrF(H,O)(en),][X](A = K, Na, NH,; X = [Cr(Cr%I I cis-[Cr(nic-O),(en),] Br [Cr(CN)(NCS)(en),] SCN CrX,N, (X is anionic or neutral) [Cr(DMSO),(en)l (cIo4)3 [cr(D~),(en)l(c1o,), A][CrF,(en)] (A = Na, cis-[CrF,(en),]) [ e n q ll CrF,(en)]CI" [CrF,(H,O),(en)]X (X = C1 (~rum-F,),~ Br)
[Cr(SCH,CH2NH,),(en)]CI0, [CrC1,(H,O),en]C1, (trm [CrBrFz(H,O)(en)l K[Cr(CN)(NCS),(en)l
c~~-[C~(OSO,CF,)~(~~)~]CF~SO~
Ref.
33 6,33 33 6 33 6 6 6 6,33 6 33 6 6 6 33 33 33 17 17 36,37 37,38 29,39 40,41 42 43 41,44 45
46 47 48 49
49
17,50 51 51,52,53,54 43
55 51 48 56
single crystal structure determinations; see also refs. 57 and 58 for other [Cr(en),]3+ salts. Cyanato and selenocyanato complexes are dealt with in Section 35.4.2.8.
a X-Ray
1. Yu. N. Shevchenko and N. K. Davidenko, Russ. J . Znorg. Chem. (Engl. Trami.), 1979, 24, 1193. 2. Yu. N. Shevchenko, N. I. Yashina, K. B. Yatsimirskii, R. A. Svitsyn and N. V. Egorova, Russ. J . Znorg. Chem. (EngL Tram/.), 1983,28,216. 3. R. D. Gillard and P. R. Mitchell, lnorg. Synth., 1972, l3, 184. 4. F. Galsbd, Znorg. Synth., 1970, 12,269. 5. E. Pedersen, Acta Chem. Scond., 2970, 24, 3362. 6. W. W. Fee, J. N. MacB. Harrowfield and W. G. Jackson, J. Chem. Soc. {A), 1970,2612. 7. K. Akabori and Y. Kushi, J. Znag. Nucl. Chem., 1978, 40, 1317. 8. H. P. Jensen, Acta Chem. Scand., Ser. A , 1981, 35, 127. 9. S . E. Harnung and T. Laier, Acta Chem. Scand., Ser. A, 1978, 32, 41, 10. P. Andersen, F. Galsbd, S. E. Harnung and T. Laier, Acta Chem. Scand., 1973.27, 3973. 11. H. Toftlund and T. Laier, Acta Chem.Scand., Ser. A , 1977, 31, 651. 12. H. Toftlund and E. Pedersen, Acta Chem. Scand., 1972,26, 4019. 13. C. F. C. Wong and A. D. Kirk, Znorg. Chem., 1978,17, 1672. 14. J. W. Vaughn, J. M. DeJovine and G. J. Seiler, Znorg. Chem., 1970,9, 684. 15. J. Glerup, J. Josephsen, K. Michelsen, E. Pedersen and C. E. SchHffer, Acta Chem. Scand., 1970, 24,247. 16. J. V. BrenW and I. Leban, 2.Anorg. Allg. Chem., 1981,480,213. 17. J. W. Vaughn and A. M. Yeoman, Inorg. Chem., 1976, 15,2320. 18. J. W. Vaughn and A. M. Yeoman, Synth. React. Inorg. Metal-Org. Chem., 1Y77, 7 , 165. 19. J. W.Vaughn and G. J. Seiler, Znorg. Chern., 1979,18, 1509. 20. G. Wirth, C. Bifano, R. T. Walters and R. G.Linck, Znorg. Chem., 1973,12, 1955. 21. C. F. C. Wong and A. D.Kirk, Can. J . Chem., 1976.54, 3794. 22. C. F. C. Wong and A. D. Kirk, Can. J . Chem., 1975,53,3388. 23. J. W. Vaughn, Znorg. Chem., 1983, 22, 844. 24. Yu. N. Shevchenko and V. V. Sachok, Russ. J. Inorg. Chem. (Enel. Trans[.), 1983, 28, 1118. 25. A. D. Kirk and T. L.Kelly. Inorg. Chem., 1974, l3, 1613.
792
Chromium Table 49 @wtnotes continued)
26. J . C. Hempel, L. 0. Morgan and W. B. Lewis, Inorg. Chem., 1970, 9,ZOfA. 27. C. Bifano and R. G. Linck, Znorg. Chem., 1974, U,609. 28. D. A. House, J . Inorg. Nucl. Chem., 1973, 35, 3103. 29. M. Nakano and S. Kawaguchi, Bull. Chem. Soc. Jpn., 1979, 52, 3563. 30. W. G. Jackson and W. W. Fee, Inorg. Chem., 1975,14, 1154. 31. J. Ribas, M. Serra and A. Escuer, Transition Met. Chem., 1984, 9, 287. 32. W. G. Jackson and W. W. Fee, Znorg. Chem., 1975,14, 1174. 33. W. G. Jackson, P. D. Vowles and W. W.Fee,Inorg. Chim. Acta, 1977,22, 111. 34. D. A. Palmer and D. W. Watts, Inorg. Chim. Acuc, 1972,6, 197. 35. S. Kaizaki, Bull. Chem. Soc. Jpn., 1983, 56, 3625. 36. A. P. Sattelberger, D. D. Darsow and W.B. Schaap, fnorg. Chem., 1Y6,15, 1412. 37. S. Kaizaki, J. Hidaka and Y. Shimura, Bull. Chem. SOC. Jpn., 1975,48,902. 38. A. D. Kirk and G.B. Porter, Zmrg. Chem., 1980, 19, 445. 39. I. J. Kindred and D. A. House, J. Inorg. N w l . Chem., 1975,37, 1359. 40. C. J. Weschler, J. C. Sullivan and E. Deutsch, Inorg. Chem., 1974, 13,2360. 41. I. K. Adzamli and E.Deutsch, Znorg. Chem., 1980,19, 1366. 42. R.C. Elder, L. R. Florian, R.E. Lake and A. M. Yacynych, Znorg. Chem., 1973, 12,2690. 43. C. J. Weschler and E. Deutsch, fnorg. Chem., 1973, U ,2682. 44. J. C. Sullivan, E. Deutsch, G. E. Adams, S. Gordon, W. A. Mulac and K. H. Schmidt, Inorg. Chem., 1976,15,2864. 45. G . J. Kennard and E. Deutsch, Znorg. Chem., 1978,17, 2225. 46. J. Ribas, M. Monfort and J. Casabo, Tmnsition Met. Chem., 19M, 9, 407. 47. J. C. Chang, L. E. Gerdom, N. C. Baenziger and H.M. Goff, Inorg. Chem., 1983,22,1739. 48. A. Botar, E. Blasius and H. Augustin, Z. Anorg. A&. Chem., 1976, 422, 54. 49. W. G. Jackson and W. W. Fee, horg. Chem., 1975,14, 1161. 50. J. W. Vaughn and J. Marzowski, Inorg. Chem., 1973,12,2346. 51. C . D i k , A. Segul, J. Ribas, X. Soians, M. Font-Altaba, A . Solans and J . Casabb, Transition Met. Chem., 1984,9, 469 52. J. W. Vaughn and G. J. Seiler, Synth. React. Inorg. Metal-Org. Chem., 1979,9, 1. 53. J. W. Vaughn, J. Cryst. Spactrosc. Res., 3983, W, 231. 54. S. Kaizaki, M. Akaho, Y.Morita and K. Uno, Bull. Chem. SOC.Jpn., 1981, 54, 3191. 55. R. Stomberg and I. Larking, Acta Chem. Scund., 1969, 23, 343. 56. N. E. Dixon, G. A. Lawance, P. A. Lay and A. M. Sargeson, h o g . Chem., 1984,23,2940. 57. C. Brouty, P. Spinat and A. Whder, Acta Crystallogr., Sect. B, 1980, 36, 2037. 58. P. Spinat, A. Whuler and C. Brouty, Acm Crystallogr., Sect. B, 1979, 35, 2914.
Table 50 Complexes of Diamines Excluding Ethylenediamine Except for Mixed Diamine Complexes and Some Oxalato, Malonato and Acetylacetonato Complexes (see also Table 49)
Ref
Complex
CrN6 [Cr(en),(pn)]X, (X = CI, Br, SCN) [Cr(en)(pn),]X, (X = Cl, SCN, CIO,) [Cr(en),(tmd)]X3 (X = C1, Br, SCN, (+),&A conf.) [Cr(en)(tmd),]X3 (X = Cl, Br, I, SCN, (+),-Br A conf.) [Cr(pn)(tmd),]X, (X = C1, S C N ) [Cr(pn),(tmd)]X, (X = C1, S C N ) [Cr(en)(pn)(tmd)]X, (X = CI, SCN) [Cr(en),(ibn)]Br, fCr(en),(chxn)]Br, [Cr(tmd),(ibn)]Br, P(pn),(ibn)lBr, [Cr(pn),(chxn)lBr,
1,273 2,4
CrX,N, trans-[CrF,(tmd),]X, (X = CI, I, ClO,) trans-[CrBrF(tmd),]ClO, trarn-[CrF(NH3)(tmd),1X,(X = ClO,? X, = Br, CIO,) fram-[Cr(NH,)(H,O)( tmd),]BrzC1O, tram-[CrCl,(tmd),]X (X = CI. Br, Clod, NO3) rrans-[CrBr,(tmd),]X (X = Br, C104) trans- and cis-[Cr(NCS),(tmd),]SCN, (+)ss9- and (-),,-isomers, A and A conf. cis- and trans-[Cr(H,O),(tmd),] (NO,), cis- and trans-[Cr(ONO),(tmd)2]C104, (+)480-C104 rrans-[Cr(OH)(H,O)(tmd)~]X~ (X= ClO,, NO,) cis-[Cr(OH)(H,O)(tmd),]X, (X = NO,, X, = &Ob) cis-[CrCl,(tmd),]X (X = C1, C10, and (+)D-CIO,) and (-),,-isomers, former A conf. tmn.s- and cis-[Cr(N3),(tmd),]C04, trm-[Cr(CH3C0,),(tmd)z]X (X = C1, I, C104) rra~-[Cr(nic-0),(tmd),]C1 (nic;O = 0-tmnded nicotinate) tram-[A][CrF(H,O)(tmd),][X][A = K,Na, NH,; X = Cr(CN),, CrNO(CN),] tram-[CrX,(en)(tmd)]X [X = C1, Br, SCN] cis- [CrCI2(en)(tmd)]CI
truns-[CrCl2(en)(tmd)]C10, rrans-[Cr(OH)(H,O) (en) (tmd)]CLO,
6,7 8 8
8 9,10 10 2,11,12 11 11,13 10,11 11,14 9 11
7 15 16 2,17 2,17 18 18
Chromium Table 50
(continued)
Complex tram-Cr[F,(pn),]X (X = Br, CIO,, [CrF,(pn)]; also (R)-pn) cis-Cr[F,(pn),]X (X = CIO,, BPh,) tram-[CrF(H,O)(pn),]Xz (X = Br, I, CIO,) cis-[CrFX(pn),]X (X = Cl, Br, I) cis-[CrX,(pn),]X (X = C1, Br) tram-[CrBr,(pn),]Br, also R-pn tram-[CrCl,(pn),]X (X = CI, CIO,; also R-pn) tram-[Cr(NCS),(pn),]SCN cis and tram-[Cr(NCO (pn)JNCO trans-[Cr(ONO),( R -pn),]CIO, cis- and t.am-[CrF,(en)(pn)]Br, also R-pn traw-[CrX,(en)(pn)]X (X = C1, Br, SCN) tram-[CrF(W,O)(en)(pn)]X, (X = CI, Br, I, C10,) tram-[CrBr,(en)(pn>]ClO, trum-[CrFCl(en)(pn)]ClO, tram-[Cr(ONO),(en) (R -pn)]ClO, cis-[CrFCl(en) (pn)]Cl cis-[CrFBr(en)(pn)]Br cis-[CrX,(en)(pn)]X (X = CI, Br) tram-[CrF,(pn)( tmd)]Br, also R-pn trum-[CrF(HzO)(pn)(tmd)]Xz (X = C1, Br, I, NO3, C10,) tram-[Cr(ONO),(R -pn)(tmd) ]ClO, trm-[CrX,(pn)(tmd)]X (X = C1, Br, SCN) cis-[CrX,(pn)(tmd)]X (X = Cl, Br) cis-[CrFCl(pn)(tmd)]C1 ci.s+[CrFBr(pn)(tmd)]Br trum-[CrFz(bn)z]C104 tram-[CrX,(bn),]X (X = C1, Br) cis-[CrX,(bn),]X (X = C1, Br) cis-[CrClz(AA),]C1O, (AA = ibn, meso- and (f)-bn) rrum-[CrF,(ptn),]ClO,, also R,R- and S,S-ptn trans-[CrCl,(ptn),]X (X = C1, ClO,, also S,S-ptn) tram-[CrBr,(S,S-ptn),]Br cis-[CrCl,(ptn),]CL tram-[CrF,(en)(R, R-ptn)]Br trum-[Cr(ONO),(en)(R, R-ptn)]ClO, tram -[Cr(ONO)z( R, R -ptn),] Clod trans-[CrF,(R-pn)(R,R-ptn)lBr, also S-pn or R,S-ptn tram-[Cr(ONo),(R-pn)(R, R-ptn)]C104,also R,S-and S,S-ptn tram-[CrF,(chxn),]C104, frans-[~F,(R,R-~hxn)~]Cl trans-[CrF(ONO)(R,R-chxn),]ClO, tram-[Cr(ONO),( R, R -chxn),]ClO, trans-[CrCl,(chxn),]Cl cis-[CrCl,( chxn) ,] CI tram-[CrF,(R, R-chxn)(en)]Br cis-[CrF,(chxn)(pn)]X (X= Br, CIO,) tram-[CrF,(R, R -chxn)(R-pn)]Br, also S-pn cis- and tram-[Cr(NCO),(chxn),lNCO tram-[CrF,(S, S-stien),]ClO, tranr-[CrX,(S,S-stien),lX (X = C1, Br) [CrC12(opd),]Cl, [CrCl,( opd),]CI (monodentate amine)
-
CrX,N, [C~Fz(HzO)z(Pn)l(X= a,Br, I) IPnH,I[CrF,(Pn)lC1 [CrF,(pn),][CrF,@n)], also Na salt [NBu4l[Cr(NCO),(Pn)l A[Cr(NCS),(tmd)l (A = K, Ag) [NBu4l[Cr(NCoh(cb)1 [chxnH,][CrF,(chxn)]cl [CWH,O),(chxn)lBr [CrBrF,(H,O)(aa)] (aa = pn, chxn) Oxalato-, malonato- and acegylacetonato-amine complexes [Cr(ox)(L),]C104 (L = en, pn, tmd; (-),,-cations, A conf.)
[cr(ox)(en)zl[c~(~x),(en~la
[Cr(mal)(en),]X (X = C104, [Cr(mal),(en)]) [Cr(ox)(&O),(en)lBr K[Cr(ox),(en)]~KI~2HzOa
793
Ref.
1,6,19,20 19,21 22 22 23 24,29 25,29 2 2,30 13 1,20,21 2.17 1;21 21 21 13 21 21 2,17 1,20 1,21 13 2, '17 2,17 21 21 26 26 26 27 20,26,28,29 26,29 29 26 20 13 13
20 13 20,26 13 13
26 9 20 1 20 30 29 29 31
1,21,32 32 1 30 12 30 32 32 32
35 36 37 37 38
Chromium
794
Table 50 (continued)
Complex
[Cr(L)(en),]CI2 (L = acac, 3-C1- and 3-Br-acac) [Cr(L),(en)]CI (L = acac, 3-CI- and 3-Br-acac) [Cr(L)(L')(en)]Cl (L = acac, 3-C1- and 3-Br-acac) [Cr(acac)(tmd),]I, [Cr(acac),(tmd)]I, (-),,-cation, A conf." [Cr(ox)(NH,),IC104 a Crystal
Ref.
39 40 40 40 41 41,42 35
structure determinations; also for rran.s-[CrF(H,0)(tmd),1(C104)~ and ( -)589-1Cr(PICS),(imd),l.0.5[Sb,(+)-ta~t,]~
1. J. W. Vaughn and J. Marzowski, Iwrg. Chem., 1973.12,2346. A. Uehara, Y. Nishiyama and R. Tsuchiya, Inorg. Chem., 1982, 21, 2422. J. A. McLean, Jr. and A. Bhattacharyya, Inorg. Chim. Acta, 1981, 53, L43. K. Kiihn, F. Wasgestian and H. Kupka, 1. Phys. Chem., 1981, 85,665. M. Rancke-Madsen and F. Woldbye, Acta Chem. Scand., 1972, 26,3405. J. Glerup, J. Josephsen, K. Michelsen, E. Pedersen and C. E. Scheer, Acta Chem. Scand., 1970, 24, 247. J. W. Vaughn, G. J. Seiler, M. W. Johnson and G. L. Traister, Inorg. Chem., 1970, 9, 2786. J. W. Vaughn, Inorg. Chem., 1981,20,2397. 9. E. Pedersen, Acta Chem. Scund., 1970, 24, 3362. 10. M. C. Couldwell and D. A. House, Inorg. Chem., 1972,11, 2024. 11. M. Nakano and S. Kawaguchi, Bull. Chem. SOC. Jpn., 1979, 52, 3563. 12. E. Blasius and E. Mernke, Z. Anorg. A @ Chem.. 1984,509, 167. 13. S. Kaizaki, Bull. Chem. SOC.Jpn., 1983,56, 3625. 14. T. Laier, B. Niclscn and J. Springhorg, Acta Chem. Scund., Ser. A , 1982, 34, 91. 15. C. A. Green, R. J. Bianchini and J . I. L e g , Inorg. Chem., 1984,23, 2713. 16. J. Ribas, M. Montfort and J. Casab6, Transition Met. Chem., 1984, 9 , 407. 17. S. Mitra, T. Yoshikuni, A. Uehara and R. Tsuchiya, Bull. Chem. SOC.Jpn., 1979, 52, 2569. 18. M. C. Couldwell, D. A. House and H. K. J. Powell, Inorg. Chem., 1973, 32, 627. 19. J. Casab6, A. Solans and J . Real, Synth. React. lnorg. Metal-Org. Chem., 1983, W, 835. 20. S. Kaizaki, Bull. Chem. SOC.Jpn., 1983,56,3620. 21. J . W. Vaughn and G. J. Seiler, Inorg. C h m . , 1974, W, 598. 22. J. Ribas, M. L. Martinez, M. Serra, M. Montfort, A. Escuer and N. Navarro, Transition Met. Chem., 1983,8,87. 23. J. A. McLean, Jr. and N. A. Maes, Inorg. Nucl. Chem. Lett., 1972, 8, 147. 24. J. A. McLean, Jr. and R. I. Goorman, Inorg. Nucl. Chem. Lett., 1971,7, 9. 25. J. A. McLcan, Jr. and R. Barona, Innorg. Nucl. Chem. Lett., 1969, 5 , 385. 26. R. Tsuchiya, A. Uehara and T. Yoshikuni, Inorg. Chem., 1982, 21,590. 27. C. P. Madhusudham and J . A. McLean, Jr., Inorg. Chem., 1975, 14, 82. 28. H,Toftlund and E. Pedersen, A C ~Chem. Q Scand., 1972, 26, 4019. 29. S. Kaizaki and Y . Shimura, Bull. Chem. Soc. Jpn., 1975, 48, 3611. 30. E. Blasius and G. Klcmm, Z. Anorg. Allg. Chem., 19778,443, 265. 31. J. C. Chang, J . Inorg. Nucl. Chem., 1975, 37,855. 32. C. DiaZ, A. Segui, J. Ribas, X.Solans, M. Font-Altaba, A. Solans and J. CasaM, Transition Met. Chem., 1984, 9, 469. 33. X. Solans, M. Font-Altaba, M . Montfort and J. Ribas, Acra Crysrallogr., Secr. B, 1982, 38, 2899. 34. K. Matsumoto, S. Ooi, H. Kawaguchi, M. Nakano and S. Kawaguchi, Bull. Chem. Soc. Jpn., 1982, 55, 1840. 35. D. A. House, Inorg. Chim. Acta, 1982, 60, 145. 36. J. W.Lethbridge, L. S. Dent Glasser and H. F. W. Taylor, J . Chem. Soc. ( A ) , 1970, 1%2. 37. J. W . Lethbridge, J . Chem. SOC., Dalton Tram., 1980, 2039. 38. L. S . Dent Glasser and J. W . Lethbridge, 1. Chem. SOC., Dalton Trans., 1976, 2065. 39. S. Kaizaki, M. Akaho, Y. Morita and K. Uno, Bull. Chem. SOC.Jpn., 1981, 54,3191. 40. S. Kaizaki, J. Hidaka and Y. Shimura, Inorg. Chem., 1973,12, 135. 41. M. Nakano, S. Kawaguchi and H. Kawoguchi, Bull. Chem. SOC.Jpn., 1979,52, 2897. 42. K. Matsumoto and S. Ooi, Bull. Chem. SOC. Jpn., 1979.52, 3307. 2. 3. 4. 5. 6. 7. 8.
provides aqua derivatives, e.g. fr~rn-[CrBr(H~O)(en)~]Br~, with rentention of configuration since the Cr-0 bond does not break. Anation in dipolar aprotic solvents such as DMF, DMSO and DMA, and solvolysis induced by the addition of AgCIQ4, HgFz or Hg(MeC02)2 are also useful preparative procedures.404,40sThe inversion in relative solubility of perchlorates and halides in passing from water to these solvents has facilitated the isolation of certain water-soluble complexes. Examples of the preparations of ~is -[C r(C N)~(en )~]~+ salts, mono- and di-ammine- and fluoro-bis(ethy1enediamine) salts are given in Schemes 39 to 46. (b) Cyano series. The preparation of a series of cis -dicyanobis(ethylenediamine)chromium(II1) salts (Scheme 39) starts with the reaction of green tr~ns-[CrCl~(H~O)~]C1-2H and liquid hydrogen cyanide, but no trans-dicyano complex was obtained, presumably because of the lability of systems with CN trans to CN.327The purity of ~is-[Cr(CN)~(en)~]' was established by ion exchange chromatography, and its reactions and visible and IR spectra, and particularly the resolution of the iodide, support the cis assignment. The v ( E N ) absorption of 2139 cm-' was unexpectedly unsplit, but there were four v(Cr-N, en) bands at
Chromium
795
550, 536, 491 and 425m-' and trans complexes show fewer bands in this region. Unusually, spin-forbidden bands were observed in the solution spectrum. From the CD spectra the ( )589 isomer was assigned the A configuration (see also Section 35.4.1.l.ii).
+
HCN (OOC) + CrCl,.6H20
cis-[Cr(CN),(en),]CI
green
I
NaI or NaCIO,
~is-[Cr(CN)(H,O)(en)~l~+ + HCN
acis-[Cr(CN),(en),\Cl
LlCl
then p I l 4
DMF
I
cis-[Cr(CN),(en),]X lemon yellow
I
HCI or HBr
(NH4N + )D-~-[CIUHMSO&~
cis -[CrX,(en),]
( + )w[C~(CN),(~~)~II %%[Cr(CN>,(en),l[C,,H,,SO,BrI+ filtrate
+
%% - )58dCr(CN)~Ien)zlI
Scheme 39
A safer procedure, starting from aqueous [Cr(en),]Cl, and NaCN in the presence of charcoal, has been reported- to yield both trans- and cis-[Cr(CN),(en),]Cl, but others330have not been successful with this method. ( c ) Reactions with liquid ammonia. Ammonolysis of [CrX$(en)$+ salts sealed in Carius tubes with liquid ammonia has afforded many mono- or di-ammine derivatives (Table 49 and The preparation of the cis isomers is straightforward, but thermal Schemes 40-45).w7substitution with trans + cis rearrangement is common. Thus, ~is-[CrCl~(en)~]' reacts smoothly to yield ~is-[Cr(NH3)2(en)~]~+ with a few per cent of the tram compound, whereas trans-[CrC12(or Brz)(en) gives 95% cis-[Cr (NH3),(en)z]3+ ; trans- [Cr(NH3)z(en)2]3+has been isolateda8 in 30% yield from the less accessible complex tr~ns-[CrCl(NH~)(en)~](ClO~)~ (Scheme 44) by ammonolysis followed by fractional crystallization. For a particular pair of cis and trans isomers the geometrical assignment was based on IR spectral data and on the overall molar absorptivities, the cis having the more intense bands, because the electronic spectra are otherwise very similar. An easier route to ci~-[CrF(NH~)(en)~]~+ salts has been devised (Scheme 45), and the isolation of one chiral form has confirmed the cis geometry (A configuration).m HPLC was useful in determining the cb/truns ratio (4:l) in the crude ammonolysis product. +
trans-[CrBrF(en)zlC104
NH3W
,Iace NH4Ct04,
0°C
'
60/40cis/trans-[CrF(NH3)(en),]BrC10, HCI. then fract. cryst. with
HC104
'
trans- and cis -[CrF(NH3)(en)2]( C104),
Scheme 40
i , HCI (N0,r) ii. HBr
-
Scheme 41
cis-[CrC12(en),]C104 trans -[CrCI(H,O)( en),]B r2
NaCi04
trans-[CrCI(NH,)(en),](CIO,)~
NH3(1)
NHdU
cis-[Cr(NH3),(en),](C104),
Scheme 42
trans- [Cl(en),CrOHCr( en),C1] (C104), NH3(I), LiNHZ, 10°C then HBr
Scheme 43
truns-[Cr(NH,)(HzO)(en),]Br,
-
2 : 1 ~is/truns-[Cr(NH~)~(en)~]CI(C10~)~ fract cryst.
HCI, HC104
Scheme 44 COC3-Z*
cis-[CrXBr(en),]Br2 X = CI, Br
trans-[Cr(NH,),(en),](ClO,),
Scheme 45
analion. MeOH
anation, MeOH
Chromium
797
empirical, but when X-ra single crystal investigations have been carried out, they have confirmed the assignments.To4 The four possible isomers of [Cr(cis-l,2-~hxn)~]~+ (mer-A, mer-A, fac-A and fac-A) have been separated by cation exchange using disodium hydrogen phosphate and disodium tartrate, and isolated as the perchlorates. They were identified from their electronic, CD and ESR behaviour and comparisons with the Co"' analogues. The ligand has two dissymmetric centres of opposite chirality (R, S) and fac and mer refer to the spatial arrangement' of the dissymmetric centres.416Some correlations of X-ray powder photographs and absolute configurations of ICr(trans-1,2-chxn),]C13 have been made.417 Developments in instrumentation have allowed measurements of the single-crystal CD spectra of [( )D-Cr(en)3]"+doped in [Ir(en)3]C13between 7 and 293 K. Transitions to the excited states 4T2,'TI, 'E, '& and 'T2 were observed.418The discussion refers to much earlier work. The polarized electronic spectra of single crystals of several trar~-Cr[XY(en)~] complexes have been assigned and ligand field parameters evaluated.419CD, absorption and circularly olarized emission spectra have been reported for [Cr(en),],+ in the region of the 4AZg*'Eg Kg transition^.^^ Thermal deamination of tris(ethylenediamine)chromium(III) complexes is a standard preparative method for cis - and trans-diacidobis(ethy1enediamine) complexes4z1~4Z2 and the thermal behaviour of the starting materials has been related to their crystal structures.423The cyano com lex ~is-[Cr(CN)~(en)~]ClO~ in DMSO undergoes stepwise reduction 111- 11- 1 at the DME.14 The standard redox potential for the Cr"'/Cr'I couple is -1.586 V (versus SCE). From Raman investigations the strong metal sensitive band at 450-600 cm-l shown by [Cr(en)3]3+compounds has been assigned to the totally symmetric M-N stretching vibration and the band at 280-320crnp1 to a totally symmetric N-M-N bending mode. Studies of electronic spectra suggest that the vibration responsible for the latter band has at least as much M-N stretching character as the former.4z It appears that mixed ring conformations can be identified from the IR spectra of tris(ethy1enediamine) complexes.426 The XPES of a number of ethylenediamine and cyano complexes, including [Cr(en)3]C13 and K3Cr(CN)6, have been measured in order to obtain the difference between the Nls and Cls binding energies. For the en complexes the A E values are related to (T donation, and for the cyano corn lexes are mainly controlled by the nitrogen charge, which depends on the IT back-donation.
+
9
gu
(ii) Bidentate diamines excluding ethylenediamine Synthetic methods with bidentate amines generally follow those developed for complexes of ethylenediamine, but there can be differences in detail, e.g. thermal dehydration of tr~ns-[CrF(H~O)(en)~]X~ (X = C1, Br, I, SCN) produces cis-FX isomers, whereas the major products from tr~ns-[CrF(H~O)(trnd)~]X~ are the trans-FX isomers (Table 50). Similarly, the reaction of li uid ammonia with tr~ns-[CrFBr(en)~]ClO~ provides cis- and transCrF(NH3)(en)Z!+ in 60 :40 ratio; with tr~ns-[CrFBr(tmd)~]ClO~ the only product is trans/CrF(NH3)(tmd)Z]2+;tmd complexes appear more resistant to stereochemical change than en complexes. Reactions of diamines with CrCI3-6H2Oin aqueous HF, or tr~ns-[CrF~(py)~]ClO~ in 2-methoxyethanol under reflux are general synthetic methods for trans-[CrF~(AA)$ camplexes. However, the less accessible isomers, ~is-[crF,(AA)~]'(AA = en and pn) have now been isolated from the mother liquors left by the less soluble trans complexes or by using longer reaction times to allow trans +cis isomerization to Q C C U ~ . ~ ' , ~ ~ The structure of trans-[CrF(NH3)(tmd)2](C104)zhas been confirmed crystallographically.429 In the absence of crystallographic data, assignments of geometry in [CrX2(diamine)Z] complexes can be aided by IR spectroscopy. Commonly, trans isomers show essentially three bands in the 390 to 55Ocm-' region, while cis isomers show four or more (Table 51),430 although these criteria did not work well with a series of cis and trans complexes [CrXz(tmd)2]"' (X= NCS, ONO, N3, H20; XZ= OH, €€20).431 Complexes containing Cr-F groups give strong bands in the 350-370 cm-l range due to Cr-F stretching vibrations coupled with other metal-ligand modes. The bands are low by comparison with other chromium(II1) fluoride complexes (Table 51) and other chromium-halogen modes, Le. C r - C l , 320-350 cm-', Cr-Br, 250-280
Chromium
798
Table 51 IR Spectra (600-200cm-') of Complexes [CrFX(AA),]+ and [CrF(H,O)(AA),]z' Complex trans-[CrF,( en),]Cl trans-[CrF,(en),]ClO, trans-[CrClz(en),]C104 tr~ns-[CrBr,(en)~]C10, trans-[CrC1F(tmd),]C104 tram -[CrBrF(tmd),]C10, trans-[CrFz(pn),JC1 tram-[CrCI2(NH,),]ClO,
Chelate ring deformation 552s, 525sh 560s, 520sh 558s, 535sh 552s, 531sh 550s 550s 545sh, 52Sm
552s 565w, 55Ow ~ ~ ~ ~ - [ C r F ( o ~ , ) ~ P n ) , l 570sh, ( ~ ~ ~523s 4~~ 538 trans-[CrF(NH,)(trnd),](ClO,), 545vs cis-[CrClF(tmd),]Cl
tram -[CrF(OHz)(en),](C104)2 tram -[CrF(OHz)(en),]Br,
cis-[CrBrF(tmd)JBr
545m, 52Ow
cb-[CrCIF(en),]CI
575sh, 55Ow, 525w 570sh, S5Ow, 525w 575sh, 550m, 525w
cis-[CrBrF(en),]Br cis-[CrFI(en),]I
Y ( Cr-N)
492m, 452m 488w, 442m 4%, 445m 488mw, 445m 4Wm, 435m 490m, 435m 462w, 442m 46Sm, 450m, 435m 475w, 445m 485w, 445m 495sh, 450m 492, 435 49Omw, 445m, 42Ow 488w, 445w, 44Ow, 42Ow
49Ow, 445m, 42Ow 48Sw, 445111, 42Ow 48Ow, 445m, 42Ow
v( Cr-F)
Y ( Cr-X) (X = Cl, Br, I )
370s 370s
370s 370s 370s
35ovs 285vs 325m 250s
330s
350sh 365ms 365s 365s 358 36ovs
325m
365s
240s
355m
32Ow
3S5m
240111
355m
225m
489 535 522
(iii) Mixed bidentate amines including ethylenediamine
I
At the time of the Garner and House review3'' there were only five known complexes of CrlI1 with two different amines; this is no longer the situation (Table 50) and there seem to be no particular difficulties in obtaining mixed amine complexes.432"'" Thermal deamination in the solid state of the diamine and mixed diamine complexes [Cr(AA),]X,, [Cr(AA)*(BB)]X3 and [Cr(AA)(BB)(CC)]X3 (AA, BB and CC represent the amines en, pn and tmd and X = C1 or SCN)takes place with the loss of one diamine molecule (the lowest boiling amine with the mixed complexes). The thiocyanates give transbis(diamine)bis(isothiocyanato) complexes but the chlorides form cis complexes although isomerization to trans complexes occurs on further heating, especially when cis-[CrClz(tmd),]Cl is the initial The cis c,trans thermal isomerization has also been investigated in cisand trans-[CrX,(AA),]X"' (AA=bn, chxn or ptn; X=C1, Sr). The presence of two six-membered chelate rings or large cations seems to encourage the formation of trans p r o d ~ c t s .Since ~ ~ ~intermediates ,~~~ such as mer-[CrC13(bn)2].H20 have been isolated, isomerization goes through bond rupture. A number of complexes with two identical chiral diamines or two mixed diamines of the types truns-[CrFZ(N4)]+and trans-[Cr(ONO),(N,)]+ have been prepared. The diamines are en, tmd, (R)-and (S)-propylenediamine, (lR, 2R)-1,2-cyclohexanediamine (chxn) and (2R,4S)-2,4pentanediamine (ptn) (Table 50). The trans configurations follow from the spectral resemblances to trans-[CrXz(en)$+ complexes. Contributions to the CD of complexes with chiral diamines can come from the conformation of the puckered rings and the vicinal effect of asymmetric atoms on the ligands. With trans-[CrF2(N4)]+ complexes the CD contributions are separable and additive, except for bis(R-pn), bis(R,R-chxn) complexes and mono@-pn) complexes with en and tmd. The overall CD arises mainly from the effect of the 1 ring conformation^.^^^ By studying the CD spectra of the complexes frans-[Cr(ON0)z(N4)]+ with the same amines in the region of intraligand n -+ n* nitrito absorption bands, the existence of chiral rotational isomers of the nitrito ligands has been deduced. Marked solvent variations of the intraligand CD s ectra are attributed to the effects of solvent bulk and donor number on rotamer equilibria. 439 From the crystal structure of cis-[Cr(NCS)z(tmd)2][Sbz(tart)2]z~2H20, the cation has the A configuration and this agrees with the assignment from the CD ~ p e c t r a . ~ ~ ~ , ~ ~ '
Chromium
799
The Pfeiffer effect (the shift in a chiral equilibrium on the addition of an optical isomer of a different compound) of racemic [ C ~ ( O X ) ~has ] ~been examined usin for the first time optically stable metal complexes c~S-[MXY(AA)~]"+ (where M = C$+ or Cog+,AA = en or tmd and X and Y = anionic monodentate ligand). It was found that the chiral equilibrium of [ C ~ ( O X ) ~ ] ~ was always displaced in favour of its A enantiomer in the presence of A enantiomers of the cis complexes, and it is proposed that the absolute configurations of cis complexes could be of an aqueous inferred from the equilibrium shift induced in [ C ~ ( O X ) ~ Laser ] ~ - . ~irradiation ~ solution of racemic [Cr(~x)~(phen)]-or [Cr(~x)(phen)~]+ in the presence of ( + )- or ( - )-cinchonine hydrochloride rapidly shifts the chiral equilibrium in a direction opposite to that induced by the usual Pfeiffer effect in the dark.439
(iv) Ethylenediamine and related diamines -polynuclear
complexes
Many analogues of the dinuclear ammines (Table 47) are known in which ammonia molecules may be considered replaced by ethylenediamine and other bidentate ligands (Table 52). The inter-relations among the complexes are similar to those in Schemes 35 and 36. There are additional examples in the review by Garner and House302and other bridged complexes are covered in Sections 35.4.1.1, 35.4.2.3.vi7 35.4.2.4, 35.4.2.5, 35.4.4.2, 35.4.4.10, 35.4.7.4 and 35.4.8.3. Cyano-bridged complexes are listed in Table 45. In a recent investigationu0 the racemic binuclear cations A ,A-A,A-[(en)2Cr(OH)zCr(en)2]4+ and A,A-A,A-[(H20)(en)2Cr(OH)Cr(en)2(0H)]4+ have been isolated from the meso isomer A,A-[(en)zCr(OH)zCr(en)2]4t as various salts (Table 52). The equilibria between the cations have been studied. The binuclear cations could not be separated into their enantiomers A,A and A,A because they react with classical resolving agents to form y -carboxylate compounds which have been isolated,441e.g. A,A-[(en)2Cr(OH)(MeC02)Cr(en)2](C104)4. They contain symmetrical carboxylate bridges and deprotonate in strong base to give y-carboxylato-p-oxo takes complexes. De rotonation of [(en)zCr(OH)zCr(en)2]4+to [bg~)~Cr(O)(OH)Cr(en)~]~+ place similarly' and sulfato-bridged systems are also known. As with the dinuclear ammines, most research has been aimed at relating the antiferromagnetic behaviour and spectroscopic properties of the dinuclear complexes to their structures. Great variations in J have been observed for the same cation [(en)2Cr(OH)2Cr(en)2]4+ (R = H in (96))370 in different environments3'* and between these cations and [(NH3)4Cr(OH)zCr(NH3)4]4+(table in ref. 374). Thus external factors such as hydrogen bonding must have an appreciable effect on the exchange in these complexes, probably through altering 8. It has been possible to derive J for several salts of [(en)2Cr(OH)zCr(en)Z]4+from low-temperature single-crystal absorption and powder luminescence spectra in the region of the 4A24A2+4A22Etransition of the paired ~ystem.~" The difference of up to 10% from the values derived from powder magnetic susceptibility data indicates a slight temperature dependence of J. The complex [(en)zCr(OH)2Co(en)2](S206)2has been prepared by heating the cocrystallized optically active dithionates (equation 30).44sIt is an internal active racemate because the chiral centres are different, and it contains CrI" in a dinuclear system not subject to magnetic interaction so that the ESR spectrum is simple.
-
h-~is-[Cr(OH)(H~O(en)~]S,0, + A-cir-[C0(0H)(H,O)(en)~]S~0~ (-)589-A. A-[(en),Cr(OH),Co(en),j(Sz06)2
(30)
A crystal structure determination has confirmed that A,A-[HO(en)zCrOHCr(en)20H](C104)3.H20 formed in the reversible ring opening of A,A-[(en)2Cr(OH)2Cr(en)2]4+ has the structure inferred from its chemical properties.* Analogous complexes are apparently formed by tmd.4Q7 The linear trinuclear cation (100)and the tetranuclear cations (101) to (103)44w50(Table 52) exist with mononuclear complexes in aqueous ethylenediamine buffer solutions in which equilibrium has been established by the catalytic effect of CS+ and charcoal under well-specified condition^."^ The cations were separated by ion exchange chromatography and fractional crystallization, and isolated as various salts (Table 52). The tetranuclear (rhodoso) compound (104)451 has been repreparedW8by a modification of Pfeiffer's method. The antiferromagnetic behaviour of (101) and (104) has been correlated with their structures. For (101) all models indicate that Jlz = J34>Jn and that a spin singlet lies lowest with a triplet
800
Chromium
W N 3 I-*
n
5. k
h
s s s
m
v
8 ri
0
0
@E E
6
801
Chromium
Y
s
e
(8 f It
2
f
802
Chromium
803
Chromium
and a quintet at about 10 cm-' and 32 cm-l respectively.@' In (102)the dominant exchange is between the central and each of the three peripheral chromium ions. An S = 3 level lies lowest and there is a complicated temperature variation of peH.450
(v) Bidentate ligands with primary amine and N heterocyclic donor groups: ( 2-pyridy l)methylamine and 1 - (2-pyridy1)ethylamine
The geometrical and optical isomerism exhibited by tris(amine) complexes of the bidentate ligands (105) and (106) has been investigated. The fac and mer isomers of [ C r ( ~ i c ) ~ (107) ]~+ and (108) have been prepared by utilizing slightly hfferent starting materials and solvents (Table 53). The fac complexes are yellow and less soluble than the orange mer complexes. The latter have more intense, less symmetrical electronic absorption bands to lower energy. The assignments are confirmed by the isomorphism with corresponding cobalt(II1) complexes for which the 'H NMR spectra of the methylene groups clearly indicate the isomeric types.45Z Table 53 Complexes of (2-Pyridy1)methyIarnine (2-Picolylamine) Complex f~C-[Cr(Pic),lX3 mer-[Cr(~ic)~]X~ a&-[ CrX,( pic),]Y
Ref.
Comments
Yellow, X = CI, Br, I, trans-[CrBr,(py),]I in ZMeO(CH,),OH/EtOH [Cr(pic)J3+, log K , = 5.65, log K, = 3.16, log K3= 2.21, 25 "C Orange, X = 3r, I, trurzs-[CrBr,(py).,]CI in py/EtOH X=F, Br, Y = Br, I; X = C1, Y = C1, I, X=HZO,Y = C1; Xz = OH, H,O, Y = CI,O.SS,O,; in last Cr-N(py (tram), 2.064; Cr-NH,, 2.054,2.089 C r a H , 1.926; Cr-OH,, 1.998 (-)o-~-cis-[CrCl,(pic),]CIO~, A conf. X = c1,Y =Cl, I, 0.5Sz0,; X = B r , Y = I X = H,O; Y = (NO,),, X = Y = Cl or Br
1
P-cis -[CrX,(pic),]Y trans-[CrX,(pic),]Y
1 2 1 3 4
5 3 6
1. K.Michelsen, Acta Chem. Scand., 1970,24,2003. 2. J. R. Siefker and R. D. Shah, Tdmrcr, 1!379,26,505. 3. K. Michelsen, Acta Chem.Scand., 1972,26, 1517. 4. S. Larsen, K! B. Nielsen and I. Trabjerg, Acta Chem. Scand., Ser. A , 1983,37, 833 5 . K. Michelsen, Acta Chern. Scand., Ser. A , 1976,30,521. 6. K. Michelsen, Acta Clwn. Scand., 1973, 27, 1828.
I
I
NH,
NHz (105) (2- Pyridy1)methylamine (pic)
facial
(106) 1-(2-Fyridyl)ethylamine (mepic)
meridional
F = pyridine N, M = methylamine N (107)
(10s)
The possible geometrical isomers of diacidobis(diamine) complexes containing two unsymmetrical amines such as pic are shown in (109) to (ll3). The (Y series are differentiated from the fi series by an approximate Cz axis, and from each other by the relative positions of the pyridine P and methylamine M nitrogen atoms. The preparative routes for cfi-u and ch-p series are outlined in Schemes 47 and 48. A1thoa:gh tran~-[CrX~(py)~]+ salts, where X = F, C1 or Br, were used as starting materials, only cis products were produced. In many cases lithium salts LiX (X = C1, Br or I) were added to aid crystallization. The 6 series were prepared from
Chromium
804
the chromium(I1) halide by oxidation with I2 in pyridine in the presence of pic, or from the tr~ras-[CrX~(py)~]+ salts in hot 2-methoxyethanol. The a-cis and p-cis assignments were and p-cismade from isomorphous relationships between a-~is-[CrCl~(pic)~]C1.H~0 [CrClz(pic)z]2Sz06,and the corresponding cobalt(II1) complexes ('H NMR). A decision between the cis trans cis or cis cis trans arrangements could not be made453but from the formation of members of the a series on acid treatment of related diols of crystallographically determined structures (see Section 35.4.2.3.vi below) the cis trans cis arrangement was ~O~, proposed and confirmed by a structure determination on [ C ~ ( H O ) ( O H ) ( P ~ C ) ~ ] Swhich showed that the cation belongs to the a-cis series with the pyridine nitrogen atoms trans.454The Cr-OH bond is considerably shorter than the Cr-OH2 bond (Table 53). cis-a series Aa20
CrC13 DMSO
c~-cis-[CrCl,(pic),]+y red
a-cir-[Cr(Hz0),(pic),13' orange OH-
'H* a-~is-[Cr(OH)(H,O)(pic),]~' A
-
tram-[CrF,(~y)~l+
pic, A
Z-MsO(CH2j2OH
pink
~u-cis-[CrBr,(pic),] + red
n-cis-[CrE2(pic),]+ red Scheme 47
cis-p series
s x
(10s) trans trans trans
X (ll0)trans cis cis
(111) cis trans cis
(112) cis cis rram 1y
series
/3 series (113) cis cis cis
In the trans series (Scheme 49) only one geometrical isomer was obtained, presumably the rrum trans trans because steric hindrance would prevent the formation of the tram cis ck isomer. The assignment was based on comparisons of colours and spectra with those of the bis(en) analogues.455 Complexes of the related amine 2-(2-pyridyl)ethylamine (or 2-(2-arninoethyl)pyridine,AEP) [CrX2(AEP)2]Y(where X = Y = Cl or Br) have been pre ared by methods similar to those in Scheme 48, but cis or trans assignments were not made.4 5 9
Chromium
805
trans series
c~-cis-[CrCl~( pic),]*
Ag2O
truns- and a-ck-[Cr( OH)(H,O)(~~C)~]'+
trans- [CrX,( pic),]Cl
green
e
trans-[Cr(H,O),(pic),] (NO& yellow-orange
Scheme 49
(vi) Di-p-hydroxo cumplexes containing (2-pyridyE)methylamine and
1 -(2-pyridyl)ethylarnine In the preparation of @-ci~-[CrBr~(pic)~]I a by-product assumed to contain the diol cation [ ( p i ~ ) ~ C r ( O H ) ~ C r ( p iwas c ) ~obtained.453 ]~~ Improved preparative procedures gave the optically active cations (- )D- and ( +)D-di-p-hydroxobis{bis(pic)chromium(III)} (Table 54).457Comparisons of CD spectral data suggest that the absolute configurations are A h and M respectively. The binuclear cation can in principle exist in several geometrical and configurational isomeric forms, but from models all except (114)and its antipode are grossly sterically hindered and this explains why only one pair of isomers is obtained. Acid treatment of the diols gives ( - )D- and ( + )D-~-cis-[CrClz(pic)z]+, identified by their absorption spectra; consequently they also have A and A configurations, assuming that cleavage occurs with retention of configuration. It is also believed from these results that the a-cis series have the cis truns cis configuration.
(114)AA isomer
The diols prepared from racemic mepic (Table 54), purified by column chromatography and resolved with sodium antimonyl ( )D-tartrate, t u q ed out to be identical with the pair prepared directly from (R)( + )D-mepic and (S)( - )D-mepic.458There are more potential geometrical isomers than in the pic series because of the introduction of the methyl group, but again almost all are ruled out because of steric hindrance. From models it appeared that the most stable diol would have the pyridine nitrogen atoms trans, and from the absorption and CD spectra it was concluded that the diol resulting from the preparation with (S)( - ),-mepic has the M absolute configuration. This last assignment was based on empirical rules for mononuclear complexes since all previously reported structures of chromium(II1) diols have been either meso forms or racemates. An X-ray analysis of ( +)n-[{(S)( -)Dmepi~)~Cr(OH)]~(S20&2H~0 supports the assignment (Table 54).459 The cation has almost C2symmetry with the pyridine N atoms trans. The interplanar angle between the pyridine rings is 50" indicating considerable strain within the binuclear complex because in the related mononuclear complex cis-aquahydroxobis(pic)chromium(III) dithionate the pyridine planes are almost perpendicular. The hydrogen atoms of the bridge lie in the coordination plane. The action of sodium hydroxide on the dioI gives the p-oxo-p-hydroxo- arid di-p-oxo complexes [L&r(O)(OH)Crbl3+ and [L&r(0)2CrL#+ (L = mepic; Table 54).460 The visible and CD spectral parameters have been obtained in media of appropriate pH. The structure of [(mepi~)~CrO(OH)Cr(mepic)~]Br~.5H~O has been determined by X-ray methods. Equal numbers of enantiomorphic cations (AA and AA) are present and, as was the case with the diol, only the isomer with the pyridine nitrogen atoms trans has been obtained. The Cr-Cr separation of 2.883 8, is substantially less than in any dihydroxo brid ed dimer so far reported (2.95-3.06 A). The Cr-N distances are in the range 2.057 to 2.130 and the oxo ligand may exert a trans effect since it is truns to the long Cr-N bond. The Cr202 bridging unit is significantly non-planar. The dinuclear complexes generally are antiferromagnetic and values of
+
i,
806
Chromium
Table 54 Dinuclear Complexes of (2-Pyridyl)methylamine, 1-(2-Pyridyl)ethylamine and Tetradentate Amines Complex
Related Open Chain
Cornrnem
X = C1, Br: from [CrX2(H2O),JX.2H,O with some CrCI, in 2MeO(CH,),OH; Zn gives X = 0.5ZnC14;resolved via (+)-
Ref.
1
[SbOC4H,0,] salts; also X = I, CIO,
(-ID-[{(
)D-mepic}2Cr(oH)1Z(C104)4
[(mepi~)~CrO(OH)Cr(rnepic)~]Br, Olive green
[(mepic)~Cr(0)zCr(mepic),lC1, Brown yellow
Open chain tetramines (see also Section 35.4.2.4) [(bispicen)Cr(OH)]Z(CIO,), Blue violet
[(bispicbn)Cr(OH)]Z(C104)4
From racemic mepic, [CrBr,(H,0),]Br.2H20 in 2-MeO(CH2),0H and Zn, ion exchange to remove Zn salts, also I and CIO, salts X = Br, CIO,, from racemic complex above via (+),-[SbOC,H,O,] salts; cations also from (+),- and (-)D-mepic respectively (below) From (-)D-mepic as above; also CIO, salt and crystal structure of S,O, salt: Cr-O-Cr, 101.9"; C r 4 , l . W (av); Cr-N (py and NH,), 2.059 (av), Cr - Cr , 3.021 A, (S)-mepic present, M configuration,singlet-triplet splitting 533 cm-' From (+),-mepic as above From racemic diol above and NaOH (2 M); antiferro., singlet-triplet sepn. 46 cm-',C r - O - C r , 100.6", Cr--OH-€r, 95.0", C r - 0 , 1.874 (av), C r - O H , 1.955 (av), Cr-N, 2.057 to 2.130, C r - C r , 2.883 A; pyridine Ns trans, AARRRR and M S S S S cations in equal nos. ( - ),-isomer from ( - ),-diol and NaOH (2 M) From diol chloride and NaOH (4M); antiferro., singlet-triplet sepn. 83 cn-',Cr-0 ca. 1.89 A
2
Antiferro., singlet-triplet sepn. 24 cm-', C r a r , 103.0" (av), Cr-0, 1.91-1.98, Cr-NH, 2.075 (av), Cr-N (py), 2.072 (av), C r - 0 , 3.046 A, racemic mixture, A b of Min cation, one cis-cu, one cis+ ligand Antiferro., singlet-triplet sepn. 44 to 50 m-' for various isomers from optically active and racemic bispi n ,'32 na Cr--O--Cr, 101.9" (av), Antiferro., singlet-triplet sepn. Cr-0,1.967 (av), Cr-NH, 2.080(av), Cr-N (py), 2.085 (av), C r - C r , 3.054 A, racemic mixture, A A or M in cation, cis-@ligand; also red isomer and [Cr{Cr(bispictmd}(0H),},](C1O4), From S,S-picbn, Mconf., also S,S-picchxncomplex
5
2 3
4
4
6,7 8
7
1. K.Michelsen, Acta Chern. Scand., Ser. A , 1976,30, 521. 2. K.Michelsen and E. Pedersen, Acta Chem.Scand., Ser. A, 1978, 32, 847. 3. S. Larsen and B. Hansen, Acta Chem. Scand., Ser. A, 1981,35, 205. 4. K.Michelsen, E. Pedersen, S. R. Wilson and D. J. Hodpon, Inorg. Chim. Acta, 1982,63, 141. 5 . M. A. Heinrichs, D. J. Hodgson, K. Mkhelsen and E. Pedersen, Znorg. Chern., 1984,23,3174. 6. K. Michelsen and E. Pedersen, Acta Chem. Scand., Ser. A , 1983, 37, 141. 7. Y . Yamamoto and Y. Shimura, Bull. Chem. Soc. Jpn., 1981, 54, 2924. 8. H. R. Rscher, D. J. Hodgson, K. Michelsen and E. Pedersen, Inorg. Chim. Acta, 1984, 88,143.
the singlet-triplet separations are given in Table 54. The chemistry of the diols in Table 54 which contain open chain tetramines is discussed in Section 35.4.2.4.v.
35.4.24 Open-chain polyamines
( i ) Diethylenetriamine (dien) and related ligands The complex [Cr(dien)21C13may be prepared by a number of methods. These include heating or heating a CrC& and dien directly,& or heating a solution of CrC13 and &en in solution of CrC1,.6H20 and dien in absolute The perchlorate [Cr(dien)4AC1O4)3has been obtained in crystalline form from a solution of the chloride in dilute HC104. Although three geometric isomers, two of which should be optically active, are possible for this corn lex, no resolutions or structural assi ments have so far been made. The kinetics of aquation''and the phosphorescence behaviour% have been reported. A number of complexes of the type [CrC1(AA)dienI2+ (AA = en, pn or tn) have been
Chromium
807
prepared by the reaction in equation (31).465 Only the a(C1)-b,c(AA)-d,f,e(dien) isomer was isolated in each case. These complexes undergo spontaneous, Hg2+- and base-catalyzed hydrolysis at similar rates and all seemingly by an associative interchange mechanism.M6 Details of the solution electronic spectra of the complexes and their hydrolysis products are in Table 55. Cr.1,.6H,o i, ii, iii, iv [CrC1(AA)dien]ZnC1, (31) i, DMSO, heat; ii, dien (1 :l), heat; iii, en (1 :l), heat; iv, [ZnCl,]'Table 55 Electronic Solution Spectra of [CrX(AA)dien]"+ Complexes".'
'+
[CICl(en)dien]
[CrCl(pn)dienr: [CrCl( tn)dien] [Cr(en)(dien)H,OI3+ [Cr(pn)(dien)H,O [Cr(tn)(dien)H,O]
r++
514 (79.3) 514 (74.5) 526 (71.5) 482 (69.4) 482 (65.3) 490 (58.9)
428 (24.5) 417 (22.9) 432 (21.3) 410 (21.7) 410 (19.5) 414 (19.4)
373 (91.4) 372 (85.7) 375 (87.6) 365 (55.7) 362 (52.9) 362 (54.9)
"In 0.1M H+ solution. I. D. A. House, Iflorg. Nucl. Chem. Lett., 1976, 12, 259.
Dissolution of the chromium(IV) diperoxo compound, [Cr(02)zdien].H20,in concentrated HCl affords the mauve crystalline product mer-[CrC13(dien)].467The fac isomer may be obtained as green crystals by the addition of concentrated HC1 to the oily residue obtained after evaporating a solution containing [Cr(urea),]C& and dien in EtOH.468The configurations of the complexes were initially assigned on the basis of spectralM9and kinetic data470and later confirmed by X-ray crystallography, which showed the green material to be the fac isomer.471 The bond lengths (pm) in this com lex are shown in (115). Hydrolysis of these complexes proceeds as shown in Scheme 50.$89470 The fact that the partly unwrapped dien complex [Cr(H20)4dienH]4fmay be obtained by hydrolysis of either 1,2,3-or 1,2,6-[Cr(dier1)(H~O)~]~+ implies that protonation occurs on a primary amino group. Protonation of the secondary amino group followed by isomerization is an unlikely alternative since such reactions are generally very slow +468 1,2,3-CrC13dien green
n 5 M HCIO,
1,2,3-[CrCl,(dien)H20]*a
blue
1
0.05 M HCIO, 55-60°C, 5min
1,2,3-[Cr(die11)(H~O),]~+
.
1,2,3-[CrCl(dien)(H,O),]"
magenta
red
I
H + , 69T
\Cr(dienH)(H20)4]4f pink
T
ICr(dienH2)(H20)5]5+
'
purple
M HCIO, 20-25 OC, dark, 5 h
0 1-1
1,2,6-[C~(dien)(H,O)~]~' orange
1,2,6-Cr(OH),dien blue
0 1 M NaOH 10 nun, 20-25 T 0 05 M HC104
1,2,6-[CrCl,(dien)H20]' pink
2 M HCIOa, 60 "C dark. 3 h
2 M HC104.60 ' C , dark, 3 h
5m'"
T
1,2,6-CrC13(dien)
purple
"This is thought to be the 5,6-dichloro isomer. Scheme 50
20-25
OC'
'low
Cr(OH), + dien
[Cr(H20),
Chromium
808
The mononuclear complex [CrC12(tame)DMSO]I.H20 and the binuclear complexes [Cr(pOH)OH(tame)]21~2H20 and [H~O(tame)Cr(p+-OH)2CrOH(tame)]Ij~H20 containing the tridentate ligand tris(aminomethy1)ethane (tame) have been reported.472
(ii) Triethylenetetramine (trien) The isomers cis-a-[CrClz(trien)]C1 (116) and cis-P-[CrC12(trien)]Cl (117)are prepared in 7040% yields according to Scheme 51."' The use of S0Cl2 as solvent in the preparation of ck-P-[CrC12(trien)]C1offers the following advantages: (a) it dissolves the starting complex but not the product, (b) its reaction with H20provides a homogeneous source of HCI and (c) it provides an anhydrous medium and thus prevents the formation of chloroaquachromium(II1) products. Although the isomers of cis-[CrC12(trien)]+ are stereorigid under anhydrous conditions, the presence of water causes rapid conversion of the ch-p to the cis-a form, probably via the stereomobile aquation product [CrCl(trien)H20I2+ (Scheme 52). Indeed the first step in this reaction, the aquation of cis-p-[CrCl,(trien)]+, occurs I@-fold more rapidly than that of the a isomer473and is one of the most rapid aquations reported for chromium(II1) complexes. Additional support for this mechanism is provided by the fact that isomerizations of analogous cobalt(II1) complexes proceed much more readily after aquation. The lability of cis-~-[CrCI,(trien)]' relative to its a isomer is consistent with an 1, mechanism in which HzO attacks the metal ion at a position trans to the leaving group. In the cis-a isomer this site is blocked and rendered hydrophobic by methylene groups in the trien chain. In the cis-p isomer, however, this site is open and is also hydrophilic because of the proximity of amino groups.47'
(117)
(116)&-LY
c~-P
m-[Cr(C,O,)trien]X
X = Br or C10,
I
SOCl,/H,O (50 : 1, vjv)
cir-B-[CrCl,(trien)]ClO,
hCIO,/MeOH
f
ci.y-p-[cr~l,(trien)l+
NaC'04(q)
+ ci.y-a-[~r~l,(trien)~~l~,
1
HCl(aq)/MeOH
AOHiHCI
cis-a-[CrCl,(trien)]CI-H,O
cis-fl-[CrCl,(trien)]Cl Scheme 51
The cis-m- and -p-[CrCl,(trien)]' isomers may be distinguished by spectroscopic methods. The electronic spectrum of the cis-a isomer has a long wavelength shoulder on the first ligand field band similar to that observed in the spectra of other cis-a-[MCI2(trien)]+ (M = Co, Rh) complexes. No such shoulder is observed in the spectra of the ck-p isomers. The differences in
Chromium
809
cis-~-[CrCl,(trien)]+% c~-8-[CrCl(trien)Hz0]”+C1-
cis-a-[CrCl,(trien)]+
cis-a;[CrCl(tnen)H,O]f+ Seheme 52
the IR spectra of the complexes, shown in Figure 4, provide support for the assignment of configuration^.^^^ Details of the electronic solution spectra of these and other chromium(II1)trien complexes are given in Table 56. While the [Cr(G04)trien]+ starting material used in the above reactions was, on the basis of IR spectroscopy, originally thought to have the &-a configuration, recent evidence based on chiral interconversions indicates that it is in fact the cis-fi isomer.474The primary step in the aquation of this complex involves Cr-N bond rupture and leads to the formation of [Cr(&04)(H20)trienH]2f.475 I600
7
CIS -
cis
I 200
IO00
800
IO00
800
a
-p
asym NH, region I200
5 (cm-’1
Figure 4 Differences in IR spectra (Nujol mulls) between ck-a-and cis-/3-[Cr(trien)Cl,]Cl (adapted from Znorg. Chem., 1977,16, 1154)
Reaction of CrC13-6Hz0 with trien gives a blue-violet mixture from which some chromium(II1)-trien com lexes (electronic spectra given in Table 56) may be isolated as indicated in Scheme 53.47 While the conversion of cis-~~-[Cr(OH)(trien)H~O]Cl~ to the cis-p isomer in the temperature range 160-225 “C is reversed on cooling, the process is irreversible if the reaction is carried out above 225 “C. A number of chromium(II1) complexes containing ‘partially unwrapped‘ trien ligands have been characterized. The complexes [Cr(trienH,)(H20),+2]”+3 are intermediates in the stepwise aquation of cis-a-[CrC12(trien)]+ (Scheme 54) and can be isolated by cation exchange ~hromatography.~ The ~ ~ a uation of cis-cu-[Cr(N,),trien]+ similarly affords a series of [Cr(N3)(trienH,)(H20),+I]X+P ions as outlined in Scheme 55.478 The colours and electronic spectra of these trien complexes are also included in Table 56. Empirical4” and theoreticala0 models have been developed to predict the photochemical behaviour of chromium(II1) complexes. These rules predict that photolysis of cis-[CrN4C12]+ complexes should proceed with aquation of the amino ligands trans to the weak field C1-. While the photoreactivity of ci~-[CrCl~(en)~]+ is, as expected, dominated by loss of amine, the behaviour of cis-a-[CrCl,(trien)]+ offers a crucial test to these models since photolabilization of the secondary amino groups should occur but because of the geometry of the complex these are constrained from leaving the primary coordination sphere of the metal ion. Indeed photolysis
r
Chromium
810
Table 56 Electronic Spectra of Chromium(II1)-trien Complexes
Complex
Colour
cis-cu-[CrCl,(trien)]+ ck-P-[crcl,(trien)]+ ck-[CrCl(trien)H,OI2+ cir- ~r-[Cr(OH)(tnen)H,O]'+ cir-cu-[CrOH(trien)]:+ cir-a-[Cr(NCS),( tien)]+ cir-a-[Cr(N,),(trien)]* [cr&o,(trien)]+ 1.2,3-[Cr(HZ0)3trienH]4+ 1.2 ,3-[Cr(N3)(H,0),trienHl3+
I,,(~)/nm(dm~ mol-' cm-')
Purple Purple
396 (90.5) 403 (85) 391 (70) 386 (70.8)d 397 (123)" 372 (86.7) 402 (126) 372 (96.5) 375 (35.6) 403 (102) 368 (45.5) 390 (120) 389 (27.5) 409 (82.1) 398 (22.0) 423 (73.5)
535 (97.2) 542 (100) 518 (105) 521 (91.2) 543 (129) 488 (157) 518 (185) SO0 (139) 513 (72.0) 532 (158) 493 (82.9) 502 (189) 522 (48.7) 543 (117) 552 (22.3) 560 (75.6)
Pink Pink 3he-violet Orange Brick red
Pink Red
Magenta 1,2,6-[Cr(H,0)3trienH]4+ Orange 1,2,6-[C1(N~)(H,O~pien~~+ Deep red Magenta [Cr(HzO)4trienW,] [Cr(N,)(H20)3trienH,]4+ Purple [Cr(HzO)5trienH3]6+ Purple [Cr(N3)(H,0)4trienH3]5+ Pink ~
Lin(E)
455 (28.8)" 461 (35)b 442 (38)a 420 (36.9)a 452 (70.0)a 425 (28.0)8 430 (13.8)' 453 (51.8)8 418 (20.4)' 437 (59.7)9 440 (10.1)f 467 (33.7)g 465 (5.1)g 488 (26.3)h
Ref. 1 1
1 2 2 3 3
3 4 5 4
5 4 5 4 5
~
O.leMHCI. bInfMeOH. 'This is the product 5m'f after dissolving cis-B-[CrCl,(trien)]+ in O.1MHCI at room temp. KO. In DMSO. In 2 M HCIO,. IR 3 M HCIO,. In 4 M HCIO,. 1. W. A. Fordyce, P. S. Sheridan, E. Zinato, P. Riccien and A. W. Adamson, Inorg. Chm., 1977,16,1154. 2. R. Tsuchiya, A. Uehara, K. Noji and H. Yamamura, Bull. Chem. SOC.Jpn., 1978, 51, 2942. 3. D. A. House and C. S. Garner, J. Am. Chem. Soc., 1966,88,2156. 4. R. L. Wilder, D. A. Kamp and C. S. Garner, Inorg. Chern., 1971,10,1393. 5. S. C. Tang, R.L. Wilder, R. K. KUdmOtD and C. S. Garner, Synrh. React. Inorg. Meweld-Org. Chern., 1971,1,ZW.
CrC13.6H,0 + trien
I
180 "C,6 h
blue-violet product
\
H,O, stand overmghl
NaI. H,O
crs-cu-[Cr,(OH),(trien),]I4.2H2O cb-cu-[Cr(OH)(trien)H,0]C12-H20 5w135"C
cis-cu-[Cr(OH)(trien)H,O]C+ pink: 6,(NH) = 157Ovs cmcool to 1.t.
11
heat to 16&225'C
cis-/?-[Cr(OH)(trien)H20]C12 violet: 6,,(NH) = 1592w, 1575s, 1 5 5 8 cm-' ~~ Scheme 53
Cr(OH),trien 0.1 M N /
0.25 M Hg(N0,)d
\
HCIO,, 0-10 "C
0.1 M HClO,
cis-cu-[CrCl,(trien)]+
31,2,6-[Cr(trienH)(H,0),14'
[Cr(trienH,)(H,O)$+
1,2,3-[Cr(trienH)(H,0),]4+ Scheme 54
+ [Cr(trienH,)(H20)4]5+
Chromium
811
[Cr(N,)(HZ0)3rienR,]"'
T3M HCIO,,
0 01 M NaOH 25 'C. 120 min 11, HC104 to pH 1 I.
ci~-a-[Cr(N,)~trien]Br
+ [Cr(H,0),trienH,]5'
9
W ; ,
1,2,6-[Cr(N,)(H,0),trienH]3'
/ +
3 M HCIO, 0 "C. 15-20 h
1,2,3-[Cr(N,)(H20),trienH]3+ [Cr(N3)(H,0),trienH,]4' Scheme 55
of ci~-lu-[CrCl~(trien)]+ was observed to involve negligible amine loss, with C1- aquation being the dominant process. Since the axial coordination sites in this complex contain primary amino groups which are as unconstrained as the photolabile amines in cis-[CrCl,(en),]+, the fact that amine loss is not observed confirms that ligand labilization is confined to the xy plane. The photolysis proceeds with retention of configuration.
(iii) 2, 2', 2"- Triaminotriethylamine (tren) A number of chromiurn(II1) complexes of tren, a tripodal isomer of trien, have been prepared from [CrCl,(tren)]Cl according to Scheme 56.@' This complex is conveniently prepared by the addition of tren to a solution of [CrClz(H20)4]Cl-H20in DMSO which had previously been heated to boiling. In acid solution [CrCl,(tren)]+ undergoes a two-step aquation, the first involving loss of C1- and the second involving Cr-N bond rupture to give the partly unwrapped tren complex [CrC1(H20)trenH]3+. Solvolysis of [Cr(Cl),tren]+ in formamide, N-methylformamide, DMF and DMSO appears to occur via dissociative rnechanisms. A number of these solvolysis products have been isolated and c h a r a c t e r i ~ e dReaction .~~~ of tr~ns-[CrF~(py)~]C104 with tren in 2-methoxyethanol gives cis-[CrF,(tren)]ClO, as From this the tren complexes [CrF(X)tren](C104), (X = H2 0 , NCS, Br, N3, MeC02; x -0, 1 or 2) have been prepared.487 In acid solution cis-[CrF2(tren)]C104 is converted to cis-a-[CrF(tren)H20I2+ (HzO trans to the tertiary N) both by acid-catalyzed thermal aquation and by ligand field p h o t o l y s i ~ .A~ ~second but pH-independent thermal aquation process involving rupture of a Cr-N bond has also been observed. The electronic solution spectra of chromium(II1)-tren complexes display d-d bands of greater intensity than those of analogous Cr-N4 complexes having higher symmetry.481 Dissolution of [Cr(Cz04)tren]+ in dilute HC104 at 15°C leads to the rapid formation of [Cr(C204)H20(trenH)]2f.484 Heating the product solution to 60 "C causes stepwise red-shifts in the d-d spectral maxima until the formation of [Cr(C204)(H20)4]+is complete. In concentrated HC1 solution, however, [Cr(C204)tren]+ reacts to give [Cr(Cl),tren]+ in a two-step process.
[Cr(N,)fren]Br
[Cr(Cl)Br(tren)]Br
[Cr(C,04)tren]C10,.H,0
0 1 M tlClOn
[CrCl(NCS)tren]Cl
[CrCI(NCSe)tren]SeCN Scheme
[CrC1(%O)~trenH]~'
56
Reaction of CrCI3(Me3N)Zwith tren in benzene (Scheme 57) leads to products in which tren behaves as a tridentate ligand coordinated to the metal through the primary amino (Section 35.4.2.2).
812
Chromium
/ ) , : ;
CrCl,(tren) blue-grey, air-sensitive, non-electrolyte in CH,Cl,, pcB= 3.83 BM (295 K), v(Cr-Cl) = 338, 300cm-', Am- = 591, 408 nm (reflectance)
(Me,N),CrCl,
~nhpink, [cr(tren>,lc1, hygroscopic,
prtr= 3.86 BM (294 K), = 498, 380 nm
3: 1 electrolyte in HzO, A,,
(reflectance)
Scheme 57
(iv) Linear tetramines other than trien The linear tetramine ligand, 179-diamino-3,7-diazanonane (2,3,2-tet), a homologue of trien, forms octahedral complexes which can adopt several isomeric configuration^.^'^ The complex cis-P-RR,SS-[Cr(G04)2,3 ,Ztet]+ has been synthesized and resolved and then used to prepare racemic and chiral cis-a-[Cr(X)(Y)(2,3,2-tet)ln+ (X = Y = C1, NCS, CF3C02, H2O; X = Q, Y = H20) c ~ m p l e x e s .The ~ ~ meso complexes trans-[CrX2(2,3,2-tet)]+(X = C1, F, NCS) have also been prepared and the secondary NH isomeric confi urations for two of these (X = F, NCS) have been confirmed by X-ray crystallography. 48L89 The complexes cis-pRR,SS-[CrX2(3,2,3-tet)]+ (Xz= G O f - or 2Cl-) and rrans-RR,SS-[CrY2(3,2,3-tet)]+ (Y= C1, NCS-) which contain the ligand 1 ,lO-diamino-4,7-diazadecane(3,2,3-tet) have been synthesized and resolved.490Details of the visible absorption spectra of the above complexes have been reported as well as CD spectra and aquation kinetic parameters in some cases.486,4w93 The complex trans- [CrF2(3,3,3-tet) ]C104 (3,3,3-tet = 1,11-diamin0-4~8-diazaundecane) has been prepared and its electronic spectrum compared with those of other Cr111)G2% complexes (X=a tetramine or two diamines; Z = F , C1, Br, I, DMF or COT) using crystal field and angular overlap models494 (note that the tetramine ligands are incorrectly named in this reference). A spectrochemical series in Cp , the second-order crystal-field parameter, shows trends in chromium-ligand interactions which are not apparent from the conventional series in
4(v) 1,6-Bis(2'-pyridyl)-2,5-diatahexane(bispicen)and related ligands Mononuclear and binuclear chromium(II1) complexes containing bispicen and related tetradentate ligands have been prepared. The isomers cis-a- (118) and -P- (119) [CrC12(bispicen)]+are obtained by the methods outlined in Scheme 58495,496 but complexes with the trans geometry are unknown.
(118)cis-&
(119)cis-p
Addition of bispicen to a solution of [CrBrz(Hz0)4]Br.2H20in 2-methoxyethanol and in the presence of zinc powder gives the binuclear di-p-hydroxo complex [CrOH(bispicen)];+ (see Table 54 for details of the structure).495In principle, 13 possible isomers of this complex exist but the use of molecular models shows that 11 of these are grossly sterically hindered. The remaining two isomers, (120)and (l21), were isolated by the methods outlined in Scheme 59. Treatment of M-( )D-[CrOH(bispicen)]2(C104)4*4H20 with concentrated HCl affords {+IDcis-ru-[CrC12(bispicen)]C104, which has therefore been assigned the A onf figuration.^^ The stereochemistries of the bispicen complexes were established using electronic and CD spectroscopy and by comparison of their X-ray powder photographs with those of analogous cobalt(II1) complexes with structures known from 'H NMR spectroscopy. A number of complexes of the type cis-a-[CrX2(S-bispicpn)]Y(X = Y = C1, A and A;
+
Chromium CrCI,
813
-+ bispicen
I +
DMSO. heal
violet solid
mother liquor
\
/
i, wash with EtOH ii, extract with 1 M HCI iii. cool filtrate
i, cool in ice ii. add ether
- ry- [CrCI2(bispicen)]Cl
I\
I
i . H,O ii, 1 M HCIO,
i. dissolve in EtOH ii, add ether
conc. HCI
precipitate
ck-a-[CrCIZ(bispicen)lC1O,
1
i , H,O
ii, conc. HNO,
cki-~-[CrF,(bispicen)]Br.2.5HZ0*O.3LiB~
T T
"
sticky product
cis-/3-[CrCl,(bispicen)]N03~0.75 H,O
I
i. Liar ii. cation exchange purification
Na,S20,~2H,0
ck-a-[ CrF,(.bispicen)]'
cis-/3-[CrCl,(bispicen)]0.5S,06.2H,0
T T
2-methoxyethanol, reflux
trans-[CrF,(py),jBr
+ bispicen
i, AgN03 ii, Na,S20,-2H20
cis -cy-, p - [CrCIZ(bispicen)]I 2-rnrthoxyethanol. hcat
trunu-[CrCl,(py),]I
+ bispicen
Scheme 58
[CrOH(bispicen)],I,-3H20 + (+),-NaSbOC,H,O, H'O1
+
(-),-[CrOH(bispicen)],( (+)D-SbOC4H40S}212. 1l H z O
1 i, 1M
mother liquor
NaOH
IFtOIl
+
ii, EtOH iii. iv. 4Nal M HCI
(+),-[CrOH(bispicen)],( (i-),-SbOC4H40,} 14H20 I
I
.1
i, hot H,O
i i , NaC10,
AA-(-)D-[CrOH(bispicen)l,I,.3H,0
AA-( +),-[ CrOH(bispi~en)]~(C10,),~4H,O Scheme 59
(EO)M
(121) AA
X=C1, Y = I , A ; X = B r , Y = I , A; X=C1, Y=C104, A; X = F , Y = B r , A; X = Y = B r , A; X = Y = NCS, A) containing the unsymmetrically substituted and optically active ligand 1,6-bis(2'-pyridyl)-3-methyl-2,5-diazahexane(bispicpn) have been prepared by methods similar to those in Scheme 58 and characterized by electronic and CD spectra.497An X-ray crystal structure determination of A-cis-Lt.-[CrClz(S-bispic n) C1 confirmed the assignment of a A configuration, initially based on CD spectroscopy:' number of analogous cis-cu-[CrC12(Rbispicpn)]+ complexes and also cis-/3-[CrC12(R-bispicpn)]I have been prepared and
p!
Chromium
814
~ h a r a c t e r i z e dThe . ~ ~ ~complexes [CrC12(S,S-bispicbn)]C104 (bispicbn = 1,6-bis(2'-pyridyl)-3,4dimethyl-2,5-diazahexane) and [CrC12(RJR-bispicchxn)]C104 (bispicchxn = N , N'-bis(2-pyridylmethyl)-l,2-cyclohexanediamine) have been assigned A-cis-& structures on the basis of CD spectral comparison^.^^ Details of the electronic spectra of a number of bispicdiaminechromium(II1) complexes are presented in Table 57. Table 57 Electronic Spectra of Bispicen and Related Chromium(II1) Complexes" Complexb
A,,
cis- CY-[CrCl,( bispicen)] cis-j3-[CrClZ(bispicen)]
+
+
A-cis-a-[CrCl,(S-bispicpn)] A-ck-a-[CrCl,(S-bispicpn)] +
+
A-cis-a-[CrCl@ -bispicpn)]+
A-ck-a-[CrCl,(R-bi~picpn)]~ A-cis-@ - [CrCl,( R -bispicpn)] A-ck-a-[CrBr,(S-bispicpn)]+ +
A-cis-a-[CrBr,(S-bispiqn)]+ A-cis- a-[ CrF,( S-bispicpn)] A-cis-a-[Cr(NCS),(S- bispicpn)] +
+
A-cis-a-[CrCl,(S,S-bispicbn)]* A-cis E-[CrCI,( S, S-bispicchm)] [CrOH(bispiccn)]: M-[CrOH(S-bispicpn)I4+ M-[CrOH(R-bispicpn)$+ Ab-[CrOH(R-bispicpn)],+ 1
+
+
M-[CrOH(S, S-bispicbn)];+ M-[CrOH(S, S-bispicchxn)]:
+
(E)
(nm, dm3 rn~l-~ctn-')
Ref.
545 (104), 407 (99) 538 (131), 407(100) 540(102),406(102) 550 (loo), 408 (93) 543 (103), 407(101) 554 (95.4), 409 (90) 538 (129), 406(104) 558 (117), 418 (145) 565 (105), 420 (107) 523 (81),378 (49)' 501 (117), 319 (3890)F 547 (91.2), 407 (83.2) 553 (93.3), 410(87.1) 534 (197), 385 (150) 530 (174), 384 (162)' 533 (220), 384 (148) 529 (173), 386(147) 533 (186), 386(148)' 527 (174), 386(148)'
1 1 2 2 3 3 3 2 2 2
2 4 4 1 4
5 5 4 4
In 0.1 M HCl unless otherwise indicated. Complexes are either red, blue or violet coloured. 1. K. Michelsen,Acta Chem. Scund., Ser. A, 1977, 31, 429. 2. Y. Yamamoto and Y. Shimura, Bull. Chem. SOC.Jpn., 1980, 53, 395. 3. K. Michelsen and K. M. Nielsen, Ac?u Chem. Scand., Ser. A , , 1980,34,755. 4. Y. Yamamoto and Y. Shimura, Bull. Chem. SOC.Jpn., 1981, 54,2924. 5. K. Michelsen and E. Pederseri, Acta Chem. Scund., Ser. A , 1983,37, 141.
a
In H,O.
The di-p-hydroxo complexes [CrOH(bispic n)]2(C104)4,s00~S01 [Cr(OH)(S,Sbispicbn)12(C104)4, [Cr(OH)(S,~-bispi~chxn)l,(ClO~)~~ and [CrOH(bi~pictmd)]~(CIO~)~ (bispictmd is 1,7-bis(2'-pyridyl)-2,6-dia~aheptane)~~~~~~~ have been prepared by methods similar to those in Scheme 59. Once again, steric repulsion appears to be responsible for the limited number of the potential isomers isolated. From X-ray crystallography (Table 54) the violet form of [CrOH(bi~pictmd)l~(ClO~)~ is shown to be a racemic mixture of A h and M isomers, and the singlet-triplet separation in this antiferromagnetic species is 33 cm-1.502For various salts of [Cr(OH)(bispicpn)]i+ the values are in the range 44-50 ~ r n - ' . ~ ' ~
(vi) Tetruethylenepentumine (tetren)
b,,,
The complex [Cr(tetren)(H20)]3', prepared by hydrolysis of a,p,R-[CrCl(tetren) C12, undergoes aquation with successive rupture of Cr-N bonds in acid solution (Scheme 60).5 The N-thiocyanato complex [CrNCS(tetren)](SCN)z may be obtained by heating the chloro complex with KSCN in aqueous solution.so5
[CrltetrenH,)(H20)4I6+
4 M HC104, M "C
a
ICr(tetrenH,)(H20),17+
4MHClO
MI'C
ICr(H,O),]"
Rate constant extrapolated from 10-35 "C values. Scheme 60
+ tetrenHZ+
Chromium
815
35.425 N heterocyclic ligands ( i ) Pyridines
A large number of chromium(II1) complexes containing pyridine (py) ligands are known. The addition of py to a solution containing high concentrations of Cr3+ and F- gives truns-[CrF2(py),]+, which, owing to the high stability of the Cr"'-F bond in the absence of acid, is a convenient starting material for a variety of [CrF2N4]+complexes (Schemes 32, 33, 47, 58 and 61).392,496These reactions are usually carried out in boiling 2methoxyethanol. Under acid conditions, the F- ligands can be substituted to give complexes of the type [Cr(X)(Y)NJ. A detailed analysis of the electronic spectra of the complexes trans[Cr(X)(Y)(py),]' (X, Y = F, C1, Br) using an angular overlap model to describe the Cr-py bond has led to the conclusion that py in these cases acts as a JC acceptor.5o6However assumptions leading to this conclusion have been d i s p ~ t e d ,since ~ ~ ~it *has ~~ been ~ pointed out that such behaviour is inconsistent with that of py in other complexes, e.g. M(X)(Y)(P~)~ (M = Fe, Co, Ni), where it acts as a n donor."' Frozen solution X-band ESR spectra have been recorded for a series of complexes of the type tr~ns-[Cr(X)(Y)(py)~]~+ ( X = Y = F , C1, Br, I, H 2 0 , OH; X = F, Y = C1, Br, HzO, OH; X = C1, Y = H2 0 , OH; X = Br, Y = H20).509 The dihalogeno complexes show tetragonal symmetry with 1.97 ig i1.99 and zero field splittings 0.30 d 2 0 S 3.5 cm-'. The aqua and hydroxy complexes have rhombic symmetry. trans-[CrF,(RNH,),]X
?!; cis-[CrF,N,]X trans-[CrF,N,]X
(N4 = trien, tren)
01
(R= alkyl, allyl)
N.3,
x = CIO,
a trans-[CrF,(py),]X
1
(N4 = 2,3,2-tet or 3,2,3-tet)
o ,i ::: cis-[CrF,(N-N),]X trans-[CrF,(N-N),]X
(N-N = phen, bipy) or (N-N = en, pn, tn, chxn)
Scheme 61
Reaction of CrC13 and pyridine at reflux tern eratures and under anhydrous conditions f The far-IR spectrum of this complex shows affords the green-coloured product CrC13(py)3.397,51 three well-separated v ( C r 4 1 ) bands indicative of a mer configuration."' Subsequent X-ray studies established that both this and CrBr3py3 are isostructural with the known merM ~ C l ~ ( p yThe ) ~ complexes .~~~ CrX3(4-Mepy)3 (X = C1, Br) obtained from CrX3 and 4-Mepy were also shown to have mer configuration^.^^^ Reaction oi CrC13-6H20with a ten-fold excess of each of the cyanopyridine isomers (CNpy) in either 1- or 2-butanol at 80°C affords the complexes CrC13(CNpy),, although configurations have not been assigned.'14 Thermal decomposition of the complexes mer-[CrCl3(R-py),] (R = H, 3-Me, 4-Me) and pyH[CrC14(py)2]gives the dimers Cr2C16(R-py)4.515 The complexes mer-Cr(CF3C02)3py3 and CpCr( CF3C02)2py have been prepared by the methods outlined in Scheme 62.516An X-ray analysis of the former complex showed it to have a slightly distorted octahedral structure with almost identical Cr-0 (mean 195 pm) and Cr-N (mean 210 pm) bond lengths, Reaction of Cr(MeSO3I3, a Cr06 complex containing bridging MeS03 ligands, with py and the appropiate amount of bipy gives the green complexes Cr(MeS03)3py3, Cr(MeS03)3bipy and Cr(MeS03)3(bipy)2 all of which appear to contain octahedrally-coordinated chromium(II1)71'. Reduction of RpyCr(0)(02)2 (R = 3-Me, 3-C1 and 3-CN) with Fez+ under acid conditions gives the species [Cr(Rpy)(H20)5]3+ in solution.518 For the aquation of these and the unsubstituted py complex (equation 32) a linear relationship exists between -log k and the pK, values of the py ligands. [c~(RPY)(w,o),I~++ H,O -L+[ c ~ ( H ~ o ) ~ + I ~RPY +
(32)
Some multinuclear chromium(II1) complexes which contain pyridine ligands have been described. The amide (122 = H,L) reacts with CrC1,-6H20 in pyridine solution containing Na2C03 to give the dimer [Cr2(HL)2(py)4].2py, the structure of which is described in Section 35.4.2.11.519
Chromium
816
CpCr(CF3COz)zpy
violet: pee = 3.44 BM (temp. independent)
(CpCrSCMe&S
1, C F 3 m 2 H in 1HF.heat ii. py
i, CF3C02H in THF,reflux
ii, py
mer-Cr(CF,CO,),(PY),
>
violet: pcff= 3.96 EM (77-295 K): vco2= 1770, 1410 ern..' Scheme 62
0
\\C-NHn HN-C /
/;" \
Reaction of chromium(I1) acetate with various other divalent metal acetates in py gives a series of oxo-centred trinuclear complexes which contain bridging acetate ligands (Scheme 63) .52" The reaction between various poly-4-vinylpyridines and ci~-[CrCl~(en)~]Cl in water at 70 "C gives complexed poIymers which, despite having degrees of coordination between 0.36 and 0.71, show no magnetic interactions between the chromium(II1) ions along the polymer chains.521Addition of py, 3-Mepy or py-0 to benzene solutions of the chromium(II1) alkoxides Cr(C1)20R(ROH)2 (R = Et, Pri, Bun) gives green or brown solids Cr(C1)20R(py)2, Cr(Cl)~OR(3-Mepy)~ and Cr(Cl)zOR(py-0)2, all of which appear to be dimeric with alkoxide bridging ligands."2 Cr:"M"O( OAc)&
M = Mg, Co, Ni; L = py M = Mn, Zn; L3= py f 2H,O
y Cr"'Fe"'M"O(OAc),L, M = Mn, Fe, Co, Ni; L = py
Scheme 63
Other reported chromium(II1) complexes containing py and related ligands are listed in Table 58. Complexes of 2-pyridylmethylamine are dealt with in Section 35.4.2.3 and of pyridine carboxylic acids in Section 35.4.8.
(ii) Bipy ridy Is, phenan thdines and terpyridyIs A number of chromium(II1) complexes with bipy, phen, terpy and related ligands have been prepared and their photochemical and photophysical properties investigated, mainly because of their potential applicability as photosensitizers for solar energy conversion and storage. Much of this work has recently been thoroughly and critically r e ~ i e w e d ~ and ~ ~ .a' ~later ~ article
Chromium
817
Table 58 Chrornium(II1)-Pyridine and Related Complexes Complex
Ref.
Complex
Ref. 7 7 7
8 8 1 1 1 9 10 10 1 1 1 1 1
a
tu = thiourea.
1. C. S . Garner and D. A. House, Tramition Met. Chem. ( N . Y.),1970,6,59. 2. A. Bakac, R.Marcec and M. Orhanovic, Inorg. Chem., 1974,W, 57. 3. S. T.D.Lo and D.W ,Watts, A u t . J . Chem., 1975,28,1907. 4. A. Bakac and M.Orhanovic, 2. Natuflorsch., Teil E , 1974,29, 134. 5. L. K. Mishra, H. Bhushan and N. K. Jha, Indian J . Chem., Sect. A , 1982,21,639. 6. K. M.Purohit and D. V. Ramana Rao, J . Indian Chem. SOC., 1983,60,1009. 7. M. M.Khan, I . Inorg. Nucl. Chem., 1975,37, 1621. 8. K.S. Nagaraja and M. R . Udupa, Indian J . Chem., Sect. A , 1983,22, 531. 9. S. J. Patel, Bol. SOC. Chil. Quim., 197'0, 16, 18;Chern. Abs., 1971, 75, 144%. 10. S. J. Patel, Bol. Soc. Chil. Quim., 1971.17,61;Chem. Abs., 1972.77, 83056m.
describing a pulsed laser photochemical study of [Cr(bipyI3l3+ includes some additional references.525 The ability of these heterocyclic ligands to stabilize a wide range of oxidation states of coordinated metals also renders their complexes suitable for participation in series of one-electron reversible charge transfers. Such electron transfer chains containing four reversible redox couples have been observed for chromium complexes containing bipy (Scheme 64), phen, terpy and related ligands in acetonitrile s ~ l u t i o n . The ~ ~ ,first ~ ~ three ~ couples for each complex involve redox orbitals which are primarily metal-centred while the last couple in each case involves a molecular orbital with mainly ligand character.53 All five members of the bipy series, as well as the further reduced complexes [ C r ( b i ~ y ) ~ ]and ~ - [Cr(bip~)~B-, have been isolated and their magnetic moments and electronic spectra (the latter with the exception of the anionic complexes) reported.45 [Cr(bipy),13+ I [Cr(bipy)#+
C [Cr(bi~y)~]+ e Cr(bipy),
[Cr(bipy)J
Scheme 64
A convenient synthesis of J C r ( b i ~ y ) ~ and ] ~ + its aquation behaviour under various conditions are outlined in Scheme 65.52-529 The complex [Cr(phen)3](C104)3can be prepared in a similar manner527and its thermal- and photo-aquation reactions parallel those of the bipy complex under similar conditions.530 Both complexes have been successfuIly resolved using ( )Dtris{ ( - )-cysteinesulfinato)cobaltate(III) as the anionic resolving agent.527Similar tris chelates containing the ligands 5-Clphen, 4,7-Mezphen, 4,7-Phzphen, 3,4,7,8-Me4phen, 4,4'-Me,bipy and 4,4'-Ph2bipy as well as the bis complex [Cr(ter )2 C10&, which was shown by X-ray &e obtained by chlorine oxidation of crystallography to have a lstorted mer confi uration!' the corresponding chromium(I1) complexes!32 The influence of various factors, e.g. nature of
+
818
Chromium
oxidant and its rate of addition, on the oxidation of [Cr(bipy)$' and [Cr(~hen)~]'+ have been summarized in a recent article.533Details of the visible absorption spectra of these complexes are presented in Table 59.
I
1.i i, Zn amalgam;
iii
iii, air; iv, hv, pH9; v, I&O, OH-; vii bipy, O,, pH4; vii, HCIO,.
ii, bipy, NaCIO,, HClO,;
Scheme 65
Table 59 Visible Absorption Spectra of [Cr(N-N),]'+ Complex ion [Cr(bipy)313+ [Cr(4,4'-Me,bipy)3fC [Cr(4,4'-Ph bipy),] [Cr(phen),lS+ [Cr(5-Clphen),13+ [Cr(4,7-Me2phen),P+ [Cr(4,7-Ph2phen),] ' +
[Cr(3,4,7,8-Me,phen),I3+ [Cr(terpy121" [C~(4'-Phterpy),]~+ [Cr(4,4' ,4"-Ph,terp~)~]~+
I,,
(E)
and {Cr(N-N-N),J3+
(nm,dm'mol-' cm-')
458 (269), 428 (676), 402 (933)" 446 (269), 418 (631), 394 (933)" 445 (1740), 422 (36301,404 (5370)" 454 (324), 435 (603), 405 (871)a 466 (389), 436 (708)' 450 (501),424 (891), 402 (1047)" 484 (1230), 445 (2040)" 456 (741), 428 (1150), 400 (1350)" 473 (1410), 443 (22391,422 (2040)" 472 (1470), 442 (239U), 418 (2220)b 467sh (1540), 438sh (2860), 397sh (4750)b 465sh (2540), 428sh (5320), 399sh (11
Complexes
Ret 1
1 1 1 1 1 1 1 1 2 2 3
In 1 M HCI, In MeCW solution. 1. N. Serpone, M. A. Jamieson, M. S. Hcnry, M.Z . Hofhnan, F. Bolletta and M. Maestri, J . Am. Chem. Soc., 1979, 101, 2907. 2. J. M.Rao, M.C. Hughes and D. J . Mawro, Znorg. Chim. Actu, 1976,18, 127. 3. M . C. Hughes, D. J. Macero and J. M. Rao, Inorg. Chim. Acta, 1981, 49, 241. a
Despite a few earlier reports to the contrary it now appears that complexes of the typc [Cr(N-N)2(X)(3Y)J"', (N-N = bipy or phen; X, Y = monodentate ligands) invariably have ci configurations.' 4,5 The dichloro complexes (X = Y = C1-) are conveniently prepared b heating CrC13 and the amine in boiling ethanol in the presence of zinc dust catalyst.'34 Thi preparation, resolution, reactions and properties of a number of such complexes have beel thoroughly investigated and some of the reported chemical interconversions are outlined ii Scheme 66.535Visible spectral data for these species and for other heterochelates containin; bipy or phen ligands are presented in Table 60 and pK, values of the aqua complexes are listed haw in Table 61. Details of the crystal and molecular structure of ~is-[CrCl(bipy)~H~O](ClO~)~ been reported.533The binuclear di-p-hydroxo complexes [CrOH(N-N),]4+, some reactions o which are included in Scheme 56 and the pK, values of which are given in Table 61, can bl obtained by the gradual neutralization of a refluxing solution containing Cr(N03)3.9H20 and two-fold excess of the amine in 1M HC104.s36Because of the acidic properties (pK, values ii Table 61) and robustness of these complexes the corresponding p-oxo-y-hydroxo (124) ani di-y-oxo species (125) can easily be isolated after the addition of base to (123; Scheme 67).5' Magnetic susceptibilities of salts containing these complex ions over the temperature rang 5-300 K are indicative of antiferromagnetic exchange interactions, which lead to singlet-triple separations of between 4Ocm-' in (123) and llOcm-' in The com lexe [CrOH(bipy)2]zC14.9H20and [CrOH(phen)2]2(N03)4.7H20 have both been resolved." Th crystal and molecular structures of [CrOH(phen)2]214-4H20and [CrOH(phen)2]2C14-6H20hav been reported.539 Addition of phen-HN03 to an aqueous solution of CrC13(bipy)DMF giver I
Chromium
819
after the addition of base, the complexes [CrOH(bipy)phe11f~(N0~)~+6H~O, which has been resolved, and [CrOH(bipy)(phen)H20](N03)2-1.5H20.538 The addition of one equivalent of base and then KI to an aqueous solution of ~is-[Cr(bipy),(H~O)~]~+ gives a crystalline product which was shown by X-ray analysis to be the H&-bridged dimer [ C ~ ( H ~ O ~ ) ~ ( b i p y ) ~ ] ~ 14-2Hz0.s40 The existence of this dimer in concentrated aqueous solutions has been demonstrated by three-phase vapour ten~iometry.'~~ The tartrate-bridged complexes Crz(tart-H)2(N-N)2 and Na[Cr2(tart-2H)(tart-H)(N-N)z] (tart = tartate = C4H402-; tart-H = C4H302-; tart-2EI= C4H20:-)containing either two optically active or two meso bridging ligands have been prepared and ~haracterized.'~~ The complexes [Cr(C2O4)(N-N),]C1, K Cr G04)2N-N] (N-N = bipy or phen) and [Cr(C2O4)(bipy)phenjC1have been r e ~ o l v e d . ~ 54L ! 545 The kinetics of racemization of a number of chelates described in this section have been studied and evidence for both intermolecular
5.
Table 60 Visible Absorption Spectra of cis-[Cr(N-N),(X)(Y)Y+
Complex ion
Complexes
LIara(Eb)
LinYEb)
Medium
Ref.
492 (44.8) 448 (94.2) 4% (43.7) 517 (42.0)
468 (39.2) 442 (89.2) 458 (29.4) 465 (35.1)
0.1 M HCIO,
1
0.1 M HNO, 0.1 M NaOH
1 1
519 (41.5) 519 (48.6) 443 (57.3) 415 (144) 522 (46.5) 553 (43.7) 572 (47.3) 558 (40.1) 578 (42.5) 574 (52.7) 518 (45.6) 598 (49.8) 522 (42.8) 520 (56) 522 (41.2) 550 (65) 497 (66.5)
468 (34.8) 461 (22.0)
0.1 M NaOH HZO
1 1
HZO 0.1 M HC1 DMF 0.1 M HCl DMF
1 1 1
445 (I 32)
h20
1
421 (227)
DMF
1
0.01 M HCl 0.1 M HNO, 0.01 M NaOH HZO
2 1 1 1'
501 (62.6) 543 (77) 397 (133) 505 (72.4) 496 (61.7) 535 (86) 392 (178) 537 (106) 447 (158) 540 (114) 537 (108) 447 (160 ) 540(-) 537 (1083 447 (158 ) 540(-)
454 (33.0)
455 (16.6) 474 (18.9) 448 (24.0) 478 (17.5) 492 (22.8) 526 (43.9) 484 (29.0) 538 (36.5) 495 (32.0)
440 (69.7) 424 (160)
420 (69.2) 445 (90.0)
461 (17.0) 464 (48.7)
448 (90.4) 417 (220)
1
1'
HZO
3
h20
4
420 (178) 444 (126)
3
HZO M HCI
467 (45.6)
1
Id
0.1 M HNO, 0.1 M HCI
5
462 467 (46 )
0.1 M HCI O.1MHCl
5
462(-)
0.1 M HC1
5
462 (42.9) 467 (46')
(-2
422 (303)
1"
In nm. In dm3mol-' cm-'. Spectra of these complexes are also reported in ref. 3 and are in moderately good agreeyent. Spcctra of thesc complexes are also reported in ref. 5 and are in excellent agreement. tart = tartrate. The absorption cocfhicnts quoted in ref. 5 are per chromium(II1) and have been adjusted here to molar absorptioncoefficients. 1. M. P. Hancuck, J. Josephsen and C. E. Schiiffer,Acta Chem. S c u d . , 1976,30,79. 2. W. A. Wickramasinghe, P. H. Bird, M. A. Jamieson, N. Serpone and M. Maestri, I w r g . Chim. Acto, 1982, 64, L85. 3. J. A. Broomhead, M. Dwyer and N . Kane-Maguirc,Inorg. Chem., 1968, 7 , 1388. 4. A. Tatehata, Inorg. Chem., 15'77, 16(5), 1247. 5. J. Josephsen and C.E. Schaser, Acta Chem. Scund., 1970,24,2929. a
COCI-M
Chromium
820
Table 61 pK, Values for Ionization of Aqua and p-Hydroxo Ligands in lJ0-Phenanthroline and 2,2'-BipyridyI Complexes of Chromium(II1) Complex ion
Medium
PK, ~
~is-[Cr(bipy),(H,O),]~+ ~ir-[Cr(phen),(H,O),]~+ cis-[CrCl(bipy) (HZO)]*+ ICrOH(biPY)zl!+ {CrOH(phen),]$+
~~
Re)? ~~~
pK, = 3.5; pK, = 6. la pK, = 3.0 f 0.2; pK, = 6.0 f 0.2azb p K , = 3.4; pK, = 6.0"
0.1 M NaNO, 1 .O M NaNO, 1 M NaNO, 1.OM NaNO, 1 M KCI 1 M KCI
4.6 f 0.2" pK1=7.6;pK,= 11.9' pK, =7.4;pK, = 11.8
1 2 1 2 3 3
Temperature not stated but assumed to be 298 K. values w$:e erroneously attributed to [Cr(phen),(H2Cl)2]3+ in ref. 2; clearly on the last line, column 1, L86, [Cr(phcn),(H,O),] was mistakenly written for [Cr(l~ipy)~(H~O)~] . T = 297-300 K. 1. R. G. Inskeep and J. Bjerrum, Acta Chem. Scmd., 1961,15,62. 2. W. A. Wickramasinghe,P. H. Bird, M. A. Jamieson, N. Serpone and M. Maestri, Inorg. Chim. Acta, 1982, 64, L85. 3. J . Josephsen and C. E. SchtiEer, Acta Chem. Scand., 1970, 24,2929. a
bThcsc
i, K,C,O, in HzO, 100°C, 4min; ii, conc. HCl/HCl(g), 50°C overnight in sealed tube; iii, 0 . 3 M HC1, 100 "C, 3 h, (N-N = phen); iv, conc. HCl, 70 "C, overnight in sealed tube; v, Ag,O in H,O, 25 "C, overnight; vi, Hg(OAc), in HF, 25 "C, overnight; vii, conc. HCI, 100 "C,5 h in sealed tube; viii, HF, 25 "C overnight; ix, conc. HC104, 60 "C, 24 h, adjust p H to 4;x, Na,C,O,/H,GO, in HZO,80 "C, 2 h; xi, conc. HBr/HBr(g), 50 "C, 24 h in sealed flask; xii, conc. HC104, 60 "C, 24 h.
Scheme 66
3+
0 (N-N)
,Cr
' 649 (45), 450 (W), 361 (340)d
'
As Nujol mulls. Room temperature. 'In THF solution. In MeCN solution. 1. D. Wester, R. C. Edwards and D.H.Busch, Inorg. Chem., 1977,16, 1055.
(g) Cr(N0)4. The binary nitrosyl Cr(N0)4, a dark red volatile solid which is both isoelectronic and isostructural with Ni(C0)4, may be obtained by photolyzing a solution of Cr(CO)6 in pentane while purging with NO.5w The evidence from IR and Raman spectroscopy suggests that the molecule has Td symmetry both in solution and in the solid state. Photolysis of Cr(C0)6 and NO in frozen argon and methane matrices also leads to Cr(NO), via the intermediate Cr(CO)3(N0)2.600 The chemistry of a number of carbonyl and cyclopentadienylchromium-nitrosyl complexes has been discussed in the companion series.'
(ii) Thionitrosyls The relative scarcity of transition metal complexes containing NS ligands compared to the number containing NO has been attributed to the lower stability of NS and to the lack of reagents for introducing it into complexes.@" The preparation and reactions of the first reported organometallic thionitrosyl complex, CPC~(CO)~NS, are shown in Scheme 80 Gas-phase core-electron binding energies of this compound have been determined.m Addition of a nitromethane solution of NS+PF;, obtained in situ from N3S3C13 and A PF6, to CsH6Cr(C0)3 in ethanenitriie gives the thionitrosyl complex [CrNS(MeCN),](PF6),.1' Reactions of this complex and some properties of it and its reaction products are outlined in Scheme 81. The v(NS) band in the IR spectrum of [CrNS(ButNC),]PF6 occurs at lower wavenumbers than the corresponding band in the spectrum of its oxidation product consistent with greater electron density on the metal and increased back-bonding into a a* orbital on NS.
Chromium
833 [CpCrCO(NO)NS]PF,
Na[CpCr(CO),]
+ N3S3C13
-
/ +
NOPF, in MeCN/CH,CI,
CpCr(C0)2NS
CO
+ NaCl CpCr(NO),CI
Scheme 80 [CrNS(MeCN)&PF&
blue; v(NS) = 1245 cm-'; v(CN) = 2325, 2305 cm-l Bu'NC, Zn in MeCN
[CrN!3(Bu'NC),jPF6 green; v(NS) = 1135 cm- ; v(CN) = 2203,2135,2070 cm-'
1
NOPF, in MeCN
[CrNS(Bu~~),I(PF,), blue; v(NS) = 1220 cm-'; v(CN) = 2225 cm-'
Scheme 81
(iii) Azo ligands Complexes of chromium(II1) with multidentate ligands containing azo linkages as well as other donor groups are of interest because of their a plications in spectrophotometric analysis and because of their uses as commercial dyestuffs.' Some ligands used in the analysis of chromium(II1) are listed in Table 65. Tnble 65 Azo Ligands Used for the Analysis of Chromium(II1) Ligand C,H2NBrT-N=N41Jl15N0
C~~ASO~-N=N--C,,,H~O~S~-N=N-C,H,RR~R~ (R,R', R = H, CO,H, PO,H,, As03H2,NO,, Cl) ~H50~N"-C17H1108s2Na2
Ph-N=N-C&O, cla7-N=N-c6H502 C,H,N-N=N+SO, C,HZNS-N=N-H5OZ
Application
Refi
Cr"' and Crm at pH 3.3-4.0 Cr"' at pH 2
1 2,3
C I " at pH 3-4.5 Cr"' at pH 3-6
4
C p ' at pH 3-6 CP' at pH 5-5.5 Cr"' at pH 5
5 6 7
5
1. F. Zhang and G. Chen, Cham. Absw., 1985,102, 67121h. 2. S. Fusheng and L. Xu, Chem. Abstr., 1984,100,15M40a. 3. V. K. Guseinov and 0. A. Tataev, Chem. Abstr., 1977, 87, 177042n. 4. L. Xu and F. Li, Chem. Abstr., 1983,90, 118679e. 5. I. K. Guseinov, N. E%. Rustamor, Ya. A. Asimov and M. M. Agamalieva, Chem. A srr 1978,89,35914r. " 6. K. L. Chemg, K. Ueno and T.Imamura, 'Handbook of Organic Analytical Reagents,$ , CRC, Florida, 1982, p. 197. 7. K. L. Cheng, K. Ueno and T. Imamura, 'Handbook of Organic Analytical Reagents', CRC, Florida, 1982, p. 208.
The structures of a number of chromium(II1)-azo complexes have been determined by X-ray diffraction methods. The ligands 2,2'-dihydroxyazobenzene and 5-nitro-2-(2hydroxynaphth 1azo)phenol form 2 :1 complex anions with chromium(II1) which have mer configurationsJ5 In the 2 :1 complex anion formed between 3-methyl-2-[3-methyl-l-@bromophenyl)-5-oxo-2-pyrazolin-4-ylazo]benzoic acid and chromium(II1) ( 0 9 ) the azo, carboxylate and enolate groups of one ligand are arranged tram to the corresponding groups in the other ligand.606 A number of isomeric 2 :1 chromium(II1) complexes containing ligands derived from
834
Chromium
F
-
I
N=CMe
.
039)
(140) n = 2, 3 or 6
2,2'-dihydroxyazobenzene, 0-(2-hydroxyl-l-naphthylazo)phenol, 2,2'-dihydroxyazonaphthalene and o-(2-hydroxy-1-naphthy1azo)pyridine have been prepared.607 Chromium(LI1) complexes containing the tridentate ligands 4-methylsulfonyt2-(4,5-diphenylimidazol2-y1azo)phenol and 4-chloro-2-(4,5-diphenylimidazol-2-ylazo)be1uoic acid have also been prepared.608 The effects of centra1 ring size on the electronic spectra and stabilities of the pentacyclic chromium(II1) complexes (140) have been investigated.- The preparation and properties of a number of 1:1 chromium(II1) complexes containing substituted naphtholazopyrazolone ligands (L) have been reported.604These compIexes appear to be of the type f a ~ - [ c r L ( H ~ O ) ~and ] + fuc-CrL(C1)(H20), and react with various bidentate ligands such as acac, salicylaldehyde, amino- and hydroxy-acids (Y) to give the ternary complexes, Cr(L)(Y)H20. The effects of substituents on the spectra of the dyes and their complexes are discussed. (iu) Hydrazine, hydroxylamine and related ligands Addition of alcoholic Nz&H20 to an aqueous solution of Cr(C104)3 affords a violet crystalline compound of composition Cr(N21€,)2(C104)3.610The IR spectrum of the product shows split absorption bands at 1100 and 630 cm-l characteristic of chelating C10; and a band at 950 cm-l characteristic of a monodentate hydrazine ligand. On this basis structure (141) is proposed for this complex.
Addition of Cr2(GO4),.6H20 to a cold alcoholic solution of N2H4 gives a violet powder of composition Cr2(C204)3.4N2fi4H20.611 This product is also obtained when the starting complex is stored in a desiccator over a 60% alcoholic solution of N2&. Exposure to a higher partial pressure of N2H4 leads instead to the product Cr2(G04)3-7N2&.H20.Evidence based on spectroscopic611,612and magnetic613studies indicates that the former product is polymeric containing an infinite -N2&4r-Nfichain in the equatorial plane, while the latter complex contains C201- as well as N2& bridging ligands. Acetylhydrazide (AcNHNH2) and benzoylhydrazide (BzNHNH2) form complexes with CrCl34H20 having compositions CrCl3(AcNHNH2),*3H2O and CrC13(BzNHNH2)3 re~pectively.~'~ IR and electronic spectral data suggest that each ligand is coordinated to the metal through the unsaturated nitrogen atom of its imido1 form (142).
Chromium
835
Chromium(II1) forms a number of complexes with NH20H depending on the reaction conditions as outlined in Scheme 82.615 Reaction of KCr(S04)2.12H20 with NH20H.HC1 (12-fold excess) and KOH (&fold excess) in aqueous solution affords the lilac-violet complex [CrCl(SO,)(NH20H)4].2H20 (for which s(N0H) = 1275 cm-l). All of these complexes contain N-coordinated hydroxylamine as shown by the characteristic shift to higher wavenumbers in the S(N0H) band relative to that in NH,OH.HCl. The electronic spectrum of [CrOH(NH20H)5]C12,the only one of these complexes to dissolve in water, shows maxima at 552 and 407 nrn; the position of the first implies that the N&OH ligand lies between NH, and H20 in the spectrochemical series. Reductive nitrosylation of CrO2- by NHzOH at pH 4-5 in the presence of pyridine-2,6-dicarboxylicacid (H2dipic) gives the products CrNO(dipic)(H20)2 (red, peff= 1.70 BM at 298K), [CrNo(dipi~)(NH~OH)~].3H~0 (yellow-green, peff= 1.71BM at 298 K) and Crdipic(N€l~o)(NH~OH)~ (dark green, peff= 3.6 BM at 298 K, Table 62).578The modes of coordination of hydroxylamine and the hydroxylamine anion in these complexes have not been established.
[CrC1(NH,OH),(H20),](OH),~EtOH 6(NOH) = 1200, 1270 cm-' six-fold excess
n 25 M
CrCl,.6H,O
N H ~ O HID E ~ O I Istr-fold . excess, warm
' [CrOH(NH,OH),]Cl,
NH20H.HCI, 12fold excess. KOH. six-fold excess aqueous soh
[Cr(OH),(NH,OH),H,O]OH~l.5H20 B(N0H) = 1270 cm-I. Scheme 82
4 '
Hydroxylaminechromim I11 complexes have been observed amongst the products of reduction of [CrNO(NH&] + by chromiurn(I1) in acid solution .579 Ion exchange experiments show that at least 50% of the NH20H produced (by the two-electron reduction of NO-) in this reaction is initially coordinated in highly charged complexes such as [(NH3),CrXCr(H20),Y]"+ (X = OH- or NH20H; Y = HzO or NH20H). In the complex tris(N-benzoyl-N-phenylhydroxylamine)chromium(III) the ligand acts as a bidentate two 0 donor.616 3542.7 Amido ligands
This section is dominated by dialkyl- and disilyl-amide ligands, which, because of steric factors, constrain chromium(II1) to the unusual coordination numbers of three or four. The reaction between LiNPrk and CrC13 in THF solution leads to the air- and water-sensitive red-brown solid Cr(NP&)3.5PZ The complex is monomeric in solution and in the solid state (peff= 3.80 BM at 298-123 K). An X-ray crystal structure analysis shows that the molecule contains a planar CrN3 and three planar CrNG units with 68-70" dihedral angles between the former and each of the latter planes (143).6'7 The planarity of the nitrogen atoms suggests involvement of their lone pairs in n bonding to the metal, an idea which is further supported by the relatively short Cr-N bond lengths. A number of other chromium(II1) dialkylamide complexes, all of which are highly sensitive to air and water, have been prepared and their coordination properties clearly depend on the steric requirements of the specific ligand. Hence in solution both Cr(NMe& and Cr(NEt& appear to exist as dialkylamido-bridged dimers in Under vacuum and at which each chromium is surrounded by a tetrahedral ligand field mild temperatures the dimers disproportionate according to equation (35). This reaction has been used to synthesize a number of Cr(NR2)4compounds (R = NEt2, NPrY, NBuZ, piperidide)
836
Chromium
(Section 35.5.4.3).618The inability of chromium(II1) to achieve its preferred hexacoordination because of steric hindrance accounts in part for this unusual disproportionation. In the tetrahedral chromium(1V) compounds the d2 electrons occupy low energy orbitals. In the dimeric chromium(II1) complexes, however, one of the d electrons on each chromium must occupy a high energy orbital. Transfer of an electron from one chromium to another results in formation of the volatile Cr(NR2), and the polymeric residue [Cr(NR2)2]n.618The monomeric complex Cr(NPr&)?), is stable thermodynamically with respect to d i s p ro p o rt i o n at i ~nWhile . ~~ many metal-dialkylamide complexes react with CS2 to give dithiocarbamate complexes, this insertion reaction is not successful with Cr(NPr:),. It does, however, react with C02, N O (see Section 35.4.2.6) and O2 (Scheme 83). The species Cr(02)(NPr&)?),may be a peroxo chromium(V) compound. At low temperatures the reaction with O2 produces an explosive compound of composition Cr03(NP&592 [cr2(NR2)6]
-
Cr(NR2)4 + [Cr(NR2)2]n
Cr(NPr\),(O,CNPrb)
(35)
CrNOQWrf),
cox
CrlNPr;),
A0
k
CrO,(NPr;), Scheme 83
The reaction of LiN(SiMe3)2 with CrC13 in THF affords the bright green complex Cr[N(SiMe3)2]3.5w A single crystal X-ray analysis showed that the complex is similar structurally to Cr(Pr;), with a trigonal planar CrN2 and planar CrNSi2 units. Like the dialkylamide complexes, Cr[N(SiMe3)& is rapidly hydrolyzed although it is less susceptible to insertion reactions as shown by its inability to react with either CS2 or C02. Since Cr(NPrL), and Cr[N(SiMe3)2]3 are both volatile and can be sublimed without decomposition, they are amenable to study by photoelectron spectroscopy. The gas phase He-I photoelectron spectra of these species show Cr-N u bond ionization energies in similar regions (9.9 and 10eV re~pectively).~” However the nitrogen lone pair ionization and the d (,A, bands occur at significantly lower energies in the spectrum of the dialkylamido complex. This confirms that the nitrogen lone pair is stabilized by delocalization on to the silicon atoms in N(SiMe3)2, thus making the ligand unable to act as a ~d donor to the metal. No such delocalization is possible in the dialkylamido ligand, and so in the complex Cr(NPr:), x donation from ligand to metal can occur, with a consequent lowering of metal d electron ionization energies. A study of the electrochemical reductions of Cr(NPri), and c~[N(SiMe~)~]3 in acetonitrile solution shows that the reductions are either quasi-reversible or irreversible and that the dialkylamido complex has a slightly lower reduction potential,594 Magnetic susceptibility measurements confirm that Cr(NPr‘,), and Cr[N(SiMe,)& behave as magnetically dilute species with pen values of 3.80 and 3.73BM respectively over the
Chromium
837
temperature range 123-298K.620 Neither complex gives an ESR signal in solution at room temperature. However, frozen solutions of the former in toluene at 130 K and polycrystalline samples of the latter show g anisotropy with principal values g,,= 2.0, g , 74.0 corresponding to an axially symmetric system with a large zero-field splitting of the chromium ion."' 35.4.2.8 N-Thiocyanato, N-cyanato, N-selenocyanato, azido and related ligands
The vast majority of known chromium(II1) complexes with the ambidentate chalcogenocyanate ligands NCO-, NCS-, NCTe- are those containing N-bonded thiocyanate.
(i) N- Thiocyafiate.~ (a) [Cr(NCS)613-.A variety of salts containing the [Cr(NCS)6I3- anion have been prepared.621,622 Prior to its structure determination by X-ray diffraction methods, criteria based mainly on IR spectroscopy were used to establish N coordination of the ligand to CI? in this anion. Hence the IR spectrum of &[Cr(NCS)6] (Table 66) has NCS bending and CS stretching vibrations at 483 and 820 cm-l respectively, positions which are characteristic of N-bonded thiocyanate complexes.621 In addition a band at 364cm-' in the IR spectrum of (Et4N)3[Cr(NCS)6]has been assigned to the triply degenerate (Tl,) Cr-N stretch of the Cr(NCS)6 group. Some properties of [CT(NCS)~]~are listed in Table 66. Table 66 Some Properties of [Cr(NCS),I3Cation [Ho(C,H,NCO,H)(H,O)J (Et4W3 K3 K3 K3
R@.
Property
Bond lengths (pm): Bond angles IR spectrum (an-'): IR spectrum (cm-'):
Cr-N, 200.2; N--C,ll4.4; C-S, 161.9 NCS, 176.6;CrNC, 164.3 v(CN),2078; 6(NCS),483; v(CrN), 364 (Nujol mull) v(CN) = 2058s, 2098s, 2118s;G(NCS) = 474s; v(CS)= 8Mw, [KBr disc)
Electronic spectrum [nm (dm3mol-' cm-')I: Magnetic moment (BM):
550(102),41Osh(95),38osh(79),310(17380) in 96% MeOH 3.79at 295 K, 3.74 at 82 K
e):
1
2
3 4
5
1. A. H. Norbury, Adw. Imrg. Chem. Radiochem., 1975,17,231. 2. A. Sabatini and I. Bertini, Inorg. Chem., 1965, 4,959. 3, M. A. Bennett, R. 3. W. Clark and A. D. J. Goodwin, Inorg. Chem., 1%7, 6,1625. 4. R. Ripan, I. Ganescu and C. Varhelyi, 2.Anorg. Allg. Chem., 1968, 357,140. 5. J. J. Salzmann and H. H. Schmidtke,Ihorg. Chim. Acta, 1969, 3, 2 M .
( 6 ) Mixed ligand N-thiocyanates. A large number of mixed ligand Nthiocyanatochromium(II1) complexes, many of which are of the type [Cr(NCS)4L$+ and similar to the original Reinecke's salt (L = NH3), are Generally they are prepared by refluxing a solution of [Cr(NCS)6]3- and the appropriate ligand L in alcohol and give trans isomers except when L=py, PhEt2P or of course when LZ is a bidentate ligand. Some representative reactions of [Cr(NCS)6I3- are shown in Scheme 84.55%5607623a30 The electronic spectra of the products have been used in many cases to deduce the positions of ligands L in the spectrochemical (the first spin-allowed ligand-field band 4Azg3 4T2g is a direct measure of 1ODq) and nephelauxetic series. The reaction of [Cr(NCS)6]3- with secondary and tertiary phosphines (R2R'P) gives, after acidification, one of the products R2R'PH[Cr(NCS)4(R2R'P)2] (A), (R2R'PH)2[Cr(NCS)SR2R'P] (B) or (R2R'PH)3[Cr(NCS)6](C) depending on the steric requirements of R and R'.628 Hence straight chain alkylphosphines give products of type A, EtPh2P gives B, while branching in the alkyl group increases the likelihood of obtaining product C. Neither PPh3 nor PHPh2 undergoes any of these reactions. The replacement of ligands in [Cr(NCS)6]3- by imidazole and its derivatives in 95% EtOH has also been i n ~ e s t i g a t e d . ~ ~While ' ' ~ ~ benzimidazole (bzim) and 2-methylbenzimidazole (2-Mebzim) give Reinecke type salts such as [Cr(NCS)4(bzim)2]-, imidazole (Him) and its 1- and 2-substituted derivatives give instead complexes of the type [Cr(NCS)3(Him)3].H20 or [Cr(NCS)3(2Etirn)~(H,O)]. Details of the IR and electronic spectra of some mixed ligand Nthiocyanatochromium(II1) complexes are presented in Table 67.
838
Chromium
Cr(NCS),(L-L)(P-dik)
K[Cr(NCS)4(R~~)z]
K[Cr(NCS),bipy]
Cr(NCS),( Rim),.H,O or Cr(NCS),(Rim),H,O or CrNCS(OH),(Rim), H[Cr(NCS),(Rbzim),]
[Cr(NCS),(Pyim),]NCS.H,O
Me,PH[Cr(NCS),(Me,P),], K[Cr(NCS)4(PhEt2P),l, (EtPh,PH),[Cr NCS),EtPh2P], (C.&ii)d\Cr(NCS)6]
K,[Cr(NCS),AA](AA = gly, ala, his, glu, tyr, met, asp) or K,[Cr(NCS),AA] (AA = ida) or K[Cr(NCS),(AA),] (AA = leu, isoleu)
i , R = H; heat with py at 110 "C: R = 3-NH,, 4-NH2; reflux in EtOH; ii, Rim = imidazole or substituted imidazole ( R = H , l-Me, 143, 1-Vi, 2-Me, 2-Et, 2-Pr'); reflux in 95% EtOH; iii, Rbzim= benzimidazole or substituted benzimidazole (R = H , 2-Me); reflux in 95% EtOH; iv, Pyim = 2,2'pyridylimidazole; reflux in EtOH; Y, Gnbzim = 2-guanidinobenzimidazole; reflux in EtOH or Pr"0H; vi, AA = amino acid; 1 : 1 or 2: 1 mole ratio as appropriate, in H,O; vii, phosphine in Pr'OH, reflux, H'; viii, R3P0 (R = E t , Bun, C,H,,) in Pr'OH, reflux; ix, diars in EtOH, reflux, Et4NCl; x, bipy in EtOH, reflux; xi, /3-dik = P-diketone (benzoylacetone, dibenzoylmethane); L-L = bipy, phen, (4-NH2py),; reflux in EtOH. Scheme 84
A successful separation of the species [Cr(NCS),(H20)6-n1(3-n)+ (n = 0-6) including the cis, pans isomers of [Cr(NCS)2(H20)4]+ and the mer, fuc isomers of Cr(NCS)3(H20)3has been The complete series of anions accomplished by adsorption chromatography on Sephadex [Cr(CN)6-n(NCS)n]3d(n = 0-6) has been prepared and separated by gel electrophoresis and by chromatography on alumina.632Ligand substitution in [Cr(NCS)6]3- by CN- in acetonitrile affords the trans (n = 2) and mer (n = 3) mixeb ligand intermediates, while substitution in [Cr(CN)6I3- by NCS- in the same solvent leads to the cis and fac isomers (Scheme 85).633The position of the low energy 4A2,(F)+4T2g(F) band undergoes a step-wise blue shift of about 30 nm on decreasing n (Figure 5 ) and log E for the charge transfer band at -310 nm increases almost linearly with n, the number of thiocyanate ligands. These observations suggest that no change in the mode of NCS- coordination occurs throughout this series. The reaction of [Cr(CN)(NCS)sI3- with en in acetonitrile affords the complexes rne~[Cr(CN)(NCS)~enl-and cis-[Cr(CN)(NCS)(en)$. 634 The solid state deamination of [Cr(en)3](SCN)3on its own and in the presence of trace NWSCN catalyst, which functions by protonating and breaking loose one end of the coordinated amine in the rate-determining step, gives in a two-step process trans[Cr(NCS)2(en)2]SCN.635At high catalyst levels the cis product is obtained. Other Nthiocyanatochromium(II1) complexes prepared by solid state deaminations include cis[Cr(NCS)2(tn)2]SCNand C~~-[C~(NCS)~(~~)~]SCN,~ while the trans isomer of the former was obtained by deamination of a suspension of [Cr(tn)3](SCN)3in acet~nitrile.~~' A number of N-thiocyanatoammine and amine chromium(II1) complexes are referred to in Tables 46,49 and 50. The complex [{ Cr(NCS)(TPyEA)}20](BPh4)2 contains N-bonded thiocyanate (DO). (c) Bridged NCS- complexes. Complexes containing both bridging and non-bridging NCSligands have been prepared as outlined in Scheme 86.638Evidence for formation of bridged complexes is based mainly on the IR spectra which show CN stretching bands at -2135cm-l and at -2100 cm-' corresponding to bridging and terminal N-thiocyanate ligands respectively. The homonuclear dimer is both photochemically and thermally the more stable of the two complexes. Evidence from IR and electronic spectroscopy points to the existence of bridging thiocyanate ligands in the salts M3[Cr(NCS)6](M3 = Cu3, Ag3, CdSn, Hg3,2,T13, Pb3/2)and also in adducts formed between Reinecke's anion and a number of h.12' ions of the first row d-block
Chromium *
3
r4N
839 m.3-
u-
N
840
Chromium
Chromium [Cr(NCS),l3-%
[CrCN(NCS)5]3-
rruns-[Cr(CN),(NCS),]’-
841
~ ~ u ~ ~ - [ C ~ ( C N ) , ( N C S ) , ] ~ -rner-[Cr(CN),(NCS)3]3-
-
CN-.
% [CI-(CN)~NCS]~cN-\ [Cr(CN),]’- SCN-, ~ u c - [ C ~ ( C N ) ~ ( N C S ) ~ ] ’ SCN-
[Cr(CN),NCSI3- SCN-, cis-[ Cr( CN),(NCS)$-
Scheme 85
X (om) 650 I
500
400
350
I
I
1
-
CCr(NCSQ3-
.-......CCr (CN) (NCS)J3-
.. trons- ECr(CN),(NCS),I3-
-..-..
trans- CCr(CN),(NCS)273-
300 I
CCr(CN)J(NCS)J’-
--.--.
CCr(CN)(NCS)513-
---- CCr(CN)J5F i r e 5 Electronic spectral changes (in aqueous solution) accompanying stepwise substitution in [Cr(NCS),J3- by CN- (adapted from 2.Anorg. AUg. C h . ,1975,417, 55)
The measured formation constant of [(NH3)5Cr-NCSAg]3+ (log K = 5.11, I = 0.03M, T = 298 K) is comparable to the formation constant of AgSCN (log K = 4+8)under the same conditions.621 ( d ) NCS- linkage isomers. Although the thiocyanate ion is a potentially ambidentate ligand, examples of linkage isomerism in thiocyanatochromium(II1) complexes are rare. This can be attributed to the class a nature of the metal ion and its almost exclusive preference for N over S donor atoms. One of the few known S-thiocyanatochromium(II1) complexes, [Cr(S N)(H20)5]2+,is obtained by the inner-sphere reductions of trans-[CoX(NCS)(en)$+ (X = 20,NH3, C1-, SCN-), [Fe(NCS)(HZO),)” (remote attack) or [Co(SCN)(NH&I2+ (adjacent attack) by aqueous C?+.639*640 In aqueous solution [Cr(SCN)(H20)5]2+undergoes (as well as aquation) spontaneous and metal-ion-catalyzed isomerization, Catalysis by C4’ occurs by a redox mechanism involving initial metal ion attack at the N atom of the S-coordinated ligand while catalysis by H$+ occurs by the non-redox pathway shown in Scheme 87.639
i
Chromium
842
&[Cr(NCS),].4H,O [~(NH3)5H2OI,(SO& dilute HCIOI. 40 "C
[(NH3)5Cr-SCN-Cr(NCS),\- 3H20 v(CN); 2135,2098,2080 cm-
[(NH3)5Co--SCN-Cr(NCS)51.3H20 v(CN); 2137,2100,2083 cmScheme 86
Hg \Cr(SCN)(H20),I2' + Hg"
[HgCl]
+
+ [Cr(N CS)(H20),I2+
c1-
I
[(H,O),CrSCNI4'
J [ (H,O),Cr-N
CS-Hgj4+
Hg2+
Scheme 87
The products of reduction of Fe(NCS)'+ by Cr2+ in aqueous solution containing thiocyanate ions (equation 36), include the isomers of [Cr(NCS)(SCN)(H,O),]+, which contains both Nand S-bonded thiocyanate ligands.641These isomers undergo spontaneous decomposition by parallel aquation (loss of the S-thiocyanato ligand) and isomerization (Cr-SCN 4Cr-NCS) reactions. Details of the solution visible spectra of these isomers as well as those of similar chromium(II1) complexes for comparison are listed in Table 68.From the compiled data it is apparent that the S-bonded thiocyanate ligand lies very close to Br- in the spectrochemical series while the N-bonded form lies between N, and NO. Fe(NCS)'+
+ C?' a Cr3++ Cr(NCS)*++ Cr(SCN)" + cis, tram-Cr(NCS):
+ cis,tram-[Cr(NCS)SCN]+ + Fez+
(36)
Table 68 Visible Spectra of Isomeric Thiocyanato and other Chrornium(II1) Complexes"
A,
Complex [c~(Ncs)(H20~51z+ [Cr(SCN)(H O)$+ [crI(HzO)51'+ [CrBr(HzO)5]2' [CrCl(H20)5J2' [CrF(HzO),I + [CrN3(H20),l2+ [CW)(HZO)g+ [CrNO(HzO)51 [CdHzO)5N&I2+ c~s-[C~(NCS),(H,O)~]+ tr~rn-[Cr(NCS)~(H,0)~]+ [Cr(NCS)(SCN)(H,O),] '
'
570 (31.4) 620 (26) 650 (36.1) 622 (19.9) 609 (16.4) 595 (12.2) 585 (67.5) 560-545" 559 (28) 545 (22,l) 562 (-50) 575 (-47) 605 (55)
(nm)(e in dm3mol-' cm-') 410 (33.5) 435 (20) 474 (32.6) 432 (22.4) 428 (20.8) 417 (11.8) 434 (66.4)
449 (120) 397 (21.8) 420 (-54) 425 (-47) 440 (43)8
262 (8000)
Ref. 1 2b 1 1 1 1 1 1 1 1 3 3 4
bNote that values of E for this complex in ref. 1 have been corrected in ref. 2. 'An undetermined composition of cip, tmns isomers. 1. A. Haim and N. Sutin,3. Am. Chem. SOC., 1%6,88,434. 2. M.Orhanovic and N.Sutin, J. Am. Chem. SOC., 1968,90,4286. 3. J. T.Hougen, K. Schug and E.L. King, J. Am. Chem. SOC., 1957,79,519. 4. L. D.Brown and D. E. Pennington, Znorg. Chem., 1971,10,2117. "In dilute HCIO,.
(ii) N-cyanates and N-selenocyanates Compared with N-thiocyanatochromium(II1) complexes, those containing N-cyanato and N-selenocyanato ligands are relatively few in number. Some crystalline salts M3[Cr(NCSe)6].nS ( M = N a,K) have been prepared by heating acetone or dioxane (S) solution mixtures
843
Chromium
containing MSeCN and CrC13,followed by slow evaporation of solvent.642The purple-coloured complex (Me4N),[Cr(NCSe>,] was obtained by addition of Me4NC1 to a cooled solution containing CrC13 and a ten-fold excess of KSeCN which had been refluxed for four hours.642 The green-coloured complex (Ph4As)3[Cr(NCO)6]was obtained by the addition of AgCNO to a solution of CrC13 and Ph4AsC1in acetone containing a trace of zinc Details of the IR and electronic spectra of these complexes together with those of [Cr(NCS),I3- for comparative purposes are listed in Table 69. The suggested presence of N-selenocyanato ligands in the anion [Cr(NCSe)6]3- is based on these data. Hence the appearance of v(C-Se) in the region 600-700 cm-' indicates N-coordination of this ligand to the metal. In selenium-bonded selenocyanato complexes the v(CN) and v ( C 4 e ) bands are generally observed above 2080 cm-I and in the region 500-600 cm-' respectively. The almost identical values to 10 Dq for [Cr(NCS)6]3- (18 080 cm-l), which is known to contain N-bonded ligands, and [Cr(NCSe)6]3- (17 920 cm-') further suggest N-bonding by the ligand in the latter species. The situation regarding [Cr(NC0)6]3- however is less clear. While the v(C0) band at 1336 cm-' in the IR spectrum of the complex is in the range expected for N-cyanato ligandsw and the strong band at 345cm-' has been assigned to a Cr-N stretching vibration, the very low value of 1ODq (16390cm-') is more typical of a Cr"*06 rather than a Cr"'N, chromophore. Spectroscopic evidence for the existence of N-cyanato ligands in the complex merCr(NCO)3(py)3is somewhat more convir~cing.While ~~ the v(C0) and v(CrN) bands in the IR spectrum of this complex occur at positions close to those reported for [Cr(NC0)6]3--,its visible spectrum is quite similar to that of rner-[Cr(NCS),(py),] , from which it is prepared by oxidation with B r 0 3 (equation 37). mer-[Cr(NCS),lpy),] -t 4Br0;
PY
546 nm (E = 121 dm3mol-.' cm-') 399 nm ( E = 93 dm3mol-' cm-l)
Am:
mer-[Cr(NCO),(py),] i4Br- -t 3S03
(37)
556 nm (110) 393 nm (83)
Table 69 Main Bands in the IR and Electronic Spectra of [Cr(NCX),I3- Complexes v(CN)
Complex [Cr(NCS),I3[Cr(NCSe),r[Cr(NCO),] ~~~~
~~
~~~~
2U78s
2067s 2205s ~
~
IR S(NCX) v(Cr-N) (cm-') 483m 420m
619111 601m
364s -
345s
Electronic
v(CX)
820w" 663m' 1335m'
4A2g+1T&
553
559 610
4A2g-+4T,g(F) ( 4
CT
409
309b 342" -
L
436'
Ref. 1-3 4,5
6
~~~
"Et4Nf salt as KBr disc or Nujol mull. In MeOH. 'Me4N+ salt in Nujol. acetone. "In DMSO. 'Ph4As+ salt in KBr disc. Reflectance. 1. M. A. Bennett, R. I. H. Clark and A. D. J. Goodwin, Inorg. Chem., 1967, 6, 1625. 2. A. Sabatini and I. Bertini, Inorg. Chem., 1965, 4,959. 3. V. Alexander, J. L. Hoppt and M. A. Mdati, Polyhedron, 1982,1, 191. 4. A. I. Brusilovets, V. V. Skopcnko and G . V. Tsintsadzc, Rurs. J . Inorg. Chern. (Engl. Traml.), 1969, 14, 239. 5. K.Michelsen, Acta Chem. Scand., 1963, 17, 1811. 6. R. A. Bailey and T. W. Michelson, J . Inorg. Nucl. Chem., 1972, 34,2935.
The reaction of K3[Cr(NCX)6] or (BU:N)~[C~(NCX)~] (X= 0, Se) with chelating diamine ligands in acetonitrile solution followed by column chromatography on alumina gives salts of the complexes [Cr(NCX)4AA- and cis,tr~ns-[Cr(NCX)~(AA)~]+ (AA = en, pn, chxn) all of which have been characterized by electronic and IR spectroscopy."6 Details of the preparations and spectra of [Cr(NCX)(NH3)5]2+(X = 0, S, Se) complexes are presented in Tabie 70. (iii) Azides
The reaction of [Cr(NH&H20](C104)3 and NaNl in dilute acetic acid solution at 60°C affords the monoazido complex [CrN3(NH3)5](C104)2.647 The addition of less than one equivalent of KN3 to a solution of chrornium(1II) in molten KCNS at 185°C produces a bathochromic shift in the visible spectrum consistent with the formation of the thermally stable complex [Cr(NCS)gN3]3-.648The neutral triazido complexes C T ( N ~ ) ~ ( Nand H ~ )Cr(N3)3(py)3 ~
Chromium
844
Table 70 Prepararions and Spectra of [Cr(NCX)(NH,),]2' (X = 0,S, Se) Complexes
Complex
"(-1
Preparation
ICr(NCO)(NH,),](NO,), ICr(NCS)(NH3)sI(ClO4)~
Cr(N0,)3.10H,0 + urea at 170"C [Cr(NH3)sH*01(N03),'NH,N03 + NaSCN in H,O at 60-65 "C; NaCIO, [Cr(NH,),H,0](C10,)3 + NaSeCN in DMSO at 80 T
+
[Cr(NCSe)(NH,)s](C104)2 .. .. -
Electronic spectrum' ( s in d m mol cm I)
IR spectrumb
R#.
493 (63) 491 (83.1)
366 (38) 3TOsh (52.5)
v(CN) = 2240 crn-' v(CN) = 2084 cm-'
2,3
476 (67.6)
-
v(CN) = 2075 crn-'
4
1
.-
KBr disc. sh = shoulder. In H,O. 1. H. H. Schrnidtke and T. Schhherr, 2.Anorg. AUg. Chem., 1978,443,225. 2. E. Zinato, R. Lindholm and A. W.Adamson, J. I n o g . Nucl. Chem., 1969, 31, 449. 3. C. S. Garner and D. A. House, Tranriiion Met. Chem. (N.Y.),1970, 6,59. 4. N. V. Duffy and F. G. Kosel, Inorg. Nucl. Chem. Len., 1969, 5, 519. a
have been prepared, the first by refluxing an aqueous solution containing [Cr(NH3)6](clO& and NaN3 and the second by the addition of pyridine to a hot ethanolic solution containing Cr(N03)3-xH20and NaN3.302The green crystals of the latter complex are only slightly water soluble and should be treated with caution because of their potential explosive nature. When an aqueous solution mixture of [Cr(en),]Cl, and NaN3 is heated the colour changes to rose-red and the salts ck-[Cr(N,),(en),]Cl and ~ i s - [ C r ( N ~ ) ~ ( e n )are ~]N coprecipitated ~ from solution on cooling.302The brick-red complex cis-a-[Cr(N&(txien)]Br has been prepared by heating an aqueous solution of ~is-a-[Cr(Cl)~(trien)]Cl.H~O in the presence of excess NaN, and then adding KBr.649 The configuration of the complex was established on the basis of its IR spectrum. Hence in the asymmetric NH2 bending region (1560-1660cm-l) it shows a split absorption, whereas in the CH2 rocking region (860-940 cm-') it shows two bands of medium intensity, features which distinguish the cis-a from the c i s 6 and trum configuration^.^' The violet complex ( B u ~ N ) ~ [ C ~ ( N (peR ~ ) ] at ~ 295 K = 3.76 BMMO), in which a large counterion is employed to stabilize the [Cr(N3),I3- anion, was obtained by the addition of BGNC1 to the filtrate from a reaction suspension containing CrC13.6Hz0 and NaN3 which had been heated at 50-60 "C for one hour.651Evidence from IR spectroscopy points to the fact that [Cr(N3)6]3- contains bent M-N-N groups. Comparison of band positions in the electronic spectrum of the complex with those for other hexacoordinated chromium(II1) complexes indicates that the azide ligand occupies a position slightly lower than F- but higher than (Et20)PSF in the s ectrochemical series and roughly the same position as I- in the nephelauxetic series.6g Electronic spectral data corresponding to the 4A2g--*4T,(F) and 4A2g +4T,g(F) electronic transitions in a number of azido-chromium(II1) complexes are compiled in Table 71. Table 71 Electronic and IR Spectroscopic Data for Some Azidochromiurn(II1) Complexes
Complex
v,(N3-) (cm-') 2094
2090,2055
2102,2048
I,,
(nm) ( E indrn3mol-' cm-') 500 (154). 382 (93.4) 502 (166), 366 (99) 515 (186), 399 (121) 509 (154), 392 (93) 551(213), 407 (128) 585 (67.5), 434 (66.4) 514 (195), 397 (129) 515 (224), 398 (148) 514 (195). 397 (129) 515 (224), 398 (148) 518 (185), 402 (126) 520 (220), 400 (135) 603 (87.5), 443 (87.2) 667 (204), 502 (170)
Ami.
(E)
446 438 (60)
447(68.5) 447(68.5) 452 (70.0) 450 (70)
Medium H2O 2 M HCIO,
172 3
0.01 M HClO, 1.5 M HCIO, 1M NaCIO, 1M HCIO, 0.01 M HC10,
3 2
H,O 0.01 M HCIO, H,O 0.1 M HCI 1M NaC10, 1M HCIO, CH,Cl,
I . R. Davies and R. B. Jordan, Inorg. Chem., 1971,10, 11M. 2. C. S. Garner and D. A. House, in 'Transition Metal Chemistry', ed. R. L. Carlin, Dekker, New York, 1970, vol. 6, p. 59. 3. I. J . Kindred and D. A. House, J. Iwrg. Nucl. Chem., 1975,37, 1320. 4. H. Ogino, T. Watanabe and N. Tanaka, h o g . Chem., 1975,14, 2093.
5 . T. W. Swaddle and E.L. King, Inorg. Chem., 1964, 3,234. 6. W. Beck, W. P. Fehlhammer, P.PUmann, E. Schuierer and K. Feldl, Chem. Ber., 1%7,100,2335.
Ref
4 5 2 2 2 2
2 2 5
6
Chromium
845
The photochemical conversion of azidochromium(II1) to nitridochromium(V) complexes has in recent years attracted considerable interest.652 The complexes [CrN3(edta)lz-, [CrN3(nt a)(H20)]- , [Cr(N3)'(nt a)]'-, [CrN3(VDA) (H20)]- and [Cr(N&( VDA)I2- (VDA = N,N'-valinediacetate) when exposed to UV radiation liberated N2 and gave products having ESR spectra (narrow line, no fine structure) characteristic of species with one unpaired electron and visible spectra similar to those of oxochromium(V) complexes.652It was therefore concluded that the products were nitridochromium( V) species. UV irradiation of CrN3(salen)(H20)653and Cr(TPP)N3-nH20654(Scheme 88) also give the corresponding nitridochromium(V) complexes. The evidence for formation of an intermediate nitrene complex [CrN(NH&I2+ in the 313 nm photolysis of CrN3(NH&I2+ has been disputed and the results reinterpreted to indicate formation of [Cr (N3-)(NH&I2+, which undergoes rapid aquation to the nitridopentaaquachromium(V) cation.653
b
Cr(TPP)OH.nH20
.%
Cr(TPP)N,.nH,O 5 [CrV(TPP)(N3-)] Scheme 88
The kinetics of aquation of a number of azidochromium(II1) complexes have been inve~tigated.~'~ Compared ,~~~ with other acidochromium(II1) complexes, the chromiumazide bonds in these species seem remarkably stable to thermal substitution. Hence in the base hydrolysis of [CrN3(NH3)#+ a pathway involving initial loss of NH3 concurs with the usual base hydrolysis pathway involving loss of N,. The aquation of azidochromium(II1) complexes is H+-assisted with protonation of the azido ligand accounting for the enhanced reactivity. While the action of oxidants on coordinated azide is generally quite complicated, the peroxymonosulfate ion HSO, reacts cleanly with [CrN3(NH3),I2+ in a ueous solution according to equation (38).656Tracer experiments using HSO, enriched with ' 0 at either the terminal peroxide position or at all other positions indicate that the oxygen transferred from oxidant to reductant in reaction (38) is the terminal peroxide of HSO;. Peroxymonosulfate also reacts cleanly with uncoordinated HN3 and N; (to give N,O and N2) and the observed reactivity follows the order HN3 N’-C6HbbgH> N’-PhbgH. Replacement of the aqua ligands in c i ~ - [ C r ( b g H ) ~ ( H ~ O ) ~ ] ~ + by phen, bipy676and GO$- 677 in aqueous solution proceeds by a pathway which is first order in ligand concentration but with values of A H Z which are similar for ail three incoming groups. A mechanism involving outer-sphere association of reactants followed by transformation of outer-sphere complexes into products by a dissociative pathway has been proposed for these reactions. The products have been isolated as the perchlorate salts [Cr(bgH),L](CI04), (L = phen, bipy, n = 3; L = GO,, n = 1) and characterized. Some chromium(II1) complexes with dibiguanide ligands which contain ethylene, hexamethylene or phenylene bridges linking two biguanide residues have also been reported.674 When an aqueous paste containing Cr(bg),-H2O and NKSCN is heated at 80°C until evolution of ammonia ceases, the red crystalline complex \Cr(bgH)2(NCS)21NCSis obtained.678 While evidence from IR spectroscopy ( Y C = ~ 2095 cm- , vCs = 770 cm- , &CS = 490 m-l) supports the presence of N-thiocyanato ligands, no information regarding the geometrical configuration of the complex can be deduced from its 1R and electronic spectra. By comparing the positions of the 4Azs4‘Tr, bands in the electronic spectra of appropriate complexes (Table 75) it may be inferred that biguanide is slightly higher in the spectrochemical series than either the N-thiocyanato or biguanidato ligands.
Table 75 Electronic Spectra and Magnetic Moments of Chromium(II1) Biguanide Complexes Complex
4A2g-+‘TTZB 4Azp-+‘7& (F) (nm)
pcfi (277-278 K) (BM)
Xef.
Nujol mull supported on filter paper. Reflectance spectra. In aqueous solution. 1. A. Syamal, 2. Nar~?fomch.,Ted B, 1974,29,492. 2. R. H. Skabo and P. W. Smith, Awt. J . Chem., 1969,22, 659. 3. D. Banerjea and S. Sengupta, 2.Anorg. Allg. Chem., 1973,397,215. 4. S. Sengupta and D. Banerjea, 2.dnorg. A&. Chem., 1976,424, 93. 5. D. Banerjea and C. Chakravarty, J . Inorg. Nucl. Chem., 1964,26, 1233.
Complexes of chromium(II1) with tetramethylguanidine, HN = C(NMe2)2, have been prepared according to Scheme 94 and although they have not been fully characterized because of the presence of free ligand impurities, the appearance of C=N stretching bands at lower wavenumbers in the IR spectra of the complexes suggest that the ligand is coordinated to the metal through the imine nitrogen.679
CrCI,
+ HN=C(NMe,),
v(C=N) = 1609 cm-’
CrCI,-4EIN=C(NMeJ2
green, v(C==N) = 1563 cm-’ Scheme 94
COC3-BE
exass HN==C(NMe2)2. 120 “c
b
CrC13-6HN=C(NMe2)Z grey-green, v(C==N) = 1562 cm-‘
Chromium
852
(iii) N-coordinated ureas and amides In most of the known urea complexes of chromium(II1) the ligand is O-bonded to the metal ion.679 However the complexes [CT(NH~)~{ OC(NH2)2}]3f and [Cr(NH3)5{OC(NHMe)2}]3+, both of which can be prepared in high yield by appropriate substitution of the labile triflate ligand in [CT(NH~)~(OSO~CF~)]'+, undergo isomerization without competitive hydrolysis in basic solution to give deprotonated N-bonded urea complexes (Scheme 95).680 Reacidification of the product solutions regenerates the O-bonded isomers rapidly and quantitatively. The IR spectrum of the O-bonded urea complex possesses characteristic absorptions at 1570 and 1640 cm-l both of which are absent from the spectrum of the deprotonated N-bonded form. A similar change from O-bonded to deprotonated N-bonded formamide occurs when a solution of [Cr(NH3)5(OCHNH2)]3+ is made basic. [(NH,),Cr-OC(NH2j2I3+
a,
504 ( E
= 58),
11
+ I(NH3j4(NH2)Cr-OC(NH2)2]2+ = [(NH,>,(NH,)Cr-NH2CONH2]'kf
OH-
Ifas,
kb
370 ( E = 38)
pKa=13.5,298K, I=O.lMNaCIO,
[(NH,),Cr-OC(NH,)NH]*+
+ H+
[(NH3)$r-NHCONHJ2+
A,, A,,
490 ( E = 48), '372 ( E = 43)
in nm,E in dm3mol-' cm-' Scheme 95
Addition of amide (122,H4L) and Na2C03to a boiling solution of CrCl3.6Hz0 in pyridine results in formation of the complex [Cr(HL)py2I2.2py (1541, which contains chromium(1II) coordinated to the amide oxygen and deprotonated amide nitrogen of different molecules.519 This is the first reported example of chromium(II1) coordinated Ito a nitrogen of a deprotonated amide. The structure consists of discrete dimers and disordered solvent molecules of pyridine. C1
35.4.3
C'
Cr-NW1 = 2.145 A
= 2.097 A = 1.931 8, Cr-O2 = 1.915 8, Cr-Omi, = 1.976 A Cr-Nmid, = 2.030 8,
Cr-N,z Cr-0'
Phosphorus, Arsenic and Antimony Ligands
Complexes of hard Cm' with ligands containing soft P and As donor atoms have not been easy to prepare. It has usually been necessary to use weak donor solvents in the absence of oxygen and moisture, although some hydrates have been obtained by admitting traces of moisture. Structures have generally been inferred from conductance and molecular weight measurements, and IR and electronic spectra. The Cr'" is invariably six-coordinate with magnetic moments close to 3.9 BM. Complexes of a few monodentate tertiary phosphines are known. There are dimers polymers [CrC13(PR3)], (R = Et, [CrX3(PR3)2]2(X = C1, R = Et, Bun; X = Br, R = Bun, Ph) and anionic complexes [PPh4][CrC14(PR3)2]62(Table 76). The tris(phosphine) complexes CrC13(PHEt2), and CrC13(PH2Ph), have also been rep0rted,6*~but a number of
Chromium
a53
organophosphines would not react with chromiurn(II1) halides. From K3[Cr(SCN)& several salts [R3PH][Cr(SCN),(PR3),] have been obtained (see also Section 35.4.2.8.i.b). Monodentate phosphines with [Cr(THF),I3+ solutions (see below) produce green Earlier work with ditertiary arsines and phosphines produced the compounds: CrX3[oC~H~(ASM~&]I.~ [CrX2{o-CsH4 , AsMe2)2}2]CQ (X= C1, Br, I), [crX3(HZo){o= c1, Br),6B6 [CrX3{o-Cs&(AsMe2)2}]2 (X = Br, I), [cr12{oC6H4(ASMe2)2}] (X= C1, Br, n = 2; X = I, n = 4; C ~ H ~ ( A S M ~ Z ) Z }CrX3(Ph2PCH2CH2PPh2)1.5.nH20 ~]~~, X = NCS, n = 0) and [ C ~ X ~ { O - C ~ H ~ ) ( PClU, M ~ ~(X ) ~=}C1, ~ ] Br).689Complexes of multidentate tertiary phosphines and arsines were also obtained: [CrX3{P(o-C6H4PPh23}] (X= C1,Br), [Cr12{P(o-C6€&PPh&}]13$90 CrX3{PPh(o-C6H4PPh2)2) (X= C1, Br, NCS),69! CrX3{AsMe(oand CrC13{CMe(CHZAsMe2)3} .693 C&ASMe2)2} (X = CI,Br, I)6923693
'
Table 76 Complexes of Phosphorus, Arsenic and Antimony Ligands Electronic s ectra'
P
(m- 1
Complex
R&
329s, 325s 334sh, 328s 333sh, 327s 370s, 345s, br
12 BOOsh, 15 800, 20350 12 800, 13500, 15 900, 20 450
1 1 1
13 MWkh, 15 600,20 500
1
353, 341s
13 SMIsh, 15 150,20 900
1
22Ow, 306w
15 SO0
1
345s, 319s
12800, 1 7 4 0
1
349s, 321s
13 150, 17550
1
364sh, 356sh
13 300, 18450
1
355111, 330s. 305sh
15580, 19 840
2
358sh, 3331% 325% 312m
15000(502), 19300(328)
2
312m, 285s, 258s
14 800 (993), 18ooo (601)
2
13440, 16 450
2
354,326s, 302s
15 530,20 800
2
34Os, 315s
14 660 (378), 19 500 (170)
2
14 800 (408), 19ooO (150)
2
366sh, 3 4 h , 315sh, 300s
14 700 (543), 19 700 (343)
2
2%
14700 (402), 18800 (337)
2
14 490,17 540
2
346m. 335s, 302s
14 OOO, 18 200
2
326s, 319s
15 250 (367), 19 600 (273)
2
3 6 2 , 3 4 0 ~ 316 .
15 660 (253), 21 660 (173)
2
290,250
14 790, 19 460
2
14 970, 18 240
2
366, 336, 316
15 660 (352), 21 Mx) (163)
2
298,252
15 150 (206),20 400 (110)
2
2%
-
2% 25Osh
255s
-
Chromium
854
Table 76 (continued) Electronic s ectraa
P
(cm- 1
Complex
Ref.
375, 324
15 800,21300
2
349, 336, 317
16 130,20 830
2
269,250
15 820.20
ooo
2 -
337hr
2
335m
16 I m h , 18300sh, 20 OOO, 22 530
2
355m
16 ZoOSh, 18400sh, 19 960,22 700
2
282m
16 61Osh, 18 150,21300sh, 22 830
2
28om
16720sh, 18 180, 21 400sh, 22700
2
38om
15 970sh, 17010,21 500sh, 24 OOO
2,3,4
3%
15300sh, 16550,2OOOOsh, 22Mx)
2,3,4
290sh, 275s
2
355m, 352sh, 338111, 306sh
2
3Wm, 333sh, 321s, 307sh
2,3
314, 285s, 258sh
2,3
-
2,s
375sh, 352s, 338, 320sh, 308sh
2
306,295, 275s
2
375w, 345s, 333sh, 308m
2
320br, 280s
2
370sh, 345s, 306s
2
-
2
350s, 338s
14 500, 17 OOO, 21 OOO
2
29obr
14 120, 17 OOO, 20 OOO
2
375sh, 345sh, 3&h, 315111
Isomer mer
5
354, 334, 320sh
mer
5
36Osh, 340, 318
mer
5
350,334
fac
5
364br
fac
5
346,340sh
fac
5
333sh, 326
foc
6
298,265,230
mer
5
305,282
mer
5
362, 342, 320
855
Chromium Table 76 (conrinued) Electronicspectra'
@-'I
Complex
fw
290sh, 263,245sh
P(CH,CH,PPhz)3 Blue CMe(CH,PPh,), Green AsMe(CH,CH,CH,AsMe,),
290, 256
288br
hrrple
286br
As(CH&H,C&AsMe-& Purple CMe(CH,AsMe,), Blue
298, 278
[CrWI
PPh(CH,CH,PPh,), Blue P(CH,CH,PPh,), Purple CMe(CH,AsMe,), Blue
544, 517, 505
mer
542, 519,507
mer
538, 492
fac
[CrX,L]BF,
X
P(CWH,PPb), Dark blue P(CH$HZPPh& Dark blue P(CH,CH,PPh,), Turquoise [-CHzP(Ph)CH,CH,PPh2]2 Dark blue As(CH,CH,CH,AsMe& Purple
Cl
350, 310
CiS
Br
296,255
Cir
I
Cis
CI
347. 318
CiS
C1
346, 326
CiS
(dm3 mol-' cm-') in parentheses. First examples of C r S b bonds. :[~~][CrCl,(cis-Ph,PCHCHPF~)],Cr-P, 2.485, 2.511, Cr-CI, 2.331, 2.319, 2.318 A. mer-[Crcl,{P(CH,CH,PP$),I, Cr-PE'h,, 2.466, 2.489, Cr-P(CH,),, 2 . 3 9 , Cr-Cl, 2.306, 2.292 and 2 . 3 3A. 2. M. A. Bennett, R . J. H. Clark and A. D. J. Goodwin, 1. Chem. SOC. (A), 1970, 541. 2. L. R. Gray, A. L. Hale, W. Levason, F. P. McCullough and M. Webster, 3. Chem. Soc., Dalton Trans., 1983,2573 3. R. S. Nyholm and G. J . Sutton, J. Chem. SOC., 1958, 560. 4. R. D. Feltham and W. Silverthorn, Inorg Chem., 1968, 7 , 1154. 5. L. R. Gray, A. L. Hale, W.Levason. F. P. McCullough and M. Webster, J . Chem. SOC., Dalton Trans., 1984, 47. 6. R. J. H. Clark, M. L. Greenfield and R. S . Nyholm, J . Chem. SOC. (A), 1%6, 1254. aE
The above complexes were generally prepared from poorly soluble chromium(II1) chloride, or [CrC13(THF)3]in situ in THF, although a few were obtained by oxidation of phosphine- or arsine-chromium carbonyl^^^^,^^,^^ with halogens. By using CrX3 THF)3] (X = C1, Br or I) in dry CHzC12694instead of THF, competition for the hard C r I by an 0 donor solvent has been avoided, and extensive series685,695@96 of complexes of multidentate P, As and S (see p. 886) donor ligands have been obtained (Table 76). The pseudooctahedral structure of [CrC4(cisPh2PCHCHPPh2)]- has been confirmed cry~tallographically.~~~ The diphosphine does not exert any trans influence, consistent with weak bonding to the hard Cr"' ion, which is also evident from the electronic spectral parameters of the complexes generally. It has been confirmed that the complexes typified by CrC13[o-C6H4(A~Me2)2]1.5 have the tran~-[CrX~(L~)~]cis-[Cr&(L~)] structure favoured earlier,686but for CrX3(Me2As(CH2)3A~Me2)1.5 a ligand-bridged structure [X3(Lz)Cr(L2)Cr(L2)X3]has been put forward because the v(Cr-421) absorptions are different from those in [NP~]-l[CrCL{Me2As(CH2)3AsMe2}] (Table 76). Repeat preparations CI€ CrX3(Ph2P(CHz)2PPh2)1.5.nH20, for which similar diphosphine-bridged structures were proposed,@@ failed. The weaker donors Ph2PCH2CH2PPh2, cis-PhzPCHCHPPh2 or PhzAsCHzCHAsPhz produce complexes [CrX3(L2)2],which are believed to be meridional, containing one uni- and one bi-dentate diphosphine and diarsine, and give [CrX3(H20)(Lz)Jin the presence of moisture. It remains necessary to generate in situ solutionsw presumed to contain [CrF3(THF)3] and [Cr(TIIF)6][BF4]3for the preparation696of the fluoro complexes CrF3L (Table 76), and
6 (
856
Chromium
the salts [Cr(L2)3][BF4]3 695 (L2 = Ph2PCHzCH2PPh2,MezPCHzCHzPMez,O - C ~ & ( P M ~ 0 ~-) ~ , C&(AsMe2)2 or cis-Ph2AsCH-CHAsPh2) and [Cr(L3)2][BF& (L3= PhP(CH2CHzPPh&, MeAs(CHzCHzCH2AsMe&, MeC(CHzPPh2)3or MeC(CH2AsMe2)3)because attempts to isolate solid starting materials caused decomposition. Of these octahedral complexes,695only [Cr(MezPCHzCH2PMez)3][BF4]3 undergoes reversible electrochemical one-electron reduction in DMSO (as does [CrXz{o-C&(PMe2)2}2]C104 in MeCNm9)and reactions with LiX lead to partial displacement of the neutral ligands to give, for example [CrC13(PhP(CH2CH2PPh2)2}2]. An attemptm6to prepare [ C ~ { ~ - C & ( A S M ~ ~ ) ~ } ~ ]from [ C ~[Cr(H20)6][C104]3 O~]~ failed. In the neutral complexes CrX3(L3 or L4) (X= C1 or Br) those P or As ligands which are potentially quadridentate act as trident ate^.^^^ An X-ray study has established that [CrC13(P(CH2CH2PPh2>3}]has a mer arrangement of the P3C13 donor set with one PPh2 group not bonded. This phosphine is usually quadridentate in complexes of other metal ions. Structures have been assigned to the other complexes from the number of v ( C r 4 1 ) vibrations. Tripodal tridentates, e.g. CMe(CH2PPh&, constrain monomeric complexes to form fac isomers and this provides a further check on the IR data. No 2 : 1, L3 or L4 to Cr complexes were obtained when X = CI or 3r. The reaction of [Cr13(THF)3] with multidentate ligands in a 1:l ratio gavem6 the Complexes Cr13L [L = PPh(CH2CH2PPh2)2, P(CHzCH2PPh2)3, CMe(CH2AsMe2)3, AsMe(CH2CHzCH2AsMe2)2or A S ( C H ~ C H ~ C H ~ A ~Few M ~physical ~ ) ~ ] . data are available, but Cr13L are considered analogous to the corresponding chlorides and bromides. Reaction of [CrI,(THF),] in a 1 : 2 molar ratio gave two complexes of the type Cr12L3: with L = PPh(CH2CH2PPh2)zthe complex is formulated as [Cr{PPh(CH2CH2PPh2)3}]13and with L = CMe(CHZAsMe& as [CrIz{CMe(CH2AsMe2),}2]1,the arsine being bidentate. Treatment of the corresponding halide in CHzClz with AgBF4 gave the complexes [CrX2L][BF4] (Table 76). The two v(Cr-X) stretches are consistent with cis cations and tetradentate L. The Cr-P bonds in [NPr~][CrC14(cis-Ph2PCHCHPPhz)] and rner-[CrC13{P(CH2CH2PPh2)3}]696are long (Table 76) and this is attributed to steric repulsions and the weak binding of the neutral phosphine li ands to the hard C P ' ion. The C r - C l bonds are ~ , ~ with ~ , octahedral ~ ~ , ~ Cfl", ~ ~ and , ~ ~ Analysis of the electronic: s p e ~ t r a ~ ~ , is ~consistent the derived Racah parameters are always much lowered from the free ion value; this also is attributed to weak binding of the soft ligands. An alternative assignment697 has been proposed695for [CrX2{o-C6H.,(AsMe,),),] -t which places the diarsine lower in the spectrochemical series.
35.4.4 Oxygen Ligands 35.4.4.1 Aqua complexes
The hexaaquachromium(1II) ion (155) is a regular octahedron and is found in aqueous solution and in many hydrated salts.698*6w~700~701 Typical examples are the violet hydrate [Cr(H20)6]C13and the chrome alums, MCr(S04)2-12H20.The acidity of the aqua complex is marked; the first hydrolysis has p K , = 4.0 (25 "C). Further hydrolytic equilibria are complicated by polymerization and are dealt with in the following section. The most recent detailed reports on [Cr(Hz0)6l3+ in the solid state are IR702and crystallographic studies6w of cesium alum. In the vibrational study, metal-ligand vibrations were assigned at 555 cm-' (-vas stretch) and 329cm-' (vas bend). In the structural paper, the classification of alums is discussed; the C r - 0 bond length was found to be 1.959A. The hexaaqua ion is also found in crystals of Cr4Hz(S04)7.24H20;the 0-0 bond distances are in the range 1.915-1.991 A.703
(155)
The rate of exchange of H20at [Cr(H20)6]3+is given as rate = k[Cr(H20)6]3+;this reaction has been extensively studied.m The most reliable results are summarized in Table 77. The
Chromium
857
substantial and negative value of AV* together with AS* near to zero suggests an 2, mechanism for exchange. Table 77 Kinetic Data for Water Exchange at Chromium(II1) k X IO7 Compound
b-')
AH* (kJ mol-')
ASt (J K-' mol-')
AV* (cm' mol-')
Ref.
~
1. J. P. Hunt and R. A. Plane, 1. Am. Chem. Soc., 1954,76,5960. 2. D.R. Stranks and T.W. Swaddle, J . Am. Chem. SOC., 1971,93,2783. 3. N. V. DuEy and J . E. Earley, J . Am. Chem. SOC., 1967,89,272. 4. T.W.Swaddle and D. R. Stranks, J . Am. Chem. SOC., 1972,94, 8357.
3544.2 Polymeric hydroxy complexes
The existence of such complexes was first postulated by Bjerrum in 1908.705There have been a number of studies in recent years. Several early members of the series of hydrolytic polymers formed on the addition of base to aqueous chromium(II1) solutions have' been isolated;706 purification was achieved by ion-exchange chromatography on Sephadex-SP C25 resin. The structures suggested for these complexes are illustrated (W6-162). The structural unit (162) was held to be singularly important, both as a constituent of higher polymers and in the mechanism of dimerization. Equilibrium data for these complexes are summarized in Table 78.
(158)
(158)
(159)
I
(159)
Among the polymeric hydroxy complexes, the dimer has been the most extensively studied species. There is no totally unambiguous crystal structure but the complex is almost certainly as illustrated (156). In addition to this doubly bridged dimer a singly bridged [(H20)5CrOHCr(H20)5]"C(163) species has been reported.707 The magnetic properties of both (156) and (163) have been studied. Coupling in the doubly bridged dimer (156) was
Chromium
15+
(162) Table 78 pK, Values for Coordinated Water in some Chromium Complexes Complex
Cr3+
Cr,(OH),
Cr,(OH),
Cr4(OH),
pK, values
4.29
3.68
4.35
6.10
6.04
5.63 6.01
2.55 5.08
'After Marty (ref, 706); c h a r g ~on complexes omitted, I = LOMNaUO,, 25T, structures are given in (155)-(159).
characterized by - I l k = 7.5" and g = 1.96, and for the singly bridged species (163) by - J / k = 16" and g = 1.96. A more favourable bond angle was suggested to explain the stronger coupling in the latter case; direct Cr/Cr interactions were not believed to be important. The dimer (156) has also been the subject of an extensive NMR study, in solution, by the inert probe method.708Changes in structure at high proton activities were investigated; p-dioxane was used as the additive. Magnetic moments were measured, in solution, by the Evans method; values of 3.94 BM and 3.89 BM were obtained for the monomer and dimer respectively. There is obviously no strong spin interaction at ambient temperatures. At high concentrations of perchlorate (>8M), the complex turns from bluejgreen to green (t0.5 15min). Leffler suggested708that perchlorate complexation (164)causes this change (equation 41) based on the fact that monobridged dimer (163) and dimer (l56) should be distinguishable in NMR line broadening experiments.
(163)
[(HZO'"C'\
/OH,
,
Cr(H,0)J4+ + C10;
,
OH
F==?
/
[(H20)4Cr,
\
Cr(H20)3(C104)]3+ + H,O
OH
OH
(156)
(1W
(41)
Chromium
859
The mole ratio A trimeric complex has also been the subject of a detailed [Cr]: [OH] was found to be 4:3; the degree of polymerization was assessed, by freezing point depression, to be three. A triangular structure, as shown in (157), was suggested on the basis of magnetic studies. The closely related oxo-bridged complex [(H20)5CrOCr(H20)5]4+is obtained in low yields on the reduction of 1,4-benzoquinone by aqueous solutions of chromium(I1).710 The sole product of the hydrolysis of this dimer7*l(HC104/LiC104 I = 1.0) is hexaaquachromium(II1). 35.443 Oxides and oxide hydroxides
The oxide systems deriving from chromium(II1) are numerous. Chromium(II1) oxide, Crz03, is the final product of the calcination of many chromium(II1) complexes. Chromium(II1) oxide has the A1203 structure (D5)with hexagonal lattice parameters a = 4.95, c = 13.66A and eta = 2.76L712 The oxide is also the product of the spectacular ignition of (NH&Cr207. The hydrated form of the oxide Cr203.2H20 is an important pigment,'I3 Guignet's green. Chromites formally contain the Cr20:- ion and are generally prepared by fusing CrzO3 with a binary oxide.714They are typical spinels; chromium(II1) occupies octahedral sites. The parent spinel, chromiteFeCrz04-is the only commercially important chromium mineral. Calcium chromite is not a spinel and exists in two modifications: the j3 form, which is isomorphous with calcium ferrite, space group Pnam17" and the CY form, which is a high-temperature stable rnodificati~n,~'~ prepared by fusing CaC03 and Cr203(1350 "C, 24 h). The spinels formed by chromium are all of the normal t e; lattice energy calculations for a large number of such species have recently been reported. Another important class of oxide-containing solids are the compounds of general formulation Cr02H, referred to as oxide hydroxides or, occasionally, as chromous acids. The system is polymorphic; a rhombohedral form is obtained by the decomposition of an a ueous solution of Cr203 (Mallinkrodt analytical reagent) in a high pressure vessel at 300°C.79 The material is red/brown and has a layer structure; oxygen atoms are coordinated to chromium(II1) in a distorted octahedron. Each octahedron shares six edges with six surrounding coplanar octahedra to form a continuous sheet of close-packed oxygen atoms. The sheets are superimposed, oxygen atoms in one sheet falling directly above those of the sheet below. The structure as a whole is not close-packed; the layers are held together by short H bonds.719A second form of Cr02H has been prepared by the hydrothermal treatment of CrO, (derived from Cr03, 72 h, 450 "C, 40 000 psi); this form is olive green. The compound is orthorhombic ~ ~ ~coprecipitation ,~~~ of and isomorphous with In02H, Le. a deformed rutile s t r u c t ~ r e . The y-(Al, Cr)02H and H(Cr, M)O2has been investigated.n2
8
35.4.4.4 Other oxide system
The novel material CrzTe4011contains the binuclear ion [Crz010]14-,as isolated units, formed by two Cr06 octahedra sharing an edge.723The exchange interaction between the chromium centres has been modeltd by the Heisenberg-Dirac-Van Vleck phenomenological model, with the exchange parameter JsAsB= -6 (f0.5)cm-'. The substitution into the M"'Te06 .~~~ system of V02+ has been investigated for a number of metals including ~ h r o m i u m ( I I 1 )The transition between monoclinic and rutile forms can be modelled on the basis of the size of the M"' ion. The substitution of chromium(II1) into yttrium, iron and gallium garnets has been i n v e ~ t i g a t e dIn . ~the ~ system [Y,][CrxFe2-x][Fe3]02single-phase specimens were obtained with values of x = 0.5. For the gallium system, x < 0.75 gave a single phase. Perovskites An'B"'O 3 involving indium or thallium and chromium(II1) have been synthesized.726The coupling of the chromium(II1 centres in the trirutile compound MCr2O6 (M =T e or W) has been inve~tigated.~The coupling is dependent on the second metal ion and has been correlated with slight structural variations.
1'
35.4 45 Peroxides
Two green peroxychromium(II1) species have been reported.728The products of the reaction of Hz02 and Crw in 2-6M perchloric acid were purified by ion-exchange chromatography COC3-BB.
860
Chromium
(Dowex 50W-X2).The compounds were tentatively identified as Cr02Cr4+and Cr02Cr02C9+ species; their decompositions were not simple processes. Attempts to synthesize these or similar species by the oxidation of chromium(I1) with ox gen were unsuccessful. The compound [(H20)5Cr02Cr(H20)5]4+ was also reported earlier. 7 L
35.44.6 Alcohol and alkoxide complexes
(i) Alcohol complexes Alcoholate complexes of the general formula CrC13.nROH (R =M e, Et, Pf, Bun and n-hexyl) have been reported .730 These purple insoluble complexes were prepared by refluxing CrC13.3THFin the anhydrous alcohol and are typical chromium(II1) s ecies. The structure of the related complex tran~-[Cr(MeOH)~Cl~]Cl has been ~Ietermined.’~ PIt is similar to that of the related aqua complex and is of approximately C,, symmetry. A detailed study of the complexes formed by ethylene glycol and chromium(II1) has been reported;732both monodentate and bidentate coordination of chromium by the glycol was postulated. (ii) Alkoxide complexes
Alkoxide complexes including those of chromium(II1) have been extensively and excellently re~iewed.~~ Methoxy ~ . ’ ~ ~ corn lexes, [Cr(OMe)Cl2],,.2nMeOH and [Cr(OMe)C12],.nMeOH, have been thoroughly studied?” The former is prepared by the reaction of anhydrous CrCb with methanol, the latter by heating the former at 100°C for several hours. Adducts of the dimethanoates can be prepared with acetone, acrylonitrile and dioxane. These compounds are unusual among alkoxides in showing marked solubility: the acetone adduct appears to be a dimer in both solution and the solid state. A tetrameric structure was suggested for the monomethanoate (165).
The physical properties of samples of Cr(OMe)3 prepared by a variety of methods have been investigated.736 Good crystals could not be obtained but limited crystallography suggested a layered lattice with chromium occupying octahedral sites. A useful summary of earlier preparations and attempted preparations of chromium methoxides can be found in this paper. The insertion of various isocyanates into chromiurn(II1) alkoxide M-0 bonds has been reported.737The complexes are prepared by refluxing the isocyanates with a suspension of the alkoxide in benzene. No structural data were given for the products. Unusual bimetallic alkoxides have recently been prepared738by the reaction of Cr[Al( OPT^)^]^ with alcohols and acetylacetone (166). A wide range of spectroscopic methods were used to study them. In general, the results were in accord with a monomeric formulation similar to (166) below: Cr[A1(OMe)4]3was grossly insoluble; the small size of the methyl groups may permit extensive polymerization.
Chromim
861
35.4.4.7 j3-Ketoenolates and related ligand
(i) Synthesis The tris and bis complexes of acetylacetone (2,4pentanedione) (167) with chromium(II1) have been known for many years (168, 169).739The tris compound is generally prepared by the reaction of an aqueous suspension of anhydrous chromium(II1) chloride with acetylacetone, in the presence of urea.74oRecently a novel, efficient s nthesis of tris(acety1acetonato)chromium(111) from Cr 03 in acet lacetone has been reportedx1 The crystal structure of the tris complex has been determine~l.'~' A large anisotropic motion was observed for one of the chelate rings, attributed to thermal motion, rather than a slight disorder in the molecular packing.
Derivatives of, or ligands closely related to, acetylacetonate have been extensively studied. Compounds of analogous structure are, in general, obtained. Ligands studied include l-phenylbutane-l,3-dione,1,1,l-trifluoropentane-2,4-dione,1,1,1,5,5,5-hexafluoropentane-2,4dione, 2,2,6,6-tetramethylheptane-3,5-dioneand l-phenyl-5,5-dirnethylhe~ane-2,4-dione.~~~ Preparations include the original one detailed above740 and methods from chromium(II1) acetate and metallic chromium.744The earlier work and advantages of the various methods are summarized by Dilli and R o b a r d ~ ?who ~ ~ noted, particularly with derivatives of acetylacetone, that in the classic reaction of the free ligand with urea may markedly reduce the yield. The closely related tris complex of malonaldehyde (€70) has been synthesized by the reaction of 1,1,3,3-tetramethoxypropaewith anhydrous CrC13 in an ether The complex was purified by sublimation and contains the simplest structural unit capable of enolizing to form a P-ketoenolate chelate.
(170)
Several unusual complexes containing neutral acetylacetonate have been reported.746The complex [CrBr~(acac)(acacH)] was obtained from the reaction of HBr with [Cr(acac)3]; [CrCl,(acac)(acacH)] was obtained from the reaction of CrC13(THF)? with acetylacetonate. Other unusual f3-diketonates include the tris(rhenaacet0nates) (171).747,748 Two different forms of the tris complex of 2-nitroacetophenone with chromium(II1) have been rep~rted;'~'these are probably geometric isomers.
cis.
(171)
Complexes containing two different ligands of the P-diketonate type have also been
Chromium
862
including the series [Cr(CF3COCHCOMe),(MeCOCHCOMe)3-,] (n = 1 or 2); cis and trans isomers were isolated. The linkage isomerization and disproportionation of the unsymmetrically substituted acetylacetonate complex (172)have been studied751and were said to be intermolecular. H MeC
M
Camphorate complexes of chromium(TI1) have been studied. The four possible isomers of the tris complex of ( + )-3-acetylcamphorate (173)were i s ~ l a t e d , ’ ~ ~and * ” ~absolute configurations were tentatively assigned, The photoisomerization of these complexes has been in~estigated;~’~ quantum yields of the order of lop3 were obtained with visible or ultraviolet radiation at temperatures around 100 “C. Bond-breaking processes were held to be important in the reactivity of cis isomers.
(173) R = H or Me
Dimeric complexes of bis(acety1acetonates) have also been reported: di-p-diphenylphosphinatoacetylacetonatochromium(111) prepared by the reaction of diphenylphosphinic acid with an excess of [Cr(acac),] has been investigated.755It crystallizes in the triclinic space group P1, the bridge (CrOPOCr), forming a puckered eight-membered ring. The crystal structure of the alkoxy-bridged complex bis(~-methoxy)bis[bis(2,4-pentanedionato)chromium(III)] has Other alkoxy-bridged been determined; it crystallizes in a racemic rather than meso complexes have also been synthesized.757 ( i i ) Physical studies
Acetylacetonates have occupied a central position in the study of complexes by physical methods. Some of this work is summarized in Table 79. Table 79 Some Physical Properties of [Cr(acac),] Property
Value
Conlmni
Ref.
C r - 0 bond energy lODq lODq
256 kJ mol-’ 217 kJ mol-’ 17.8 x lo3cm-’ 17.5 x lo3cm-’
1 2 3 4
835 835
0.55 cm-’ 0.79 cm-’
Eo.,
-1.73V
Combustion calorimetry Combustion calorimetry Electronic spectra Also substituted acacs All 17.5( f 3 ) x Id cm-’ Nephelauxetic parameters Also for substituted acacs us. SCE in DMSO Also 17 substituted acacs
1. M.M. Joncs, E. J. Yow and W. R. May, Inorg. Chem., 1962, 1, 164. 2. J . L. Wood and M. M. Jones, Inorg. Chem., 1964,3, 1553. 3. R. L. Lintvedt and L. K. Kernitsky, Inorg. Chem., 1970,9, 491. 4. C. J. Ballhausen, ‘Introductionto Ligand Field Theory’, McGraw-Hill, New York, 1962, p. 239. 5. A. M. Fatta and R. L. Lintvedt, Inorg. Chem., 1971,10,478. 6. R. F. Handy and R . L. Lintvedt, Inorg. Chem., 1974,l3,893.
5
6
863
Chromium
( a ) Vapourphuse chemistry. A remarkable feature of the chemistry of the complexes of the tris(acety1acetonate) type is their extreme stability in the vapour phase: there are hence many mass spectral and gas chromatographic studies of them. Much of this emanates from the group of Sievers. The complexes of hexafluoroacetylacetonate are considerably more volatile than those of the parent acetylacetonate. The partial resolution of tris(l,l,1,5,5,5-hexafluoro-2,4pentanedionato)chromium(III) by gas chromatography on pulverised d-quartz has been rep~rted.~~ The ' , ~cis ~ ~and trans isomers of the tris(trifluoroacety1acetonate) complexes of chromium(II1) can be se arated efficiently by gas chromatography (5% silicone grease on Chromasorb-W, 115 0C).7 To provide better understanding of such studies, vapour pressure ~~ related measurements have been made on a number of metal P - d i k e t o n a t e ~ .Earlier measurements are reviewed in this paper. In typical gas chromatographic phases (Apiezon-L, QF1) the results of the above study indicated that the solution behaviour of these tris octahedral complexes is determined solely by non-specific van der Waals type forces; there was no evidence for a direct interaction such as H bonding. A rate and equilibrium study of the cisltruns isomerization of tris(l,l,l-trifluoro-2,4pentanedionato)chromium(III) in the vapour phase has been reported.76' The equilibrium constant 3.56 was independent of temperature in the range 118-144.8 "C, the same vaIue being measured by both static and kinetic methods. The activation energies for isomerization were both less than 115 kJmol-l; the Cr-0 bond energy is 210kJmol-'. On these grounds, a bond-rupture mechanism was rejected. A similar mechanism may operate for the isomerization in solution. The mass spectra of various 8-diketonates have been extensively s t ~ d i e d . ' ~The ,~~ availability of ionization energies, measured by mass spdctra, has led to a fairly detailed debate as to the nature of the orbital from which the first electron ionizes,7B4 Gas phase rearrangement^,'^^ often involving fluorine migration to the metal, have been observed. The SIMS spectrum of [Cr(acac),] is particularly simple,'@ yielding the base peak [Cr(acac),]+. The structure of tris(l,l, 1,5,5,5-hexafluoro-2,4-pentanedionato)chromium(III)has been determined by gas-phase electron diffraction.767A most interesting feature of this study was 30.1'1 was very close to the value found that the normalized ligand bite angle [(O-O/M-0) in the solid state by X-ray diffraction (30.8') but quite different to the value predicted by Kepert's m0de1.'~ The results of this and related work on vapour phase structures are summarized in Table 80.
P
Table 80 Summary of Electron Diffraction Data" __
-
Compound
C-0
iCr(hW3l ICr(hfa),l [Cr(acac),l hfa acac
1.270 1.276 1.263 1.259 1.287
C-C (ring) 1.409 1.392 1.388 1.407 1.405
Method
ED ED X-Ray ED
ED
Ref. 1 2 3 4 5
Abbreviations: hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione,acac = 2,4-pentanedione, ED =electron difiaction. All bond lengths in A. 1. Ref. 767. 2. B. G . Thomas, M. L. Morris and R. L. Hilderbrandt,1.MOL Shuct., 1976,35, 241. 4. A. L. Andressen, D.Zcbelman and S. H.Bauer, I . Am. Ckcrn. Sm., 1971,93, 1148. 5. A. H. Lowery, C. George, P. D'Antonio and J. Karle, 1. Am. Chem. SOC., 1971, 93, 6399. a
( b ) Optical activiry. As tris P-diketonates are neutral D3 complexes, resolution by the more-popular , less-soluble diastereoisomer and ion-exchange methods is not possibIe. Various methods have been reported. Potentially most useful are: a stereospecific synthesis from an unisolated ( )-tartrate complex of chromium(1II) ,769 chiral chromatography using (2R,3R)( - )-dibenzoyltartaric acidm0and chromatography on an L-nickel(II)tris(l ,lo-phenanthroline)/ montmorillonite column."' Other methods of resolving [Cr(acac),] are extensively reviewed in these papers. The crystal structure and absolute configuration of L-( - )-[Cr(a~ac)~] has been reported.772 The? circular dichroism of ~-[Cr(acac),]has been studied in the solid state and in solution.773 The rotational strengths of the d-d transitions are extensively discussed; competing rather than reinforcing effects lead to the smaller rotational strengths observed for tris(acety1acetonates) as compared to tris-(ethylenediamine) or -oxalate complexes.
+
864
Chromium
The Cotton effects in mixed amino acidatelacetylacetonate complexes [Cr(aca~)~L] (L = Lalanine, L-valine or L-phenylalanine) have been absolute configurations were assigned by reference to the parent tris(acety1acetonate) complexes. Synthesis was achieved by the photolysis of mixtures of the amino acid and [Cr(acac>,]. The partial photoresolution of both cis- and trans-(1,l ,l-trifluoro-2,4-pentanedionato)chromium has been accomplished by irradiation with circularly polarized light (5461 A) in chlorobenzene solution.775The results indicated that both bond rupture and twist mechanisms were important. A number of other B-diketonates have also been inve~tigated.’~~ ( c ) NMR. The complex [Cr(acac)3] has found use as a spin relaxant in 13CFTNMR ~ ] methanol, acetonitrile, benzene and The solution dynamics of k C r ( a ~ a c ) in DMSO have been studied in some detail.7 Different models were proposed for solvation by the various solvents. In methanol, the hydroxyl proton is apparently closer to the chromium than the methyl group. An outer-sphere dipolar mechanism of exchange was suggested in DMSO, the oxygen atom of DMSO pointing toward the octahedral face of the [Cr(acac),]. In benzene a random orientation of the flat and cleft positions, with respect to the solvent, best fits the data. In acetonitrile the results, surprisingly, suggest that the methyl group points toward the complex; weak H bonding may account for this observation. A summary of other spin relaxation studies can be found in this paper.778 A detailed understanding of the many factors involved when [Cr(acac),] is used as a spin relaxant is required. It has been shown that line widths are affected by the reagent.779 If temperature dependent features are to be studied, it was concluded that the ratio of relaxant to substrate must be kept as low as possible. Caution in the use of [C r(a~ac)~] as a spin relaxant was also emphasized in a study of cholesterol chloride.780Despite the problems with the use of this complex as a spin relaxant, it remains widely sed.^^^,^^^ (iii) Chemical reactivity
The bromination of tris(acetylacetonato)chromium(III) was first reported by Reihlen.781 There have been many studies of electrophilic substitution at com lexes of both acetylacetonate and its derivatives; this work has been extensively r e ~ i e w e d . ~ Some typical reactions are outlined below (equation 42). In this section, we shall briefly mention some more recent work; the interested reader is recommended to study the extensive, although somewhat dated, review by C ~ l l m a nand , ~ ~Mehrotra’s ~ 7783
Me
Me
Me
Me
(174) (175) X = I, Br, C1, SCN, SAr, SC1, NO2,
CH,Cl, CH,NMe, COR, CHO
The electrophilic substitutions of acetylacetonate complexes have been taken as suggesting ‘aromatic character’ in the chelate ring. Results with seventeen different 1,3diketonatochromiurn(II1) complexes were recently held to support this suggestion (176-178; equation 43).784 The bromination of tris(l,1,l-trifluoro-2,4-pentanedionato)chromium(IiI), previously claimed to be unreactive ,’85 has been r e p ~ r t e d . ” ~ Ar
\
R’ (176)
Ar \
d’ (177)
Ar \
R
(1781 X+ = CI’, Br+, or NO; Ar = 4-FC6H, or a derivative R = Me, Et, Pf,CF, or Ph
Chromium
865
An isolated study of the tris tropolonate complex (179) of chrornium(II1) has appeared;787 mass spectra were recorded and fragmentation patterns discussed.
35.4.48 Catecholates, quinones, tetrahydrofuran and 0-bonded urea
The chemistry of the transition metals including chromium(II1) with these ligands has been the subject of a recent and extensive review,788with references to the early literature. The close relationship between the catechol (180), semiquinone (181) and quinone (182)complexes may be appreciated by considering the redox equation below (equation 44).789The formal reduction potentials for the chromium(II1) complexes (183-186; equation 45) are +0.03, -0.47 and -0.89 V (us. SCE in acetonitrile) respectively.
M @ 3 ) ] 0\
3
(45)
(i) o-Semiquinones The preparation of tris(o-semiquinone) complexes of chromium(II1) has been rep~rted;'~' . ~ P ~Cr(phenSQ)yanisole were prepared complexes of formulae C ~ ( O - C ~ , S Q ) ~ and (o-C14SQ = tetrachloro-l,2-benzoquinone;189). The complexes exhibit temperature-dependent magnetic properties, p& 1.1BM at 286 K dropping to 0.35 BM at 4.2 K. This is the result of intramolecular spin-coupling between chromium(II1) and the semiquinone. The complexes were prepared by the reaction of chromium hexacarbonyl with the quinone (equation 46) but are correctly f~rrnulated,'~' by comparison with authentic samples, as semiquinones of chromium I11 . The above reaction has proved difficult to repeat in the absence of photolysis>g' 'and it was hence suggested to be a photochemical, oxidative substitution. In related work, the reaction of C r b i ~ y ( C 0 with ) ~ a number of quinones was studied.7Y2The resulting redox series: lCr(SQ)2(bipy)]+/[Cr(SQ)(Cat)(bipy)]/[Cr(Cat)2(bipy)lwas studied; the oxidation state of the chromium remains at three during these reactions. Cr(CO), + quinone (187)
-
Cr(quinone),
(1W
Chromium
866
(ii) Cacecholutes The resolution of tris(catecholato)chromate(III) has been achieved by crystallization with ~-[Co(en)~]";the diastereomeric salt isolated contained the ~-[Cr(cat),]'- ion.793Comparison of the properties of this anion with the chromium(II1) enterobactin complex suggested that the natural product stereospecifically forms the L-cis complex with chromium(II1) (190). The tris(catecho1ate) complex K [Cr(Cat)3].5H20 crystallizes in space group C 2 / c with a = 20.796, b = 15.847 and c = 12.273 and p = 91.84'; the chelate rings are planar.794 Electrochemical and spectroscopic studies of this complex have also been undertaken.79s Recent molecular on quinone complexes are consistent with the ligand-centred redox orbital chemistry generally proposed for these sy~terns.~''
A
(190)A
diagram of the A-cis chromium(II1) enterobactin trianion
(iii) Hydroxamates
Chromium(II1) complexes of a number of polyhydroxamic acids, microbial iron sequestering and transport agents (siderochromes) have been r e p ~ r t e d . ' ~The , ~ ~kinetic ~ inertness of the chromium(II1) complexes allows the facile separation of isomers; for the model complex tris(N-methyl-( - )-methoxyacetylhydroxamato)chromium(III), D - c ~ , L - C ~and the LID-trans isomers have been ~eparated.~"The chromium complexes of desferrioxamine B (191) have been investigated; the possible isomers are illustrated below (192-196). The cis isomer was isolated in relatively pure form.799 Thiohydroxamate'"" and dihydroxamate (rhodotorulic acid) complexes have also been studied.*'' NK
CONH
\
(CW5
\
N-C
I
/ !/CH,)2
I1
OH 0
\
(cy', /""Y( c y , p
N-C
Id
&I
z
,,Me
N-C
I II OH 0
(191)
A-C-trans, trans
A-N-cir.,trans A-N-trans,cis (1%) (1%) The five enantiomeric geometrical isomers of ferrioxamine B. The oxygen donor atoms of each hydroxamate group have been omitted for clarity. The A optical isomer is shown in each case. A-N-cis,&
(192)
A-C-trans,&
(193)
(194)
Chromium
867
(iv) THF The complex [Cr(THF)3C13]was first prepared in 1958.*02However pure bromide, iodide and thiocyanate complexes were reported only in 1983.803 The preparation of [Cr(THF)6]3+by the reaction of [Cr(THF),C13] with AgBF4 in THF was attempted; only an impure brownlpink oil was obtained. The tris complexes are useful starting materials in both organometaflic and coordination chemistry.
(v) Urea (O-bonded) Chromium is unusual in that it forms a stable hexakis O-bonded urea complex. The complex was first prepared as the chloride salt by PfeifferW and a crystal structure of the complex salt [Cr{OC(NH2)z}6][Cr(CN)6]*2DMS0.2EtOH has recently been r e p ~ r t e d . ~Coordination ’ at chromium(II1) is octahedral; r(Cr-0) is in the range 1.96-1.98A. The reduction of the perchlorate salt of this complex to a chromium(I1) species has been studied polarographically.m Detailed studies of the luminescence spectra of several salts of the chromium urea complex have been r e p ~ r t e d . ~ ’ ~ , ~ ~ 35.44.9 Non-C oxo anions
(i) Nitrogen-containing Anhydrous chromium(l1I) nitrate may be prepared b the reaction of dinitrogen pentoxide with chromium carbonyl in dry carbon tetrachloride.“ The product has rather low thermal stability, is involatile and decomposition begins at 60 “C. The complex 1,2,3-trinitrotriamminechrornium(III) has been prepared and is a nitrito isomer.810 The nitrito complexes of cis- and trans-bis(ethylenediamine)chromium(111) have been prepared,811 ~ii-[Cr(en)~(ONO)~]C10~ and tr~n;s-[Cr(en)~(ONO)~]X (X = C10; or NOT). The nitrito complexes cis- and trans-[Cr(en>,(ONO)(X)]+(X = ONO-,OH-, F-, Cl-, Br-) have also been studied.’I2 (ii) Sulfur-containing
of chromium(lI1) sulfate There are old reports of tris sulfate c ~ r n p l e x e s ~and ~ ~ .a~ number ’~ hydrates are known. In recent years monosulfatopentaa uachrorniurn(II1) complexes have been prepared”’ and characterized by IR spectroscopy?l6 sulfate probably behaves as a monodentate ligand. The basic chromium(II1) sulfate Cr(OH)S04.H20 crystallizes in the non-centrosymmetric, monoclinic, space group Cc.817 Each chromium atom is coordinated octahedrally by two hydroxo groups, three sulfate 0 atoms and a water molecule. The metal atoms are joined by single hydroxo bridges forming an infinite chain. In acidic aqueous solutions the reaction between hexaaquachromium(II1) and sulfate has been studied in detail; ion-pairing can be detected in this system.818Polymeric species involving p-hydroxy and p-sulfato groups have been i s ~ l a t e d . ~ ~ ~ , ~ ~ Trifluoromethylsulfateforms weak complexes with chromium(III);”l this anion may provide a viable alternative to perchlorate as a ‘non-coordinating anion’ (for Cr(O3SCF& see Section 35.7.2). Tris( 0,U’-sulfinate) complexes of chromium(II1) have been prepared (RSOZ-, R = Me, C6H5,p-CH,CJd,); typical monomeric complexes are obtained.8z2 (iii) Phosphorus-containing ( a ) Complexes of H3P03 and H P 0 2 . Ebert prepared a complex which he formulated as H3[Cr(HP03)3]3- (trisphosphitochromic acid) by refluxing chromium(II1) hydroxide with H3P03.8UThe complex was said to be tris-chelated (197)and a resolution as the strychninium salt r e p ~ r t e d . ~ * ~It. may ~ ” provide one of the very few examples of molecular optical activity in Good a non-carbon-containing molecule. Its 1R spectrum has been studied in analyses, particulariy for phosphorus, have not been ~ b t a i n e d ; until ~ ~ ~such ? ~ ~data ~ become available there must be some doubt as to the exact composition of these species. A number of other corn lexes involving this ligand, including a series of bisphosphites, have been reported. 828327
868
Chromium
A mono hosphite complex has been prepared by the reaction of either chromium(II1) perchloratS8 or chromateag with the ligand. The related monohypophosphite complex [Cr(H20)4H2P02]2+has also been ( b ) Phosphates. Although chromium phosphates have been known for there are few that are well characterized. A crystalline hexahydrate is the best-characterized species and exists in at least three forms.s32 Simple complexes of phosphate and chromium(II1) may be important in the absorption of chromium(II1) by biological system^.^^^^^^ Phosphate derivatives have been studied and simple complexes have been prepared from chromium(II1 in combination with: tri-p-t0lylphosphate,8~~ alkoxyalkylphosphate,m6isopropylphosphate~”)trieti~ylphosphate~~~ and dimethylmethylphosphonate (for Cr(02PF2), see Section 35.7.2) .R3y Chromium(II1) forms stable complexes with adenosine-5‘-triphosphate These are kinetically inert analogues of magnesium ATP complexes and may be used to study enzyme systems. The complexes prepared are chiral and may be distinguished in terms of chirality at The related complex of chromium(II1) with adenosine-5‘-(1the metal centre (198,199) thiodiphosphate) has been prepared; the diastereoisomers were separated.@’ The stereospecific synthesis of chromium(II1) complexes of thiophosphates has been reported845by the method outfined in equation (471, enabling the configuration of the thiophosphoryl centre to be determined. The availability of optically ure substrates will enable the stereospecificity of various enzyme systems to be investigated. E45 .w78417842
0
(198) A-Cr(H,O),ATP
0 I!
(199) A-Cr(H,O),ATP
( c ) Phosphinates. Typical chromium(II1) phosphinates are polymeric materials soluble in non-polar solvents (203). Poly{di-~-(diphenylphosphinato)}aquahydroxychromium(III) may be prepared by the aerial oxidation of the chromium(I1) precursornW This method leads to a molecular weight of 6000 (number average). Polymers of identical composition may also be prepared from chromium(II1) in 1:1 water/THF m i x t ~ r e s . ~ ~Polymers ’ , ~ ~ of the general formula [Cr(H2O)OH(0PRR’O),]n (R = R‘ = C6H5, R = Me, R’ = Ph and R = R‘ = C8H17) were also prepared; on heating to constant weight in vacuo these materials are converted to the corresponding hydroxooxo polymers.
869
Chromium
Many polymeric chromium(II1) complexes in this general class exist. For the general formula [Cr(L)(OPRR'0)2],, compounds have been re ared involving hydroxide, perfluorocarboxylate, alkoxide, aryloxide and c a r b o ~ y l a t e s50 ,~~~ ( d ) Miscellaneous. A perchlorate complex of chromium(II1) has been reported; IR evidence was used to suggest bidentate coordination of the anion.=' Chromium(II1) borates have been prepared by solid-state methods;852Cr203was reacted with molten B203 at 1100"C. The solid isolated had the calcite structure. A monomeric complex [Cr(02SeMe)3 has been prepared; the seleninate was a stronger ligand than DMSO but weaker than urea.85
B P
3
35.44.10 Carbon-containing oxo anions ( i ) Carboxylates
Simple carboxylates of chromium(II1) find industrial application as catalysts for the polymerization of w - a l k e n e ~ and ~~~ in , the ~ ~ preparation ~ of chrome-tanning solutions .s56,857 There seems to be no simple carbonate of chromium(II1). Compounds formed on the surface of Cr203,sometimes formulated Cr2(C03)3-nH20,are best viewed as carbon dioxide adsorbed on the oxide.@* Carboxylate complexes of chromium(II1) will now be considered in terms of the various ligand types.
(ii) Acetates and simple carboxylates Early investigations8" showed the product of the reaction between acetic anhydride and chromium(II1) chloride to have the composition [Cr3(MeC02)6(OH)2(H20)]+.OrgelsM]suggested a structure of the kind illustrated below (204) for the trimer and crystallographf61 has confirmed this. The three carboxylate-bridged chrornium(II1) atoms form an equilateral triangle around a shared oxygen atom. The vertex of each chromium(II1) octahedron trans to the central oxygen atoms is occupied (in the trihydrate) by a water molecule. This is the only ligand in this complex which will readily undergo substitution.
R
I
(204)
The physical properties of a large number of trimeric acetates of this type have now been investigated. Magnetic coupling between the chromium centres within the trimer OCCUTS, e.g. in [Cr30(MeC02)6(H20)3]C1.1-E20, .To = 10.5cm-I and J1= 2.5 cm-1.862How such results are best interpreted is controversial. Brown and co-workers suggested that inter-trimer coupling is i m p ~ r t a n t . ~In ~ ~contrast, ,'~ Tsukerblat et al. find the majority of results865to be interpretable in terms of models which do not invoke inter-trimer interactions. The gross features of the electronic spectrum of trinuclear chromium(II1) acetates may be explained866 in terms of tetragonally distorted chromium(II1) centres; fine structure in the 650-750 nm region is the result of exchange coupling. The electronic spectra of a series of acetates [Cr@(MeC02)6R3]+ (R = H20, NH3, py, DMSO) have also been successfully explained in terms of ligand field ~ ~ 7 ~valence870 ~~. Related complexes of chloroacetic acids have been s t ~ d i e d . ~Mixed and mixed metalf11,5macetates of this type are also known (see also Sections 35.4.2.S.i and 35.7.2). Other simple derivatives of acetate RC02H; R = Me, Et, Pr, CH2Cl, CHCI,, CC13) give trimeric complexes of similar structure.87 Simple monomeric acetate complexes have been reported to form in solution from the
1
870
Chromium
reaction of and ammine874complexes of chromium(II1) with acetate, Chloro carboxylates CrCl2(O2CR)+THFand CrC1(02CR)2 (R = C17H15-&1&3) have been prepared in THF. These are dimeric and trimeric respectively, in benzene.s75 Basic trimeric formate complexes of chromium(II1) are formed when anhydrous chromium(II1) chloride is reacted with HC02H.s76 (iii) Oxalates Oxalate complexes of chromium(II1) were first characterized at the turn of the century by Rosenheim and C ~ h e n . ~The ~ ' most extensively studied are the tris species and the cis- and trans-bisoxalates (205-207): these formulations were first suggested by Werner.878All may be made by the reduction of chromate with oxalate. Reliable preparations have been reported for tris by Kauffman and F a o r ~ *and ~ ~ for cis- and trans-diaquabisoxalatochromate(II1) by Bailar 1 3 -
1-
In aqueous solution, the equilibrium between the cis- and tram-diaqua complexes lies almost completely toward the cis isomef"' (K = 0.17, pH 3-4). The sparingly soluble potassium salt of the trans isomer ma however, be prepared by the slow evaporation of a saturated solution at room temperature, RYd,8soand the cis isomer by cooling a hot solution or by allowing potassium dichromate and oxalic acid to react in the presence of a minimal quantity of ~ a t e r . The ~ ~ ~ * tris complex was resolved by Werner in 1912,s83 providing the first example of an anionic optically active coordination complex.
(iv) Tris(oxalato)chroma te (211) Structural and other studies of the salts of this ion are numerous. The crystal structure of potassium tris(oxalato)chromate(III) has recently been rein~estigated."~EPR studies of [ C ~ ( O X ) ~do] ~ed - in K3[Al(~~)3].3H20 had revealed four magnetically distinguishable sites for substitution,""a result in conflict with the reported crystal structure886of K3[Cr(ox)3]+3H20 is isomorphous. The earlier crystal structuress6 was found to be with which K3[A1(~~)3]-3Hz0 unsatisfactory. Crystals of K3[Cr(ox)3]-3H20 are monoclinic (a = 7.714, b = 19.687, c = 10.316A and p = 108.06"). The structure consists of discrete [M(ox)~]~-ions, K+ ions and water molecules. One potassium and one water are cooperatively disordered over two possible pair sites (termed cy and /3) with an occupancy ratio of 3 :1. The result is in excellent agreement with the EPR study.'*' The structure of (NH4)3[Cr(~~)3].2H20 has also been reported.887 The electronic s ectrum of [Cr(ox),]'- has been studied extensively both in solution and in the solid state.8s8,89 The assignment of the spin-allowed transitions from solution electronic spectroscopy is a popular undergraduate exercise.s90 The low energy spin-forbidden 4A2-+ ' E transition is readily observed for this complex.8s9~s90 Table 81 summarizes the most significant features of the electronic spectrum. Werner resolved the tris oxalate using strychninium ion in 1912.#l Other effective resolving agents for the tris complex include: ( 4- )-tris(l,10-phenanthroline)nickel(II),892 ( + )- or ( - )-tris(ethylenediamine)cobalt(III)s93 and ( - )-tris(ethylenediamine)rhodium(III).896 The ( - ) - [ ~ O ( O X ) ~ion ] ~has - been studied b single-crystal methodssw and has the D configuration. It has been related to ( + )-[Cr(ox),l'by X-ray powder ph0tography,8~' so its absolute configuration is also established as D by the more exact form of the rule of least-soluble dias ter eois~mers.~~~
r
Chromium
87 1
Table 81 The Electronic Spectrum of Potassium Tris(oxalato)chromate(III) (after ref. 889) E,,,
A (nm)
(dm3mol-' cm-')
711 697 676 672
663
0.13 2.65 0.31 0.23 0.41
570 488 481 420
75.8 0.13 0.09 96.4
Assignment '4'
Assignment
O-O 0-0
++ 543)267 810
O-O 310)95 0-0-k 405 0-0-t (2 X 130) or 0-0 + 405 + 225 Spin-allowed +
1
215
2A2+4T2
0-0 + 543 Spin-allowed
4A2+ 4T1
The 'Pfeiffer effect' is a term used to describe changes in the optical activity of solutions containing a chiral compound (the 'environmental substance') on the addition of a racemic dissymmetric complex. The effect is generally attributed to a shift in the position of the equilibrium between D and L isomers for the racemic complex. The exact mechanism involved in mediating the chiral interaction is unknown. Perhaps surprisingly, both 'environmental substance' and complex may simultaneously be cations. Studies of the 'Pfeiffer effect' usually involve a moderately labile racemic complex; [Cr(ox)$- is a popular choice for such studies, summarized in Table 82. Other studies of the optical activity of tris oxalates include work on photoinduced o tical photoracemizationgW and the solid-state racemization of K,[C~(OX),].~,
2
Table 82 Pfeiffer Effect Studies on [Cr(ox),] Environmental compound (+)-Cinchmine (+)-[cO(Phe"3l3' (+)-Cinchonine Chiral "-hydroxy esters
Solvent Water Water/dioxane Water/THF Water/DMSO Water/ester
% Resolution
Re$
2-6 6.0
3
-
4
70 (max)
1,2
4 5
1. S. Kirschner, Rec. Chem. Prog., 1971,32, 29. 2. K. T. Kan and D. G. Brewer, Can. J . Chem., 1971,49,2161. 3. K. Miyoshi, K. Wada and H. Yoneda, Inorg. Chem., 1978, 17,751. 4. K. Miyoshi, Y. Kuroda, J . Takeda, H. Yoneda and H. Takagi, Inorg. Chem., 1979,18,1425. 5. A. F. Drake, J. R. Levey, S. F. Mason and T. Prosperi, Inorg. Chim. Acta, 1982,SI, 151.
The thermal decomposition of K3[Cr(ox)3].3H20proceeds, after dehydration, via the loss of CO and C02 to C r ~ 0 3When . ~ ~ [Cr(HzO)6][Cr(ox)3] was heated at 303K in the solid state, along with decomposition, some exchange of the chromium(II1) centres could be detected by 'lCr labelling.y03
(v) Bis oxalate complexes Crystallographic studies of the bis oxalates of chromium(II1) are not abundant. However, the structure of both transm and cis905 isomers has been confirmed crystallographically. Potassium trans-bis(oxalato)diaquachromate(III) is monoclinic (space group P 2 / c ) ; the oxalates are has strictly coplanar. The crystal structure of the complex salt [Cr(en)2(ox)][Cr(en)(ox)z] been this red salt is obtained as an intermediate in the preparation of salts of mixed ethylenediamineloxalate chrornium(II1) complexes. The structure consists of discrete complex ions linked by H bonding to water molecules and neighbouring ions. The bis oxalato complexes are readily assigned to the cis or trans geometry on the basis of their electronic spectr~m.~"~~~"' The EPR spectra of a large series of trans isomers have been studied (axial ligands studied included H20, OH, py and NH3).W8The spectra are quite
Chromium
872
insensitive to the nature of the axial ligand, but ESR spectra are useful in distinguishing cis and rram isomers. Potassium cis-bis(oxalato)diaquachromate(III) has been resolved using strychnine.909The rate of cis/trans isomerization paralleled that of racemization, suggesting a single mechanism. Many reports cuncern the kinetics of isomerization and/or aquation of bis oxalato complexes. A number of the more recent and/or significant papers are collated in Table 83. Table 83 Studies of Bis Oxalato Complexes
Ligand
H*O H20
bo
SCN-
N;
CH,CO; NH3
DMF
DMSO
c0;-
en l,lO-phen/2,2’-bipy
Nature of work
Ref.
Reaction with oxalate, cis/rrans synthesis and kioerics of anation with oxalate, nitrate catalysis Aquation and isomerization of cis and trans complexes, isomerization. (Also for malonate). Fourteen references to studies of the isomerization. Solid state isomerization Monothiocyanato, monoaqua, synthesis and kinetic study Mono and bis aquation kinetics Synthesis, cis /tram, hydrolysis kinetics, isomerization kinetics Synthesis Cis, synthesis and a study of aquation kinetics Cis, preparation and aquation kinetics Monocarbonato, cis and aquation kinetics Synthesis Photoresolution/preparation Preparation/resolution Racemization kinetics Preparation and acid hydrolysis
1
2 3 4
5 6 7 8
9 10 11 12 13 14
15
1. T. W. KaUen and E. J. Senko, Inorg. Chem., 1983,22,2924. 2. P. L. Kendall, G. A. Lawrance and D. R. Stranks, Inorg. Chem., 1978,17,1166. 3. M. Malati, M. McEvoy and M. W. Raphael, Inorg. Chim. Acta, 1976,19, L5. 4. K. R. Ashley and S. Kdprathipanja,Inorg. Chem., 1972,11, 444. 5. K. R. Ashley and R. S. Lamba, Inorg. Chem., 1974, W, 2117. 6. M. Casula, G. Illuminati and G. Ortaggi, Inorg. Chern., 1972, 11, 1062. 7. E. Kyuno, M. Kamada and N. Tanka, Bull. Chem. SOC.Jpn., 1967,40, 1848. 8, K. R. Ashley, D. K. Pal and V. D. Ghanekar, Inorg. Chem., 1979, l8, 1517. 9. K . R. Ashley and R. E. Hamm, Inorg. Chem.,1966,5,1645. 10. D. A, Palmer, T. P. Dasgupta and H. Kelm, Inorg. Chem., 1978, 17, 1173. 11. A. Mead, Tram. Furaday Soc., 1934,30, 1052. 12. K. Miyoshi, Y.Matsumoto and H. Yoncda, Inorg. Chem., 1982,21,790. 13. J. A . Broomhead, A u t . J . Chem., 1962,15,228. 14. J . A. Broomhead, N. Kane-Maguireand 1. Lauder, Inorg. Chem., 1970,5, 1243. 15. T. W. Kallen and R. E. Hamm, Inorg. Chem., 1977,16, 1147.
Brid ed hydroxy complexes (208 of the bisoxalate type have been known for many years.’ Both a trihydrate910,911~912 and an anhydrous913form are known. The trihydrate is isorn~rphous’~~ with the corresponding cobalt(II1) complex (Durrant’s salt) and the hydrous complex913is slowly converted to the hydrate on exposure to moisture.912Only a preliminary crystallographic characterization has a t ~ e a r e dit; ~crystallizes ~~ as a meso rather than racemic complex. IR9” and magnetic studies are in accord with the hydroxy-bridged structure. Despite the crystallogra hic meso structure, the resolution of the complexes has been reported either photochemically’ or with cis-( - )bis(ethylenediamine)dinitrocobalt(III)’16 and leads to optically active complexes with spectra similar to those of the corresponding tris oxalate.
K
*3’11
P’
Chromium
873
(vi) Malonates Malonic acid CH2(C02€Q2(H2mal) (209) has a coordination chemistry with chromium(II1) closely resembling that of oxalate. Malonic acid is a slightly weaker acid than oxalic acid and slightly more labile complexes are formed. The tris complex is the most extensively studied, prepared by the reduction of chromate solutions or the reaction of chromium(II1) hydroxide with m a l ~ n a t e . ~ ~The ' ' ~cis ' ~ and ~ ~ trans ~ ~ diaqua complexes may be prepared by the reduction of chromate with malonate; the isomers are separated by fractional crystallization. The electronic spectrum of the tris complex is similar to that of the tris oxalate and a detailed analysis of these spectra has appeared.g89
The resolution of [Cr(mal)3]3- was achieved with L-[CO{( - )-1, Z d i a m i n o p r ~ p a n e } ~ ] B r ~ ; ~ the only diastereoisomer to precipitate contained ~-[Cr(mal),]". A crystal structure of the diastereoisomer was reported, permitting the correction of earlier work which had suggested the ( + )sg9-[Cr(mal)3]3- ion to have the configuration.^',^^^ Polarized crystal spectra of [Cr(mal)3I3- doped into [NH4]3[Fe(mal)3]are extremely similar to those of [ C r ( o ~ ) ~ under ]~similar conditions.923 Solution studies of malonate complexes include the unusual observation that [(H20)&r(Hmal)l2+ has considerable kinetic stability.924The mechanism of ring closure has been the subject of detailed The methylenic protons of [Cr(mal)3]3- under o bromination when a suspension of the potassium salt is treated with bromine in ether (2 h). $26 Legg and c o - w o r k e r ~have ~ ~ ~recently used a number of malonate complexes to demonstrate that the 2H NMR spectra are both readily observed and useful for solving structural problems. The resolution that may be obtained by this method may be appreciated by the fact that in the complex [Cr(mal-d2)2(bipy)]2- the geminal protons of the malonate are clearly resolved.
(vii) Squarates Complexes of chromium(II1) with the squarate ligand (210) have been prepared."' Subsequently the complex Cr(C404)(OH).3H20 was investigated crystallographically and shown to contain pairs of chromium atoms joined by two y-hydroxy bridges and two bridging C40$- ions.929In the same study, a compound of formulation Cr(C404)3/2-7H20 was isolated, a salt of the hexaaquachromium(II1) ion.
"x0 -
.*'
1-
, -
'1
0
0 (210)
35.4411 Hydroxy acids
(i) Tartrates and aliphatic hydroxy acids
+
While undertaking a study of alkaline solutions of chromium(II1) and ( )-tartrate in 1896, Cottong3' first observed the anomalous rotatory dispersion of light in the region of an absorption band-'The Cotton Effect'. The solutions used probably contained a tris tartrato species. Mason concluded that complexation in such solutions was stereospecific;931 the complex formed in the presence of an excess of doubly deprotonated tartrate is probably fac-~-[Cr(R,R-tart)~]~-.~~~ This diastereoisomer may be markedly stabilized by hydrogen bonding when the ligand is doubly deprotonated, a suggestion supported by the fact that the addition of a single proton to the complex greatly decreases the intensity of the CD spectrum (211,212). AlkaIine solutions of chromium(II1) in the presence of racemic tartrate become
Chromium
874
optically active when irradiated with circularly polarized light.933 Photo inversions at the chromium(II1) centre are postulated to account for this observation. In the dark, the optical activity slowly decreases to zero.
(211) A - ~ U C
(212) A-fuc
Mixed dinuclear complexes of tartrates and 2,2’-bipyridyl or 1,lo-phenanthroline and chromium(II1) have been p r e ~ a r e d ; ~ ~the ~ . ’typical ~ ~ structure is illustrated below (2l3), Stereochemical correlations have been carried out by oxidatively cleaving the tartrate bridge (214,215).936 The crystal structure of sodium hydrogen bis(y-meso-tartrato)bis(2,2’bipyridyl)dichromate(III) heptahydrate has been reported.937
n
N \
N I
1’-
(215) B-AA(rneso-rneso)
An isolated report of the s nthesis and characterization of a number of citrate complexes of chromium(II1) has appeared. a3fJ
(ii) Aromatic hydroxy acids Complexes of salicylate with chromium(II1) have not been reported but the tris complexes of salicylaldehyde and chromium(II1) may be prepared by refluxing [Cr(THF),Cb] with salicylaldehyde and sodium acetate in The acid hydrolysis of this complex was studied in detail, but the isomerism obviously possible for this complex was not apparently considered. Khan and Tyagi9‘“‘studied the formation of phthalate complexes of chromium(II1).
35.4.4.12 Sulfoxides, N-oxides and f-oxides
(i) DMSO The complex [Cr(DMSO),][ClO4], was first prepared by Cotton in 1959941as green needles by reacting concentrated Cr(C104)3 solutions with DMSO. Subsequently Drago established the spectrochemical series for chromium(II1) to be H20> DMSO > TMSO > C6H5N0.942 A
Chromium
875
detailed IR study of the hexakis DMSO ion as its perchlorate, and the corresponding d6-DMS0 complex, suggested that the point group of the cation was D3d or &.943 The preparation of [Cr(DMS0)6]3f from anhydrous CrBr3 has been reported.944 The solvolysis of hexaaquachromium(II1) in DMSO proceeds via the series of complexes [Cr(DMSO)n(H20)6-n]3+ (n = 1 to 6) .945,946 The anation and solvolysis of several chromium(II1) complexes in DMSO have been in general the reactions proceed by Id mechanisms. A direct electrochemical synthesis of [Cr(DMS0)6][BF4]3 has been reported.949 A comprehensive review of metal ion complexation by DMSO contains an extensive section on c h r ~ m i u m ( I I I ) . ~ ~ ~
(ii) DMF The complex [Cr(DMF),][C1O4I3 has been prepared from the reaction of chromium(II1) perchlorate with DMF 190 "C, 10 h).951 Solvent exchange and substitution proceed by I, mechanism^.^^^,^" The complex [Cr(DMF)6]Br3-DMF was prepared by the reaction of chromium metal with bromine in DMF;953the electronic spectrum of this complex was very similar to that of the perchlorate salt.
(iii) N-Oxides An extensive, if somewhat dated, review of the coordination chemistry of aromatic N-oxides is available.954Most studies of the N-oxide complexes of chromium(II1) involve the preparation and characterization (usually by magnetic and spectroscopic methods) of monomeric or hydroxy-bridged complexes and are summarized in Table 84. Table 84 N-Oxide Complexes Ligand
Complex
Re$
Trimethylamine N-oxide Pyridine N-oxide 4-Substituted pyridine N-oxide 2,6-Lutidine N-oxide
tCr(L~,l[C~O4l3 Icr(L)61[c'0413 [cr(L)61[c104b [Cr(L)611BF413
1 2
Picolinic acid N-oxide Pyridine carboxamide N-oxide 1,lo-Phenanthroline N-oxide 2,2'-Bipyridyl N-oxide Quinoxaline N-oxide 3-Methylisoquinoline N-oxide Quinoxaline 1+dioxide
[Cr(LMLH)(OH,)lCQ4 [Cr(L)3]C13.2Hz0 ICr(U31[CQL ICr(L),l[C~0413 [c~(L)4(0Hz)(0c~03)1 [C1041z.%o [Cr(L),(OCIO,)(OHz)I(C1041* [C&I~L(OH~~'HZO Polymeric CI(C1O4),~-HZ0 [Cr(C104)3]zL.12Hz0 PL3(~*0)1 [Cr(LH),(EtOH),( OC10,)2]C10, [Cr(LH),(OC103)~(EtOH)z]C104 Icr(L~3I[clo413
6
Phenazine 5,lO-dioxide
3,4,5-Trimethyl-l-hydroxypyrazo~e 2-oxide Purine N(l)-oxide Adenine N(1)-oxide N,N,N', N'-Tetramethylethylenediamine N,N'-dioxide
3
4-5
[Cr(L),Cl,(OH2),]Cl~4Hz0 7 8 8
9
10 11,12 13,14 15 16 17 18
1. R. S. Drago, J. T. Donogue and D. W. Hcrloker, Inorg. Chem., 1965,4,836. 2. C. Valdemoro, C.R. Hebd. Seances Acud. Sci., 1961,253,277. 3. L. C.Nathan and R.0. RaMale, Inorg. Chim. Actu, 1974,10, 177. 4. C.M.Mikulski, L. S. Gelfand, E. S. C. Schwartz, L. t.Pytlewski and N. M. Karayannis, h o g . Chim. Acta, 1980, 39, 143. 5. C. M. Mikulski, L. S. Gelfmd, L. L. Pytlewski, N. S. Skryantz and N. M. Karayannis, Inorg. Chim. Actu, 1977,21,9. 6. F.J. Iaconianni, L. S . Gelfand, L.L.Pytlewski, C. M. Mikulski, A. N. Speca and N. M. Karayannis, Inorg. Clrim. Acta, 1579,36, 97. 7. A. E.Landers and D. J. Phillips, Inorg. Chim. Acta, 1982,59,125. 8. A. N. Spcca, N. M. Karayannis and L. L. Pytlewski, Inorg. Chim. Acta, 1974, 9, 87. 9. D.E.Chasan, L. L. Pytlewski, C. Owens and N. M. Karayannis, J. Imrg. Nucl. Chem., 1977,39,585. 10. A. N. Speca, L. S. Gelfand, F. J. Iaconianni, L. L. F'ytlewski, C. M. Mikulski and N. M. Karayannis, Inorg. Chim. Acla, 1979,33, 195. 11. D.E. Chasan, L. L. Pytlewski, C. Owens and N. M. Karayannis, Inorg. Chim. Acta, 1977,24,219. 12. D.E. Chasan, L. L. Pytlewski, C. Owens and N. M. Karayannis, J. Inorg. N u d Chem., 1978,40, 1019. 13. D.E. Chasan, L. L. Pytlewski, C. Owens and N. M. Karayannis, Trumition Met. Chem. (Weinhim, Ger.), 1!976,1,269. 14. D.E. Chasan, L. L. Fytlewski, C. Owens and N. M. Karayannis, J. Inorg. Nucl. Chem., 1979,41,13. 15. D.X. West and M. A. Vanek, J. Inorg. Nucl. Chem., 1978,40, 1027. 16. C. M. Mikulski, R. DePlince, T. BaTran, F. J. Iaconianni and N. M.Karayannis, Inorg. Chim. Actu, 1981,56,27. 17. C . M. Mikulski, R. DePrince, T.BaTran, F. J. Iaconianni, L. L. Pytlewski, A. W. Speca and N. M. Karayannis, Znorg. Chirn. Acra, 1981,56,163. 18. M. J. Bigley, K. J. Radigan and L. C. Nathan, Inorg. Chim. Acta, 1976,16,209.
Chromium
876
Interesting complexes have been prepared from the chiral ligands derived from 3?3'dimethyl-2,2'-bipyridyl N,N'-dioxide (216,217).955,956 A number of diastereomeric tris complexes were isolated. These could be interconverted photochemically.
(iv) P-Oxides The complex of tri-n-butylphosphine oxide (tbo), [Cr(tb0)~(ClO~)~]Ci0~, has been ~repared.'~'Crystalline samples were obtained, and, on the basis of IR and electronic spectra, monodentate coordination of the perchlorate was suggested. The geometric isomerism possible for this complex was not considered. The hexakis complex of tris(hydroxymethy1)phosphine oxide and chromium(III), [Cr(L)6][C104]3has been prepared;"' it was concluded that bonding in these complexes was weaker than in the corresponding complexes of trimethylphosphine oxide.959Tris(dimethy1amino)phosphine oxide forms complexes CrL3C13, for which a facial arrangement of the ligands has been s~ggested.~"The reaction of Cr(CO)6 with diisopropylmethylphosphonate results in a series of polymeric complexes.961A complex of tri henylphosphine oxide has been said to contain a trigonally bipyramidal chromium(II1) ion.9 8
35.4.5
Sulfur Ligands
354.5.1 Thiolate, disulJid0 (utd tlrioether ligands
The chemistry of chromium(II1) complexes containing sulfur donor ligands, although until recently unexplored, is now an area of considerable research activity. Mechanisms for reactions of these complexes are described in a recent review."
(i) [ C r ( W ( ~ 2 0 ) 5 l 2 + The tendency of chromium to capture whatever ligands are in its first coordination sphere on being oxidized from the +11 to the +I11 state963has been exploited in the preparation of [Cr(SH)(H20)5]2+in aqueous solution (equation 48) .964,965 Hence oxidation of [ C T ( H ~ O ) ~by ]~+ PbS, Ag2S or S 2 0 $ - under N2 gives the desired product, although in low yield. However the use of polysulfide as oxidant increases the yield of green-coloured product to 10-20%. The scope of this reaction can be extended to include the synthesis of a range of alkyl-, aryl- and chelating-thiolatochromium(II1) complexes by choice of an appropriate disulfide oxidant. However, complexes containing simple alkylthiolato ligands cannot be synthesized by this method due to the inability of chromium(I1) to reduce the corresponding disulfides.% [Cr(H20)6]2+ + various S-containing oxidants -----, [Cr(SH)(H,O),]"
(43)
Some reactions of [Cr(SH)(H20)5]2+ are outlined in Scheme 96.-%' The ion exchange behaviour of this complex is characteristic of a 2+ species, thus confirming the presence of HSrather than H2S as the sulfur ligand. No spectroscopic evidence for protonation of this ligand was observed even at pH=O. This, however, is not too surprising, since by analogy with [Cr(H20)6I3+,which is more acidic by a factor of 5 X 10" than free H20, the p K , of H2S in the complex [Cr(H20)5H2S]3fshould be considerably less than 0. However, kinetic evidence has been obtained for the existence of this species. While the complex [Cr(SH)(H20)5]2+is remarkably stable under anaerobic conditions in aqueous solution at room temperature,964it undergoes aquation by both acid-independent and acid-dependent pathways, the latter attributed to a rapid protonation equilibrium preceding the rate-determining Aquation
877
Chromium
rate constants of 3.1 X loL5s-l and >>3.8 X 10-5s-’ have been estimated for [Cr(SH)(H20)5]2+ and its conjugate acid respectively, both at 313 K, I = 1.0. [Cr(SH)(H,O),JSO,
[Cr(NCS)(SH)(H,0)4]’
H+
12
[(H,O),CrS2Cr(H2O)5l4+
+-
H’
e [Cr(NCS)(H,O),H,S]”
[Cr(SH)(H,0),J2’
F=
[Cr(H,O),H,Sl3’
The UV-visible spectrum of [Cr(SH)(H20)5]2+in aqueous solution (Table 85) is typical of thiolatochromium(II1) complexes. The bands at 575 and 435nm are identified with the 4A2g+ ‘2& and 4A2g+ ‘Tb transitions respectively, while the very intense band at 258 nm is due to an S ligand to metal charge transfer transition.% The aquation of [Cr(SH)(H20)5]2+is characterized by the disappearance of the charge transfer band from the spectrum. Addition of H2S04 to a concentrated solution of [Cr(SH)(H20)5]2+ leads to the isolation of the brown-green solid [Cr(SH)(H20)5]S04.965 The IR spectrum of this complex has bands at 2560 (vSH) and 340cm-’ (vMS), while the Raman spectra of aqueous solutions show a band at 337 cm-l (vMS).
(ii) [ C ~ ( S A I - ) ( H ~ O(Ar ) ~ ]=~anilinium) + The aromatic thiolatochromium(II1) complexes [Cr(SAr)(H20)5]3+ (Ar = 4-PhNH:, 4PhNMe:) have been prepared in solution under N2 (equation 49) and purified by cation exchange c h r ~ m a t o g r a p h y .Initial ~ ~ ~ elution with 1M LiC104 solution separated species of charge +3 from more highly charged polymeric products, while subsequent elution with 2M LiC104 separated the green thiolato complexes from the blue and slightly faster moving [ c ~ ( H ~ o ) ~ ] Despite ~+. repeated chromatography, however, the complex ICr(4SPhNH3+)(H20)5]3fcould not be obtained free from the hexaaqua species, in any greater than about 95% purity. The solution spectrum of this complex (Table 85) shows an internal ligand band at 230 nm, a very intense charge-transfer band at 295 nm which masks the 4A2g+4Tk d-d band and an unsymmetrical band at 595 nm with a shoulder at 640 nm. The shape of this band is due to splitting of the ‘Ggexcited state and the absorptions at 595 and 640nm may be attributed to the 4B1(4A2g)+4B2,4E,(4T2g) transitions respectively. [Cr(H,O),]*+
+ ArSSAr
HClOq, Nz
[Cr(SAr)(H,0),I3+
(49)
As in the case of [Cr(SH)(H20)5]2+, the complexes [Cr(SAr)(H20)5]3+undergo anaerobic aquation with loss of the S ligands by both acid-independent and acid-dependent pathways, the kinetically active species in the latter resulting from protonation on the S atom. Aerobic aquation of the thiolatoanilinium complex gives [Cr(H20)6]3+and p-aminophenyl disulfide as products. This reaction is autocatalytic (p-aminothiophenol acting as catalyst) and is also catalyzed by H202 and Fe3+, all seemingly by a mechanism which involves oxidation of the thiolate ligand to a labile coordinated free radical.
(iii) [ C I ( S R ) ( H ~ O ) ~(JR~= + aliphatic) Because of the thermodynamic inability of chromiurn(I1) to reduce simple alkyl disulfides, the synthetic method used for the previous complexes is unsuccessful for chromium(lI1) complexes containing simple alkylthiolato ligands. Hence, whereas complexes containing chelating thiolate and 4--SPhNH: ligands are obtained rapidly by chromium(I1) reduction of the corresponding disulfides, the complex containing -SCH2NMe3f is produced only slowly and those containing the ligands MeS-, EtS- and -S(CH2)2NH: are unobtainable by this I
878
Chromium
879
method.966 However some of these complexes, e.g. [Cr(SCH2CH2NH3)(H20)5]3+, can be prepared by chromium(11) reduction of the alkylthiolatocobalt(II1)complexes, a reaction which occurs by an inner sphere mechanism and which results in ligand transfer (Scheme 97).% The product in solution can be partly purified by cation exchange chromatography but cannot be obtained free from [Cr(H2o)6I3+,which is present to an extent of 5% in the final effluent. The solution electronic spectrum of this complex (Table 85) contrasts with that of the thiolatoanilinium complex in that the charge transfer band is at a shorter wavelength (274nm) and does not obscure the 4A28-, '7ig band, which is clearly observed at 438 nm. Using this and a series of related complexes [CrX(H20)5]2+(X = F, N3, OAc, CN) evidence is presented that in the case of ligands derived from weak acids (HX) the acid-independent term in the aquation rate law, like the acid-dependent one, depends on the ability of the coordinated ligand to be protonated and corresponds to the removal of HX from [Cr(OH)HX(H20)4]2frather than the removal of X from [CrX(H20)5]2+.70,966 Potential ligands have been found to enhance the rate of Cr-S bond cleavage in [Cr(SR)(H20)5]2+and [Cr(SAr)(H20)5]2+complexes and to be incorporated into the coordination sphere of the metal at great1 enhanced rates, indicative of a high trans effect associated with thiolate ligands (equation 50).20
[CT(SR)(H~O)~]~+ + L"- + H'
-
[CrL(H20)s]'3-"'++ HSR
(50)
r
-
H30f
[Cr(SCHzCHzNH,)(H,0),]3+ + [Co(H,O)$'
+2enw
Scheme 97
(iv) Complexes containing S, N and S, 0 chelating ligands Complexes containing these ligands are also dealt with in Sections 35.4.2.3 and 35.4.8. The thiolatoamine complexes [Cr(SCH2C02)(en)z]C104 and [Cr(SCH,CH,NH,)(en),](C104)2 have been prepared by chromium(I1) reduction of the disulfides in the presence of en (Scheme 98).w1 In the preparation of the aminoethanethiol complex a second product , [Cr(SCH2CH2NH2)2en]C104,was also produced. These complexes could be separated by either fractional crystallization of the perchlorate salts from dilute NaC104 solution or by cation exchange chromatography. The proportion of unipositively charged species in the product mixture was found to increase by using excess disulfide in the preparation. The electronic solution spectra of these complexes are presented in Table 85. The aquation reactions of the complexes in dilute HC104 solutions proceed predominantly through H+-catalyzed C r - 4 bond fission, although acid-independent processes involving C r 4 and Cr-N bond breaking are also competitive.971The aquation of [Cr(SCH2C02)(en>,]+in HCI04 proceeds via several steps the first of which gives C~~-[C~(O~CCH~SH)(~~)~H~O]~~. Subsequent aquation steps involving Cr-N and at a later sta e Cr-carboxylate bond fission (analogous with aquation reactions of oxalat~polyammine~~~~' and acetatopentaamminechromium(III)m complexes) result in [Cr(en)(H20)4]3+ the predominant product following extensive hydrolysis.w1 The aquation of [&(SCH2CH2NH2)(en)2]2+ in 4 M HC104 gives [Cr(en)z(NH2CH2CHzSH)H20]3+ as the primary aquation product and is much slower than aquation of [Cr(SCH2C02)(en),]+. Both complexes however are some lo4-fold more reactive than their cobalt(II1) analogues. Since X-ray crystallographic evidence413indicates that (after allowance is made for the different ionic radii of the metal ions) the Cr-S bond is actually stronger than the C o - 4 bond in the [M(SCH2CO2)(en),]+ complexes,970and since the larger crystal field activation energy of cobalt(II1) would account for only a lo1-lo2 decrease in rate it appears that the greater lability of the C r 4 bond may be due to the greater basicity and ease of protonation of sulfur bound to Cr. The metal-ligand bond distances in the [M(SCH2C02)(en)2]+(M = Co, Cr) cations are compared in Table 86. Oxidation of some of the afore-mentioned complexes as well as [Cr{S(CH&C02} (en)2]C104 and [Cr{SC(Me2)C02}(en)2]C104(electronic spectra of these complexes are included in Table
880
Chromium Cr
[Cr(Sligand)(en)l](CIOd),"
[Cr(H,0),I2+
i, excess 20% HC104, anaerobic conditions; ii, 10% deaerated en solution, inert atmosphere; iii, deaerated solution of disulfide, inert atmosphere, so that Cr :en :disulfide = 2: 6 : 1; iv, HCIO, after removal of Cr(OH), by filtration. a
S ligand = -SCH,CO;,
-SCH2CH2NH2,x = 1, 2, respectively.
Scheme 98
Table 86 Bond Distances (pm) in [M(SCHzCOz)(en)z]f(M = Co,Cr) Complexes'
M
M S
M-0
Cr
233.T 224.3'
196.6 191.8
co
M-NIa
M-Pb
M-iV3"
209.6 197.1
209.9 195.8
211.2 200.5
M-N4 207.6 196.5
N'2 =nitrogen trans to oxygen. N =other nitrogen in same en ring as N'. 'N3= nitrogen trunr to sulfur. d = other nitrogen in same en ring as N3. e In the complex Cr(S2COEt)3the 0-4 distances are 238.7 and 239.9 pm.' 'In Co(S,COEt), thc &S distances are 227.6 and 227.7 ~ r n . ~ 1. R. C. Elder, L. R . Rorian, R.E. Lake and A. M. Yacynych, lnorg. Chem.,1973, 12, 2690. 2. S. Merlin0 and F. Sarton, Acta Crystdlogr., Sect. B. 1972, 28, 972. 3. S. Merlino, Acta Crystdlogr., Sect. B, 1969, 25, 2270. a
b
85) by H202has been investigated.412Reaction of [Cr(SCH2C02)(en)$ produces a complex product mixture, the composition of which has proved difficult to elucidate. However, the thiolate ligand is oxidized and it appears that the oxidation state of sulfur changes from -11 to +I1 (RSO;, sulfinate) and possibly +IV (RSOT, sulfonate), with the likelihood of a transient 0 (RSO-, sulfenate) state. The mechanism of oxidation by Hz02 involves nucleophilic attack by coordinated sulfur on the 0-0 bond. The reactions follow second-order kinetics and while reactivity is relatively insensitive to the nature of the thiolato complex, steric crowding at the sulfur atom causes decreased reactivity for the complexes [Cr(SCMezC02)(en)z]+ and [Cr(4-SPhNH3)(en)2]3+. The nucleophilicity of coordinated sulfur in the complex [Cr(SCH2CH2NH2)(en)2]2+ towards Me1 (equation 51) has been measured in DMF/H20 and compared with sulfur nucleophilicities in thiolatocobalt(II1) systems.975As in the case of the H2OZoxidations, the nucleophilicity of thiolate coordinated to chromium(II1) is only slightly less than when it is coordinated to cobalt(III), implying that nucleophilic attack by coordinated sulfur does not involve any appreciable distortion in the first coordination sphere of the metal.
[Cr(SCH,CH,NH,)(en),]2*+ Me1
-
[Cr{S(Me)CHzCHzNH,}(en),13+ + I-
(51)
(v) Cr(SR)3,CrX3(SR2),,CrC13(THT)3( X = CZ, Br; R = Me, Et; TKT = tetrahydrothiophene) The preparation and properties of a number of these thiolato and thioether complexes are presented in Table 87. X-Ray powder photographs of Cr(SMe)3 show that the com ound has The a layer-type lattice with chromium(II1) occupying octahedral Cr(SMe), chemical behaviour of the mixed halogenothioetherchromium(II1) complexes is illustrated using CrC13(SMe2)3as an example in Scheme 99.978It was not possible to isolate solid samples of the dimer [CrC13(SEt2)& or the monomers CrBr3(SRz)3(R=Me, Et). The complexes CrC13(SR2)3(R = Me, Et) have been assigned mer geometric configurations on the basis of their far IR spectra. Hence in the Cr41 stretching region these complexes show three absorption bands, which for the mer isomer (C, symmetry) is expected, since the three C r - C l stretching modes (2al+b1) are IR active. For the fuc isomer, however, only two C r - C l stretching modes (a1 + e of C s ) are a ~t i v e. ~"These criteria have also been applied to distinguish between the isomers of CrCl,(THT), (THT = tetrahydr~thiophene).'~'
Chromium
881
882
Chromium CrCl,(SMe,),
1 [crc13,1,:;:;:t dissolve in C6H6
Scheme 99
(wi) Complexes containing
S%- and
HS, ligands
The oxidation of [Cr(SH)(HzO)5]2+by I2 or Fe3+ under aerobic conditions in acid solutions gives the disulfido-bridged complexes [ H20)5CrSzCr(Hz0)5]4+and [(Hz0)5Cr(S2H)Fe(HzO)5]4+ respectively (Scheme 100).967,966The latter complex can also be obtained by substitution of chromium(II1) in the former complex by iron(I1) under acid conditions. The product distribution in the iron(II1) oxidation of [Cr(SH)(HzO)$+ is pH dependent and at 298 K, pH = 1 the heteronuclear dimer [(H20)5Cr(S2H)Fe(H20)5]4+ constitutes over 80% of the product mixture. The rate of this reaction shows a [H+]-l dependence, an observation consistent with [CrS(H20),]+ being the kinetically active species. [ ( ~ ~ 0 ) ~ ~ r ~ ~ ~ r ( ~ ~ 0 ) ~ 1 ~ + = 399 (2660), 305 (3600), 230 (6450)8 yellow-brown: A,
I\
[Fe(H20)J3+, n+
I(H,o),c~(s~H)F~(H,o)~~~+ yellow-green: A,,
'Amax in nm
(E
in dm3 mol-' cm-').
= 580 (70), 355 (2940), 310 (3600)"
This reaction also gives ~(H,0),CrS,Cr(HzO)s]4' as a minor (pH-dependent) product. Scheme 100
(vi) Binary and ternary sulfides A number of binary compounds in the chromium-sulfur system have been identified and their structures established. These include CrS , Cr2S3,Cr3S4,Cr5S6and Cr5S8.980,9s1 The origin of ferromagnetism in Cr& and Cr& lies in the ordered arrangement of cation vacancies with resulting uncompensated spins.w Since the magnetic and electrical roperties of Cr& indicate that the d electrons of chromium are localized, the formula CrYICr Ss has been suggested for this compound with chromium(II1) occupying the filled cation layers and chromium(1V) occupying the partly filled layers.98' The ternary sulfide NaCrS, which can be repared by fusion of either NazCr04, Na2C03and sulfur983or K2CrO4, NaKC03 and sulfur:' contains octahedral Cr"'S6 groups in which the C r - S distances are 2 4 4 ~ m On . ~ lowering ~ the temperature from 300 to 20K a shoulder at 13350 cm-' on the 4A2g-+'T~2g band (14 600 cm-') in the reflectance spectrum becomes prominent.985Although this shoulder was originally attributed to splitting of the 'TZgstate in a trigonally distorted octahedral field it now appears that it is instead due to a spin-forbidden transition to the 'Eg state.% The sulfides CuCrS2 and AgCrS2 have been prepared by heating the mixed binary chalcogenide^.^^ The crystal structures of these compounds show that the sulfur atoms are arranged in a deformed cubic close pack with chromium occupying octahedral sites and copper or silver in tetrahedral sites. Like NaCrS2, these sulfides are antiferromagnetic and Nee1 temperatures for the three compounds fall in the range 19-40K.987 Ferromagnetic interactions occur in the chromium layers of NaCrS2. The structure of LiCrS2 is of the NiAs
R
Chromium
883
type with lithium and chromium atoms occupying octahedral sites.988In Crus3 the chromium atoms have distorted octahedral coordination, while each uranium is located at the centre of a bicapped trigonal prism of sulfur atom^.^^^.^ The semiconducting thiospinels MCr2S4(M = Mn, FeyCo and Zn), which can be prepared by the high temperature combination of the metals and sulfur, contain MI'S4 and Cr"'S6 However, CuCrzS4 is metallic and appears from crystallographic and magnetic evidence to contain copper(1) and not copper(I1). The chromium atoms occupy octahedral Cr"'S6 sites. The metallic properties of this compound have been attributed to a conduction band arising The structure of CrPS4, from the absence of a fraction of the valence electrons of determined by X-ray crystallography, consists of puckered hexagonally close-packed sulfur layers stacked parallel to (100) with chromium in octahedral and phosphorus in tetrahedral intersticwW2This is the first reported metal thiophosphate having the metal ion in octahedral sulfur coordination.
35.45.2 Bidentate S donor ligands: dithiocarbamates, dithiophosphates, 1,l- and l,2dithiolates, etc.
The chemistry of the complexes formed by the dithio ligands (218)to (225) with transition metals including chromium(II1) has been well described in several review^.^^^^^^^^^^ OR "0 R (219)Dithiophosphate
(218)X = NRz, dithiocarbamate
(220) 1 ,bDithiodiketone
X = OR, xanthate X = SR, thioxanthate X = R, dithiocarboxylate
R s\ / 2- e c . S' \R (222) 1,l-Dithiolate
(221)Dithiophosphinate
(223) X = 0, dithiocarbonate X = S,trithiocarbonate X = NCN, N-cyanodithiocarbimate
-.>c//0
S
'r
2-
'
s5c\o (224) Dithiooxalate
(225) 1,2-Dithiolates (dithiolenes)
In general, the complexes are prepared by mixing aqueous or alcoholic solutions of the sodium, ammonium or potassium salt of the ligand and CrC13*6H20,sometimes with Zn dust p r e ~ e n t . Ho ~ ~wever, , ~ ~ in some cases anh drous conditions are necessary to prevent the formation of polymeric hydrolysis productsYg and dithiophosphate is much more resistant to temperature and extremes of pH than dithiocarbamate or xanthate. In the preparation of C r g , where L is 4-aminophenazonedithio~arbamate~ aqueous C T ( N O ~was ) ~ added to the reaction mixture from the preparation of the ligand.999 Another general method is illustrated by the preparation of [Cr(SzCNRz)3](where R = Me, Et, Pr, Bun, Bui, CH2Bu; Rz = (CH&O, (CH,),, (CH,),, MePh or EtPh; equation 52). In this a chromium(I1) salt is added to the ligand and the mixture allowed to oxidize.lm An alternative is to reduce a precursor, e.g. the 3,5-dirnethyl-l,Zdithiolium cation, with a chromium(I1) salt to the ligand which then coordinates to the Cr"' produced (equation 53).'Oo1
i
COC 3 -cc
884
Chromium
Chromium(II1) complexes of some strong acids, e.g. HSzPF2, have been obtained directly from the metal and the a ~ i d . ~ ~ *The ~ ' complex "~ [Cr(S,PF,),] can also be made by reduction of CrO2ClZ by the acid.'004 A specific method is the insertion of CSz into the Cr-N bond accompanied by reduction of CrIVto Cr'" (equation 54).'lS An account of the redox behaviour of many complexes containing the CrSs core also rovides many references to the preparation of the ligands and their chromium(II1) c~mplexes!'~ [Cr(NEt,).,J + 4CS2
-
[Cr(S,CNEt,),]
+ f(S,CNEt,),
(54)
A new method1006is the oxidative decarbonylation of Cr(CO), with [Hg(S2CNEt2)2]in 2 :3 molar ratio to give the chromium(I1) complex followed by aerial oxid2tion of the reaction mixture (Scheme 101). Although the unoxidized intermediate solution has the electronic spectrum of the chromium(II) complex (Section 35.3.6.1) the reaction pathway is not completely clear.
Scheme 101
The oxidation of Na(NH4)S2CNR2(R = Me, Et; Rz= (CH& with K2Cr207 produces a mixture of [C T(S ~C NR ~)~] and the dithioperoxycarbamato complex [Cr(SzCNR2)z(0S2CNR~)], which has been separated chromatographically. The new dithioperoxy derivatives are in the second fraction and have additional bands in their IR s ectra at 1009 and 488 cm-l, assigned to the VIS-0) and v(Cr-0) vibrations A redetermination of the crystal structure of [Cr(SzCNEt2)2(OS2CNEt2)] taking into account the disorder of the 0, S-chelate has confirmed the gross structure, improved the refinement and provided distances comparable with those expected from other chromium(II1) cornplexes.'O1' The X-ray results make unlikely the assignment of a dinuclear structure [CrzO(SzCNR2)2(R2NCSS2CSNR2)] to the second fraction."" The tris(dithiocarbamales) with Rz = (CH&S and (CH2)flMe have also been prepared by the oxidative method.1°12 Some data,from X-ray investigations are given in Table 88. The CrSs octahedra show small trigonal distortions but these are less than in related MS6 complexes, and the Cr-S bund distances fall within a small range, Perfluoroalkyl derivatives [R&r(S2CNR2)2py] (R = Et, RF= C2F5,C3F7, C&; R = Me, Pr', Bz, RF = C3F7)and [C3F7Cr(S2CNEt2)2(c~clohexylamine)] have been prepared in the same way as [C2F5Cr(salen)py](Section 35.4.8.1).l l3 The complexes are light sensitive and produce the cations [RFCr(Hz0)5]24in water, but are otherwise stable a-bonded chromium(II1) species; no stable alkylchromium(II1) complexes were obtained. The doublet found for the methylene protons in the 'H NMR spectrum of [C3F7Cr(SzCNEt2)2py] suggested that enantiomers with a high barrier to interconversion were present in solution. A cis configuration was found in [C3F7Cr(S2CNMe2)zpy]and the bond trans to the C3F7group is considerably longer than the other C r - S linkages. The crystal structure of [Cr(S2COEt)3] is composed of enantiomorphous molecules with trigonal symmetry. The short S2C-0 bond distance (1.297A) is taken to mean that the resonance structure S 5 - d - R contributes c~nsiderably.'~'~ The magnetic moments of the complexes are usually near 3.8 BM at room temperature. The moment of [Cr(S2CNEt2)3]is essentially independent of temperature down to ca. 40 K. Then it falls slowly to approximately 3.6 BM at 4 K and more rapidly to 3.5 BM at 2.5 K.1015 It was not
Chromium
885
Table 88 Structural Data for Complexes of Bidentate S Donor Ligands Complex [Cr&CNC4H,0)3] CH,C1, [Cr~&2mc4H80)31'2C&6 [c~(s~cNc,H,),I.o.~c~H,
[cr(Sz~b)31
[Cr(S,CN(CH,),},] .2CHC13 [Cr(~C~~)z(oszCNEtz)l
cr-s (A) 2.40 2.3% 2.404 2.39 2.397 (av) 2.405 (S,O ligand) 2.405, 2.403 2.379, 2.370
~c3F7c~~s,~e2)2PYl
Cr-0, 1.988 tram, 2.457
[Cr(S,COEthl [0(sp(oEt)&l
others, 2.392 2.393 2.421-2.430
Ref. 1 2 3 4 5 6
7 8 9
1. R. J. Butcher and E. S i n , J . Chem. Soc., Dalton Trrms., 1975,2517. 2. R. J. Butcher and E. Sinn, 1. Am. Chem. SOC., 1976, 98,2440. 3. E. S i n , Inorg. Chem., 1976,15,369. 4. C.L. Raston and A. H. White, Aust. J . Chem., 1977,30,2091. 5. V . Kettman, J. Garaj and J. Majer, Collect. Czec. Chem. Commun., 1981,46, 6. 6. R.L.Martin,J . M. Patrick, B. W. Skelton, D. Taylor and A. H. White, Aust. 1. Chem., 1982,35,2551.
7. A. M. Van Den Bergen, K. S . Murray, R. M. Sheahan and B. 0. West, 1. Organomet.
Chem., 1975,90,299. 8. S . Medino and F. Sartori, Acta Cryxlallogr.,Sect B, 1972, 28, 972. 9. H. V. F. Schousboe-Jensenand R. G. HazeU, Acrn Chem. Scand., 1972,26, 1375.
possible to decide whether the low moments at low temperature are due to small zero-field splittings or very weak exchange interactions, or both.1°15 The complex [Cr(SacSac)d (220; R= M e) has a moment which decreases steadily as the temperature is lowered.' The polymers [Cr2(&CNHGH&NHCS2)3] and [Cr2(S2CNHC6H12NHCS2)3] show weak antiferromagnetic interaction, with 8 values (negative intercepts on T axis) of 32" and 8" respectively. They are amorphous, and insoluble in water and common The temperature de endence of the 'H NMR spectra of many [Cr(S2CNR2)3]complexes has been investigated.lm?T)O'' There were broad double resonances from the N 4 H 2 protons indicating that inversion between the A and A isomers is slow on the 'HNMR time-scale. No exchange broadening was observed up to 84 "C when [Cr(SzCNEt2)3] decomposed. Dithiocarbamato complexes of other tervalent metal ions do not exhibit this stereochemical rigidity. Assignments of spin-forbidden bands in the spectra of [Cr(S2CNR2)3]and [Cr(S2COR)3] (R = Me or Et) have been made and the emission spectra studied.l0lg MCD spectroscopy has been useful in the location of the spin-forbidden transitions. That the Cr-S bond is comparatively strong has been adduced from the presence of relatively intense molecular ion peaks in the mass spectra of several CI" dialkyldithiocarbamates.'020 The complexes [Cr(S2CNMe2),], [Cr{S2P(OEt)2}3]and [Cr(S2COEt)3] have Pfeiffer CD activity but this develops only when there is a large ratio of optically active environment compound to the racemate.lml A normal coordihate analysis of the IR spectra of [Cr(S2CNR2),] (where R = Me, Et, Pr or Bun) has shown that the alkyl substituent affects the thioureide band at ca. 1550 cm-' through kinematic as well as electronic effects.lm2 Complexes E(L)zCl2]Cl in which L is a tetraalkylthiuram sulfide or disulfide have been characterized. Other typical chromium(II1) complexes of bidentate S donor ligands are the xanthates [Cr(S2COR),] (R = Me, Et, Bz, Ph), thioxanthates [Cr&CSR),] (R = Me, Et, Pr, But, Bz), dithiocarboxylates Cr S2CR)3] (R = Ph, c6H4Me, C6&OMe, C ~ H ~ N MC~ 6ZM , t 2 , C&O, c5H3NH2, Bz)y1 d,lok'mg dithiooxalate [Cr(C&02)3]3-, trithiocarbonate [Ph4Pj3 or [Ph4As]3[Cr(CS3)3~,103~1031 dialkyldithiophosphates [Cr(S2P(OR)2}3] (R = Me, Et, PI", Pr', Bus, B U ' ) , ' ~ ~ , ~33' ~dithiophosphinates Cr S2PR2)3](R = F, CF3, Me, Ph, OEt)1002,1005 and, for example, di!hio-@-diketonates [Cr(SCRCHCR'S)3] (R = R' = Me, Ph; R = Me, R' = OMe, OEt) l,loo5and dithiocarbazates such as [Cr(S2CNHNH&] and [Cr(S2CNHNPh2)3].lMS Complexes of Se donor ligands, [Cr(Se2CNR2)3](R = Et; R2 = (CH2)5, (CH&O or (CH&S) and [Cr{Se2P(OEt)2}3]are similar to the correspondin S donor complexes, but their electronic spectra exhibit a pronounced nephelauxetic e f f e ~ t . ~ ~ ~ , ~ ' ~
'
J
Chromium
886
A number of chromium(II1) complexes of 1,l-dithiolato dianions (222) are known: [Cr(SX=X)3]3- (where X = C(CN),, C(C02Me)2, NCN, C(CN)COZEt, C(C02Et),, C(CId)C02Me and C(CN)CONH2)583~w3~1""5and the thioselen 1 analogues [Cr{ SSeC=C(CN)2}313- and [Cr{ SSeCO(CH2)20Me}3] have been obtained. lo3' The complex [Cr(S2PF2)3] is volatile and monomeric in the gas phase.'""' Its He' photoelectron spectrum has been reported. lo3*The absence of interfering ionization bands of substituent groups outside the coordination sphere permitted detailed assignments. The UVPES spectrum of [Cr{S2P(OEt),},] has been ~b t ai n ed , '~'and the symmetric v ( C r 4 ) vibration at 282 cm-' has been identified in the RR spectrum of [Cr(S2PPh2)3].*040 Complexes of 1,Zdithiolate dianions (225) are of interest, although because they undergo successive one-electron transfer processes which involve orbitals of ligand character so unlike those of the dithiocarbamate type of ligand, it is unprofitable to attempt to assign oxidation s t a - t e ~ . ~The ~ ~ ~chromium "'~ complexes are shown in Table 89. Table 89 1,2-Dithiolato Complexes Complex
A = Ph4As, peff= 3.90 BM A = Ph4P, pen= 3.88 BM (95 K), 0 = 9" A = Ph,BzP A = Ph4As, pCff= 2.89 BM A = Ph4P, peff= 2.76 BM (88 K), B = 8' pLe,=2.95BM
[A13[Cr{SzG(CN~z~31 Brown [AIz[Cr{SzG(CN)2}31 Yellow-brown tPh4AsIdCr{ ~ZGtcF,>*} 31 Olive green ~
~
~
4
~
~
Dark green [Cr{Sz&(CF3)2) 31 Red-purple [~~(%&Phz)31 Dark red [Ph4PI[Cr(Sz&Ph,),l [AIz[Cr(SZC6C14)31 Green [Bu4NI[Cr(SzC6F4)21 [Me"[Cr"(~zGK),1 Gold
ReJ
Comments
1
~
~
~
~
peE ~ =Z1.89 G BM ~
~
~
~
>
Z
~
3
1,2
3 4 132
3 1 1
1
Diamagnetic
1
Diamagnetic
1,s
g = 1.996 A = Et4N or BuZN, p = 2.88 BM
4,5 6
Unusual bis(che1ate) complex
7
perf= 2.50 BM, polymer? peff=3.85 BM (DMSO)
8
1:A. Davison, N. Edelstein, R . H.Holm and A. H. Maki, J . Am. Chem. Soc., 1964, 86,2799. 2. E, 1. Sticfel, L. E. Bennett, 2. Don,T. H. Crawford, C. Simo and W. B.Gray, Inorg. Chem., 1970,9,2$1. 3. J. A. McCleverty, J. k k e , E. J. Wharton and M. Gerloch, .I. Chem. Soc. ( A ) , 1968, 816. 4. P. Vella and J. Zubieta, J . Inorg. Nrccl. Chem., 1978, 40, 613. 5. G. N. Schrauzer and V. P. Mayweg, J . Am. Chem. Soc., 1966,88,3235. 6. E. J. Wharton and J. A. McCleverty, J . Chem. SOC. (A), 1969, 2258. 7 . A. Callaghan, A. J. Layton and R. S. Nyholm, Chem. Commun., 1969,399. 8. J. R. Dorfman, Ch.P. Rao and R. H. Holm, Inorg. Chem., 1985,24,453.
The general electrochemical behaviour of the tris(che1ate) complexes of Cr"' with the ligands (218)to (225) has been extensively in~estigated.~~~~'""~~'~'~'@'~ New data have been correlated with early work.lm In general, the CrI'I complexes are resistant to oxidation and reduction consistent with the stable & subshell. Comparisons have been made between complexes of different ligand types and substituent effects analyzed for a particular ligand type in terms of the Hammett CT+ constant. The dithiocarbamate ligand stabilizes high and low oxidation states and the redox processes are modified compared with the tris(dithi0carbamates) when one ligand is OS2CNR2 and by the presence of bipy in sohtion, which forms mixed ligand complexes in the lower oxidation states. Substituent effects are prominent in the 1,lethyleneditholate series. With these ligand systems the redox processes are metal based whereas in complexes of the 1,Zdithiolates (dithiolenes) the redox processes are primarilj ligand based. Published data on the volatility of complexes of the transition metals including chromium with bidentate sulfur and sulfur-oxygen donor ligands have been summarized. The octahedral complexes [C~(C~~-M~SCH=CHSM~>~](BF~)~, [Cr(S(CH2CH2CH2SMe)2}z](BF4)3,685 mer-[Cr{S(CHzCH2CHzSMe)2}X3], (X = C1, Br or I), fac-[Cr{CMe(CH2SMe)3}X3: (X = C1 or Br) and ~UC-[C~(CH~SCH~CH~SM~)~C~~]~'~ have been prepared and characterizec by methods used for similar P and As donor ligands.
Chromium
Chromium
888 35.4.5.3
Thiourea
Complexes of thiourea (tu) with chromium(II1) have not been studied in detail and the lack of structural investigations in particular has led to uncertainties regarding the ligand donor sites and geometrical configurations of some of the reported species. The complex CrC13(tu), is conveniently prepared by evaporating a methanolic solution containin CrC13.6Hz0 and tu (1:4) to dryness, and then washing the residue with hot acetone.' 43 Evidence based on the IR spectrum of this complex (Table 90) indicates that the ligand is bonded through sulfur to the metal ion. Hence the v(NH) bands in the spectrum of the complex occur at higher wavenumbers than in the spectrum of free tu, indicating a reduction in H bonding and therefore S coordination of the ligand to the metal. Similarly the r(CS) band at 733cm-l in the free ligand spectrum is shifted to a lower wavenumber in the spectrum of the complex consistent with a reduction in carbon-sulfur bond order and coordination through the sulfur atom.1043,1044 Furthermore, the expected increase in the carbon-nitrogen bond order due to S coordination is confirmed by the shift to higher wavenumber of the Bl NCN band in the complex (1490 an-') relative to free tu (1476 cm-I). A strong band at 282cm-' assigned to v(MS) lends additional support to the presence of S-thiourea ligands. The reflectance spectra of CrC13(N,Nf-Et2tu)3and CrC13(N,N'-B~nt~)3 are strikingly similar to that of CrC13(tu), (Amax = -680, 510 nm) implying similar ligand fields in all three ca~es.~"''~ No mention is made of the geometrical disposition of the ligands in any of these complexes. CrC13(tu)3 reacts with various Lewis bases in hot methanol solution as outlined in Scheme 102.l'" Salient features of the IR spectra of a selection of these complexes are presented in Table 90. Application of the afore-mentioned criteria to the IR spectra of these products leads to the conclusion that in some cases the complexing site of tu changes from sulfur to nitrogen when heterocyclic amines or phenylarsonate ligands are introduced into the complex. In other cases, however, e.g. [CrC12(py)z(tu)2]C1,for which both vasym(NCN)and v(CS) occur at higher wavenumbers than the corresponding IR bands for free tu, no definite conclusions can be reached on the basis of IR spectra regarding the mode of ligand coordination. Clearly the IR data on their own are insufficient to establish structures with certainty and X-ray crystallographic studies are needed to clarify both the ligand donor sites and the geometrical configurations of these complexes. The preparation and spectra of complexes of general formula M[Cr(NCS)4(tu)2] (M = K or various Nif'-amine complexes) have been reported. 1046m47
Q
[CrC12(Rpy)z(tu),lC1
1
RPya
[Cr(L-L)z(tu>zlc1,
G L C
Rq"l"b
CrCl,(t~)~
I
[CrCI,(Rq~in)~(tu),]Cl
PhAsO,H,
[Cr(PhAsO,H)z(tu),]CI a
Rpy = pyridine; 2-, 3- and 4-methylpyridine; 2,4- and 2,6-dimethylpyridine. isoquinoline and acridine. L-L = phen or bipy.
Rquin = quinoline,
Scheme 102
A number of substituted tu complexes of chromium(II1) have been prepared and characterized. These include [cr(N-Met~)~](ClO~),, [Cr(N,N'-Me2tu)6](C104)3, CrC13(NMetu),, CrX3(N,Nf-Me2tu)3(X = C!, Br) ,lw8 and the N-phenylthiourea complexes CrC13(NPhtu), and CrCl,(N-RPhtu), (R =2- or 3-Me; 2-Me0; 2-, 3- or 4-C1; 3-Br; 4-1; 2-OH).'049 Evidence from IR (Table 90) and electronic spectroscopy strongly suggests that in all these complexes the substituted tu ligands are coordinated through sulfur to the metal ion. Details of the electronic spectra of some of these complexes are given in Table 91. The ligand 3-(4-pyridyl)triazoline-5-thione(L) reacts with CrC13.6H20 in refluxing EtOH to give the product tr~ns-[Cr(H~O)~Lz]Cl~ in which it is S-bonded to the metal ion.1050The mixed ligand chromium(II1) complexes [Cr(H20)5Etu](OAc)3and [Cr(OAc)OH(T-5-T)].2Hz0, containing ethylenethiourea (Etu, an S donor) and 1-substituted tetrazoline-5-thiones (T-5-T; N,S donors) have been prepared and characterized.'"' Dissolution of thioureachrornium(II1) complexes in coordinating solvents such as MeOH, DMF, DMSO, MeCN generally results in solvolysis with replacement of the tu ligands.
Chromium
889
Table 91 Reflectance Spectra of Thioureachromium(II1) Complexes
Complex [cr(N-Met~)~](ClO,), [Cr(N , N '-Me,tu),] (C104)3 CrCi,(tu), CrCl,(N-Metu), CrCl,(N,N'-Me,tu), CrCl,(N-Phtu),
690 714
CrC13(N-3-MePhtu),
682
CrC13(N-3-ClPhtu), [~rCl~]'-a
675
680 699
725 700 730
526 549
510 518 521 510 510 5 10 529
~
a
Included for comparison.
1. P. Askalani and R. A. Bailey, Can. J. Chem., 1969,47, 2275. 2. E.Cervone, P. Cancellien and C. Furlani, J. horg. N u l . Chem., 1968,30,2431. 3. M.H.SOMIand A. S. R. Murty, J . Inorg. N u l . Chem., 1977, 39, 2155. 4. C. K.Jergensen, 'Absorption Spectra and Chemical Bonding in Complexes', Pergamon, Oxford, 1962, p. 290.
35.4.6
Selenium and Telhuium Ligands
A number of binary and ternary compounds in the chromium-selenium and the chromiumtellurium systems have been investigated. These include CrSe, Cr2Se3,Cr3Se4,Cr7Ses, Cr5Se8, CrTe, Cr2Te3, Cr3Te4, Cr5Te6, Cr5Te8, Cr,Teg, Cr5S4.8Se3.2,981 NaCrSe2, CuCrSez and AgCrSe2.986Some Se donor ligands are included in Section 35.4.5.2. 35.4.7
Halogens as Ligands
35.4 7.1 Simple halihs CrX3
These have been known for many years. 1052-1054 Chromium(II1) is approximately octahedral (peE=3.69-4.1BM); the compounds have a layer structure. In the chloride, r(Cr-Cl) is 5.76A between layers and 3.46w within layers. The iodide is isomorphous with the chloride and the bromide has a similar but distinct structure. All may be prepared by the direct halogenation of the metal. Other methods are available, e.g. CCI3 may be prepared by heating Crz03.xH20 in CC1, vapour at 650°C.1055The anhydrous halides are insoluble in water, however reducing agents such as zinc catalyze dissolution. The trichloride reacts with liquid ammonia to form ammine complexes.
35.4.7.2 [CrX4]- and related ions There has been some controversy'0s6~'0s7 concerning the correct formulation of [CrCLJ ions in salts such as [PC14][CrC14J.In general the evidence supports polymeric six-coordinate This is supported by a crystal structure of K[CrF4],1058which contains columnar [CrF4]- ions. Related compounds such as NaCrF41059and NH4CrF41m also involve sixcoordinate chromium(II1); these compounds are weak antiferromagnets. A related antiferromagnetic species is MnCrF5, where a neutron diffraction study shows chromium(1~I)to be octahedral with a single bridging fluoride;lo61 this is an example of a general class of chromium(II1) fluoro complexes M"Cr'"F5 .235 The anions [CrC14(MeCO2H)& and [CrC15(MeC02H)]'- are known (Scheme 92). 35.4 7.3 [CrX6] 3- anions
Complexes containing anions of the above formulation have attracted a large number of studies because of their alleged simplicity. This is illustrated by the central position such complexes have played in the evolution of crystal field, ligand field and molecular orbital models of bonding in transition metal complexes. In a study of the heats of formation of first row transition metal fluorides, AHf for K3CrF6 was found to be 2977kJmol-1.1062The usual double-humped distribution of AH, 21s. d" was
Chromium
890
observed. Semi-empiri~al'~~~ and self-consistent-field molecular-orbital calculationslW give similar results for the [CrF6I3- ion and are in accord with the observed electronic spectrum.lM5 These studies are summarized in Table 92. Table 92 Electronic Spectra of Hexafluorochromate(II1) (all energies are cm-'
X
lo3, after ref.
1063) One electron 2ta-3e, Uncorrected (correction ref. 1)
3t1, a, 3t1,+3e, -+
1ODg
t2a +
t2,+%,
nU+tB
2 3 7
16.54 12.68 (16.4)
6.88 (12.7) 16.44
27.8
36.76
40.50
49.44
16.10 15.20
Re5
I 7.50
4,6 1
1. Spin corrections after A. Dutta-Ahrnedand E. A. Boudreaux, Inorg. Chem., 1973, It, 1590. 2. R. F. Fenske, G. Caulton, D. D. Radtke and C. C . Sweeney, Inorg. Chem., 1966,5,960. 3. P. 0. Offenhartz, 1. Chem. Phys., 1%7,47,2951. 4. D. L. Wood, 3. Ferguson, K.Knox and J. F. Dillon, Jr., J . Chem. Phys., 1963,39,890. 5 . C. K.Jdrgenson, 'Absorption Spectra and Chemical Bonding in Complexes', Pergamon. London, 1962. 6. Rcf. 1065, and G. C. Allen and K. D. Warren, Struct. Bonding (Berlin), 1971, 9, 49. 7. Rcf. 1063.
Luminescence spectrocopy is potentially a powerful technique for studying chromium(II1) complexes and a series of fluoro and aqua complexes have been studied.1066The luminescence correlates well with ligand field strength and, at liquid air temperatures, the lifetime of the doublet state from which phosphorescence originates is 2 X lO-'s-'. ion in K3[CrF6] has A detailed study of the spin density and bonding of the [crF6 appeared.1067The results of polarized neutron diffraction experiments"' were reinterpreted. A chemically based model of the [CrF6I3- ion has been fitted to the 92 observed magnetic structure factors by a least s uares procedure. The spin density in the chromium(II1) orbitals has tZg symmetry (t:;P"(')',e>-ud(5)). There is also a region of spin density centred on the chromium atom, radially more diffuse than the theoretical 3d orbital and containing 0.4(1) spins parallel to the spin density of t2, symmetry. There are small parallel spin densities in 2p, of fluoride and antiparallel spin populations in fluoride 2p, and along the Cr-F vector. There is no significant spin population in fluoride 2s. The results of this modelling are in agreement with &,lffi9 semi-empirical'mo and ab initio molecular orbital The magnetic circular dichroism of chromium(II1) in dicesium yttrium hexachloride has been studiedlo7' down to 6 K. Transitions to all three quartet terms, Z& and 'Tk (I and II), were observed. The parameters D = l!280cm-' and a = 78.2 cm-' were determined. At 6 K rich vibronic fine structure was observed, which could satisfactorily be explained only in terms of Oh geometry. 35.4.7.4 Dimeric structures
Many complexes contain the [Cr2X,]'This unit consists of two distorted octahedra which share a face; there are three bridging and three terminal halides (226). The distortion of the octahedron is such that the two metal ions are displaced from each other. The magnetic properties of these complexes have been the subject of extensive study summarized in Table 93.
(226) Typical structure of a [Cr2X9I3-ion
The complexes undergo weak antiferromagnetic interactions with exchange proceeding via the halide bridges. The counterion may have a marked effect on the structure; for [Cr2C19]'-, change of the counterion from potassium to tetraethylammonium is accompanied by a decrease
Chromium
891
Table 93 Magnetic Properties of [Cr&$
M-M (A) 3.05 3.07 3.12 3.89
Peff (3M)
-J (cm-')
Ions
-
3.26,3.32
4.00
Ref.
Complex
2,6 2 1
-
~-
1. R. Saillant and R. A. D. Wentworth, Inorg. Chem., 1968.7, 1606. 2. G.J. W e d and D.J. W. Ijdo, Acta Crystallogr., 1957, 10, 466. 3. I. E. Grey and P. W. Smith, Aut. 1. Chem., 1971,24,73. 4. P. C.Crouch, G. W.A. Fowles and R. A. Walton, J. Chem. Soc., 1969, 972. 5. A. Eamshaw and J. Lewis, J . Chem. SOC., 1961,396.
6. R. SaiUant, R. B. Jackson, W. E. Streib, K. Folting and R.A, D. Wentworth, Inorg. Chem., 1971, 10, 1453.
in J from 16 to 5 cm-'. An extreme example of similar behaviour is [EtJI3[Cr2Br9] which obeys the Curie-Weiss law. The vibrational spectra of [Cr2X9I3- have been variously studied. 1077,1078 The agreement between observed and calculated snectra was excellent for hhsymmetry; na evidence was found for any C r - C r interacti~n.""~In related work, the doping of chromium(III) ions into CsMgC13 and related materials bas been investigated. 1079~1080 ESR spectroscopy has shown that there is a distinct tendency for chromium ions to cluster in pairs [for single crystals with 1 part in 1000 chromium(II1)j. CsMgCl, adopts the CsNiC13 structure (227); the structure proposed for the impurity centre is illustrated. The forces controlling the distribution of ions were held to be electrostatic; other trivalent metal ions should behave in a similar way to chromium(II1).
T
C axis
vacancy
(227) chain showing the propJ X pair. The corners of the
A perspective view of an MX;
osed structure of the Cr"-Cr'
octahedra are occupied by halide ions.'080
354.7.5 Solution chemistiy
Werner's early work on coordination chemistry involved the preparation of many of the isomers in the CrX3-nH20 system.1081,1082 Crystal structures are available for several of these coc3-CC'
Chromium
892
complexes. The green complex trans-[Cr(HzO)4C12]C1~2H20, the normal com ound supplied as chromium(II1) chloride, has been studied on a number of Recently a low-temperature structure of the complex dicesium trans-dichlorotetraaquachromium(II1) trichloride1085was determined; the electronic spectrum was also studied. The mechanism of the geometric isomerization of the dichloro complex has been investigated.1086The pure cis complex may be prepared by the aquation of trichloro complexes followed by purification by ion-exchange chromatography."% The formations of c h l ~ r o , ~ bromo1089 ~ ' , ~ ~ and iodolm complexes have been studied. Recently, electrochemical methods have been used to measure equilibrium constants for the formation of chloro (0.086), bromo (1.1 x and iodo (1.1 x lop5) complexes of chromium(III) .lo9' 35.4.8
Mixed Donor Atom Ligands
35.4.81 S c h i f s bases, p-ketoamines and related ligands
Compared with other transition elements,lW2 few complexes of Schiff s bases or /3-keto amines with chromium(II1) are known. Most work has been done with ligands (228) to (231) and their complexes and the interrelations between the more important of them are set out in Table 94 and Schemes 103 and 104. Me X% O H Y
-N R'
Me
N'
"
Me
(230) X = Y = H , B = (CH,),: salenH, X = Y = H, B = 0-C,H,: salphenH, X = Y = Me, B = (CH,),: MesalenH, X = 3 - c o 2 H , Y = H, B = (CH2)2:3-C0,salenH2
Me (231) acacenH,
( i ) Syntheses
Many preparative methods have been used, with variable success: oxidative decarbonylation of Cr(C0)6 by the ligand,'W3,'W4reaction of the appropriate amine with [Cr(sal),] (salH is salicylaldeh de), reaction of the ligand, either produced in the reaction mixture or preformed,r095 with hydrated or anhydrous CrCl,, hydrated Cr"' acetate,lm or [CrC13(THF)3],1097-1W9 and aerial oxidation of the chromium(I1) complex produced in situ.1099-1101 No well-established chrornium(I1) complexes, except of a few /3-keto amines (Section 35.3.9.1), have been isolated. The source of Cr2+ can be Crz MeC02)4(H20)2],1100 aqueous C?' produced by the reduction of C?+ electrolyticail$'O1 i r by Zn/Hg in hydrochloric acid,1099or anhydrous or hydrated CrC13 reduced by zinc dust in suspension in the reaction mixture.1100,1102 Presumably the ligands can displace solvent molecules more readily from the labile C? species. Several hours reflux in organic solvents is usually required for preparations directly from chromium(II1) starting materials. No method is of general applicability; for example, [Cr(~alen)(H~O)~]Cl (Scheme 103) is readi2 prepared from [Cr2(MeC02)4(H20)z]but this method affords [Cr(acac),] with acacenH2.l'
Chromium
893
Tabk 94 Preparations of Some Schiff BW and @-KetoamineComplexes co?nplex
Synthesis
Ref.
Tris(che1ates) Ligand (228). bidentate, X = Y = H R=Me Cr(CO), R = Pr", Bu', p-C6H4Me,p-C6H,Br R = Me, Et, Pi", Pr', ,
+ C&(CHNMe)OH,
toluene, reflux Cr(sal), f amine in benzene, or CrCl3.6&0 Schiff base, Na,CO, Hydrated C?" acetate, suspended in ethylene glycol, salH, amine, 2/3 theoretical Na,CO,
+
1 2 3
Ligand (m), bidenrare, X = S,b-bmro, Y = H R = Me, Et, Pr", Pr', Ph
As above
3
Ligand (a), bidentate, X = 5 N 4 ,Y = H R = (R or S)-u-CH(Bz)Me R = (Ror S)-a-CH(Ph)Me
As above
4 5
Ligand (a), bidentate R, = Me, R = CH,Ph, Ph, 0-, p-tolyl, 0-, m-, p-C6H,Cl, p-anisyl, p-C6H,-C6H4, p-naphthyl Re = H, R =p-tOlyl Bir(cklatt3) Ligund (228), tridentate,X = Y = H R = CH,CH,NH, [Cr(en-sal),]X R = O-C,H,CO,H,~ o-C&I,OH, CHZCOZH R = CH,CH,OH
KOBh in BbOH, ligand, Zn,CICI,(THF)~
6
Cr(sal), fen, MeOH, add NaX, X = Br, I or '30,
2
[Cr,(CH,CO,),~Z€&O], sdN, anthranilic acid, 0-aminophenol or glycine As above, add NaClO,
7 7
[Cr{C6H4(CHNCH,CH,0H)O}3]C104 Ligand (a), tridentate, R = CH2CH2-2-pyridyl Y = H, X = H, 5-C1, 5-N02, 5,6-benzo X = H, Y =Me, Et, Pr a
Complexes formulated H[Cr(C,H,(CHM)O},]
2-(2'-Aminoethylpyridine), salH in EtOH + CrC1,.6H,O, NaOAc; formulae [CrLJCl as proton from one R group must remain to balance charge.
1. F. Calderazzo, C . Flotiani, R.Hcnzi and F.L'Epplattenier, J . Chem. Sac. ( A ) ,1969, 1378. 2. R. Lancashire and T.D. Smith,I . Chem. Soc., Dalton Tram., 1482,693. 3. S. Yamada and K. Iwasaki, Bdl. Chem. SOC.Jpn., 1968,4t, 1972. 4. J. E. Gray and G. W.Everett, Ir., Inorg. Chem., 1971,10,2087. 5. K. S. Finney and G. W.Everett, Jr., Inorg. Chim. Acta, 1974, U, 185. 6. J. P. CoUman and E. T. Kittleman, Imrg. Chem., 1%2,1,499. 7 . K. Dey and K. C. Ray, Inorg. Chim. Acta, 1974, 10, 135. 8. D. K. Rastogi and P. C. Pachauri, lndinn I . Chem., Sect. A , 1977,15,748.
ICr(salen)(MeOH),]Cl
[Cr(salen)(H20),]NCS
KNCS
[Cr(salen)(H,O),]X X = PF.5, BPh4 t
[Cr(salen>(H,O),]Cl
K[Cr(salen)(ONO),]
120"c
[Cr(salen)Cl]
[Cr(salen)pyCl]
[Cr(salen)L,]Cl
L = NH,, py
Ot
[Cr(salen)OH]
RNH, Scheme 103
[Cr(salen)(acac)]
8
Chromium
894
CrCI,
acacenll?, KOEtlEtOH
fKX1 [Cr( acacen)LJCI L = NH,, RNHZ [Cr(acacen)(bipy)]C10, [Cr(acacen)(acac)]
K[Cr( acacen)X, J X = ONO-, CN-, NCS-
DMP is 2,2-dimethoxypropane Scheme 104
Since oxidation of [Fe" salen] gives {Fe(salen)},O, the formation of [Cr(~alen)(H,O)~]Cl from Cr" solutions is unexpected, but it is consistent with the observation'103 that aerial oxidation of aqueous chromium(I1) leads to the formation of hydroxo or a ua species. The elusive {Cr(salen)}20 was eventually prepared by oxidative decarbonylation' of Cr(C0)6 by [Hg(salen)], a method which could have more general application. The reaction of salenHz with Cr(C0)6 gives a product similar to [Cr(~ a len)(OH)]-0 SH~ 0Dichromium .'~ and mixed metal p o x 0 complexes such as (TPP)CrOFe(TPP) are now known (Section 35.4.9.1). Complexes obtained by methods similar to those in Scheme 103 include [Cr(salen)(HZ0)2]C1Q4,1101 [Cr(salen)(X)(H20)] (XH = nicotinic acid or benzoic acid),"" [Cr(salen)X(H20)] (X = NCS- or N;),"05 [Cr(salphen)(Hz0),]X (X= Cl- or O A C - ) , ' ~ ~ [Cr(salen)(AA)]BPh4, [Cr(sa1phen)(AA)]C1O4 (AA = bipy, phen or en).1107Complexes related to those in Scheme 104 are [Cr(acacen)A2]BPh4,[Cr(acacen)ClA], [Cr(acacen)(NCS)A], [Cr(acacen)Az]C104, [Cr(acacen)(H20),]SCN (A = py or 4-methylpyridine)110s and The salts [Cr(L)(NH&]SCN [CrL(H,O),]Cl, in which L is formed from acac and (L = salen, salphen or Mesalen) have been prepared''" from NH4[Cr(NCS)4(NH3)2].
3
(ii) Properties Photolysis of the azido complex CrN3(salen).2H20 produces" the nitrido chromium(V) complex CrN(salen).H20 and the reactionll'l of [Cr(salen)(HzO),]+ with iodosylbenzene gives the oxo chromium(V) derivative [CrO(salen)12+;chromium(II1) porphyrin complexes behave similarly (Section 35.4.9.1). As expected for chromium(II1), the complexes generally possess magnetic moments near 3.8 BM at room temperature. A few, (H[Cr{C6H4(CHNR)O}z] (R= o-C6H40H or CH2CHZ0H)'1" and [Cr(salen)(OH)].0.5H20) have low moments ( ~ 3 . BM) 5 ascribed to antiferromagnetic interactions. The magnetic susceptibility of [Cr(salen)(OH)].0.5HZ0 deviates markedly from the Curie law, but as the results were sample-dependent, no detailed interpretation was attempted.lW9The room temperature magnetic moment of {Cr(salen))20 is 2.2BM, consistent with an oxo-bridged structure. The ESR spectrum confirms that the chromium is trivalent,lW4so further magnetic and structural investigations are desirable. The complexes have yellow to red colours and the electronic spectra are dominated by intense ligand absorptions. A weak band ususally found in the range 18 000 to 21 000 cm-l has been assigned to the 4A2g+4T2gtransition and provides an a roximate value for A. The trans structure of the cation in [Cr(salen)(H,O),]Cl'"gPand the mer arrangement of the tridentate ligands in [Cr(en-~al)~]I [en-sal is N-(2-aminoethyl)salicylaldimine)] have been confirmed by single-crystal X-ray investigations (Cr-O(av) = 1.93, Cr-Nev) = 2.08 A) .1112 In [Cr(~alen)(H,O)~]Cl, the Cr-N bond lengths are 1.977 and 2.005 A, but the Cr-0 distances vary considerably (1.916 and 1.9518, (salen); 1.923 and 2.085 A to HzO) and the angles at the chromium atom are distorted from right angles. The ground-state distortion has been related to the lability of the axial ligands."13 There must be considerable distortion of salen and acacen from their usual essentially planar geometry in complexes such as [Cr(acacen)(acac)]. The complexes [Cr(en-sal)z]X were prepared (Table 94) in conditions which could have given [Cr(salen)X], but, as indicated by other data and confirmed by the crystal structure of the
Chromium
895
iodide, en and saIH condensed to give the tridentate ligand en-sal. The solubility of the nitrate has been exploited1’14in the resolution of [Cr(en-sal),]+ as the hydrogen ( R , R ) - 0 ,0’dibenzoyltartrate. The diastereoisomers were converted into the iodides which formed twinned crystals unsuitable for X-ray investigation. When (R)-1 ,Zdiaminopropane (R-pn) is used instead of ethylenediamine in the reaction with Cr(sal)3, the salicylaldehyde may condense with either NH2 group. The product was (232), with the ligands meridional, as expected, and the -CH2NH2 groups condensed.1115 Me /
Tris[N-(R or S)-a-benzylethyl-5-nitrosalicylaldiminatoJand tris[N-(R or S)-a-phenylethyl-5nitrosalicylaldiminato]-chramiurn(ll1) have been prepared1’16,11’7from the ( R ) - or (S)-amine as appropriate. For ligands of a given chirality, four diastereoisomers are possible (fac, A and A; mer, A and A). Chromatographic separation of the complex with R = (S)-a-CH(Bz)Me gave two products A and B (the more rapidly eluted), and with R = (R)-a-CH(Bz)Me, A’ and B’ were obtained. A and A’ have identical absorption spectra but from their CD spectra have opposite configurations about the metal. Since they differ in ligand chirality also, they are enantiomers. Steric effects would favour the mer(trapls) configuration, and the corresponding cobalt(II1) complexes, which chromatographically separate similarly, have been assigned mer configurations from their ‘H NMR spectra. Analogous relationships were found among the pair of diastereoisomers obtained with R = (R)- or (SI-m-CH(Ph)Me. The more rapidly eluted isomers are thought to have the A-mer configuration from analogy with the cobalt(II1) case, and because studies with the chiral P-diketonate (+)-3-acetylcamphor have shown that diastereoisomers with the same absolute configuration have the same relative rates of elution. The isomers did not interconvert under reflux in the presence or absence of charcoal. The complexes C r b , where LH is formed by condensation of salicylaldehyde with 2-furfurylamine or 2-thenylamine7contain the ligand in isomeric form (233).1118
N
9
(233) X = 0 or S
The addition of en or 1,3-diaminopropane to a mixture of chromium(II1) acetate hexahydrate and diacetyl under reflux, followed by addition of NaC104, has given the complexes [Cr(dia~en)(H~O)~]ClO~ and [Cr(diacpn)(H20),]C104, which contain the tetradentate Schiff bases (2M).1119 The thiocyanates [CrL(H20)(NCS)](NCS), were also isolated. From their IR spectra, and because the coordinated Schiff base would not react with another molecule of diacetyl to form a macrocycle, they have been assigned cis configurations. In the chromium(l1E) complex of the potentially heptadentate ligand (235, X = 5C1) the apical nitrogen atom is not bonded, the Cr-N distance being 3.229 A. The Cr-0 (1.979 A) and Cr-N (2.137 A) bonds are longer than in [Cr(~alen)(H~O)~]Cl and [Cr(en-sal)2I see above) and the ligand seems to fix a minimum sized ‘hole’ into which the metal ion fits.’ XJ The
I (
Chromium
896
MeAN(CW,),NH,
(234) diacen, n = 2 diapn, n = 3
Mn"' and Fe"' complexes are isomorphous with the Cr'" com lex. The chromium(II1) complexes with X = H , 3-OMe, 5-C1, 5-Br, S-Me1121or 3,5-C1211* have been prepared by methods similar to those for the salen derivatives.
P
(235)
Chromium(II1) does become seven-coordinate in two complexes in which the organic li ands are formed by condensation respectively of 2,6-diacetylpyridine and semicarbazide,"" and
2,6-pyridinedialdehyde and 6,6'-bis(a-2-hydroxyethylhydrazin0)-2,2'-bipyridyl~~~~Sevencoordinate complexes are formed by chromium in several oxidation states (Table 95). Table 95 Seven-coordinate Chromiuma Complex [C~'(CNBu'),ItPF& [Cr"(CNR)S(dppe)l(PF6h Cr"[P(OMe),15H, CrNO(S,CNEt,),
[Cr1"(dapsc)(H2O),]OH(NO3)~ [Cr"(CNPh),(d~Pm)lIPF,), [C~L(H,O),ICI,
Structure
4: 3 Tetragonal base-trigona1 cap (1) (or 4: 3 piano stool, CJ R = But, C6HI1 Pentagonal bipyramid Pentagonal bipyramid, axial NO (w3) Pentagonal bipyramid (23s) Unknown structure Pentagonal bipyramid, see (l28)
Re5 1,2
2 3 4 5 6
7
* Several mixed ligand complexes containing carbon monoxide' or peroxide (Section 35.7.7) are seven-coordinate. dapsc is 2,6-diacetylpyridinebis-semicarbazone. 'L is the planar pentadentate macrocycle formed by condensation of 2,6-pyridinedialdehyde and 6,6'-bis((u-2hydroxyethylhydrazino)-2,2'-bipyridine. 1. J. C. Dewan, W. S. Mialki, R. A. Walton and S. J. Lippard, J . Am. Chem. Soc., 1982,104, 133. 2. W. S. Mialki, D. E. Wigley, T. E. Wood and R. A. Walton, Znorg. Chem., 1982,21,480. 3. F. A. Van-Catledge,S. D. Ittel, C. A. Tolman and J. P. Jesson, J. Chem. Soc,, Chem. Commun., 1980, 254. 4. S. Clamp, N. G. Connelly, G. E, Taylor and T. S. Louttit, J . Chem. Soc., Dalton Tram., 1980,2162. 5. G. J. Palenik, D. W. Wester, U.Rychlewska and R. C. Palenik, Inorg. C h m . , 1976,15, 1814. 6. F. R. Lemke, D. E. Wigley and R.A. Walton, J. Organomer. Chem., 1983,248,321. 7 . L.-Y. Chung, E. C. Constable, M. S. Khan, J. Lewis, P. R. Raithby and M. D. Vargas, J . Chem. Soc., Chem. Commun., 1984, 1425. 8. M. G . B. Drew, Frog. Inorg. Chem., 1977, 23, 67.
Chromium(II1) is also in an unusual situation in the heterobinuclear complex (236 synthesized by the addition of CrC13-6H20to Cu(3COz-salenHz) There is ferromagneti interaction between the CUI' and Cr"' ions because of the orthogonality of the magneti orbitals (quintet-triplet separation = 120 cm-'). Chromium(II1) complexes of Schiffbases derived from pyridoxal and glycylglycine have bee prepared in solution in attempts to find complexes that show good intestinal absorption'125(se also Section 35.4.8.3).
(iii) Miscellaneous SchiJSs base and related complexes Often, in studies of complexation between metal ions and the great variety of Schiff's base and related ligands, chromium(II1) complexes are included amongst many from the fir?
Chromium
a97
(236) [CuCr(3-CO,~alen)(H~O)~]Cl.3H,0
transition series generally. These Cr**' complexes are typical of the oxidation state with no unusual properties. Consequently, only examples are collected in Table 96.
(iv) Organometallic derivatives Although tetradentate Schiff s bases stabilize Co"'-alkyl 0 bonds this behaviour is less common with C?. Several perfluoroalkyl derivatives have been obtained by Scheme 105 and the same method yields complexes of bidentate ligands too: RFCr(sal-NR)2py,RF = C2F5,C3F7, R = 4-Me&H4; RF= C3F7,R = Bun; and C3F7CrLpy where LH = salH, acacH or bzacH. The intermediates 'RpCrCi2(MeCN),' have not been isolated, but pyridine, bipy, phen and terpy analogues have (Scheme 105). An incompletely characterized mixed chloro-iodo-chromium pyridinate separates first in the reaction with pyridine and is removed before crystallization of [C3F7CrClz(py),].The perfiuoroalkyl-chromium(If1) derivatives are stable, their electronic and ESR spectra resemble those of other Cr"'-salen complexes but the magnetic moments are higher than usual. 1126~1127 Alkyl halides would not react with [CrC12(MeCN)2], but the alkyl derivatives [RCr(salen)(H20)1 and [RCr(salphen)(HzO)] (R = Me, Ph) are said to form on reaction of the organic hydrazines with the Cr"'-Schiff's base complexes in MeCN under nitrogen followed by oxidation with oxygen and hydrolysis.'lB 35db2 Miscellaneous mired donor atom ligan& ( i ) Complexes of N--0
chelating ligands
The majority of papers concerned with such complexes are synthetic studies of six-wordinate species with octahedral ligation. In general, such complexes have magnetic moments in the range of 4.0BM and electronic spectra which have usually been assigned under pseudooctahedral symmetry. Such studies are summarized in Table 97. A number of ligands have received considerably more attention; these are now considered individually. There have been a number of studies of ethanolamine (237) complexes of chromium(II1). A tris complex with deprotonated alcohol groups [Cr2(Eta *3Hz0 and a more complicated complex [Cr2(Eta)3(HEta)3][C104]3 have been prepared.l12b The l tris complex was prepared by reacting anhydrous CrC13 with 2-aminoethanol for 24 h, followed by recrystallization. The complex [Cr,(Eta)@Eta),][C1O4l3 was prepared by hydrolysis of the tris complex in aqueous ammonium perchlorate. Electronic spectra of [Cr(Eta),] were consistent with chelated amino alcohol ligands, though isomerism was not considered. The solid complex [Crz(Eta)3(EtaH)3](C104)3 was nearly isomorphous with the corresponding cobalt(II1) complex. The similarities would suggest a structure involving a hydrogen-bonded dimer with three protons holding two facial [Cr(Eta)3] molec'des together. Other studies of chromium(II1) 2-aminoethanol complexes have led to the isolation of tris complexes and dinuclear complexes of various compositions.1130,1131,1132,1134,1135
898
Chromium Table % Mixed Donor Atom Ligands: Semicarbazides, Hydrazones, Thiosemicarbazides,etc. Ligand
(228), X = Y = H; R = NHCONH,(ssaHA; tridentate, -2Hf in base X = 5-C1,5-Br, Y = H, R = NHCONH, (a), X = H , Y = H, Me, Et, Pr, X = 5,6-bcnzo Y = H; R = NHCOPh; -2H+ in base (228), X = H, Y = Me, R = NHCO-2-py (tetradentiate?) or NHCO-4-py (tridentate?) (a), X = Y = H, R = CHMcC(0H); NOH, tridentate (ZZS),X = Y = H, R = NHCSNHZ(stscH2); tridentate, -2H' in base Mixed cnmplexes of stsc and LH, LH = oxine, picolinic acid or glycine (a), Y =Me, X = 3-Me, 4-Me, S-Me, R = NHCSNH, (US), LH,: X = Y = H, R = N: C(SMe)NH, (Us), X = Y = H , R = NHCSeNH,, (sesaH,) acacH + picolinic acid hydrazid,e(acac2ph) acacH + isonicotinicacid hydrdzide, (acac2iH) acacH + 2,6-diaminopyridine
+
4NMe,C6H,CH0 2-HOC,H,CONHNH2 4-NMe,C,H4CH0 + C,H,CONHNH, saw + NH,NHCO(CH,),,CONHNH,, n = U, 1. '1 5,5-Mcthylenebis(salicyladchyde) + PhNH, 5-XC6H,CH0 + NH,NHCSNH, Diacetyl monoxime + hydrazides: RCONHNH,, R = Bu, Ph, 3-ClPh, 2-N02Ph: dihydrazides, R = CONHNH,, (CH,),CONHNH,: Diacetyl monoxime semicarbazide, thiosemicarbazide or selenosemicarbazide Pyruvic acid + NH,CSNHNH, (thpuH,)
+
Phenylpyruvicacid + NH,CXNHNH, 4-Mephenylpyruvicacid + NH,CXNHNH2
Compkxes [Cr(ssaH),]Z, Z =NO,, CI; [Cr(ssa)(ssaH)I,'NH,(K)ICr(s=)Zl Analogous 5-C1 and 5-Br complexes [CrLJCI octahedral; [CrLX], in base, X = C1, Br, NO,, five-coordinate?fiea -3.0 BM [CrLCl,](pH 2-3), p, -3.8 BM; [CrLCl],(pH 5) pea -3.3 BM, all non-electrol es [CrL(NO3)(H,O)J,2H,O, p G K = 3.6BM [Cr(stscH),CI], [Cr(stsc)(stscH)], NH,[Cr(stsa),] [Cr(stsc)L(H,O)], non-electrolytes, p =3.8BM CrL, [Cr(LH),]Z, Z = C1, NO,, [Cr(LH)L] [Cr(sesa)(sesaH)]~H20 [Cr(acac2ph)X2],X = CI,Br, NO,, NCS [Cr(acac2ih)Xz], X = CI, Br, NO,, NCS [CrLX], L = macrccycle (tetradentate?), X = CI, Br, NO,, NCS [CrLCl,]Cl, L neutrai keto form [CrLCI,]CI, L neutral keto form [CrL(OAc)l [CrL(H,O),]Q [CrL,], X = C1, NO,, MeO, NH, [ C r ( W L l , [CrL(H,O),I~,, [ ~ C ~ , ( L H , ) I [Cr(LH,)Ll, [ C ~ 2 W W ) 6 1 C [Cr2Cb(LH4)I L [Cr(LH)L] ; [Cr(LH)(H,O),]CI, (semicarbazone only)
Ref. 1
2 3 4
5 6 7 8 9 10 11 12 13 14
15 16 17,18,19
20
[Cr(thpuH)(thpu)l, NH,[Cr(thpu),l, pew -3.8 BM; [Cr(thpuH),], peB -3.1 BM, low spin ~ r "
21
x = " o][Cr(LH),]X, x=s,o
22
X = C1, Br, I, NO,
B = NH,NHCSNH,: [CryNO,),(H,O),] B = RC,H,NH,, R = H, 2-Me, 3-Me. 4-Me: CrL, Further derivatives
23 24 25
26
1. N. M. Samus and V. G. Chebanu, Russ. I . Inorg. Chem. (Engl. Transl.), 1969,14, 1097. 2 . V. G. Chebanu and N. M. Samus, Rurs. 1. Inorg. Chem. (Engl. Transl.), 1976,2l, 1807. 3. D.K. Rastogi, S. K. Dua and S. K. Sahni, J. Inorg. Nucl. Chem., 1980, 42, 323. 4. V. B. Rana, J. N. Curtu and M. P. Teotia, 3. Inorg. Nucl. Chem., 1980, 42,331. 5 . M. K. Ghosh, B. Sur and A. K. Chakraburtty, J. Indian Chem. Soc., 1984, 61,282. 6. A. V. Ablov and N. V. Gerbeleu, Rust. J . Inorg. Chem. (Engl. Traml.), 1965, 10, 33. 7. K. M. Purohit and D. V. Ramana Rao, Indian J . Chon., Sect. A , 1983,22,520. 8. M.S. Patil and J. R. Shah, J. Indian C k m . Soc., 1981,58, 944. 9. V. M. Leovac, N. V. Gerbeleu and V. D.Canic, Rum. J. Inorg. Chem. (Engl. Truml.), 1982, 27, 514. 10. A. V. Ablov, N. V. Gerbeleu and A. M.Romanov, Russ. 1.Inorg. Chem. (Engl. Transl.), 1968, W, 1558. 11. M. P. Teotia, I. Singh, P. Singh and V. B. Rana, Synth. React. Inorg. Metal-Org. Chem., 1984, 14, 603. 12. V. B. Rana, P. Singh, D. P. Singh and M. P.Teotia, Tramifion Met. Chem., 1962, 7 , 174. 13. Y. M. Temerk, S. A. Ibrahim and M. M.Kamal, Z. Nafurforsch., Ted E , 1984,39,812. 14. K. K. Narang and U. S . Yadav, Cum. SEi., 1980, 49,852. 15. A. M. Karampurwala, R.P. Patel and J. R. Shah, Angew. Makromol. Chem., 1980,89,57. 16. U. N. Pandy, 3. Indian Chem. Soc., 1978, 55, 645. 17. V. Yu. Plotkin, M. G. Felin and N. A. Subbotina, Rms. 3. Inorg. Chem., 1983, 28, 668. 18. V. 1; Spitsyn, M. G. Felin, V. Yu. Plotkin, A. I. Zhirov and N. A. Subbotina, Russ. J. Inorg. Chem. (Engl. Truml.), 1980, 25, 1654. 19. V. I. Spitsyn, M. G. Felin, V. Yu.Plotkin, N. A. Subbotina and A. I. Zhirov, Rum. J . Inorg. Chem. (Engl. Traml.), 1982, 27, 1278. 20. V. Yu. Plotkin, M.G. Felin, N. A. Subbotina and V. V. Zelentsov, R u s . J. Inorg. C k m . (Engl. Trawl.), 1983, 28, 825. 21. A. V. Ablov and N. V. Gerbeleu, Russ. J . Inotg. Chem. (Engl. Transl.), 1970, 15, 952. 22. S . Chanda and K. B. Pandeya, Gazz. Chim. I t d , 1981,111,419. 23. A. K. Rana and J. R. Shah, Indian J. Chem., Sect. A, 1982, 929. 24. A. K. Rana and J. R. Shah, Indian J . Chem., Sect. A , 1982, 21, 177. 25. N. R. Shah and J. R. Shah, Indian J . Chem., Sect. A , 1982,21, 312. 26. H. S. Verma, A. Pal, R. C. Saxena and J. L. Vats, J. Indian Chem. Soc., 1982,59,11f34.
Chromium [CrC12(MeCN)2]
I
I
Nz. RFI
‘RFCrC1,(MeCN),’ f ‘CrCl,-,I,(MeCN),’
orange
LH,, M e a . NEt,, then remove solvent
residue
C , F , C ~ C ~ , ( P YO)r~C3F7CrC12(bipy)2
1
green
\
C&I, EtOHIC6H6, Nzv 0°C. then py or bipy
bipy. phen or tcrpy
899
I
’
py
bipy) [C3F,CrCl(bipy)2]PF,
[RFCrLpyl red
L = salen, RF= CF3, CZF5,C3F7
L = salphen or acacen, RF= C2F5, C3F, Scheme 105
Table 97 Complexes with N,O Chelate Donors Ligand
Form
Complex
Ref.
1. E. Garcia-Espana, 3. Moratal and J. Faus, J . Coord. Chem., 1982, E?, 41. J. J. Habeeb, D. G . Tuck and F. H. Walters, J . Coord. Chem., 1978, 8, 27. P. K. Biswa, M. K. Dasgupts, S. Mitra and N. R. Chaudhuri, 1. Coord. Chem., 1982, 11,225. A. G. Gllinos, J. M . Tsangaris and J. K. Kouinis, 2.Nuwfonch., Ted B, 1977,32,645. $. K. Sengupta, S. K. Sahni and R. N. Kapoor, Synrfi. React. Inorg. Meld-Org. Chem., 1983, U, 117. 6. C. Preti and G. Tog, Aut. J . Chem., 1980,33,5?. 7. B. Chatterjee, . I . Inorg. Nucl. Chem., 1981,43,2553.
2. 3. 4. 5.
Picolinic acid (pyridine-2-carboxylicacid) complexes of chromium(II1) have been the subject of a number of studies. Complexation by picolinic acid in watedethanol (30% v/v) follows an ion-pairing, Eigen-Wilkins type mechanism.1136 Activation parameters suggest an associative character for the reaction of the aqua complex. Chelated complexes of chromium(II1) and picolinic acid are the products of the rapid, inner-sphere reduction of [Co’II (pico)(NH3)5]2+ with chromium(I1).l13’ The reaction of the related 4-carboxylic acid complex of cobdt(I1I) with chromium(I1) is also rapid; in contrast, pyridine-3-carboxylic acid (nicotinic acid) complexes undergo slower reactions. A p-hydroxy-bridged dimeric complex [Cr2(pico),(OH)z] has also been prepared. A study of magnetic properties in the temperature range 16-300K leads to J = -6 cm-I and g = 2, typical for such complexes.’138 Complexes of 8-hydroxyquinoline and chromium(II1) have been known for many years. 113’ Complexes have been mercurated and the deuterated quinoline isolated.113’ The bromination of chelated 8-hydroxyquinoline proceeds about thirty-five times faster than that of the free ligand.’lN Tris(8-hydroxyquinolinato)chromium(III) absorbs large amounts of hydrochloric, hydrobromic and hydrofluoric acids. Chemical reaction with the complex was considered a more likely explanation than solid solution or clathrate formation, even though more than one mole of acid was absorbed per mole of complex.114’ The complex diaqua~2,6-diacetylpyridinebis(semicarbazone)]chromium(III)hydroxide dinitrate hydrate (238)has a most unusual Chromium(II1) is coordinated in an approximate pentagonal bipyramid (PBP), with the ligand forming the pentagonal plane and
Chromium
900
two water molecules in the axial positions. The solution magnetic moment is 4.05BM. An interesting feature of such a complex is that a regular PBP would lead to a single unpaired electron in the degenerate d,z+ or dxy orbitals. The distortion of the ligand plane removes this degeneracy and provides the first examples of the Jahn-Teller effect in a pentagonal bipyramidal complex (Table 95).
The coordination of chromium(II1) is also unusual in the dimeric complex [Cr{H(chbaEt)) (py)&.2py (239)519(H4(chba-Et) = 1,2-bis(3,5-dichloro-2-hydroxybenzamido)ethane).The structure of this complex is described in Section 35.4.2.11.
O \
(ci
Cl’
n HN-C
C-NH
‘OH
/O
H O b C l
c1
C’.
(ii) Chelating ligands containing sulfur and related donor arom There are a number of papers reporting the synthesis and characterization of monomeric complexes of this type; the more significant and/or recent are summarized in Table 98.
Table 98 Complexes with S,O and N,S Chelate Donors
Ligand
Form
Complex
Re5
1. R.A. Haines and J . W. Louch, Inorg. Chim. Acta, 1983,n,1. 2. W.Shibutani, K . Shinra and C. Matsumoto, J . Inorg. Nud. CAem., 1981,43, 1395. 3. S. Chandia, K.B. Padeya and R. R. Singh, J. Inorg. Nucl. Chem., 1980,4,1075. 4. V. V. Savant, J. Gopalakrishnanand C. C. Patel, Inorg. Chm., 1970,9,748. 5 . S . Chandra and K. K. Sharma, Synrh. React. Inorg. Metal-Org.Chem., 1982,12, 647. 6. R. J. Balahura and N. A. Lewis, Inorg. Chem., 1977, 16,2213.
The complex bis(2-mercaptoethylamine)ethylenediaminechro~~m(III) perchlorate has beer prepared by the reaction of cystamine with aqueous solutions of chromium(I1) in the presencc
Chromium
901
The complex crystallizes from aqueous solution to form opaque violet of rectangular bipyramids, the crystal structure of which has been determined. The reaction was said to go via an intermediate (240) to the chromium(II1) complex (241) which underwent rapid ring closure. The stereochemistry of (241)is dictated by the strong Jahn-Teller effect in the chromium(I1) precursor (240). A series of thiolato complexes, e.g. [Cr(H20),SCH2CO2]', has been oxidized with H202.976The chromium(II1) complexes thus formed, in contrast to the cobalt(II1) analogues, decompose, presumably via an unstable sulfanato-chromium(II1) intermediate.
I
0
H' H' (240)
The complexes of the monathio-p-diketonate RC(SH)=CHCOR' (R = R' = phenyl; R = phenyl, 2-thienyl, @naphthyl; R' = CF3), which are acetylacetonate analogues have been synthesized (M2).1'43 Dipole moment measurements are consistent with facial structures (243). Mass spectra indicate no metal-containing peaks for the complex [Cr(PhC(S)=CHCOPh),)]: however, for the fluorinated monothio-j3-diketonates, various metal-containing peaks, e-g. M-2L==F7 were observed. Such ions involve fluoride migration. Monothiooxalate complexes of chromium(II1) have been prepared; fairly unusual complexes, exemplified by [Cr((QSO3)Cu(Ph3),},], were reported. On refluxin under chloroform (7 h) reactions of the kind illustrated were alleged to occur (equation SS).'fh"
(244)
(245)
(246)
The complexes formed between thioglycolic acid (mercaptoacetic acid) and chromium(II1) have been in~estigated."~~ In alkaline solutions above 60 "C, a blue green complex [Cr(SCH2C02)3]3- is formed. In acidic solutions, two red compIexes [Cr(H20)4(SCH2C02H)]2+and [Cr(H20)2(SCH2C02H)2]+are predominant with a species [Cr(H20)2(SCH2C02)2]-detectable at neutral values of pH. The rates of ring opening and closure for the chromium(I1I) mercaptoacetate complex [Cr(H20)4SCH2C02]+have been measured."& A large number of Complexes with arsenic-containing ligands have been reported (247). Three types of cornpiex of formula CrO(o-R2As&H4C02).nH20, Cr(Ph2AsC&C02)2(OH).2.5H20 and C ~ ( O - R ~ A S C ~ C O ~ ) ( O H )were ~ - ~ H prepared. ~O The authors concluded that arsenic was probably not coordinated to the chromium in any of these c~mplexes.'~~'
0::;; (247)
902
Chromium
35.483 Amino acids
(i) Potentially bidentate amino acids Complexes of simple amino acids with chromium(II1) were first prepared by Ley.114BThe isomers possible for tris chelated complexes of this type are illustrated below (248-251). The consequences of such isomerism were first seriously considered by Gillard.1149 Red complexes of the formulae [C r(gl~)~] and [Cr(~-ala)~] were prepared by neutralizing refluxed solutions of hexaaquachromium(II1) and the amino acid in ratios between 1 :5 and 1:10. These complexes were shown to be isomorphous with j3-[C0(gly)~]and ~-B-[Co(~-ala),] respectively. The crystal structure of red b-[Cr(gly)s] has also been reported.llS0
(250) A-b
(248) A-LY
(251) I\-@
There is some controversy concerning the existence of the cy isomers of these tris complexes. Israilyllsl reported a purple complex to be a-[Cr(gly),]; however, subsequent workers have shown that this substance most probably was the dihydroxy dimer [Cr2(gly)4(0H)2] Careful chromatography, on potato starch, of solutions from chromium(III)/glycine reactions yielded red and purple fractions,’153the electronic spectra of which were consistent with /3 and CY isomers respectively. Solutions of the ‘acomplex’ were unstable even in the dark and cold. Hoggard has recently the preparation of the a isomer of the glycine complex by a fractional crystallization. The complex was anhydrous, unlike its cobalt(II1) analogue. X-Ray powder methods could hence not be used to confirm the identity of the complex; the luminescence spectra were held to be consistent with meridional coordination. There have been a number of studies of the physical properties of /3-[Cr(gl~)~], summarized in Table 99. .1149,115231153
Table 99 Physical Properties of Tris Amino Acidates
Crystal structure AH; (kJ mot-’) Pcff
(BM)
Electronic spectrum 70% HClO, Amax (nm) 9 (em)
CD, 70% HCIO, Arnax/min,
(As,)
o-/3-[Cr(Ldah]
B-lC4dYAI
Ref.
-
P2,lc, 2 = 4
1 2
2380 3.83 534 (46.6) 396 (39.5) 512(+1.00) 452 (-0.25) 418 (+0.08) 382(-0.19)
2264 3.82 3.89
3 4
539 (44.6)
400 (37.8) -
-
3
-
1. Ref. 1150. 2. C. E. Skinner and M. M. Jones, h o g . Nucl. Chem. Len., 1967, 3, 185. 3. Ref. 1153. 4. T. Morishita, K. Hori. E.Kyuno and R. Tsuchiya, Bull. Chem. Soc. Jpn., 1965,88, 1276.
Many other amino acids form complexes which are red fi isomers with ‘glycine-like’ coordination at chromium(II1). The complexes are most frequently prepared by refluxing the amino acid with chromium(III), followed by neutralization. The problem with this kind of method is that significant quantities of the corresponding bridged hydroxy complex are formed; indeed the solid tris complexes are converted to such complexes on standing under water. Repetition of the literature preparations of these complexes is often dimcult and the ease of hydrolysis/kinetic inertness of chromium(II1) probably leads to some of the discrepancies in the literature, An approach which potentially avoids the problems of hydrolysis is solid-state synthesis. Amino acid complexes have been prepared by heating (135°C) solid hexaamminechromium(II1) nitrate with the amino acid followed by recrystallization. Complexes of glycine, ( f)-alanine, ( f )-isoleucine, ( f )-leucine, ( f )-aminobutyric acid,
903
Chromium
( f )-norvaline and ( iz )-valine'15s have been prepared by this method as have the corresponding L-amino-acid complexes.'156 In general, P-fm isomers were obtained. In related work, complexes of various basic amino acids were pre ared.1157The tris complex of ( f )-methionine has been prepared by conventional methods.' 58 A number of simple mixed complexes of amino acids with chromium(II1) and aminoethanol or 2,2'-iminodiethanol have been reported. 'I5
P
(ii) Hydroxy -bridged Complexes These are extremely common and can be a problem when the synthesis of the related tris complexes is being attempted, articularly as they are of rather limited solubility. They have been known for many The isomers possible for these species are profuse.1L49 For the general formula [(aa)zCr-(p-OH)2Cr(aa)2] there are three possible geometric configurations about each metal ion: trans N cis 0, cis N trans 0 and cis N cis 0. In addition, each metal ion is a dissymmetric centre bringing the total number of g6ometric and optical isomers to 21. A crystal structure of the all-trans isomer of [Cr2(gly)4(OH)2]has been reported.'l6' Its low temperature magnetic susceptibility has been fitted to both the Van Vleck and modified Van Vleck models. The uncorrected model leads to peff= 3.80 BM with 2J = 8.4 cm-1.11623116371164 with the inclusion of quadratic exchange W =7.4 cm-l.l'" Relate: studies valine, phenylalanine, leucine proline1166z1167and histidinella comof plexes have appeared. The proline complex is unusual in that it is soluble in methanol and DMSO. Circular dichroism spectra have been measured; the X-ray structure shows the complex to be the ~-tran~(N), ~-truns(O)isomer. 'Ih6 More complicated dimers of unusual stoichiometry have been reported. 1169
year^."^""^^^"'
,116373149
(iii) Solution studies Stability constant determinations are few;117othey are summarized in Table 100. Complexation by acidic amino acids is obviously of relevance to the tanning of leather; the stability constants for L-glutamic and aspartic acid1171complexes are much greater than those for glycine or alanine."^^^^^^^^^"^ This is probably because the acidic amino acids form tridentate complexes. In contrast, cysteine1173appears to form glycine-like complexes in moderately acidic solution; however, in the solid state L-cysteine is known to be tridentate (vide infra). Table 100 Typical Equilibrium Constants of Amino Acid Complexes" Ligand L-Aspartic
acid L-Glutamic acid cr-A 1anine Glycine
Kl
K*
12.15 10.1 11.39
8.98 9.5
8.53 8.62
Ref.
50 "C
7.57
L-Cysteine oL-Methionine
8.05
7.44 7.65 6.4 7.45
7.45
6.45
Glycine
7.60
8.4
Conditions
-
25 "C 25 "C
40 "C 0.4 M NaCIO,
alogl, values in 0.1 M NaCIO,, except ref. 1174. 1. Ref. 1171. 2. M. Mizuochi, S. Shirakata, E. Kyuno and R. Tsuchiya, Bull. Chem. SOC.Jpn., 1970,43, 397. 3. Ref. 1172. 4. A. A. Kban and W. U. Malik, J . Indian Chern. Soc., 1963, 40,565. 5. Ref. 1173. 6. Ref. 1174.
Kinetic studies have concentrated on the glycine system.1174,'175,1176 In the pH range 3.0-3.8 (40 "C), the reaction proceeds by acid-dependent and -independent paths; reaction via the hydroxy complex is probably I,, whereas the aqua complex appears to undergo I, substitution.
904
Chromium
In more acidic solutions, monodentate 0-bonded complexes are formed.'177,1175~1178 Ring closure is rapid if the pH is raised; y, S and E amino acid complexes do not undergo ring closure.1179 (iu) Potentially tridentate amino acids For tridentate amino acids with three non-equivalent donor atoms such as L-aspartic acid or L-cysteine, the isomers possible are illustrated below (252-254). There have been a number of reports of the preparation of L-aspartic acid complexes.1180,"g'~1182. In the earlier work the isomers were not identified, however in the later study, the complexes were tentatively identified by comparison of their spectroscopic properties with those of the corresponding cobalt(II1) The order of elution of the complexes on HPLC was also similar to that observed for the corresponding cobalt(II1) complexes. Mixed complexes containing L- or maspartate and L-histidine were also prepared.1182A crystal structure of one salt obtained from this kind of system, bis(L-histidinato-O,N,N')chromium(III) nitrate, has been determined.Ilp4
Sodium bis(L-cysteinato)chromate(III) dihydrate has been prepared by refluxing L-cysteine with chromium(II1) nitrate and neutralizing the solution.ll8'. The product is a dark blue solid in which cysteine is coordinated with the sulfur atoms trans (see 252-254). A number of chloro and other complexes of cysteine and related amino acids have been A related complex L-histidinato-D-penkillaminatochromium(II1)has been prepared'lS7 and its crystal structure reported. The reaction of chromium(111) with D-gencillamine has been investigated.'lxXChromiurn(II1) complexes are also rapidly obtained as the products of the reaction of chromium(V1) with D-penicillamine. Several complexes were prepared including a monomeric (S,N,O chelate); equilibrium constants were measured. These studies may be relevant to the treatment of chromium(V1) toxicity with D-penicillamine (see biological chromium(V1) chemistry p. 947). Unusual S-bonded complexes of chromium(II1) have been prepared by reacting aqueous solutions of chromium(I1) with [ C ~ ( e n ) ~ ( ~ - c y s )or ] ~ +the corresponding methionine complex.1189Under the fairly acidic conditions used, S,O [excess of chromium(II)] or S,N [limiting chromium(II)] complexes were obtained; these both eventually rearranged to give 0 , N species. ( v ) Chromium(IIZ) and glucose tolerance
Chromium was recognized as an essential trace element in 1955.1190Rats fed a chromiumdeficient diet developed an impaired tolerance for intravenous glucose, which could be reversed by an insulin-potentiating factor present in brewer's yeast, meat and various other foods. The insulin-potentiatingfactor was found to be a complex of chromium(III)1191and such substances have been termed Glucose Tolerance Factor(s) (GTFs). Chromium was demonstrated to be essential for humans in 1975."92 There are several reviews of the chemistry of chromium(II1) and its relationship to glucose A detailed procedure for the extraction of GTF from Saccharomyces carlsbergenis has been reported,1197involving ethanolic extraction, hydrolysis and ion exchange chromatography. Nicotinic acid (255) was detected in the complex by its characteristic UV and mass spectra. Glycine, glutamic acid and cysteine together with trace amounts of other amino acids were found in the purified sample of GTF. This is probably the most refined GTF yet isolated. Although IR spectra were recorded, no detailed spectroscopic studies were undertaken, which is unfortunate as studies of the ligand field spectra might help to suggest likely structures.
Chromium
905
Nicotinic acid is present in the more active GTF preparations isolated from yeasts. The coordination chemistry of this ligand is particularly relevant to glucose tolerance and the presence of this substance is apparently essential for the maximal activity of complexes in tests in vitro. The instability of highly purified GTF fractions has frequently been noted; this may arise because the substance in vivo is stabilized by a protein. The synthesis and characterization of materials showing biological activity similar to that of GTF isolated from yeast is a logical objective. As already mentioned, Mertz found aqua and similar Complexes of chromium to be more active than chelates with strong ligands. An exception to this was an unspecified cysteine complex,1193prepared by C. L. Rollinson, which showed marked, but erratic, behaviour in GTF tests, Further investigation of this observation would be interesting, particularly as the crystal structure of a cysteine complex is now known.1185 by reacting A complex with considerable activity in tests in vitro has been synthe~ized"~~ chromium(II1) acetate in 80% alcohol with two equivalents of nicotinic acid followed by the addition of one equivalent, in turn, of glycine, glutamic acid and cysteine. A complex containing nicotinic acid was isolated from the reaction mixture by ion exchange chromatography. The physical properties and biological activity of this complex were very similar to those of GTF isolated from yeast.1197Although this material is probably a mixture, it should be further investigated as it is one of the most potent synthetic materials yet obtained. The amino acids believed to be involved in GTF are the constituents of the naturally occurring tripeptide glutathione (256). Anderson et al. have reported a complex of glutathione/nicotinic acid and chromium(II1) to be particularly active in their in vitro assay.1198 The complex was described as being purified by HPLC, but the details of the preparation are not available.
(256) Glutathione (GSH) (shown as [€I&]-)
Recently, three different kinds of compounds containing chromium(II1) and glutathione have been synthesized:llWa bisglutathione complex K2[Cr(H3L)(H2L)],mixed complexes with the amino acids L-cysteine, L-glutamic acid and t-aspartic acid K2[Cr(HtL)(A)] (A is the dianion of the amino acid), and a mixed complex with glycine Kz[Cr(H2L)(gly-O)(OH)].All exhibit an intense UV charge transfer band characteristic of a Cr-S bond. The sulfhydryl to chromium linkage undergoes an acid-catalyzed hydrolysis. The complexes have been characterized by elemental and thermogravimetric analysis, electronic and IR spectroscopy and circular dichroism. Comparison of these properties with those of known chromium(II1) complexes leads to the conclusion that glutathione is bound to chromium(1II) by the Q-terminal glycine group {N,Q} and the deprotonated sulfur of cysteine. The glutamic acid residue does not apparently interact with the chromium centre; the suggested mode of coordination of glutathione is illustrated in (257). The lack of structural information means that any suggested structure for GTF must be highly speculative. Mertz has suggested12mthat two trans nicotinic acids are coordinated to chromium as illustrated in (258).The first coordination sphere is completed by the various amino acids involved. There is little evidence to support this and it seems unlikely that the hard chrornium(II1) centre would bind preferentially to the nitrogen group of nicotinic acid; the idea of N coordination has attracted support and influenced other workers. One product of the reaction of nicotinic acid and chromium(II1) (mole ratio 3:1, pH3.2) has been said to be
Chromium
906
(257) Suggested coordination mode of glutathione R = NHCOCH,CH,CHNH2COzH
tetraaqua-trans-bis(nicotinate).1200It showed some biological activity but no spectral or other characterization was reported. It would be surprising if chloro ligands were not coordinated in this corn lex and nicotinic acid is most likely to be 0-bonded. Indeed, Legg has recently shown ,lZ8 using deuterium NMR, that carboxyl-bonded nicotinic acid complexes are readily formed in aqueous solution. A crystal structure of a pentaammine nicotinic acid chromium(II1) complex shows 0-bonded nicotinate,'m which again supports the dominance of 0-bonded nicotinic acid in chromium(II1) chemistry.
glycine NH2?
cysteine COT? SH?
co;?
(258)
The view that N-bonded trans nicotinic acids are involved in GTF has also been ~ h a l l e n g e d ~ ~from ~ ~ *a ~more ~ " biological viewpoint; an 0-bonded nicotinic acid complex has been shown to have some biological activity. The biological significance of chromium(II1) was suggested to relate to the ability of the relatively hard chromium(II1) centre to form stable complexes with the carboxylate group of nicotinic acid. The crucial grouping recognized by the biological system would hence involve the nitrogen of the 0-coordinated nicotinic acid (259-262). The surface topology of the complex may be the most important factor and the effect of 1,Qdiguanidinobutane (262) on yeast metabolism, which is similar to that of chromium(II1) complexes, may be explained on this basis.'204
6 H +'
Chromium
907 NH
\
/
“H
’CH~-CH-CNH~ II
y
N
H
2
II
Suggested structures for biological activity
( v i ) Chromium and fanning Knapp discovered in 1858 that chromium chloride converted raw skins into leather, but he failed to realize the commercial significance of his discovery. Others developed his ideas. A two-bath process due to Schultz (1894) and an adaptation of Knapp’s original one-bath process by Dennis (1893), meant that by the early twentieth century chromium tannage was commercially important. At present it accounts for the vast majority of leather production. In the tanning process hides are first washed or soaked, hair and keratinous debris are removed, bated (enzymes are used to break down non-collagenous components, which are washed out) and the hide is acid-pickled to prepare for the addition of the chromium salt. Contemporary processes are exclusively based on one-bath procedures and utilize chromium(II1). The older two-bath process is now obsolete, mainly because it involved the in situ reduction of chromate, a major environmental and toxicological hazard (cf. chromate toxicity p. 947) to chromiurn(II1) on the hide. A useful review of the history of chromium tannage processes is a~ai1able.l~” The most popular tanning solution in current use is termed 33% basic chromium(1II) sulfate and corresponds to the empirical formula CrOHS04. In older procedures, chromate was often reduced at the plant using glucose/sulfuric acid mixtures. At present, 33% basic chromium sulfate is rovided as either a solution (chrome liquor) or as a commercially available powder,”’of constant, but at present slightly uncertain, chemical composition. The composition of such tanning liquors has been investigated.lZw As many as 12 components have been identified, including monomeric di- and oligo-nudear complexes involving hydroxide andlor sulfate bridges. Chrome tanning involves the careful control of the hydrolysis of chromium(II1) (cf. 0 donors p. 857). The tanning mixture is introduced to the pelt at fairly acidic values of pH (2.5-3.0). The pH is then increased and the chromium reacts with and is fixed to the collagen. The low pH of introduction minimizes the processes of olation and oxolation (see 0 donors), which could easiry lead to oligomeric and even insoluble material unable to penetrate the hide. At low pH the collagen is protonated and this also favours the penetration of the chromium complex into the hide. The hydroxy complexes, formed on raising the pH, are more reactive than aqua due to the trans effect, a point not generally mentioned in the literature of tanning. The unfavourable effects of hydroxide are thus balanced against the requirement for a reasonably rapid reaction. Olation may further be limited by the use of a complexing (‘masking’) agent. However, the reaction of chromium with collagen will be less favourable if the metal is strongly complexed.1208,1209 Carboxylates of various kinds have been used as masking agents in tanning,lZmbut the present trend seems to be to use ‘simple basic sulfate solutions’ or relatively low concentrations of carboxylates such as formate under carefully controlled conditions. An exception to this is the preparation of special leathers, where a suitable chromiumconplexing agent, e.g. a hydrophobic ligand, may be used to impart a required property, e.g. shower resistance or lubrication, to the finished hide*208and similarly the use of fiuorocarboxylates to produce soil-resistant leathers.
9008
Chromium
Gustavson first recognized1210that chrome tanning was due to the reaction of chromium with the carboxylic acid groups of collagen. Collagen, the princi a1 component of hide, is an unusual protein consisting of a thin rod cu. 30008, long and 14 thick, composed of three peptide chains of equal length each containing about 1000 amino acid residues. Glycine accounts for about 30% of the amino acid residues and there are large quantities of proline and hydroxyproline. Metal ion binding to collagen has recently been reviewed.'*'l Collagen fibrils tanned with 33% basic sulfate show a distinct cross striation (electron microscopy, 50 OOO :1), presumably owing to an increased ordering of the collagen r n o n o m e r ~ . ' ~ 'The ~ , ~exact ~ ~ ~ way in which chromium is incorporated into collagen is not yet fully understood. In the tanning process, chromium may be envisaged as reacting with the free carboxylates of the protein to give three distinct kinds of coordinate linkage: (1) with a single carboxylate (W),(2) with cross-linking between strands via a single chromium (M), or (3) with cross-linking between strands by a di- or oligo-nuclear chromium species polymerized either via 01, oxo or sulfato bridges, or a combination of them (265). The relative importance of the different kinds of interaction has not been fully established. Bridging by di- and oligo-nuclear complexes, as illustrated in (269, is believed to be the most important interaction for the tanning of leather, although as much as 90% of the bound chromium may be bound only to a single collagen strand as in (263).
w
TProtein
T P r o t c i n
Y P r o t e i n
0
0
\ /
c
Schematic representations of possible collagen interactions, L may be a masking ligand, water or hydroxide
In (265) the bridging ligands, shown as 01, could
be oxo or sulfato. There are obvious possibilities for isomerism in this interaction
The chemistry of tanning is very complicated. Although reliable procedures have been developed and the general principles of the chromium-collagen interaction are well understood, there is the possibility for much new work in this area.
35.4.8.4 Complexones: edta, pdta, nta and related ligands
(i) Ethylediaminetetraaceric acid (edtu) A violet complex [Cr(Hedta)H20] was first prepared in 1943 by Brintzinger, Thiele and Muller.1214It was transformed on the addition of base to a blue complex, then formulated as [Cr(Hedta)OH]-. Thus, even from the earliest studies, edta in its chromium(II1) complex was thought to be pentadentate; this has now been confirmed crystallographically (266). 1215 Coordination at chromium(1II) is a distorted octahedron; the average deviation from 90" is 5.1". The currently accepted protonation schemes for chromium(II1)-edta complexes are Deuterium NMR spectroscopy has been used to confirm illustrated in equations (56)-(61) these solution eq~ilibria.'~'' As well as the quin uedentate ones,1214,1215 complexes in which edta is of lower denticity have been prepared."'$ [Cr(Hedta)H20] reacted with hydrochloric and hydrobromic acids to give complexes of formulae [Cr(H3edta)Xz(H20)]*3H20. The most likely structure was considered to have one of the nitrogens and two of the carboxylates of an edta ligand uncoordinated. The equilibrium constant (log K) for the formation of the quinquedentate complex is 23.40.I2l9Perhaps the most remarkable feature of chromium(II1)-edta chemistry is the kinetic
909
Chromium
O H
(266) Molecular structure of [Cr(H,O)Hedta]
Protonation Scheme for Chromium(II1) edta Complexes (after ref. 1216)
--
€6
[Cr(Hedta)H,O] [Cr(edta)H,O]- + H' [Cr(edta)H,O][Cr(edta)(OH)I2-+ H+ [Cr(H2edta)(H20)2]+ [Cr(Hedta)(H,O),] + H+ [Cr(Hedta)(H,O),] [Cr(edta)(H,O),]- + H+ [Cr(edta)(H,O)J[Cr(edta)(OH)(H,O)]*- + H ' [Cr(edta)(OH)(H,O)]'ICr(edta)(OH)2]3-+ H'
3.1
(56)
7.5
(57)
2.49 (58)
--
2.92 (59)
6.16 (60) 8.0
(61)
lability of the coordinated water of [Cr(Hedta)H20]. The exchange of this water occurs on the stopped-flow time-scale, and has hence attracted a large number of kinetic studies, summarized in Table 101. Chromium(II1)-edta complexes are typically preparedluO by refluxing the ligand with a suitable chromium(II1) salt for about half an hour followed by recrystallization. The formation of [Cr"'(Hedta)H20] from acetato, formamato and trifluoroacetato chromium(II1) complexes Table 101 Rate Constants of the Aquation Reactions [kbr(s-I)] of [CrX(H,O),l2', Complexes at 25 "C and I = 1 M (after ref. 1224)
X-
K
Complex
[CrX(NH,)5]2+ and fCrXY]("-2)
OAc-
ONO-
c1-
NCS-
Br-
~~~
[CrX(H O),],': [CrX(Nh,),] [CrX(edtra)][CrX(medtra)][CrX(hedtra)][CrX(aeedtra)][CrX(Hedtajj[CrX(edta)]
4.1 x lo-' 3.6 X
a
9.2 x
4.1 x 1 0 - ~
8.7 x (2.40f 0.17) X
(1.63 f 0.13) X (5.89 0.28) x 10-58 (8 3) x 10-4h 0.189 f 0.01ZhSi 0.450 0.03oB 6.24 f 0.32h 0.244 f 0 . 0 1 9
** 0.38f 0.06'
13.4"
5.4 f 0.6g
*
2.8 x 7.4 x
a a
4.3 x 10-6 a 8.1 x 1 0 - ~
0.697 & 0.044jsk 2.0'
(3.17 k 0.33) X lo-' 26.8 It l.gh
'Ref. 203. bE.Deutsch and Hi Taube, Inorg. Chem., 1968, 7, 1532. C. Postmus and E. L. King, J.h Phys. C+m., 1955, 59 1216. I = 0.2 Mi. ' I = 0.07M. T. Ramasami and A. G. Sykes, Inorg. Chem., 1976, 15, 2885. 'Ref. 1223. Ref. 10. A H f p 60.4 i 1.2kJmol-', AS1=-55f43mol-'K-'. Ogino, M. Shimura and N. Tanaka, Bull. Chem. SOC. Jpn., 1978, 51,1311380. AH*= 53.9 f 3.3 kJ mol- , AS* = 68 f 11J mol-' K- . I H. Ogino, M. Shimura, A. Masuku and N. Tanaka, Chem. Len., 1979,71. Y. S. Sulfab, R S. Taylor and A. G . Sykes, Inorg. Chem., 1976,15,2388;edtra = e t h y l e n e d i a m i n e - N , ~ ' , ~ - t nmedtra a ~ ~ ~=, N-methylenediamine-N,N',N'-tri acetate, hedtra = N-(hydroxyethyi)ethylenediamine-N,N',N'-t~a~ta?e, aeedtra = N-(acetoxyethyl)ethylenediamine-N,N',N'-t~acetate.
'p.
Chromium
910
has been studied;lZ2’ the acetato complex reacted 103 times faster than hexaaqua chromium(II1). lzz” The thermal decomposition of [Cr(NH3)6]Naedta-3H20 provideslZ2* an interesting method of synthesizing [Cr(Hedta)H20].
(ii) Other complexones derived from ethylenediamine Ligand exchanges with a variety of ligands closely related to edta (267,268 and 269) are rapid .1223,1224 Typical results are summarized in Table 101. Chrornium(II1 ethylenediamineN,N‘-diacetato complexes [Cr(edda)(HZO),]+ have been synthesized.lZ2 CrC13-6Hz0 was reacted with Hzedda and neutralized; separation of the isomers (270, 271 and 272) by ion-exchange chromatography was attempted. On the basis of electronic spectra and comparison with the cobalt(II1) system, the first complex to elute was assigned as &-cis(94%), the second band as p-cis (6%). The equilibrium constant for this system is log K = 12.42 (room temperature) ,lZz6 presumably an overall value for both isomers.
3
-OXCHz.,
,CH,CHzOH
-0ZCCHz /
NCHZCHZN,,
(267)
-O,CCHZ,
/
CHZCO,
-O,CCH,
/
NCHZCHZN,
/
H
CH,COi
(268) edtra3-
hedtra3-0FCH2 ,_
CK
:1
;NCHzCH2N -
O,CCH,’’
’‘CH,COY (269) medtra3-
(270) trans
(272)
(271) a-cis
P-ks
(iii) Other complexones derived from polyamines In contrast to edta, the complexes of 1,3-propyienediaminetetraacetic acid are sexidentate.12” This may well be because ring strain is reduced (273) by the additional -CH2 group in each chelate ring. The complex (2S,4S)-2,4-pentanediaminetetraacetatochromate(III) monohydrate has been preparedIm and forms stereospecifically. By comparison with Co”’ analogues, absolute configurations have been assigned. Complexes with ethylenediamine“,“-diacetic-N,N’-dipropionic acid (H4edda) and S,S-2,2-(ethylenediimino)disuccinic acid (H,S,S-edds) were also made.’228 An extensive study of triethylenetetraaminehexaaceticacid complexes of chromium demonstrated both mononuclear, dinuclear and heteronuclear chelates with copper(II).1229 0 II
0
II
-O-CCH, ,.
-WCH,’
CcH2-0-
/
N( CHZ)3N,
II
CCHrO-
I1
0
0
(273) pdta
(iu) Imino acetic acids Several complexes with iminodiacetic acid (Hzida) and methyliminodiacetic acid (Hgnida) have been ~repared.’”~ The bis mida complex exists only as the trans isomer, ida forms bis cis N complexes; this is also the arrangement adopted in the mixed mida/ida complex.
91 1
Chromium
Spectroscopic measurements were used to support these assi nments. Related studies of truns-bis(N-isopropyliminodiacetato)chromium(III) dihydrate123qJ231 support the earlier suggestions'225of specificity in the coordination of mida. The oxygen-18 exchange of fruns-(fuc)-bis(N-methyliminodiacetato)chromate(III) has been studied in detail. -'. Acid-dependent and -independent paths are involved; one-ended ligand dissociation was held to be important. The equilibrium constants for complexation of ida are ( I = O , 25°C) logK1 =9.30 and logK2=7.14.'233 The chromium complex of 2hydroxyethyliminodiacetic acid (H20H-A), K[Cr(HO-A),] ,1234 has been made. It is approximately octahedral and the hydroxy groups are not deprotonated.
(v) Nitrilotriacetic acid
Complexation of chromium(II1) by nitrilotriacetic acid (Hznta) has been investigated. Values of equilibrium constants (I = 0, 25 "C) are log K1 = 10.66 and log K 2 = 8.73. 1234 A large number of mixed complexes with nta/chromium(III) have been prepared, including mixed aminoacidates,1235acetylacetonates, catecholates, oxalates and complexes with 1,lo-phenanthroline and 2,2'-bipyridyl. 1 2 3 ~ 1 2 3 8
( v i ) Sulfur-containing ligands Two different isomers (274 and 275) have been obtained from the reaction of thiobis(ethylenenitri1o)tetraacetic acid (tedta) with CIT(CIO~)~, depending on the pH of the reaction. The cis isomer has pentadentate (cf. edta) ligation, the trans isomer is sexadentate; the equilibrium is pH-dependent . The related complexes of thiobis(ethylenenitri1o)tetraacetic acid have also been investigated.1240 The above studies were directed to finding ligands which would bind to the surface of electrodes by spontaneous absorption. Although the chemistry of these systems is complicated, this objective was achieved in both studies.
'0 (274)
35.4.9
(275)
Multidentate Macrocyclic Ligands
35.4.9.1 Porphyrins and corrins: oxidation states 11 to V Chromium porphyrins constitute a comparatively new and important class of compound. In order to avoid an artificial separation, complexes of various oxidation states starting with chromium(I1) are dealt with in this section and some chromium(V) salen derivatives are included. Electronic, ESR and IR spectra are routinely recorded to characterize chromium porphyrin complexes and chromatography is extensively used in their purification; for details the original references should be consulted. The chemistry of the porphyrins is covered in a recent series.lU1
(i) Chromium(II)porphyrin complexes The chromium(I1) complex of mesoporphyrin IX dimethyl ester (276) can be prepared by metal insertion with an excess of chromium hexacarbonyl under nitrogen.12" After removal of solvent and exttaction with toluene, Cr"(MPDME) is crystallized by the addition of n-pentane. Presumably tht weakly acidic NH protons of the porphyrin oxidize Cro to Cr" and form H2.
Chromium
912
The oxidation of Cr(MPDME) in solution by 0 2 or HzOz can be reversed by sodium dithionite. Since peff= 2.84 BM in the solid and 5.19 BM in CHC13, there are axial interactions in the solid state which are removed on dissolution.
Me (276) R' R' R' R'
Me
= RZ= Et, mesoporphyrin dimethyl ester, H,MPDME = R2= H, deuteroporphyrin dimethyl
ester, HzDPDME
= R2 = CH=CH2, protoporphyrin IX dimethyl ester, HzPPDME = R2 = CH(OH)Me, hematoporphyrin dimethyl ester,
H2HPDME
The meso-tetraphenylporphyrinate, Cr(TPP), first reported as a tetrahydratelM3 with HzTPP (277) with Cr(C0)6 in n-butyl ether (Scheme 106). The reaction was carried out under nitrogen containing small quantities of air, the need for which was not explained, and the formulation is in doubt because products from other proceduress0 have &Berent visible spectra. It can be prepared by reduction of CrCl(TPP) with NaB& in EtOH/CHC13,S95activated zinc in THF/ /H20,80 or [Cr(acac),] in toluene or THF. The last method is of general application to MElCl(porphyrin) complexes (equation 62) because [C r(ac a~)~] is a powerful one-electron reducing agent, soluble in toluene, which is oxidized to a soluble CI" species, presumably [CrCl(acac),], which firmly binds the C1-. The solvate Cr(TPP)-2PhMe is obtained and Cr(0EP) (278) can be prepared similarly. A disadvantage is that Cr(acac), is very air-sensitive and an excess is required because it does not react stoichiometrically. pea= 4.90 BM, was prepared by metal insertion into
M"'Cl(porphyrin)
+ Cr(acac),
Cr(TPP)(py12
Cr(CO),
HzTPP
-
M"(porph*n)
+ CrCl(acac)*
(62)
Cr(TPP)NO
Cr"(TPP)
OPPh, + CrCl(TPP)
02 Cr'"O(TPP) toluene
69 +
+0
0 + CrCl(TPP)
Scheme 106
From its ma netic moment (4.9BM), Cr(TPP).2PhMe is high spin. A crystal structure determination"' shows that the molecule is centrosymmetric and the Cr atom essentially four-coordinate; each toluene interacts very weakly with the metal and a pyrrole ring. Planar Cr(TPP) will add two molecules of bases such as pyridine axially to form intermediate
Chromium
(277) R = Ph, tetraphenylporphyrin, H,TPP R = p-tolyl, tetra-p-tolylporphyrin, H2?TP R =mesityl, tetranresitylporphyrin, H,TMP R = p-C,H,SO;, tetra-p-sulfonatophenylporphyrin,H,TPPS
913
(278) Octaethylporphyrin, H,OEP
spin complexes (Table 102). The spin-pairing has little effect on the average Cr-N (porphyrin) distances since they are 2.033 A in Cr(TPP).2PhMe (S = 2)lN and 2.027 A in Cr(TPP)(py)z (S= l).12" However, the Cr-N distances to the axial pyridine molecules (ca. 2.13 A) are considerably less than in isoelectronic high-spin manganese(II1) complexes (ca. 2.4 A). This is attributed to the depopulation of the 3dzz orbital on spin-pairing in the Cr" complex. The alternative formulation of the Cr" porphyrin complexes as CI" porphyrin radical anion complexes is excluded by their visible spectra and oxidation-reduction behaviour.80'1246 The rate of oxidation of Cr(TPP) in solution can be lowered by cooling and by having high axial ligand concentrations, but no evidence has been found for reversible oxygenation. Powdered Cr(TPP)(py), (the five-coordinate structure earlier proposed1246was in error12A4) irreversibly forms an O2adduct which was thought from its magnetic moment (2.7 BM) and IR spectrum to be a C P superoxide complex Cr 02)(TPP)(py).1M In view of recent results (see l below) which show that Cr'vO(TPP) and (Cr"(TPP),O exist, it is ossible that the adduct is a mixture. Carbon monoxide does not react with Cr(TPP),'22 but Cr(TPP)NO can be obtained .59531247
(ii) Chromium(II1) porphyrin complexes Evidence is growing that chromium is an essential trace element (Section 35.4.8.3). If mammalian diets are supplemented with simple 'lcr-labelled salts it is found that less than 1% of the dose is absorbed because at intestinal pHs (>6) insoluble C9" hydrolysis products are produced. Complexes which remain intact in the acid conditions of the stomach as well as in more alkaline conditions are required. As Crl" porphyrin complexes are stable in these respects, methods for synthesis of the complexes and incorporation of 51Cr have been surveyed.lm Chromium(II1) porphyrin complexes have been synthesized via aerial oxidation of the chromium(I1) complex prepared from Cr(C0)6,'x2 from CrClz and the porphyrin in refluxing DMF,1249and directly from CrC13 or [Cr(acac),] in DMF1250and other high boiling solvents such as b e n z ~ n i t r i l e . ~Since ~ ~ ' the 'lCr radiolabel is sup lied as CrC13 in 0.1 M HCI, the carbonyl method and methods needing an excess of metalL' are unsuitable. To incorporate the label the 5 1 C ~was + reduced essentially completely by a 20-fold excess of CrCl2.4H2Oin a 1 :1 EtOH/H20 mixture according to the rapidly established equilibrium (63) for which the equilibrium constant is 1. The dilution of the 51Crwas acce table. DMF and the porphyrin were then added in an a da ptati~n' ~'of~the earlier method.'% The labelled complexes, shown in Table 102, were purified chromatographically. The method does not work well for porphyrins with free carboxylate side chains.
F
*Cr"'CP+
+ Cr" e
*Cr*'+ Crn1Cl2+
(63)
Kinetic studies have shown that axial ligands in the complexes Na,[Cr(TPPS)(H,O)z], are unusually CrCI(TPP)L LL = monodentate base) and [Cr(Schiffs base)(H20),]' labile,111371253*' and equilibrium constants for proton dissociation from coordinated water in the first complex'252have been determined. The chloro complex CrCl(TPP) is a non-electrol e in DMSO and neutral N, 0 and S donor ligands will bind to give six-coordinate species.' There is little difference in the stability
2
Chromium
914
Table 102 Some Synthetic Details and Properties of Porphyrin and Crv-Salen Complexes Complex Oxidation state II Cr(MPDME) Violet Cr(0EP)
purple
Cr(TPP).2PhMe Pu le Cr8-PP)(py)Z.PhMe Green-purple Cr(lTP)(L),.PhMe Green-purple Oxidation state III CrCl(MPDME)
CrCI(DPDME) CrCI(PPDME) CrCI(HPDME) Crcl(TPP) Deep blue CrBr (TPP) Na,[Cr(TPPS)] Cr(OMe)(TE'P).2MeOH Cr(OEt)(TPP).2EtOH Cr (OH)(TPP).2H2O Cr (Ci04)(TPP) Dark green Cr(O,CMe)(TPP) Cr(TPP)NO Red [Cr(TPP)IzO Purple (TPP)CrOFe(TPP) Purple CrCI(ITP) Green CrCKThQ) Green ' CrOH('ITP)H,O.H,O Violet )"C Violet CrOH(OEP).0.5H20 Violet CrBr(0EP)py Dark brown Cr(OPh)(OEP)PhOH Violet CrCl(0EP)
Ref.
Comments
cr(co),, HzMPDME in n-decane (170 "C) or decalin (205 "C), peE=2.84 (solid), 5.19 BM (CHC13) [Cr(acac),] reduction of CrCl(OEP), pes= 4.8 BM
1 2
[Cr(acac),] reduction of CrCI(TPP), pea= 4.9BM (av), 2.033 A; planar CrN, Cr(TPP).2PhMe and py in toluene or CrCl(TPP), Zn, THF/py/H,O, p,, = 2.93 BM indep. of T ; Cr-N(TPP) (av), 2.033, Cr-N(py) (av), 2.13 8, Cr(7'PP)-2PhMeand L in toluene, L = 3-pic, 4-pic, 1-MeIm; p,=2.9BM
2 3 2
c d 2 * 4 H z o ,HzMPDME, reflux DMF, expose to air as for CrCI(TPP) below, "Cr incorporated As above As above As above CrCI, or crCly4HzO, H,TPP, reflux DMF, expose to air. "Cr incorporated (see text) From CrCl(TPP), excess [NBu4]Br in CHCI, CrCL24H,O, Na,[TPPS], reflux DMF, expose to air; also from Cr(CO),; sometimes as (HzO)z, pea = 3.87 BM Cr(CO),, H,TPP, reflux decalin, expose to air, crystallized from MeOH, pea= 3.63 (294 K), 3.60 BM (79 K) As above, crystallized from EtOH, peff= 3.60 (294 K), 3.59 BM (79 K) As above, reaction mixture left one month, no alcohols in work-up CrCl(TPP) and AgClO, in THF; monodentate C10,
5
Cr(O,CMe),, H,TPP, reflux PhCN v(N0) = 1700 cm-', S = 3 (ESR)
12 11,13
Monohydrate, CrCI(TPP) in CH,Cl,, NaOH; CrO(TF'P) + Cr(TPP); p2yK= 1.61BM Per dimer, p = 3.12 (300 K), 1.9 BM (4.2 K), J = -153 cm-', g = 2.04; intermolecular; J = -1.45 err-', g = 2.14;Cr+Fe, 8 4 2 m - l . Also mixed porphyrins and (TPP)CrOFe(PC); C r U Fe linear CrCI,, H,TTF', reflux DMF, NaCl aq, HCl, Poor analyses, occludes CH,Cl, As above, poor analyses, decomp. to ?CrOH(TMP)
14,15,16
Cr-N
CrCI,, H,TTF', PhCN, reflux in stream of N, to remove HCl, chromat., add NaOH
4
2
5 5 5
6,7,5 7 5,&9 10 10 10 11
7,18
14 14 19
-
20
CrC13,H,OEP, PhCN, reflux stream of Nz to remove HCI, chromat., add NaOH [Cr(acac),], ZT,OEP, reflux diethylene glycol
19 21
[Cr(acac),],
22
H,OEP, PhOH, 250 "C, pem= 3.2 BM
Prepared as CrCI(TPP)
7
CrOH(OEP).OSH,O, NaOCl and NaOH, v(Cr=O), 1015 cm-', dim. Cr"(TPP), 0,in toluene, V(CI=O), 1020cm-', diam.,C r - 0 , 1.62; Cr-N,2.036, Cr ca. 0.5 8, above N4plane; also CrO(TFP), CrO(TXF9 CrCi(TPPj, PhIO in CEIzClz,KOH; similarly from NaOCI, Bu'0,H or m-chloro ero benzoic acid, v(Cr=O), 1025 cm-', v(C1="0), 981 cm- , diam. CrCI(TMP), PhIO in CH,CI,, similarly from Bu'O,H and KOH, v(Cr=O), 1023cm-', diam.
23
Oxidation state(N)
CrO(0EP) Red Cro(TPP) Red
15,16 14
P"y
CrO(TMP) Purple
14
915
Chromium Table 102 (continued)
CrO(TTP) Lavender Red Oxidation stare(V ) CrO(TPP)CI [CrO(TPP)]+ Red solution CrO(TETMC) Dark red brown CrN(0EP) Red CrN(TTP) Red-purple
Salen complexes [CrO(salen)]PF6 Dark brown CrN(sa1en) Red brown
c r c l ( m P ) , PhIO in c&, V(CI=O), 1mOcm-', diam., C r A , 1.572, Cr-N(av), 2.032, Cr 0.469 A above N, plane Cr(OH)(lTP).2H,O, PhIO in CH,Cl, or NaOCl and NaOH, v(C1=0), 1020 cm-', diam. CrCl(TPP), PhIO in CH,Cl,, peH= 2.05 BM, v(Ct=O), 972 cm-' generated electrochemically
14 23
14,24,25 30
CrCl,, H,TETMC, NaOAc, DMF, exposure to air, pcE= 1.7tBM, v(C1=0), 964cm-',g=1.9)37 CrOH(OEP)*O.SH,O as below, g = 1.9825
26
CrOH('LTP).2H,O, CH,CI,/EtOH, aq. NH,, NaOCI, g = 1.9825 Photolysis CrN,(TTPl, g = 1.985, v ( C d 4 N ) , 1017cm-', v ( C d S N ) ,991 cm- , CEN, 1.565, Cr-Na(TI'P)(av), 2.042, Cr 0.42 A above N, plane (C& adduct), RR spectrum, CrN(TPP) and CrN(TMP) also known
19 20
[Cr(salen)(H,O),]PF,, PhIO in MeCN, g = 1.977, v(Cr=O), 997 cm-'; also CrO(salen)X, X = F, Cl,Br CrN3(salen).2H,O, photolysis, v(Cr=-N), 1012 cm-', g = 1.974
28
19
27
29
1. M. Tsutsui, R. A. Vekapoldi, K. Suzuki, F. Vohwinkel, M. Ichikawa and T. Koyano, 1. Am. Chem. SOC., 1969,9l, 6262. 2. C. A. Reed, J. K. Kouba, C. J. Grimes and S. K. Cheung, Itwrg. Chem., 1978,17,2666. 3. W. R. Scheidt and C. A. Reed, Inorg. Chem., 1978,17,710. 4. W. R. Scheidt, A. C. Brioegar, J. F. Kirner and C. A. Reed, Inorg. Chem., 1979, 18,3610. 5. D. P. Riley, Synth. React. Inorg. Metal-Org. Chem., 1980, 10, 147. 6. A. D. Adler, F. R. Longo, F. Kampas and J. Kim, J . Inorg. Nucl. Chem., 1970,32, 2443. 7. D. A. Summerville, R. D. Jones, B. M. Hoffman and F. Basolo, I. Am. Chem. Soc., 1977,99, 8195. 8. J. G. Leipoldt, R. van Eldik and H. Kelm, Inorg. Chem., 1983, 22,4146. 9. E. B. Fleischer and M.Krishnamurthy, J. Coord. Chem., 1972,2, 89. 10. E. B. Fleischer and T. S . Srivastava, Inorg. Chim. Acta, 1971,5,151. 11. S. L. Kelly and K. M. Kadish, Inorg. Chem., 1984,23, 679. 12. T. N. Lomova, 8. D. Berezin, L. V. Oparin and V. V. Zvezdina, Rws. J. Inorg. Chem., 1982,27, 383. 13. B. B. Wayland, L.W. Otson and Z. U. Siddiqui, J . Am. Chem. Soc., 1976, %, 94. 14. J. T.Groves, W.J. Kruper, Jr., R. C. Haushalter and W. M. Butler, Inorg. Chem., 1982,21,1363. 15. J. R.Budge, B. M.K. Gatehouse, M.C. Nesbit and B. 0. West, I. C h . Soc., Chem. Commun., 1981,370. 16. D.J. Liston and B. 0. West, Inorg. Chem., 1985, 24, 1568. 17. D. J. Liston, K.S. Murray and B. 0.West, J. Chem. Soc., Chem. Cornmun., 1982, 1109. 18. D. J. Liston, B. J . Kennedy, K. S. Murray and B. 0. West, Inorg. Cham., 1985,24, 1561. 19. 5 . W.Buchler, C. Dreher, K.-L. Lay, A. Raap and K. Gersonde, Inorg. Chem., 1983, 22,879. 20. J. T. Groves, T. Takahashi and W. Butler, Inorg. Chem., 1983,22,884. 21. J. W. B'uchler, G. Eilcelmann, L. Puppe, K. Rohbock, H. H. Schneehage and D. Weck, Liebigs Ann. Chem., 1971,745,135. 22. J. W. Buchler, L. Puppe, K. Rohbock and H. H. Schneehage, Chem. Ber., 1973,106,2710. 23. J. W. Buchler, K.-L. Lay, L. Castle and V. Ullrich, Inorg. Chem., 1982, 21,842. 24. J. T. Groves and W. J. Kruper, Jr., J . Am. Chem. Soc., 1979,101,7613. 25. J. T. Groves, W.J. Kruper, Jr., T.E. Nemo and R. S. Myers, 3. Mol. Cotul., 1980, 7 , 169. 26. Y. Matsuda, S. Yamada and Y.Murakami, Inorg. Chim. Acta, 1980, 44, W W . 27. C. Campochiaro, J. A. Hofmann, Jr. and D. F. Bocian, Inorg. C h . ,1985, 24, 449. 28. T. L. Siddall, N. Miyaura, J. C. Huffman and J. K. Kochi, J . Chem. Soc., Chem. Commun., 1983, 1185. 29. S. I. Arshankow and A. L. Poznjak, 2. Anorg. A&. Chem., 1981, 481,201. 30. S. E. Creager and R. W. Murray, horg. Chem., 1985, 24, 3824.
a.
constants for binding of s-BuNH2 (log K = 4.28) and n-acceptor ligands such as pyridine (logK=4.28) to CrCI(TPP) in acetone (equation 64).'*% It therefore seems that metal to ligand n bonding is unimportant. However, 1-methylimidazole is anomalously strongly bound (log K = 6.71), consistent with some ligand to chromium n bonding. N donors will displace 0 donors and S donors but not the coordinated chloride. x
CrCl(TPP)(acetone) + L e CrCl(TPP)L + acetone
(64)
Cyclic voltammetry shows that the electrochemical reduction of CrCl(TPP) in non-a ueous media is a function of the solvent.1257In non-coordinating solvents, e.g. CH2C12,the Cl% /CS1 reduction (equation 65) is irreversible and the Cr"/Cr"-anion radical reduction (equation 66)is reversible. In weakly coordinating solvents, e.g. MezCO, the solvent L modifies process (65) through weak coordination to give CrCl(TPP)L, and in coordinating media such as DMF is COC I -DD
'
916
Chromium
involved in both reductions and renders equation (65) reversible. The effect of N donors was studied, and in C2H4C12and DMF the C6' and Cr"-anion radical species axially coordinate two substituted pyridine molecules. From the reduction potentials, this stabilized the Cr"' oxidation state over the Cr" state, and the Cr" state over the Cr"-anion-radical state. There was no indication that coordinating ligands would displace C1- from CrCl(TPP)L. In Cr(C104)(TPP) the axial interactions between the metal centre and the counterion are much less than in CrC1(TPP).1247Consequently it is easier to reduce and less easy to oxidize and species Cr(TPP)(L); (where L = DMF, DMSO and py) form in solution. Cr"'CI(TPP) Cr'I(TPP)
+
Cr"(TPP) + C1[Cr(TPP)]-
The reaction of NO with Cru(TPP) affords Cr(TPP)NO which is a red solid with v(N0) = 1700cm-' and an ESR spectrum typical of an S = 4 species.595When solutions of Cr(OMe)(TPP) in CHClJMeOH are exposed to NO, spectroscopic changes indicative of the formation of an NO complex occur. On degassing, the original spectrum reappears. Since the electronic and ESR spectra of the NO complex are similar to those of Cr(TPP)NO, it is believed that reductive nitrosylation has taken place according to equations (67) to (69). It is not stated whether Cr(TPP)NO as the solid or redissolved loses NO. More recent work'a7 shows that DMF, DMSO and py coordinate axially to Cr(lTP)NO in CH2C12 and formation constants have been determined; at high concentrations some displacement of NO occurs. Cr(OMe)(TPP)+ N O e Cr(OMe)(TPP)NO Cr(OMe)(TPP)NO+ MeOH + Cr"(TPP) + MeONO -k MeOH Cr"(TPP) + NO
-
Cr(TPP)NO
(67)
(68)
(691
(iii) Chromium(N) and (V) porphyrin complexes
Chlorotetraphenylporphyrinatochromium(II1) dissolved in CH2C12is oxidized by iodosylbenzene or m-chloro erbenzoic acid to CrVO(TPP)C1,which is stable in solution for several hours (Scheme 106).125K) The addition of alkenes rapidly regenerates CrCl(TPP) and gives alcohols arid epoxides; similarly cyclohexanol gives cyclohexanone. The same products are obtained with catalytic amounts of CrCI(TPP), and FeCl(TPP) behaves in a like manner.1259The red complex CrO(TPP)Cl has been well characterized in solution by IR, magnetic (Table 102) and ESR investigations. As it forms in CH2C12,the typical ESR spectrum of CrCl(TPP) is replaced by a sharp signal near g = 2 with the expected hyperfine splittings for Crv.1260Details of the ESR spectra show that the unpaired electron is primarily on the metal so that CrO(TPP)Cl is an oxochromium(V) species and not a chromium(IV) porphyrin n cation radical. Coupling effects in the ESR spectra consequent upon the formation of Crt70(TPP)C1 (from PhI170) and substitution of C1 by BuNH2, OH- or F- confirm the formulation. On standing at room temperature CrO(TPP)Cl decomposes in CH2C12 to give a brigbt red solution containing a diamagnetic oxochromium(1V complex. This happens so readilr that the spectrum of CrVO(TPP)Clfirst published' 2) is, in fact, that of red CIvO(TPP);12 ' CrO(TPP)Cl is dso red but the spectrum is different.1250,12M, CrIVO(TPP) can be directly obtained by oxidation of CrCl(TPP) by iodosylbenzene, Bu*02H, NaOCl or rn-chloroperbenzoic acid, followed by treatment with base (Scheme 106); CrO(TMP), CrO(TTP) and Cr0(OEP)1250,1261 and others have been prepared similarly (Table 102). In the crystalline state, the oxochromium(1V) complexes are stable for months. Unlike oxochromium(V) species they do not oxidize hydrocarbons, even in the presence of PhIO, although CrO(TPP) will oxidize PhCHzOH to PhCHO.lZ1 Another route to CrO(TPP) is from CI'(TPP) by reaction with oxygen.1262The structures of CrO(TPP)126zand CrO(TTP)1250are similar to those of other five-coordinate oxometalloporphyrins. In both complexes the metal atom is -0.5 A out of the porphyrin plane towards the oxygen atom, and the Cr-0 distances are similar: 1.572A [CrO(TTP)] and 1.62A [CrO(TPP)]. The average Cr-N distances are almost identical at cu. 2.03 A. The Cr"(lTP) was prepared by zinc amalgam or Cr(acac)z reduction of CrCl(TPP) and its oxidation to CrO(TPP) was monitored spectrophotometrically. The reaction (equation 70) takes place in two distinct stages believed to involve a p o x 0 intermediate ,1262 although others consider that autoxidation of the solvent toluene is
917
Chromium
involved.1250C P O TPP oxidizes Fe"(TPP)(pip), (pip = piperidine) to the mixed-metal p o x 0 complex (TPP)Cr"loF&(TPP) which contains a strongly antiferromagnetically coupled (S=$,S = 3) pair (Table 102). Similar complexes can be obtained with two different tetraphenylporphyrins, and Cr"O(TPP) can also be prepared by stirring CrCl(TPP), CHC13 and aqueous HC1 in air.'263 4Cr"(TPP) 2{C8"(TPP)}20 54CrWO(TPP) (70) Oxidation of CrCI(TPP) in CH2C12by the radical cation from phenoxathiin hexachloroantimonate produces a species with peff= 2.8 BM. The electronic spectrum is consistent with Cr"'(TPP?) or CrIV(TPP)and the former is favoured.'264An IR band in the 1270 cm-' region is indicative of a ring-oxidized metalloporphyrin, and there is a strong band at 1285 cm-' in the spectrum of [CrCl(TPP)][SbCb].1265 One oxochromium(V) complex, CrO(TETMC) , containing the trinegative anion of a corrole (279), has been characterized as the solid.1266It is prepared (Table 102) simp17 by exposure to air of a solution presumably containing a Cr" complex. Aerial oxidation of Cr '(TPP) produces the oxochromium(1V) complex CrO(TPP) so the corrole ligand apparently facilitates autoxidation. The redox behaviour of CrO(TETMC) has been examined by cyclic v ~ l t a m e t r y . ~ ~ ~ ~
(279) 7,8,12,13-Tetraethyl-2,3,17,18-tetrarnethylcorrole, H,TETMC
Nitridochromium(V) porphyrins, stable to air, moisture and mild reducing agents unlike oxochromium-(IV) and -(V),have been prepared by two methods (equations 711=' and 72).654,'269The C$n porphyrins probably take up NH3 as one axial ligand and oxidative dehydrogenation produces the nitride (equation 73). CrOH(ITP) + NaOCl + NH, crN,(m)
+ CrN(lTP)
CH2C12or C&
Cr"', d3
hv
+ NaCl + 2Hz0
CrN(TTP)+N,
(71) (72)
Cr",d'
The nitrido complexes have been thoroughly characterized. The oxidation state has been confirmed by ESR and ENDOR spectroscopy. The C = P + group can be considered weakly electron donating; only the dry orbital is not involved in D or JG bonding, and it accommodates the single d electron. The stability to reduction compared with Cr=02+ species is ascribed to the high energy and low electron affinity of the empty d orbitals.1268The expected square pyramidal structure has been confirmed by X-ray methods for CrN(TTP).C6&.1269 It is isostructural with CrO(TPP). Coordination shell bond distances are given in Table 102, and the CEN distance is short, consistent with a triple bond.
(iv) Chromium ( V ) - d e n derivatives Chromium(II1) sakn will form chromium(V) derivatives in much the same way as porphyrin complexes do. By reaction of iodosylbenzene with chromium(II1)salen oxochromium(V) complexes are obtained (equation 74) which will effect oxygen atom transfer to phosphines and alkenes such as norbornene (equation 75) in stoichiometric and catalytic systems.'270 The
Chromium
918
halides [CrO(salen)]X (X = F, C1, Br) can be prepared from [CrO(salen)]PF6 by metathesis with [N%]X.
[Cr"'(salen)(H,O),]PF,
+ PhIO
-HzO MeCN
+
[CrVO(salen)jPF, PhI
(74)
There is disorder in the crystal structure of [CrO(salen)]PF6, but the average out-of-plane displacement (which takes place on oxygen transfer to the metal) of the Cr atom is 0.52 A, and the average Cr=--0 distance 1.56 A. The nitrido complex CrN(sa1en) can be obtained from [CrN3(salen)].2H20 by photolysis (equation 72).653The Y(CEN) frequency is 1012 cm-'. 35.4.9.2 Polyazamacrocyeles: cyelam, tet a, tet b and related ligands
In comparison with some other metal ions, limited synthetic work has been carried out on chromium(II1) complexes with macrocyclic ligands. The vast majority of reported complexes involve tetraazamacrocycles.
(i) Cyclum A number of complexes of the type [Cr(cyclam)X2]+ (cyclam = 1,4,8,11tetraazacyclotetradecane, 82) have been prepared as outlined in Scheme 107.1271-1273The difficulty in synthesizing trans-[CrX2(cyclam)]+ complexes directly from the free cyclic ligand is in marked contrast to the behaviour of other ions of the first transition series, e.g. cobalt(III), which forms stable trans complexes. The colours and spectroscopic properties of these cyclamchromium(II1) complexes are presented in Table 103. The use of IR spectroscopy to differentiate between cis and trans cyclam complexes has been widely explored and the region 790-910 cm-' was found to provide confirmatory evidence of structure, independent of metal or counteranions present.'272 Hence tvuns complexes generally show two groups of bands in this region, a doublet near 890crn-I associated with the secondary amine vibration and a singlet near 810 cm-' due to the methylene vibration. The spectra of cis isomers on the other hand are more complex with at least three bands between 840 and 890cm-' and two between 790 and 810 cm-' (Table 104). Table 103 Colours and Electronic Spectra of Cyclarnchromium(III) Complexes
cis-l~r~~(cyclam)l* cis-[CrBr,(cyclam)]+ cis-[Cr(OH),(cyclam)]+ cis-[~r(cyclam)(~~)~]~+ cis-[Cr(NCS),(cyclam)]+ cis-[Cr(ONO),(cycla~t$] ~is-[Cr(N~)~(cyclam)] cis-[CrClz(Mc,cyclam)]+ +
Dark red Grey-violet Orange
Orangered Red-brown Red-violet
cis-[Cr(Me,cyclam)(H,O)~]~+
mans-[~r~l,(cyclam)I+ nnns-[Cr(NCS),(cyclam)]+
Purple Orange
rrans-[Cr(cy~lam)(H~0)~1~+ trans-[CrCI,(Me,cyclam)]+
aans-[Cr(CN),(cyclam)]+
Yellow
529 (111) 404 (106) 527 (94) 408 (72) 547 (84) 370 (66) 483 (126) 370 (38) 486 (189) 368 (101) 481 (134) 355 (211) 517 (276) 400 (144) 559 (123) 412 (97) 506 (75) 380 (53) 567 (20) 404sh (30), 364 (33.5) 483 (139) 364sh (117) 510 (24) 405 (39) 350 (53) 571 (20) 386 (31) 414 (62.5) 328 (62.5)
A in nm of aqueous solutions. E in dm3 mol-' cm-'. pK, = 3.8, pK, = 7.0, 20 "C,I = 0.1M. is similar to that of cis-[Cr(en),(ONO)2]+ which is known to contain O-bonded NO; ligands. 1. I. Ferguson and M. L. Tobe, Inorg. Chim.Acta, 1970,4, 109. 2. D. A. House, R. W. Hay and M. A. Ah,Znorg. Chim. Acta, 1983, n,239. 3. C. K.Poon and K. C. Pun, Inorg. Chem., 1980,19,568. 4. N. A. P. Kane-Maguire, J. A . Bennett and P. K. Miller, Znorg. Chim. Acta, 1983,76, L123. 5. R. G. Swisher, G. A. Brown, R. C. Smierciak and E. L. Blinn, Znorg. Chem., 1981,20, 3947.
468 (26) 455 (48) 356 (38)
1
1
1 417 (26) 418 (35) 415 (29) 445 (60)
1
365 (22)
4
1 1 1 2
Dinitrito complcx, since spectrum
Chromium
919
Table 104 IR Spectra of [Cr(cyclam)X,]+ Isomers Between 790-910 cm-’ N H vibration
Complex
CH, vibration
trcm -[CrCl,(cyclarn)]Cl cis-[CrCl,(cycIam)]Cl trans - [Cr(NCS),( cyclam)]SCN
8%, 882s 872m, 862m, sh, 854111
cis-[Cr(NCS),(cyclm)]SCN
870m, 860m,%Om, sh
804s 815w, 805m 802m 818m, 810m
885s, 878m
1. C. K. P w n and K. C . Pun, Inorg. Chem., 1980,19, 588.
CrCl,(THF),
I
+ cyclam
trans-[Cr(CN),(cyclam)]CIO, NaCN. NaCIO, DMSO. 62 ‘C
DMF, reflu
I
(-90%)
(-10%)
1
cis-[CrBr,(cyclam)]Br \ P
-. ck - [Cr(cyclam)(N0,)H20](N03)2
i. vH 7. reflux
1’ 11,
AgNO,, H + reflux
NaSCN ’
trans-[Cr(NCS),(cyclam)]SCN.O. 5H20 NaSCN in H20
reflux
cis-[Cr(NOZ)2(cyclarn)]N03
py.
s,o$-in H,O
cis-[Cr(N,),(cyclam)]N3
cis-[Cr(NCS),(cyclam)]SCN
cis-[CrOH(cyclam)(H,0)]Sz06-3HZ0 LHBr
cis-[Cr(cyclam)(H20),]Br,~3H,0 Scheme l(t7
In the attempted preparation of trans-[Cr(~yclam)(NH~)~]~+ from the dichloro corn lex in liquid NH3 only a crude product contaminated by an orange material could be isolated.’P74 This impurity was shown to contain the 0-carbamato complex tr~ns-[Cr(OCONH~)~(cyclam)]+. Suitable crystals of trans-[Cr(OCONH2)z(cyclam)]C104~1.5H20 were grown and the structure determined by X-ray diffraction methods. The structure contains two independent centrosymmetrical complex ions, which are almost identical except for the orientation of the carbamate ligands. The macrocyclic ligand has a conformation in which the five-membered ring has a gauche and the six-membered ring a chair conformation. The average C-C (151.3pm), C-N (148.0 pm) and i-N (205.9 pm) distances in the complex are similar to those in the doubly protonated ligand (150.9, 148.2, 204.5 pm respectively). The average Cr-N bond length (205.9 pm) is close to the ideal value (207 pm) calculated for trans-cyclam complexes1275and also similar to those in chromium(II1) complexes with other amines.*274 T h e relative may be due to substitution inertness of tram-cyclamchromium(II1) complexes in steric hindrance by the two axial C-H groups of the six-membered rings towards the entering group in an associative interchange mechanism.1274 The photochemistry of cis- and trum-[CrC12(cyclam)]+ on irradiation of the d-d bands in dilute acid solution involves monosubstitution by solvent to give the corresponding isomeric [CrCl(cy~larn)(H~O)]~+ product^.'^'^ However, the photobehaviour of [Cr(CN)z(cyclam)]+ in solution at room temperature is in marked contrast to that normally displayed by chromium(II1) ~ o r n p l e x e s . ~ ”For ~ ~ ’this ~ ~ complex, ~ like rrun~-[Cr(dN)~(NH,)~]+ ,329 the 4B2g component of the first spin-allowed quartet excited state lies at lower energy than the ‘EE component, an ordering which in the tetraammine complex restricts photolabilization to the in-plane NH3 positions. However, in the cyclam complex the nature of the in-plane ligand prevents the occurrence of such a reaction with the result that even after extensive ligand field photolysis no substitution photochemistry is detected. Furthermore the complex exhibits an
Chromium
920
intense long-lived ’ER+4B1,phosphorescence which shows equal enhancements in intensity and lifetime on deuteration of the NH groups. These properties of its ’ER state make trans-[Cr(CN),(cyclarn)]+ an attractive compound to study in terms of energy- and electrontransfer with various substrate molecules. (ii) Tet a and let b A number of chromium(II1) complexes of the diastereoisomeric C-meso- and C-racemo-
5,7,7,12,14,14-hexamethyl-l,4,8,11-tetraazacyclotetradecane (tet a and tet b respectively; 280 and 281)have been prepared.12’’*12p0While the tet b isomer like cyclam can readily fold to give cis-[CrXz(tet b)]+ complexes, tet a only folds with difficulty because of unfavourable interactions between the methyl substituents and axially coordinated ligands. Hence these isomers react with CrC13(DMF)3, prepared in situ by dehydration of CrC13.6Hz0 in DMF, according to Scheme 108. Replacement of C1- by other anions provides a convenient route to a range of trans-[CrXz(teta)]X and cis-[CrX,(tet b)]X complexes. Details of the electronic spectra of these products are presented in Table 105.
M
e
w
E
e
M
e
m
E
e
fNH “7 Me &%, Me Me (280) teta u (C-meso)
MeMe&Me I
(281) tet b (C-racemic)
frons-(CrCl,(tet a)]Cl-HCl.2H20 = 574 (25), 440sh (27), 387 (47)nm (dm3mol-’ cm-’) green; ,Imax(&)
CrCI3.6H20
DMF’
CrC13(DMF)3 let b
in DMF,
cis-[CrCl,(tet b)]Cl sea-green; Am= = 598, 430 nmb a
In DMF.
Nujol mull spectrum.’ Scheme 108
The first stey in the base hydrolysis of rruns-[CrClz(teta]]+ (Scheme 109; kl= 145 dm’ mol-’ sat 298 K, ionic strength 0.1 M; AH = 114 kJ mol-’, AS* = 179 J K-’ proceeds some 112-fold faster than the corresponding reaction of trans-[CrClz(cyclam)]+ (k,= 1.3 dm3mol-’ s-l at 298 K).’276 On the basis of the steric acceleration due to C-methyl substitution, the first-order dependence of rate on [OH-] and the large positive entropy of activation an &lCB mechanism is suggested for this reaction.’”’ Acid hydrolysis of the tet a complex is also considerably faster than its cyclam analogue.’”’ The crystal structure of one of the possible isomers of trarar-[CrCl(tetU ) H ~ O ] ( N Ohas ~ ) ~been reported.lB2 Addition of base to an aqueous solution of cis-[Cr(mal)(tet b)]+ (mal = malonate) gives rise to a bathochromic shift in the longer wavelength band of its visible spectrum.lBOThis spectral change has been attributed to ionization of the CH2 group in the malonate ligand (equation 76), although such an explanation must be treated with at least some scepticism since it implies an abnormally large increase in the acidity of this group due to coordination of the neighbouring CO; groups.
921
Chromium Table 105 Colours and Electronic Spectra of Chromium(II1)-tet Complexes'. *
Complex
A,,
Colour
(nm) ( E in dm3mol-' cm-')
Medium ~
rram-[CrC1,( tet a)] tram-[Cr(NCS),(tet a)]+ trans-[Cr(tet a)(HzO)J3+ trans-[Cr(tet a)(H20)J3+ tram-[CrBr,(tet a)]+ tram-[CrBr,( tet a )]* cis+[CrCl,(tet b)]+ cis-[Cr(NCS),(tet b) I+ cis-[CrBr,(tet b)]+ cis-[Cr(NO,),(tet b)]* c&-[Cr(NO,),(tet b)]* cis-[Cr(N,),(tet b)]+ cis-[Crox(tet b)]' c&-[Crox(tetb ) ] + cis-[Cracac(tet b)J2+ cis-[Cracac(tet b)]" cis-[Cr(OH),(tet b ) ] + cis-[Cr(OH)(tetb)(H O)]" cis-[Cr(tet b)(H,O),]'+ cis-[CrCl(tet b)(H,O)]'+ cis-[CrF,(tet b)]' cis-[Crmal(tet b ) ] +
+
574 (25) 440sh (27) 387 (47) 500 420 322 530 408 340 535 (65) 423 (96) 602(30) 41Osh 374(47) 600 (33) 41osh (38) 382 (43) 430 598 526 (210) 340 (99)
Green Orange Pink Fink Green Green Sea green Maroon Magenta Pink Pink Bhe-violet Pink Pink Red Red Dark blue
595
438
525 392 524 (202) 390 (116) 570 420 522 390 516 (1571 384 (102) 512 385 325 535 (161)390 (256) 609 (1111380 (73) 572 (noj 407 (53j 529 (169)388 (82) 554 (173) 405 (105) 541 (145) 379 (58) 527 (167) 357 (82) I
-
Violet Violet Violet
_
.
~~
~
DMF Nujol" Nujol" MeOH DMF MeOH Nujol" MeCN Nujol" Nujol" DMF Nujola Nujol" H,O or 1M NaOH Nujola 1M HCl or NaOH 1 M NaOH 1M NaCl HClO,/NaClO, 1M WCI 1 M HCl or NaOH 1M HCl
I
'Because of insolubility in the common solvents, spectra of Nujol mulls on filter paper were recorded. 1. D. A. House, R. W. Hay and M. A . Ah, Knorg. Chim. A m , 1983,72,238. 2. J. Eriksen and 0. Mvjnsted, Acta Chon. Scund., Ser. A , 1983,37,$79.
+
&ans-[CrCl,(tet a ) ] + OH-
A
tram-[CrCl(OH)(tet a)]'
I
+ C1-
OH-,k,
trans-[Cr(OH),(tet a)]'
+ C1-
Scheme 109
+OH-
A,,,
-
= 527 nm
r
/O\,/P
A,,,
-1
= 570 nm
(iii) [12]aneN4, unsymmetrical [14]aneiV4, [15]aneN4 The preparation and properties of some chromium(II1) complexes of 1,4,7,10tetraazacyclododecane ([12]aneN4) are outlined in Scheme 110.lZs3 The reaction between CrC13.3THF and the unsymmetrical 14-membered macrocycle 1,4,7,1l-tetraazacyclotetradecane (1,4,7,11[14]aneN4) in DMF leads to equal amounts of cis- and trans[CrC12(1,4,7,11[14]aneN4)]C1and these can be separated on the basis of the insolubility of the former and the solubility of the latter in methanol (Scheme 111). The reaction between CrCl3-3THFand the 15-membered macrocycle [15]aneN4(81)in DMF produces only the trans isomer of [CrC1z([15]aneN4)]C1~2Hz0 (green; A, = 586 MI, E = 24 dm3mol-' cm-l; 476 nm, E = 48 dm3 mol-' cm-'; 410 nm, E = 63 dm3mol-' cm-I), which undergoes a slow one-stage hydrolysis in aqueous solution.lW The reaction between the strongly reducing complex [Cr([15]aneN4)(H20)2]2+and organic halides (RX) produces the monoalkylchromium(111) complexes trans-[RCr([15]aneN4)H20l2+, equation (77).305*1284 The reaction follows second-order kinetics and the reactivity of alkyl halides follows the pattern tertiary > secondary > primary and RI > RBr > RC1 (Table 106). This and other information suggests a mechanism which involves rate-limiting halogen atom
922
Chromium
N -
o,
W d
F u
Y
'i 0
r"
P
I
d
h
'-'
I
n
Chromium
923
abstraction from RX by the chromium(I1) complex to generate a carbon-centred free radical which rapidly couples with a second chromium(I1) complex (Scheme 112). In the presence of electrophiles such as Hg2+ or MeHg+, the alkylchromium(II1) complexes undergo electrophilic dealkylation as in Scheme 113.297These reactions also follow a second-order rate law and the rate constants fall sharply with increasin bulk of R and by two to three orders of magnitude as the electrophile is changed from H& to MeHg+ (Table 106). On the basis of such observations a bimolecular electrophilic substitution mechanism, SE2 is proposed for these reactions. Furthermore these complexes are somewhat less reactive towards Hg2+ than the corresponding [RCr(HZO)5]*+species1285owing to the considerable steric bulk added to the site of reactivity by the macrocyclic ligand. Details of the electronic spectra of trum[RCr([15]aneN4)H20]z' and related complexes are listed in Table 107.
+ RX
2[Cr([15]aneN4)(H,0),12'
-
trans-[RCr~[l5]aneN4)H,O1"
+ truns-[XCr( [151aneN,)HzOIz'
Table 106 Second-order Rate Constants for Alkylation of [Cr((15]aneN4)(H,0)z]2+by RX (kRX) and for Eiectrophilic Dealkylation of the Chromium(II1) Products by Hgz+(kHg)and MeHg+(k,,,,).
RX
kHga"
kmarb
Me1 EtBr Eff PPBr Bunk PhCHzCl
PhCH,Br WBr CyBr 1-AdamantylBr
kMdf,"."
3.1 x lo6 2.53 x 103
0.046 0.164 0.413 0.169 0.138 3.23 x lo2 1.91 x io4 1.91 0.836 0.98
-
8.21 X 10' 4.88 x 10' 1.14 x 10' 4.3 x io-' 1.6 x 10-3 3.1 x 10-3
5.2
-
"In dm3m01-'s-'. bAll at 298K in l:lButOH/H,O, I=O.ZM. 'All I = 0.5 M, [H+] = 0.25 M. 1. G. J. Samuels and J. H. Espenson, Inorg. C h m . , 1979; 18, 2587. 2. G. J. Samuels and I. H. Espenson, Inorg. Chem., 19813,19, 233.
at 298K,
Table 1W Electronic Spectra' of truns-[~r(~15]aneN4)H,0]*~(L = alkyl, halide or aqua) Complexes' A-
L
Me Et
(nm) ( E in dm3 mol-' cm-')
468 (69) 467 (66) 468 (71) 468 (69) 510sh (49) 5M)sh (67) 463 (88) 353 (2170) 564 (22) 560 (23) 562 (26) 540sh (28)
w
By" Pr'
CY
I-Adamantyl PhCH, C1
Br ib
H,O'
375 (227) 383 (387) 383 (465) 383 (459) 396 (550) 400 (422) 383 (347) 297 (7470) 462 (62) 460 (68) 462 (75) 454 (87)
258sh (3300) 264 (3100) 265 (3440) 268 (3300) 287 (3280) 298 (3070) 268 (3070) 273 (7920) 393 (76) 393 (78) 394 (90) 377 (88)
In aqueous solution pH 3. Complex thermally unstable. 3+ complex. 1. G. J. Samuels and 3. H. Espenson, Imrg. Chem., 1979, 18, 2587.
a
[Cr([l5]aneN4)(H,O),]'+
+ Rx
-
&am [XCr([151aneN,)H,O]'+
+ R-
[Cr([15]aneN4)(H,0),]*++ R. % tr~nr-[RCr(t15]aneN,)H,O]~ ' Scheme 1U COC3-DD'
(77)
924
Chromium RHg'
+ trans-[Cr([15]aneN4)(H,0),]'*
tr~m-[RCr([lS]aneN.,~H~O]~+
\
MeHg+
MeHgR + trans- [Cr ([ 15]aneN4)(H20)2]3+
Scheme 113
(iv) Other macrocycles A green chromium(I11) complex (282) containing the dianionic unsaturated macrocyclic ,11,13-tetraenate7Me2[14]tetraenateN4, ligand 5,14-dimethyl-l,4,8,11-tetraazacyclotetradeca-4,6 has been prepared by the method in Scheme 114.'% This complex reacts with NaN02 in refluxing alcohol to give the mononitrosyl Cr(Me2[14]tetraenatoN4)N0(l37),which on the basis of experimental evidence appears to contain chromium(1) and the cationic NO+ ligand, Section 35.4.2.6. Some chromium(II1) complexes with macrocyclic ligands derived from N heterocycles are mentioned in Section 35.4.2.5. A number of chromium(II1) complexes of the ligand 1,4,7-triazacyclononane7 [9 aneN3, and its N,N,N-trimethylated derivative, Me3[9]aneN3, have been in~estigated.~ 2+1286 From [Cr([9]aneN3)(H20)3]3' the binuclear complex [Cr(p-QH)([9]aneN3)H20]i+ and the trinuclear complex [Cr3(pOH)40H([9]aneN3)3]4+have been obtained and the crystal structure of the latter as the iodide has been determined by X-ray diffraction methods. This cation (283)contains salt (+5H20) two different pairs of p-OH bridges with Cr-0-Cr bond angles of 98" and 126" and a very short H bond which links the terminal and one of the bridging OH ligands. The crystal structures of the binuclear complexes [CrZ(p-QH)2(p-C03)([9]at1eN~)~]I~-H~0 and [Cr2(pOH)3(Me3[9]aneN3)2]13-3Hz0, the latter representing the first genuine tris(y-hydroxo) complex of chromium, have also been reported.
Scheme 114
35.49.3 Phthalocyanines
Relatively few chromium complexes with phthalocyanine ligands have been reported and, ir view of the chemical relationship between the few known compounds, all of them, irrespectivr of the metal oxidation state, are dealt with in this section. In the following discussion PI represents the dianionic phthalocyanine ligand.
Chromium
925
(i) CrPc and [CrPc(p-O)]2 The chromium(I1) complex CrPc can be prepared by either of the routes in Scheme 115.1287-1290 While the Cr(CO)6 method conveniently gives the b polymorph of CrPc (40% yield) and this on sublimation at 300-350 "C can be converted to a-CrPc,1288the chromium(II1) acetate method gives, after washing the product mixture with organic solvents, both CrPc and an oxidized s ecies which was long thought to be the chromium(II1) complex C~ P C ( O H ) .'~ ~,'~' Repeated sublimation of this mixture at 400 "C/10-6 mmHg was reported to give the pure hydroxide. In contrast to a-CrPc, which is air stable, the /l-polymorph is air sensitive and under anhydrous conditions picks up O2 (0.5 O2 per chromium) to give a deep blue product, which on the basis of available experimentai evidence seems to be the di-poxochromium(1V) dimer (284) rather than CrPcCOH).lm8Hence the product: (i> lacks characteristic OH vibrations in its IR spectrum, (ii) shows no evidence for CrPc(0H)' or the characteristic fragments of this ion in its mass spectrum, and (E) is formed in the absence of water. Furthermore lSO isotopq substitution effects on the IR spectra support a structure containing a C-0 group and eliyinate possible structures such as those with peroxo bridges or those with polymeric -Cr-0-Cr& groups. The electronic spectra of the oxidation product and CrPc are quite similar in band positions and shapes, thus eliminating the phthalocyanine ring as the possible site of oxidation.
Scheme lf5
The magnetic moment of the oxidation product is 1.9BM at room temperature (pepinoaly for a d2 ion = 2.9 BM) and is temperature-dependent, pointing to the existence of antiferromagnetic coupling between the chromium ions.la8 The reflectance spectrum of the product has bands at 755sh, 720, 66Osh, 490 and 380 nm. Its Raman spectrum has a band at 1041 cm-', which shifts by 45cm-' on "0 isotopic substitution and which seems to be due to a Cr=O stretching vibration. The fragmentation pattern in the mass spectrum is similar to that of PcCr except for a group of peaks between 578 and 582 thought to originate from CrPcO+. When '*O is used as oxidant these peaks are observed in the range 580-594. Some of the many reported The magnetic moment reactions of the oxidation product are illustrated in Scheme 116.128771291 of the high-spin CrPC (3.49 BM at 298 K) falls sharply with decreasing temperature (0 = 306 K) to below the spin-only value for a low spin complex.1290This behaviour has been attributed to antiferromagnetic interactions (J = -38.2", g = 2.0) in stacks of planar molecules. By contrast the relatively air-stable bis(pyridine) adduct, CrPc(py),, is iow spin (pes at 298 K = 3.16 BM) and shows a much lower temperature dependence (0 = 35 K).
(ii) CrPc(N0) Solid /?-CrPc reacts reversibly with gaseous NO at ambient temperature to give a 1: 1adduct, which reacts with both py and O2(Scheme 117).5" No structural information is available on the oxidation product of the nitrosyl complex. In contrast to the 6 form, solid a-CrPc is unreactive towards either O2 or NO, a difference ascribed to the solid-state structures of the two polymorph^.^^' Although the distance between the phthalocyanine planes is similar (340 pm), in both cases the metal-metal distance increases from 340 to 480pm on going from the a to the form. These distances are sufficient to permit access of O2or NO in the /l form, while excluding them from the a form. The addition of py to CrPc(N0) causes the NO stretching band in the IR spectrum to intensify and to sharpen considerably. The diikse nature of this band in CrPc(N0) indicates the presence of a perturbed NO ligand but the introduction of py causes lattice expansion and eliminates the perturbation.
926
Chromium K[CrPc(CN)OH]
\
CrPc(0Ac)
reflux MeOH/BuOH, KCN,
CrPc(py), I
PY. N2
CrPc(OAc)(H,O)
1)20/dH20
Oxidation" uroduct
R2P02H' MeoH' reflux
R = M e . Ph
>
[C~PC(M~OH)~]R,PO~ I
h 8 0 "C. 15 mmHg MePhP0,H
[CrPc(MeOH)(R,PO,)] f MeOH
J H[CrPc(el),]
[ CrPc(H,O)(MePhPO,)]
In the references to this work the oxidation product was described as CrPc(0H) but was later described as the dimer [CrPc(p-0)lz. a
Scheme 116
CrPc + py + NO
-
Cr(Pc)(NO)py v(N0) = 1680 cm-'
Cr(Pc (N0)OH v(NO] = 1690 cm-
Scheme 117
(iii) Oxidation and reduction of CrPc Reduction of CrPc with Na gives different products in THF and HMPA.129231293 In THF the product obtained is [Cr'Pcj-, which has an ESR spectrum consistent with the low-sph chromium(1) configuration efbh. The product in HMPA on the other hand has an ESW spectrum consistent with the low-spin configuration &e,: The inversion of orbital energies is due to axial binding of a SG donor solvent molecule in HMPA but not in THF. Two-electror, reduction of CrPc or CrC1(Pc)H20 by dilithium benzophenone in THF gives the green crystalline solid Li2[CrPc]-6THF.1293,1294 The magnetic moment of the complex indicates that it has two unpaired electrons and this oints to the formulation [Crt(PG)12-, which has the electronic configuration edbig eg(n*) (as Pc represents the doubly negatively charged phthalocyanine ligand, P F represents the radical anion obtained by one-electron addition tc Pc). Oxidation of CrPc by S0Cl2 affords the complex CrC12Pc, which is two oxidation equivalents above the starting complex.665The magnetic moment of the complex (4.40BM at 298 K) is consistent with the formulation Cr"'CI2(Pb), which contains two magnetic centres, one (Cr"') having three unpaired electrons and the other (Pc';) one unpaired electron. It alm eliminates the alternative formulation Cr'"C12Pc for the oxidation product. Furthermore? thc Cr-C1 stretching bands in the far IR spectrum of the product occur at positions close to those in the spectra of known Cr"'C1-containing complexes. Since the reactivity of thionyl chloride towards MPc complexes parallels that of NO, it appears that an initial stage of the oxidation involves coordination of thionyl chloride to the metal.
7
(iu) Other complexes The binuclear complexes H2U(Pc)CrOCr(Pc)H20 and NH3(Pc)CrOHCr(Pc)OH form when [Cr20(NH3)ln]C14~H20 and [Cr2(0EI)(NH3)lo]C15,respectively, are allowed to react with phthalonitrile at 543 "C. lZg5 The 4' ,4",4"',4""-tetrasulfonated phthalocyanine (TSPc) complex, OH(TSPc)CrOHCr(TSPcjHzO was obtained by fusing [Cr20H(NH3)lo]C15with ammonium-4sulfophthalate and urea.' 95
927
Chromium
The complex Na3[Cr(TSPc)].9H20 has been prepared from cobalt(I1) acetate, sodium 4-sulfophthalic acid, ammonium chloride and ammonium molybdate in nitrobenzene .I2% Electrochemical reduction of this complex at a platinum electrode at -0.7 V us. SCE gave the highly air-sensitive [Cr(TSPc)14- in DMF solution. The action of S0Cl2 on Na,[Cr(TSPc)l affords the corresponding tetrasulfonyl chloride which on refluxing with octylamine gives the tetraoctylsulfonamide (TOPc) derivative C ~ ( T O P C ) O H . ~The ' ~ ~ ability of this and related complexes to act as photosensitizers in the reduction of methyl viologen in THF/HzO (15 : 85) solutions containing triethanolamine has been investigated. Irradiation of the system containing the chromium(II1) complex did not result in formation of reduced methyl viologen. However the corresponding chromium(I1) complex, prepared in situ by irradiation of the chromium(II1) complex in the presence of cysteine, was found to be a more effective photocatalyst than any of the other metallophthalocyanines studied, but the quantum yield even in this case was low (4 = 2.6 x 10-3).
35.5
CHROMIUM(W)
Chromium(1V) does not have any aqueous solution chemistry except for the formation of intermediates in the reduction of Cr"' to Cr"'. Chromium(1V) compounds tend to disproportionate into CI" and CrW species (equation 78) and the metal ion in this oxidation state is powerfully oxidizing towards organic compounds. An eight-coordinate complex [CrH4(dmpe)4] is known (Section 35.3.4.1). 3Cr'"
+
2CI" + Crv'
(78)
35.5.1 Anhydrous Halides and Complex Fluorides Chromium tetrafluoride can be prepared by fluorination of the metal, CrF, or CrC13 at high temperature.13@It is an amorphous green solid and turns brown with traces of moisture, but is unexpectedly inert towards some reagents at room temperature. It is of intermediate volatility and is antiferromagnetic (p =3.02BM, 8 =78"); it is therefore likely to consist of CrF6 units sharing edges. The preparations of several CrF4 adducts such as XeF&rFd are shown in Scheme 120 (Section 35.61). CrOF2 (peE= 1.1BM) is produced on thermal decomposition of CrOZFz,and a complex (N02)2Cr02Fzis known (Section 35.7.1). At high temperature in the gas phase the chloride and bromide exist in equilibrium with the halogen and solid CrXJUz(equation 79) and from their Raman spectra CrCL, and CrBr, are tetrahedral. 2CrX3(s) + Xz(g)
.
-1303K
2CrXk)
(79)
The hexafluorochromates(IV), M2[CrF6](M = Li, K, Rb, Cs) and M[CrF6] (M = Ba, Sr, Ca, Mg, Zn, Cd, Hg, Ni) are prepared by fluorinating argopriate mixtures of MC1 and CrC13.'299 from CrF5 and NOF and characterized The nitrosonium salt (NO),[CrF,] has been obtained by its Raman spectrum. The symmetric stretch of the [CrF6I2- unit was found at 61Ocm-l. A band at 20 200 cm-' in the reflectance s ectra of A2[CrF6](A = K, Rb, Cs) has been assigned to the first spin-allowed d-d transition PTlg+ 3T2g. The second transition (3T1,+ 3Tk) may be superimposed on a charge transfer band at -30 OOO crn-l. 1301 Pentafluorochromates(IV) M[CrF5] ( M = K , Rb, Cs"), considered to contain condensed CrF6 units, and the ink heptafluorochromates(1V)Rb3CrF7 and Cs3CrF7,which have the (NH4)3SiF7~tructure,'~'are also known.
35.5.2 Chloro and Cyano Complexes Alcoholic reduction of K[CrO&] in the presence of pyridine, the pyridinium hahde and a little water is reportedL303to give the stable dimeric complexes [Cr(OH)2X2py]2(X = C1, Br, I) (pzz7K= 2.8 BM); and hygroscopic K2[CrQ(CN)4py](pa = 3.0 BM) is obtained on addition of KCN to [pyH][CrO&l] in pyridine to which ethanol has been added.13@'
rn 35.5.3
Chromium
Chromates(1V) and CrO2
"here are several types of mixed oxide or chromate IV , These are MCr03, M2cro4, M3Cr05 and &Cr06 (where M = Sr, Ba) and Na4CrO4;lLsLquations 80-82 illustrate typical preparation^.^,^^,'^^ The reaction of Cr03 with liquid sodium does not yield Na2Cr03 as previously thought. The compounds M2Cr04 are air-stable, coloured substances, which contain tetrahedral C a t - groups and have magnetic moments of 2.8 BM at room temperature. Crz03+ SrCrO, + 5Sr(OH)z
NaCrO, + 2Na,0 Ba,CrO,
7 3Sr2Cr04+ 5H20 9ca *C
-
+ CrO,
410'C "PC.
1 w o 'C
Na4Cr0, + Nat
(80)
(81)
2BaCr03
High pressure and temperature are re uired to produce the compounds MCr03, e.g. 38-1310 The CrIVis octahedrally coordinated in these BaCr03, which exists as several p01ytypes.l~ the structure consisting of pairs of fsce-sharing octahedra linked to other pairs by sharing corners. The C r - C r distances are cu. 2.6 A. Chromium(1V) oxide can be repared in various ways, for example by thermal decomposition of Cr03 and Cr02C12?*1' Two recent methods are the thermal decomposition of Cr(N03)3.9H20 and the oxidation of Cr2O3 with NH4C104under pressure in a heated sealed gold Its ferromagnetic behaviour (Tc = 120"C) makes it suitable for magnetic tapes. The formula Cr$Cr1vS8 has been suggested for Cr5Ss (Section 35.4.5.l.vii). For peroxo complexes see Section 35.7.7. 35.5.4
Alkyls, Alkoxides and Amides
355.41 Alkyls The chromium(1V) tetradkyls Cr% are air and light sensitive and thermally unstable to different degrees dependent upon R, but if kinetic paths are biocked through the absence of j3-hydrogen atoms, or the presence of bulky organic groups, many can be isolated. The chemistry of these tetrahedral monomers has been covered in the companion series.' A new tetraalkyl is Cr(l-adarnantyl~nethyl)~~~~~ and there is fuller information on the alkenyl compound Cr(CPh=CMe2)4.l3l5 355.4.2 Alkoxides
The stability problems with the tetraalkyls also apply to the tetraalkoxides; in addition, the high oxidative power of CI? means that alcoholysis with primary alcohols and most secondary alcohols leads to oxidation to the aldehyde or ketone and the formation of a chromium(II1) alkoxide. Relatively stable chromium(1V) alkoxides are obtained only from tertiary alcohols and some heavily substituted secondary alcohols.
(i) Syntheses and properties The tertiary butoxide C r ( 0 3 ~ ' )can ~ be prepared by the following methods: (1) the action of di-t-butyl peroxide on Cr(C&&, (2) alcoholysis of the alkylamide, e.g. C T ( N E ~ ~(3) )~, oxidation of C~(OBU')~ (prepared in situ from Cr(NEt2), and Bu'OH in excess) by various reagents, e.g. Cr02(OBu')z, Br2, Bu;O2, P ~ ( O A C or ) ~ 02,(4) oxidation of a mixture of NaOBu' and [CrC13(THF)3]with CuCl and ( 5 ) the addition of CuCl to a suspension of L~[C~(OBU')~] in refluxing THF. These methods, particularly (Z), have been used in the preparation1316 of other chromium(1V) tertiary alkoxides (Table 108). These are blue liquids or low melting solids which can be distilled in vacuum and are monomeric in cyclohexane. Their magnetic moments are close to 2.8BM and that of C ~(OB U')~ is almost independent of temperature as expected for tetrahedral Cr'" (d', p, = 2.83 BM). The electronic absorption spectrum of blue Cr(0Bu')d
Chromium
929
contains bands centred at 9100,15 200 and 25 000 cm-' assigned to the 3A2(F)+ "(F), T 1 ( F ) and 3T1(P)transitions in Td symmetry and a band at 41 000 cm-' assigned to charge transfer. No ESR signal was detected for C ~(OB U' )~ in toluene solution down to 98K due to fast relaxation in the zero-field split levels of the 3A2ground term; a small zero-field splitting was confirmed by the detection of ESR signals from frozen toluene samples at much lower ~ ) ~considerable thertemperature (10 K). From thermochemical measurements, C ~ ( O B Uhas modynamic stability (A& = -1275 kJ mol-'). Table 108 Chromium(1V) Alkoxides and Amides; Preparation and Properties
comm
Compound Cr(OR), Blue
Cr(NR,), Green
R = Bu', CMe,Et, CMeEb, CEt,, SiEt,; Cr(OBu'),(OCMe,Et),
v(Cr0) ca. 600cm-l; from Cr(NEt,), and almhol R = CHMeCMe,, from Cr(OBuf), and CMt$fIMeOH R = 1-adamantoxo, see Scheme 118 R = CHB& see Scheme 118, tetrahedral monomer: C r 4 (av), 1.773 A, O - C r - 0 , 108.8-112.4", v(Cr0) = 718 cm-' R = Et, Pr, R, = MeBu, C,Hlo; liquids, except R, = QH,,, m.p. ca. 60 "C peBca. 2.8 BM, 0 = -10" (R = Et), -3" (R2= C5Hl0)
Ref. 1 2 3 4 5
1. E.C. Alyea, J. S. Basi, D. C. Bradley and M. H. Chisholm,1. Chem. SOC. (A), 1971, 772. 2. G. Dyrkaa and J. Rocek, J. Am. Chem. Soc., 1973,95,4756. 3. M. Bochmann, G. Wikinson, G . 8.Young, M.B. Hursthouse and K. M. A. Malik, J . Chem. Suc., Dalton Tram., 1980, 901. 4. M.Bochmann, G . Wilkinson and G . B. Young, J . C k m . Soc., D u h z TMW., 1980,1879. 5. J. S. Basi, D. C. Bradley and M.H. Chisholm, J . Chem. Sac, (A), 1971, 1433.
Chromium(II) borohydride is f~rrned'~''by reduction of Cr(OBu'), with B2€&(equation 83).
B2WJ
Cr(OBu'),
THF
Cr(B&),.2THF
(83)
Although the alcoholysis of C ~(OB U' with )~ several primary and secondary alcohols leads to oxidation by CrTVto the aIdehyde or ketone,1316 alcoholysis by the secondary alcohol 3,3-dimethyl-2-butanoI affords a Cr'" alkoxide (equation 84)l3I8 which is sensitive to oxygen and moisture, but otherwise stable. Presumably the bulky t-butyl groups prevent the molecule from achieving the conformation required for hydrogen transfer in the oxidation step. Cr(OBu'),
+ 4HOCHMeCMe3 7Cr(OCHMeCMe3), + 4Bu'OH 7u "C
(84)
The methods for the preparation of chromium(1V) alkoxides are well illustrated by Scheme 118, which shows the preparations of adamantoxo (1-adamantanol = adoH, 285) and bis(tbutylmethoxo) complexes of Cr"' and CrIV.1319~1320The blue chromium(1V) complex Cr(1ado), is air-stable, presumably because the small covalent radius of CrIVallows close packing of the bulky adamantyl groups which protect against encroaching ligands. Owing to its stability and easy preparation from Cr(NEt&, and because it does not undergo redox or disproportionation reactions, Cr(l-ado)4 is preferable to Cr(OBu'), as a precursor for organic derivatives of chromium(IV), e.g. Cr(CHZSiMe3),(Scheme 118). The room temperature magnetic moment of 2.68BM is consistent with the CrIV formulation; the IR spectrum is consistent with a monomer, and a molecular ion has been observed in the mass spectrum. As with C ~ ( O B U ~ ) ~no, 'ESR ~ ' ~ signal has been detected down to 98 K.
(285) The sterically demanding secondary alcohol bis(t-buty1)methanol (2,2,4,4-tetrarnethylpentan3-01) does not react with Cr(N(SiMe&}, or Cr(NEtz)4, but the chromium(II1) complexes C~(OCHBU:)~.THFand LiCr(OCHBui)4, and the chromium(lV) complbx Cr(OCHBui)4 have
Chromium
930
Cr(1-ado),NO r(N0) = 1715 cm-'
\I
Cr'V(CHzSiMe3)4
\
Cr(NPr\),
I-adoH
LiCr(l-ado)3C1
tiazSMe3
-
-
-6O'C
\
LiNEtz
L~OCHBU~ Et*O
a
CUCl
blue-purple solid
;""" /r
/
CrCl,(THF),
LiOCHBuI
CUCl
LiCr(OCHBu;)4-THF
1-adoH
CrIv(l-ado),
[Cr(l-ad~)~]~(LiCI),
Cr(NEt2)3' in situ
Cr(OCHBu:),.THF I
CUCl I-adoH
Cr(1-ado),
from filtrate
Cr'V(NEtZ)4
Cr(OCHBu:),
4
CrCI,
Low yield of Cr(1-ado), by this route. Scheme 118
been prepared as in the lower part of Scheme 118.1320All are soluble in non-polar solvents and Cr(OCHBu5)s is a monomer in benzene, but the solid has a low moment (pCeE= 3.2 BM at 296 K) and the broad IR band, ~(Cr-o), at 555 cm-I also suggests that it is a dimer. The absence of strong bands between 450 and 750 cm-' suggests that Cr(OCHBufi), is a monomer, and this has been confirmed by an X-ray investigation. The U r - 0 angles are close to tetrahedral and the crowding in the molecule prevents coordination oligomerization (286). The short Cr-0 bond lengths (1.773 A) indicate 04Cr p n - d n bonding, which may account for the rather large Cr-0-C bond angles (140.5, 141.1').
355.4.3 Amides
Equations (85) and (86) express the general r n e t h ~ d ~ for ~ * ~the ~ * preparation of chrmnium(1V) dialkylamides. The chromium(II1) dialkylamide (Section 35.4.2.7) is extracted into pentane from the residue after removal of the solvent; careful evaporation of the pentane gives Cr(NR& which disproportionates on further heating in vacuum because of the volatility of CrW(NR2),. The method has been successful with R = Et, Pr and Rz= MeBu, C5Hlo, but failed with higher homologues because of the low volatility of the CrIV species, and with
Chromium
931
R = Me because the initial reaction is then more complicated than expressed by equation (85). In dimeric Cr2(NR2),, steric hindrance prevents chrornium(II1) from obtaining its preferred octahedral coordination; this, the ligand field stabiliiation in Cr(NR2)4 (tetrahedral, d'), the covalency of the Cr'v-NR, bond, and the polymeric structure of involatile Cr"(NEt2)2 all contribute to the driving force for reaction (86). The structural changes are represented for R = Et in equation (87). THF CrC13+ 3LiNR2 CrIn(NR2),+ 3LiC1 (85) anhydrous
2Cr"1(N&)3
40-100 T
10-3
E 2
/ \
(Et2N)2Cr111
h '
Cr"'(NEt,),
H=
CrTv(NR2)4r + Cr"(NR2),
e-
transfer
,,NEtt
(Et,N),Cr'"
\NEt,
+ [Cr1'(NEt2)21,
(87)
Et2
The magnetic moments (ca. 2.8 BM, Table 108) vary little with temperature as expected for tetrahedral CI?, and molecular ions were found in the mass spectra. Like Cr(OBu')4, no ESR signal was detected, over the temperature range 98-298 K. A band in the electronic absorption spectra (for R = Et, 3 = 13700 cm-l, E = 1200) could not be assigned with confidence to simple d-d transitions because of the covalent nature of the Cr-N bonds in which there is likely to be N+ Cr IT and p,-d, donor character. The He' UVPES of C T ( N E ~has ~ ) been ~ interpreted; it remains unclear why Cr(NEt2)4is paramagnetic and M O ( N E ~diamagnetic.1321 ~)~ An unstable blue complex, which may be Cr(02)(NPf2)3 or the chromium(1V) nitroxide complex CrO$NPr:),(ONPr5), is formed in a rapid reaction between oxygen and Cr(NPr',), in pentane. 592~132 Reactions of Cr(NEt&, some examples of which have been discussed elsewhere, are presented in Scheme 119. The unusual reactions with C02, which produce the chromium(II1) and chromium(I1) carbamato complexes (287) and (288), are believedla to proceed by C 0 2 insertion into a Cr-N bond, which promotes P-hydrogen elimination from a coordinated diethylamido ligand, and then reductive elimination of Et2NH produces a reactive chromium(I1) species Cr1I(02CNEt2)(NEt2). The subsequent reaction is dependent upon the relative concentration of C 0 2 . Et,NH
EtzNH + R'R'CO
+ Cr(OR'),
+ Cr(OCHR'R2)3
Cr(OSiR3)4f Et,NH
- R~R*CHOH
Cr(NEt&
Cr:"(02CNEt2)4(p-NEt,), dark green
cs2
Cr(S,CNEt,),
+ (S,CNEt,),
Cry(02CNEt,),(NEt2H), red-orange
(287) Scheme 119
35.5.5
Porphyrins
Chromyl(1V) porphyrins are included in Section 35.4.9.1.
35.6
CHROMIUM(V)
Chromium(V) chemistry is not extensive. Species of this oxidation state are frequently postulated as reactive intermediates during the reduction of chromium(V1). However, compounds of this oxidation state with reasonable stability are being discovered; there are
Chromium
932
several classes of well-defined complex: the pentafluoride, oxyhalides and various oxohalochromates; oxo and peroxo complexes; water-soluble chromyl(V) derivatives of &-hydroxy carboxylates; and chromyl(V) complexes of porphyrins and other multidentate ligands (Section 35.4.9.1). There is a recent review.1323
35.6.1
Halogen Compounds
Earlier work on CrF5, CrOF,, CrOC13 and the oxohalochromates(V), e.g. K[CrOF4] and K2[CrOC15], is described in refs. 222 and 1306. More recent information is outlined in Schemes 120-122 and Table 109. CF,
+ SF, f Cr"'FJ A
N02CrVF,
CsCrVF, dec llO'C
dec 142°C
Cr
Cr"'(SO,F),
XeF,,.Cr'"F, -k OSFz
F2
+ S206F2
XeF,
Cr"*F, t AF, (A = P, As, Sb)
XeF,
/\xeF4+cr 1'F3
+ XeF,.2CrLvF,
+ Cr'I'F,
V
CrF5.2SbF,(I)
//
Oz(Cr'VF,Sb$ll) --+ -02 pale yellow m.p. 175-8 "C (dec.)
I
i.e. CrVF,Sb2F,,
-
C,F,(Cr1VF4Sb2F,,) yellow green dec. 20°C
- Xc
Xe(Cr1VF,Sb,F,,)2 cream m.p. 128-134 "C (dec.)
IF5(1) in excess
IF4(Cr'VF4Sb2FII) brown dec. 153°C
Scheme 1u)
3561.1 Chromium(V) fluoride The pentafluoride is a red solid (m.p. 30°C) which can be prepared1324*13zs by the action of fluorine on chromium powder and probably has a distorted trigonal bipyramidal structure in
Cr03 + CIF or CrO,F2
0 "C
Chromium
933
CrOF3.0.1-0.2C1F brick red -
-'
0.5O2+CrF5 t- CrOF, 1w "C purple
-
CrF, +- 0.50,
K[CrOF2]
KF'HF
Scheme 121
[AsPh,][CrOCl,]
1 : 1 (AsPh,]CI
[phenH][CrO,Cl,] CrC13(acacH)(H,O)
the vapour phase.13z The solid and the liquid are fluorine-bridged polymers.L364 CrF5 is much more reactive than MoFs and WF5; it is readily hydrolyzed and attacks glass, and is a powerful oxidant and fluorinating agent: for example, it will oxidize Xe to XeF2 and XeF4;1326and in an unusual reaction with XeF6, fluorine is evolved and the brick red solid XeF6.CpF4, which decomposes ra idly in air, obtained. Curie law behaviour and a magnetic moment of 2.76 BM support the C J V formulation. The action of XeF6 in excess on CrF2 also gives XeF6CrF4.227 The viscous Reaction of SO3 and CrF, produces peroxydisulfuryl difluoride and Cr(S03F)3.1327 deep brown liquid CrF4Sb2FlI, which can be obtained from SbF, and CrFs and distils unchanged in vacuum, is another vigorous oxidizing and fluorinating agent comparable to PtF6 in its ability to oxidize dioxygen and xenon (Scheme 120). It does not react with krypton or molecular nitrogen. There is one strong absorption band at 453nm in its visible spectrum consistent with a d' system. The IR and Raman spectra support the presence of SbzFTl but not CrF:; thus the preferred structure is CrF4Sb2Fl1with fluoride bridges and partial charge separation. A strong absorption at 1860cm-' in the Raman spectrum of 02(CrF4Sb2Fll)is assignable to the dioxygenyl cation, and various bands at lower frequency in the IR and Raman spectra of this and related complexes (Scheme 120) to the (CrF4Sb2Fll)- group. The magnetic moment of 3.08 BM at 295 K is low for three unpaired electrons (3.87 BM), suggesting that there is some interaction between the unpaired electrons of 0,' and (CpF4Sb2Fl1)-. Bands characteristic of the cations are found in the IR and Raman spectra of W6(CrF4Sb2Fll) and IF4(CrF4Sb2Fll). The two T,, modes of the octahedral CrF; ion are found at 6OOcm-' and near 280cm-l in the spectra of N02CrF6 and CsCrF6, and the bands of the former at 2315 and 1360cm-' are assigned to the symmetric and asymmetric stretches of the [O=N=O]+ group.1328
35.632 C h r o m y l ~huiides It is difficult to obtain pure CrOF3 by the reaction between Cr03 or Cr02F2and ClF because t s remove the residual C1F from CrOF3.0.1-0.2ClF requires repeated treatments with F2 (Scheme 121). The urple CrOF3 is slightly hygroscopic; it attacks certain organic solvents and glass above 230 OC.l8 An apparently purer red-purple product, which hydrolyzes readily but is unreactive towards organic solvents, has been prepared by treatment of the adduct CrOF3.0.3BrF3 with fluorine under mild conditions in which reaction to form CrFs (Scheme 121) does not Chromylly chloride (Scheme 122) slowly decomposes at room temperature and in organic s01vents.l~~ 32 From its IR spectrum Cr0Cl3 in an argon matrix is a molecular species of C% symmetry (Ci-Cr-CI angle -lOSO), but an additional v ( C r 4 1 ) band, and the shift of other
Chromium
934
Table 109 Chromyl(V) Compounds Ref. Chromyl(V) halides CrOF, Purple
1.82 2.87
lo00 -990
CrOC1, Red-black
1.8
1022 1024
740-600 (Cr-F) -670 (Cr-F) -565 (Cr-F-Cr) 620-4k (Cr-F&)
”
-
435sh, 408, 333
condensed 0% CsI, 250 K 1018 462,410~
/
4
Y
Ar matrix, 12K Oxotetrahalochromates(V)a K[CrOF,]
1.76
-
-
1.78
1019
640 Cr-F 500 Cr-F-Cr? 650 Cr-F 505 Cr-F-Cr 398,343sh
1.75
1016
397,347sh
7
1.84
1016
398,348z.h
7
1.80 1.79 1.78 1.78 (1.64)c 1.74 (1.77)c
1016 1020 1022 1035,960 1005,965
408sh, 380br 400 395,348sh 395 400
1.74‘
1025,950
385
1.73‘
1020,940
415
1.62 1.87(1.96)‘ 1.91
1019 1020,940 1017
348 405 401,348~
1.52”
960,890
1.77 1.90 1.78
-
1.97
950 927 945 933 945 950 956
Cs[CrOF4]
Dark red [ q u i d ][CrOC14] Dark red [iquinH][CrOCI,] Dark red [NMe4][CrOC14] Brown red [NEt,][CrOCI,] Dark red [NPr,l[CrOCl,I Gold [PBu,] [CrOCl,] Red [PPh,Bz][ CrOCl,] Dark brown [AsPh,] [CrOCI,] Yellow brown [Pcbl[crocbl Red-brown Oxopentahalochromates(V) ‘[NEt,],[CrOF,]’ Yellow [bipyH,][CrOCl,lb Brown [4,4’-bipyH,] [CrOCl,] Dark brown [phenH,] [CrOCI,] Cs,[ CrOCIS] Dark red Rb,[CrOCl,] Dark red K,[CrOCI,] Amine adducts of CrOC1, CrOCI,( bipy) Brown, prep. by thermal dewmp. Cr0Cl3(bipy) Direct reaction (Scheme 122) from CrOCI, CrOCl,(phen) CrOCl3(Z-CNpy) Red
1005
1.85 1.92
955 -
334,312sh
340 320
2 6 7
10 7 10 7 8 11 8 11 11
954
-
12
-
898 964
360
3 3
1.76 1.87 1.86 1.78
945 957 945 930
1.94
-
“NO-CrO,F,and S0,.2Cr0,F2 (Section 35.7.1) may contain Cr”. bRef. 14 contains the preparation of additional salts of [CrOCIJ and [CrOcl,]’-.
‘Solution measurement (NMR).
354
1,5
1.85
Petiodato complex
Cr(IOd(phe4,
1020
-
-
10 12 10 10
13
935
Chromium Table 109 (continued) 1. P. J. Green, B. M. Johnson. T. M. Loehr and G. L. Gard, Inorg. C h m . , 1982,21,3562. 2. E. G. Hope, P. J. Jones, W. Levason, J. S. Ogden, M. Tajik and J. W. Turff, 1. Chem. SOC., Dalton T r m . , 1984, 2445. 3. E. A. Seddon, K. R. Seddon and V. H. Thomas, Tramition Met. Chem., 1978,3,318. 4. W. Levason, J. S. Ogden and A. J. Rest, J . Chem. Soc., Dalton.Tmm., 1980,419. 5 . C. Rosenblum and S. L. Holt, Transition Met. Chem., 1972, 7 , 87. 6. R. Colton and J. H. Canterford, 'Halides of the First Row Transition Metals', Wiley-Interscience,London, 1969, chap. 4. I. K. R. Seddon and V. H. Thomas, 1. Chem. SOC., Dalton Tram., 1971,2195. 8. 0 . V. Ziebarth and J. Selbin, J. Inorg. N u d Chem., 1970, 32, 849. 9. K. R. Seddon and V. H. Thomas, Inorg. Chem., 1978, 17, 749. IO. H. K. Saha and S. K. Ghosh, J. indim Chem. Soc., 1983,60,599. 11. J. E. Ferguson, A. M . Grccnawey and B. R. Penfold, Inorg. Chim. Acta, 1983, 7l,29. 12. S. Sarkar and J. P.Singh, I . Chem. SOC., Chem. Commun., 1974, 509. 13. Y. Sulfab, N. I. Al-Shstti and M. A. Hussein, Inorg. Chim. Acto, 19S4,86, W9. 14, H.L. Krauss, M. Leder and G. Miinster, Chem. Ber,, 1963.96, 3008.
bands to lower frequency, in the spectrum of the poorly volatile solid suggest polymerization.1332The IR spectrum of CrOF3 also indicates halide bridging (Table 109).
35.6.1.3 Oxohalochromates(V) and other derivatives of chromyl(V) halides CrOCl,(bipy) exists in two forms because the product from direct reaction (Scheme 122) exhibits a Cr--0 stretching frequency at 964cm- , but the product from thermal decomposition of [bipyH2][CrOCi5]has a broad band at 898 cm-l suggestive of polymerization through C-0 * Cr interactions. The protons needed to € o m [phenH][CrOzClz]and [pyH][CrOCl,] (Scheme 122) are presumed to have come from oxidation of the amine. The magnetic moments, ESR and electronic spectra of the oxochlorochromate derivatives are characteristic of Cr" complexes, and they all disproportionate in water.t331The ionization energy (10 eV) of the d electron in CrOC13 has been derived from He' and He" PES.1333 In a typical preparation of a tetrachlorooxochromate(V) salt, the chloride AC1 in glacial acetic acid is added to Cr03 in the same solvent saturated with hydrogen chloride (reaction 88). With some cations pentachlorooxochromates(V) can be obtained but the relation between cation size and coordination number is unclear. 1334~1335 A pentafluorooxochromate(V), [NEt4l2[Cr0F3] was prepared by the reaction of AgF/HF with [NEt4][CrOC14] in dichl~romethane,'~~~ but its composition is in doubt.'"' Care is needed to prevent contamina~ ~ K2[CrOC15] ~ . ~ ~ ~ ~ is usually contaminated with tion by Cr"' or CF1 i m p ~ r i t i e s , ~e.g. K2[CrC15(H20)],although pure samples have been obtained,1336and attempts to prepare bromooxochromates(V) from Cr03 Yoduced perbromide (Br;) , although the preparation of [bipyH2][CrOBr5]has been claimed.' 37 The v(Cr=O) and v(Cr-Cl) vibrations (Table 109) of [CrOC14]- and [CrOC15]- are in the ranges expected for isolated anions, and decrease with increase in coordination number from five to six as commonly found in other systems. An X-ray study of [AsPb][CrOC14 has confirmed the presence of square pyramidal anions (Cr-0 = 1.519, Cr-Cl = 2.240 ).1338 The short Cr-0 distance indicates considerablep, + d, bonding. From a single crystal the electronic absorption bands of [CrOC14]- at ca, 13 000 cm-l and 18 000 cm-' have been assigned to d, +dxr,yrand Cr-O(n)-+ Cr--O(u*) transitions, and bands at higher frequency to charge transfer, but other calculations1340assign the band at 18 000 cm-l to halogen-to-metal charge transfer. The ESR spectra have been interpreted. The fact that the addition of [NEt4]CI to a solution of [AsPh4][CrOCI4]changed 9: from 1.988 to 1.970 indicates that compounds earlier thought to contain [CrOCl5I2- contained [CrOC14]-.13,'
A
+
Cr03 ACl
MeC02H
[A][CrOCl,]
HClW
The product of reaction between Cr02C12and PCls (equation 89) at high temperature or in the presence of a reducing agent has been identified as [PC14][CrC&](Section 35.4.7.2). The product at room temperature has been shown to be the reactive chromyl(V) salt [PC14][CrOC14] The pyridine adducts, (Table 109) and not the chromium(V1) adduct Cr02Clz.PC15.1342 CrOzClz(py)zand Cr02C12@y),are believed to be mixtures of chromyl(V) species, and from reaction in acetic acid [pyH][CrOC14],[pyH][CrOzCla]and [Cr30(MeC02)6(py)2(MeC02H)]Cl have been isolated; similarly [bipyH][Cr02C12]has been obtained rather than Cr02C12(bipy),
936
Chromium
and Ph4AsC1 and CsCl with CrOzC12 in acetic acid yield mixtures of [AsPh4][Cr03C1] and [AsPh4][CrOC14],and Cs[Cr03C1] and Cs2[CrOC15],re~pectively.'~~ Cr02C1, + 2PC15
20°C
[PC14][CrOC14]+ POC&+ 0.5C1,
039)
Solutions containing [CrOC14]- and [CrOCl5I2- anions slowly generate Cr"'. This has been exploited in the preparation*334of the Cr"' anions [CrC&(MeCOa),]- and [CrCl5MeCO2HI2-, and various bipy complexes, e.g. [CrC13(bipy)(NCMe)], from [bipyH2][CrOC15].1344 Oxidation of ~is-[Cr(H,O>,(phen),](NO~)~ with sodium periodate at pH 4 produces the (Table 109). periodato complex [Cr(IO,) (pl~en)~]
-
35.6.2 Complexes of Oxide and Peroxide There are some well-defined chromium(V c~mplexes'~" in which oxide is the ligand: Mii)[(Cr04)2] (M= Ca, Sr, Ba),1306M:'(Cr04)30H M i c r o 4 (M = Li, Na,1305 K, Rb, (M = Ca, Sr, Ba) and M5(Cr04)3X(M = Ca, Sr, Ba, X = F or Cl). In these the Cr occupies a tetrahedral site. There are also the peroxides'346 M:[CT(O~)~](M=N€&, Na, K), which are obtained by the action of 30% HzOz on alkaline chromate solutions. They are red-brown, paramagnetic salts (peB= 2.94 BM) of some stability, and in K3[CrOs] the anion has a distorted dodecahedral arrangement (289) with the oxygen coordination occurring in pairs, and the -0 distance close to that for peroxo bonds (0%= 1.49 A),1306,1346*1347 Not much coordination chemistry of these complexes has been published since the reviews b Rosenblum and H ~ l t and ' ~ ~Connor and E b s ~ 0 r t h . lSome ~ ~ investigations of the ESR'Y48 and electronic spectra1349of CrOi- doped in chloroapatites [(P04)3C1- salts] have been reported. The belief that Sr5(P04)3C1is isotypic with apatite has been disproved by a single crystal study.'350 The three equilibrium phases in the ternary system CaO-CrM0&&n03 (the ternary compound, thought to have a chromate chromite formulation such as Cs(Cr~Cr$")O,, calcium chromate CaCrw04 and monocalcium chromite CaCr$"04) have been further studied. Comparisons of the solid state IR, diffuse reflectance and ESR spectra of the three phases suggest that Cfl" is not present in the ternary phase, but Crv02- anions are. Hence, the solid ternary phase is best described as Ca3(Crv04)2rather than Cag(Cry'Cri'')024. The latter represents what ha ens when the ternary phase is dissolved in acid, where Cr" disproportionates into CrW and C 13" Disproportionation of Li3Cr04 (equation 90) takes place in a KCl/LiCl melt.1352The thermal decomposition of some rare earth chromates(V) has been related to their structure^,^^^^ and magnetic measurements on KSrCr04 and KBaCr04 confirm the V oxidation
P.
3Li,Crv0, -----, 2Li,CrWO4+ LiCr'"02
+ 2Li20
(90)
35.6.3 Complexes of Tertiary a-Hydroxy Carboxylates
The formation and decomposition of Crv in aqueous and non-aqueous media during the oxidation of organic substrates such as oxalic acid and ethylene glycol by potassium dichromate has been recognized for some time. No study resulted in the isolation of a stable, well-characterized chromium(V) complex until 19'78 when potassium bis(2-hydroxy-2methylbutyrato)oxochromate(V) monohydrate was prepared from chromium trioxide and the tertiary ar-hydroxy acid in dilute perchloric acid according to equation (91). The Cr", which is
Chromium
937
produced in equimolar quantity with Cr"', undergoes further reduction, first slowly and then very rapidly, Good yields of the potassium salt are obtained after neutralization with KHC03, provided the separation procedures are carried out expeditiously, at low temperature as far as possible, and with careful control of pH.1355A simpler, generally applicable route with good yields to this type of complex is the reaction of anhydrous Na2Cr207 with the tertiary a-hydroxy acids RR'C(OH)C02H (R = R' = Me, Et, Bu; R = Me, R' = Et, Ph, R,R'= (CH2),, n = 4, 5 ) in acetone (equation 92).1356The complexes are insoluble in non-polar solvents, soluble in water and acetone and heat stable. Whilst higher alkyl groups stabilize the complex, phenyl groups have a destabilizing effect. Many other hydroxy acids form unstable Crv complexes in solution. 2HCrO;
+ 4EtMeC(OH)CO,H
4H+, -16°C
1
[OCr(OzCOCMeEt)2]-+ Cr" + 2MeCOEt + 2C0, + 5Hz0 (91) NazCr20,+ 5RRC(OH)CO2H 2Na[OCr(OZCOCRR'),]+ RR'CO + COz+ 5H20 (92) The salt Na[OCr(02COCEt2)2 has been used in recent mechanistic studies of the oxidation of NH30Ht and N2H; by Cr ; the former yields Cr-NO derivatives (see also Section 35.4.2.6) and the latter N2 and Cr"'.1357 The structure of K[0Cr(O2COCMeEt),] (290), obtained with difficulty from a poor crystal, contains pentacoordinate Crv and tram bidentate ligands of the same chirality, aIthough enantiomers of the complete anions are present in the crystal. The short apical Cr-0 bond distance (Table 110) is compatible with a C r V 4 group. The other 0-0 distances are greater and differ from each other, and all O-Cr-0 bond angles depart considerably from 90" or 180" so that the coordination polyhedron is much distorted from a square pyramid. In the solid the ethyl groups close off the osition trans to the chromyl oxygen. The oxidation state of (290), and of the sodium salts,135Bhas been established by analytical methods, including a new iodometric method which determines CrV separately from its ability to oxidize iodide rapidly at much lower acidities than Crw does. The electronic spectrum, magnetic moment and ESR data (Table 110) confirm the Crv assignment, and the C-0 stretching frequency is consistent with a terminal Cr-0 multiple bond. The stability of these Crv complexes to oxidative decomposition has not yet been explained.
Table 110 Complexes of Tertiary cu-Hydroxy Carboxylic Acid Dianions Complex
pd(BM)
K[OCr(O,COCMeEt),J~HzO'
2.05 (H,O)
Dark brown
M[OCr@fp)J [M = K, Cs (purple), NEt4 (blue)]
1.79 (K) 1.71(Cs) 2.33 (NEb)
v(Cr=O) (cm-')
&7
Ref.
994(Ksalt) 1005 (Na salt)
1.9780
1 2 3,4
-
-
1.9794
-
'Bond distances: C!==Cl, 1.554; Cr-0 (carboxyl), 1.911 (av), Cr-0 (OH-derived), 1.781 A (av). 1. M.Krumpok, B. G. DeBoer and 3. Rotek, 1. Am. Chem. Soc., 1978,100,145. 2. M.Krumpok and J. RoEek, J. Am. Chem. Soc., 1979, lM,3206. 3. N. Rajasekar, R. Subramaniam and E. S. Gould, Znorg. Chon., 1983, 22,971. 4. C. 5. Willis, 3. Chem. SOC., Chem. Commun., 1972,944.
938
Chromium
Other chromium(V) complexes stable to reduction and hydrolysis can be prepared1358-1359 by reaction of chromate with pefiuoropinacol (CF3)zC(OH)C(OH)(CF3)2(Hzpfp) (equation 93). Hydroxy acids slowly displace two chloride ligands of Csz[CrOCl5] in hot MeCN to give Cr0(AB)Cl3 in which AB is tartrate, hydroxymalonate or malate. ESR data indicate that these are Cr" complexes but the magnetic moments are high."", IR bands near 940cm-' and 340 cm-I have been assigned to the v(Cr0) and v(CrC1) vibrations respectively. A dibenzoato complex Cr(02CC&)zC13 has also been d e s ~ r i b e d . ' ~ ~
35.6.4
Porphyrins and Schiff's Bases
Chromium(V) complexes of these ligands are included in Section 35.4.9.1. 35.7 CHROMIUM(VI)
Although there are several very important chromium(V1) compounds, the powerful oxidizing ability of this oxidation state leads to the oxidation of many potential ligands, and its coordination chemistry is limited. However, the oxidizing abilit is exploited in many organic syntheses: and a new oxidizing system is Cr03/HBF4/MeCN."J A safer method of preparing the reagent t-butyl chromate has been re~0rted.l~"Aqueous solutions of CrO? react with excess NHzOH in the presence of various coligands to generate complexes containing the [Cr(NO)]'+ moiety (Section 35.4.2.6). References to the many extensive accounts of the preparation, properties and reactions of chromium(V1 compounds, and their industrial and analytical importance have been collected. 29136
I
35.7.1
1
Hexafluoride and Chromyl(VI) Halides
Chromium hexafluoride (Table ill), the only hexafluoride of the first transition series, was first prepared in small yields together with CrF5 from the metal powder and fluorine at 400°C and 350 atm. It decomposes rapidly to CrF5 and F2. Recently, it has been prepared from Cr03 and Fz under less extreme conditions, Le. 170 "C and 25 atm. The lemon yellow CrF6 sublimes at room temperature from the CrF, which is also formed. IR spectra (N2matrix) showed Cr-F stretching vibrations centred at 759cm-l. Weaker bands due to some Cr02F2, CrOF4, CrF, and CrF4 were also identified in the spectra.1364It has been suggested that the fluorination of Cr 03 by F2 at 4atm is stepwise, because as the reaction temperature increases the main products are Cr02F2 (150 "C), CrOF4 (220 "C) and a mixture of CrF5 and CrF4 (250 "C) believed to come from the decomposition of CrF6.1365This is confirmed by the new preparation Of crF6. The oxotetrafluoride CrOF4 is an easily hydrolyzed solid obtained as a by-product when fluorine is passed over chromium,'365but it is better prepared from Cr03as just outlined. It is a powerful fluorinating and oxidizing agent, is fluoride bridged in the solid state, molecular (GU) in a low temperature matrix, and forms the salt Cs[CrOF5] (Table lll).1366 The dioxofluoride Cr02F2 can be obtained in many ways. The methods starting from C r0 3 outlined in equations (94) to (97) are generally more convenient1367than earlier ones which involve F2365,HF, SF4, SeF4, C O F or ~ IF5. In reactions (96)and (97)purification is simplified because CrOZFzis volatile and WOF, and MoOF4 are not. Treatment of Cr02C12with F2 or C1F also produces Cr02F2. The reaction between Cr02F2 and Cr02Clz gives CrOZClF,which from its IR and mass spectra is a true com 0 ~ n d . Decomposition l~~ of CrOZF2to CrOF2 occurs at high temperature (500---525"C).136bgIt is a powerful oxidizing agent and reacts vigorously with NO, NO2 and SOz as equations (98) to (100) show. CrO,
+ excess C1F
CrO, + COF,
0 "C
185 'c
CrO,F,
+ Cl, + 0,+ C10,F
Cr02F2+ CO,
194) (95)
Chromium
939
Table 111 Chromium(V1) Fluoride, the Chrornyl(VI) Halides and Cs[CrOF,]
Comments
Compound
Ref.
Unstable above - 100 "C,dec. to CrF, and F,, v(Cr-F), 759 em-' M.p. 55 "C, separated by fractional sublimation, readily bydrolyzed, reacts with Pyrex, v ( c I = o ) , 1020, Cr-F, 760-690, Cr-F-0, -500 cm-l M.p. 31.6"C, reactive, attacks glass, sublimes 29.6"C,stable in dark; v ( C r - O) , 1006, 1016 cm-', v(Cr-F), 727,789 cm-' B.p. 117 "C, fumes in moist air, heatand light-sensitive,ignites some organic compounds; C-0, 1.581 A, Cr-CI, 2.126 A, OCrO, 108.5", ClCrCl, 113.3", OCrCl, 108.7" Unstable even below room temp. CsF, CrOF,, 100 "C,N,; v(Cr=O), 955, v(Cr-F), -690 cm-'
CrF, Yellow CrOF, Dark red
CrO,F, Dark violet-red CrO,Cl, Deep red (1)
Cr02Br2 Cs[CrOF,] Orange-brown
6 3
E. G. Hope, P. J . Jones, W. Lcvason, J. S. Ogden and M. Tajik, J . Chem. Chem. Commun., 1984, 1355. 2. A. J. Edwards, W. E. Falconer and W. A. Sunder, J. Chem. SOC., Dalton Trans., 1974,541. 3. E. G.Hope, P. J. Jones, W. Levason, J. S. Ogden, M. Tajik and J. W. Turff, J . Chem. SOC.,Dalton Trans., 1985, 529. 4. I. R. Beattie, C. 5 . Marsden and J . S . Ogden, J . Chem. Soc., Dalton Trans., 1.
Soc.,
1980, 535. 5. P. J. Green and G. L. Gard, Znorg. Chem., 19?7,16, 1243. 6. R. Colton and J. H. Canterford, 'Halides of the First Row Transition Metals', Wiiley-Interscience, London, 1969, vol. 1, chap. 4. 7. C. J. Marsden, L. Hedberg and K. Hedberg, Inorg. Chem., 1982,21, 1115.
CrO,
+ WF6
00,
+ MoF,
125 "C
CrO,F,
125 'C
f
WOF,
(96)
Cr02F2+ MoOF,
(97)
NO + CrO,F, = NO.CrO,F, SO, + 2CrO,F, = SOz-2Cr0,F, 2N0, + CrO2F2= (NOz),CrO,F,
Magnetic moment measurements suggest that the solids produced in equations (98) and (99) contain Cr" and that produced in equation (100) contains Cr1v.1370 Other reactions of Cr02F2 are shown in Scheme 123. N02[Cr02F,]
Cr0,(CF3C0,)2 + CF3COF
Cr02(SOP),
- NOF
NO[CrO,F,]
r
t
Na,(Cr0,F,(N03)2]
.1Sd
A
MFZO~
Cr02F2
M[CrO,F,] M,[CrO,F,]
M = Ca, Mg M = Na, K,Cs
CrO,(NO& -tNa,[Cr02F41, also K, Cs salts Scheme 123
940
Chromium
Chromyl chloride Cr02C12can be prepared by distiliation from a mixture of an alkali metd chloride and concentrated sulfuric acid with an alkali metal dichromate or Cr03. It can also be prepared from dichromate and HC1, Cr03 and an acid chloride such as PCls or HS03C1,222or Cr 03 and AlC13 in molten LiCl/NaCI/KC1.1371 Chromyl bromide Cr02Br2is thermally unstable and little is known of its properties.m It can be prepared from Cr02C12and HBr in excess at low temperature, and by reaction of Cr03 with CF3COBr and HBr in the resence of P4Ol0.The existence of the iodide has been claimed but as mentioned in a review'3k of chromyl(V1) compounds this is unlikely. Since the metal in the chrornyl(VI) halides Cr02F2 and CrOzC12 is formally in the do oxidation state, the structures of these mzlecules w o u s be expected to follow the valence shell electron pair repulsion theory, Le. O=Cr=O > X-Cr-X (X = F or CI). From t& gas phase Raman and matrix IR spectra'3n of Cr02F2 the angles are O G O 102.5" and FCrF 124', in agreement with the earlier values from electron but-not with those hzrn microwave spectroscopy. Later electron diffraction values137s are OCrO 107.8' and FCrF 111.9". Irrespective of the discrepancies, it is clear that molecular Cr02F2, in common with many do complexes of the transition metak, does not folloxthe VESPR theory. The angles in Cr02C12 the same pattern: OCrO = 108.5", ClCrCl = 113.3' (electron diffraction). The IR spectrum of Cr02C12in an argon matrix has been reported.1377 The Raman spectrum of liquid Cr02F2 can be assigned in Ch,symmetry, and a band at 708cm-' arisin from the symmetric Cr-F stretch is replaced in the solid-state spectrum by one at 540 cmThe lowered frequency suggests polymerization through fluoride bridges in the solid, and this is consistent with the similar unit cell parameters of and VOF,, which is known to be polymeric.
B
35.7.2
Other Chromyl(VI) Compounds
Chromyl derivatives Cr02X2(X = nitrate, sulfate, perchlorate, fluorosulfate, borate, acetate, trichloroacetate, azide and benzoate) have long been known.2 The perchlorate Cr02(C104)2 has been obtained as a minor product in the reaction of C1206 with CrC13. From its IR spectrum Cr02(C10& contains monodentate perchlorate groups so the chromium has a tetrahedral environment like CrOzC12.851 Chromyl nitrate is a red, easily hydrolyzed liquid that can be prepared from N205and Cr03, CrOZCl2or KCr03C1. A method which avoids the use of N205 is the reaction of Cr0& with sodium nitrate (Scheme lB).'3m 3esides the.chromy1 derivatives in Scheme 123, CrOzFzforms the adduct CrO2F2*TaF~, Cr02F2*SbFsand Cr02Fy2SbF5 which may be [Cr02F]" salts. The fluoro- and chloro-sulfates can be isolated as green solids, stable if dry, by reactions (101) and (102). Brown crystals can be sublimed from green Cr02(S03F)2,1382 and if the reaction mixture in equation (101) is heated, dark brown crystals separate.1383In each case the brown form and the green form have substantially the same analyses, IR spectra and powder photographs { Cr02(S03F)2}and are diamagnetic; the different colours may be due to different degrees of polymerization. Besides Cr02C12, Cr03, K2Cr04 and K2Cr207 are solvolyzed to Cr02(S03C1)2 by HS03C1.1383,'384In disulfuric acid Cr02(HS04)2 forms, identified by conductance and freezing point data and electronic ~ p e c t r o s c o p y . ~ ~ * ~ , ~ ~ * ~ CrO,Cl,
+ S,OP,
CrO,Cl, f 2HS0,Cl
--
CrO,(SO,F),
+ C1,
Cr0,(S03Cl),
(101)
+ 2HCI
(102)
The reactions (103) to (110) afford further chromyl derivatives: Cr02(02CR)2 (R = CF3, CClF2 or C3F7),1386 Cr02(03SCF& Cr02(N03)(02CCF3)i~~ C r 0 ~ ( 0 2 P F 2 ) 2 , ' ~ ~ CrOz(OTeFs)2,1389Cr02(OSeF5)2,1390 Cr02C1N3,13'l Cr02C12(ICN)2 and [Cr02C1(NCS)3]2. The reaction of Cr02C12in CS2 or C C 4 with various donor molecules (acetone, DMSO, THF, Ph3P, etc.) does not produce stable adducts of higher coordination number but oxidation of the base and ill-defined Cr'" products. 1393
- -
CrO, + (RCO),O CrO,(O,CCF,), CrO,(O,CCF,),
+ HNO,
+CF3S03H
+ B(OTeF,),
-
Cr0,(02CR),
Cr02(03SCF&-+ CF,CO,H
+
C r 0 2 ( N 0 3 ) ( 0 2 C C F 3 ) Cr0,(NQ3)z
00,I- P,O,F, CrF,
_+
+ CF3C02H
CrO,(O,PF,),
CrO,(OTeF,),
+ BF,,
(103)
(W (105)
(1061 TeF,, CrF,?
(107)
---
Chromium
+ Hg(OSeF&
CrO,Cl,
+
CrOzClz ICN
+ (SCN),
CrO,Cl,
941 CrO,(OSeF,),
CrO,Cl,(ICN), [CrO,CI,(NCS),J,
The chromyl derivatives generally hydrolyze readily to chromate and the acid corresponding to the coordinated anion. Thermal decomposition of yellow Cr02(03SCF3)2 and C T O ~ ( O ~ C C Fproduces ~)~ oxygen and the green anhydrous chromium(I1I) salts is much less stable and readily yields Cr(03SCF3)31387and C T ( O ~ C C F ~ ) ~CrO2(O2PF2), : Cr(0zPF2)3; its solutions in C C 4 are moderately stable, like those of Cr02(SCN), and Cr0z(NC0)2, and have eiectronic spectra similar to those of other chromyl complexes. 13" Chromyl chloride reacts with monocarboxylic acids to form trinuclear cations [Cr30(02CR)6(RC02H)3]+(R = H, Me); the low moments of the formate and chloride salts and of derivatives such as Cr30(02CMe)6L3]Cl (L = py, NH3) suggest similar structures to other trinuclear complexes' 94 (Section 35.4.4.10). The anionic difluorophosphato complex K2[Cr02(02PF2)4]is prepared by reaction of KZCrO4 with P203F4.1388From their IR and N)~a cis structure with Raman spectra, it is concluded that the adduct C I-O~C ~~(IChas N-bonded ICN and [Cr02C12(NCS)3]2 has an -NCSSCN-bridged structure.'39z
I
35.7.3
Chromiam(VI) Oxy Compounds
35.7.3.1 Chromium (W)oxide Most chromium(V1) compounds may be considered to be derived from the orange-red trioxide Cr03, the anhydride of chromic acid, which can be crystallized from aqueous solutions of chromates and dichromates by the addition of sulfuric acid. When heated above its melting point, Cr03 loses oxygen to form Cr308, Cr205, CrOz and finally Crz03. It consists of infinite chains of distorted Cr04 tetrahedra sharing two corners; the bridging Cr-0 distances are 1.748 8, and the terminal Cr-0 distances 1.599 A. The angle at the bridging oxygen is 143°.1306 Electron diffraction data for aseous Cr03 can be best interpreted by assuming that it consists of trimers and tetramers.13 Depolymerization occurs on dissolution in water (to form HzCr04) and in the few organic solvents in which it is soluble. It is a powerful oxidizing agent. Adducts, Cr03py and Cr03pyz, used as organic oxidants in CH2C12,are formed by pyridine bases, and with alcohols chromate esters are ~ b t a i n e d . ~ , ' ~ ~
F
35.7.3.2 Chromafa, dichromates and polychromates
When Cr03 is dissolved in water the equilibria (111) to (115) are set up. Above pH8 the main species is yellow chromate CrOz-; in the p H range 2-6 HCr0; and orange-red dichromate CrzO$- are in equilibrium; and below pH 1 H2Cr04predominates. Chromates and dichromates of ammonium and the common metal ions are known: the ammonium, magnesium and alkali metal chromates are soluble in water, but those of silver, lead and the alkaline earth metals are only slightly soluble. Whether chromates or dichromates crystallize depends on the pH and the cation present. At high Cr"' concentrations trichromates and tetrachromates form and some have been isolated as solids, e.g. KzCr3Ol0and K2Cr4013,but, unlike molybdenum and tungsten, chromium does not exhibit an extensive iso- or hetero-polyanion chemistry. Many double chromates and basic chromates are also known.' HLCr04 e H C r O ; + H ' HCrO;
e
Cr,O:-+H,O Cr,O:-
+ OH-
HCrO;
Cr@-+H'
e 2HCrO; HCrO;
+ OH- e
Cr@-
+ Cr0:+ H,O
Chromates and polychromates of organic cations can be prepared by careful hydrolysis of the appropriate ester with the organic base in organic solvents. Examples are (NBunH3)2Cr04, (pyH)2Cr3010and ( N B I G ) ~ C ~ ~ O ~ ~ . ' ~ ~ Whereas the ester bis(trimethylsily1) chromate is explosive and readily hydrolyzed ,13% the crystal of the triphenyl derivative has been obtained (Table 112). The germanium, tin and lead analogues have been prepared by reactions (116) to (118). These compounds are
Chromium
942
better regarded as chromate esters than chromyl(V1) compounds .1400 Electrochemical reduction of (Ph3XO)zCrOz(X = Si, Ge, Sn or Pb) is generally irreversible and of little use in the characterization of chromium(V1) complexes.1401 Chromates of the type Me3SbCr04 and [hae3Sn12[Cr04]are also ~ I I O W I I . ~ ~ ~ ~ 2Ph3GeBr+ Ag,CrO, 2Ph3SnC1+ Ag,CrO, 2Ph3PbC1 Na,CrO,
+
--
(Ph,GeO),CrO, + 2AgBr (Ph,SnO),CrO, + 2AgCl (Ph3PbO),CrOz+ 2NaCl
(116) (117) (118)
Table 112 Bond Distances in CrO, and Representative Chromates and Polychromates Bond dktunces (A)
Compound
(av), 1.599, C r - 0 (br), 1.748& C r s r , 143" (av), 1.658 A, 0 - 0 4 (av), 109.7" Cr-0, 1.583 8, Cr-0.. . . .Ce4+, 1.665, 1.685, 1.6368, Cr-4 (av), 1.644 A CaCrO,.H,O C r - 0 , 1.69, 1.67, 1.63 8, Cr-O.-.-Ag+, 111.6, 110.1, 109.3, 106.1A angular distortion through interaction Ag+ C r 4 , 1.65, Cr-0. . . .Zn2', 1.678, (bidentate CrOi-) Cr-0 (av), 1.64 A; uncommon coexistence of Cr0:- and Cl-. C r 4 (av), 1.65 A, chains of edge-sharing pentagonal bipyramids of HfO, interconnected by CrO, tetrahedra; Zr compound isomorphous M'=K, M'I'=Fe, C r a ( a v ) , 1.638, M'M:11(OH),(Cr0,)2 MI= Na, M"' = AI, C r - 0 (av), 1.69 8, CrOZ- linked to Si by 0 bridges (Ph,SiO),CrO, Cr-0, 1.54, Cr-O(Si), 1.74A Si--Q--Cr, 133.1" and 162.7" [CrO,], C r - 0 (av), 1.653 8, KBi(Cr04)(Cr,0,)-H,0 [Cr,O,], C r - 0 (av), 1.623, C r - 0 (br), 1.782 A, C r - M r , 118.2' C r 4 (av), 1.65 8, C T(br),~ 1.76 8, P2Jn form, C r 4 (av), 1.596 A C r 4 (br), 1,796A, 0-0-0, 122.9" Cr-0, 1.634, Cr-0 (br), 1.781A C r - M r , 121.0" Cr-4 (av), 1.595, C r - 0 (br), 1.764 8, C r - O - C r , 129.0' C r 4 (av). 165. C r - O - C r (av), 1.75 8, Cr--O-Cr, 132.7' C r - 0 , 1.576-1.621, C r - 0 (br), 1.691-1.8468, C r a r , 147.2", 139.3", 1 k . k BaH[O,CrOPO,OCrO,] .H,O C r ( l v , 1.585, 1.630, 1.600, Cr(l)-O. . .P, 1.826A C r ( 2 b - 0 , 1.616, 1.611, 1.602, Cr(2)-0. .P, 1.828 A
Cr03 (NH4)2Cr04 Ce(Cr04),.2H,0
Cr-0 Cr-0
+
1. J. S. Stephens and D. W. J. Cruikshank, A m Crystullogr., Sect. B, 1970, 26, 222. 2. J. S. Stephens and D. W. J. Cruikshank, A& Crystullogr., Sect. B, 1970, 26, 437. 3. 0. Lindgren, Acta Chem. Sand., Ser. A , 1977, 31, 167. 4. 0. Bars, J. Y. Le Marouille and D. Grandjean,Acta Crysrullogr., Sect. B, 1977, 33, 3751. 5. M. L. Hackert and R. A. Jacobson, J . Solid State Chem., 1971, 3, 364. 6. M. Harel, C. Knobler and J . D. McCullough, Znorg. Chem., 1969, 8, 11. 7. B. N. Figgis, B. W. Skelton and A. H. White, A u t . J . Chem., 1979,32,417. 8 . M. Hansson and W. Mark, Actn Chern. Scand., 1973,27, 3467. 9. Y. Cudennec, A. Riou, A. Bonnin and P. Caillet, Rev. Chim. Miner., 1980, 17,158. 10. B. Stensland and P. Kierkegaard, Acta Chem. Scand., 1970,24,211. 11. A. Riou, Y. Gerault and Y. CudenneE, Acta CrystaUogr., Sect. B, 1962, 38, 1693. 12. Y. Cudennec,J. Znorg. Nucl. Chem.,1977, 39, 1711. 13, P. Lafgren, Acta Chem. Scund., 19771, 25, 44. 14. G. A. P. Dalgaard, A. C. Hazel1 and R. G . Hmell, Acta Chem. Scund., Set. A , 1974,28, 541. 15. F. Dahan, Acta Crystullogr., Sect. B , 1975,31, 423. 16. A. Stepien and M. J. Grabowski,Actn Crystullogr., Sect. B, 1977, 33, 2924. 17. P. Ufgren, Acta Crystallogr.. Sect. B, 1973, 29, 2141. 18. M. T. Averbuch-Pouchot,A. Durif and J. C. Guitel, Acta Crystullogr., Sect. B, 1977,33, 1431.
Ref.
1 2 3 4
5 6 7
8
9
10 11 12 13
14 15 16 17
18
Chromium
943
In recent investigations (see also Section 35.6.2) the double chromates KLn(CrO& (Ln = La-Sm), RbLn(CrO& (Ln = La-Nd) and CsLa(Cr04);? have been found to be isostructural with croconite (PbCr04),lm3 the chromates A4Pb4(Cr04)6(A = K, N&) have been obtained,"" and the reactions of chromate, dichromate or Cr03 in alkali carbonateim5 and bisulfate1= eutectics and fused sodium nitrite1407have been investigated. Interaction between Ag+ and CrOl in (K, Na)N03 melts leads to the formation of the weak complexes [AgCr04]-, [Ag2Cr04]and [Ag(Cr04)2]3-.1408
(i) Spectroscopic investigations There have been a number of investigations of the electronic and IR spectra of chromates, and attempts to account for the electronic spectrum by molecular orbital t h e ~ r y . ~ ~ ~ , ' ~ From the splittings of the IR absorption bands of CrOz- it can be determined whether the &ion is mono- or bi-dentate, e.g. it is monodentate in [Cr(Cr04)(NH3)5]Iand bidentate in [Cr(Cr04)(NH3),]I.i410 Complete vibrational assignments have been made for K2Cr04;1411and molecular K2Cr04 has been isolated in an argon matrix; the IR spectrum shows that its structure is D2dand the 043-0 bond angles are 96 f 50.1412 RR spectroscopy has been used to investigate the unusual red colour of A 2cro4.1413 Crystallographic and IR studies on the hydroxychromates M'M$''(OH)6(Cr04)2 (M'= Na, K, NH4; Mn' = Fe, Al) have confirmed that they are isomorphous.l4I4 The NMR shifts of the I7O spectra of CrO;-, CrzO:- and CrOzC12have been related to the n character of the Cr-0 linkages, and from them it has been deduced that the Cr+Cr linkage in dichromate is angular rather than linear in solution as well as in the s 0 1 i d . l ~ ~ ~
(ii) Structures of chromates and polychromates The tetrahedral structure of the anion in simple chromates such as K2Cr04 has been confirmed by many single crystal X-ray studies. Within the C r0 4 groups (291) the 0-Cr-0 angles are close to the tetrahedral value and the C r - 0 bond distances are essentially equal and close to 1.BA unless there is interaction of some oxygen atoms with other cations, e.g. Ce4+ in Ce(Cr04)2-2H20(Table 112). Chromates are often isomorphous with corresponding sulfates. The dichromates consist of two distorted Cr04 tetrahedra with a corner in Common. Rubidium dichromate, for example, exists in several crystal forms which contain anions shaped like (292); the bridging C r - 0 bonds are considerably ionger (-1.SA) than the terminal Cr-0 bonds, which are similar to those in CrOi-, and the Cr-0-Cr angles are 123". In SrCrzO7there are distinct Cr20$- units of types (292) and (293) with Cr-0-Cr angles of 133" and 140" respe~tive1y.l~~ A more accurate determination of the structure of (NH4)2Cr207has shown that Cr-O,, is 1.78 A (not 1.98A as earlier reported). The structures of the tri- and tetra-chromates, e.g. Rb&r3010 (294) and Rb2Cr40r3(295), and of polymeric CrO, (296) follow the same general pattern, although the detailed structure can vary with the cation. 0
9
.o -
0
0 I
I
0
(295) Cr,Ot;
I -0
0
0
0
-
0(2%) CrO,
0
I
?
Chromium
944
The C r - h C r angle varies between 115 and 148" and the Cr-Ob, distance between 1.7 and 1.98, in the condensed chromates. Brief structural details for examples of the different chromates are in Table 112. There are earlier results in Ref. 1306. Chromates(V1) of the divalent metals (V, Ni, Co, Cu, Zn and Cd) are isomorphous, and each Cr atom is surrounded by six oxygen atoms in distorted, edge-sharing 0 ~ t a h e d r a . l ~ ~ 35.7.4 Anionic Oxo Halides and Other Substituted Chromates The equilibria (111) to (115) exist only in HN03 and HC104 and are modified when other anions are present because of the formation of substituted chromates, e.g. chlorochromate and sulfatochromate, as in equations (119) and (120). Dichromate is slow to oxidize, otherwise no halochromates could be HCr20; + HC1 HCr,O; + HSO;
e Cr03CI- + HzO e [Cr03S04]2-+ HzO
(119)
(120)
Salts of the trioxohalochromates(V1) [Cr03X]- (X = F, C1, Table 113) can be crystallized from solutions of the appropriate dichromate and HX and have uses as oxidants of organic compounds. The fluorochromates can also be obtained from CrOs, HF and the alkali metal carbonate, and the chlorochromates from Cr03, MCl and HCl. Potassium trioxochlorochromate may also be prepared from K2Cr04and Cr02C12.Salts of [CrO,Br]- and [Cr031]- have been reported but they decompose through oxidation of the halide. Table 1l3 Fluoro- and Chloro-chromates(V1)
Cornmen&
Compound
M[CrO,F] Red
M = K, Rb, Cs, NH,, easily hydrolyzed; Cr-0, 1.60 and 1.62, Cr-F, 1.68 A (NH ) 2 C r 4 , 1 . 6 0 , 2 Cr-(0, F), 1.67 disorder (Rb) M = L i , K, Rb, Cs, NH,, PhNeAs, hydrolyzed in water, stable in acid, Cr-4 (av), 1.53 (Ksalt), 1.52 (NH,), 1.606 (Rb), 1.612 8,(Cs), Cr-Cl(av), 2.16 (Ksalt), 2.15 (NH,), 2.194 (Rb), 2.197 A (Cs)
Ref. 192
1,
M[CrO,Cl] Orange
1,3 4
5
1. R. Colton and J. H. Canterford, 'Halides of the First Row Transition Metals', Wdey-Interscience, London, 1969, vol. 1, chap. 4. 2. W. Granier, S. Vilminot, J. D. Vidal and L. Cot, 1. Fluorine Chem., 1981, 19, 123. 3. J. J. Foster and A. N. Hambly, Ausf. J. Chem., 1977,30, 251. 4. R. Amstrong and N. A. Gibson, Ausf. J . Chem., 1968,2l, 897. 5. J. J. Foster and M. Stems, J . Cryst. Mol. Sirucf., 1974, 4,149.
There are three major processes in the thermal decomposition of the alkali metal chlorochromates M[Cr03CI] (M=Li, K, Rb or Cs); these are the formation of Cr02C12, MCr-,O, and Cr203;in each case there is simultaneous formation of M2Crz07and MC1.1416The rubidium salt is isostructural with the potassium and ammonium salts, but not the cesium salt. The anions are approximately tetrahedral. The average values for the Cr-0 bond distances in Cs(Rb)[CrO3C1]are rather longer than determined earlier for K(N&)[Cr03Cl] (Table 113) Some disorder was found in the tetrahedral [Cr03F]- anions of KCr03F and the average Cr-(0, F) distance is 1.65 ~ . 1 4 1 ' 8 Salts M[Cr03X] (where X = iodate, sulfate, NH2, etc.) have earlier been described.2Adducts of KCr03X ( X = F , C1, Br, IO3) with primary and secondary aliphatic amines have been characterized in solution by their UV and IR spectra.1419 The fluorinated anhydride (CF3C0),O removes chloride from K[Cr03C1] and adds across one C-0 bond according to equation (121).1420The same anionic complex is obtained from K2Cr207and the anhydride; but K2[Cr02(02CCF3)4]is formed from K2Cr04. Other chromates and dichromates, and other fluorinated anhydrides, react similarly. The complexes are red-brown solids, easily hydrolyzed and unstable to heat and light. K[CrO,Cl] + 2(CF,CO),O
-+
K[CrO,(O,CCF,),]
+ CF,COCl
(121)
Boron trifluoride adducts approximating to K2[Cr04(BF3)2]and K2[Cr04(BF3)3]have been ~ep0rted.l~~~
945
Chromium
The salts A[Cr02F3], (A=NO+ or NO;, Scheme 123) and the other dioxo compounds below are believed to contain cis-dioxo groups and fluorine bridges because there are IR bands in the 900-1000cm-1 region, which are assignable to the vs(Cr02) and v,,(Cr02) vibrations, and bands in the regions of 550-750cm-1 and 35Ocm-I assignable to terminal and bridgin fluoride respectively.14" Salts of the anions [CrO4FI3- and [CrO2F4I2-have been described; and M2[Cr02F4](M = Na, K, Cs), M[Cr02F4] (M = Ca, Mg)13'0 and Na2[Cr02F2(N0&] can be derived from Cr02Fz (Scheme 123).1380
9
357.5 Organoimidochmium(VI) Complexes Among the t-alkylimido complexes of the do transition metals of groups V to VII is deep red (Me3SiO)2Cr(NBut)2, which can be prepared from Cr03 or Cr02Clz and tbutyl(trimethylsily1)amine and is monomeric from preliminary X-ray data. After removal of the hexane (equation 122 , the residue is dissolved in hexamethyldisiloxane and the imide separates at -40 0C.142 3) The preparation from CrO2ClZ(equation 123) has some advantages.I4% The diimide forms the oxo derivative (Me3SiO)2CrO(NBu'.)on reaction with benzaldehyde.
-
CrO, + NHBu'(SiMe3) Cr02C12+ NHBu'(SiMe,) in excess
hexane
(Me,SiO),Cr(NBu'),
(Me3SiO)2Cr(NBut)2+ 2[NH2Bu'(SiMe,)]C1
hexane
(122) (13)
The reaction of bis(trimethylsily1)diazene with chromocene or CpCrC12 affords the dark violet crystalline imide with structure (297). The imide shows distinct trimethylsilyl signals in the NMR spectrum, and in methanol forms [CpCr(NH)(NSiMe3)J2.1425 SiMe, I
35.7.6 Mixed Oxidation State (111 and VI) Oxo Compounds The compounds MCr308 have long been known. In their structures Cr"'06 octahedra and CrWO4tetrahedra are linked together in the ratio 1:2 by sharing corners and edges.'306 Several new methods of preparation have been described (equations 124 to 127). Trichromates (M2Cr3010,M = K, Cs) give the same products as tetrachromates (equation 127).14% KCrO3C1 KCrO,Cl
-275 "C 12 h
-
+ 2Cr0,
3MX -t5C1-0,
K2Cr207+ KCr308+ 2KC1+ 1.5C1,
+ O.5Cl2+ 0.502 M2Cr207+ MCr308+ 1SX, KzCr2O7+ 2KCr308+ 1.502
350-4Im 'C
250 'C
KCr,O,
(124)
(125) (126)
2K2Cr,Ol3 (127) From the XPES,Cr205 can be described as a mixed valence compound containin C?' and 0O6 Cr6+ in the ratio 1:2 as in KCr308.1427 The oxide Cr5OI2is formulated Cr2(Cr04)3.1 35.7.7 Peroxo Complexes of Chromium(IV), (V) and (VI) The action of Hz02 on acidic dichromate solutions gives an unstable blue species which can be extracted into ether (equation 128).2,518,1428 On the addition of pyridine the blue compound CrV'0(02)2py crystallizes. The molecules are distorted pentagonal pyramids with sideways; ~ workers ~ ~ ~ with less refined data have reported rather bonded peroxo groups ( ~ 8 ) some different bond distances.1430In water the blue species is considered to be CrO(02),(OH2), in ether it is CrO(O&OEtz, and amines such as aniline, bipy and phen can replace the pyridine. With bidentate bipy the coordination sphere becomes essentially a pentagonal bipyramid (B),
Chromium
946
and the phen corn ound is similar; it also has one Cr-N bond (2.23 A) much longer than the other (2.11 A).143PBy extraction into tri-n-butylphosphate (TBP) containing Ph3As0, the adduct Cr0(02)20AsPh3can be obtained. 1432 HCrO;
+ 2H20, + H+
-
CrO,.H,O blue
'0
+ 2H20
(1%)
157
(299)
(298)
In neutral or weakly acidic solutions of dichromates and H202 the violet species CrO,(OH)forms (e uation 129) and explosive violet salts M[CrwO(Oz)20Hf (M = N&, K, Tl) can be isolated.'%33The salt [Ph3MeAs][Cr0(02)20H]is more stable, and from the lack of an OH band in the IR spectrum it is believed that the anion contains a strong hydrogen bond; related blue peroxohalochromates [Ph3MeAs][CrO(02)zX] (where X is C1 or Br) are, also known (Scheme 124).1432Kinetic studies show that the blue and the violet anions are formed by similar mechanisms.1434 HCrO, + 2H,O, Cr05(OH)- + 2&0 (129)
H+
violet
-
/ I (02),CrO(OH2) blue acidic aqueous soln.
TBP
PhJAsO
blue adduct
(OWrO{OP(OBu)d 'blue' organic / soln. [Ph,MeAs]X
orange chlorochromate
'blue' anion X = C1 or Br
Scheme 124
The unstable, violet black compound (NH4)2[Cr~'02(02)5]+2H20 is by reaction of H202with Cr03 in the presence of NH&1 and HCI, and Cr(02)(NPrh)3may be a peroxo chromium(V) compound (p. 138). From the RR spectrum of CrO(02)2.H20,its instability has been related to the relatively low (952 cm-') Cr-0 stretching vibration and the relatively high (1012 cm-') 0-0 stretching ~ i b r a t i 0 n . The l ~ ~ peroxidic ~ stretch was earlier assigned to a band in the 865 cm-l region.1433 The chromium(V) complexes M:[Cr(02)4], obtained from alkaline solutions, contain dodecahedral anions (see Section 35.6.2). Brown crystals of paramagnetic, reasonably stable Cr'V(NH3)3(02)2 can be prepared1437 according to equation (130). Some reactions of C T ( N H ~ ) ~ (are O ~given ) ~ in Scheme 125, and its structure (300) has been determined. 1438 The anion in K3[Cr(CN)3(02)2]is structurally similar,1439and the 0-0 bond distance is 1.45 A. In [Cr(en)HzO(02)2].Hz0 this distance is 1.46 A.'"" 2]-H20, 2H20 and [ C r i b n H ~ 0 ( 0 ~ ) ~ ] . HThese ~ O . and Cr(NH3)3(02)2 are useful
Chromium
947
(300) amine complexes,302but their explosive nature is a disadvantage and rather few amines form this type of complex so the synthetic use is limited. Further information on peroxo chromium complexes is given in P a ~ c a l . ~
Biological Effects of Chromate(VI) The biological effects of chromate are apparent in three main areas: mutagenicity and carcinogenicity, an adverse immune response and the use of chromate in clinical chemistry to tag erythrocytes. The clinical use of chromate has recently been reviewedlU1 and although reviews of the immune response to chromate are available1U2little is known of the chemistry involved; this is surprising as chromate present in cement is a major cause of dermatitis. The carcinogenicity and mutagenicity of chromium(V1) are well e~t ab 1 i s h ed . The l ~~ toxicity is usually considered in terms of the uptakeheduction mode1la3 since chromium(V1) readily passes into the cell, via anion channels, and once within the cell it is eventually reduced by cellular components to chromium(II1) species. Figure 6 illustrates the likely fate of chromate within a mammalian cell. As can be appreciated from Figure 6 it is complexes trapped within the cell that are the agents responsible for the toxic effects of chromate. The systems which reduce the chromium(V1) are as yet unknown, as are the final products of the reaction. However, microsomes1444are capable of reducing chromium(V1) as are various n u ~ l e o t i d e s l ~ ~ and even fulvic acids.1446In these cases chromium(V) species of considerable stability have been observed using EPR spectroscopy. Within the cell reduction by a sulfide is the most probable reaction. Glutathione is an intracellular peptide important in the maintenance of redox s t a t ~ s ; ~ ~ , ~ ~ ' it is found at millimolar concentrations in typical mammalian cells and is implicated, in genera1,
35.7.8
mgUre 6 The uptake reduction model for chromate carcinogenicity (after Wetterhahn). Possible sites for reduction of chromate include the cytoplasm, endoplasmic reticulum, mitochondria or the nucleus'443 COCS-EE
Chromium
948
in the defence of cells challenged with oxidizing agents. However, in the case of chromate toxicity, cells with elevated levels of GSH have been shown to have an increased susceptibility to damage,1449.1450 which perhaps indicates that a chromium complex of glutathione is an active intracellular toxin. It is most unusual for a toxic metabolite to be generated by a reaction with GSH. The reaction of GSH with chromium(V1) has been studied in acidic s o l ~ t i o n s ' under ~~~,~~~ which the complete reduction of chromate to chromium(II1) is rapid. Mechanisms involving thiolate esters of chromium(VI), which react with either protons or GSH, were suggested to be important. A detailed study of the reduction of chromate by various biological reducing agents (in neutral, buffered solutions, at low GSH concentrations) indicates that chromium(V) species are not generated'4s3 and also suggests the importance of thiolate esters. However at neutral pH, solutions of chromium(V1) containing an excess of glutathione (1 : 10) rapidly develop a green C O ~ O U ~ ,which ~ ~ ' ~ slowly decays to a purple colour typical of chromium(II1) complexes. Concurrent with the green colour the typical EPR spectrum of a chromium(V) species is observed. Chromium(V) has also been observed in the EPR s ectrum of frozen glasses obtained from reaction mixtures of chromium(V1) and glutathione. 1 4 8 A related reaction, that of L-cysteine with CrV' at neutral pH, has been studied in the product was suggested to be the bis-truns-S complex of cysteine characterized crystallographically by DeMeester et al., 'Is7 but no evidence for CrV species could be obtained, and the rate-determining step was suggested to involve two-electron transfer. It has been that initial one-electron reduction of chromium(VI) leads to chromium(II1) complexes in which the oxidized form of the substrate is coordinated. The presence of chrornium(V) in GSH/chromate reaction mixtures suggests that GSSG may be captured in the reaction of GSH with chromium(V1 . This is in marked contrast to the results of P e n n i n g t ~ n ' ~ ~ ~ with L-cysteine, but Wetterhahn'" has some evidence for Cr"'/GSSG complexes from such reactions. The way in which the dominant reduction mechanism for chromate changes with the reaction conditions and how this is related to the toxicity of chromate is not as yet clear. As outlined above, the products of the reaction may depend on the mechanism of reduction and these, as yet, unidentified chromium complexes are probably the agents responsible for the mutagenicity of chromate. The substantial stability of the chromium(V) complexes and thiolate esters generated in the reaction of GSH with chromate suggests that if similar complexes were formed in vivo they would have time to reach many intracellular compartments and could hence be the crucial active intermediates in the toxicity of chromate. The complexes responsible for the toxicity have not yet been identified. The possibilities include: the thiolate ester, a relatively stable chromium(V) species, or a chromium(II1) complex. More work is needed in this area.
1
35.8
REFERENCES
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Chromium
949
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21. 22. 23. 24.
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Chromium
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M. Greenaway and B. R. Penfold, Inorg. Chin. Acta, 1983,71,29. 1337. A. K. Banarjee and N. Banerjee, Inorg. Chem., 1976,15,488. 1338. B. Gahan, C. D. Garner, L. H. Hill, F. E. Mabbs, K. D. Hargrave and A. T. McPhail, J. Chem. Soc., Daltun Trans., 1977, 1726. 1339. C. D. Garner. J. Kendrick. P. Lambert. F. E. Mabbs and I. H. Hillier, Inurn. Chem., 1976.15.1287. . , 1340. J. Weber and’C. D. Garner, fnorg. Chem., 1980,19,2206. 1341. C . D. Gamer, I. H. Hillier, F. E. Mabbs, C. Taylor and M. F. Guest, J . Chem. Soc., Dalton Trans., 1976,2258. 1342. K. R. Seddon and V. H . Thomas, Inorg. Chem., 1978,17,749. 1343. E. K. Mooney and K. R. Seddon, Transition Met. Chem., 1977,2,215. 1344. D. A. Edwards and S. C. Jennison, Transition Met. Chem., 1981, 6, 235. 1345. R. Scholder, F. Schwochow and H. Schwarz, Z. Anorg. Allg. Chem., 1968, 363, 10. 1346. J. A. Connor and E. A. V. Ebsworth, Adu. Inorg. Chem. Radiocham., 1964,6,279. 1347. M. Roch, J. Weber and A. F. Williams, Inorg. Chem., 1984,23,4571. 1348. K . Forster, M. Greenblatt and J. H. Pifer, J . Solid Store Chem., 1979, 30, 121. 1349. R. Borromei, P. Day and L. Oleari, J. Chem. Soc., Faraday Truns. 2, 1979,75,401. 1350. H. Miiller-Buschbaum and K. Sander, Z. Natwforsch., Teil B, 1978, 33, 708. 1351. E. A. El-Rafei, Inorg. Chem., 1981, 20,222. 1352. K. Niki and H. A. Laitinen, J. fnorg. Nucl. Chem., 1975,37, 91. 1353. A. Roy, M. Chaudhury and K. Nag, Bull. Chem. SOC.Jpn., 1978, s1, 1243. 1354. R. Olazcuaga, J.-M. Reau and G. Le Hem, C. R. Hebd. Seances Acad. Sci., Ser. C, 1972,275,135. 1355. M. Krumpolc, B. G. DeBoer and 3. Rocek, J. Am. Chem. Soc., 1978, 100, 145. 1356, M. Krumpolc and J. Rocek, I. Am. Chem. SOC., 1979,101,3206. 1357. N. Rajasekar, R. Subramaniam and E. S. Gould, Inorg. Chem., 1983, 22, 971. 1358. C. J. Willis, J. Chem. Soc., Chem. Commun., 1972,944. 1359. P. F. Bramman, T. L u d , J . B. Raynor and C. J. Willis, J. Chem. Soc., Dalton Trans., 1975, 45. 1 W . H. L. Krauss, M. Leder and G. Miinster, Chern. Ber., 1963,%, 3008. 1361. T. Takeya, E. Kotani and S . Tobioaga, J. Chem. Soc., Chem. Commun., 1983,98. 1362. W. M. B. Koenst and J. T. M. F. Maessen, Synth. Commun., 1980,10, 905. 1363. ‘Kirk-Othmer Encyclopedia of Chemical Technology’, 3rd edn., Wiley-Interscience, New York, 1979, vol. 6. 1364. E. G. Hope, P. J. Jones, W. Levason, J. S. Ogden, M. Tajik and J. W. Tu& J . Chem. Soc., Dalton Trans., 1985, 1443. 1365. A. J. Edwards, W. E. Falconer and W. A. Sunder, J . Chem. SOC.,Dalton Trans., 1974, 541. 1366. E. G. Hope, P. J. Jones, W. Levason, J. S. Ogden, M. Tajik and J. W.Turff, J. Chem. SOC., Dalton Trans., 1985, 529. 1367. P. J. Green and G. L. Gard, Imrg. Chem., 1977,16, 1243. 1368. G. D. Flesch and H. J. Svec, J. Am. Chem. SOC., 1959,81,1787. 1369. W. V. Rochat, J. N. Gerlach and G. L. Gard, Inorg. Chem., 1970, 9,998. 1370. S. D. Brown, P. J. Green and G. L. Gard, J . Fluorine Chem., 1975,5203. 1371. R. S. Drago and K. W. Whitten, fnorg. Chem., 1966, 5 , 677. 1372. W. H. Hartford and M.Darrin, Chem. Reu., 1958, 58, 1. 1373. I. R. Beattie, C. J. Marsden and J. S . Ogden, J. Chem. SOC., Dalton Tram., 1980, 535. 1374. C . D. Garner, R. Mather and M. F. A. Dove, J. Chem. Soc., Chem. Comrnun., 1973,633. 1375. R. J. French, L. Hedberg, K. Hedberg, G. L. Gard and B. M. Johnson, Znorg. Chem., 1983,22,892. 1376. C . J. Marsden, L. Hedberg and K. Hedberg, Inorg. Chem., 1982,21, 1115. 1377. E. L. Varetti and A. Mueiler, Sprctrochim. Acta, Part A , 1978,34,895. 1378. S. D. Brown, G. L. Gard and T. M. Loehr, J . Chem. Phys., 1976,64, 1219. 1379. A. J. Edwards and P. Taylor, Chem. Commun., 1970, 1474. 1380. S . D . Brown and G. L. Gard, Inorg. Chem., 1973,12,483. 1381. S. D. Brown, P. J. Green and G. L. Gard, J. Fluorine Chem., 1975,5,203. 1382. W. V. Rochat and G. L. Gard, i m r g . Chem., 1969,8, 158. 1383. Z. A. Siddiqi, Lutfullah, N. A. Ansari and S. A, A. Zaidi, J. Inorg. Nucl. Chem., 1981, 43, 397. 1384. J. K. Pun and J. M. Miller, fnorg. Chim. Acta, 1983,75,215. 1385. R. C. Paul, J. K. Pun and K. C. Malhotra, J . Inorg. Nucl. Chem., 1971,33,4191. 1386. J. N. Gerlach and G. L. Gard, Znorg. Chem., 1970, 9, 1565. 1387. S. D. Brown and G. L. Gard, Znorg. Chem., 1975,14,2273. 1388. S. D. Brown, L. M. Emme and G. L. Gard, J. Inorg. Nucl, Chem., 1975,37, 2557. 1389. P. Huppmann, €3. Labischinski, D. Lentz, H. Pritzkow and K. Seppelt, 2.Anorg. A&. Chem., 1982,487,7. 1393 K. Seppelt, Chem. Ber., 1975,108, 1823. 1391. K . Dehnicke and J. Strahle, 2. Anorg. Allg. Chem., 1965, 339, 171. 1392. V. Fernandez and J. Rujas, Inorg. Nucl. Chem. Lett., 1979,15,285. 1326. 1327. 1328. 1329. 1330.
~I
Chromium
969
1393. R. C. Makhija and R. A. Stairs, Can. J. Chem., 1969, 47,2293. 1394. R. C. Paul, 0. B. Baidya, S. K. Gupta and R. Kapoor, Z. Nufwforsch., Teil B, 1977,32, 148. 1395. A. A. Ivanov, A. V. Demidov, N. I. Popenko, E. Z. Zasorin, V.P. Spiridonov and I. Hargittai, J. Mal. Smcr., 1980, 63, 121. 1396. E. Mappus and C.-Y. Cuilleron, J. Chem. Res. ( S ) , 1979, 42. 1397. W. J. Behr and J. Fuchs, Z. Naturforsch., Teil B, 1975, 30, 299. 1398. M. Schmidt and H. Schmidbaur, Angew. Chem., 1958,70,704. 1399. B. Stensland and P. Kierkegaard, Acta Chem. Scand., 1970,24,211. 1400. K. Handlir, J. Holecek, M. Nadvornik and J. Klikorka, Z. Chem., 1980, 20,31. 1401. M. Otto, R.Wagener and H.Hennig, Inorg. Chim. Acta, 1981, 64,L11. 1402. H. C. Clark and R. G. Goel, Itwrg. Chem., 1966,5,998. 1403. T. I. Kuzina, G. A. Sidorenko, 1. V. Shakhno and T. I. Bel’skaya, Rws. J . Inorg. Chem. (Engl. 7’rmd.), 1980, 25,200. 1404. T. Fukasawa and M. Iwatsuki, Bull. Chem. SOC. Jpn., 1979,52,3697. 1405. D. A. Habboush, D. H. Kerridge and S. A. Tariq, Thermochim. Acta, 1979,28,143. 1406, B. J. Meehan and S. A. Tariq, Aust. J . Chem., 1980,33,647. 1407. R. N. Kust and R. W. Fletcher, Inorg. Chem., 1969, 8, 687. 1408. B. Holmberg and G. Thomd, Znorg. Chem., 1980,19, 2247. 1409. S. Felps, S. I. Foster and S. P. McGlynn, Inorg. Chem., 1973,l2,. 1389. 1410. J. Casabo, J. Ribas and J. M. Coronas, J. Znorg. Nucl. Chem., 1976, 38, 886. 1411. D. M. A d a m and M. M. Hargreave, J . Chem. Soc., Dalton Trans., 1973, 1426. 1412. I. R. Beattie, J. S. Ogden and D. D. Price, J . Chem. SOC., Dalton Trans., 1982, 505. 1413. R. J. H. Clark and T. J. Dines, Znorg. Chem., 1982, 21,3585. 1414. Y. Cudennec, A. Rim, A. Bonnin and P. Caillet, Rev. Chim. Miner., 1980,17, 158. 1415. R. G. Kidd, Can. J. Chem., 1967,45,605. 1416. J. J. Foster and A. N. Hambly, A u t . J. Chem., 1977, 30,251. 1417. J. J. Foster and M. Sterns, J. Cryst. Mol. Srrucr., 1974, 4, 149. 1418. W. Granier, S. Vilminot, J. D. Vidal and L. Cot, J . Fluorine Chem., 1981,19,123. 1419. R. Mitzner and R. Sommer, 2. Chem., 1979,19, 76. 1420. J. N. Gerlach and G. L. Gard, Inorg. Chem., 1971,10,1541. 1421. V. Gutmann, U. Mayer and R. Krist, Synth. React. Inorg. Metal-Org. Chem., 1974, 4, 523. 1422. P. J. Green and G. L. Gard,Inorg. Nucl. Chem. Lett., 1978, 14, 179. 1423. W. A. Nugent, Inorg. Chem., 1983,22, 965. 1424. W. A. Nugent and R. L. Harlow, Inorg. Chem., 1980,19,777. 1425. N. Wiberg, H. W. Haring and U. Schubert, Z. Nufwforsch., Teil B, 1978, 33, 1365. 1426. J. J. Foster and A. N. Hambly, Aust. J. Chem., 1976, 29, 2137. 1427. T. Tsutsumi, I. Ikemoto, T. Namikawa and H. Kuroda, BULL Chem. SOC. Jpn., 1981,54,913. 1428. J. A. Connor and E. A. V. Ebsworth, Adu. Inorg. Chem. Radiochem., 1964,6,279. 1429. R. Stomberg, Ark. Kemi, 1963, 22, 29. 1430. B. F. Pedersen and B. Pedersen, Acta Chem. Scund., 1963,17,557. 1431. R. Stomberg and 1 3 . Ainalem, Acta Chem. Scund., 1963, 22, 1439. 1432. R. Armstrong and N. A. Gibson, A w l . J. Chem., 1968,21,897. 1433. W. P. Griffith, I . Chem.Soc., 1962, 3948. 1434. S. N. Witt and D. M. Hayes, Znorg. Chem., 1982,21, 4034. 1435. F. Hein and S. Herzog, in ‘Handbook of Preparative Inorganic Chemistry’, 2nd edn., ed. G . Brauer, Academic, New York, 1965, vol. 2, p. 1392. 1436. R. E. Hester and E. M. Nour, J. Raman Spectrosc., 1981, 11,39. 1437. G. B. Kauffman and G. Acero, Znorg. Synth., 1966, 8, 132. 1438. R. Stomberg, Ark. Kemi, 1963, 22,49. 1439. R. Stomberg, Ark. Kemi, 1965, 23, 401. 1440. R. Stomberg, Ark. Kern., 1965, 24, 47. 1441. C. J. Sanderson, in ‘Biological and Environmental Aspects of Chromium’, ed. S. Langard, Elsevier, Amsterdam, p. 102. 1442. D. Burrows, ‘Chromium Metabolism and Toxicity’, CRC Press, Boca Raton, FL, 1984. 1443. P. H. Connett and K. E. Wetterhahn, Struct. Bonding (Berlin), 1983, 54, 93, 1444. K. W. Jennette, J . Am. Chern. SOC.,1982,104,874. 1445. D. M. L. Ooodgame, P. B. Hayman and D. E. Hathway, Polyhedron, 1982,1,497. 1446. D. M. L. Goodgame, P. 8.Hayman and D. E. Hathway, ZAorg. Chim. Acta, 1984,91, 113. 1447. D. L. Rabenstein, G. Guevremont and G. Evans, in ‘Metal Ions in Biological Systems’, ed. H. Sigel, Dekker, New York, 1979, vol. 9. 1448. A. Meister, Science, 1983, 22, 472. 1449. K. E. Wetterhahn and D. Y.Cupo, Abstracts of the 23rd International Conference on Coordination Chemistry, Boulder, USA, 1984, p. 474. 1450. K. E. Wetterhahn and D. Y. Cupo, Carcinogenesis, 1984,S, 1705. 1451. A. McAuley and M. A. Olatunji, Can. J. Chem., 1977,SS, 3335. 1452. A. McAuley and M. A. Olatunji, Can. J. Chem., 1977,55,3328. 1453. P. H. Connett and K. E. Wetterhahn, J . Am. Chem. Soc., 1985,107,4282; 1986,108,1842. 1454, P. O’Brien, J. Barrett and F. Swanson, Inorg. Chim. Actu, 1985, lW? L19. 1455. K. E. Wetterhahn, D. Y. Cup0 and P. H. Connett, ‘Trace Substances in Environmental Health - XVIII’, Proceedings of a Conference at the University of Missouri, ed. D,D.Hemphill, June, 1984, p. 154. 1456. D. W, J. Kwong and D. E. Pennington, Inorg. Chem., 1984, 23,2528. 1457. J. N. Cooper, G. E. Staudt, M. L. Smalser and G. P. Haight, Inorg. Chpm., 1973,12, 2075.
Molybdenum A. GEOFFREY SYKES University of Newcastle upon Tyne, UK
G. JEFFERY LEIGH and RAYMOND L. RICHARDS AFRC Unit of Nitrogen Fixation, University of Sussex, Srighton, UK
C. DAVID GARNER and JOHN M. CHARNOCK University of Manchesier, UK
and EDWARD I. STIEFEL Exxon Research and Engineering Company, Annandale, NJ, USA
Because of factors beyond the editors’ control, the submission of manuscripts for this chapter was delayed. In order to minimize any delay in publishing ‘Comprehensive Coordination Chemistry’ as a whole, the coverage of molybdenum appears at the end of this volume, commencing on page 1229.
971
37 Tungsten ZVI DORl Technion-Israel
lnstitute of Technology, Haifa, Israel
37.1 INTRODUCTION
973
37.2 THE ELEMENT
974
37.3 THE CHEMISTRY OF Wv' 37.3.1 W" Halides and Derivatives 37.3.2 Complexes Containing the 0x0 Ligand 37.3.2.1 Complexes with one oxo ligand 37.3.2.2 Complexes with two oxo ligands 37.3.2.3 Complexes with three oxo ligands 37.3.3 Complexes Containing Multiple Bond to Sulfur, Selenium, Nipogen and Carbon Atoms 37.3.4 WSi-, WUSZ- and WO& as Ligands 37.3.5 Dithiolene Complexes
974 974 976 976 977 978 979
37.4 THE CHEMISTRY OF Wv 37.4.1 W v Halides and Their Derivatives 37.4.2 The Oxyhalides and Their Derivatives 37.4.3 Complexes with Terminal Sulfur and Selenium Atoms 37.4.4 Complexes with Meral-Metal Bonds 37.4.5 Thiocyanato Complexes
984 984 985
37.5 THE CHEMISTRY OF WN 37.5.1 W N Halides and Their Derivatives 37.5.2 The Oxyhalides and Their Derivatives 37.5.3 The W,OCl:, lon 37.5.4 Complexes with Metal-Metal Bonds 37.5.4.1 Complexes wifh metal-metal double bonds 37.5.4.2 Trinuclear clusters 37.5.5 Eight-coordinuteComplexes 37.5.6 Complexes with Multiple Bo& to Sulfur, Nitrogen and Carbon 37.5.7 Cyanide Complexes
988 988 990 990
37.6 THE CHEMISTRY OF W'" 37.6.1 Mononuclear Complexes of W" 37.6.2 Dinuclear Complexes of W" 37.6.2.1 The yW2X9]'- ion and related compounds 37.6.2.2 Complexes of the type w&
998 998 998 999
962
983
9% 986 988
95Q 991 993 996 997 997
1d00
37.7 THE CHEMISTRY OF W" 37.7.1 Monomeric Complexes 37.7.2 Complexes with Quadruple Bonds 37.7.3 The Hexanuclear Clusters /w6X8]4+
1005
37.8 COMPLEXES OF DINITROGEN 37.8.1 Reduction of Coordinated Dintbogen 37.8.2 Reactions Leading to Carbon-Nitrogen Bond Formnsdon
1011 1012
37.9 NITROSYL COMPLEXES
1014
37.10 HYDRIDO COMPLEXES
1014
1005 1008 1011
1013
1015
37.11 REFERENCES
-
37.1 INTRODUCTION The chemistry of tungsten is varied and complex not only because it covers nine oxidation states (-2 to +6), but also because of its ability to form complexes with different coordination numbers and geometries, and because of its tendency to form clusters and polynuclear complexes with a variety of metal atoms. The chemistry of this element has been studied since the characterization of tungstic acid in
973
974
Tungsten
1781,' and the oxychlorides, the hexachloride and the hexabromide in 1857. Until the 1930s most of the development centered around the higher oxidation states of the element. The development of the lower oxidation states had to await the discovery of W(CO)6. The successful synthesis of cyclopentadienyl compounds has led to the development of its organometallic chemistry. This was followed in the late 1960s and the 1970s by the synthesis and development of the chemistry of complexes containing metal-metal bonds.' This chapter, which is based on the literature that had appeared through the end of 1983, surveys the coordination chemistry of the element with special emphasis on structure, bonding, synthesis and chemical behavior. The discussion is limited to well-characterized complexes and well-understood systems.
37.2 THE ELEMENT Tungsten was discovered in 1781 in a mineral now known as scheelite,' whose principal constituent is CaW04. Tungsten steel was invented in 18553 and the first alloys containing tungsten were manufactured in 1868. These 'high speed steels' have been used ever since as cutting tools because of their hardness even at high temperatures. Because of its high melting point, it has been used as a filament for incandescent lamps4 since 1904, and for a long time this was its principal application. More recently, there has been increased interest in alloys containing tungsten, as materids for high temperature applications. Tungsten carbide, when cemented, is almost as hard as diamond and is therefore very useful for dies, tools and other applications requiring extreme hardness and wear resistance. The element is obtained primarily from the two commercially important ores, scheelite (CaW04) and wolframite [(Fe,Mn)W04]. W 0 3 is produced from these ores and then reduced to the metal with h y d r ~ g e n . ~ 37.3 THE CHEMISTRY OF Ww
In addition to complexes containing the WO, WOz and W0 3 structural units, the element shows quite a varied chemistry in this oxidation state, undoubtedly because the hexahalides are reasonably stable and give rise to a variety of substitution products of the type [W&--nLn] (n = 1-6). In addition, coordination numbers higher than six can also be obtained. Wv' also shows considerable tendency to form bonds of order higher than one with good n donor atoms such as S, Se, N, NR, CHR and CR. 37.3.1 Ww Halides and Derivatives The three hexahalides w F 6 , WC16and WBr6are known and synthetic routes to their preparation are summarized in Table 1. w F 6 is a colorless gas while WC16 and WBr6 are blue-black moisture sensitive solids. Under well-defined conditions, the octahedral hexahalides undergo substitution to give products ot the type [WX6-,&,] (n = 1-6). Mixed fluoro-chloro compounds have been reported to result from reaction of WC16 with F2 or from w F 6 with Me3SiCl.' Table 1 Synthetic Routes to Wv' Halides
Compound wF6
wc1,
WBr,
Preparation W with F, W with CIF, ClF, or BrF3 W 0 3 with HF,BrF, or
SF.4 W with C1, or S2Cl, WO, with CI,, ?&I2, HCI, CCL, PCl, WO, with hexachloropropene W or W(CO)6 with Br,
Ref. 124 125 75
126 75 127
75
Tungsten
975
Figure 1 The structure of [W(NMe;),(0,CNMe,),]14
'H and 19F NMR data have established the existence of cis- and truns-WClZF4, cis- and tram-WF4Cl2, rraer-WC13F3 and fuc-WC13F3. W F 6 also reacts with B(OTeF5), at 120 "C to give [WFn(oTeF5)6-n]. [WF5(OTeF5)] and C ~ ~ - [ W F ~ ( O T ~have F ~ > ~been ] isolated and ~haracterized.~ Halide substitution by ligands containing nitrogen, oxygen and sulfur donor atoms is also known. The reaction of WCk, with NCS- leads to [W(NCS)6].8WF6 reacts with Me,SiNEt, to give [WFSNEt3] and ~is-(Wd(NEt&].~ With Me3SiN3one obtains [WF5N3]."' An interesting six-coordinate complex results from the reaction of WC16 with LiNMe2." Crystal structure analysis12 of [W(NMe&] shows that the complex is octahedral with an average W-N distance of 2.025 A. It reacts with ROH (R = Me, Et, Pr", Pri)I3 to give the known W(OR), compounds. With MeOH, under certain conditions, f ~ c - [ W ( N M e ~ ) ~ ( 0 Mise ) ~ ] ~ b t a i ned.'~ [W(NMez)d reacts with C 0 2 to give ~uc-[W(NM~~)~(O~CNM~~)~] whose structure is shown in Figure 1.l The W-N distance of 1.922A is significantly shorter than the one observed in [W(NMe2)6] indicating significant N+ W n donation. This is believed to be an important factor in limiting the insertion to only three COz molecules. It is not surprising that with CS2 insertion is coupled with reduction to the known WIV compound [W(S2CNMe2)4].13 Both WC16 and WF6 react with Me3SiOR ( R = M e , Ph, C&) to give [ W X - n ( O R ) n ] (n = 1-4)"' For pt = 2 and 4 both cis and tram isomers are and for n = 3 both mer andfuc isomers have been reported. Further substitution to give [WX(OMe)S], [W(OMe and [W(OPh)6] is also known. Crystal structure determination of truns-[WC12(OPh)4$1 and ~ i s - [ W C l ~ ( O M e ) ~has l ~ *revealed relatively short W - 0 distances of 1.82 and 1.84 A respectively. These distances are just 0.1-0.15A longer than the ones found for W 4 , indicating considerable O+ W n donation. Another example of a short W-0 distance (1.908 A) is furnished by the structure of [W(OzC2H4)3].19 With Me3SiMe, WC16 reacts to give [WC15SMe]." Methyl derivatives of Ww have also been prepared. Dimethyl mercury reacts with WC16 to give [WMeC15],21 while with A1Me3, [W(Me)6] is obtained." This red air-sensitive material reacts with several reagentsz3to give known compounds such as [W(OMe)6],24[W(NM€&],ll [W(OPh)6]2sand [W(tdt)?].% With NO, the compound [WMe4(02N2Me)2]is obtained. Crystal structure analysis has shown that the W atom is eight-coordinate with a geometry between square antiprismatic and dodecahedral (Figure 2).27 In solution, this complex is stereochemically non-rigid down to -50 "C.
C
Figure 2
The structure of [WMe4(02NzMe)z]"
Tungsten
976
The stability of complexes of the type [W(OR)6] is quite remarkable. It has been shown2' that short exposure of [W2(OPr')6py2]to approximately two molar equivalents of CO gives two products, one of which is (1).This structure can best be viewed as a substituted tungsten carbonyl in which two cis CO groups have been replaced by donor atoms of the bidentate neutral ligand [W(OPri)6]. This complex is unusual in th3t it contains two atoms of the same element in oxidation states that differ by six units. [w(OPri)4(~-~Pi)2W(CO)~l
(1) Several other WvTcomplexes with coordination number greater than six have been reported. [W(Me)6] reacts with tertiary phosphines to give the seven-coordinate adducts [W(Me)6L] (L=PMePhz, PMe2PH, PEtPh,, PMe3).29*23 The best characterized of these is the PMe3 adduct, which was shown by '€3 and 13CNMR to be fluxional at room temperature, although a rigid structure such as a capped biprism exists at low temperature. With excess MeLi, [W(Me),] gives the bright yellow eight-coordinate compound Liz[W(Me)8j which is reasonably stable as the dioxane solvate." [WMeC15] was reported to react with octamethylphosphoramide and diphos to give the adducts [WMeC15-L]which are believed to be eight-coordinate.21 WF6 is known to react with TIF and CuF, to yield3' WFT1 which has physical properties similar to those reported for (NO)[WF7].31WFF can also be prepared from LiN3 and two equivalents of WF6.32 The existence of WF2- has also been ~ l a i m e d .WC16 ~ ~ , ~reacts ~ with Me2& 2,5-dithiahexane (dth) and tetrahydrothiophene (tht) to give the seven- and eight-coordinate adducts [WC1&] and [wc16L?j.34 [WF,&- has been prepared from WF6 and NHqN3.32
37.3.2 Complexes Containing the Oxo Ligand The oxo ligand forms a strong bond with W"' by utilizing both (T and n donation resulting in a short W-0 bond length. Complexes containing the WO, cB-W02 and cis-W03 structural units are known. 37.3.21 Compltxes with one oxo ligand
Synthetic routes to the oxohalo complexes of Ww are summarized in Table 2. The oxohalo Complexes with one terminal oxygen atom are of the general type [WOG] (X = F, C1, Br). The structures of [WOC14] and [WOBr,] consist of chains of sli htly distorted octahedra connected by oxygen bridges with W-0 distances of 1.8 and 2.1 A.3 In contrast, WOF, is tetrameric in the solid state with fluoro bridge^^^,^^ as depicted in Figure 3. Chloride substitution by NCS- to give [WO(NCS),] has been reported.' The coordinatively unsaturated [WO&] complexes react with ligands such as alkyl cyanides, THF or py to give the six-coordinate [WOXL] complexes.38g39[WOXs]- (X = F-, C1-, Br-) are also k n ~ w n . ~ Interesting ',~~ adducts result from the reaction of WOF, with XeF2 in HF. White stable crystalline materials of composition [WOF4-XeF2]and [(WOF4)yXeF2]have been
B
T8bie 2 Synthetic Routes to Ww Oxyhalides Compound
WOF, WOCl, WOBr, W0,Cl2 WO,Br, WO212
Prepadon
Ref.
WOCl, with HF WO, with either F2, IF,, SeF, W with F2and 0, WO, with SOCl,, CCl,, 0, octachlorocyclopentene WC1, with SO, or WO, WO, with CBr, WBr, with WO, WCb with WO, WO,, WS3, W0:- with CCl,, CIz and 02,CC14 and O2 W with Cl, in air WBr, with WO, W with Br, in air WO, with Br, or CBr, W with NO3 and I2 in sealed tube
7s 7s 75 128, 129, 130 40, 131 75 131 131
75 75 131 7s 75 32
Tungsten
977
Figure 3 Proposed structure for [WOF,]?
On the basis of Raman measurements and 'T NMR spectra it has been suggested that the XeF2 molecule is bonded to the sixth coordination position of the W atom through the fluoro atom, forming a bent W-F-Xe bond. Adduct formation between [WOF4] and SbF5 is also known.44 WOCl, reacts with bidentate Schiff bases to give complexes of the type [WOCi3(Schif€ base)].45 These compounds show one v(W-0,) at around 970 cm-l. With tridentate Schiff bases, [WOC12(Schiff base)] is obtained. Complexes of the type [WOF3L]- (L = bidentate hydroxy acids) have also been rep~rted.~' The structure of the azoxybenzene adduct of WOCl, has been reported.47 The azoxybenzene ligand is coordinated through the oxygen atom trans to the oxo ligand with a bond length of 2.276A. The W-0, distance is 1.669A. The structure reported for the diars adduct oE WOCL,48 appears to be in error. In this structure, the W-Cl bond trans to the oxo ligand is shorter than the cis W X I bond being 2.26 and 2.40A respectively. This resuIt is of course contrary to our knowledge of the trans influence of the oxo ligand.49 With hydrogen peroxide, Ww, like molybdenum, forms peroxy compounds in which the ratio of peroxy groups to metal is 4 :1, 3 :1 , 2 :1, and 1: 1, but most are not well ~haracterized.~' A 2: 1 peroxy complex, obtained from acidic solution containing alkali metal tungstate and a high concentration of HzOz was shown by X-ray crystallography to be an oxygen-bridged dimer51 with formula [(02)2W(0)OW(O)(02)2(H20)2]2-. The coordination geometry is best viewed as a pentagonal bipyramid with two pentagonal planes rotated by 90", one with respect to the other. The structure of the 1: 1 complex ion [WO(O2)F4l2-has been determined" and is best described as a distorted octahedron in which the center of the 0-0 bond occupies a corner. A 2: 1 and a 1:1 peroxy complex can also be obtained in the presence of the ligands pyridine carboxylate and pyridine 2,6-dicarboxylate re~p ect i v el yPhosphine . ~~ and arsine oxides have been shown to stabilize peroxo complexes of WVr.59Thus, complexes of the type [W(0)(02)2L] (L = OPR,; R = Pr", Bun, Ph; OAsR3, R = Pr", Bun, Ph) have been prepared and characterized. These compounds show the expected v(W-Ot) around 950 an-', and ~ ( 0 - 0 )around 850 an-'.
37.3.22 Cornplaes with two oxo ligands The oxohalo complexes containing two terminally bonded oxygen atoms are of the type with oxygen W02X2 (X = F, Cl, Br, I). WO2CI2was shown to be polymeric in the solid bridges and W-0 bond lengths ranging from 1.63 to 2.34 A. W02F2, W02Br2and WO& are believed to be polymeric as well.56 WS2Clzis also known.57 In spite of its polymeric structure and insolubility in common solvents, it has been that complexes of the type W02C12L and W02C12(L-L) can be prepared with mono- and bi-dentate ligands respectively. W02C12Lcan also be prepared with Schiff bases.45 [W02(S2CNR2)2] (R=Me, Et, Prn) have been prepared5' by an oxo transfer utilizing a dinuclear Mo" complex as depicted in equation (1). Because of their moisture sensitivity, these
Tungsten
978
Table 3 Complexes Containing the W o , Structural Unit.
a
Compound
v(ciS-WO,) (cm-')
Ref.
~ w o z c ~ z @ ~ ) d [WOzClz(Me~)2I [wozc~z(opp$)zl ~W~ZC1Z@iPY)l [W02(S2CNEtJJ (Et4N)[WW3(OPMe3)I (Me,N)2IWO,CI,I (Me,N)[WOzCl,(acac)] (Et4N)[WOzFdHzO)1 IWO2BmPY)l [WO2(0~),1" [Wo2(toX>zlb [wo2(acacM
913,965 922,972 915,960 915, 953 890, 933 905, 950 895, 943 895,940 900,997 907,954 918,958 910,950 912,960
58 58 58 58
OX = 8-hydroxyquinoline.
60 133 45 45 133 134 134 134 134
tox = 8-mercaptoquinoline
complexes cannot be prepared from WOZ- in a manner analogous to the preparation of [MoO~(S~CNR~)~] All complexes containing the WOz structural unit have cis configuration and exhibit two strong IR absorption bands around 950 and 900crn-l. The structure of dichloro-dioxopentane-2,4-dionato Ww has been determined and it contains the expected cis arrangement of the oxygen atoms with a W-0, distance of 1.730k61 Table 3 lists well-characterized compounds containing the W 0 2 structural unit.
37.3.23 Complaes with three oxo ligands Complexes containing the W03 structural unit are the least studied. Of the oxyhalides, W03F$-, W03E2-,W O P - and W03Cl- have been prepared. The isolation of [WO3(dien)I6' has been reported. The complex is unstable in solution and hydrolyzes to WOf- and protonated dien. Formation constants fox WO5- with aspartic acid and glutamic acid have also been rneas~red,'~but because of the competitive formation of isopolytungstate in acidic solution and the stability of W0:- in basic solution, the pH range over which formation constants of such complexes can be measured is very small. The reaction of W03 with oxalate has been reported@to give [W03(ox)J2- and [W~OS(OX)~]~-. Slow air oxidation of an acidic W solution results in the formation of the mixed valence Ww-Wv complex65 [W408C18(HzO)4]2-.It is believed that this ion is obtained by an equilibrated reaction between [W02C14]'- and [WOC15]2-. Its structure consists of four W03C12(H20) octahedra sharing corners as depicted in Figure 4. The four W atom form a
R@me 4 The structure of [W40,Cl(HzO)4]2- 65
near1 planar square and are joined through linear oxygen bridges. The W-Ob distance of 1.861, although short, is significantly longer than the W - O t bond length of 1.711& as expected. Substitutions without disruption of the W40t4 core led to the isolation66of (2), (3) and (4).
Tungsten
P [W,08(NCS),~I*-
979
P Iw,o,cl,(H.t,o)l'-
Figure 5 The dispositions of 0, around the W40, squareM
The structure of (3) is quite similar to that of [W408Cls(H20)4]2- with the exception of the spatial disposition of the terminally bonded oxygen atoms as depicted in Figure 5. As expected from the trans effect of the 0, ligand, the W-N bond lengths trans to 0, are on the average 0.2A longer than the W-N bond lengths cis to 0,. Both structures reveal no significant differences between the four tungsten atoms, which, together with theoretical ~onsideration,6~ suggest that these mixed valence complexes are best described as delocalized systems.
37.3.3 Complexes Containing Multiple Bonds to Sulfur, Selenium, Nitrogen and Carbon Atoms Several good n donor ligands, in addition to the oxo ligand, have been shown to form multiple bonds with Ww. This tendency undoubtedly results from the do electronic cunfiguration of W"' and its high formal positive charge. Thus, the reaction of WX, (X = C1, Br) with Sb2S3and Sb2Se3leads to complexes of the type WSX, and WSeX.+68,69These compounds are moisture sensitive and evolve H2S and H2Se on exposure to moist air. Structure analysis of WSK7' shows that they are dimeric in the solid state with two WSX, square pyramidal units linked through unsymmetrical halogen bridges as depicted in Figure 6. The short W-S, distance of 2.090 A clearly indicates a W-S bond order higher than one. The IR bands at 569 and 555cm-l for [WSCl,] and [WSBr4] respectively have been assigned to v(W-St). It is believed that [WSeX] has similar structures and an IR band at 396 cm-' has been assigned to ~ ( W - s e , ) . ~The ~ slight paramagnetism of these compounds (-0.4 BM at 25 "C) is believed to arise from Wv impurities, or, at least in part, from TIP.38 m 6 also reacts with S&S3 to give WSF4.71The reaction of [WSCL] with XeF2 leads to the mixed fluoro-chloro complexes. These have not been isolated, but 19FNMR has established71 the presence in solution of [WSF2C12], [WSFCb] and [wSF3C1] in addition to the ions [WSFJ, [WSF4Cl]-, [WSFClJ and [WSF3C12]-. The reaction of [wYC14] (Y = 0, S, Se) with S h y 3 in CS2 leads to complexes of the type WS2C12,WSSeC12 and WOSC12.72 Compounds of the type [W&]are coordinatively unsaturated and thus react with a variety of ligands or with donor solvents to give simple six-coordinate adducts.73aAn example of such
Figure 6 The structure of [WSC14]70 COC3-PP
Tungsten
980
F w7
The structure of [WSCl4(p-dth)WSCl4lnb
an adduct is shown in Figure 7. Important structural parameters of this and other structurally characterized complexes of Ww are given in Table 4. Table 4 Structural Parameters of some W"' Complexes
W--x,"
Compound
(A)
W-A,,b
(A)
2.07 (F) (qH)z[WO(Oz)F4I 1.74 (0) [WOCl,(azoxybenzene)] 1.67 (0) 2.28 (0) 2.29 (0) ( M ~ ~ N H ) Z [ W ~ O , C I , ( ~ , O ) ~ ] 1.71 (0) (CS~NH,)[W~O~(NCS),~].~H~O 1.68 (0) 2.28 (N) 1.68 (0) 2.36 (0) KZ[W~OdO~)~I4WO 1.73 ~ ~ , " ~ , , ~ , ~ ~ ~ ~ Z ~ ( ,0) , ~ ~2.16 ~ (0) ~ ~ ~ l [WCI4(N&CI,)] +NCCC& 1.70 (N) 2.37 (N) 2.42(Cl) 1.68 (N) (P~&)[WC~~(N&CIS)~ 1.71 (N) 2.43 (Cl) (P~~A~)Z[WZNC~JI 2.10 (S) 3.05 (Cl,) IWSC141 3.03 (Brb) 2.08 (s) [WSBr4l 2.35 (Cl) (Ph4As)[WSCI,] 2.17 (S) 2.10 (S) 2.26 (Ob) [WSCl,][WOSCI,(DME)] 2.79 (S-thioether) 2.07 (S) [WSCl&dth) 2.26 (Se) 2.46 (0-ether) [WSeC13(CH,0C,H,0)] a
(A)
Ref.
1.92 (F) 2.29 (Cl) 2.40 (a) 2.08 (N) 1.93 (0) 2.39 (CI) 2.30 (Cl) 2.33 (Cl) 2.30 (Ci) 2.37 (CIb) 2.55 (Brb) 2.33 (Cl) 2.32 (Cl) 2.30 (Cl) 1.93 (0)
52 47 65 66
W-ACwb
51 61 86a
89
80 70 70 74 73a 76b
Terminally bonded atom. 'Atom W M or cis to X.
In some cases, the interaction of [WY&] with donor solvents leads to oxygen abstraction or cleavage of an ether linkage. Thus, [WSC4] abstracts an oxygen atom from 1,Zdimethoxyethane (DME) to form a complex that was shown by X-ray ~rystallography~~ to consist of the [WOSCl,DME] moiety bonded to [WSCL] through an oxygen bridge. This oxygen abstraction is characteristic of the high oxidation state halides of the early transition metals,75and in some cases can be used as a simple route to oxyhalide complexes. For example, [WOF4.Et20]can be prepared from WF6 and d i e t h ~ l e t h e rThe . ~ ~reaction of [WSeC14]with the same solvent leads to cleavage of the ether linkage and the formation of a W-alkoxide bond as shown in Figure 8. A similar reaction was reported for WOCL,]. Several nitrido complexes of W I are known. WCl, reacts with C1N377 to give [WNC13]whose structure is believed to be tetrameric similar to that of its molybdenum analogue? [WNQ] can also be prepared from WC4 and Impure [WNBr3]has been repared from WBr6 and IN3.79The preparation of [WNClJ has been reported and [W2NCllof- was isolated from this
6
Figure 8 The structure of [WSeCl,(2-methoxyetho~ide)j~~~
Tungsten
981
Figure 9 The structure of [WC&(NC&l,)] CC1,CN86'
preparation," The structure of this dinuclear complex consists of Wv' and Wv linked by a linear asymmetric nitride bridge." With py, the adduct wC 13*3py], of unknown structure, is obtained.'l Other characterized complexes derived from EWNC13] are [WNC13.PPh3], [WNC15]- and [WNC13(bipy)]." The reaction of [WNC13] with Poc13 gives the tetranuclear compound [WNC13(OPC13)]4.This structure contains a planar square W Y 4 eight-membered ring with alternating W-N bond lengths. The oxygen atom of the POC13 ligand is coordinated trans to the shorter W-N bond.83 The presence of the nitride ligand is easily detected in these complexes by a strong IR absorption band around 1050 cm-I assigned to Y(W-N~). ~ Complexes containing the WNR group are also known. The reaction of WCI6 with aliphatic and aromatic nitriless5 leads to the formation of imido W" complexes. Strong TR absorption around 1280cm-' is characteristic of the WNR group. These complexes are thought to be intermediates in the alkylcyanide reduction of WC16. The structures of several of these complexes have been determined crystallographically. The structure86aof [WC14 NCZCl5)].NCCCl3,which is depicted in Figure 9, reveals a short W-N bond length of 1.70 , as expected for a bond order of two or higher. When CC13CN reacts with WClS, a dichloro-bridged Ww imido complex is obtained (Figure 10). Complexes of the type [W(NPh)(OCMe3)4]and [W(NPh)C15]- have also been ~rep ared . '~ In all known structures containing WNR groups, the trans influence of the imido group is clearly seen (Table 4). The coordinatively unsaturated complex [WCL@GC15)] reacts with monodentate ligands to form six-coordinate with structures similar to the one depicted in Figure 9. When WClb is reacted with cyanogen in POC13, a dinuclear complex in which NC2C14Nbridges two [WC14(OPC13)]moieties is obtained.g0 With bidentate ligands it is suggested that sevencoordinate complexes are formed.88 Oxygen abstractions are also observed for the imido Wvl complexes. Thus, in addition to the 1: 1 adduct of OPPh3 with [WC14(NGC15)]the complex ~ C 1 2 0 z ( 0 P P h 3 ) is z ] formed." Also, W(NMe& reacts with BdOH to give [WO(OBU')~].'~ It has been shown recently that alkylidene and alkylidyne complexes of Ww also exist, and that some of these are important catalysts for the metathesis of terminal and internal alkene^.'^,^' Crystal structure analysis of the alkylidene complex [W(O)(CHCMe,)Cl,PEt,] shows that the geometry around the metal atom is distorted trigonal bipyramidal with the W-0, and W=CHCMe3 units in the equatorial plane. The short W - C distance of 1.882 A is in accord with a W=C double bond.92[W(NPh)(CHCMe,)Cl&] has also been prepared. Here the tungsten atom is six-coordinate with the W=NPh and W=CHCMe3 units in cis position, as expected, similar to complexes containing the WOz structural nit.^^,^^,^^ The alkylidyne complexes are best prepared by cleavage of a W=W triple bond" as depicted
8,
Figure 10 The structure of [W,Cl&-Cl~(N&C15)2]86b
Tungsten
982
schematically in equation (2). (These reactions will be discussed in more detail in Section 37.6.2.2.) Thus, the reaction of [Wz(OBut)6with RC%N and RCr-CR leads to cleavage of the
W=W triple bond and formation of both nitrido and alkylidyne complexes of Ww. Crystal structure dete rrnina ti~n~~ of [(OBU~)~W=N],and [ ( O B U ' ) ~ W = C M ~has ] ~ shown that the nitrido complex is polymeric with unsymmetrical W-N-W bridges, The alkylidyne complex is dimeric with a short W-C bond length of 1.76A. As expected, this distance is significantly shorter than the one observed for the W-C double bond.
37.3.4
WSi-, WOSg- and WO&-
as Ligands
The thiotungstate ion WSZ- has been known since 182695and its exact chemical composition was determined in 1886.'' Its structure97showed the expected tetrahedral geometry with a relatively short W-S distance of 2.165A. The mixed oxothio anions are also known.98 WSZ- and to a lesser extent WOSg- and WO&- have been shown in recent years to act as ligands for several metal ions. Complexes of the type [M(WS4)z]2-(M = Ni2+, Co2+,Zn2+,Pd2+, Pt"), where the thiotungstate ion acts as a bidentate ligand, have been prepared.9931""*101~1" Similar compounds with WOSg- and W02S$- are also kn~wn,'"~~~@'~~@' and in all cases the coordination is through sulfur. It is interestin to note that with trivalent ions such as C?', Eu3' and Dy3+, complexes such as [M(W02S2),f- are not obtained, but rather WO2S- hydrolyzes to the tungstate ion>06 Of particular interest are those complexes which serve as model compounds for biological systems such as the active center of the MoFe protein in nitr0gena~e.l'~These complexes will be discussed in detail in the relevant chapters. It should be noted here, however, that WSf- is a non-innocent ligand, and thus complexes such as [Co(WS,)$(n = 2, 3)108*109 and [Fe(WS4)$- (n = 2, 3) can easily be prepared.110,111,112In addition, some unusual complexes have been synthesized with WS:- as ligands. One such example is [Fe3W3Sl2I4-which contains the Fes(y3-S)2center as depicted in Fi ure 11. Complexes of the type Au2 WS&f- and [(PhzMeP)2Au]z[WS4]have also been prepared and structurally analyzed.''3~114(In the first case, two WS2- ions are bridged by gold atoms as shown in Figure 12 (identical structure is obtained with WOS$-) and in the second, WSf- acts as a bridging group for two PhZMeP),Au] moieties. WSl- and WOSZ- have also been shown to act as tridentate ligands. 111,116~117 One such complex is shown in Figure 13.117 Crystal structure analysis of complexes containing the WSf-, W0S:- and WO2S;- ligands have shown that these ions are tetrahedral with the W-Sb bond length slightly but significantly longer than the W-S, bond length. Finally it should be mentioned that both WSef- and WOzSe%-are known'" and [Zn(WSe4)]'- has been prepared.'lg
Figure 11 The structure of [Fe3W3S,J4-
Tungsten
S
983
b
Figure 12 The structure of IAy(WS,)J2-
37.3.5 bithiolene Complexes Dithiolene complexes attracted considerable interest during the 1960s and early 1970s because of their unusual electronic and structural properties. Tungsten complexes of the type W(L-L)3 are known €or L-L = sdt, bdt, tdt, S ~ G M Q , S2&H2, S Z G ( C F ~and ) ~ S%&(CF3),. These highly colored complexes undergo two reversible Structurally, the one-electron reductions leading to mono- and di-anionic neutral complexes represent examples of a trigonal prismatic coordination in six-coordinate complexes similar to the analogous Re and Mo complexes.12' The interesting problems associated with this class of compounds, such as their electronic structure and the relation between this and geometry, have been thoroughly reviewed.121~120 Of the reduced complexes several have been isolated, Hydrazine reduction of [W(sdt),] gives the dianion which reacts with two equivalents of MeI. It is interesting that the methylation reaction occurs only on one of the three ligands. The methylated product reacts with diphos to give [W(sdt)2(diphos)]. The mixed ligand complex [W(sdt),(SzCNEt2)]- was also reported. Crystal structure analysis'" of (Ph,As),[W(mnt>,] shows that the symmetry of the WS6 framework is close to D3 and that the geometry is midway between octahedral and trigonal prism. The complex [W{Se2&(CF3)2}3]has also been preparedla and like its sulfur analogue, it undergoes two reversible one-electron reductions. The dianionic complex was isolated as the tetrabutylammonium salt.
Q' 0
b pigure W The structure of [CU,(WOS~)~(PP~,),]; phenyl rings are omitted"'
Tungsten
984 37.4 THE CHEMISTRY OF Wv
Wv, unlike Mo", is not dominated by dinuclear species. Most well-characterized complexes of Wv are monomeric and there are only a few reported examples of dinuclear compounds. These, with the exception of [WzCllo], all contain an M-M bond of order one.
37.4.1
Wv Halides and Their Derivatives
The three halides WFS, WCls and WBrs are known. WF5 can be prepared from WF235and it was shown to be isostructural with MoFs, NbF5 and T E ~ F , , 'so ~ ~therefore probably has the same tetrameric structure with fluoride brid es. The dark green WCls can be prepared by reduction of WCl,5137or by disproportionatio~f~~ of WC4. It can also be prepared by reduction of WC16 with tetrachloroethylene in the presence of strong light.139The black-green WBr, is prepared by reduction of WC& in HBr, or by thermal decomposition of WBr6.140 WCl, is dimeric in the solid state14' and contains bridging C1 atoms. The long W-W distance of 3.814 A, and the magnetic moment of -1.0 BM per W142at room temperature, are consistent with the absence of a W-W bond. The octahedral W&- anions (X= F, C1, Br) are best prepared by reaction of the higher halides in a suitable ~ o l v e n t . ~ ~In, ~some ~ ~ ,cases ~ " they can be prepared by the reaction of the Crystal structure determination of C,[WCl,] and pentahalides with halide ions.'42,14s*146 (Et4N)[WC16]14'has shown that all the W-C1 bond lengths are practically the same, with an average value of 2.33 A. When dry, they are quite stable, but easily hydrolyzed. The WX, ions have low magnetic moments compared with the spin-only value for a d' electronic configuration. This is believed to result from large spin-orbit coupling. The reaction of W(CO)6 with IFs in the presence of KII4* gives the WFi- ion which is believed to be eight-coordinate Wv. Several adducts of WX, (X= Cl,Br) have been r e p ~ r t e d . ' ~ ~The , ' ~ best characterized is [WCls(diars)]'49 which is obtained by reduction of WCL with excess diars. This paramagnetic complex (1.19 BM) is isomorphous with the seven-coordinate [NbCls(diars)] and [TaClS(diars)]. It is well known that nitrogen-containing ligands can reduce W"' and Wv halides to complexes of W".152,153,154However, several substitution products of WX, (X = C1, Br) have been pre ared, and they are best formulated as cis-[WX&]+ (L= Me,py, py, PhCN, bipy, diphos).'P8~155 These adducts are all paramagnetic with magnetic moments in the range of 0.63-1.45 BM. Although magnetic moments below the spin-only value are expected for Wv complexes, some of the reported moments seem to be unreasonably low, perhaps as a result of diamagnetic WIVimpurities, or magnetic interactions of adjacent d' centers. Monomeric alkoxo complexes of Wv are also known. The dinuclear alkoxo complexes will be discussed in Section 37.4.4. The blue paramagnetic complex [WC13(OR)2](R = Me, Et) were first isolated from the reaction of WC16 with the corresponding alcohols. 156~157 When as ethanol is used, the reaction liberates chlorine which oxidizes the ethanol to a~etaldehyde',~ depicted in equations (3) and (4). Chloroalkoxo complexes of Wv can also be prepared directly from WClS and alcohols at -70 "C.Thus, M[WCl,(OR),] and M[WC15(0R)] (M = tetraalkylammonium; R = M e , Et, Pf') as well as the seven-coordinate M[WC&(OEt)] have been i s ~ l a t e d and ~ ~ characterized. *~~~~ Similarly, [WBr,(OMe),]- has been prepared from WBr5 in methanol. The monomeric chloroalkoxo complexes are paramagnetic and exhibit electronic absorption spectra similar to the Wv oxyhalides.
'''
WCls + 2EtOH C1, + EtOH
--
4C1, + [WCI,(OEt),] + 2HC1 MeCHO + 2HC1
It has been suggested previously (Section 37.3.1) that the relatively short WV1-OR bond length is indicative of bond order greater than one. If this holds true for the WV-OR bonds, then for complexes of the type [W&(0R)2]- the n donor ligands OR- will prefer trans stereochemistry since the d electron can be accommodated in the dxy atomic orbital phich is orthogonal to the orbitals involved in the bonding of the 36 donor atom. Indeed, ESR spectra of the dialkoxo complexes are consistent with axial symmetry.'60 This idea has also been put forth to explain the preference of the dioxo complexes with the d2 electronic configuration for the tram stereochemistry. An interesting solid state reaction has been reported for (%N)[WC15(OR)]'57 as depicted in
Tungsten
985
equation (5). For a given W' cation, the rate of alkyl chloride elimination follows the order Me > EtCl > PrCl. The rate of evolution of ethyl chloride decreased with increase in the size of the cation. (%N)[WQ(OR)I
-RCI, (R,,N)[WOC&]
+ unknown products
(5)
37.4.2 The Oxyhmlides and Their Derivatives
Of the oxyhalides of Wv, both [WOCl,] and [WOBr3] are known. These can be prepared by reduction of [WO&]161 or by the reaction of WX5 with Sb203162as depicted in equation (6). 3WX5 + Sb,O,
-
3W0X3+ 2SbX3 (X = C1, Br)
(6)
The structures of WOX3 are believed to be similar to that of [NbOC13]163which reveals the presence of infinite chains of Nb-0-Nb bonds. The absence of a strong v(W-0,) around 950 cm-' and the low magnetic moment of -0.5 BM are in agreement with this structure. In addition, WOX3 are antiferromagnetic, which is suggested to occur by superexchange mechanism through the W-0-W bridges. 16' The monomeric [W0X5]'- (X = C1, Br) anions have been isolated with a variety of +l cations. They are easily prepared from WCls or K3[W02(&04)2] in concentrated solutions of HC1 or HBr in the presence of the desired cation.!" [WOXJ are also known.lM These are all paramagnetic with magnetic moments close to the s in-only value, presumably because tetragonal distortion destroys the spin-orbit coupling.'' Their electronic spectra have been interpreted. Complexes of the type WOX3L] and [WOX3(L-L)] are known for a variety of mono- and bi-dentate ligand^.^^*'^^* '* They are best grepared by reacting [WOX3(THF),] or [WOX3(MeCN),] with the desired ligand. 16',' With some nitrogen containing ligands, complexes of this type can be prepared by reduction of WO&. Thus, the reaction of WOX, with py or bipy , gives [WOC13pyz] and [WOC13(bipy)] re~pectively.~'All the oxytrihalide adducts are paramagnetic and exhibit the characteristic ~ ( w - 0 ~around ) 950 cm-' (Table 5). Three different isomers are possible, in principle, for the [WYX3L] complexes (Y = 0, S, Se, NR) (Figure 14). ESR spectra of [WOCl&] (L=Ph3P, Me2PhP) was reported to be consistent with the cis-mer isornerlM in agreement with the solid state structure of [WOC13(OPPh3)2].However, the complexes [WOCI3(PEt&] and [W(NPh)Cl&] (L = OPMe3, PMe3, PMeZPh)have been shown to have trans-mer stereochemistry in the solid state.169>'70q171 With bidentate ligands, (Figure 14) it has been suggested that the fac isomer is preferred.167 However, structure determination of [WSC13(mte)] (mte = 172-bis(methylthio)ethane) has shown that it has the mer stereochemistry in the solid state.172Whether this is indeed the preferred stereochemistry for complexes of the type [wyX,(L-L)] will have to await further studies. The electronic spectra in the visible region of the [WOX3b] and [WOX3(L-L)] complexes are similar to those reported for WOX, and similar assignments can be made.ls6 Several porphyrin complexes containing the WO unit are known. 173~174 The paramagnetic 1647165
I
Table 5 Properties of Some W v Oxo Complexes
1.51 1.48 1.52 1.44 1.44 1.52 1.22 1.48 1.20 1.34 1.30
992 970 980 998 990 955 960 970 950 949 943
166 166 166 166 166 167 167 167 167 189 189
Tungsten
986
trans-mer
cis-mer
fac
mer
Figure 14 The possible isomers of [WOCI,LJ and [WOC13(L-L)]
complexes [WO(OAP)L] ( O M = octaethylporphyrin; L = OPh, OMe) are obtained by reduction of H2W04 in the presence of PhOH or by oxidation of K3W2C19 in methanol. For L = OPh, peff= 1.4BM and v(W-Ot) = 946 cm-l and for L = OMe, peff= 1.7 BM and v(W-0,) = 910 cm-l. The reaction of WF, with OAPH:! leads to the oxo-bridged dimer [WO(OAP)],O. [WO(TPP)OMe] arid [WO(TPPS)H20] are also known. Base treatment of these leads to the oxo-bridged dimers The TPP complex undergoes electrochemical reduction to the WIv compound (-0.85 V us. SCE independent of solvent).176 37.4.3 Complexes with Terminal Sulfur and Selenium Atoms
The pentahalides WXs (X= C€, Br) react with Sb2S3and Sb2Se3’” to give WSCl,, WSeC13 and WSeBr3. These complexes are insoluble polymeric materials and much less reactive than the corresponding WSX, and WSeX, complexes (Section 37.3.3). They show no IR bands which are expected for v(W-St) and v(W--Se,). Whether their structures are similar to those of WOX, is not known. The only adducts that have been isolated and characterized are [WSCl,(rnte)] and [WSCl,(bipy)]. Both are paramagnetic and exhibit the expected IR absorption for v(W-S,): for [WSCl,(mte)], peff= 1.52 BM and v(W-St) = 535 cm-l; for [WSC13(bipy)],pcff= 1.45 BM and v ( W 4 , ) = 531 cm-l. On heating, both WSC13 and WSeBr3 disproportionate to give the W” compounds WSClz and WSeBrz. Attempts to prepare WSBr3 were unsuccessful since the reaction of WBrS with Sb& leads to the WIV compound WSBr2.’77 Several complexes containing the W2S3 and W2O3 structural units have been prepared with thioxanthates and dithiocarbamate^.^^^,'^^,'^^ Whether these have structures similar to the analogous molybdenum complexes is yet to be determined.
’”
37.4.4 Complexes with Metal-Metal Bonds
A dominant structural feature of complexes of MoV is the M O ( ~ - X ) ~ Mbridge O which can be either planar or bent. It is believed that in these complexes there exists a Mo-Mo bond of order one which arises from the overlap of the dxy atomic orbitals (Figure 15) (each with one electron) leading to a bonding orbital that accommodates the two electrons, in agreement with their diamagnetism. The same type of structures have only recently been shown to exist for Wv. Structural determination of the barium salt181,’82of [W 04(edta)]*- shows the presence of a bent W(,U-O)~Wbridge. The W-W separation is 2.5421, the dihedral angle between the
F@W 15 Overlap of W dxy orbitals across the [W(p-X),W] bridge; z axis is perpendicular to the plane of the paper
Tungsten
987
Figore 16 The structure of [ W z ( ~ - S ) , ( ~ ~ ~ 4 ) 2 ( O M e ) , ] ' 8 6
two WOz planes is 147', the W atoms are displaced by 0.34 8,from the plane defined by the six oxygen atoms towards Ot, and the W-W-0, angles are obtuse. These structural parameters are consistent with the presence of a W-W bond. Electrochemical or tin reduction of WOZ- in the presence of oxalic acid and potassium oxalate leads to the isolation of K3[Wz04(C204)2.5].5H20183which contains the Wz02+ moiety. This oxalate complex reacts with KF in HF to give K3H[W204F6].1'4In this complex, the WzOZ- unit is almost planar with a W-W separation of 2.62w. It has been ~uggested''~ that electrolytic reduction of WOi- leads to the formation of compound (5) which decomposes to compound (6). Wv complexes containing the W(p-S)zW bridge are also known. Compound (7), prepared by [W,0,(~03,(H20),16--
[W~O,(CZO~)Z(H,O),]'-
(5)
(6)
[W2Sz(S2CNEt2)2(0Me)4] (7)
reacting [W(CO)3(MeCN)3]with tetraethyldithiuramdisulfide in methanol,lS6 contains a planar W(P-S)~W bridge (Figure 16). The W-W distance of 2.79185, coupled with the other structural parameters, is indicative of a W-W bond. This conclusion is particularly clear when this structure is compared with that reported for [WzCllo]f41which has no W-W bond. In [WzCllo], the W-W bond length is 3.814& the angles at clb are obtuse and the angles Clb-w-clb are acute. These features are expected for a non-bonded structure, since in the absence of an attractive force, there must be a net repulsion that causes the bridge to stretch along the M - . M direction. The synthesis of the mixed valence W", Wv' complex [W4Sl2I2- has been reported. Its structure analy~is'~'reveals the presence of the W&+ structural unit with a planar W(P--S)~W bridge, and a W-W distance of 2.95085. The coordination sphere around each W atom is completed by a terminal sulfur atom (W-S, = 2.10 A; v(W-St) = 495 cm-') and the bidentate ligand WSZ-. The W-W distance of 2.95085 is somewhat longer than expected for a W-W bond of order one which is believed to result from the interaction of the Wv and Wv' centers. The structure of [C12W(0)(p-S)2W(O)C12]2-has also been reported."' This complex contains a bent W(P-S)~Wbridge (dihedral angle of 149") with a W-W separation of 2.844 A. An interesting dinuclear complex of Wv has been obtained from the reaction of [WSeC13] with Ph4AsC1 in a 1: 1 mole ratio. The diamagnetic w2Cl8Se3]'- contains both an Se2- and Sez- bridging units with a W-W distance of 2.862A. The presence of a W-W bond in this compound is further supported by the acute angle of 73.3" subtended at the bridging selenium atom. lWb Complexes containing the bridge' W(p-SR)z(p-X)W (X= C1, OMe)'8q"90 have also been prepared. The structure of compound (8) which is depicted in Figure 17lWreveals the presence of a W-W bond of order one with a W-W separation of 2.854A. Complexes of this type have been shown to have interesting e1ectrochemistry.lw +
Reaction of WC16 with ethanol gives the blue paramagnetic [WC13(OEt),] compound (see Section 37.4.1). Treatment of this with ethanol affords the red diamagnetic dimer of composition [W2C14(OEt)6]. Crystal structure analysis shows191that the compound contains a planar W(pOEt)2W bridge (Figure 18) with a W-W bond of order one (W-W separation of 2.715A). It is now known that the best method of preparing the ditungsten species (see Section 37.5.4.1). [WzCb(OR)6] is by AgN03 oxidation of the Ww complexes coc3-PP'
988
Tungsten
Figure 17 The structure of [WZOZ(~--SBui)Z(~-Cl)C~~-
CI
CI
p%ure 18 The structure of [W2C14(OEt)J1"
37.4.5
Thiocyanato Complexes
Reduction of WOZ- with thiocyanic acid leads to colored species which have been used for tungsten ana1y~is.l~~ The color of these species are pH dependent and the structures of some of them are still uncertain. The following sequence of equilibria have been suggested on the basis of several isolated amine salts (equations 7-11).88*194Of these species, it is believed that the
+ NCS- 4 [W02(NCS)$- + H 2 0 [WO,(NCS),]'- + H+ + NCS- e [WO(OH)(NCS)4]2" [WO(OH)NCS}.$- + H+ e [WO(NCS>,]"- + H20 [WO(UH),(NCS),]-
2[WO(OH)(NCS)4I2[W,O,(NCS),]"-
+ 2H+ + 2NCS-
e e
[Wz03(NCS)J4- + H20 2[WO(NCS),]*-
(71 (8)
(9) (10) (11)
orange diamagnetic [W203(NCS)x]4-and the green paramagnetic [WO(NCS)5]2- have structures which are similar to the analogous molybdenum complexes. The complex [W(NCS)& can be prepared'', by direct reaction of WCl, and NCS-.
37.5 THE CHEMISTRY OF WW Monomeric complexes of Wm such as the halides, the oxyhalides and their derivatives have been known for quite some time. In most cases these are six-coordinate, but higher coordination numbers are also known. During the last two decades, new structural types have been reported. Of these, the most interesting are the dinuclear complexes containing W=W double bonds and the trinuclear clusters containing W-W single bonds.
37.5.1
W w Halides and Their Derivatives
All four tetrahalides, WF,, WCL,, WBr4 and W14, are known. WF4 can be prepared by the reaction of WF4 with benzene in a bomb at 110"C.'% Its structure is not known, but is is certainly polymeric with W-F-W bridges. WC1, and WBr4 are best prepared as black needles
I Tungsten
989
by aluminum reduction of the higher halides in a controlled temperature gradient. 154 These are diamagnetic and isomorphous with their Nb and Ta analogues which consist of infinite linear chains of octahedra sharing a common edge. The alternating short and long metal distances imply the existence of direct metal-metal bonds,lW in agreement with the diamagnetism of these compounds. W14 is prepared by the reaction of WOz with Al13 or by the reduction of WC16 with HI. The tetraiodide is unstable and decom oses even at room temperature to WIz and Iz. The hexahalide complexes W G - (X = C1, Br) are easily prepared144,'98by reduction of the higher halides with alkali iodide. These octahedral complexes have low magnetic moments and high f3 values, suggesting antiferromagnetism. WClg"" has been reported to react with vicinal diols199to ive alkenes and s ecies containing the W@+ unit. The hexaisothiocyanate complex [W(NCS)j- is also known. ' 5 The six-coordinate adducts W a (X = C1, Br) can be prepared by reduction of the higher halides, by direct addition to or by oxidation of lower valent complexes. Certain nitrogen ligands have long been known to reduce W& and WX5 to complexes of the type WX& (L = py, MeCN, EtCN, PrCN).l@The acetonitrile complex, which is a useful starting material for the preparation of other adducts, can also be prepared by the reaction of WCI, with W(CO)6 or by the oAdation of W(CO)6 with Brz in acetonitrileZmas depicted in equations (12) and (13).
m,
+ W(CO), W(CO), + Br,
4WC1,
S[WQ(MeCN),] McCN
> [WBr,(MeCN),]
+ 6CO
(12)
+ 6CO
(13)
Phosphine and arsine adducts of W C 4 are conveniently prepared by reduction of WCls with amalgamated zinc in the presence of the desired ligandmmlSome of these complexes can also be prepared by Br2 or Clz oxidation of cis-[W(CO)&] (L =tertiary phosphine).z02 The stereochemistry of the WX& complexes depends on the nature of L. Crystal structure ,203 and [WCI&'Ph,),]" (prepared by reduction of analysis of [ W & ( P ~ ) ~ ] [WC14(PMe2Ph)z]z04 WYC14 (Y = 0, S, Se) with excess phosphine) have established trans stereochemistry. Far IR measurements suggest the same stereochemistrfm for L = RCN and C4HzS. However, with L = MeCN, EtCN and Et2& cis-[WX&] is formed. The six-coordinate [WX&] complexes, with the d2 electronic configuration, are paramagnetic with magnetic moments in the range of 1.7-2.1 BM (Table 6). The orange complex trans-[WC14(PMezPh)2]reacts reversibly with PMezPh to gve the red paramagnetic (p c =~2.68 BM) seven-coordinate compkx &WCl4(PMezPh)3] .201*2mOther sevencoordinate WIV complexes have been re orted.l4With an excess of PMe3, [WC14(PMe3)3]is obtained directly from WC16J7 No structural information is available for the seven-coordinate Ww complexes, but it appears that this coordination number is accessible for tungsten in this oxidation state, and that in this respect, the six-coordinate WW complexes need not be considered coordinatively saturated. Several P-diketonate complexes of the type [W(L-L)X2] (X= C1, Br; L = acac, 6-methyl2,4-heptanedione, dibenzoylmethane) have been prepared from reaction of the /3-diketone ligands with WX5. The suggested cis stereochemistry, based on IR measurements, must be considered tentative. Table 6 Properties of Some W'" Complexes Compound
Color
peR(BM)
v(W-0) (cm-')
Ref.
Orange Red Orange Blue Blue Brown Pink Green-brown Purple
2.05 2.68 1.88 Diamagnetic
-
202 202 201
Diamagnetic 1.67 Diamagnetic 1.43 Diamagnetic
945 952 +
955
-
960
201 201 206 167 149 201
990
Tungsten
37.5.2 The Oxyhalides and Their Derivatives
The oxyhalides WOF2, WOCl, and WOBr2 are known. WOFz is an inert black non-voiatile powder. It is prepared by the reaction of WOz with HF at 500°C. W0Clz was reported to result from the thermal decomposition of WOC1262 as a black-purple solid. Golden-brown crystals of WOClZ have been obtained by SnCl, reduction of W O c 4 followed by sublimation,zw and by the reaction of a mixture of W, W0 3 and WCls followed by chemical transport.210 It is.likely that WOClz exhibits polymorphism, and that the different preparative routes simply lead to different polymorphs. Nonetheless, its structure, and that of WOBrz, is polymeric, probably involving both oxygen and halogen bridges. The oxohalo derivatives of Wlv can be prepared by reduction of the oxo complexes of Wv and Wv1,267J"1by reduction of WC16 in wet ethanol, or by the reaction of WClg- with the appropriate ligand in wet Thus the complexes [WOX&] (L = PMezPh, PEtzPh and PMePhz) are prepared by reacting (Et4N)[WC16], WOCI, or WCls with the desired ligand in wet ethanol. These complexes exhibit the expected strong IR band around 950cm-' which is associated with ~ ( w - 0 ~ )The . [WOX2L3] complexes can undergo halide substitution and complexes such as [WOX2(PMe2Ph)3j(X= NCO, NCS) result from the reaction of the halide derivatives with NaNCO and JSNCS. It has been shownzo7that the seven-coordinate complex [WCL(PMe3)3] can abstract oxygen to give [WOC12(PMe3)3].By this method, [W0ClzL3](L = PMe2Ph, PMePh,, P(OMe)3) have been prepared. The [WOX2L3]complexes are diamagnetic. Their proton and phosphorus NMR and far IR spectra suggest the cis-mer stereochemistry (see Section 37.4.2). This suggestion is strongly supported by the crystal structures of blue [MoOCIZ(PMezPh)3 and green [MoOClz(PEtZPh)3] which show that the halide ions are indeed cis to one another.A I With certain bidentate ligands, it has been shown167 that Wv complexes of the type [WOC13(L-L)] (L-L = diphos, cis-PhzPCH=CHPPhz, 1,2-bis(diphenylphosphno)benzene) can be reduced with excess ligand to [WOCl(L-L)z]+. These diamagnetic pink complexes, isolated as the tetraphenylborate salts, are 1:1 electrolytes in nitromethane and exhibit the expected ~ ( w - 0 ~ )around 950cm-I. 31P NMR data clearly favor the structure in which the two bidentate ligands occupy the equatorial plane and the C1- ligand is truns to Ot. 677z07
37.5.3 The WzOClf Ion
The incomplete reduction by tin of KzW04 in concentrated HC1 leads to a very deep purple solution which was suggested to result from a compound of apparent composition K2[W(OH)Cl5].'12 The high intensity of the electronic absorption band of [W(OH)Cl5lZ- at 19 100 cm-l ( E = 10 000) is inconsistent with a mononuclear formulation (especially with ligands such as OH- and Cl-), and indeed, the suggestion has been madez13 that this ion should be formulated as [WzOC110]4-,an oxo-brid ed W"'-Wv mixed valence dimer. The dinuclear structure of [WzOCllof was established by ~rystallography.2~~ The W-0W bridge is linear and the W-0 separation is quite short (1.871A) indicating a bond order higher than one. However, the mixed valence formulation has been questioned. Careful spectral and magnetic measurements suggest that the two tungsten atoms have equivalent d2 electronic configurations and that they are antiferroma netically coupled with an exchange energy of -75 K.'15 The intense absorption at 19 100 cm- is believed to be associated with the W-0-W chr~mophore.~~'
H
37.5.4 Complexes with Metal-Metal Bonds As mentioned previously, one of the more interesting developments in the chemistry of W" has been the synthesis and the characterization of dinuclear complexes containing W-M double bonds and trinuclear clusters containing W-W single bonds. In discussing complexes containing two interacting metal atoms, which are considered tc have a metal-metal double bond, and those that contain three interacting metal atoms and haw metal-metal single bonds, it should be pointed out that the assignment of bond orders i! problematic since these complexes, almost invariably, also contain bridging ligands. For Wv (d electronic configuration), the W-W bond order cannot exceed unity, but for W'" with a d
Tungsten
99 1
electronic configuration, a metal-metal double bond may be at least postulated. However, arguments for deciding between a double bond on the one hand, and a single bond together with indirect pairing of the spins of the remaining two electrons through the bridging atoms on the other hand, are rarely, if ever, conclusive, because the only data on which to base them are structural. For an isolated case, these structural data can be used to show that there is a metal-metal bond, but they do not generally allow one to conclude that the bond order must be two rather than one. However, there are today a few reported cases in which a W-W double bond aan be assigned almost unambiguously on the basis of comparison of structural parameters of two related complexes, which differ from one another only by two electrons, i.e. dinuclear complexes of WI' and Wv. 37.5.4.1 Complexes with metal-metal double b o d The synthesis and structural characterization of a WIV complex containing a planar W(P-S)~Wbridge1% has provided a rare opportunity to develop an unambiguous argument for the existence of a double bond between two W atoms when bridging atoms are also present. Compound (9) (Figure 19) was prepared by allowing [W(CO)3(MeCN)3] to react with tetraethyldithiuramdisulfide in acetone. The W(P-S)~Wbridge is strictly planar (there is a crystallographic inversion center between the two metal atoms), with a W-W separation of 2.530& indicating a direct metal-metal bond. The presence of such a bond is further supported by the very small W-Sb-W angle of 65.5" and the obtuse sb-w-% angle of 114.5". The chemical environment and geometry of the metal atoms in this complex are almost identical to those found for the Wv compound (7) (see Section 37.4.4) where the W-W separation was found to be 2.791 A. The shortening of the W-W separation by 0.26A clearly indicates that a considerably stronger force exists between the metal atoms. This conclusion is further supported by the contraction of the W-Sb-W angle and expansion of the sb-w4b angle (for the Wv compound, these angles are 73.2" and 106.8" respectively), whereas the w-& distances in both are practically the same. Thus it seems clear that in this case we are dealing with a double bond between the two tungsten atoms. Other examples which strongly support the existence of a metal-metal double bond are furnished by the dinuclear alkoxo-bridged dimers (10).216 Several synthetic procedures have been reported for the preparation of these compounds. They include the oxidation of quadruply bonded tun ten compounds217 in alcoholic HCl solutions, the oxidation of [W2Cl9I3- in alcohols?''the electrolytic reduction of alcoholic solutions of WC16219and the alcoholysis of WCl.+220Other alkoxy derivatives can be prepared by a simple alcohol exchange reactiomZ1 [W2C14(P-OR)~(OR),(ROH),I (R= Me, Et) [W~(~L-S)~(~-S~CNE~Z)Z(S~CNE~Z)~]
(9)
(10)
consist of two edge-sharing distorted octahedra. The metalCrystal structures of metal vector bisects the edge formed by the bridging oxygen atoms and the complete Cl2W(p-0)2WCl2 unit is planar (Figure 20). Above and below this plane, there is one OR on one W atom and HOR on the other, with a strong hydrogen bond between the two oxygen atoms (0* - . O = 2.48& and therefore the W-0 bonds are perpendicular to the plane
Figure 19 The structure of [Wz(p-S),(S2CNEtJ4]'86
-
Tungsten
992
CI
CI
figure 20 The structure of [W,C14(0Et)4(EtOH)2]216
instead of bending away from each other because of 0 0 repulsion as is observed in the Wv angle is 95.7") (see Section 37.4.4)). structure (0 * 0 distance is 3.04 and the W-W-OR The structures of the alkoxo-bridged dimers of W'" and Wv provide an excellent opportunity to make a detailed analysis of M-M single and double bonding; The number of compositional and structural changes on going from the Wrv to the W compound are minimal. In composition, only two hydrogen atoms are removed, and structurally, other than the bending away of the two OR groups, all changes are those caused by the increase in the M-M distance from 2.482 8, in the Wrv complex to 2.715 A in the Wvcomplex. These changes are associated with the W-0b-w and ob-wab angles. Thus by going from the d'-d* to d2-d2 electronic configuration the bond order increases from one to two. Moreover, qualitative considerations suggest that the double bond consists of the (T component responsible for the d'-d' single bond and a 3t bond. Fenske-Hall calculations support this interpretatiom216 As mentioned above, a variety of complexes of type (10) can easily be prepared by alcohol exchange and, in this way, compounds with R = Pr", Bun, Pen", Oct" have been isolated and characterized.221However, when secondary alcohols have been used for exchange with (lo), mixed alkoxy species were obtained. 'H NMR showed that the bridging ethoxide ligands had not been replaced, a result that was confirmed by the crystal structure determination of the mixed ethoxide-isopropoxide and ethoxide-sec-pentoxide derivatives. The reductive coupling of acetone by (10) to give a Wv compound in which the ditungsten center (W-W = 2.701 8,) is bridged by the resulting organic molecule (Figure 21) is another manifestation of the relative ease by which compounds (10) can be converted to dinuclear alkoxo-bridged Wv compounds.222 In addition to compounds of type (IO)there is another class of WIValkoxides with a different structure. The oxidation of [W2(NMe&] by methanol and ethanol in hydrocarbon solvent leads to the tetrameric [W4(OR)16]compounds.223These molecules do not possess the localized W-W double bonding as found in (LO) but, rather, have a structure similar to that of
-
+
CI
c1
Figure 21 The structure of [W,Cl,(p-OEt),(p-L),] (L=MC+COCOM%)"'
Tungsten
993
Figure 22 The structure of [W,(OR),,]; carbon atoms are
[T4(OEt)16]224 With distortions due to M-M bonding. Crystal structure analysis of [W4(0Et)16](Figure 22) reveals the presence of two short W-W bonds (2.645 and 2.76 A) and two long bonds of 2.93 A. In this structure, there is a total of five possible W-W interactions, and thus, to form five bonds of order one, ten electrons are required. Only eight metal electrons are available for metal-metal bonding, which leads to the observed distortions. It has been suggested that this distortion results from a novel second order Jahn-Teller effect.225 The reaction of [W2(NMe&] with excess Pr'OH leads to yet another t e of tetranuclear Ww alkoxy compound [W4(p-H)2(OPi)14]as depicted in Figure 23." The molecule is centrosymmetric with alternating short (2.446 A) and long (3.407 A) W-W distances, consistent with the presence of a W-W double bond and a non-bonding distance, respectively. Finally, WO2, which has a modified rutile ~tru ct u re, 2exhibits ~~ alternate W-W distances of 2.475 and 3.096 8, across shared edges of W 06 octahedra. Thus, the short distance of 2.475 A which indicates a W-W double bond makes a recognizable dinuclear unit in the idnitely extended array of the metal oxide. Dinuclear complexes with a W-W double bond containing three bridging groups are also known. The reaction of [WCl&] (L = THF, Me& with Me3SiSR (R = Me, Et, Bu', CHzPh, Ph) has been shown to give compounds of type (ll),n8~n9~uo9231 Crystal structure analysis of
P
[LCl,W(p-S)(p-SR),WCl,L] (L = THF, MezS, C1; R = Me, Et, Bu', CH2Ph) (11)
several of these have revealed a distorted confacial bi-octahedral structure with W-W separation of close to 2.5 A. The acute bond angles subtended at the bridging atoms agree with a strong metal-metal interaction. A
(3" Figure 23 The structure of
[W4(p-H)2(OFY)14]; carbon atoms are not shownzz6
37.54.2 Trinuclear clusters
During the past several years it has been shown that both molybdenum and tungsten in oxidation states of +4 or thereabouts, have marked predilection to form trinuclear cluster species. For the tungsten cluster species, there are three different structural types with respect to the ligand arrangements (Figure 24 a, b, c) which are based on an equilateral triangle of M-M bonded atoms. Structure (a) contains the [W3(p3-X)(p-Y)3]nucleus, where X = 0, C1,
994
Tungsten
0
A
Q
Figrue 24 Structural types of trinuclear tungsten clusters
OR and Y = 0. This structure is basically the same as the one observed several decades ago for the mixed metal oxide system Zn2M0308,232and is exemplified by the complexes (U),(13)and (14).
Complex (12) has been prepared by treatment of [W204(&04)2.5]3- with 40% HF at 70°C. Crystal structure analysis has established the presence of the W & L ~ - X ) ( ~ - nucleus Y)~ (X= Y = 0) with an average W-W distance of 2.515 A. Without counting the M-M bonds, the coordination number of each metal atom is six and the geometry is distorted octahedral.233 Treatment of W2Cl, ( P ~ U ? )with ~ ] acetic acid in diglyme at 160 "C leads to the isolation of (l3) (Figure 25).23 In this structure X = C1, Y = 0, and the average W-W distance is 2.609 A. In both these complexes the metal is in the +4 oxidation state, and therefore each metal atom
d
R
Figure 25 The structure of [W,O3Cl,(O,CMe)(PBu;),1; the butyl groups are omittedu4
Tungsten
995
has only two d electrons to use for M-M bonds. Thus, the two bonds formed by each metal atom cannot exceed a single bond. Compound (14)has been prepared by refluxing 1: 1:2 mole ratio of W(CO)6, Cr(C0)6 and pivalic acid in o-dichlorobenzene.us In this structure X = OCH2C(Me), and Y = 0, and the average W-W separation is 2.610 8,.The average oxidation state of the tungsten atoms is +34 or formally W'II, WU', W'". In this eight electron system, the extra pair of electrons occupies a non-bonding or slightly antibonding molecular orbital. It is therefore reasonable to suggest that two electron oxidation to a six electron configuration may be possible without disrupting the cluster nucleus. A comparison between the three W-W distances shows that with (p3-CI), the W-W distance is approximateIy 0.18, longer than that with (p3-0), as expected when a larger capping atom is pIaced above the metal triangle. It is tempting to suggest that the longer bond length observed in the eight electron cluster is due to the additional pair of electrons. However, it is important to realize that the W-W bond length is also affected by the nature of the bridging atoms. Trinuclear clusters of type (b) contain two capping groups, one above and one below the triangular plane. In addition, each pair of tungsten atoms is bridged by two carboxylates, and the coordination sphere is completed by a radial ligand which is bonded to each metal atom. With capping oxygen atoms, the [W302(02CR)6]unit can be the basis for a varied family of compounds. First, the R group of the carboxylate is variable. Among the four compounds isolated to date are those having R = Me, Et and But. Second, the radial coordination site may be occupied by neutral ligands, anionic ligands or a mixture of both, to give cationic clusters such as (E), neutral ones such as (16) and anionic ones such as (17).236
These clusters are prepared by refluxing W(CO)6 in the appropriate acid with or without its anhydride. Cationic clusters are best purified by ion exchange chromatography. The structure of (15) is depicted in Figure 26. Structure analysis has shown that the W302 nucleus is essentially invariant to the carboxylate ligands and that the W-W distance which averages 2.75 A is only slightly affected by the radial ligands. In these clusters, the oxidation state of the metals is +4 (d2 electronic configuration) and, therefore, the W-W bond order cannot exceed unity. The stereochemistry of the tungsten atoms in these species is of particular interest. Each tungsten atom has a coordination number of nine, counting the neighboring metal atoms as well as the coordinated oxygen atoms. The geometry can be regarded as either distorted capped square antiprism or a distorted tricapped trigonal prism. The metal atoms are coordinatively saturated which accounts for the fact that the radical ligands are substitutionally inert.
Figare 26 The structure of [W30,(0,cMe),(a,0)3]"
Tungsten
996
Recently, clusters (18) with one capping oxygen atom and one capping ethylidyne moiety have also been prepared.=' The +1cluster is prepared by refluxing W(CO)6 with a mixture of acetic acid and its anhydride in MeCN. A one electron oxidation of the +1 ion leads to the +2 ion, whose crystal structure analysis reveals a W-W distance of 2.81A. This distance is significantly longer than the one observed for (V),in agreement with the fact that the +2 ion is a five electron system and therefore the W-W bond order is less than unity. [W,O(CMe)(O,CMe),(H,O),j"+ (n = 1,2) (18)
[W,0(0,CR),(H,0)3]2' (19)
(R = Me,Et)
Clusters of type (c) (19)have been prepared by refluxing W(CO)6 in a 1 :1 mixture of the acid and its anhydride in the presence of small amounts of triethylamine.u8 These clusters might be considered a variant of (b) in which one capping group is absent. The structure of (19) (Figure 27) is virtually identical with the previously reported structures of tritungsten clusters with two capping oxygen atoms except for the absence of one capping oxygen. Of the quantitative differences between the hemicapped and the bicapped structures, the largest and most unambiguous by far is in the W-W distances. In the four bicapped structures,236the W-W distance averages 2.75(1) A, while in the present coumpound, the average distance is 2.710 A. This is clearly a significant difference in both the statistical and chemical senses.
Fignre 27 The structure of [W30(0,CMe),(H,0)JZC
238
The hemicapped cluster is an eight electron system and a Fenske-Hall type molecular orbital shows that in addition to the six M-M bonding electrons comparable to those in the bicapped species, an additional electron pair occupies an orbital which is weakly M-M bonding, in agreement with the observed shortening of the W-W bond length. Finally, although the difference in the W-W distances between structures (a) and (b) are approximately 0.2& they both represent W-W single bonds. On the basis of molecular orbital calculations it has been suggested that this is due in part to the difference in the distribution of charge density in these m o l e ~ u l e s However, .~~ there is no doubt that changing the bridging ligands will have a considerable effect on the M-M distance, especially when dealing with metal-metal bonds of low order. 375.5
Eight-coordinate Complexes
Eight-coordinate dodecahedral complexes of the general type W(L-L)4 are known for several bidentate ligands such as dithiocarbamate, 8-quinolinol, substituted quinolinols, picoline and substituted picolines. The reaction of [W(C0)3(MeCN)3]with tetraethyldithiuram disulfide in refluxing chloroform gives [W(S2CNEt2)4]which can also be prepared from [WCl,(MeCN),] and the dithiocarbamate ligand.=' It was also reported that the addition of CS, to [W(NMe&] leads to
Tungsten
997
[W(SZCNMe2)4].241[W(S,CNEt,)4] can be oxidized with BrZ to the eight-coordinate Wv complex [W(S2CNEt2)4r2. Crystal structure analysis of this compound reveals the expected dodecahedral geometry with W--S(A) distance of 2.529 8, and W-S(B) distance of 2.494 A (A and B refer to the Hoard-Silverton notation).243Recently, [W(S2CSR)4](R = Et, P t , But) have also been prepared and I2 oxidation leads to the corresponding Wv complexes [W(S2CSR)4]15.'78 With 8-quinohol or its derivatives, W(L-L), com lexes have been prepared by a sealed-tube melt reaction with either K3[W2C19] or W(CO)6.' Crystal structure analysis of W(L-L)4 (L-L = 5-bromoquinolinol) has established the dodecahedral geometry for this and related compounds.245The four quinolinol ligands span the Hoard-Silverton m edges, with the oxygen atoms occupying A positions and the nitrogen atoms the B positions. Another series of eight-coordinate complexes has been prepared with picolinates. All eight-coordinate complexes of the type W(L-L), to date are diamagnetic, with the two electrons of the dZconfiguration paired in the low-lying dx2-y~level of the DU d ~ d e c a h e d r o n . ~ ~ The IR electronic spectra consist of intense low energy bands ( E > lo4), attributed to d 3 x* charge transfer transitions. [W(pi~olinate)~] and several quinolinate derivatives can be oxidized to the corresponding Wv complexes.246These are paramagnetic with moments of about 1.7 BM, close to the spin-only value. The two low-intensity near IR transitions are reasonably assigned to d-d transitions from the d+ z level. Liquid nitrogen ESR measurements suggest lower symmetry for these Wv complexes.246YThey are reasonably stable in the solid, but disproportionate rapidly in solution to the WIV compounds and tungstate.247
37.5.6 Complexes with Multiple Bonds to Sulfur, Nitrogen and Carbon The tendency of Ww to form multiple bonds to good ~tdonor atoms is much less pronounced than for Wv and Ww; however, several such complexes are known. The reaction of aqueous WSZ- with dilute HC1 ghes the [w30s8]2- ion.248Its structure consists of the WO unit bonded to two WS$- ligands with a short W-0, distance of 1.68(2) A. [W3S9lZ-has also been prepared."8."9 Its structure is very similar to that of [w30s8]2- with a W-St bond length of 2.07(1) A, In this complex, the geometry around the central WIV ion is that of a distorted square pyramid. The multiple bond between Ww and a carbon atom is exemplified by [W(CR)L,Cl] (R = H, CMe3; L = PMe3)=' and by [W(CPh)(CO)z(py)2Br].251 Crystal structure analysis of this last compound revealed the expected short W - C bond length of 1.84 A. Complexes containing multiple bonded nitrogen ligands are usually obtained from dinitrogen complexes. The complexes [W(NH)X(diphos)]+ (X = F, C1, Br) are knownzs2 and in basic The nitrido group is readily methanol yield ammonia via trans-[W(NH)(OMe)(diph~s)~].~~~ attacked by electrophiles to give imido, dialkylimido or thionitrosyl complexes. Thus, [W(N)(N,)(diphos),] reacts with Et1 to give [W(NEt)(N3)(diphos)2]I. Na/Hg reduction of [W(NR)C14] in the presence of the desired li ands leads to the complexes [W(NR)C121+] (R = Ph, Et; L = PMe3, PMe,Ph, CNBu').'' C stal structure analysis of [W(NP~I)CI,(PM~,)~] has revealed a short W-N distance of 1.755 in agreement with v(W-NR) around 1080cm-'. The phosphine ligands occupy mer positions cis to the W(NR) unit. This stereochemistry is similar to that suggested for the analogous complex [WOC12(PMeSh)3].ml
x
37.5.7 Cyanide Complexes The octacyanotungstate ion [W(CN),I4- has been known since 1914.254It is easily obtained by reacting a variety of starting materials containing WIII, WIV and Wv with aqueous cyanide.=' The trivalent metal is usually oxidized by either air or solvent, whiie the pentavalent starting materials disproportionate to [W(CN)8]4- and WOt-. The [W(CN),I4- ion can be easily oxidized to [w(CN),l3-, but it does not appear possible to obtain it directly from Wv compounds. [W(CN),I4- is diamagnetic while [W(CN)8]3- is paramagnetic with petr= 1.76 BM.=' Both [W(CN)8I4- and [W(CN),l3- are stable in solution in the absence of light and no exchange of I4CN- is detected. This exchange is strongly catalyzed by light and the rate is
Tungsten
998
dependent on the light intensity. Exposing the yellow solution of [W(CN)8]4- to light leads to a rapid change to red-brown, and then to purple. On the basis of crystallographic the red-brown color is due to trans-[WO2(CN),I4- and the purple color to trans[W(0)(OH)(CN)4]3-. Recent1 , the [W(O)(0H2)(CN),]'- ion has been isolated and characterized as its cadmium salt?"and protonation of [W02(CN),j4- was reported to lead258to [W~O~(~)~I~-. The photolysis of [W(CN)*I3- has been i n ~es t i g at ed in ~~'the pH range 1-13. The photolysis consists of a two-step sequence involving a primary intermolecular redox process and a consecutive thermal process both leading to [W(CN),I4-. The rate of the oxidation of [W(CN)8]4Lwith several oxidizing agents has been studied. With IO; for example,2M)the reaction is first order with respect to both reagents and is catalyzed by H+. Inorganic cations have an accelerating effect on the oxidation in the order, Mg >> Cs > Rb > K > Na > Li. The reaction of [W(CN)8]4- with H2 at 400 "C was reported to give a green-black product of composition &[W(CN)6].25 Recently the same reaction has been shown to give [W(CN),HI4-, and in the presence of base the following equilibrium is established,261as depicted in equation (14). [W(CN),I5- can also be prepared directly from [W2Cl9I3-with an excess of cyanide. The sharpness of the 13C NMR lines suggests that on the NMR time scale there is no exchange of coordinated cyanide and that both structures are fluxional in solution. [W(CN),HI4-
+ O H - e [W(CN),p- + H 2 0
(14
Salts of [W(CN)8]4- are dodecahedral both in solution and in the solid state with the exception of the cadmium salt which is believed to have the square antiprismatic structure in both phases.zs5 ESR and vibrational spectra suggest that the Wv ion [W(CN),I3- has DM symmetry in solution.z62 However, evidence for both the dodecahedral and square antiprismatic ground state for this ion has also been reported.263 Mixed cyanoisonitrile complexes have been reported to result from the reaction of Ag4[W(CN)8] with RNC ( R = M e , Et, Pr", Pr', But and CPh3).z64 The resulting [W(CN),(CNR),] complexes are dodecahedral, with the more strongly 3t accepting isonitrile ligands occupying the B position. These complexes can also be obtained by the reaction of the silver salt with alkyl iodide.255
37.6
THE CHEMISTRY OF W"'
With a few exceptions, the chemistry of WIT' is almost entirely that of dinuclear species containing a metal-metal bond of order three. There are only a few well-characterized examples of mononuclear complexes. This undoubtedly results from the tendency of WI" to form metal-metal bonds and the susceptibility of mononuclear complexes to oxidation. Of the halides of WII1, only WBr3 and W13 are known.z69The thermally unstable WBr3 is prepared by the reaction of WBrz with excess Br2 in a sealed tube. It is inert to HzO and concentrated HCI. W13 is prepared from W(CO)6 and I2 at 120 "C.
37.6.1
Mononuclear Complexes of W"'
Camplexes of the type WX3py3 (X=C1, Br) have been preparedz6' by treatment of [W(CO)&] with py at 140 "C.These are paramagnetic with peff= 3.39 and 3.40 BM for the chloro and bromo derivatives respectively. [WXL ]- (X = C1, Br; L = Py, 4-Mepy) have also been prepared by pyridine reduction at 350K of the W" compounds [WX&]. Crystal structure analysis of [WC14py2]- and [WBr,(Me~y)~l-shows that both have the trans ~ t e r e o c h e m i s t r y .Recently, ~ , ~ ~ ~ it has been shown that [LW(CO),]- (L = polypyrazolylborate) react with thionyl chloride to give [LWCl,] which can be reducedzm to the W"' complex [LWClq-. 37.6.2
Dinuclear Complexes of Wtn
The dinuclear complexes of W"' can be divided into two groups. In one, the metal-metal bond is supported by bridging ligands as exemplified by the [W3Cl9I3-ion, while in the second,
999
Tungsten
the dimeric species are of the W2& type, recognized by the a2n4electronic configuration, where the two metal atoms are held together by metal-metal bonds only. Structurally, what set these two groups of complexes apart are the M-M bond length, the coordination number and the geometry around the metal atom, and chemically, there is very little that the two groups have in common.
37.62.1 The Iw&g]
3-
ion and related compounds
The [W2X9J3-(X = Cl, Br) ions were first prepared years ago b electrochemical or chemical reduction of tungstates in concentrated HC1 or HBr solutions.’ More recently, it has been shown that [W2X9l3- is readily prepared by passage of HX gas through a solution containing CsX and [W2(mhp)4](mhp = anion of 2-hydroxy-6-methylpridine). It is interesting that when CsX is excluded, compound (IO)is formed.”” The bromide derivative can also be prepared by halogen exchange (e uation 15). Reaction (15) reaches equilibrium after approximately three hours and [W2ClBr8#- is the major tungsten-containing specie^."^ A continuous flow of HBr throughout the course of the reaction leads to complete exchange within 24 hours. Of the several possible mechanisms for this halogen exchange, the most likely seems to involve an intermediate with a W(p-C1)2W bridge, and four terminal chlorides bonded to each metal atom. This is a viable intermediate in view of the structure of [W2C16py4](Figure 28).
2
[W,Cl,]’-
+ 9Br-
[W2Br$-
-t9C1-
(15)
[W2C19]’- has the confacial bioctahedral structure with a W-W bond length of 2.418 A. (This structure was first solved in 1958272and then re-refined in 1983).273This experimental result coupled with the electronic configuration of the metals is consistent with a W-W bond order of three. [W2Cl9I3-can be oxidized with oxidants such as C12, Br2 and 12.274The violet mixed valence complex [W2C19l2- is paramagnetic with peff= 1.87 BM per formula unit. The dianion can also be prepared in low yield by the reaction of [W(CO),Cl]- with WC16 in methylene ~hloride,”~ or in 40-50% yield by the reduction of WC4 with Na/Hg in THF273in the presence of [(Ph3PNPPh3)C1].Structurally [W2Cl9I2-is very similar to [W2Cl9I3-with a W-W bond length of 2.5408+ in agreement with a bond order of 2.5. The increase of 0.122A in the W-W distance in going from [W2C19]3Lto [W2Cl9I2-is believed to result not only from the decrease in the bond order, but alsb because of the increase in the effective charge of the metal atoms.’73 The electronic absorption spectra of [W2X9]”- (X= C1, Br; n = 2, 3) have been interpreted recently.276[W2Br9]’- is also known, and is obtained in reasonable yield according to equation (16). Its structure is similar to that of [W2Cl9I2-with a W-W bond length of 2.601 A and a magnetic moment of 1.72 BM per formula unit.277 4[W(CO),Br]-
+ 7C,H4Brz
-
2W2Br9]’-
+ 7 G H 4 + 2C02
(16)
The reaction of [ M o ~ ( O ~ C Mwith ~ ) ~a] warm concentrated solution of HC1 or HBr affords the [MoZX8Hl3-anions. These anions contain the M~(p-€€)(p-Cl)~Mo bridging and possess structures similar to that of [W2Cl9I3-.This type of oxidative addition is also possible with the quadruply bonded dimer [ M o W ( O ~ C C M ~ ~This ) ~ ] .molecule when treated in the same way279 affords [MoWCI,H]~- with a Mo-W distance of 2.495 A. [W2Cl9I3- reacts with heterocyclic organic basesz8* to give the diamagnetic compounds
CI
N
CI
”
Figure 28 The structure of [W,C&y4]; pyridine rings are not shown”’
Tungsten
lo00
Figure 29 The structure of [Cl,(p-CI)(p-SPh)zWC13]z-; phenyl carbon atoms are omittedu'
[WzC&] (L = py, pic and 4-isopropylpyridine). Structure andysis of [W2Ckpy4] has establishedzs1the presence of a planar W (P -C ~)~W bridge with a W-W distance of 2.7378, (d3-d3) (Figure 28). This distance is considerably larger than that expected for a W"'-W"' dimer of this type with a triple bond. In fact, this distance is consistent with a W-W single bond. It can be argued that the situation here is similar to the one described for the d4-d4 [RezCllo]4-ion.Z82The complexes [W2Ck(PMe3)4]and [W,Cl,(THF),] which are prepared by Na/Hg reduction of WC14 in THF with or without PMe3,283may possess structures which are related to that of [wZcl6py4\ and (21).231 Isoelectronic with [W2C19J- are the paramagnetic thiolato-bridged dimers They possess the confacial biottahedral structure (Figure 29) with W-W distances of 2.505 and 2.519 8, respectively. These distances are not significantly different than those found for compounds (11)(see Section 37.5.4.1) which contain a W-W bond order of two. This result lent strong support to the suggestion that the difference in bond length of 0.122 8, observed in going from [W2Cl9I3-to [W2Cl9I2-is not caused only by the decrease in the bond order.zn The reaction of [WCL(MeS)2] with EtzSiH gives compound (22) from which the anion (23) can be prepared.m5The structure of (23) possesses the W ( P - - H ) ( ~ - S M ~ )bridge ~ W with a W-W distance of 2.410 A, consistent with a W-W bond of order three. [(Me,S)CI,W (p-SEt)3WCl2(SMeZ)] (20)
37.6.2.2 Complexes of the cype W a 6 The metal-metal triple bond is formed by overlap of the metals' d,z orbitals to give a u bond, and the d,, and dyz orbitals to give the x bonds. In a d3-d3 situation, the six metal electrons are accommodated in the three bonding molecular orbitals resulting in the u2n4 electronic configuration. In the absence of the 6 component, the triple bond allows free rotation about the M-M axis, and thus the adopted staggered configuration is determined by the steric requirement of the ligands. The first W"' complex of the W2& type286was prepared by the reaction of WC16 with LiCH2SiMe3.The orange-brown organometallic compound [w2(CH2siMe&] can be sublimed in vacuum at 120°C. It is stable in air for short periods, but oxidizes very rapidly in solution. The proton NMR spectra indicate that the six Me3SiCH2 groups are equivalent. [W2(CHzSiMe3)&crystallkes with two independent but virtually identical molecules in the asymmetric unit with a mean W-W bond length of 2.255A. The stereochemistry in the solid is staggered as expected (Figure 30). The compound [W2(NM&] was first obtained as a minor product from the reaction of WC16 with LiNMe2.88 Attempts to prepare this material by reacting LiNMez with lower valent starting materials such as K3[W2C19]or W6CIl2were not s u c c e ~ s f u lIt . ~is~ now known that the best preparative route to the compounds [W2&] (X = W e 2 , NMeEt, E t 2 ) is by reacting X with a form of WCL, which is prepared by the reaction of W(CO), with two equivalents of
Tungscen
1001
W
Figure 30 The structure of [W,(CH,SiMe3).J; methyl groups are omitted='
WC16 in refluxing chlorobenzene.284As can be seen from Figure 31, [W2(NMe&] has the expected staggered, ethane-like stereochemistry with a W-W bond length of 2.294 A. In addition, the methyl groups are not equivalent. One set of methyl groups lie approximately over the W-W bond (for example C-1; these are called the proximal methyl groups), while the other set, called the distal methyl groups (for example C-2), lie away from the W-W bond. Compounds of the type [W2(0R),] are also known,290but they are much less stable than their molybdenum analogues. For example, [MO~(OBU')~] can easily be purified by vacuum sublimation at 100 "C, while the corresponding tungsten compound is thermally unstable and is very sensitive to both oxygen and moisture. In addition to [W~(OBU~)~], [Wz(osiMe3)6(N~e&] and [W2(oP~)6py2]2whave been prepared by the reaction of [W2(NMe2)6]with Me3SiOH in hydrocarbon or with Pr'OH in pyridine. These compounds are also thermally unstable. The reaction of [Wz(NMeZ),] with MeOH or EtOH gives the tetranuclear cluster compound [W4$0R)16]223 while the reaction with PfOH in the absence of pyridine gives [W4(p-H)2(0Pr')14]22(see Section 37.5.4.1). Compounds of the type [W2C12(NR2)4\(R = Me, Et) are known, and are best prepared by the reaction depicted in equation (17). 91 The lability of the W - C l bond has led to the preparation of [WX2(NEt2)4](X= Br, I),292 and a series of dialkyl derivatives [W2R2(NR;)4] (R = Me, Et, Bun, Pf, CH2CMe3,CMe3, CH2SiMe3;R' = Me, Et).293The dialkyi derivatives are red-orange, thermally stable, and in most cases can be sublimed in vacuum around 100 "C. The structures of these molecules are very similar; they have the anti rotational conformation with bond length close to 2.3 A (Table 7). [W,(NR,),]
+ 2Me3SiC1
-
[W,Cl,(NR,)4]
+ 2Me3SiNR2
(17)
The reaction of U ~ ' - [ W ~ C ~ ~ ( with N E ~two ~ )equivalents ~] of LiCH2SiMe3is stereospecifi$94 giving exclusively ~ ~ L ~ - [ W ~ ( C H ~ S ~ M which ~~) then ~ (slowly N E ~isomerizes ~ ) ~ ] to an equilibrium mixture of anti and gauche rotamers. The stereospecificity clearly requires that the W=W bond remains intact during the reaction, and that the Me3SiCH2ligand replaces C1 with retention of configuration at the metal atom. NMR studies of this reaction suggest that there is a trans labilizing effect which is transmitted through the triple bond. [W2Me2(NEt2)4]295 was also
Figure 31 The structure of [W,(NMe,),]lZ
Tungsten
1002
Table 7 Triple Bond Distances Between W Atoms
W-W(A)
Compound
Ret
W-W(A)
Compound
Ref.
shown to exist in solution in an approximately 3:2 equilibrium mixture of anti and auche rotamers. The gauche rotamer predominates as the bulk of the R group increases.' !f It is interesting that alcoholysis of the [W2Rz(NR2)4 compounds with bulky alcohols leads to [W2R2(OR)4]with retention of the W-R bonds.' Insertion of C 0 2 into M-NR2 and M-OR bonds occurs quite readily in certain cases, leading to dialkylcarbamato and alkylcarbonato groups respectively. Thus, [Wz(NMe&] reacts completely with excess COZ to give [W2(02CNMe&], while the reaction of [W2Me2(NEt&] with C 0 2 leads to [W2Me2(O2CNEt2),].The structures of these molecules (Figures 32 and 33)297 are quite similar in that both have two bridging and two chelating carbamato ligands similarly disposed around the metal atoms, and in the one case, the methyl groups are trans to the chelating ligand while in the other, an oxygen of a unidentate carbamato group is trans to the chelating ligand. In both, the W-W triple bond is retained (Table 7). These compounds are fluxional in solution and below -60 "C,the NMR spectrum of [W2(02CNEt&,] is consistent with the solid state structure. The NMR spectra of [W2Me2(02CNEt2)4]has not been completely interpreted.289 were Until recently, the only two examples of complexes of the type &WE= In these complexes, the coordination [Wz(OPri)6py2]2wand [W2(NMe2)4(PhNNNPh)2.zg8 number of each metal atom is four, and the W-W bond lengths are only slightly longer than those found for the W2& compounds. It is now known that when [W2(NMe2)2]is allowed to react with EtOH in the presence of Me(H)NCH2CH2N(H)Me, the adduct (24) is formed. In this structure (Figure 34) the W-W triple bond is retained (2.296 A), and the two ends of the molecule are almost perfectly staggered. [W2(NMe2)6] was also shown to react with alcohols in the presence of PMe3 to give the phosphine adducts [W2(OR)6(PMe&] (R = Pr', CH2But and Et). The structure of [W2(0CH2But)6(PMe3)2] reveals two square planar W03P units joined by a W-W triple bond of distance 2.342 A.
J
[wz(oEt),(Me(H)N~H,N(H)Me)] (W
1, 2 - [ W z { S i ( S ~ ~ ) ~ } ~ ( N M e , ) , l (25)
As mentioned previously, [WC12(NR2)4] reacts with LiR to give the 1,2-dialkyl-substituted derivatives. Recently, this reaction has been extended to other anionic ligands.300Thus, the reaction of [WzC12(NMe2)4]with two equivalents of [(Me3Si)Si]- or [CpFe(CO)z]- results in the
0 0
Pigure 32 The structure of [W2(02CNMe&]2w
Tungsten
1003
C
0
C
Fire 33 The structure of [W2Me2(02CNEt&4]zw
formation of compounds (25) and (26) respectively. These are the first examples of a W-W triple bond supporting a metal-metal single bond between two different metal atoms. Of considerable interest are reactions in which the metal-metal bond order changes. So far, there are no known examples in which a W-W triple bond is converted to a W-W quadruple bond, a reaction which is known for molybdenum.m However, several reactions in which the W-W triple bond is oxidized are known. The preparation of alkoxo clusters of WIV have been mentioned (Section 37.5.4.1)and the reaction in which WGW reacts with R W R or R e N to give mononuclear complexes containing the W d R and WEN units have been discussed (Section 37.3.3). Recently, several new and important reactions, in which the dinuclear unit remains intact, have been reported. The reaction [W2(0B~t)6] in the presence of pyridine or [w2(oPr')6pyZj with ethyne gives (27).301 The structure of the isopropoxide derivative contains the W(pOPri)2(p-C2H2)W bridge with a W-W distance of 2.567A. In solution, this compound is fluxional showing rapid terminal bridge OR exchange. At low temperature, the NMR spectra is consistent with the solid state structure. The addition of CzMez to [Wz(OCH~But)~pyZ] in hexane yields the blue compound (28). In this structure there is only one bridging alkoxy group
(W-W = 2.602 A), and the C2Me2unit is closer to one of the W atoms (Figure 35). It has also been shown that changing the alkoxy groups and/or the alkyl substituents greatIy influences the course of this reaction. Thus, reacting [W2(OPTi)6py2]with an excess of C2Me2 leads to compound (29) in which two alkynes are coupled and a third is present as a terminal ligand. In
C
Figrue 34 The structure of
[W,(OEt),(Me(H)NC,H.,N(H)Me)]2w
1004
Tungsten
N
Figure 35 The structure of [W2(02CH2Bu'),py(p-C2Me,)]; the tertiary butyl groups and py ring are omitted3"
this molecule (W-W distance = 2.85 A), as with the others, W-W interaction is retained. When compound (28) is reacted with MeCN, compound (30) is obtained.3o2aIts structure (Figure 36) reveals the presence of the deprotonated bridging ligand (p-N(CMe)&T) whid results from coupling of the (p-C2Me2) moiety with two equivaIents of MeCN. The W--U bond length of 2.617A is consistent with a bond of order one. It has also been shown thai [W,(OR)6] reacts with a-diketones to give compounds in which the reduced organic moiety i! bonded to a dinuclear complex with a W-W bond of order one.302b The reaction of [W2(OBut)6] with NO in pyridine leads to cleavage of the W-W triplt and the formation of [W(OBu'),(NO)py]. In this structure, the geometry around thr metal atom is trigonal bipyramidal, with the py ligand tram to NO. The reaction of [W,(OR),] (R = Pr', CH2CMe3) with CO in the presence of excess pyridint leads to [Wz(OR)6py2(p-CO)].304The structure of the kopropoxide derivative is that of i confacial bioctahedron, with the CO ligand and a pair of OPr' groups making up the bridgini face. The W-W distance of 2.4998, is consistent with a bond of order two. However, wher [W2(OPt)6py2] is reacted with CO without additional pyridine,305 two different products art obtained depending on the amount of CO added. With four equivalents of CO compound (1)i: formed (Section 37.3. l ) , while with two equivalents of CO the tetranuclear compount [W2(OPrf)6py(C0)]2 is obtained. The structure of this molecule consists of twt [W2(OPr')6py(CO)] units which are bridged through the oxygen atoms of the CO group (Figun 37), an unprecedented bonding arrangement for the CO ligand. The W-W bond length o 2.654A is significantly longer than that found for [W2(OPt)@yZ(CO)] and falls within thd range typical of single rather than double W-W bonds.
W Figure 36 The structure of [W,(O,CH,Bu'),py(p-N(CMe),N)]; the tertiary butyl groups and py ring are omitted3u2n
1005
Tungsten f7
/--.
W
Figore 37 The structure of [W,(OPr'),py(CO)],;
propyl groups are omitted3''
37.7 THE CHEMISTRY OF Wn Until several years ago, most of the known compounds of W" were monomeric complexes, which exhibit mainly coordination number seven, and cluster compounds containing the octahedral W6 nucleus with W-W single bonds. The most important advance in the chemistry of W" has undoubtedly been the preparation and characterization of dinuclear compounds containing W-W quadruple bonds, 37.7.1 Monomeric Complexes There are two general procedures for the preparation of monomeric W" complexes.m In one, substituted tungsten carbonyls are oxidized by halogens under controlled conditions as exemplified by equations (18) and (19). In the second, W(CO)6 is first oxidized by Clz or Br, at low temperature, followed by reaction of the oxidized product with the appropriate ligands. This preparative procedure is exemplified by equation (20). IW(CO),@iPY)l+ Br2 [W(CO)2(diars)2] Br,
+
[W(~O),Cl,] + 2PPh,
--
-
[W(CO),(bipY)Br,l+ co [W(CO),(diars),Br]Br [W(CO),(PPh,),Cl,] + CO
There are numerous examples of this type of complex. The majority are seven-coordinate; some representatives are given in Table 8. With bidentate and tridentate ligands, when the Table 8 Properties of Some Monomeric W" Complexes compound
Carbonyl stretches (cm-')
2080,2041,2000, 1925 2110,2025, 1980 2015, 1940, 1900 1960, 1875 1853 2030, 1942,1905 2037, 1959, 1908 1930, 1835 1960 1916, 1824 2039, 1963, 1909 2036,1972, 1247 (CS)
Visible spectra (nm) ( E or color) Yellow Yellow Yellow Blue Red Yellow Yellow 685 (223); 490 (1060) 689 (90); 360 (3440) 506 (38); 419sh 405 (1960) 417 (1814) Orange -
Ref
269 309 309 309 307 269 269 59 269 269 316 316 269 322
Tungsten
1006
stoichiometry suggests a coordination number higher than seven, it is invariably found that not all the donor atoms are coordinated to the metal. The halocarbonyl anions [W(CO)Z3]- (X = Cl, Br, I) are prepared by the addition of the desired halide to [W(C0),X2] or by oxidation of [W(CO)&]-. Mixed halogen derivatives are prepared ~ i m i l a r l yFor . ~ ~example, oxidation of [W(CO)51]- with Br2 leads to [W(C0)41Br2]-. Oxidation of ~is-[W(CO),(diphos)~] with I2 leads to the unexpected one electron oxidation product [W(CO)2(diphos)2] + . This paramagnetic complex exhibits only one carbonyl stretching Reaction of [W(C0)4diars] with excess frequency, indicating a trans stereochemi~try.~~' bromine also results in the seven-coordinate complex [W(C0)3(diars)Br2]+,which appears to be a WII' derivative.308It is unusual that three carbonyls are retained by the metal in this oxidation state. Refluxing of [W(C0)3(PPh3)2X2](X= C1, Br) in CH2CI2 gives the deep blue crystalline derivatives [W(CO)2(PPh3)2X2] .m The structure of this six-coordinate complex is probably ) ~ B ~ ~ ] as having an unusual nonoctahedraI similar to that of [ M O ( C O ) ~ ( P P ~ ~ described six-coordinate geometry.310The complex reacts reversibly with CO as depicted in equation (21). [W(C0)2(P~h3)2X21+C O
e
[W(C0)3(PPh&X21
(21)
A series of seven-coordinate complexes of the type [W(CO)2(PPh3)(L-L)2] (L-L = S2CNR2, S2COMe, S2P(OEt),) have been prepared by the reaction of [W(CO)(PPh&ClZ] with excess ligand.m With excess HS2P(E'r')2 or with an equimolar amount of HS,P(OEt),, only one Cl is displaced and [W(CO),(PPh3)(L-L)Cl] is obtained. The visible absorption spectra of these complexes are solvent dependent. This, together with conductance measurements, strongly suggests the presence of the cationic species [W(CO)2(PPh,)(L-L)]+ in polar solvents such as methanol. [W(CO)3(PPh3)\S2CNEt2)]has been shown to react with acetylene to afford [W(CO)(C2H2)(S2CNEt2)2]. Crystal structure analysis reveals a distorted pentagonal bipyramidal geometry around the metal with sulfur atoms S-2 and S-3 (Figure 38) occupying the apices of the pyramid. The CO and C-C bonds are nearly coplanar, enabling the acetylene ligand to act as a four-electron donor. More recently it has been shown that [W(CO)3(S2CNR2)2] also reacts with alkynes to give [W(CO)(R1-R2)(S2CNR2)2] where R1= R2 = H, Me, Et, Ph and R1= H, R2= Ph.312 The seven-coordinate complexes [W(CO)3(YPPh3)X2](Y = P, As, Sb; X = Cl, Br) can undergo a two-electron reduction in CO atmosphere as depicted in equation (22). No intermediates are observed in this reaction.313 [W(CO),(YPh3,),X,]
+ 2CO + 2e-
-
[W(CO)&]-
+2 W h 3 + X-
(22)
Several examples of W" complexes with no carbonyl ligands are also known. The reaction of W13 with excess diars at 160°C gives the paramagnetic complex [W(diar~)~I,]with peff= 2.70 BM.314 A related complex WC12(PMe2Ph)4]can be prepared by Na/Hg reduction of [WC14(PMe2Ph)2]under argon.31 This paramagnetic complex (peE= 2.30 BM) exhibits only one v ( W 4 1 ) band at 280 cm-' suggesting the trans configuration. A general procedure has been outlined for the preparation of isocyanide complexes of Wn (equations 23, 24).316[W(CNR),I2+ (R = CMe3, C6Hll) can also be prepared by reaction of the
5
C
Figure 38 The structure of [W(CO)(~Hz)(SzCNEtz)J3"
Tungsten
1007
Table 9 Structures of Seven-coordinate W" Complexes
Compound ( E t 8 1[W(COhBr3] IW(CO)ddPam),Br21 [W(CO)3(PMe,Ph),I]BPh4 ~w(c0)3(d~Pe)I,I PVO)3(difas)r,l [W(CO),(dPamP,l [W(CO),(bipy)(GeBr,)Br] [W(CO),(diars)qI [W(CO)3Idth)(SnC13> [W(COMdml3e),III [W(CO)(C&)(S&NEt,),] [W(COMBu"C),I,I [W(CO)3(S2CNMe2)21
Suggested structure
co co CO/m co co co CO/CTP CTP
co
Copping group
co co I co co co Br I Sn I
CTP Distorted PBP 4: 3 piano stool 4: 3 piano stool
-
Ref. 269 269 269 269 269 269 269 269 269 269 311 322 323
RNC ligand with [Wz(mhy)4].318 NMR measurements show that the isocyanide complexes are fluxional down to -30 "C. l6 W(CO), -tX, % [W(CNR)a]X (X = C1, Br)
(23)
The structures of several of the seven-coordinate complexes of W" have been determined and these are listed in Table 9. There are three recognized geometries for coordination number seven,319*320 the pentagonal bipyramid (PBP), capped octahedron (CO) and capped trigonal prism (CTP). The 4 :3 piano stool arrangement, which consists of a plane of four donor atoms parallel to a plane of three donor atoms, is also recognized.321Two extreme cases for this geometry, both with C, symmetry, are considered, and these are related to one another by a rotation of the triangular face. It has been suggested however, that the 4:3 geometry is not sufficiently different from the CO and CTP to warrant its inclusion as a separate geometry. This point was recently questioned, and it is argued that when the dihedral angle between the two planes defining the 4:3 arrangement is close to 0", the 4:3 geometry is a better representation of the molecular ge~rnetry.~" A case in point is the structure of [W(C0)2(CNBuf)312](Figures 39 and 40). The 4:3 geometry is also considered as the best description for the structure of [W(CO)3(S2CNMe&].323This molecule was also shown to be fluxional, and dynamic 13CNMR measurements suggest two distinct intramolecular rearrangement processes.
Ro
Figure 39 The structure of [W(CO),(CNBU~),IJ; tbe tertiary butyl groups are omitteda2
1008
Tungsten
Figure 40 The two planes which define the 4: 3 geometry in fW(CO)2(CNB~f)312]322
The structures of [W(CO),Cl(dcq)(PPh,)] and [W(CO)2(dcq)z(PPh3)] (dcq = 5,7-dichloro-8quinolinato) have recently been determined.324 These are best described as having the CTP geometry. The positions of all the donor atoms in the coordination sphere of these complexes are in agreement with electronic argument^.^^ The first example of a PBP geometry in which a bidentate ligand s ans one of the pentagonal P~Z)]. edges is furnished by the structure of [ W ( C O ) ~ I ~ ( P ~ ~ P C H , P32%
37.7.2 Complexes with QnadrupIe Bonds The existence of quadruple bonds was first recognized in 1964289and since then a great many compounds of rhenium, technetium, chromium and molybdenum have been shown to contain them. Both bridged and unbridged compounds are known with a large variety of ligands. The bonding has been described in detail.28g The first W-W quadruply bonded complexes characterized were [W2MenC18-n]4-prepared by treatment of WC14 with methyllithiurn. Crystal structure analysis has established the short W-W distances of 2.264 and 2.263 8, for the methyl and the mixed chloromethyl derivatives respectively. In addition, these compounds have the expected eclipsed s t e r e o ~ h e m i s t r y . ~ ~ ' , ~ ~ The reaction of WCL with K2c& in THF gives a reasonably air-stable material that was shown to be [W2(CgH&]. The W-W bond length of 2.375 A, although long, is consistent with the presence of a quadruple bond.329However, it is recognized that this compound (and the related chromium and molybdenum) is electronically different from other quadruply bonded species. Treatment of MO(CO)~with 6-methylpyridone (Hmhp) and 2-amino-6-methylpyridine (Hmap) in refluxing diglyme has led to the isolation of [ M ~ ~ ( m a pand ) ~ ] [ M ~ ~ ( m h pwhich )~] contain very short M M C F -o distances.289 mh )4 was also prepared by the same way. It reacts with Li(map) in THF to give [W2(map)4].33 [w( *33 Both compounds have been structurally characterized and have been shown to contain short W-W distances of 2.164 and 2.161 8,for the map and mhp compounds, respectively. These are the shortest known internuclear distances between any two atoms in the third transition series. [W2(mhp)4]was the first and remains the only quadruply bonded tungsten complex to be obtained by oxidation of W(CO)6. The structure of [ W * ( m a ~ )is~ ]shown in Figure 41. The arrangement of the ligands around the W, unit is such that the idealized symmetry of the complex is &.
81
Figure 4 1 The structure of [W,(map),$'
Tungsten
1009
A special case in this family of complexes is represented by the tetracarboxylates of the form [M2(02CR)4]. Following the successful preparation of [ M O ~ ( O ~ C Mfrom ~ ) ~ ]MO(CO)~in refluxing acetic acid-acetic anhydride mixture ,289 several attempts to prepare the analogous tungsten derivative by the same procedure have failed. This reaction leads invariably to the trinuclear clusters of type (E)(see Section 37.5.4.2). However, the continuous efforts to prepare complexes of the type [W2(02CR)4]have recently been rewarded by the synthesis of [W2(TFA)4]. This was achieved by Na/Hg reduction of [W2a6(THF)4]in THF followed b addition of NaTFA, or by Na/Hg reduction of [WC4L in the presence of NaTFA at 0 0C?32,33y [W2(TFA)I4 easily adds two ligands along the W-W bond and the structure of both [W,(TFA),.gdigly] and [W2(TFA)4.2PPh3]have been determined. The compounds have the expected geometry with short W-W bond lengths of 2.209 and 2.242A respectively. The increase of about 0.03 8, in the W-W bond length most likely indicates a stronger M-L u bonding and a weaker M-M bonding in the phosphine adduct. On the basis of the Raman spectra of [Wz(TFA)4], [Wz(TFA)4-2PPh3]and [M02(TFA)~-2pylit is predicted that the W-W bond length in pure [W2(TFA)4]should be in the range of 2.20-2.21 A,333some 0.1 A longer than that found in [Mo,(TFA)~], indicating a weaker M-M bonding in the tungsten dimers. This is in agreement with the chemical behavior of these two compounds. In contrast to the unsuccessful early attempts to produce [W2(02CR)4],the heteronuclear compound [ M O W ( O ~ C C M ~was ~ ) ~ ]obtained by reacting a 3: 1 mixture of W(CO)6 and Mo(CO)~ in refluxing d i c h l ~ r o b e n z e n e .The ~ ~ ~ heteronuclear complex was freed from [MO~(O~CCM~~ by) ~ ] careful oxidation with Iz. Structure analysis of [MOW(O~CCM~~)~]I,M shows ~ C Nthe expected idealized Da symmetry with a short W-Mo separation of 2.194 A. The iodide ion is coordinated to the W atom and the MeCN molecule is coordinated to the molybdenum atom. [MoW(O2CCMe3)J is reduced with zinc dust to [MoW(02W(02CCMe3),]whose structure reveals a W-Mo distance of 2.080 The longer distance observed for the iodo derivative is consistent with the fact that in [ M o W ( O ~ C C M ~ ~ )the ~]+ bond , order is formally 3.5. [MoW(mhp),] is also known. It is prepared by the reaction of a 1.5: 1 mole ratio of W(CO)6 and Mo(CO)~with Hmhp in refluxing diglyme. [MoW(mhp),] is purified from the other products by I2 oxidation followed by reduction with zinc amalgam.336The potentials (us. SCE) of the one electron reversibie oxidation of [ M ~ ~ ( m h p )[M~W(rnhp)~] ~], and [W2(mhp)4]of 0.2, -0.16 and -0.35V respectively, indicates that in a mixture of these, the heteronuclear compound can be selectively oxidized and thus purified. The Mo-W distance of 2.091 A336is shorter than the average M-M bond lengths of 2.065 and 2.161 found in [Moz(mhp)4] and [W2(mhp)4] re~pectively,~~' suggesting a stronger metal-metal bonding in the heteronuclear species. Raman spectra and force constant calculations are in agreement with the crystallographic results. Also in agreement with this conclusion are the results of photoelectron spectra, which have been measured for a series of M-M quadruply bonded complexes.337 Attempts to prepare the octahalo anion [W2Chl2- were not successful until 1980, when a general procedure for the preparation of mixed halo-phosphine dimers was reported."' In this procedure WC14 is reduced with Na/Hg in the presence of a phosphine ligand, as depicted in equation (25). With PMe3, if only one equivalent of reductant is used, the red crystalline [W2C16(PMe3)4]is obtained. Treatment of this compound with an additional equivalent of reducing agent leads to [W2C14(PMe3)4].339 With the bidentate phosphines, dmpe and diphos, reaction (25) fails to give the desired products [WzC14(dmpe)z]and [W2C14(diphos)2]I However, these can be obtained by treating [W2C14(PBu")4]with the bidentate ligands in toluene at 80 "C. With diphos, a green a isomer and a brown p isomer are formed in an analogous way to molybdenum. [W2C14(dmpe)2] and the green isomer [WzCL(diphos)2] possess a centrosymmetric eclipsed structure as depicted in Figure 42. The p isomer is structurally similar to [ M ~ ~ C b ( d i p h o s and ) ~ ] ~contains ~ intramolecular diphos bridges. The tungsten compounds adopt a staggered conformation and thus can be considered to have a bond order of three.340In contrast, the molybdenum analog has a torsional angle of 30". This difference undoubtedly reflects a weaker 6 component in the W-W quadruple bond and therefore steric requirements are able to enforce a fully staggered conformation. WCL,
+ ZNa/Hg + 2L
$[WzCl,L,] L = PMe,, PM%Ph, PMePhz, PBug
(25)
When the reduction of [W2C16(THF)4]is carried out at low temperature without the phosphine ligand, [W2Cl8I4- is formed.339Its structure reveals the expected eclipsed stereochemistry with a W-W bond length of 2.259 A.341
Tungsten
1010
Fignre 42 The structure of [ W , C i , ( d ~ n p e ) ~ ] ~ ~
The complexes [W2C1&4] undergo a reversible one-electron oxidation. A comparison of the oxidation potential of analogous molybdenum and tungsten compounds clearly shows that the tungsten compounds are considerably easier to oxidize. This ease of oxidation is believed to be responsible for the fact that it is difficult, if not impossible, to obtain quadruply bonded tungsten compounds by synthetic procedures which are known for molybdenum.339 Chemical oxidation was also noted for [W2C14(PBu;)4]. Treatment of [WzC12(PBu:)4] with [Ag(MeCN)4]+ leads to the paramagnetic [W2C14(PB~;)4]+.Reaction of the butyl phosphine derivatives with acetic acid at 160 "C in diglyme affords the trinuclear cluster (W) (see Section 37.5.4.2), and with benzoic acid to give compound (31) having a W-W triple bond with a W-W distance of 2.423 A.'" [W,(P-H)(~-C~)(PB~,)~C~~(O~CP~),I (31)
The ease of oxidation of the quadrupl bonded tungsten dimers is also exemplified by the reaction of [Wz(mhp)4]343or [wdTFA)4{33 with HCl to give [W2Cl9I3-, which is also readily obtained from [WzC18]4- even at -78°C. It is believed that these reactions proceed vin the hydride intermediate [W2Cl8HI4-. Recently, it has been shown that the tetranuclear compound [W4C18(PB~;)4]344 can be prepared by Na/Hg reduction of [w2c16(THF)2(PBU:)2]. The structure of this molecule is similar to that of [ M O ~ C ~ ~ ( P E with ~ ~W-W ) ~ ] ' ~distances ~ of 2.309 and 2.840 8,along the edges of the rectangular cluster. The structures of several of the quadruply bonded tungsten dimers have been determined (Table 10) and in all cases the W-W bond length is significantly longer than those found in analogous molybdenum compounds. The crystallographic results coupled with theoretical,Ja s p e c t r o s ~ o p i c ~and ~ ~ e~l~e ~ c t' r o ~ h e m i c a studies l ~ ~ ~ clearly indicate that the W-W quadruple bond is indeed weaker than the M e M o quadruple bond as was intuitively suggested originally by consideration of the core size of these two elements.
Table 10 Quadruple Bond Distances Between W Atoms Compound
Li4[WzMe8]*4Et,0 Li4[W,Me,-,CI,].4THF ~W~(CF&)~] [Wz(mhP)4l*CH2C1, IWZ(ChP),l [Wzbap)41*THE: [W2(dmhp)z(PhzNNNPh,),1
w-w(A) 2.264(1) 2.263(2) 2.375(1) 2.161(1)
2.177(1) 2.164(1) 2.169(1)
Ref. 327 327 329 330 289 331 289
W-W(A)
Ref.
2.262(1) 2.287(1) 2.280(1)
340
Na,(TMEDA),[W,Cl,] 2.259(2) [W,(TFA),].$di&hgm 2.209 [ W Z ~ ~ A ~ ] * ~ P P ~2.242 ~ tW2(02CC6H3)4.2~ 2.196
341 333 333 357
Compound W,Ch(PMe,), [W,Cl,(dmpe)&toluene [W,C~(dppe),].$H,O
340 340
Tungsten 37.7,3
1011
The Hexmuclear Clusters [W&I4+
The dihalides of Wn, WC12, WBr2 and W12 are in reality the hexanuclear compounds W2X12 which contain the [WgXSl4+cluster (Figure 43). The metal atoms occupy the corners of an octahedron with a bridging halogen atom above each triangular face. The entire unit has full 0, symmetry and it is connected to other units by halogen atoms (four per unit). There are two non-bridging halogen atoms per cluster.
Figure 43 The [W,J,]*'
core
W6Cl12and W6BrI2 are best prepared by disproportionation of the tetrachloride at about 500 "C or by reduction of WC15 and WBr5 with aluminum in a temperature gradient.lS4W6112is obtained by fusion of W6ClI2with a large excess of a KI-LiI mixture at 550°C. As might be expected from the structures, the external halogens are readily replaceable by other halogens. Thus, W6C18]Br4, [WhC18]14, [W6Br8]C14and [W,Brs]F4 have been The [W&f+ clusters also readily form complex anions of the type [W6Xl4I2- (X = C1, Br, I) and [w,&8]Yz- (x= Y = cl, Br, I) .34g These clusters are fairly good reducing agents in aqueous solutions and are oxidized even by water.350They can also be oxidized by halogens at elevated temperatures. With W a r l 2 the products are W6Br14,W a r l 6 and W6Brls.351,352 In all these products, the W a r 8 unit is oxidized by two electrons. Thus, W a r l 4 is best formulated as [W&rs]Br6 while the others contain bridging B$I ions. The reaction of C12 with W6CIl2 at 100°C leads to a product of stoichiometry WC13 which has been shown to contain the [W6C112]6+ isostructural with the Nb and Ta clusters. clusters are diamagnetic and with the 24 available metal electrons there is a The metal-metal bond of order one between each pair of tungsten atoms, in agreement with theoretical treatment of the electronic structure of these cluster^.^^^^^^^ Recently, it has been shown that [W6CIl4j2- is luminescent and that it undergoes a facile electrochemical oxidation at 1.14V us. SCE. The resulting monoanion is a powerful oxidizing agent .356 37.8
COMPLEXES OF DINKROGEN
Dinitrogen complexes of tungsten have been studied in great detail in relation to the function of nitrogenase. They show considerable versatility both with respect to reduction of the coordinated N2 and in reactions leading to carbon-nitrogen bond formation. Dinitrogen complexes of tungsten were first reported in 1970.358 Na/Hg reduction of [WC12(PMe2Ph)4]and [WC12(diphos)z]in the presence of excess phosphine under dinitrogen and C ~ ~ - [ W ( N ~ ) ~ ( P M ~The , P ~ )IR , ] . spectra of led to the isolation of t~uns-[W(N~)~(diphos)~] the trans compound show one band at 1953cm-l assigned to v(N2). The cis complex exhibits two such bands at 1931 and 1998 cm-l. Magnesium reduction of tr~ns-[WC4(PMePh~)~] under N2 in the presence of excess ligand leads to tran~-[W(N~),(PMePh,)~] .359 The complex reacts with nitrogen donor ligands to give [W(N2)2(PMePh2)3L](L = py, 3-Mepy, 4-Mepy and 4-(C0,H)py). It can also be oxidized to the monocation with FeC13. Reaction of trans[W(N2)2(PMePh2).+] with diphos gives the mixed phosphine complex [W(N,),(PMePh2)2(diphos)]. The py-substituted complex exhibits only one v(N2) band in the IR indicating a trans configuration and a mer arrangement of the phosphine ligands. The reduction of [WC14(PMe3)3]with dispersed sodium leads to c ~ ~ - [ W ( N ~ ) ~ ( P which M ~ ~reacts )~] with excess PMe3 to give [W(N2)(PMe3)5].360 When the reduction is performed under ethylene, ~ ~ U ~ S - [ W ( G H . + ) ~is( Pobtained. M ~ ~ ) ~The ] compound tr~ns-[WCl(N,)(PMe~)~] is also known.361 were prepared for X = NCS, CN Anionic dinitrogen complexes tr~ns-[WX(N~)(diphos)~]coc3-GG
1012
Tungsten
Figure 44 The Structure of ~(W(NJ2(PEt2Ph),},(pN,)1; ethyl groups are
and N3. The effect of ligands L (uncharged) or X- on the redox properties of trans-[W(L or X-)N2(diphos)] as well as the correlation of the redox properties with v(NZ) have been determined. These properties were also correlated with the susceptibility of the complexes towards attack by electrophilic reagents.36z Recently it has been shown that magnesium reduction of trans-[WC14(PEt2Ph)2]under N2 in THF in the presence of excess ligand leads in low yield to complex (32)which contains both terminal and bridging dinitrogen ligands.363Its structure is depicted in Figure 44. It has also been shown that C ~ T - [ W ( N ~ ) ~ ( P M reacts ~ ~ Pwith ~ ) ~A1C13 ] in py to give (33) in which two units of [WClpy(PMe2Ph)3N2]are bridged by two AlC12 moieties through the end nitrogen [{W(N,),(PEt,Ph),},(~L-N31
(32) [WXPY(~Me,Ph)3(~,-N2)12(A1CI,),@ = CA Br) (33)
There are two types of interesting and important reactions involving the coordinated dinitrogen, its reduction to hydrazine and ammonia, and the reactions leading to carbonnitrogen bond formation. 37.8.1 Reduction of Coordinated Dhitrogen
The protonation of coordinated dinitrogen in a tungsten complex was first observed in the reaction of t r ~ n s - [ W ( N ~ ) ~ ( d i p hwith o~)~ HCl ] as depicted in equation (26).365In the resulting complex, one chloride ligand is labile, and in the presence of BPh; IWCl(dipho~)~N~H~]BPh4 is obtained. The N2H2 group can coordinate to the metal as a diazene (W-NH=NH) or as a hydrazido(2-) (W=N-NH2) group. Spectroscopic data suggest that in the parent complex the N2H2 ligand is coordinated as a diazene, whereas in the cationic complex it coordinates as a hydrazido(2-) group. The stabilization of diazene by coordination to a tungsten carbonyl complex has been n ~ t e d . ~ , ~ ~ ’ [W(NJ,(diphos)]
[WCl,(diphos)(N2H2)J-t-
N2
(26)
Crystal structure analysis of [WCl(dipho~)~(N~H~)]BPh4~~’ clearly shows that the NZHZligand linkage is linear with W-N is indeed coordinated as a hydrazido 2-) group. The W-N-N and N-N distances of 1.73 and 1.37 respectively. The short W-N distance clearly suggests a considerable multiple bond character to the W-N bond. Treatment of [W(N2)z(diphos)2] with two equivalents of HC1 gives the hydrido complex [WH(N2)2(diphos)2&HC12) , which in refluxing methanol yields the dihydride of W’” [WIi2C12(diphos)~]. Treatment of [WX2(diphos)2(N2H2)] (X = Cl, Br) with NaBH4 in ethanol, or irradiation of the dinitrogen complex under H2 atmosphere leads to the tetxahydrido complex [WH4(diphos),] .370 Deprotonation of [WXZ(diphos),(N2H2)]and [WF(diphos),(N,Hz)]+ with Et3N or aqueous K2C03 under N2 yields [WX(diph~s),(N,H)].~’~ The resulting diazenido complexes (W=N=N-H) are characterized by a strong IR band at 1890cm-I assigned to v(N-N). This reaction can be reversed by an acid. The diazenido group is formally analogous to NO and thus the complex reacts smoothly with NO to give [WX(NO)(diphos)z] (X= F, C1, Br).371 Ammonia is produced in good yield from dinitrogen complexes containing monodentate
Ii
Tungsten
1013
phosphine ligands.3n Thus, treatment of C ~ S - [ W ( N ~ ) ~ ( P Mor~ ~tran~-[W(N~)~(PMePh2)4] P~)~] with H2S04in methanol gives approximately 1.9 moles of NH3 per gram-atom of tungsten. The fact that hydrazine is a minor froduct in these reactions indicates that ammonia is formed by splitting of the N-N bond." The amount of hydrazine can be increased if, prior to the addition of H2SO4, tr~ns-[W(N~)~(PMePh,)~] is treated with anhydrous HCl to produce the hydrazido(1-) complex [WC13(PMePh2)2(NHNH2)].Thus it appears that hydrazine production depends on the formation of the W-NHNHz m~iet y . ~" The reaction of [WBrZ(PMe2Ph),(NNH2)Jwith one equivalent of HC1 gas in DME affords [WC1Br2(PMe2Ph),(N2H3)] , which on acid treatment produces hydrazine in preference to ammonia. The complex has one labile halide, and its correct formulation as [WClBr(PMe2Ph)3(N2H3)]Brhas been established by crystd structure analysis. The linear W-N-N linkage indicates that the N2H3 group is bonded to the metal as W-N-NH3 rather It has also been shown that c ~ - [ W ( N ~ ) ~ ( P M reacts ~ ~ P ~with )~] than as W-NH-NH2.374 excess HC1 to give hydrazine in moderate yield.375[WBr2(PMe2Ph),(NNH2)] reacts with one equivalent of HC1 to give the hydrido, hydrazido(2-) corn lex (34).It reacts with NaBPh4 to give a novel diazenido complex (35) with an N-B bond.3 2
[WHClBr(PMe,Ph),(NNH2)]Br
[WHC1BrlPM~Ph>,(NN(H)BPh3)1 (35)
(34) 37.8.2
Reactions Leading to Carbon-Nitrogen Bond Formation
The reactions of tm.~-[W(N~)~(diphos)~] with acid chloride in the presence of HCl were the first reported in which a stable dinitrogen complex was converted to an organonitrogen compound.376The products, [WC12(diphos)2(N2HCOR)](R= Me, Et, Ph, p - M e O C a ) , are pink diamagnetic complexes that have no v(N2) stretching bands in their IR spectra. In the presence of a base such as Et3N these compounds lose HCl to give the diamagnetic complexes [WCl(diph~s)~(N,COR)lin which the azo ligand is also coordinated through the carbonyl oxygen. The reaction can be reversed by addition of HCl. [WC12(diphos)2(NzH2)]mand [WX(diph~s)~(N~H)]react in an analogous way with MeCOCX to give [WX,(dipho~)~(N~HC0Me)] (X = C1, Br).371 Visible light irradiation of a benzene solution containing alkyl bromides and tram[W(N,),(diphos)] produces the alkylated complexes [WBr(diph~s)~(N~R)] (R = Me, Et, Bu').377,378 These can be protonated to give complexes containing the (W=N-NHR) unit. with [WBr(di~hos)~(N~HMe)] can also be obtained from the reaction of [WBr(dipho~)~(N~HU MeBr.37 When the irradiation is carried out in THF rather than in benzene3 the w-diazobutanol compound (36) is obtained. Crystal structure analysis is consistent with this hydrazido(2-) formulation. The W-N-N linkage is linear and the W-N and N-N distances are 1.778 and 1.306A respectively. In the PF; salt these distances are 1.772 and 1.32A.380 Several structures of complexes containing the W=N-NHR or W=N-N=CR2 groups have been determined. They are all similar in that the W-N-N linkage is linear with a short W-N distance suggesting a considerable multiple bond character to the W-N bond. It should be noted that although there are small differences in the W-N and N-N bond lengths in these structures, the trend expected is usually that observed, name1 the lengthening of the W-N bond is accompanied by the shortening of the N-N b ~ n d . ~ . ~ ' ~ [WBr(diph0~)2(N-N=CH(
CH2),OH)]Br
(36)
The reactions of [W(N2)2(diphos)2]with RBr are clearly catalyzed by visible light. T h e homolytic fission of the R-Br bond that takes place at the metal cente9'l is preceded by the loss of one N2 molecule. The resulting C-N bond is formed by an alkyl radical attack on the remaining coordinated dinitrogen. Product distribution in these photocatalyzed reactions depends on the solvent and the stability of the free r a d i ~ a l . This ~ ~ ~mechanism ,~ is strongly supported by flash photolysis experiment^.^" When gem-dibromides are used in the photocatalyzed reaction, diazoalkanes are produced. With CHzBrz €or example, the diazomethane complex [WBr(diphos)z(NZCHz)]Br is obtained.383More recently it has been shown that some of the diazoalkanes do not react with protonic acids, but that the unique carbon atom is attacked by nucleophiles such as MeLi to yield diazenido comple~es.~" The reaction of [WBr(dipho~)~(N~H~)]+ and (Ph21)C1 in the two phase system C H C k aqueous K2C03 leads to [WBr(diphos),(NN=CCl2)]+, which reacts with a variety of
1014
Tungsten
nucleophiles to give novel organonitrogen ligands. It reacts with MeNH2, for example, to give [WBr ( d i p h ~ s{)N-N=C(NHMe ~ )} and with F- to give [WBr ( d i p h ~ s )N=N-CF3)]. ~( 385 [WBr(dipho~)~(N~H~)]+ also reacts with cyanoalkenes to give vinyldiazenido complexes or, after loss of the hydrazido(2-) ligand, to form nitrile-derived methylene amino complexes.386 Hydrazido(2-) and hydrazido(1-) complexes have also been shown to condense with aldehydes and ketones to give diazoalkane complexes containing the W=N-N=CR1R2 nit.^^^,^^^ Treatment of these complexes with LiAlH, gives secondary amines and ammonia, whereas treatment with acid produces hydrazine, keto azines and N2-free tungsten compounds. Amines can also be produced from organohydrazido(2-) c o m p l e x e ~ . ~ ~ ~ * ~ ~ +
37.9
NITROSYL COMPLEXES
Nitrosyl complexes of tungsten were first prepared in 1964 by reaction of W(CO)6 with nitrosyl 'chloride in an inert atmosphere. [W(NO),C12], is a dark green hygroscopic, air-sensitive material, soluble only in coordinating solvents.391The analogous [W(N0)2Br2], can be prepared by using NOBr392or by the reaction of NOBr with [W2(C0)8C14].393 [W(N0)2Clz], reacts with a variety of ligands to form octahedral complexes of the type [W(NO)zC12L] (L = PPh3, AsPh3, py, MeC6H4NH2,C&Il1NH2, Ph3PO).391,393 The IR spectra of these derivatives consist of two v(N0) bands around 1750 and 1650cm-', indicating a cis dinitrosyl arrangement. The positions of these bands sug ests the NO+ formulation and thus formally can be considered as octahedral complexes of With the bidentate dimethyldithiocarbamate ligand, [W(NO)zBr2], gives C~-[W(NO)~(SZCNM~&].~'~ Several nitrosyl complexes have been prepared by the reaction of nitrosyl chloride with anionic carbonyls. Thus, [W(CS>,Cl]- reacts with NOC13'5 to give [W(CO),(NO)Cl] while [W(NO)Cl,L] and JW(NO),ClL] (L = RB(P,),) were prepared from [LW(CO),]- with variable amounts of NOCl. y6 [w2(co)&b] also reacts with NOCl to give in addition to [W(N0)2C12]n the mononitrosyl [W(NO)C13]393which exhibits only one v(N0) bond at 1590cm-'. WC16 or WCIJ reacts with NO in benzene to give red air-sensitive material which reacts with phosphine ligands to give [WCl3(N0)(OPR3),] (PR3 = PPh3, PMePh2, PEtPh2).397 [WC13(NO)(OPPh3)2]can also be obtained from [W(C0)4(PPh3)2]and NOC1.393Careful choice of reaction conditions also gives the [WC13(NO)L;?]compounds with L = PMePh2, PEtPh, and (X= CI, Br) are also known.371 py.397The mononitrosyl complexes [WX(NO)(diph~s)~] Cationic nitrosyl complexes have also been prepared. The reaction of [W(CO),(MeCN),] with NOPF6 in acetonitrile affords cis-[W(NO),(CO)(MeCN)3] (PF6)Z which exhibit two v(NO) 1R bands at 1867 and 1775cm-l. The proton NMR spectra consists of two bands of relative intensities of 2 : l consistent with both mer and fuc arrangement of the MeCN ligands." C~~-[W(NO)~(CO)(M~CN),]~+ reacts with NaS2CNEt2 to give the known cis[W(N0)2(S2CNEt2)2]and with acac in the presence of base to give c i ~ - [ W ( N O ) ~ ( a c a c ) ~ ] . ~ ~ It reacts The reaction of [W(CO),(diphos)] with NOPF6 gives [W(cO),(No)(dipho~)]PF~.~~~ with halides and dithiocarbamates to yield [W(CO),(NO)(diphos)X] (X = C1, Br, I) and [W(CO)(NO)(diphos)(S2CNR2J(R = Me, Et). Spectroscopic data suggest that in the last compound the CO and NO ligands are cis to one another. Similar complexes can also be obtained with other bidentate Treatment of [W(N0)2C12], with AgBF4 in MeCN leads to C.~-[W(NO),(M~CN),](BF~)~. NMR measurements have shown that this complex undergoes stereospecific exchange of coordinated MeCN via a dissociative pathway. The two labile MeCN groups are those tram to the nitrosyl ligands.399 Anionic nitrosyl complexes were prepared by the reaction of [W(NO)zC12], with Ph4A~C1,391 and Ph4PBr. [W(N0)2C14]2- and [W(N0),Cl2Br2I2- are green materials with cis dinitrosyl arrangement. They react with dithiolene type ligands to give C~-[W(NO)~(L-L)~]~(LL=m nt, i-mnt, tbd).400 The two IR bands associated with v(N0) decrease in the order dtc > mnt > i-mnt > tbd. This trend has been rationalized in terms of the effects of charge and n acceptor ability of the various dithiolene ligands.
d
37.10 HYDRIDO COMPLEXES Transition metal hydrides are clearly an important class of metal complexes, and a number of books and review articles have appeared during the last 15 years. Hydrido complexes of tungsten contain in addition to the hydride such ligands as C5HS,phosphine, NO and CO.
Tungsten
1015
With phosphine ligands, complexes with a different number of hydrides are known. [W&L4] (L=PMezPh, PMePhz) were first prepared by the reaction of [WC14L4] with NaBH4 in alcoholic solutions in the presence of excess phosphine.401 The crystal structures of [W(PEtPh2)4] and [WH4{P(OPri)3}4]402 have been determined. These are all yellow air-stable materials, and exhibit a complex IR spectrum in the 1700-1850 cm-' region associated with v(WH). The proton NMR spectrum of [WH4(PMePh2)4]at room temperature is consistent with a ri id structure. A fluxional behavior develops above 50°C. [WH4(PMePh)4] behaves similarly%' With other phosphines the same fluxional behavior is observed in solution.403An intramolecular rearrangement pathway has been suggested to explain this behavior. With the diphos ligand, [W&(diphos)21 is obtained by the reaction of [WX2(diphos)2(N2H2)](X = C1, Br) with NaBH4.368 moieties are also known.404 transComplexes containing the WH5 and W& [WX4(PMe2Ph)2] (X=Cl, Br) complexes react with NaBH4 in ethanol to give [W&(PMe2Ph),]. NMR measurements show that the six hydrides are coupled equally to all three phosphorus nuclei. A reasonable structure for this complex is a trigonal rism of hydrogen atoms with phosphorus donor atoms capping the square faces of the prism. 4B3 Hydrogenolysis of [ W e 6 ]in the presence of phosphine ligands leads to different hydrido complexes depending on the size of the phosphine ligands. Thus, while PMeaPh gives primarily [W&L4], with the more bulky PP$Ph and PPrS ligands [WH6L3]is obtained. [WH6(PPf2Ph),l has been shown by 31P NMR and X-ray diffraction to have approximately C, symmetry in the solid and in solution.4u5 The pentahydrido complex [WH5(PMePh2)4]+ is obtaineda" from the reaction of [WH4(PMePh2)4]with acids such as HBF4, HPFs and TFA. The isolated salt with TFA, loses hydrogen on melting to give the dihydride [WHz(TFA)2(PMePh2)3]. The dihydride [WHzC12(diphos)2]is also known.369 The photolysis of a hexane solution of [WL6] (L = P(OMe)3) in an atmosphere of H2 yields a mixture of [WHzL5] and [WH4L4].407 Several monohydrido complexes have also been prepared. Treatment of ci~-[W(CO)~(dpm),] with 02/HC104yields tran~-[WH(CO)~dpm)~]+ which is believed to be seven-coordinate with the capped octahedral geometrySa8 Similar complexes with other bidentate phosphine ligands are also k n o ~ n . ~The , ~ ~preparation * of [WH(Nz)2(diphos)z]f and [WH(CNR)2(diphos)z]f have been r e p ~ r t e d . ~ ~Although ~,~" [WH(N2)z(diphos)2]f has a pentagonal bipyramid geometry in the solid state, with the hydride ligand in the pentagonal plane, NMR data are compatible with both the pentagonal bipyramid and capped octahedron structures in solution.411 Recently it has been shown that Na/Hg reduction of [WC14(PMe,),] under H2 gives the dihydride [WH2C12(PMe3)4and that the reaction of [WC12(PMe3),] with methanol in THF leads to the same p r ~ d u c t . ~ Finally, it should be mentioned that several p-hydrido-bridged tungsten carbonyl and mixed carbonyl nitrosyl complexes are known. When only one bridging hydride is present the W-H-W bond is always Bridging hydrides are also found together with other bridging groups in several tungsten complexes. For example, the reduction of [W2C14(PMe3)4] in THF with Na/Hg leads to a species which is best formulated as [W2H4(p-H)( PMe2)(PMe3)5]. This complex is described as a WT1-WTvdimer (W-W distance = 2.588 ) with four terminal hydrido ligands and one bridging hydride.414
1
k
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1016
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Tungsten
1017
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Tungsten
1021
345. R. N. McGinnis, T. R. Ryan and R. E. McCarley, J. Am. Chem. SOC., 1978,100,7900. 346. F. A. Cotton, J. L. Hubbard, D. L. Lichtenberger and I. Shim, J. Am. Chem. SOC., 1982,104,679. 347. G, M. Bancroft, E. Pellach, A. P. Sattelberger and K. W . McLaughlin, I . Chem. SOC., Chem. Commun., 1982, 752. 348. F. A. Cotton, R. M. Wing and R. A. Zimmerman, Inorg. Chem., 1967, 6, 11. 349. R. D. Hogue and R. E. McCarley, Inorg. Chem., 1970,9, 1354. 350. K. Lindner and A. Kohler, 2. Anorg. Allg. Chem., 1924,140, 364. 351. H. Schiifer and R. Siepmann, Z . Anorg. Allg. Chem., 1968,357,273. 352. R. Siepmann and H.G. von Schnering, Z. Anorg. Allg. Chem., 1968, 357,289. 353. H. Schaefer and J. Tillack, J. Less-Common Met., 1964, 6,152. 354. F. A. Cotton and T. E. Haas, Inorg. Chem., 1964, 3, 10. 355. F. A. Cotton and G. G. Stanley, Chem. Phys. Lett., 1978, 58,450. 356. A. W. Maverick, J. S. Najdzionek, D. MacKenzie, D. G. Nocera and H. B. Gray, J. Am. Chem. SOC., 1983,105, 1878. 357. F. A. 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1022
Tungsten
411. J. Chatt, A. J. L. Pombeiro and R. L. Richards, J . Chem. SOC., Dalton Tram., 1979, 1585. 412. K.W.Chiu, D. Lyons, G . Wilkinson, M.Thornton-Pett and M. B. Hursthouse, Polyhedron, 1983,2,803. 413. R.A. Love, H. B . Chin, T. F. Koetzle, S. W. Kirtley, B. R. Wittlesey and R. Bau, J. Am. Chem. SOC., 1976,98,
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1981, 1892.
Isopolyanions and Heteropolyanions MICHAEL T. POPE Georgetown Uniuefsity, Washington DC, US 38.1 INTRODUCTION
102.3
38.2 ISOPOLYANTONS 38.2. I General
1025 1025
38.2.2 Isopolyvanadates 38.2.2.1 Aqueous solutions 38.2.2.2 Other isopolyvawdates 38.2.3 Isopolyniobates 38.2.4 Isopolytantalates 38.2.5 Isopolymolybdates 38.2.5.1 Aqueous solutions 38.2.5.2 Nonaqueous solutions 38.2.5.3 Other isopolynwlybahtes 38.2.6 Isopolytungstatar 38.2.6.1 Paratungstatar 38.2.6.2 Metatungstates 38.2.4.3 Decutungstaie and kexatungstate
1025
1025 1027 1028 1029 1029
1029 1031 1032 1032 1033 1034 1034
38.3 HETEROPOLYANIONS 38.3.1 Fourcoordinate Primary Heteroatoms 38.3.1.1 The Keggin structure and derivatives 38.3.1.2 Non-Keggin metallophosphates and metalloarsenates 38.3.2 Sixcoordinate Primary Heteroatoms 38.3.2.1 The Anderson and related structures 38.3.2.2 Other structures incorporating octahedral heteroatoms 38.3.3 Eight- and Twelvecoordinate Primary Heteroatoms 38.3.3.1 Decatungstometalates, [XWlo036]nL 38.3.3.2 Dodecnrnolybdometalafes,[XMo,* O,,l"-
1035 1035 1035 1042
38.4 HETEROPOLYANIONS AS LIGANDS: SECONDARY HETEROATOMS
1a46
38.4.1 Complete Polyanbn Ligandr 38.4.2 Lacunary Polyanwn Ligands 38.4.2.3 Derivatives of [XM,,, O,,]"- and [X,M,,06,]"38.4.2.2 Multisubstituted heteropolyanions 38.4.2.3 Heteropoly cryptands
38.5 HETEROPOLY BLUES AND OTHER REDUCED SPECIES 38.5.1 Electrochemistry 38.5.1.1 Polytungstates 38.5.1.2 Polymolybdates 38.5.2 Spectroscopy and Electronic Structures
1043 1043
1045 1045 1045 1045 1047 1047 1047 1049 1049 1049 1050 1050 1051 1051
38.6 ORGANIC DERIVATIVES OF POLYANIONS 38.6.1 Organophosphonute Complexes 38.6.2 Organoarsonate Complexes 38.6.3 Diorganoarsinateand Related Complexes 38.6,4 Octamolybdate Deriuatiues
1052
38.7 REFERENCES
1055
1053 1053 1054 1054
38.1 INTRODUCTION Hexavalent molybdenum and tungsten, pentavalent vanadium and, to a more limited extent, niobium and tantalum form a very large number of pol oxoanions ('heteropolyanions' such as [PW120~l3and 'isopolyanions' such as [Mo7024]6-). Unlike the polyoxoanions of the post transition elements the heteropolyanions for the most part are discrete, compact species of high
1023
1024
Isopoiyanions and Heteropoiyanions
symmetry and low basicity. Salts and free acids of heteropolyanions are usually highly soluble in water and other solvents. Their extensive acid-base and oxidation-reduction chemistry, and their structures, which resemble fragments of close-packed metal oxide lattices, make heteropolyanions attractive as potential (and actual) catalysts? Other demonstrated applications of heteropolyanions include their use as analytical reagents, electron microscope stains, and antiviral and antitumoral agents. Why should the early transition metals form so many polyoxoanions? The answer lies in the size of the M5’6+ cations and their x-acce tor properties.’~~ The effective ionic radii of V5’ (0.68A), Mo6+ (0.77A) and w6’ (0.74 ) are consistent with the observation that these cations adopt four-, five- and six-fold coordination by oxide ion. With very few exceptions V, Mo and W atoms in heteropolyanions are six-coordinate. On the other hand Cr6’ (0.58 A) hap a maximum coordination number of four in oxides and oxoanions. Few isopoly- and heteropoly-chromates are known and they are all based on groups of corner-shared CrO, tetrahedra: [Cr2O7I2-, [ C I ~ O ~ O ] ~ -[Cr4OI3l2-, , [ 0 3 s o ~ 3 ] 2 -[O2IOCro3]-, , etc. These species bear little similarity to the other heteropolyanions of [HOAS(OC~O~)~]*-, Groups 5a and 6a, and, with the exception of the dichromate anion, have a limited or nonexistent solution chemistry. Several post transition elements have ionic radii similar to those of Mo6+ and W6+: Ge4+ (0.67 A), Sb5+ (0.74 A), Te6+ (0.70 A) and 17+ (0.67 A). The polyoxoanions of these elements are quite unlike the molybdates and tungstates and are often polymeric chains and networks of x06 octahedra. The: typical bond lengths found in oxoanions of Mo6+ and 17+,shown in Figure 1, reveal the essential Merence between transition and post transition cations. Note that M&, by virtue of its accessible vacant d orbitals, forms stronger (shorter) bonds with terminal oxygen atoms than does 17+.The greater bond order of the M d bonds reduces the basicity of the oxygen atoms that lie on the exterior of the heteropolyanion. The consequent low charge density on the polyanion surface minimizes protonation, cation binding (insolubility) and further polymerization. Some years ago Lipscomb6 speculated that the structures of heteropolyanions might be limited to those in which each M 0 6 octahedron has no more than two unshared vertices. At present, no structure has been found that contradicts Lipscomb’s proposal, and it is a simple matter to rationalize this. In view of the large trans influence of the M-O(terminal) bonds, see Figure 1, an M 0 6 octahedron with three terminal oxygens would be attached to the remainder of the polyanion structure by three long, and therefore weak, bonds. It seems probable that such a polyanion would be readily susceptible to the loss of the M 0 6 octahedron either directly as a neutral MO, group or hydrolytically as HOMO;.
8:
HW, m
CO41@24H:;
H,Io;-
e 1 Comparison of the tram influence of terminal oxide ligands in octahedral complexes of Mo6+ and 17+
Much synthetic and descriptive chemistry of heteropolyanions dates from 1860-1920 when the terms ‘isopoly acid‘ and ‘heteropoly acid’ were introduced. Reviews of the early work are available7 and they provide much valuable, if descriptive, information. In the following sections we shall discuss the chemistry of isopolyanions (general formula: [ K O Y I p - )as well as that of heteropolyanions ([XxMmOr]‘?-;x C m).Excluded from consideration here are substances with similar formula that are the result of high temperature solid state or melt reactions and which are polymeric mixed oxides with no defined solution chemistry.
Isopolyanions and Heteropolyrsnions
1025
38.2 ISOPOLYANIONS 38.2.1
General
The chemistry of isopolyanions has to a large extent developed from the study of hydrolytic equilibria in aqueous solution (equation 1).8 The stoichiometry and equilibrium constants for such reactions are generally evaluated from precise EMF measurements (i.e. of the hydrogen ion concentration as a function of metal ion concentration and added acid or base). Measurements of this kind, pioneered by SillBn and his scho01,~ have to be made with extraordinary precision, to permit distinction between the large number of species that are often present in polyanion solutions. Even when all experimental difficulties (attainment of equilibrium, liquid junction potentials, etc.) appear to have been overcome or allowed for, the EMF method has other limitations. An equilibrium such as equation (1) is defined only by the values of p and q ; the number of oxygen atoms in the isopolyanion cannot be determined. For example, the species [H2V3010]3-and [V309f3-cannot be distinguished. Furthermore, it is becoming clear that factors such as the nature of the counterions and the ionic strength can have significant effects upon polyanion equilibria. In general, investigations have been made at one ionic strength, and the possibilities of specific counterion binding (as observed in some heteropolyanion systems) have not always been excluded. pHf
+ q(M04]'- =T
[H&O,j"-
+ (4q - y)H,O
(1)
Virtually every experimental chemical technique that can be applied to solutions has at one time or another been used for the investigation of isopolyanions. The often conflicting results that pervade the literature are caused by the investigators' failure (a) to appreciate the complexity of the equilibria and kinetics involved and (b) realistically to assess the experimental limitations of the technique used. Clearly, there is no one experimental method that can reveal the complete story, and the literature must therefore be read extremely critically. Although aqueous hydrolytic equilibria such as equation (1) provide an overall framework for the discussion of isopolyanions, several species have been synthesized in nonaqueous media, and are unstable in water. Other isopoIyanions that are crystallized from aqueous solution, may be incongruently soluble and have polymeric structures. It will, however, prove to be convenient to treat all isopolyanions as though they were derived by equation (l),and characterize each by its 'acidity', 2 ( = p / q ) . Note that 2 for a specific anion is not necessarily equal to the value of p / q for the solution from which the anion was crystallized."
38.2.2
Isopolyvanadates
38.2.2.1 Aqueous solutions Vanadium(V) oxide is amphoteric, dissolving in alkali to give salts of tetrahedral V0:) ~ in ]+, anion, and in acid to give VO,'. The latter cation is almost certainly C ~ - [ V O ~ ( H ~ O as [VOZ(OX)Z]~and [V02(edta)l3- (ox = oxalate dianion; edta = ethylenediamine tetraacetate tetraanion). All dioxo complexes of do metals have cis configurations, an arrangement that maximizes d x - p n interactions. Salts of the orthovanadate anion are frequently isomorphous with the analogous phosphates and arsenates, e.g. Na&04*12HzO. In dilute solutions, cu. M, the protonation equilibria parallel those of PO:- but with pK values that are greater by 1-2 units (see Table 1). In solutions more concentrated than M, a multitude of isopolyvanadates is formed. These may roughly be divided into colorless species that predominate above pH6-7, and orange species in the range pH 2-6. Interconversion between colorless and orange polyanions is kinetically controlled, and equilibrium EMF measurements become unreliable in the region where both types of polyanion exist. The distribution of isopolyvanadate species according to the 'best' fit of potentiometric data for pH6-14 is shown in Fi ure 2 for [VI,, = 1.0 and lom3M." These conclusions are supported by high field 51Vand 0 NMR spectroscopy. The predominant polyanions are listed in Table 2. The di- and noncyclic tri-vanadates are presumed to be analogous to the corresponding phosphates, and their protonation constants (Table 1) are consistent with this. NMR spectroscopy indicates the presence of traces of linear tetramer also. The divanadate anion has been structurally characterized in several salts, e.g. Ca2V2O7.2H2O
Q
Isopolyanions and Heteropolyanions Table 1 Comparison of Protonation Constants for Phosphates and Vanadates'
[VOd[PO41 -
[v20714[p2o7I4[V,O,O~P30,Ol
logK1
logK2
13.95 12.0 9.73 8.0 9.39
7.8 6.5 8.78 5.8 7.4(?) 5.2
7.5
-
lOgK3
.
3.8 1.7
'In 0.5M Na(C1); 25°C (K.H. Tytko and J. Mchmke. 2. Anorg. AlIg. Chem., 1983, 503, 67).
I .o
0.8
Im
IO-' rn
I--
0.4-
-I4
-12
-IO
-8
-6 -14
-12
-8
- 10
tog C",
PEgUre 2 Distribution diagrams for vanadate(\.') species in aqueous solution at 25 "C and p = 0.5 M Na (Cl). Curves are hbelled with the values of p , q for each anion (see Table 2) (reprinted with permission from K. H. Tytko and J.
Mehmke, Z. Anorg. Allg. Chem., 1983, 503, 67)
Table 2 Major Oxovanadium(V) Species in Aqueous Solution"
Formula -541.2 -538.8 -560.4 -561.0 -563.5 -572.1 -556.3, -590.4 ca. -570 -577.6 -586.0 e e e -545
+565 +573
+405, +695 +438, +721, +852 +472, +928 +472, +928 f f f
'Minor components provisionally identified by 51V NMR include linear [V,O,,]"-, [HV4Ol3I5-, [H,V4013!4-, [V50161- and cyclic [V60,,l6-. Equilibrium constant for equation (l), p =O.$M Na(C1) 25°C (K. H. Tytko and J. Mehmke, Z . Porg. Allg. Chem., 1983, 503, 67). E. Heath and 0. W. How=&, J. Chem.SOC.,Dalton Tram., 1981, 1105; S. E. O'DonneU and M. T. !OF, J . Chem. Soc., DaIton Tram.$476,2290. Protonation constants for lVloO,] in 1.0 M NaClO,, 25°C; tog K, 5 . 8 log K23.6 (P. J. C. Rossotti and H. S. Rossotti, Acta Chem. h d . , 1956,10,957). 'pH dependent S, -421, -4%, -512 (pH6.5) (M. A. Habayeb and 0. E. Hileman, Jr., Can. J . Chem., 1980,58,2255). fpH dependent 6, +62, 406,766,780, 893, 1143 (pH6.0) (W. G . Klemperer and W. Shun, J. Am. Chem. Soc., 1977,99,3544).
Isopolyanions and Heteropolyanions
1027
with V-0 terminal) 1.69 A, V-O(bridging) 1.80 8, and (VOV, 139”. No structures containing [V3O10] - have yet been reported. The cyclic tetramer and pentamer are the major constituents of so-called ‘metavanadate’ solutions (pH6.5-8) but the only salts that have so far been crystallized from such SoIutions contain polymeric anions. Typical structures of metavanadates are (a) infinite chains of comer-shared V 0 4 tetrahedra (KV03, a-NaVO,, NI&V03, CsV03), and (b) chains of edge-shared distorted V05 trigonal bipyramids (KVO3.H20, NaV03.1.89H20, Ca(V03)2.2Hz0, Sr(V03)2.4H20).‘The cyclic tetrameric anion has been structurally characterized in ( B U S N ) ~ H V ~ O which ~ ~ , was prepared by dissolving Vz05 in alcoholic BGNOH.’, The Raman spectrum of this salt is similar to that of an aqueous metavanadate solution. Earlier UV and Potentiometric measurements had suggested the presence of a trimer in metavanadate solutions, either cyclic [V309l3- or linear [HzV3010]3-.There is no compellin evidence for significant quantities of such a species according to later measurements. The ‘V NMR data indicate that traces of a cyclic hexamer may be present and that this could be a precursor to the orange decavanadates. The decavanadates (Table 2) were first convincingly identified from potentiometric investigations of alkalinized VO: solutions, since the metavanadate-decavanadate equilibria are established very slowly and involve NMR-detectable intermediates. The structure of the decavanadate anion has been established by several X-ray investigations and is shown in Figure 3.14 The anion consists of an arrangement of 10 edge-shared V 0 6 octahedra with L& symmetry. The octahedra are distorted in order to maintain approximate valence balance at terminal and bridging oxygens and bond lengths range from 1.6OA for V-O(termina1) to 2.32A for V-O(centra1). The 51VNMR spectrum of the anion in solution has three lines in the ratio 1:2:2 and is consistent with the solid state structure. Both 51V and 1 7 0 NMR have been used to determine the protonation sites of [VloOzs]6-, with different conclusions. The crystal structure of tetrakis(4-ethyl ridinium)dihydrogen decavanadate reveals protonation at yet a third type of oxygen atom.’ The kinetics of decomposition of decavanadate by acid and alkali have been investigated,16 and l80exchange between water and the polyanion occurs with a half-life of cu. 15 h at 25 “C and pH 4-6.3.’’ A mechanism to account for the exchange and for the alkaline degradation has been proposed.
I
PY
Figure 3 The structure of [V,,0,,]6- anion shown as an arrangement of VO, octahedra
38.2.2.2 Other isopolyvanadates (i) Polymeric anions The polymeric metavanadate anions have been described above. Upon warming or ageing solutions that contain the decavanadate ion, it is possible to precipitate salts of the dark red ‘trivanadate’ anion, MV30s (M=NH4, K, Rb, Cs). The formula is sometimes written as M2V6016, and the salts called ‘hexavanadates’ in the literature. Potassium and cesium trivanadate contain infinite buckled layers of distorted VOs and V06 polyhedra. Another layer structure is found in yellow K3V5014 which also is deposited from aqueous solution. The [V5014]F;9 anion is composed of layers of corner-shared VO., tetrahedra and VOs square pyramids.
’*
1028
Isopolyanions and Heteropolyanions
(ii) Polyvanudutes containing vunadium(lV) Vanadium(1V) oxide, like VZOS,is amphoteric and dissolves in alkali to yield brown, easily oxidizable solutions. Salts obtained from these solutions were originally formulated as MzV4O9-nHZ0but were later shown to have the empirical formula, M2V307-nHZ0.The structure of the potassium salt reveals the anion [v180#consisting bf an almost spherical arrangement of edge- and corner-shared VOs square pyramids with axial V-0 bonds directed outwards (see Figure 4)." The anion is stable in solution between pH9 and 13, and is paramagnetic with about eight free spins at 22 "C. Dilute solutions of V0 2 in alkali appear to contain [VO(OH)3]- according to ESR spectroscopy. In strongly basic solution (greater than 1M OH-) vanadate(1V) disproportionates into vanadium(II1) and vanadium(V).
Figure 4 The structure of [V,,O,J'*-
anion. The central cavity has a radius of 4.5 8, and is fractionally occupied by K+ or H,O in the crystal
Several mixed-valence polyvanadates(IV, V) are reported in the literature. Almost without exception the anions have been formulated as decavanadates, often with no real justification. Thus Ostrowetsky21 reported the existence of six anions in the pH range 4-6.5. The anions were given formulas such as [V$VVyO~6H]4and contained VlV:Vv ratios ranging from 2: 8 to 7:3. More recently Johnson isolated, from similar solutions, salts of a blue-violet anion 1v v [ v 6 V130&I-€]8fand determined its structure.20 The anion has a structure of virtual C, symmetry composed of 18 VO, polyhedra (tetrahedra, octahedra and square pyramids) surrounding a central V 0 4 tetrahedron. The valence stoichiometry of the anion vanes from one preparation to another (VIv :Vv = 5 :14-7 :12), and given that Johnson's formula is approximately twice those reported by Ostrowetsky, it seems likely that some of Ostrowetsky's complexes are similarly constituted. Salts of the deep violet anion [VivVz026]4- have been obtained adventitiously from nonaqueous media. The crystal structure of the tetramethylammonium salt shows an anion of virtual & symmetry composed of a rin of eight corner-shared Vv04 tetrahedra capped on either side by VIVO5square pyramids& The polymeric anion in K2V308,which is obtained from aqueous solution, has a structure comprising V"0, square pyramids linked to ditetrahedral Vz07 units. The salt (NH&V308*0.5H20, prepared by homogeneous acidification of thiovanadate soiutions,u probably has the same anion.
38.2.3 Isopolyniobates Aqueous niobate solutions contain the hexaniobate anion, [HxNba019](8-x)-,throughout most of the accessible pH range (above ca. pH 7). Normal and acid salts of [Nb6Ol9l8-may be crystallized from aqueous solutions that are generally prepared by dissolution of freshly precipitated Nb205 in alkali. The structure of the anion, in crystals of Na7HNb6019.15Hz0, is shown in Figure 5. As in the decavanadate structure, the metal atoms are displaced towards the unshared oxygen atoms. The N b O bond lengths are 1.75-1.78 8, (terminal), 1.970-2.056 A (bridging) and 2.371-2.386 A (central).24That the same structure persists in aqueous solutions of sodium and potassium salts is strongly supported by ultracentrifugation, light scattering, Raman spectroscopy and "0 NMR. Vibrational spectra of the crystalline potassium salt have
Isopolyanions and Hereropolyanions
1029
been subjected to normal coordinate analysis. The successive protonation constants (log K ) for [NbS019]8-in 1M KCl are 12.3(1), 10.94(5) and 9.71(2).=
Figure 5 The structure of [Nb,O,,]*- in bond, polyhedral and sprfce-fillingrepresentations
On the basis of Raman and UV spectroscopy it has been suggested that [Nb6019]8-is degraded into tetrameric and monomeric species in 1-12 M KOH.= Acidification of niobate solutions in the presence of NMe: or NEtt ion is said to lead to an Nb9 species. The latter may well be a clecaniobate since hydrolysis of a methanolic solution of Nb(OEt)5 with aqueous NMe40H and NaOH yields (NMe4)6NbloOzs6Hz0and NMe4)~a2Nblo02s+8H20~0.5MeOH. The structures of both salts have been determined.2 The [Nb10028]6- anion is exactly analogous to [Vla02s]B-(Figure 3).
\
38.2.4
Isopolytantdates
The only well-established isopolytantalate is [Ta6Ol9l8-, isostructural with the corresponding niobate. The bond lengths in K&a2H2Ta6OI9-2H2O are Ta-O(terminal), 1.786-1 317; Ta-O(bridging), 1.976-2.012; Ta-O(central), 2.356-2.426 A.28Hexatantalate solutions have been examined by ultracentrifugation, light scattering, Raman spectroscopy and "0 NMR. Normal coordinate analysis of vibrational spectra reveal that the force constant for the Ta-0 stretch is about 8% larger than for Nb-0. Oxygen exchange between water and [Ta6019]'- is much slower than that for [Nb6OI9l8-and occurs more rapidly at the terminal oxygen atoms, according to "0NMRZ9Protonation constants (log K) of 12.68, 10.81 and 9.28 in 1.0M KCI have been reported and are similar to those of [Nb6OI9l8-. Solvolysis of Ta(OEt)5 with 85% H202 in the presence of Bu'NH2 leads to a product .30 Ultracentriformulated as a dodecatantalate derivative, fugation of an aqueous solution of this material shows two molecular weight fractions, one corresponding approximately to a Ta12 species and the other to [Ta6019]'-. As the solution ages, the proportion of the heavier solute species decreases. 38.2.5
Isopolymolybdates
Salts of about a dozen isopolymolybdate anions have been isolated from aqueous or nonaqueous solutions. The formulas of these anions are listed in Table 3 and their structures (see below) are based predominantly on sixcoordinate molybdenum in contrast to those of vanadates and chromates. 3lX2.5.1 Aqueorcs sobtiuns
M) the equilibria (2) are governed by log K values of 3.87 and 3.70 In dilute solutions respectively (3M NaC104, 25°C). While the MOO:- anion is certainly tetrahedral, the anomalously small value of logK1 has been taken to indicate an expansion of coordination number upon protonation. In view of the prevalence of octahedral cis MOO? units in molybdenum(V1) chemistry, formulas such as [MOO~(H~O)(OH)~]and [MoO~(H~O)~(OH)~]
1030
Isopolyanions and Heteropolyanions Table 3 Isopolymolybdate Anions Formula
Acidity, Za
Ref. for structure
0 1.00 1.00 1.14 1.20 1.25 1.25
h
1.33
1
g
1.40
1.50 1.50 1.67 1.78 1.80
j k
I m n
Z = p / q as defined in equation (1). 0. Nagano and Y. Sasaki,Acta Crystullogr., Sect. B, 1979, 35,2387. 'V. W. Day, M. F. Fredrich, W. G. Klemperer and W. Shum, J . $m. Chena. Soc., 1977,99, 6146. I. Knoepnadel, H. H a d , W. D. Hunnius and J. Fuchs. Angew. a
Chem., Int. Ed. Engl., 1974, El, 823. H. T. Evans, Jr., B. M. Gatehouse and P. Leverctt, 1. Chem. Soc., piton Tram., 1975, 505. J. h h s , H. Hartl, W. D. Hunnius and S. Mahjour, Angew. Chem., Int. Ed. Engl., 1975, 14, 644. 'M. Isobe, F. Marumo, T. Yamase and T. Ikawa, Acra Crystallogr., feet. B, 1978,34,2728. I. Boeschen, B. Buss and B. Krebs, Acta CrysfuUogr.,Sect. B, 1974, 48. H.-U. Kreusler, A. Forster and J. Fuchs, Z . Numrforsck., TeiZ B, 1980,35,242. J. Fuchs and H. Had, Angew. Chem., Int. Ed. Engl., 1976,15,375. 'H. Vivier, J. Bernard and H. Djomaa, Rev. Chim. Miner., 1977,14, e
p* '
?4. See, for example, W. Clegg, G . M. Sheldrick, C. D. Garner and I. B. Walton, Acta CrysfaUogr., Sect. B, 1982,38,2906. " B. Krebs and 1. Paulat-Bceschen,Acta Crysfaliogr., Sect. 8, 1982, $3, 1710.
B. Krebs and I . Paulat-Boeschen,Acta Crystdon., Sect. 8, 1976,
32, 1697.
have been proposed. Molybdenum(V1) is amphoteric and in 0.2-3.0M H+ yields cationic species with empirical formulas [HMoO3]+, [MoO2I2+,[Mo2O5I2+and [HMo205]+,but which also undoubtedly are based on six-coordinate Mo. MOO:-
K1
HMoO;
K2
H,MoO,
(2)
M Acidification of aqueous solutions of Na2Mo04 at concentrations greater than about leads successively to solutions containing the polyanions [Ms0,I6('paramolybdate'), The evidence for these species is based on potenP - [ M O ~ O ~and ]~tiometric and spectroscopic investigations of solutions, and on X-ray structural determinations of salts isolated from these solutions. A reevaluation of the potentiometric data31has suggested the presence of additional species, [HzMo12042]10-(analogue of 'paratungstate B', see Section 38.2.6.1) and [MO18054(oH)4(H20)*]4-, but confirmatory evidence for these is lacking at - aqueous present. No stable intermediates seem to exist between [MoO4I2- and [ M o , O ~ ] ~in solution, but (NJ%,)2M0207 crystallizes from hot solutions after some hours. The anion in this salt is polymeric however with infinite chains of Moo6 octahedra and Moo4 tetral~edra.~'A discrete ditetrahedral [M*O7l2- anion has been synthesized in acetonitrile solution and isolated as a tetrabutylammonium salt.33Although [Mo20#- is unchanged in aqueous-acetonitrile solutions, addition of alkali metal counterions caused immediate precipitation of salts of [M%OZA]~-. The heptamolybdate anion and its protonated forms, [H,Mo~O~~](~-")-, n = 1-3, predominate in solutions of pH3-ca. 5.5. The structure of the anion, Figure 6, has been determined in sodium, potassium, ammonium and isopropylammonium salts, and has been confirmed in solution by low angle X-ray ~ c a t t e r i n g The . ~ ~ anion structure can be viewed as
bopolyanions and Heteropolyanions
1031
of the decavanadate structure and involves an arran ement of octahedra with cis-Mo02 stereochemistry for each metal atom. IR, Raman and .g0 NMR spectra have been reported. Crystals of the isopropylammonium salt are photoreducible.35
'
Figure 6 The structure of [Mo,0,J6-
anion
Salts of B - [ M O ~ O ~may ~ ] ~ be - crystallized from more acidic solutions, pH 2-3. The structure of the anion (Figure 7) is also formally derivable from that of [Vlo02s]6-. The existence of significant concentrations of B-[M08026l4- in aqueous solutions has been controversial, but Raman and X-ray scattering studies provide more positive evidence.34
figure 7 The structure of /?-[MO~O~~]~anion
The presence of a very large anion in solutions acidified to H+/MoO;- -1.8 has been inferred from EMF and ultracentrifugation studies. Sodium,potassium, ammonium and barium salts of this anion were isolated from such solutions and the structure of &[M0360112(H20)16]+38-~H20 has been reported. The anion, Figure 8, consists of two MoI8 units related by inversion symmetry. Each unit contains an M07024 moiety surrounded by edge- and corner-linked Moo6 octahedra containing terminal and bridging water molecules as shown. Two of the Mo atoms in the M o ~units become seven-coordinate (quasi perita onal bipyramidal) and an identical feature is seen in the peroxo polymolybdate [Mo7022(0&] %- .31 3li25.2 Nonaqueolrs solutions
Four isopolymolybdates have been synthesized or stabilized in nonaqueous or mixed solvents: [Mo2O7I2-, [ M O ~ O ~ ~& ] ~- [- M , O ~ O ~and ~ ] ~ -[Mo5Ol7HI3-. The first of these has already been discussed. Several salts of [Mo6OI9]'- have been prepared by both rational and serendipitous routes, and five X-ray structural determinations reported.3s The anion is isostructural With [b%6019]'- and has no significant distortions from Oh symmetry, except in one case, which is attributed to crystal packing. The structure has a rare feature for an oxomolybdate(VI), namely a C4,, monooxo site symmetry for the metal atoms. As a consequence (see Section 38.5) the anion can be electrolytically reduced to mixed valence [ M O ~ O ~and ~ ] ~[Mo6OI9j4species. The oxidized anion is yellow (A,,,= = 325 nm) whereas all other isopolymolybdates are colorless. The IR, Raman and 1 7 0 and 95MoNMR spectra have been reported. The ~ - [ M O ~ O Xanion ] ~ - is precipitated by B O + from an aqueous molybdate solution, and has a structure isotypic with [ ( M ~ A S O ~ ) ~ M O (see ~ O ~Section ~ ] ~ - 38.6.2) and thus contains two
Zsopolyanions and Heteropolyaniom
1032
Figure 8 The structure of [Mo360112(H20)16]8anion. A heptamolybdate unit in one of the two Mol* moieties is emphasized. Coordinated water molecules are indicated by open circles
Moo4 tetrahedra capping a planar ring of six edge-shared ~ctahedra.~’ According to 1 7 0 NMR the MOO, tetrahedra undergo intermolecular or intramolecular reorientation in acetonitrile solution. Addition of a small counterion such as K+ to such solutions induces isomerization to /?-[M080~]~-. A unimolecular mechanism for the isomerization has been proposed.40 A structure for the unstable [Mo5OI7Hl3- anion, isotypic with [Me2AsMo4Ol5HI2(Section 38.6.3), has been proposed from IR and ‘H N M R spectroscopy. The anion is isolated as an amorphous B u 8 salt from equilibrium (3) in 7M020:-
+ H 2 0 ==
~[Mo,O,,H]~-+ 4MoO:-
(3)
X2.5.3 Other isopolymolybdates
Several isopolymolybdates which have been isolated horn solution are polymeric or appear to have structures that do not persist in solution. These include the trimolybdates M 2M 03010~~HZ0, obtained as fibrous crystals from appropriately acidified solutions. The structures of Rb2Mo3OI0.H20and RbNaMo3OI0contain infinite double chains of edge-shared Mooe ~ctahedra.~’ From more acidic solution polymeric pentamolybdates [MMo5016H(H20)] slowly crystallize. The structure of the potassium salt reveals a complex three-dimensional network of Moo6 octahedra.43 Finally, the salts ( N & ) ~ [ M O & ~ ] - ~ H ~(O N,H & [ M O ~ ~ Oand ~ ] ( P ~ ~ ~ J H ~ ) ~ [ M Oall ~O~~( contain a common MOSOBunit of edge-shared Moos octahedral (see Section 38.6.4).
38.2.6
Isopolytungstates
Isopolytungstate anions isolated from solution in the form of crystalline salts are listed in Table 4. Several other species undoubtedly remain to be characterized, but analysis of aqueous polytungstate equilibria is complicated by the extreme range of rates involved. Many apparently stable isopolytungstates may be kinetic intermediates. For this reason, no equilibrium distribution diagrams, will be given, nor would such diagrams provide a useful
Isopolyanions and Heferopolyanions
1033
introduction to aqueous polytungstate chemistry. Scheme 1 is believed to represent the sequence of reactions in aqueous solution at moderate to high ionic strength. Table 4 Isopolytungstate Anions Acidity,
Z"
Formula
woi-
[w4oI61*-
0 0
[W&l6-
1.14 [ W I ~ O ~ ~ H ~ ~ ' ~ - 1.17 1.48 ~.-KH)w,~~,I~4(H2)W1204ol6--
B-[(~2)W12040l6WlOO32~[W6015.1-
1S O 1S O 1.60 1.67
Re5 for structure b C
d e
f
g
h
Z = p / q as defined in equation (1). A. Thiele and J. Fughs, Z. Naturforsch., Teil 3,1979, 34, 145. 'A. Hiillen, Angew. Chem., 1964,76,588. K. G. Burtseva, T. S. Chernaya and M. I. Sita, Sou. Phys.Dokl. (E@. Traml.), 1978,U,784. "H. T. Evans, Jr. and E. Prince, J . Am. Chem. Soc., 1983, 105, a
pi.
M.Asami, H. Ichida and Y. Sasaki, Acra Crystallogr., Sect. C, 19W,40,35. 'J. Fuchs, H.Hartl, W. Schiller and U. Gerlach, Acta Cysrullogr., B, 1975,32,740. J. Fuchs, W. Freiwald and H. Hartl, Acra Crysrallogr., Sect. B,
Pt.
1978,34,1764.
Scheme 1
B2.6.1 Paratrrngstatas As for the molybdates, the protonation constants for WOq- (log K 1= 3.5; log K 2 = 4.6; 0.1 M NaC1, 20 "C) indicate an expansion of coordination number, e.g. to [WO,(H,O)(OH)]-, etc. Potentiometric titration of sodium tungstate solutions (> M) leads to pH inflections at H+/WO$- values of -1.1-1.2 and -1.5. The first corresponds to the formation of 'paratungstates' and the second to the formation of 'metatungstates'. At least two paratungstate species are known. 'Paratungstate A is formed rapidly upon acidification, and, after decades of formulation as '[HW6021]5-', now appears to be a heptatungstate, [W7024]6-, with a structure identical to that of the corresponding molybdate (Figure 6) ,44 although the simultaneous presence of hexatungstate species in these solutions cannot be convincingly ruled out by potentiometric data. Crystals of Na6W70m.21Hz0 are isolated from paratungstate solutions after prolonged boiling and are more soluble than those of 'paratungstate 3', Na10[W12042H2].27H20, the usual product of such solutions. Freshly prepared solutions of paratungstate B hydrolyze slowly to an 'equilibrium' mixture of A and B (equation 4). In dilute solutions ( 1.5) contain a pale yellow species (Amm = 320 nm) that may be isolated as a metastable potassium salt, &W10032(aq). Solutions of this species yeld I),B-[(Hz)W12040]6-or [W7OZ4J6-depending upon pH and ionic strength. The anion is more conveniently studied as a tetraalkylammonium salt in nonaqueous solutions. The structure of [w10032]4in the tri-n-butylammonium salt may be viewed as a
Zsopolyanions and Heteropolyanions
1035
dimer of D4,, symmetry derived from two W6019 units, each of which has lost a WO6 ~ c t a h e d r o n The . ~ ~ anion is electrochemically and photochemica1lylB reducible (see Section 38.5). from controlled The hexatungstate anion, [w6019]2-, first obtained, like hydrolysis of tungstate esters, is formed by acidification of methanolic solutions of sodium tungstate.54 Addition of water to a solution of [w6019]’- in methanol causes its conversion to [W10032]4-.In methano1 solution, [W10032]4-is slowIy (days) converted to [w6019]*-. 38.3 HETEROPOLYANIONS
At present count, some 68 elements (other than Mo and W) have been observed as heteroatoms in polyanions (see Table 5 ) . Since each element may form more than one heteropolyanion (e.g. at least 18 tungstophosphates) and there are polyanions with combinations of several heteroatoms, the total number of known heteropolyanions is extremely large. It is therefore out of the question to attempt a complete accounting of every type of heteropolyanion on the following pages. We shall restrict discussion to the main structural types. More complete reviews are a~ailable.l-~ Table 5 Confirmed Heteroatoms in Heteropolyanions
H Li Be Na Mg K Ca Ti V Cr Mn Fe Co Ni Cu Zn Rb Sr Y Zr Nb Mo Ru Rh Ag Cs Ba La Hf Ta W Re Os Pt Hg
Ce Pr Nd Th
U
B
C
A l S i Ga Ge In Sn TI Pb
Sm Eu Gd Tb Ho Er Np Pu Am Cm Cf
P S As Se Sb Te I Bi
Yb
The heteroatoms may be divided into (i) those (‘primary’) that are essential for the maintenance of the polyanion structure and are therefore not susceptible to chemical exchange, e.g. Co2+ in [ C O W ~ ~ O ~and ~ ] (ii) ~ - ,those (‘secondary’) that in principle and usually in practice may be removed born the polyanion structure without destroying it. Heteropolyanions containing secondary heteroatoms may be viewed as coordination complexes with polyanion ligands, cf. [(H3N)5Cr(OHz)]2+with [(SiW11039)Cr(OHz)]5-.Although there will be occasions in which a clear distinction between primary and secondary heteroatoms may not be possible, in most cases the classification is unambiguous. Heteropolyanions containing secondary heteroatoms will be discussed in Section 38.4.
38.3.1
Four-coordinate Primary Heteroatoms
M3.11 The Keggin structure and derivatives The structure of [PWj2040]3-in the hexahydrate of the free acid was first reported by Keggin in 1933 on the basis of powder data, and has been confirmed by more recent single crystal X-ray and neutron measurements. The same structure has been found in several other heteropolytungstates and molybdates and is shown in Figure 10. Table 6 lists the currently known Keggin anions. The structure has idealized Td point symmetry and comprises four trigonal groups of edge-shared M 0 6 octahedra, each group sharing corners with its neighbors and with the enclosed central tetrahedron. When examined in noncubic space groups, the heteropolymolybdates are found to have lower (T) symmetry as a result of alternating bond lengths for the Mo--O-Mo bridges. In every case (Mo or W) the metal atoms are displaced outwards from the center of the anion toward the unshared oxygen atoms, the displacement conferring approximate Ch local symmetry on each metal. Although the polyhedral representation may suggest otherwise, the Keggin structure, like that of most polyanions, is a close-packed arrangement of oxygen atoms.
1036
isopolyanions and Heteropoiyaniom
a
m
P
Y
8
E
n) and four possible skeletal isomers (B, y, 6, E ) formed by n/3 rotation of one, two, three and four W30,3 groups, respectively. The rotated groups are shown unshaded
e 10 The structures Of [PW,20m]3- (‘Keggin’,
Also shown in Figure 10 are skeletal isomers of the Keggin structure in which one or more of the edge-shared groups of octahedra have been rotated by n/3. The p-isomer structure has been confirmed in [SiW12040]4-and [SiMoWl1OmI4- salts and is slowly converted to the a (Keggin) form in solution, see Section 38.3.1.1.i. The y, 6 and E structures are expected to be less stable on two accounts, an increased lumber of coulombically unfavorable edge-shared contacts between M06 octahedra (M M, 3.4 us. 3.7 8, for corner sharing) and a decreased bridges resulting in decreased dn-pn interaction^.^^ It is of number of quasi-linear M-0-M interest that A13’ which cannot form a dn-pn bond and which has a smaller charge than w6’ adopts the E structure for [Al1304(0H)24(OH2)12]’+. A second form of isomerism occurs with mixed metal polyanions, such as [PW8V40mJ7-,in which the relative positions of the W and V atoms may vary.s6That such positional isomers do exist has been amply demonstrated by NMR and ESR spectro~copy.~’ The numbers of possible isomers increase with the lowering of skeletal symmetry, e.g. five for L Y - [ P W ~ ~ V ~ O ~ ] ~ fourteen for ~ - [ S ~ M O ~ W ~ , , O ~ ~ ~ - . Several deficit or lacunary derivatives of the Keggin structure exist as individual species or as fragments of other heteropolyanion structures. Removal of one M 0 6 octahedron (stoichiometric loss of MO”’) from a-or @-Kegginanions leads to isomers of [XM11039]n-.Removal of three adjacent octahedra leads to A- and B-type X M 9 0 Manions. The structures of these are shown in Figure 11.
-
(i) Silicon and germanium heteroatoms Both a-and fl-[SiW120~]~were first described over a century ago and the conversion of p + a was observed in 1909. The free acids are stable crystalline materials and may be obtained by ion exchange or by extraction as ‘etherates’ from acidified aqueous solution. The
Isopolyanions and Heteropolyanions
1037
Table 6 Keggin Structure Heteropolyanions Reported heteroatoms, Xa
H Be
B A1
V Cr Mnb Fe Co Cu Zn
Anion
Ga
Si Ge
P As (Shy (Bi)C
(Sey (Te)c
Ref. to structure d e
f g
h 1
j
k 1
m n 0
'AI1 tungstates; molybdates known with X = S i , Ge, P, As and V only. Unconfirmed. Heteroatomsin (N-2) oxidation state, no structural investigations. M. Asami,H. Ichida and Y. Sasaki, Acta Crysrdlogr., Sect. C, 1984, 40, 35. eT.J. R. Weakley, J . Chem. SOC. Pak., 1982,4,251. 'T. J. R.Weakley, Acta Crystallogr., Sect. C, 1984, 40, 16. Fuchs, A. Thiele and R. Palm, 2. Naturforsch., Ted 8,1981, 36, 161. H. Ichida, A. Kobayashi and Y. Sasaki, Acta Crystallogr., Sect. B, 1980,36, 1382. ' G. M. Brown, M. R. Noe-Spirlet, W. R. Busing and H. A. Levy, Acta Crystallogr., sect. B, 1977, 33, 1038. H. D'Amour and R. A h a n n , 2. KristaiioRr., K?istailogeom., Kristallphys., pktallchem., 1976,143, 1. K.Nishikawa, A. Kobayashi and Y. Sasaki, Bvlt. Chem. Soc. Jpn., 1975,48, 3152. ' A . RjBrnberg and B. Hedman, Acta Crysrollogr., Sect. B, 1980, 36, 1018. "N. F.Yannoni, Dks. Abstr., 1961, 22, 1032. "A. S. Barrett, D b .Abstr. B, 1972,33, 1475. " R , Strandberg,Acta Crystallogr., Sect. B, 1977,33, 3090.
EJ. '
A-a
U
P2
P3
8- a
A-P
Flgure 11 Lacunary structures, [XM11039](n+4)-and [XI&,0,]["+6)-, derived from a-and /%[XM,,O.,,,Y- anions by removal of one or three adjacent M 0 6 octahedra. No example of the B-B-XW9 structure is currently known
1038
Isopolyanions and Heteropdyaniom
latter behavior is common to all Keggin anions and certain other heteropolyanions.sRIn view of its ease of synthesis and its stability there have been innumerable investigations of CY[SiWlzOM]4-. In solution, the anion behaves hydrodynamically as an (unsolvated) sphere of radius ca. 5.5& according to viscosity and diffusion data. The many spectrosco ic methods applied' include IR and Raman (solution and solid state), electronic, 1 7 0 and '83W NMR, X-ray photoelectron spectroscopy and lS2WMossbauer ~pectroscopy.~~ When solutions of [XW120a]4- (X=Si, Ge) are treated with alkali (pH>5) a complex series of hydrolyses ensues. Based on polarographic studies the series of equilibria and reactions in aqueous solutions of tungstate and silicate (germanate) are summarized in Scheme 2. All species listed have been isolated as salts, although many are metastable in solution, and the underlined s ecies have been structurally characterized by X-ray diffraction. The structure of ~-[SiW,034)]P-is type A (Figure 11).60The anion [SiWloO& has a C, structure based on the y-Keggin isomer (Figure 10) with two edge-shared octahedra missing. The three /3 isomers of [SiWl1039]8- are distinguishable polarographically and the structure of & [ S ~ M O W ~ ~ O ~ ]prepared . ~ H ~ O from the p2 isomer shows the Mo atom in the ring of six octahedra adjacent to the rotated w3013 group (Figure 10). It is concluded that the p1 and p3 isomers have vacant sites as shown in Figure 11. All the lacunary tungstosilicate structures conform to the principle enunciated by Lipscomb that no MOs octahedron in a polyanion has more than two unshared vertices (see Section 38.1).
11
I Scheme 2
The molybdo-silicates and -germanates seem to parallel the tungsto species, but are considerably more labile, and fewer structures have been determined. Like the corresponding tungstate, a - [ S i M 0 ~ ~ 0 ~has ] ~ -been intensively studied. The /3 isomer has recently been shown'" to have the same structure as p-[SiW12040]4-. Isomerization of p+a is rapid in aqueous solution, but may be arrested in nonaqueous solvents. The reverse isomerization (a- p ) occurs when the polyanion is reduced to a heteropoly blue form (see Section 38.5.1.2). The lacunary anions [XMo1103918and ( ? ~ ) - [ X M O ~ O(X ~ ~=]Si, ~ ~Ge) - are quite unstable and difficult to isolate. They react with divalent transition metal cations to yield [ X M O ~ ~ M O ~ ] ~ and other uncharacterized products.61
(ii) Phosphorus and arsenic heteroatoms There are probably more heteropolyanions of phosphorus than of any other heteroatom (Table 7), and most of these polyanions have structures derived from the Keggin anion [PW12040]3-.Scheme 3 outlines some of the major known species. The pattern of reactions is similar to that of the tungstosilicates except that the @ isomers, with the exception of )8-[XW9034]10-,are much less stable and have never been unambiguously identified. The Keggin anion [PW13040]3- has been the subject of innumerable papers and has been thorou hly characterized by X-ray and neutron diffraction, vibrational, electronic and NMR (31P, W and I7O) spectroscopy. The free acid is very soluble in water (cu. 85% by weight) and in numerous polar solvents. The acid is levelled in water and crystals of H3PWI2Om.6H20 contain [PWl2040l3- and [H502]+ions only.62 The trimethyl ester has recently been
'B
Zsopolyanions and Heteropolyanions
1039
Table 7 Heteropolyanions with Phosphorus(V) Heteroatoms
Formula" 1 2 3 4 5
6,7 8,9 10 11 12 13 14
15 16 17 18 19 20,21 22 23,24
25 26
Ref.
S(3'P)b
-14.9 -10.4 -13.3 -10.5
C
c, d c, e f
-
g C
-12.7 (-11.0, -11.6) -9.0, -13.1 -7.1, -13.6 -
-
C
c, h c, h h i
c, h
-7.8 -9.9 -6.6
c, j
k -
1
-
f m n
-2.4 - 4 (-5.2)
n -3.4 (-->
n
+1.8
0
+1
P
Arsenate(V) analogues known for all species except 4 , 5, 13-19,25. Chemical shift (p.p.m. from 85% H,P04). R . Massart, R. Contant, J. M. Fruchart, J. P. Ciabrini and M. Fournier, Inorg. Chem., 1977, 16,2916. J. Fuchs, A. Thick and R. Palm, 2. Naturfofonch.,Teil B, 1981,36, 544. e T. J. R. Weakky, 'Synthesis, Structures and Reactivity of Polyoxoanions', NSF/CNRS yorkshop, St. Lambert-des-Bois, 1983. W.H. Knoth and R. L. Harlow, J. Am. Chem. Soc.,1981,103,1865. J. Fuchs and R. Palm,Z. Noturforsch., Ted B , 1984,39,757. h R. Contant and I. P. Ciabrini, J. Chem. Res. (S), 1982,50. YR. Contant and J . P. Ciabrini, J. Inorg. Nucl. Chem., 1981, 43, 1525. 'M.Alizadeh, S. P. Harmalket, Y.Jeannin, J. Martin-Frkre and M. T. Pope, J. Am. Chem. p c . , 19.35, 107, 2662, R. Cantant and A. Ttzt, Inorg. Chem.. 1985, 24, 4610. IS. Dubois and P. Souchay, Ann. Chim., 1948, 3, 105. m B. M. Gatehouse and A. J. Jozsa, Acta CrystaiIogr.,Sect. C, 1983, 39, 658. "L. P. Kazansky, I. V. Potapova and V. I. Spitsyn, Chem. Uses Molybdenum, Proc. Int. Conf., 3rd., 1979,67. OD. E.Katsoulis, A. N. Lambrianidou and M. T.Pope,Znorg. Chim. act^, 1980,46, L55. R. Kato, A. Kobayashi and Y.Sasaki, Inorg. Chem., 1982,21,240.
a
H + , (WO,l'-
d OH-
X = P , AS
Scheme 3
re ~0r ted.l~' Solutions of the 12-acid are rapidly converted to [PW11039]7-at pH > ca. 1.5; the latter anion appears to be stable between pH2 and 6. The corresponding AsWIz and AsWll anions are much less stable and must be synthesized and studied in aqueous organic solvents. The [XzW190ss]'4- and [X2W210,1(H20)2]6- species have no tungstosilicate analogues. Salts of the 2 :21 anion were first described in 1892, but its structure has only recently been determined.
Isopolyanions and Heteropolyanwns
1040
The anion has almost D3h symmetry with two A-a-PW9034 units (see Figure 11) that sandwich two WOs(H20) octahedra and a WOs square pyramid, i.e. the equatorial plane contains two radial H20-W=0 groups and one W=O group. The structure of [P2W190a]14- is currently unknown, but it appears to be a lacunary derivative of P2WZ1for it reacts with several divalent metal ions to give P2WI9M2species (see Section 38.4.2.2). When solutions of sodium tungstate are boiled with a large excess of phosphoric acid (P :W 4 : 1) other tungstophosphates are produced. The main species present in such solutions appear to be LY and /3 isomers of [P2Wl,0,2]6- and the cryptate [NaPsW300110]14-(originally formulated as a P3W18species). The structure of ( Y - [ P ~ W ~ ~ (the O ~ ~'Dawson' ]~anion) is shown in Figure 12 and is seen to be a fused dimer of A-a-PW90Mmoieties. Rotation of one edge-shared w 3 0 1 3 'cap' of this structure by n/3 leads to the structure.a Other possible isomers, not yet known for the phosphates, but identified via IS3W NMR for are y (both caps rotated) and 6 (DM-symmetrized fusion of PW, moieties). The y and B isomers spontaneously isomerize to the (Y form in aqueous solution.
-
Figure 12 The structure of a-[P,W,,0,,]6- ('Dawson') anion
Several lacunary derivatives of the Dawson structure can be synthesized by controlled alkalinization of aqueous solutions, as shown in Scheme 4.
T
i T OH-
*
'
H*.[W0.d2-
Scheme 4
The LYI- and ( U ~ - [ P Z W ~ , Oisomers ~ ~ ] ~ ~have - been shown by Is3W NMR and IR spectroscopy to contain tungsten vacancies in the equatorial ring and terminal cap sites respectively. Numerous metal-ion-substituted derivatives of these species are known (Section 38.4.2). The structures of the P2Wls and P2WI2anions are not known with certainty, although from NMR the two phosphorus atoms are equivalent in P2WI2.A likely structure for the latter corresponds to loss of six W06 octahedra from one long side of the Dawson anion (a 'peeled banana'); the
Isopolyanions and HeieropoEyanions
1041
anion [P8W480184]4u-which can be isolated from solutions of P2WI2consists of a cyclic tetramer of just such units.65The PEW48 anion is one of an increasing number of polyanions shown to act as cryptands for alkali and alkaline earth cations (Section 38.4.2.3). Although formally a cryptate, the sodium ion in [NaP5W300110]14is very tightly bound and is not removed when the free acid H14[NaP5W300110] is prepared by ion exchange. The heteropolyanion is one of those rare molecules with fulI five-fold symmetry, and may be viewed as an assembly of 10 corner-shared w 3 0 1 3 units forming the faces of a pentagonal bipyramid (cf. the same units in the Keggin and Dawson structures that form tetrahedral and octahedral assemblies respectively). The presence of the sodium ion in a cavity on the Cs axis of the polyanion reduces the overall symmetry from D5*to Cs,. The anion has been characterized by Vibrational, electronic and NMR spectroscopy, and is stable in solution to pH 11 (cf. pH 1.5 for a-PW12and pH ca. 6 for a-PzWls).66 Although molybdophosphates and molybdoarsenates corresponding to [PW120rnJ3-and [PZw18062]6- are well known ([PMol2OmI3- was probably prepared by Berzelius in 1826) these are usually not the most predominant species in aqueous solutions (see Section 38.3.1.2). The Keggin anions [ X M O ~ ~ O are ~ ] ~best - studied in aqueous organic solvents to retard hydrolytic degradation. Both a and /3 isomers are known, the latter being stabilized in the reduced state (Section 38.5.1.2). The Dawson anions are more stable in aqueous solution and are formed via XMo9 species (equation 5). Salts of [PM%031(OHz)3]3-and the corresponding (Figure ~~ arsenate have been isolated. The anion structure is a distorted form of A - [ Y - X W ~ O 11) in which three of the six (nominally equivalent) terminal oxygen atoms have been converted into weakly bound water molecules (Mo-€)H2, 2.23 A). The locations of the water molecules and the resulting Mo-0-Mo bond alternation confers a pronounced chirality upon the overall structure, and the chirality is retained in the resulting [X2Mo18062]4-anions (4v& D3,, for P2W18)." Numerous examples of 'mixed metal' heteropolyanions (Mo + W, V + W, V -t Mo) exist with apparent Keggin or Dawson structures, although crystallographic investigation has been limited to salts containing a-[V5W80W]7-, [ Y - [ V ~ M O ~ ~ O and ~ ] ~PI[SiWllMoOW]4-. In each case the high symmetry of the [Y or /3 Keggin framework results in crystallographic disorder. Further complications arise with the vanadium-containing anions which are mixtures of isomers (Section 38.3.1.1). 2[XM0J&i(OH&i13-
-
[ x ~ M ~ i s O a+ ] ~6HzO -
(5)
A modification of the Keggin structure is seen in the heteropolyvanadates, [XV14042]9(X = P, As). In these anions two extra V03+ groups are attached to opposite sides of the Keggin core forming a cluster of reduced charge (see Figure 13).48
Figure 13 The structure of [PV,,O,]g-
anion
(iii) Other Keggin species Several other heteropolytungstates are known or presumed to have the Keggin structure (Table 6). Two 12-tungstoborates have been known for over a century, but these prove not to be simple a or /3 isomers. The anion a-[BWI2OWl5-seems to be an undoubted Keggin species based on isomorphism, vibrational spectrum and its behavior upon reduction. The second isomer ('h'), crystallizing as the acid in hexagonal crystals, clearly is not a /3 isomer for its
Isopolyanions and Heteropolyanions
1042
polarographic and spectroscopic properties69are quite unlike those of all other P-XWl2 species. A crystal structure determination shows h to be HzlB3W390132.126 No molybdoborates are known. Of the remaining anions listed in Table 6 the transition-metal-centered species are noteworthy. The cobalt(II1) anion was the first example of a tetrahedral (high spin) d6 ~ornplex.~' Its reduction to [CouW 0 6- E = +1.00 V us. nhe) renders it an attractive outer sphere oxidant?l The correspondi; &I1 Jnion does not undergo reduction to an Fe" species but directly to a heteropoly blue (FelllWv,'c?,see Section 38.5), but [ C U W ~ ~ Ois~reducible ]~to the brown Cu' analogue (E = +0.09 V). The Mn- and Cr-centered anions have been little studied and are difficult to repare. The 12-tungstates of Sd', TeIV, Sbln and Bi'" are unlikely to have an undistorted Keggin structure in view of the unshared pair of electrons on the heteroatoms. No detailed structural studies have been made. There are however several characterized heteropolytungstates of As"' and Sb'" in which the lone pair is stereochemically active. These are summarized in Table 8. Most of these contain B-a-XW9033 units (see Figure ll), linked by extra W 0 6 or W 0 5 polyhedra. See Figure 14 for an example. Table 8 Heteropolyanions Containing Trivalent Arsenic and Antimony Ref
Anion
Tourn.6, A. Revel, G. Tournd and M. Vendrell, C. R . Hebd. d . Sa'.,Ser. C . , 1973, Z T I , 643. M. Leyrie, J. Martin-Frtre and G. Hervi, C.R. Hebd. Seances
a C.
geattces A
!"d. So.., Ser. C., 1974, 279, 895. Y. Jeannin and J . Martin-FrPre. J .
Am. Chem. Soc., 1981, 103,
1664.
dY.Jeannin and J. Martin-Frtre, Inorg. Chem., 1979, IS, 3010. 'F. Robert, M. Leyne, G. Hew&,A. TCzC and Y. Jeannin, Inorg. Chem., 1980, 19, 1746. J. Martin-FrPre and Y. Jeannin, Znorg. Chem., 1984, 23, 3394. 'J. Fischer, L. Ricard and R. Weiss, J . Am. Chem. SOC., 1976,98, 2050.
M. Michelon, G. Hewe and M. Leyrie, J . Inorg. Nucl. Chem.,
1980,42, 1583.
383.1.2 Non-Keggin metallophosphates and metallomenates
Potentiometric and 31PNMR investigations show that in aqueous solutions of molybdate and phosphate the major polyanion solute species are pale yellow PMo9 species and colorless [HnPzMo50~3](6-n)anions.71The structure of the latter anion (Figure 15) has been determined in several crystalline salts and in solution and has a Cz structure that leaves a vertex of each PO4 tetrahedron un~hared.~'The anion is quite labile and possibly fluxional (see Section 38.6.1). Protonation (pK values 5.10, 3.65 in 3.OM NaC104, 2 5 T ) occurs on the terminal oxygens of the phosphate groups. Analogous anions are formed by phosphonates (RPO;-) ,73 phosphate monoester^,^^ sulfites75 and selenites. The corresponding tungstophosphate [PzW50u]6- has been reported but it appears not to be a major solute species in aqueous solution. Aqueous solutions of molybdate and arsenate, pH 3-5, contain [ A S M O ~ O ~ ~ ( H ~and O )an ~]~As2M06anion.71The exact structure of the latter ion in solution is in some doubt, although the tetramethylammonium sodium salt contains the anion shown in Figure 15. Low angle X-ray scattering measurements of aqueous molybdoarsenate solutions are not compatible with this structure and the anion may be hydrated in a fashion similar to that observed for the analogous
Isopolyanions and Heteropolyanions
Figure 14 The structure of [As,W,,O,(H,O)]~-
1043
anion. The water molecule is indicated by the open circle
molybdoarsonates (see Section 38.6.2). The DXhstructure for [ASZMO&~]~-shown in Figure 15 is also found for WY-[MO&IZ~]~(see Section 38.2.5.2), [(MeAs)~Mo&4]~- (Section 38.6.2) and [VzMo60z6]6-.Several other molybdoarsenates are formed in acidic solutions. One long-known species is [ H + ~ S ~ M O ~which ~ O ~has ~ ] an ~ -arrangement of Moo6 octahedra like an inverted Keggin anion, a central cavity and external HOAs03 tripod groups.7BAnalogous organoarsonate derivatives are described in Section 38.6.2.
(a)
Figure 15 The structures of (a) [P,Mo,0,16-
(bl and (b) [As,Mo6O,l6- anions
38.3.2 Si-coordinate Primary Heteroatoms
38.3.21 The Andemon and related structures In 1936 Anderson proposed that several 1:6 heteropolyanions and the isopolyanion [Mo7OZ4l6-had the planar (4d) structure shown in Figure 16. The structure was verified for [TeMo6OZ4l6-in 1948, but heptamolybdate was shown to have the isomeric (Czv)form shown in Figure 6. Both D3d and Chr structures confer local C, symmetry on the molybdenum or tungsten atoms; each MOs octahedron has two (cis) terminal oxygens. The currently known anions are listed in Table 9 and include a periodato complex in which the ring of external octahedra comprises alternate IO6 and C O O ~ ( O Hunits. ~ ) ~ Protonation of the anions (see Crm, Ni" and Pt'" complexes in Table 9) is believed to take place on the oxygen atoms directly attached to the heteroatom. In most cases, attempts to neutralize the protons result in hydrolytic decomposition of the heteropolyanion, Isomerism has been observed for crystalline protonated Pt'"Mo6 anions (see Table 9). Some of the anions have been shown to be labile by isotope exchange. Thus Mo exchange between [ M O , O ~ ] ~and - [ € & , X M O ~ Oat~ ]pH ~ -2.5 and 0 "Chas ti = 35 min for X = CP1 and tt ,L(H2O)]N03.H2O (M = La-Pr) or [M(NO3)L(H20)2]z(NO3)(C104)3*4H20 (M = Nd-Er). The structures of the eleven-coordinated Ce and ten-coordinated Nd compounds were established by X-ray.495Lanthanide complexes of the macrocycle (22) which, though it contains six nitrogen atoms, is sterically only capable of four-coordination, have been also obtained.496
b
(21)
(22)
(23)
A most interesting functionalized four-nitrogen crown ligand (23; H4dota) has been synthesized.497The X-ray structure of the complex Na[Eu(dota)(H20)] (Figure 7) shows that the nine-coordinated geometry is approximately capped square antiprismatic with the four N formin a basal square, four carboxylate 0 forming an upper square, and the H 2 0 capping the latter.40 This geometry leads to very large aramagnetic NMR shifts, and the complex acts as an effective shift reagent for H 2 0 and CI-.4
r
Scandium, Yttrium and rhe Lanthanides
1097
Figure 7 Structure of the anion of Na[Eu(dota)] (H,dota is 23) (reproduced with permission from ref. 498. Copyright, American Chemical Society, 1984)
39.2 7.4 Mixed nitrogen and oxygen macrocycles There have been one or two reports of lanthanide complexes of this type of ligand. Thus three furan Schiff bases (24; R = (CH42, (CH& or CH2CHMe) have been obtained as their complexes with the series La-Nd. These slowly dissociate in water,5wThe functionalized crown ligand 1,10-diaza-4,7,13,16-tetraoxacyclooctadecane-~,~‘-diacetic acid (dacda) is 18-crown-6 with NCH2C02H units replacing two opposite oxygen atoms. It is characterized by very high association constants for lanthanide ions, determined by potentiometric titration. Thus log K values are: La, 12.21; Gd, 11.93; Lu, 10.84. Values for Group I and I1 ions are much lower.5o1
39.2.7.5 Cryptates
Lanthanides form stable complexes with cryptands typified by N(CH2CH20CH2CH20CHZCH&N (this molecule is 2,2,2-cryptate and similar ligands are named analogously). In such complexes the lanthanide ion lies completely inside the cryptate ligand which, in the example given, fills eight positions on the lanthanide coordination sphere. Depending on the relative size of the lanthanide ion and the cryptate ligand, other donors may also coordinate in positions between two -CH2CH20CH2CH20CH2CH, strands of the cryptate. The complexes MC13(2,2,1-cryptate), where M = L a , Pr, Eu, Gd or Yb, may be prepared from the ligand and MC13 in anhydrous organic solvents, and together with La(NO&(2,2,2cryptate) were the subject of the first report of this type of lanthanide complex. Cyclic voltammetry showed that the redox potentials of the europium and ytterbium complexes of 2,2,1-cryptate and 2,2,2-cryptate were significantly affected in water by complexation. Thus the redox potential decreased from 626 mV for the aqueous Eu3+ ion to 205 mV for the Eu3+ complex with 2,22-~ryptate.~~’ Using polarography, one-electron reduction kinetics for [Eu(2,2,1-cryptate)J3+and the corresponding 2,2,2 complex with Eu2+ or V+ were investigated, to ether with the oxidation kinetics of [Eu(2,2,1-cryptate)12+ when oxidized by [CO(NH$]~+. The rate constant for electron exchange was estimated to increase over the Euil/Eu,,+ rate by a factor of lo7 upon encapsulation by 2,2,1-cryptate. The results indicate
1098
Scandium, Yttrium and the Lanthanides
that the low reaction rates observed for the Eu~:/Eu:: exchange are not due to adiabaticity, but probably due to large Franck-Condon barriers.503 These complexes, unlike the crown ether complexes but similar to the ma-crown and phthalocyanine complexes, are fairly stable in water. Their dissociation kinetics have been studied and not surprisingly they showed marked acid c a t a l y s i ~ Association .~~ constant values A study in dimethyl sulfoxide solution by for lanthanide cryptates have been determined.505,506 visible spectroscopy using murexide as a lanthanide indicator showed that there was little lanthanide specificity (but surprisingly the K values for Y b are higher than those of the other lanthanides). The values are set out in Table 9.'07
Table 9
Log Association Constant Values for Lanthanide Cryptates in DMSO5"
Cryptate
Pr Nd Gd
Ho Yh
2,2,2
2,2,1
2,1,1
3.22 3.26 3.45 3.47 4.11
3.47 3.01 3.26 3.11 4.00
3.86 3.97 3.87 3.80 4.43
Other methods of preparation of lanthanide cryptates have been rep~rted,~"including the functionalized cryptate (25) whose La, Pr and Eu nitrate complexes were obtained. Paramagnetic shifts over a range of about 40 p.p.m. showed complexation with the lanthanide enclosed in the ligand cavity. Preparation was in dry acetonitrile to which triethylorthoformate was added, permitting the use of hydrated lanthanide nitrate^.^^ The preparation of an organo derivative LaC1,(CPh3)(2,2,2-cryptatej has also been r e p ~ r t e d . ~The " difficulty in straightforward preparation of lanthanide cryptates probably arises from an initial tendency to form insoluble 'external' (i.e. not enclosed) complexes and from the formation of hydroxyl ion if any water is present. Cryptates of Eu2' may be prepared in aqueous solution, however, and then converted into the corresponding trivalent complex by electrochemical oxidation.503
A number of X-ray crystal determinations have made the principles of lanthanide cryptate (Figure S ) , the La3* structural chemistry fairly clear. In [La(N0,),(2,2,2-~ryptate)][La(NO~)~] ion is 12-coordinated with two bidentate nitrate ions coordinating in two of the three spaces between the cryptate chains; the third space is thus too compressed to be occupied also.508 [Sm(N03)(2,2,2-cryptate)~[Sm(N03)5(H20)] shows only one such space occupied511 and the is similar to the samarium ~ r y p t a t e . ~ " . ~ ' ~ structure of [E~(Cl0~)2,2,Zcryptate](ClO~)~.MeCN Internuclear distances in these complexes are shown in Table 10. Interesting conformation information has been given by a 'H NMR study of the series of complexes MX3(2,2,1-cryptate), where X=C1, NO3 or C104 and M=La-Yb. A plot of paramagnetic shift against Bleaney's anisotropy constant gives a good straight line, which is evidence that all these lanthanide cryptate species are isostructural in D20. Furthermore, a single conformation is either dynamically favoured or stable, because the same AMXY pattern for all four NCH2CH20CH2CH20CH2CH2N protons is observed.514
Scandium, Yttrium and the Lanthanides
1099
O=L~@ m = o=O N= C Figure 8 Structure of the cation in [La(N03),(2,2,2-cryptate)][La(N0,),1 (reproduced with permission from ref. 508)
Table 10 Lanthanide-Ligand Distances in Cryptate Complexes
[La(N0-J2(2,2,2-cqptatc)~+ [Sm(N0,)(2,2,2-cryptate)l [Eu(C1O4)(2,2,2-cryptate)]'+
12 10 10
2.64-2.74 2.441-2.566 2.44-2.52
2.81-2.85 2.748-2.779 2.64-2.70
2.63-2.69 2.484,2.402 2.67,2.71
508 511 512,513
39.2.8 Complex Halides
Phase diagrams have been given for the NaF-MF3 systems, where M = La-Lm5I5 In all these systems, hexagonal phases NaMF4 exist below 700 "C. Above this temperature, for M = Pr-Lu, these phases become converted into disordered fluorite-like cubic phases of variable composition. Particularly for the heavier lanthanides, orthohombic phases Na5M9F32also exist at lower temperatures, Na5L~9F32 being stable from 778 "C to room temperature; other Na5M9F32are not stable below 450°C.The system NaF-YbF, has also been investigated516 by DTA and X-ray powder methods; NaYbF4 was again found, together with a fluorite phase 36 which is somewhat different from the N%.38Yb0.62F2.24 and another phase N%.32Yb0.68F2 Na5M9F32 previously reported.515 The structure of NaMF, (M=La-Tm or Y) in the lower-temperature hexagonal phase contains two t es of M3+, both nine-coordinated to tricapped trigonal prisms, with Nd-F = 2.3'77-2.5033 in the Nd3+ c0mpound.5'~ The high temperature fluorite-like phase of course shows cubic eight-coordination.518The 1: 1 stoichiometry M'MF4 has also been observed for M' = Li, M = Eu-Lu or Y and these compounds have the CaW04 In the case of the larger alkali metal cations, the 1:I complexes M'MF4 (where M' = K, M = La or Ce; M' = Rb, M = La-Pr) can be obtained, but the predominant stoichiometry is M$MF6, examples of which are K3YF6, RbMF6 (M = La-Pr, Sm, Tb, Er or Y ) , and Cs3MF6 (M = La-Pr, Sm, Gd, Dy-Er or Y), the X-ray parameters of which have been reported.5zu521 The chloride and bromide complexes show four structure types, M'M2X7, M;M2Xg, M4MX5 and M&& (M' = alkali metal, M = lanthanide or Y).'" They may be conveniently prepared by evaporation of aqueous acidic solutions of the appropriate metal ratios, followed by heating in a stream of HCl or HBr gas. The most stable phases are generally M ' W , but are considerably subject to phase transitions at fairly low temperatures so that single crystals for X-ray study are difficult to obtain and structures are dependent on powder data. Thus &Luc16 crystallizes in the K3MoC16 structures23 containing individual LuCIi- octahedra. The compounds (NH&MC16 (M = Sm-Lu or Y) and (NH,),MCl, (M = La-Tb) have been obtained as two isostructural series.s24*525 The compounds K3MC16 (M = Nd-Lu and Y) and K2MC15 (M = Nd-Tb) have also been characterized by powder diffractions26following phase studies.'"
1loo
Scandium, Yttrium and the Lanthanides
Synthesis was by heating above 450°C in evacuated quartz vessels of the stoichiometric components. The isostructural K2MCI, series underwent a phase transition in the range 350-410 "C, and became monohydrated in air. The KzMC15 series was also isostructural. The complexes M ; W , where M' = Ph3PH or C5H5NH and X = C1 with M = Pr-Sm or Dy-Tm, or X = Br with M = Pr-Eu, have been prepared from acetonitrile solution and their electronic spectra have been studied. This is of importance because there are limited numbers of lanthanide complexes where the metal ion is octahedrally coordinated to identical ligands in separated octahedra. As expected, in the absence of unsymmetrical components of the crystal field, these complexes show much weaker oscillator strengths than other lanthanide complexes. The hexaiodo complexes [Ph,PH][MI,], where M = La-Yb, and the corresponding pyridinium salts, have been prepared, working rapidly on the small scale, by action of liquid HI on the solid [MCl6I3- compound. These compounds are moisture sensitive and dissociate even in non-aqueous solvents in the presence of excess iodide ion. The series of halo complexes have been used to obtain values of optical electronegativities from the charge-transfer spectra. The values of dp, the nephelauxetic constant, from the f-f transitions, are similar for all three halide^.^^^,^^^
The structure of Cs2DyC15 has been determined from single crystal data.530It contains angle DyClZ- octahedra connected by cis corners to form chains, the connecting Dy-C1-Dy being 180". The heavier lanthanides and yttrium are isostructural. All the compounds of the type M;M2C19 which have been obtained (M' = K, M = Ce-Nd; M' = Cs, M = Ho-Lu or Y) contain paired octahedra M&$- joined at one face; they have one or other of the Cs3Cr2Cl9 or Cs3T12C19structures.531These structures are also adopted by a number of bromides or iodides, namely Cs3Y219and Cs3M2Br9(M = Sm-Lu or Y). Single crystal data are available for Cs3Y219 which has the dimensions Y-1-Y =80.8", YZ(termina1) = 2.90 8, and Y-I(bridging) = 3.13 A.532 In the case of the stoichiometries M'MzX7, structures tend not towards more highly condensed MX, octahedra but towards seven-coordinated metal ions, as in KM2C17(M = GdTm or Y), where monocapped trigonal prisms are interconnected to form layers, and in M'M2C17 (M' = Rb, M = Sm-Yb or Y; M' = Cs, M = La or Pr) where the lanthanide ions are seven- and eight-~oordinated.~~'
39.2.9
NMR Spectra of Paramagnetic Lanthanide Complexes
39.2.9.1 Introductory summary
In 1969, Hinckliy reported533the extraordinary effect produced by [ E ~ ( d p m ) ~ ( p yupon ) ~ ] the NMR spectrum of cholesterol in non-polar organic solvents. The spectrum was greatly shifted to low field, and in general the resonances of those protons nearer to the hydroxyl group were shifted more. This meant that firstly, accidental degeneracies in spectra of large organic molecules may often be removed because of differential shifts. Secondly, assignments of peaks may be confirmed, where there might otherwise be doubt, on the basis of their geometric position relative to the lanthanide ion. Thirdly and perhaps most important, structural information concerning the organic molecule may be obtained by means of a knowledge of the relative positions of the protons (or other nuclei) obtained through their differential shifts. It soon became clear that the P-diketonates themselves rather than their pyridine adducts should be and that the process depended on the formation of an adduct M(P-diketonate)sL,, where L is the molecule under investigation, usually termed the substrate, which is often an alcohol, amine or ketone. Furthermore, the use of different lanthanides produced shifts in either direction and of various magnitudes.535Another variable is of course the nature of the @-diketone. This must be sufficiently bulky to prevent polymerization by intermolecular coordination, but not so very bulky that an adduct with the substrate cannot be formed. The NMR shift is produced by an interaction between the resonating nucleus and the magnetic field produced by the unpaired f electrons of the lanthanide, This interaction may occur in a through-space dipole-dipole manner, or by delocalization of the f-electron density towards the resonating nucleus through the bonds of the substrate. Usually the dipole-dipole effect predominates. Much of the research on the mode of action of lanthanide shift reagents (LSRs) has been directed towards study of the following: the stoicheometry and geometry of the LSR-substrate adduct; the stability constant(s) relevant to this association; and the quantification of the two shift mechanisms in terms of molecular properties and the
Scandium, Yttrium and the Lanthanides
1101
establishment of the relative importance of the two mechanisms. There is of course much literature concerned with the application of LSRs to the solution of individual problems in organic chemistry, but this will not be dealt with here in detail. There are a number of reviews on the principles and applications of LSRsS3c540and on the important uses of lanthanides as NMR probes in biological It should be noted that adducts of LSRs of the tris(P-diketonate) type constitute only a special case of paramagnetic shifts of NMR resonances of lanthanide complexes. All the nucIei of any lanthanide complex (except La, Gd, Lu) will in principle show a paramagnetic shift unless the complex is cubic, and some of the most interesting work has in fact been carried out on widely differing types of lanthanide complexes, particularly complexes of uncertain structure formed in solution, including aqueous solution, by interaction of lanthanide salts with various ligands.sa
39.2.9.2 The magnetic interaction between lanthanik ions and coordinated ligands The most important magnetic interaction between lanthanide ions and nuclei which are components of coordinated molecules is probably the through-space dipole-dipole interaction, usually termed the dipolar, or in earlier work the pseudocontact, interaction. The reason for this statement is that the dipolar shift is related to the geometry of the adduct, obeying equation (9) .545 Here, I-, B and @ are the polar coordinates of the particular nucleus under study, relative to the principal magnetic axes of the molecule (whose origin of coordinates is the lanthanide ion). A is a constant which is different for each lanthanide and depends on the lanthanide electronic configuration; and FF are crystal field coefficients which depend on the nature and geometry of the ligands. AH,/H = { K , ( 3 cos’ 8 - 1) + K , sin2 8 cos2+ } / r 3 , where IC,, K2 are functions of A ( F r )
(9)
The values of A have been tabulated.545In the cases of Eu3+ and Sm3+,there are substantial magnetic contributions from excited states. Eu3+ with a ground state of 7F0,which is diamagnetic, depends totally upon the 7F1,2,3 excited states, which are thermally accessible, for its magnetic properties. From the equation it follows that for one complex of any individual lanthanide, the relative dipolar shifts of all the nuclei are determined by their geometric relationship in terms of B, @ and r. Also for an isostructurai series of complexes of different lanthanides, the shifts of analogous nuclei are related only by the values of A, assuming the crystal field parameters are not appreciably different. Since each nucleus has three parameters, 0, $I and r, but yields only one experimental datum, AH, it is not of course possible to determine the overall structure of the complex ab initio. However, uncertainty between alternative structures of substrates is capable of being resolved, as is uncertainty in alternative assignments of the NMR resonances, or uncertainty in the orientation or position of coordination of substrates of known structure. A computer optimization of agreement between observed shifts and coordination orientation (or whatever other uncertain features are to be investigated) is the normal procedure, as first demonstrated fox: the [Pr(dpm)3(borneol)] The mechanism of this type of paramagnetic shift can be understood in a purely qualitative way by considering a lanthanide complex tumbling in the magnetic field of an NMR spectrometer. This field causes the magnetic dipole of the lanthanide ion on average to provide a component opposed to the field. The magnetic field due to the lanthanide ion, as experienced by a hydrogen nucleus which is part of the complex, will be in opposite directions depending on whether the nucleus, during molecular tumbling, is on the axis of, or at right angles to, the magnetic dipole of the lanthanide ion. For isotropic complexes, it can be shown that tumbling over all orientations averages the resultant shift to zero (except for the Evans shift547on all nuclei). However, for anisotropic complexes the magnitudes of the lanthanide magnetic dipole will be different in the two situations mentioned and the resultant time-averaged field is non-zero and will provide a paramagnetic shift. It has been stated above that for a series of isostructural lanthanide complexes the paramagnetic shifts of analogous nuclei should be proportional to the parameter A, if a purely dipolar mechanism operates. The paramagnetic shift is usually taken as the difference between the chemical shift of the nucleus in the paramagnetic complex under investigation, and the
1102
Scandium, Yttrium and the Lanthanides
corresponding shift of the diamagnetic lanthanum complex (or the mean value of the lanthanum and lutetium shifts). In many series of complexes it is found that this simple relationship does not apply, certainly to all nuclei, even if the series is isostructural. Furthermore, it is almost aiways found that the geometrical relationship of the Bleaney equation is not valid for all nuclei when high levels of accuracy are concerned and in many cases it is not even approximately valid, especially for nuclei other than hydrogen or those connected to the lanthanide ion through a small number of bonds. This situation is caused by the presence of contact shifts arising from the through-bond delocalization of unpaired electron density associated with the lanthanide ion, which then directly influences the nucleus concerned. Equation (10) expresses the contact where a is the hyperfine coupling constant, while S, is the spin expectation value of the particular lanthanide. Equation (11) then expresses the total paramagnetic shift.
It is clearly necessary to estimate the relative contributions of the contact and dipole mechanisms to the overall shift in order to make use of the structural information inherent in the dipolar shifts. This may be done in different ways.54s551Firstly, the shift of any gadolinium complex is entirely contact, as the ' S, ground state of Gd3+ is necessarily isotropic. Hence a, the hyperfine coupling constant, may be found and if it is assumed constant for the series of lanthanides, then the contact component for each lanthanide complex may be found from the appropriate value of S,. Unfortunately, the very long relaxation time for Gd3+ confers considerable broadening on the NMR spectra of its complexes which usually prevents resonances from being observed. An alternative method compares the observed shifts of analogous nuclei in two complexes, in which case a is assumed constant, A and S, are known for each ion, and hence a can be calculated and AH, and AHd found. If a is constant over a series of several lanthanides, then computer optimization of such a series of complexes would be preferable. If this type of treatment, of which there are a number of variations,552gives good quantitative agreement with the observed data for a series of lanthanide complexes, then they may normally be assumed isostructural. Furthermore, the values of 8, $J and r from which follow the value of AHd for the various different nuclei can be computer optimized for consistency of the shifts with hypothetical molecular models which are structurally acceptable. The dipolar paramagnetic shifts depend, as bas been stated, on unsymmetrical terms in the crystal field expansion. An unsymmetrical crystal field can be caused by unsymmetrically disposed ligands of similar type, or by the presence in the same complex of ligands which exert dissimilar crystal fields although the coordination polyhedron may be fairly symmetrical. The practical effect of the unsymmetrical crystal field is to confer anisotropy on the magnetic susceptibility tensor by perturbing the f electron shell. The intensity of magnetization of an individual lanthanide ion thus varies with the orientation of the complex, of which it forms part, relative to the prevailing magnetic field. BleaneyS4' has given the relationship between the magnetic anisotropy and the resulting paramagnetic shift. It is as shown in equation (12), where x ~ ,are~ the , ~principal components of the molecular magnetic susceptibility tensor and 2 is their mean. AHJH = ( ( 3 c o g 2 B
- l)(xr- 2) + sin213cos 2$(xx - x,)}/2Nr3
(12)
Hence, if the magnetic anisotropy of the crystalline complex is measured experimentally, usually by single crystal measurements using the torsion fibre method, and if its crystal structure is known, then the molecular anisotropy usually can be calculated. Provided that the complex has the same conformation in solution as in the solid state, the solution dipolar shifts can be calculated and compared with the experimental data. This procedure and variations of it may be used to investigate the relative crystal fields of different li ands, and to test experimentally the validity of the theoretical approach employed.553354 A d'irect NMR measurement of anisotropic magnetic susceptibilities can be obtained from the 'H spectra of lanthanide complexes of known structure. The method depends on quadrupole coupling of the 'H nuclei, which splits absorption lines at high field owing to partial ordering of the molecules in the field.555
Scandium, Yttrium and the Lanthanides
1103
The question of the effect of molecular symmetry on dipolar shifts may be briefly discussed next. If a complex has a principal axis of symmetry of order three or more, that is, it is uniaxial, then xx is equal to xy and a simplified geometric factor (3 sin2 6 - l)F3is valid. Although some lanthanide complexes have such symmetry in the solid state, most do not and are said to be biaxial. However, if an individual ligand undergoes NMR-rapid rotation about a single bond relative to the remainder of the complex, then an averaging effect is naturally observed for the nuclei of that ligand even though the complex instantaneously may have even C , ~ y m m e t r y . This ~ ~ ~situation , ~ ~ ~ is common in solutions of lanthanide complexes and the simplified geometric factor is often capable of fitting the experimental data. There is some evidence that while mono adducts of the @-diketonate shift reagents may show time-averaged uniaxial symmetry, bis adducts often do not. Thus the spectra of [M(dpm&], where M = Pr, Sm,Dy, Ho, Er or Yb and L = 3-picoline or 3,5-lutidine, have been interpreted in terms of biaxial magnetic anisotropy. One magnetic axis is defined by the molecular Czaxis present in the X-ray crystal structure and the other two axes lie in similar orientations in all the adducts. A variable-temperature study of this system shows restricted rotation of the aromatic rings with a very solvent-dependent conformational equilibrium resulting from this .558-560
39.2.9.3 Nuclear relaation rates A further, most important, feature which does not produce a paramagnetic shift but which falls naturally into place for discussion at this point is the lanthanide contribution to the nuclear relaxation. This is a dipole-dipole interaction and hence follows an r p 4 law. The longitudinal relaxation rate of a nucleus of a diamagnetic atom near a paramagnetic lanthanide ion (except Gd3+) is given by equation (13), where l / G = longitudinal relaxation rate of the nucieus, y1 = magnetogyric ratio, z, = electron s in relaxation time, p = magnetic moment, = Bohr magneton and I = ion-nucleus distance.p61
The effect of this equation is that the ratios of the paramagnetic contributions to the relaxation rates of a set of, for example, 'H nuclei belonging to the same complex are determined by the lanthanide 'H distance only. Hence a similar type of computer optimization as for the paramagnetic shifts may be used to test compatibility with hypothetical structures which incorporate distance-dependent parameters such as the orientation or conformation of an organic ligand. A further application of relaxation rate measurements is that similar UTl ratios in a series of lanthanide complexes may be taken to indicate an isostructural series. However, this approach has the limitation that if only part of the complex is studied, perhaps an organic ligand, its T, ratios would be independent of changes, for example changes in the extent of hydration in the remainder of the complex, provided that the conformation of the ligand relative to the lanthanide ion were preserved. An excellent example of the use of TI data in a quite different way is its use to determine hydration numbers of lanthanide dipicolinate
39.2.9.4 Examples of applications of shqt or relaxation measurements A good example of an investigation into the conformation of coordination of an organic ligand is the work of Singh, Reynolds and Sherry563on lanthanide L-proline complexation. The 'H and 13C paramagnetic shifts and relaxation rates were measured for all H and C atoms of aqueous proline-lanthanide ion solutions at p H 3 for ten lanthanides. It was concluded that while the relaxation data were consistent with an isostructural series, there were inconsistencies in the dipolar shift data, after separation from contact shifts, which could be resolved either by division into two, rather than one, isostructural series, or by invoking biaxial rather than uniaxial symmetry. Adenosine-2'-monophosphate and adenosine-3'-monophosphate have been investigated
1104
Scandium, Yttrium and the Lanthanides
in aqueous solution and were indicated by shift and relaxation data to be in conformational equilibrium when bound to lanthanide ions. The lanthanides bind to bidentate phosphate groups.564 Adenosine-5'-monophosphate has also been examined similarly. It was studied in D20/(CD3)2S0 in the presence of lanthanide ions under various conditions of solvent composition and temperature. Shifts and relaxation times for the series Pr-Yb gave results more nearly consistent with axial symmetry for the first half of the lanthanide series than for the second.565The extraction of structural information from NMR data has been described for aqueous systems where lanthanide edta and related complexes interact with cytidine-5'monophosphate, ( - )-alanine566and endo -cis-bicyclo[2.2,1]hept-5-ene-2,3-dicarboxylic acid. 567 The shift reagents [M(fad),] where M = Pr, Eu, Gd, Ho, Er or Yb have been used to elucidate the solution structure of nicotine N-methiodide, I3C and 'H spectra being studied. T h e experimental data indicated effective axial symmetry, good agreement with theory being obtained. Calculation of the relative magnitudes of A H . and AH, showed that the dipolar mechanism p r e d ~ m i n a t e d .Of ~ ~course, many other examples could be given of studies of adducts of LSRs and organic substrates but these fall within the purview of organic chemistry rather than the present discussion. There has been some work on interactions between LSRs and transition metal complexes, both NMR shifts and relaxation rates being studied. Presumably the mode of interaction is by means of bridges formed by the donor atoms of one complex which bond in a labile manner to the metal ion of the second complex. Interactions examined include those between [Eu(fodwhere a 1: 1 add,,)3] and azido{N,N'-ethylenebis(acetylacetoneiminato)}~ynd~e~b~t(III), duct with a lifetime of 8*4X loL4s was formed in CDC13;'* [Gd(fod),] and N,N'-ethylenebis(acetylacetoniminato)nickel(II)57"~571 or N,N'-vinylenebis(acetylacetoniminato)nickel(II); [ P r ( f ~ d )and ~ ] [Pt(acac),];'" and [M(fod),] and cobalt(II1) tris-f?-diketonates (M = L ~ - E u ) . ' ~ ~ Other more specialized interactions of P-diketonate lanthanide shift reagents are those with alkenes, which require the mediation of a silver complex.574Thus [M(fod),], where M = Pr or Yb, together with Ag(fod) will shift the resonances of alkenes, aromatic molecules or phosphines, which are ligands which will not coordinate directly to lanthanide pdiketonates.575,576 The shifted spectrum resulting from the interaction between a platinum alkene complex and [Eu(hfc),] has also been studied (Hhfc = 3-heptafluoropropylhydroxymethylene-( -t )-camphor). In this case the alkene complex is prochiral and the shift reagent is of course optically active.577Optically active shift reagents such as E ~ ( h f c have ) ~ often been used to demonstrate quantitatively the presence of chiral isomers of organic substrates; two sets of resonances are ~ b t a i n e d . ~ ' ~Chiral . ~ ' ~ interactions in aqueous solution are also possible,580 and various types of such interaction, leading to spectral resolution of substrates or to assignment of chirality, have been Paramagnetic shifts may be induced in cations which associate with suitable lanthanide complexes. Thus in aqueous solution, [M(hpda),I6-, where M = Dy or Tm and H3hpda = 4hydroxypyridine-2,6-dicarboxylicacid, associates with 25Mg2+,39K+, 23Na+, 87Rb+ or 14NI& and produces shifts of up to 25 p.p.m. which are most marked above pH 7.582A similar effect is shown for cations (and anions and uncharged molecules) by lanthanide tetra-p-sulfonatophenylporphyrins in aqueous solutions.583This is discussed in Section 39.2.6.3, as are the large shifts produced in aqueous solution by Na[Eu(dota)(H20)] (H4 dota = Most examples of the application of paramagnetic lanthanide NMR spectra quoted here have so far been based on transient association in solution between a preexisting lanthanide species and a substrate, often organic. However, the shifted NMR spectra of stable isolable lanthanide complexes have been studied since it was established5= that in contrast to many paramagnetic complexes of the d transition metals, lanthanide complexes in general gave fair1 sharp spectra. The kinetics of ligand exchange processes were early studied by these and in more recent work, the conformations of stable isolable lanthanide complexes and their interconformational or dissociative equilibria have often been usefully investigated. Thus the paramagnetically shifted spectra of [M(dipi~)~]~[ -, M ( d i p i ~ ) ~ ( H ~ O and ) ~ ] - [M(di~ic)(H~O)~]+, where M = Ce-Yb and H2dipic = pyridine-2,6-dicarboxylicacid, have been shown to be consistent with an approximately tricapped trigonal prismatic configuration of all three types of complex ,587 similar to the X-ray crystal structure of the Yb tris complex.58gThe lanthanide diethyldithiophosphonates [M{S2P(OEt),},]- which are dodecahedral in the solid state, divide into two series in CD2C12 solution, the heavier lanthanides having square antiprismatic geometry .589 Other examples, drawn from the NMR spectra of macrocyclic complexes are given in Section 39.2.7.
Scandium, Yttrium und the Lanthanides
1I05
39.2.9.5 Studies of complwation with lanthanide shift reagents
Owing to their special utility in structure elucidation of organic molecules in non-hydroxylic solvents, the lanthanide tris(B-diketonate) complexes have undergone much investigation into their complexation with lone pair donors, mainly with the objectives of determination of association constant values and determining whether 1:1 or 2 : 1 adducts are formed. Many of these papers naturally rely on NMR measurements though other methods have been used. The complexes mainly investigated are M(RCOCHCOR')3 where M = Eu, Pr or Yb. These ions are used because they give fairly large shifts but without too much broadening. The /3-diketones are usually R = R' = CMe3 (dpm) or R = CMe3, R' = n-C3H7(fod). This topic is discussed in more detail in Section 39.2.4.2.
39.2.10 Electronic Spectra of Lanthanide Complexes This area has been the subject of recent extensive reviews ,590-59z which deal particularly with the physical and theoretical aspects. In this section, a more chemical and descriptive approach will be adopted. The tripositive lanthanide ions have the general outer configuration 4f "5s2p6and follow the Russell-Saunders L, S,J coupling scheme but with a certain amount of configuration interaction. The 5s and 5p shells have a radial dispersion which is greater than the 4f electronss93and hence shield the latter from the effect of coordinated ligands to a very large extent, but not completely. Thus the electronic spectra of tripositive lanthanide compounds can be considered as derived from the spectra of the gaseous ions by a fairly small perturbation. This perturbation may be of two general types. The first is a general nephelauxetic effect owing to a drift of ligand or 5s, 5 p electron density into the lanthanide ion, slightly expanding the 4fshell and reducing the 4f-4finteractions. The second is a crystal field splitting effect, which removes the degeneracy of the free-ion levels, and which can be represented with fair success by a point negative charge model having the appropriate molecular symmetry. The experimental energy level schemes of the gaseous free tripositive ions are obtainable with some difficulty but are available in some cases, e.g. praseodymium594or gadolinium.595 Values for Pr3+ (gaseous) versus PrC13 (solid) are shown in Table 11, and differ by about 2-5%, those for PrC13 being the smaller, as expected. Most of the basic experimental data regarding the energy levels of lanthanides must therefore be derived from the solid state, in Table 11 Positions (cm-') of Energy Levels of Pr3+ (Gaseous, Obtained from Arc Spectra) Compared with PrCI, ( ~ r y s t alline )' ~ ~
~~
'H4 3H5 E(crys!al) E(vapbur)
0 0
'H6
,F2
,F3
,F4
'G4
'4
,Po
3P1
116
'p2
2117.4 4306.3 4846.6 6232.3 6681.7 %W.6 16639.3 20383.4 20984.9 21324.5 22 139.1 2152.2 4389.1 4996.7 6415.4 6854.9 9921.4 17334.5 21390.1 22007.6 22211.6 23160.9
which the ion under study is often present in dilute solid solution in a lanthanum salt matrix. Of course, except for the ground state multiplet, identifiable from Hund's rules, the observed levels cannot be definitely assigned to particular (zs+l)LJ terms unless a theoretically derived set of energy levels is available for compdrison. Considerable effort has been devoted to theoretical prediction of the energy levels off" configurations. The total energy H of a system consisting of a point nucleus surrounded by N electrons can be represented by the Hamiltonian in equation (14), where Ho is the kinetic energy and nuclear interaction term, HE is the electronic repulsion term, and HSOis the spin-orbit interaction term.
Model Hartree-Fock calculations which include only the electrostatic interaction in terms of the Slater integrals Fb, F2, F4 and F6, and the spin-orbit interaction f , result in differences between calculated and experimentally observed levels5% which can be more than 500 cmfl even for the f Z ion Pr3+. However, inclusion of configuration interaction terms, either two-particle or three-particle, considerably improves the c ~ r r e l a t i o n s . In ~ ~this * ~ ~way, ~ an ion such as Nd3+ can be described in terms of 18 parameters (including crystal field
1106
Scandium, Yttrium and the Lanthanides
parameters) which reproduced 101 of the energy levels observed for Nd3+ in L ac4 with an rms deviation from experimental values of 8 cm-' .J99 Although the parameters can be evaluated theoretically, it is usual to obtain them from a best fit of the observed spectra. The energy levels for all the tripositive lanthanide ions in LaC13 solid solution have been given.600 Using modern computers it is possible to diagonalize a complete free-ion and crystal-field Hamiltonian. However, for the lanthanides, if not for the actinides, the effect of the crystal field can usually be treated as a perturbation on the unsplit =+lLJ levels. The levels are split into 2J + 1 components in a crystal field of C1 symmetry, or into fewer components in higher symmetries, depending on the degeneracy of the J vector re resentation in the crystal field point group. Typical values of the crystal field parameters B f i are 200-200Ocm-l. Owing to the small effect of the ligands on the 4flevels, the 4f+4fspectra of lanthanide complexes tend to have narrow bands that are also weak owing to the transitions being Laporte forbidden. The intensities of the transitions have been the subject of much study and are usually quantified by the Judd-Ofelt theory. Juddml and Ofelt602used the odd parity terms of the ligand field to mix electronic states of opposite parity, including states derived from d configurations, into the f" configurations, thus obtaining non-zero matrix elements for the electric dipole operator. The Judd-Ofelt theory also accommodates the existence of the hypersensitive transitions. These are transitions that are dramatically increased in intensity in complexes, particularly those of low symmetry and with polarizable ligands, compared with for example the hydrated ions. These transitions correspond to those having large values of Judds second rank tensor operator U"),and mostly have AI = f 2 . These selection rules correspond with those for a radiative electric quadrupolar process. It is, however, fairly certain that these hypersensitive transitions are electric dipolar in the main. Their mechanism can be described in two ph sically related ways, the inhomogeneous dielectric theory and the ligand polarization m0del?~7~'" The dipolar components of the radiation field induce a set of transient electric dipoles in the ligand environment which can couple with the 4f electrons by way of 4f electrostatic quadrupolar-ligand dipolar interactions. In a non-centrosymmetric complex, or in a centrosymmetric complex subject to an ungerade vibrational mode, large amplifications of 4f +4f transitions may be obtained from these quadrupole-induced dipolar interactions. These transitions are thus often referred to as pseudoquadrupolar. For a short readable discussion of this still controversial area, see Judd@ .*' In a recent paper,- the hypersensitive transitions 4Z15n4 2H11/2 or 4G1112 of Er3+ have been examined for complexes with five amino- or oxo-carboxylate anions. They showed four- to six-fold increase in oscillator strengths over the hydrated ion E?' aq. Using likely models for the structures of the complexes and calculating crystal field wave functions and energy levels for the 4f" configuration of Er3+, electric and magnetic dipole transition strengths were calculated. The structural models were based on X-ray solid state determinations and charge and polarizability parameters were assigned to the ligand molecules. The calculated magnetic dipole intensitities were at least two orders of magnitude less than the electric dipolar; the latter were very successful in accounting quantitatively for the observed intensitities. In a related paper, the oscillator strengths of Nd, Ho and Er acetates and propionates in aqueous solution have been related to solution structures and hypersensitivity mechanisms discussed."' In practice, the hypersensitive transitions are often used for the determination of stability constants in aqueous solution. Lanthanide absorption bands in solution do not normally change in position on complexation to such an extent that bands due to the complexed and uncomplexed ion can be clearly observed independently, as is often the case for d transition metal ions, but the marked change of intensity of the hypersensitive bands is sufficient to allow determination of K values, for example as demonstrated for various adducts of [Ho(dpm)3].611 There has been a marked increase recently in the interest shown in fluorescence, phosphorescence or emission spectra. There is little useful distinction between these terms when applied to lanthanide ions, and the term emission will be preferred here. The two ions mainly studied are Tb3+ and, especially, Eu3+. The energy level schemes for Eu3+ and Tb3+are given in Figure 9. In both these ions there is a large energy gap between the 7F and 'D levels, which corresponds with emission in the red and the green regions respectively of the visible spectrum. The main emitting state in each case is the lowest component of the excited multiplet, as higher components rapidly descend by a cascade process to 'Do or 5D4 respectively. In the case of E d + , emission corresponding to 'D04 Fo,1,2,3can be observed with 5D0-+ 7F1or 7F2 the strongest. This emission is useful in two respects. Firstly, for Eu3+,the crystal field splitting of the 'F1 and 7F2 levels can be observed and can be related to the symmetry of the complex if that is
Scaadium, Yttrium land the Lonthanides
'H4
0 L2&-
ce3+
1107
-'6 Fo pr3+ Eu3+ Tb3+ 7
-
Figure 9 Energy levels of some representative lanthanide tripositive ions. Ce and Pr, full set; Eu and Tb,partial set (energy values taken from S. Hiifner, 'Optical Spectra of Transparent Rare Earth Compounds', Academic Press, 1978)
~ n k n o w n . ~The ~ * J* values ~ ~ ~ for both the excited and ground states of Tb3+ are too large for this effect to be so useful for that ion, as it is difficult to assign the multiply split transitions. Most of these transitions could in principle be observed in absorption but are too weak to be easily studied. Secondly, the emission of a particular complex species is characteristic of that species and can be used to identify the species present. Particularly is this so if excited state lifetimes are measured, as these vary dramatically depending on the number of OH groups coordinated to the Eu3+ or Tb3+ ion. This is because multiple excitation of the OH stretching mode provides an alternative deexcitation route. Measurement of lifetimes thus can be used to determine the number of coordinated water molecules.218 Excitation of the Eu3+ or Tb3+ ions has traditionally been indirect, by broad-band UV excitation of a conjugated organic ligand which is followed by intramolecular energy transfer to the lanthanide ion f system, folowed in turn by f+f emission.614However, more recently, following the advent of tunable dye lasers, direct excitation of an excited f" level is in many cases preferable. By scanning this frequency, an excitation spectrum can be obtained whose energy values are independent of the resolution of a monochromator and not subject to spectral interferences. Excitation spectra have been of considerable use recently in studying both hydration numbers (by lifetime measurements) and inner-sphere complexation by anions (by observing appearance of the characteristic frequencies for e.g. the Eu3+ 5D0+7F0transition for the different possible species). Thus using a pulsed dye laser source, it was possible to demonstrate the occurrence of inner sphere complexes of Eu3+ with SCN-, C1- or NO; in aqueous solution, the K values being 5.96 f 2,0.13 f 0.01 and 1.41 f 0.2 respectively. The ClOi ion did not coordinate. Excited state lifetimes suggest the nitrate species is [EU(NO~)(H~O)~.~+~.~]~+ the technique here is io compare the lifetimes of the H20 and the corresponding DzO species, where the vibrational deactivation pathway is virtually inoperative.219The reduction in lifetime is proportional to the number of water molecules complexed .217*218 COC3-JJ
1108
Scandium, Yttrium and the Lanthanides
A study of hydration of the complex [ E U L ( H ~ O ) ~ ] ( Bwhere F ~ ) ~ L = { (C3H7)2N.CO-CHzOCH(Me)-}2 shows by lifetime studies of a similar sort that in aqueous acetone, x = 0.9 f 0.4 which gives a coordination number of nine. This complex was prepared from Eu(M~CN),(BF~)~, itself prepared from the action of NOBF4 on europium metal in acetonitrile. ‘15 The emission spectra of the complexes Rz[Eu(N03)5],where R = Ph3EtP+ or Ph4As+, have been investigated and it was found that the spectrum of the former could be interpreted in terms of D3hpseudosymmetry (each NO3 ion considered as occupying one ligand osition in the coordination sphere) but the position as regards the Ph,As+ salt was less c1earJ6 In a similar study, using lifetime measurements, the species Tb(N03),L3, where L = DMSO or H20, were identified in MeCN solution. Also, efficiencies were determined of energy transfer from excited Tb3+ ions in Tb(C10& or Tb(N03)3to Nd3+ ions in MeCN and other more polar organic The 4F3,2+ 4&,z emission of Nd3+ at 885 nm can also be monitored in a rather similar way, and a study of emission lifetimes together with absorption spectra for the hypersensitive Nd3+ transitions, e.g. 4Z9n+ 4G5i2,has been made in order to investigate the solvation of neodymium nitrate and perchlorate in MeCN, Me,?CO, DMSO or DMF. Excited lifetimes varied from 300 ns in MezCO to 2300 ns in CD3CN. It was found that species {Nd(N03),}(3-n)+ were formed, where n = 1-5, on addition of NO: ions to perchlorate solutions, but that addition of DMSO to Nd(ClO& in MeCN gave [Nd(DMS0),I3+, where n = 9.7 f 0.8.618 A further variation on the theme of emission is circularly polarized emission, where chiral interactions, for example between a lanthanide complex and a chiral ligand in solution, can be studied. Selection rules have been given619based on S, L and J values for 4f states perturbed by spin-orbit coupling and 4f electron-crystal field interactions, and four types of transition were predicted to be highly active chiroptically. These are given in Table 12. TabIe 12 Active Chiroptical Transitions in Lanthanide Complexes
~~~~
~
a b
0 0
0 0
C
>O
30
d
>O
SO
O,l(J+O+J’) 1(J orJ‘ = 0) 0, I ( J # O + J ’ ) I(JorJ’=O)
(From ref. 619; a = type 1, h = type 2, c = typc 9, d = type
q.
* AL # 6 .
The Pfeiffer effect, the outer-sphere interaction of a chiral substrate with a rapidly interconverting racemic solution of a chiral lanthanide complex, can be investigated by measurement of the luminescence dissymmetry factor (the ratio of circularly polarized luminescence to total luminescence) for Eu3+ or Tb3+ complexes. Thus the racemic D3 chiral complexes [ M ( d ~ a ) ~ ] where ~ + , M = Eu or Tb, interact in an outer-sphere manner with the following optically active species: cationic chiral transition metal complexes,6zoascorbic acid,621 aminocarboxylates ,622-423 tartrates 1624 a m i n e P and phenols.626Association constants can be obtained from limiting values of the dissymmetry factors. In some cases, inner-sphere complexation626 can be demonstrated, as judged by changes in the general nature of the circularly polarized luminescence spectrum and pH irreversibility of the complexation. The interaction of aspartic acid and other ligands with complexes of Tb3+ with edta and related ligands has also been studied and association constants determined.”’ The complex formation between Tb3+ or Eu3+ and (R)-( - )-172-propanediaminetetraaceticacid or (R,R)trans-l,2-cyclohexanediaminetetraacetic acid has been similarly investigated. The pH dependence of the circularly polarized and total luminescence shows a drastic configurational change of the chelate system at pH 10.5-11, corresponding, it is believed, to formation of hydroxide complexes.628The technique of magnetic-field-induced circularly polarized emission has been introduced for lanthanide the mechanisms of lanthanide transition intensities are also discussed in the paper. A useful short general review630 of the utility of Eu3+ and Tb3+emission spectra in investigations of biological systems gives also an account of the relevant basic spectroscopy of the lanthanide ions. The importance of these techniques for exploring proteins and nucleotides
Scandium, Yttrium and the Lanthanides
1109
is that Eu3+ is easily bound by such molecules, and that it readily substitutes Ca2* naturally present. The effects of coordinated ligands upon the fluorescence spectrum can then be used for structural exploration. An example is the investigation of complexes of Eu3+ or Tb3+with the ionophores lasalocid A and A23187 in MeOH. These are polyfunctional compounds containing keto, ether, carboxylate or amine groups. Mono and bis species were observed, and the high K values showed multidentate complexing. Suggestions as to the nature of this could be made.631 The bulk magnetic susceptibilities of lanthanide complexes are accurately predicted by theory, as the crystal field splitting of the ground state has little effect on these. The effective magnetic moment p(ef€) is thus given by equation (15) where gJis the Land6 factor. Values of these parameters to ether with a typical set of room temperature experimental values, for [M(N03)3(phen)2].15B are given in Table 13. In the case of Sm3+ and particularly Ed', the excited states 6 H , ~(Sm) and ' 4 2 (Eu) are thermally populated at room temperature and lead to higher observed values of p(eff) than those calculated from the ground term only. y,, = g,VJ(J
+ I),
where gJ = {S(S + 1)- L ( L + I) + V ( J
+ I)}/u(J + 1)
(15)
Table 13 Ground Terms and Magnetic Moments of the Lanthanide Ions
M"+ Ground term gJ
+
" ,. , 1) n,dJlJ Peff (=PIb ' a
La 'So
0 0 0
Ce
Pr
Nd
2F5n 3Hq 419n 617 2.54
2.46
4J5 8/11 3.58 3.62 3.48 3.44
Pm
'14 315
Sm
Eu
Gd
6Hsrz 'FO
217 2.83 0.84" 0' - 1.64 3.36
Tb
Dy
Ho
'F6 hH15,2 'I8 2 7.94
312 9.7 7.97 9.81
413 10.6 10.6
514 10.6 10.7
Er
Tm
Yb
4115n
3H6
'F,,
615 9.6 9.46
716
817 4.5 4.47
7.6 7.51
Lu
'So 0 0 0
Non-groundstate terms contribute to the observed mahnetic moments. Values are those found for M(NO,),(phen), at 20°C.'
The crystal field does, however, have a dramatic effect on the magnetic anisotropy of lanthanide complexes. For complexes of less than cubic symmetry the three principal values of the susceptibility tensor are unequal. For uniaxial symmetry, xx = xy # x Z and for biaxial symmetry xx # xy # xz. Very extensive have been carried out on the single crystal susceptibilities of the Gdlanthanide hexakis(antipyrene) triiodides over the temperature range 80-300 K, and crystal field parameters were obtained. This crystal field-induced anisotropy is responsible for the effectiveness of lanthanide complexes as NMR shift reagents, and sin le crystal anisotropies of lanthanide complexes have been determined in this connection also.5 8
39.2.11 Lanthanides m the Dipositive Oxidation State
Although all the lanthanides are stable in the solid state as M2+ ions doped into CaF2 crystals, only in the cases of europium, ytterbium and samarium is there any real coordination chemistry, and that is very limited. There is a small but developing organometallic chemistry of the lower oxidation states,"' but that is not within the scope of the present review. Much of the chemistry of the dipositive state depends on solvated speciesH2 and it is convenient to begin with these.
39.2.11.1 Hydrated species The hydrated ions of europium, ytterbium or samarium can be obtained by electrolfic reduction or by dissolution of MC12 in water, and are reasonably stable. Colourless or pale yellow-green Eu2+(a ) is stable for more than ten days in the absence of oxidizing a ents or of platinum catalysts.64 Pale een Yb2+(aq) decays in water with k = 2.4 x lO-'s-', and red Sm2+(aq) has k = 0.6 ,X'-s'-OI both reactions being first order.646These rates of decomposition follow the sequence of standard electrode potentials which are listed together with other parameters of the w + ( a q ) ions in Table 14. Of course, as not all the M2+(aq) ions exist, many of these parameters cannot be determined directly. Pulse radiolysis has been used to demonstrate the short-lived existence of red Tm2+(aq), E?(aq) and surprisingly Gd2+(aq).648 Electrochemical reduction has been used to investigate stabilities and reduction potentials of
9
Scandium, Yttrium and the Lanthanides
1110
M2+ ion^.^^,^" Thus polarography and cyclic voltammetry of 10-4-10-3 M Sm" or Gd3+ in 0.001 M H2SO4 showed, for the cyclic voltammetry, a two step reduction in each case, which was reversible for samarium with fast scan rates (50 V s-') but which showed no anodic waves for gadolinium. The reduction for the latter was interpreted as being similar to dysprosium651 where there is a two-stage reduction of water via lanthanide hydroxy species, no M2+ species being involved (equations 16 and 17). 2M3++ 2H20 + 2e-
--
2M(OH)'+
M(OH)2+-t2H20 + 2e-
+ H,
M(OH),
(16)
+ Hz
(17)
Table 14 Thermodynamic and Electrochemical Parameters for Hydrated Dipositive Lanthanide Ions
La ~~~~
Pr
Ce ~~
~~
Pm
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
YD
~
K3(ev>"*645 19.2 20.1 21.6 22.1 - 23.7 24.9 20.8 21.9 22.9 22.8 22.7 23.8 25.r -AGO M2+aq)b7646 121 268 385 406 427 515 540 184 322 418 406 381 460 54 - A G ~ ~ & V ~ ~ + ~ @ ~ - 1347 1309 1317 1346 1344 1361 1550 1463 1437 1433 1452 1451 146 -Eo(M (aq)/M2+(aq)d,646 3.8 3.5 3.0 2.8 2.5 1.5 0.35 3.6 3.5 2.6 2.9 3.0 2.1 1. Third ionization potential. Standard free energy of formationof the hydrated dispositive ion of 4p configuration(La,Ce and Gd, whose hydrated dipositive iuns woi kc 4f"-'5d1, are uncorrected). Standard free energy of hydration of the gaseous Mz.'ions. Standard electrode potential relative to NHE. a
There has been some exploration of the mechanism of reduction of d transition metal complexes by M2+(aq) (M=Eu, Yb, Sm). Both inner- and outer-sphere mechanisms are believed to o erate. Thus the ready reduction of [Co(en),]'+ by Eu2+(aq) is necessarily outer-sphere.6 However, the strong rate dependence on the nature of X when [c0(NH3)5x]2+ or [Cr(H20)5X]2+ ( X = F , C1, Br or I) are reduced by Eu2+(aq) possibly suggests an inner-sphere m e c h a n i ~ mThe . ~ ~more vigorous reducing agent Yb2+ reacts with [ C O ( N H ~ ) ~ ] ~ + and [ C ~ ( e n ) ~ ]by~ +an outer-sphere route but with [Cr(H20)5X]Z+ (X = halide) by the inner-sphere m e c h a n i ~ r n .Outer-sphere ~~ redox reactions are catalyzed by electron-transfer catalysts such as derivatives of isonicotinic acid, one of the most efficient of which is N-phenyl-methylisonicotinate, as the free radical intermediate does not suffer attenuation through disproportionation. Using this catalyst, the outer-sphere reaction between Eu2+(aq) and [ C O ( ~ ~ ) ( N H ~proceeds ) ~ ] ~ + as in reactions (18) and (19). Values found were k l = 5.8 x 102M-' s-' and k2/kl= 16.655
9
Eu2++ cat
kl
e
cat-
+ Eu3'
k-1
cat-
+ c03+ A
cat
+ co2+
(19)
The crown ether 18-crown-6 has been shown by polarography to stabilize aqueous solutions of dipositive europium and samarium relative to the tipositive state.656
39.2.11.2
Other solvated species
Solutions of dipositive lanthanide cations have been obtained in liquid ammonia, ethanol, THF, acetonitrile and hexamethylphosphoramide. Ions stabilized, or for which there is evidence for stabilization, include Nd2+, Dy2+ and Tm2+ as well as Eu2+,Yb2+ and Sm2+. The non-hydroxylic solvents are best at stabilizing M2+ ions, Thus NdC12(THF)2has been reported from the reduction of NdC13 in THF by Na(naphthalene),657 and corresponding reductions of MC13 (M = Eu, Yb or Sm) have also been achieved.658Solutions of solvated MI2 (M = Sm or Yb) in THF may easily be made by the quantitative reaction of the metal with 1,2diiodoethane, producing ethane. The solid THF adducts may be isolated.6s9 In hexamethylphosphoramide , sodium reduction of lanthanide trihalides gives solutions of Eu2+ (pale yellow), Yb2' (yellow), Sm2+ (red-violet) and Tm2' (red-brown). The latter has a half-life at 4.5 hours. The solution of SmC12 tends to be more stable (half-life, 60 hours) than those of SmBrz (30 hours) or Sm12 (25hours).660,661In liquid ammonia, Eu2+ or Yb2+ may be
Scandium, Yttrium and the Lanthanides
1111
obtained by dissolution of the metal, but sodium reduction of Sm(C10& must be used to prepare solutions of Srn2'.662
39.2.113 Complexes with nitrogen donors Some interesting examples of this class of complexes have recently emerged. Most of them are based on the bis(trimethylsily1)amido ligand. Silylamides of two types have been prepared by the action of EuI2 or YbIz on NaN(SiMe& (NaL) in diethyl ether or glyme (reactions 20 and 21). Using Yb12, reaction (21) gave YbL(Et,O), when the product was recrystallized from ether, but N a Y b S upon recrystallization from toluene. A single crystal X-ray of N a M b showed a planar three-coordinated configuration (26) with Eu-N = 2.448 A, Yb-N = 2,38 A, Eu-N' = 2.554 A, Yb-N' = 2.45 A, Eu-N" = 2.539 A, Yb-N = 2.47 A. a = 94.3" (Eu) or 97.4' (Yb). The metal-carbon distances are rather short at E 4 = 3.21 8, average and may indicate some interaction.6a
EuI,
+ 3NaL
Et20
NaEuL, + 2NaI
(26)
The X-ray structure of [EuL(glyme),] has also been determined.- The six-coordination polyhedron has only Czsymmetry, doubtless relating to the presence of two very bulky ligands = 134.5'. Bond lengths are: and two bidentate ones with rather short 'bites', giving N-Eu-N Eu-N = 2.530, Eu-0 = 2.638,2.756 A. Magnetic susceptibility measurements show the Curie law obeyed, with p = 8.43 BM. Other compounds isolated- were Eub(bipy), and E U ~ ( T H F )These ~ . were yellow or orange and soluble in hydrocarbons; doubtless they are sixand four-coordinated monomers. Complexes of EuClz with 1,lO-phenanthroline and 2,2' :6',2"-terpyridyl have been obtained from acetonitrile. They are purple EuClz(phen)2 and dark blue EuC12(terpy), both air-sensitive solids. In the NMR spectrum of the latter in Me2S0 solution, ligand peaks were broadened but not shifted, as expected for the f configuration. In Me2S0 solution, only 1:1 association occurred with the phenanthroline complex, giving an association constant value log K = 3.9. Chloride-bridged associated structures are likely for these complexes in the solid state.= I
39.2.114 Complexes with phosphorus donors The compound [Yb{N(SiMe3)2}z(Me2PCH2CH2PMe2)] is a well-established member of this rare group. It is made by the action of diphosphine on YbL(Et20)2 and the structure has been determined. It is monomeric and four-coordinated with N - Y b N = 123.6", P - Y b P = 68.4" and N-Yb-P = 101.2'. Bond lengths are Yb-P 3.012 A, Yb-N = 2.331 A, and Yb-C (silylamide) shows a probable weak interaction at 3.04 A. Also obtained similarly were Mb{PBug}2 where M = Eu (orange) or Yb (brown-red).= Phosphines also complex with divalent lanthanide organo compound^.^^
-
39.2.115 Complexes with oxygen donors A number of complexes in which ethers and silylamido ligands'shared the coordination sphere were mentioned in the preceding two sections, and there are also several examples of ethers participating in the coordination spheres of dipositive lanthanide organo compounds. 667 There seem to be no examples of well-defined complexes with coordination spheres composed completely of oxygen donors, although M2+ in solutions of water or OP(NMe2)3 must contain these. Also, though not strictly complexes, MC03, where M = Sm, Eu or Yb, adopt the
1112
Scandium, Yttrium and the Lanthanides
aragonite structure with nine-oxygen coordination.668An interesting organo complex with oxygen coordination is ((GMe5)2Yb}2Fe3(CO)ll where each (CsMe&Yb group is also coordinated by the oxygen atoms of two carbonyl groups (the carbon atoms of which bridge two iron atoms). The 3%-0 distance is quite short at 2.243 A, showin a strong interaction. The compound is made by treating (C5Me5)2Yb(Et20)2with Fe3(CO)12.6%
39.2116 Electronic spectra of dipositive lanthanides AI1 the lanthanides can be obtained as M2+ ions in dilute solid solution in the CaF2 lattice by means of either y-irradiation or by reduction by metal vapour. These materials have been instrumental in providing good spectra of the dipositive lanthanide ions670in an electrostatic field provided by the cube of fluoride ions which of course surround the M2+ions in this structure. Stabilization of dipositive lanthanides in SrF2 or BaF2 matrices is also possible. Perovskite phases such as yellow CsEuF3, bright green CsYbF3 and orange RbYbF3 can also be made by reduction of MF3 with Cs or Rb;671these ternary halides have been reviewed.6n The spectra of dipositive lanthanides differ from those of tripositive ions (except Ce") in that the excited 4f"-'5d1 configuration lies at a level up to about 40000cm-1 and dominates the spectrum with more intense absorption than is the case for the tripositive ions. The single d electron is subject to crystal field splitting of 104cm-' order of magnitude and will, in a cubic field, give d, and d, levels. This contrasts with the 4p-I configuration where the crystal field parameters are small compared with the spin-orbit coupling. There are therefore two states to be considered separately, the Russell-Saunders coupled 4Y-I shell and the crystal field affected dl state. These in turn will interact and the details of the interaction will depend on the relative energies of the 'LJ split levels of the 4F-l configuration. An approximate energy diagram for the E?' ion in CaFz is shown in Figure 10 and the experimental values for the parameters A, Eg and Efd are listed in Table 15. The La2' and Gd2+ ions have dl and f7d' ground states respectively. It will be noticed that the values of A decrease towards the centre of each half series La-Gd and Gd-Yb. This is because there is little or no f-d interaction for f o , f' or f14 ( S states) but in other cases increasing f-d coupling broadens the levels so that A as measured is no longer a sim le crystal field parameter but also depends on the nature and extent of the f -d interaction.973 - 30
- 25 - 20
- A -15
c 2 P 2
)I
-IO
1: Figure 10 Energy levels of E?' in CaF, (adapted from ref. 673).
Table 15 Energies (103 m-') Associated with Dipositive Lanthanide Ions in CaF,
La
Ce
PT
Nd
~
EidQ
Ed A"
Pm ~
-3.4 24
25
15
'As shown in Figure 10.
10.9 4.8 11.5
Sm
Eu
Gd
Tb
Dy
Ho
ET
Tm
Yb
37.7
-3.3
25.2
17.6 4.0 9.3
26.5
37.3
5.0
6.5
24.3
17.8 3.8 11.1
19.3
8.5 14.5
.
13.6
25.6
14.2 9.8
7.7 16.1
17
14.2
15
16.3
Scandium, Yttrium and the Lanthanides
1113
The spectra of a number of dipositive lanthanide species have been investigated, often in solution. The bands are fairly broad, owing to the effect of varying crystal fields on the 5d electron. Spectra have been measured in water, ethanol, acetonitrile, THF or HMPA and values of A and E f d derived. In THF, the maximum observed value of A was 14 600 cm-' for Tm2+ 661,674 The absorption spectra of EuCl phenanthroline)]+ and [EuCl(terpyridyl)]+ in dimethyl sulfoxide have been reported.&! The(absorption spectrum of the Eu2+-2,2,2-cryptate complex has been measured, together with its emission spectrum at 77 K.675Eu2+-crown ether complexes when suitably excited give a strong blue emission at -440nm.676 When excited at 365 nm, Sm2+stabilized in ternary fluorides gives a deep red to orange fluorescence believed to be due to a 'D--i, 7Ftransition within the f 6 configuration.6"
39.2.U
Complexes of Lanthanides in the Tetrapositive Oxidation State
39.212.1 Introduction This is a somewhat restricted area in two respects. Firstly, only cerium has a tetrapositive oxidation state which is sufficiently stable to coexist with organic ligands. Although praseodymium and terbium give one or two binary salts and simple halide complexes, that is the limit of their achievement as tetravalent complex formers, and the remaining elements either form no tetrapositive compounds at all, or are restricted to fluoro complexes only. Secondly, the oxidizing nature of even cerium as Ce4+ is normally sufficiently great to limit its complexes to those with ligands which are not too easily oxidized. Some data relevant to the behaviour of tetrapositive lanthanide ions are given in Table 16. The fourth ionization potential (I4) values are derived spectroscopically, and the standard reduction potential (E*) values are also derived spectroscopically, specifically from positions of charge-transfer and f + d bands particularly in hexahalo complexes [MX6l3-. It will be noticed that the correlation between these values and the stabilities of various classes of tetrapositive compounds is very good. Tabie 16 Data Concerning the 4+ State in Lanthanides Lanthanide Known compounds 14
-E(M3'(aq)/M"'(aq))
La A
-
Ce
Pr
Nd
Prn
Srn
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
D C B A A A A C B A A A A A 36.76 38.98 40.41 41.09 41.37 42.65 44.01 39.79 41.47 42.48 42.65 42.69 43.74 45.19 1.8 3.4 4.6 4.9 5.2 6.4 7.9 3.3 5.0 6.2 6.1 6.1 7.1 8.5 (1.74) (3.2) (5.0) (3.1) (5.2)
A , no compoundsknown; B, complex fluorides only; C complex fluorides, binary salts and alkaline-aqueous ions only; D, binary salts and a variety of complexes. I, values in eV. E values for M3"(aq)/M4'(aq) are in volts relative to NHE. I, and E are derived from spectroscopic data except those in parentheses which are experimental values.
39.2322 Hydrated ions
Although a single value is given in Table 16 for E- (Ce3+(aq)/Ce4+(aq)),the oxidation potential of Ce3+ ion in aqueous media varies considerably with the nature of the counterion, doubtless due to stabilization of Ce4+to different extents b complexation. Thus in 1M HzS04, Ee = 1.4435V,680 but in perchloric acid solutions, E = 1.6400-1.7310 V depending on concentation,681while a range of 1.6085-1.6104 V was observed in nitric acid solutions.682 There is some evidence for the formation of polymeric species in aqueous nitric acid solutions, and this may explain some anomalies in electrode potential determinations. Values for association constants of 17 and 2 respectively for Ce4+-Ce4+ and Ce4+-Ce3+ interactions were obtained .683 The value of E* (Ce"(aq)/Ce"(a )) is rather greater than that for the oxidation of water, since E* (H20/02)= 1.23V, and Ce%+ion is in fact capable of oxidizing water to oxygen in 0.5 M H2SO4 in the presence of RuOz catalyst. Yields are up to 73% t h e 0 r e t i c a 1 . ~ , ~ ~ Tetrapositive Pr4+ and Tb4+ are quite stable in strongly alkaline carbonate solutions. Thus electrolytic oxidation for three hours of 2-5.5 mol dm-3 aqueous M2C03 (M = K or Cs), 1mol in KOH, containing CeC13, PrC13 or TbC13 (0.1-0.01 mol dmA3) gave stable solutions of yellow Pr4+ (Amm = 283 nm), dark reddish-brown Tb4+ (Ama = 365 nm), and of
B
1114
Scandium, Yttn'um and the Lanthanides
course Ce4+. The alkaline medium provided considerable stabilization of the tetrapositive ions; E? for Ce3+/Ce4+being only +0.05 V.686The ion Tb4+ is also stabilized in solutions of sodium tripolyphosphate Na5P3OI0.The solutions show A,, at 375 nm (E = 1095) and oxidize Ce3+ or Mn" at an acid pH, while slowly reducing ascorbic acid in neutral solution.687Oxidation of Tb3+(aq) by the conducting glass electrode, which exhibits an unusually high overvoltage towards the oxidation of water, produces Tb4+in a solid form, perhaps a hydroxycarbonate, as the aqueous medium was 2.5 KzCo3/0.5 M KOH.688
39.2l23 Nitrogen donor ligands These complexes are rare, doubtless owing to the comparatively ready oxidation of amines. However, the template preparation of Ce(N03)4L.3H20 has been described; L is the eighteen-membered macrocycle formed by cocondensation of two molecules of ethylenediamine and two molecules of 2,6-diacetylpyridine. The structure of this compound is unknown and it is uncertain to what extent the macrocycle is bonded to the metal ionm9
39.2324 Oxygen donor ligands These compounds, together with the halo complexes described in the next section (39.2.7.5), form the bulk of the known complexes of tetrapositive lanthanides.
(i) Complexes of inorganic oxy anions A number of double sulfates of Ce4+,for example (NI&)4Ce(S04)4.2H20,may be obtained by concentration of acidic solutions of ceric sulfate and alkali sulfates. The ceric sulfate may be obtained by heating ceric oxide with fairly concentrated sulfuric acid (it dissolves rather slowly), cooling, filtration and washing with acetic acid. A series of hydrated double sulfates of caesium and cerium(1V) have been prepared and their IR spectra have been interpreted in terms of crystal symmetry and point group symmetry.690Cerium(1V) salts are solvolyzed in chlorosulfuric acid and the properties of the resulting solutions have been interpreted in terms of the formation of Ce(SO3C1), which, however, polymerizes at higher concentrations.w1 Ceric ammonium nitrate, the well-known volumetric standard oxidant, is formulated (N&),[ce(No&] with 12-coordinated Ce4+ ions (each NOs ion being bidentate) in approxk mately Td symmetry. The Ce-0 distances are in the range 2.488-2.530 8,.The ligand 'bite' is only 50.6-50.9' and the other M e - 0 angles are 65.0-69.9°.692 The carbonato complexes N%[Ce(C03)5]-12Hz0and [C(NH2)3]6[Ce(C03)5].4Hz0 each show cerium ten-coordinated to five bidentate carbonate ions with Ce-0 = 2.379-2.504 8, in the first complex and 2.3882.488 A in the second. The coordination polyhedra are irregular.6933694 The preceding three structures show that the ionic radius of Ce4+ is about 1.108, for 1Zcoordinatidn and about 1.04 8, for 10-coordination. Thus the ionic radius of Ce4+ is surprisingly similar to that of Ce3+, but as noted below (Section 39.2.12.4.iii) can vary according to the nature of the ligand.
(ii) Complexes of organic oxy anions exists as a and /3 modifications. Cerium tetrakis(acetylacetonate),a venerable Both these contain [Ce(acac),] monomers of D2square antiprismatic coordination but differ in the crystal packing. In the LY form,696Ce-O=2.36-2.43i% with the ring angle 0-Ce0 = 72', while in the /3 formm7Ce-0 = 2.32 8, and W e - 4 = 71.3'. The tetrakis(acety1acetonates) of Zr, Hf, Th, U and Pu also show dimorphism. The tetrakis(dibenzoy1methanate) [Ce(dbm)4] adopts a similar square antiprismatic structure with Ce-0 = 2.299-2.363 i% and 0-Ce-0 = 70.8" or 71.2°.69s There is some tendency €or Ce3+ p-diketonates to pass into the Ce4+ tetrakis(P-diketonate). A series of tri- and tetra-valent cerium P-diketonates has been examined from the point of view of the effect of additional ligands such as Ph3P0 on this process, and it was found that Ce3+ P-diketonates were stabilized by adduct formation, particularly by 1,10-phenanthroBne.6w
Scandium, Yttrium and the Lanthanides
1115
A very interesting species is the very stable catechol complex Na4[Ce(cat)4].21H20,where H2 cat = 1,2-C6H4(0H)~,which forms deep red crystals, the colour (Arnm = 517 nm) probably being due to a ligand-to-4f charge-transfer band. It has bdchelated dodecahedral s mmetry with ring 0-Ce-0 angles of 68.3" and Ce-4 distances of 2.357 and 2 . 3 6 2 2 Cyclic voltammetry showed a quasireversible one-electron reduction at -692 mV us. SCE. The complex is diamagnetic. It is rather surprising that this complex exists, when the highly reducing nature of the catecholate ligand is considered, but the [Ce(cat)14- ion is unequivocally establi~hed.~~ (iii) Ligands with uncharged oxygen donors Tetravalent cerium forms complexes with amine oxides or phosphine oxides fairly readily, as do the trivalent lanthanides. Thus the following complexes have been described: CeC&, where L = Ph3P0,701bipyridyl dioxide or hexamethylph~sphoramide,~~ CeC14(DMS0)3703 and Ce(N03)4(Ph3P0)2.704A single crystal X-ray structure is available of the orange Ph3P0 complex, which was prepared by the action of the ligand on (NH4)2Ce(N03)6in acetone. The metal ion is ten-coordinated in a pseudo-truns-octahedral manner to two trans phosphoryl oxygen atoms and four bidentate nitrate ions, but the CeOlo polyhedron is irregular. Leading distances and angles are: Ce-O(P) = 2.21 and 2.23 A, Ce-0(NO3) = 2.41-2.54 A, O(P)Ce-O(P) = 174" and P a e = 168". The fairly short C e O ( P ) distances, compared with the Ce-O(N0,) distances, are noteworthy. The corresponding thorium complex is isostructural. A number of complexes of Ce4+ may conveniently be prepared as follows. Hydrous ceric oxide is added to S0C12 in cold diglyme and yields stable H2CeC16.3(diglyme)which is soluble in organic solvents, to which solutions of suitable ligands may be added to form complexes such as CeC14{OP(NMe,) 3} 2 . 705
39.2l2.5 Complex halides
(i) Complex fluorides This is arguably the most prolific area of tetravalent lanthanide chemistry, and Ce, Pr, Nd, Tb and Dy are all represented. A variety of cerium(1V) ammonium fluorides are known. Thus the CeF4-NHP-H20 system gives (NH4)4CeF8at concentrations of N&F greater than 28.9%. At lower concentrations, a monohydrate (NH&CeF7.H20 is obtained, which on dehydration is converted into (NH4)4CeF8 and (NH4)2CeF6.The structure of (NH&CeF6 contains chains of distorted square antiprisms with Ce-F = 2.198-2.318 A, while (NH&[CeF8] contains isolated distorted square antiprisms. (NH4)3CeF7-H20contains dimeric ions [Ce2Fl4I6- in which the metal ions are eight-coordinated in a distorted dodecahedral manner with Ce-F = 2.133-2.375 A. Other complex ammonium cerium(IV) fluorides reported are (NH4)7Ce6F31and NH&eF5.7w7w The alkali metal fluoro complexes MZCeF6 (M = Na, K,Rb, Cs) and M3CeF7 (M = Na, Rb, Cs) have been obtained as colourless diamagnetic crystals by fluorination of MCI/Ce02 mixtures. They liberate iodine from aqueous iodide and decompose in water.7mThe analogous terbium compound Cs3TbF7, which is colourless and paramagnetic ( p = 7.4 BM), has been obtained similarly by fluorination at 390 "C, and its magnetic moment is much more consistent with the presence of Tb4+ (theory, 7.9 BM) than with Tb3+ (9.7 BM).710 Compounds of tetrapositive praseodymium can be made by the action of fluorine at 400-500 "C on MC1 and Pr6011mixtures. Thus obtained were M3PrF7and M&F6 (M = Na, K, Rb or Cs). They are colourless and decompose in water. The Curie-Weiss law is obeyed with observed room-temperature moments of 2.10-2.45 BM (Pr4+, 2.56 BM theory; Pf", 3.61BM).711 The compounds Na7Prfi31 and Na2PrF6 have been obtained by fluorhation of PrC13/NaC1 at 200-400°C. These products were characterized by X-ray powder pattern and iodine e q ~ i v a l e n t . ~ More ~ ' , ~ ~recently, ~ a wide variety of compounds M3LnF7 ( M = K , Rb or Cs; Ln = Ce, Pr, Nd, Tb or Dy) have been obtained, mainly by fluorination of M3LnCb at 50-150 bar and 400 "C for 2-3 hours. All the fluorides were isostructural with (NH4),ZrF7; examples are KRbCsPrF7, RbCs2NdF7 (orange) and KCs2DyF7 (orange). Compounds MPrF, COC3-JJ'
1116
Scandium, Yttrium and the Lanthanides
(M = Sr, Ba or 2Li) were also obtained starting from MPr03 (itself made from the action of O2 on the mixed lower oxides).714 The series of fluoro complexes Cs3MF7(M = Ce, Pr, Nd, Tb or Dy) have also been made by reaction of Cs3MCI, with XeF2 at temperatures of 100 "C for Ce, Pr or Tb, and 400 "C for Nd or Dy. Corresponding treatment of Cs3MC16(M = Er or Yb) yielded only CS3MF6.715 It should be noted that definitive X-ray single crystal work is really needed on some of these compounds, as structures are often based on powder correlations with type structures of non-lanthanide complex fluorides, some of which are themselves quite early single crystal studies. Good electronic spectra may be obtained from these fluoro complexes which thus provide a unique source of information about electronic energy levels in tetravalent lanthanides. The electronic spectra of Cs3NdF7 have been obtained in absorbance and fluorescence and an excellent correlation is observed with expected behaviour for the Nd4+f' ion in intermediatefield coupling. Values of F2 = 352, F4= 52.3, F6 = 5.56, and spin-orbit coupling constant f = 1109 cm-' explained the spectra, with the first nine excited states up to 3P1at 25 700 cm-l being observed above the 3H4ground state.716Similar spectra for Cs3DyF7have been obtained and agree with theoretical values calculated using only two parameters, Fz = 465.8 cm-' and f = 2270 cm-l. Only the 7F ground state multiplet and the 54excited state at 21 600 cm-' were observed.717
(ii) Complex chlorides Complex chlorides of Ce4+ may be prepared by dissolution of cerium(1V) hydroxide in methanolic hydrochloric acid followed by addition of a base €3, giving (BH)2[CeCb]. (NH&CeC16 can be used as a selective precipitant for Cs+ ions in presence of other alkali metals, with separation coefficients ranging from 5300 (Na) to 60 (Rb).'18 The thermal decomposition of Cs2 CeC& has been studied. In nitrogen at 600 K it decomposes into CeC13, C12 and CsC1,5l9 but1 in air CeOz, C12 and CsCl are formed.7z0The enthalpies of decomposition and formation were determined as: = -1974.09 f 8.37 kJ mol-' and A H (decomp. 600 K) = 25.5 f 3.3 kJ mol-'. Hexachlorocerates of organic cations vary in their thermal decomposition behaviour. Thus the pyridinium or some tetraalkylammonium compounds give chlorine, but the PPh: or AsPh: compounds do The IR and Raman spectra of R2[CeC16],where R = Cs or various substituted ammonium, has been studied and assignments to the vibrational modes of the octahedral [CeCl6I2-ion have been made.722,7uThese are: AI, stretch, 295 cm-'; T,, stretch, 265 cm-'; Tubend, 116 an-', and Tzp bend, 123cm-'. The complexes are made from (NJ$&CeC&, and RCl.724
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677. F. Gaume, A. Gros and J. C. Bourcet, ‘The Rare Earths in Modern Science and Technology’, ed. G. J. McCarthy, H. B. Silber and J. J. Rhyne, Plenum, New York, 1982, vol. 3, p. 143. 678. L. J. Nugent, R. D. Baybarz, J. L. Burnett and J. L. Ryan, J . Phys. Chem., 1973,77, 1528. 679. J. Sugar and J. Reader, I . Chem. Phys., 1973, 59, 2083. 680. A. H. Kunz, J. Am. Chem. SOC.,1931,53,98. 681. M. S. Shemll, C. B. King and R. C. Spooner, J. Am. Chem. SOC., 1943, 65, 170. 682. A. A. Noyes and C. S. Gamer, J. Am. Chem. SOC.,1936,58, 1265. 683. B. D. Blaustein and J. W. Gryder, J. Am. Chem. Soc., 1957,79,540. 684. A. Mills, J . Chem. SOC., Dalton Trans., 1982, 1213. 685. J. Kiwi, M. Gratzel and G. Blondeel, J. Chem. SOC.,Dalton Trans., 1983,2215. 686. D. E. Hobart, K. Samhoun, J. P. Young, V. E. Nowell, G. Mamantov and J. R. Peterson, Inorg. N w l . Chem, Lett., 1980, 16, 321. 687. R. Yang and X.Sun,Lmzhou Dame Xuebao, Ziran Kescuebata, 1982,18, 162 (Chem. Abstr., 97,207083). 688. R. C . Propst, J. Inorg. ‘Nucl. Chem., 1974,3h, 1085. 689. G.Wang, R. Zhang and 2.hang, Gaodeng Xuexiao Huaxuc Xuebao, 1983,4,535 (Chem. Abstr., 100,78872). 690. S. D. Nikitina and S. A. Bondar, Russ. J. Inorg. Chem. (En& Transl.), 1983, 28, 1275. 691. J. K. Puri, P. Singh and J. M. Miller, Inorg. Chim. Acta, 1983, 74, 139. 692. T. A. Beineke and J. Delgaudio, Znorg. Chem., 1968, 7, 715. 693. S. Voliotis and A. Rimsky, Acta Crystallogr., Sect. B, 1975, 31, 2620. 694. S. Voliotis, A. Rimsky and J. Faucherre, Acta Crystallogr., Sect. B, 1975, 31, 2607. 695. A. Job and P. Goissedet, C. R. Hebd. Seances. Acad. Sci., 1913,50, 157. 696. B. Matkovic and D. Grdenic, Acta Crystallogr., 1963, 16, 456. 697. H. Tutze, Acta Chem. Scand., 1969,23,399. 698. H. Barninghausen, 2. Kristallogr., 1971, 134, 449. 699. V. I. Spitsyn, L. I. Martynenko, N. I. Pechurova, N. I. Snezhko, 1. A. Murav’eva and I. Anufrieva, Zzv. Akad. Nauk SSSR, Ser. Khim., 1982, 4, 772, 700. S. R. Sofen, S. R. Cmper and K. N. Raymond, Inorg. Chem., 1919, 18, 1611. 701. F. Brezina, Collect. Czech. Chem. Commun., 1971, 36, 2889. 702. F. Brezina, Collect. Czech. Chem. Commun., 1973, 39, 2162. 703. J. G. H. Du Preez, H.E. Rohwer, J. F. de Wet and M. R. Caira, Znorg. Chim. Acta, 1978, 26, L59. 704. Mazhar-ul-Haque, C . N. Caughlan, F. A. Hart and R. Van Nice, Inorg. Chem., 1971,10, 115. 705. J. Barry, J. G. H. Du Preez, E. Eb, H. E. Rohwer and P. J. Wright, Inorg. Chim. Acta, 1981, 53, L17. 706. R. A. Penneman and A. Rosennveig, Inorg. Chem., 1969,8,627. 707. R. R. Ryan, A. C. Larson and F. H. Kruse, Znorg. Chem., 1969, 8, 33. 708. R. R, Ryan and R. A. Pennernan, Acta Crystallogr., Sect. B, 1971,27, 1939. 709. R. Hoppe and K.-M. Riidder, 2. Anorg. Allg. Chem., 1961, 313, 154. 710. R. Hoppe and K.-M. Rodder, 2.Anorg. Allg. Chem., 1961,312, 277. 711. R. Hoppe and W. Liebe, Z. Anorg. Allg. Chem., 1961,3U, 221. 712. L. B. Asprey and T. K. Keenan, J. Inorg. Nucl. Chem., 1%1,16,2ho. 713. L. B. Asprey, J . S. Coleman and M. J . Reisfeld, Adu. Chem. Ser., 1967, 71, 122. 714. R. Hoppe, ‘The Rare Earths in Modern Science and Technology’, ed. G. J. McCarthy, H. B. Silber and J. J. Rhyne, Plenum, New Yotk, 1982, 3, 315. 715. Yu. M. Kiselev, S . A. Goryachenkov and L. I. Martynenko, Russ. J. Inorg. Chem. (Engl. Transl.), 1984, 29,38. 716. L. P. Varga and L. B. Asprey, J. Chem. Fhys., 1968, 49, 4.674. 717. L. P. Varga and L. B. Asprey, J . Chem. Fhys., 1968, 48, 139. 718. A. Brandt, Yu. M. Kiseiev, L. I. Martynenko and V. I. Spitsyn, Dokl. Akad. Nauk SSSR, 1981, 257, 1189. 719. A. Brandt, Yu. M. Kiselev and L. I. Martynenko, Russ. J. Inorg. Chem. (Engl. Transl.), 1981, 26, 499. 720. A. P. Bayanov, Z. A. Temerdashev and Yu. A. Afanas’ev, Rws. J. inorg. Chem. (Engl. Transl.), 1978,23,1778. 721. Yu.M. Kiselev and A. I. Popov, Russ. J. Znorg. Chem. (Engl. Transl.), 1983, 28, 190. 722. A. Brandt, Yu. M. Kiselev and L. I. Martynenko, 2. Anorg. Allg. Chem., 1981,474,233. 723. A. Brandt, Yu. M.Kiselev and L. I. Martynenko, Russ. I . Znorg. Chem. (Engl. Transl.), 1983, 28, 1593. 724. Yu. M. Kiselev, A. Brandt, L. I. Martynenko and V. 1. Spitsyn, Dokl. Akad. Nauk SSSR, 1979, 246, 879.
The Actinides KENNETH W. BAGNALL University of Manchester, UK
1130 1138 1130 1130 1131 1131
40.1 GENERAL SURVEY 40.1.1 Introduction 40.1.2 40.1.3 40.1.4 40.1.5
Oxidation States Steric Crowding Coordination Numbers and Geometries Literature Sources
40.2 THORIUM, PROTACIINTUM, URANIUM, NEPTUNIUM AND PLUTONIUM 40.2.1 The +3 Oxidation State 40.2.1.1 Nitrogen ligands 40.21.2 Phosphorus ligands 40.2.1.3 Oxygen ligands 40.2.1.4 Sulfur ligands 40.2.1.5 Selenides and tellurides 40.2.1.6 Halogens as ligands 40.2.1.7 Hydrides as ligan& 40.2.1.8 Mixed donor atom ligands 40.2.1.9 Multidentate m n ~ ~ ~ c y cligands lic 40.2.2 The +4 Oxidation State 40.2.2.1 Group W ligan& 40.2.2.2 Nitrogen ligands 40.2.2.3 Phosphorus and arsenic ligands 40.2.2.4 Oxygen ligands 40.2.2.5 Sulfur ligands 40.2.2.6 Selenium and ellwium ligands 40.2.2.7 Halogens as ligands 40.2.2.8 Hydrogen and hydrides as ligands 40.2.2.9 Mixed donor atom ligands 40.2.2.10 Multiakntate macrocyclic ligands 40.2.3 The f 5 Oxidation Sfate 40.2.3.1 Nitrogen ligands 40.2.3.2 Phosphorus and arsenic ligands 40.2.3.3 Oxygen ligands 40.2.3.4 Sulfur ligands 40.2.3.5 Selenium and tellurium ligands 40.2.3.6 Halogens m ligands 40.2.3.7 Mixed donor atom ligands 40.2,4 The +6 Oxidation State 40.2.4.1 Group N ligands 40.2.4.2 Nitrogen ligands 40.2.4.3 Oxygen ligands 40.2.4.4 Sulfur ligands 40.2.4.5 Halogens as ligands 40.2.4.6 Mixed donor afom ligands 40.2.4.7 Multidentate macrocyclic ligands 40.2.5 The +7Oxidathn State
1131 1131 1131 1133 1133 1135 1135 1135 1135 1136 1136 1136 1136 1137 1143 1144 1170 1173 1173 1175 1175 1178 1179 1179
1179 1180 1184 1185 1185 1187 1187 1187 1187 1192 1209 1210 1212 1214 1214 1215 1215 1215 1215 1217 1217 1218 1218 1218 1218 1219 1219 1219 1219 1219 1220
40.3 THE TRANSPLUTONIUM ELEMENTS 40.3.1 The +2 Oxid&.on State 40.3.2 The +3 Oxidation Srafe 40.3.2.1 Oxygen I i g d 40.3.2.2 Sulfur fig& 40.3.2.3 Selenium and tellurium ligands 40.3.2.4 Halogens as Iigands 40.3.25 Hydrides ar ligands 40.3.2.6 Mixed donor atom ligands 40.3.2 7 Multidenta@macrocyclic ligands 40.3.3 The + 4 Oxidarion State 40.3.3.1 Oxygen ligands 40.3.3.2 Halogeras (IS ligands 40.3.4 The f 5 Oxidation State 40.3.4.1 Oxygen ligands 40.3.4.2 Halogens as ligands
1129
The Actinides
1130 40.3.5 The +6 Oxidation Stare 40.3.5.1 Oxygen ligands 40.3.5.2 Halogens as ligands 40.4
1220 1220 1220
REFERENCES
1221
40.1 GENERAL SURVEY 40.1.1 Introduction
Most of this chapter relates to two earlier members of the actinide group, thorium and uranium, both available in large quantities and relatively cheap. The radioactivity arising from the naturally occurring isotopic mixture of these two elements presents no serious handling problems, whereas ='Pa, 237Np, 239Pu,etc., are much more intensely radioactive, requiring very costly handling facilities. These isotopes are also very expensive. Consequently, basic research on these elements is restricted to government-supported research establishments, such as AERE, Harwell in the UK and the Argonne and Oak Ridge National Laboratories in the USA, where much of the effort is of an applied nature. Beyond californium (Z=98), the elements can only be produced in very small quantities and all known isotopes of 2 = 99 to 103 are short-lived and, consequently, of very high specific radioactivity. This makes it exceedingly difficult to prepare solid, characterizable compounds of any kind, and most of the scanty information that is available is based on tracer level studies. I
40.1.2 Oxidation States In contrast to the lanthanide 4f transition series, for which the normal oxidation state is + 3 in aqueous solution and in solid compounds, the actinide elements up to, and including, americium exhibit oxidation states from +3 to +7 (Table l),although the common oxidation state of americium and the following elements is +3, as in the lanthanides, apart from nobelium (2 = 102), for which the +2 state appears to be very stable with respect to oxidation in aqueous solution, presumably because of a high ionization potential for the 5fI4 NO*+ion. Discussions of the thermodynamic factors responsible for the stability of the tripositive actinide ions with respect to oxidation or reduction are available.'*' Table 1 Known Oxidation States of the Actinide Elements
Th
Pa
U
Np
Pu
Am
Cm
Bk
Cf
Es
Fm
Md
No
Lw
40.1.3 Steric Crowding Ligand field interactions in 5f transition element complexes are very small: so that there is little or no contribution to the kinetic stability of the complexes, all of which can be regarded as kinetically labile. Ligand exchange will therefore occur quite readily for there will be virtually no gain (or loss) of ligand field stabilization energy in an inter- or intra-molecular rearrangement. There will also be a marked degree of flexibility in the coordination geometries adopted by the complexes formed by these elements. Consequently, steric crowding about the central metal atom is the principal factor governing the coordination number and geometry adopted by actinide complexes. One simple way of quantifylng steric effects in actinide complexes [the cone angle factor (caf) approach] uses the sum of the solid angles subtended by the ligands to the centre of the metal
The Actinides
1131
atom in a given complex, divided by 4n (abbreviated to Z caf) to represent the fraction of the total sphere surface enclosing the metal atom which is occupied by the Obviously the value of Z caf can never be unity, which would represent complete filling of the sphere surface about the metal atom, because of ligand-ligand repulsions, and for thorium(1V) and uranium(1V) complexes of known structure, values for Z caf are 0.80 f0.04 and 0.80 f 0.03 respectively. This approach has some predictive value; for example ,'seven-coordination would be expected for complexes of thorium tetrachloride with medium sized ligands, and both sixand seven-coordination for similar complexes of uranium tetrachloride. However, prediction of the most likely geometry to be adopted in a given compound is best based on a consideration of the repulsions between the metal-ligand bonds in the complexes, and will depend on whether the ligands are mi-, bi- or poly-dentate. The actual combinations of the different ligands in individual compounds seem to be very important in determining which particular geometry and coordination number are adopted, as shown in the possible geometries for eight-coordination.6 40.1.4 Coordination Numbers and Geometries The radii of the actinide cations at the beginning of the series are, for any given oxidation state, larger than d transition metal cations in the same oxidation state, so that high (>6) coordination numbers are common in actinide complexes. Because the ionic radii decrease with increasing atomic number (the actinide contraction), the observed coordination number also decreases along the series for a given oxidation state with the same ligand. Much published work concerned with the coordination chemistry of these elements has compared the behaviour of the early and later members of the series. A useful survey of structural information is a~ailable.~ Examples of the known coordination numbers and geometries are given in Table 2. Low (6) coordination numbers are observed only with very bulky ligands, e.g. [N(SiMe3)$, or high oxidation states, e.g. [Np04]-, and the highest (12 to 14) are found only with relatively small multidentate ligands, such as bidentate NOs and bi- and ter-dentate BH4 groups. Seven-coordination is very common in dioxoactinide(VI) complexes, in which the rigid O=M=O group provides the axis for pentagonal bipyramidal geometry, but several examples of this geometry have been recorded for com lex cations (e.g. [UC13(EtCONEt&]+),* the neutral complex (U(NCS)4(Me2CHCONMe~)~~ and the anion [UF7I3- in K3[UF7].lo The analytical stoichiometry of a complex may well mislead, for UCI4.3DMSO does not involve seven-coordinate uranium(IV), but is actually [UC12(DMS0)6]2+[UClh]2-.11 40.1.5 Literature Sources Exhaustive accounts of the chemistry of thorium, protactinium, uranium and the transuranium elements appear in the most recent supplementary volumes of the Gmelin series." Unless otherwise indicated by the inclusion of references to specific papers, further information on the compounds discussed in this chapter will be found in the Gmelin volumes and in the work cited in reference 2. 40.2 THORIUM, PROTACTINIUM, URANIUM, NEPTUNIUM AND PLUTONIUM
40.2.1 The +3 Oxidation State Thorium(II1) and protactinium(II1) complexes are unknown, and relatively few uranium(II1) , neptunium(II1) and plutonium(II1) compounds have been described. This is mainly because of the ease of oxidation to the +4 state in all three cases, accentuated for plutonium(II1) by the oxidizing nature of the a-radiolysis products formed in solutions.
40.2.1.1 Nitrogen ligan&
(i) Ammonia The only reported complexes are those with uranium and plutonium trihalides. The products of composition UC13.(6.8-6.9)NH3 and UCl3*(7.0-7.4)NH3 lose ammonia at 20 or 45 "C to
The Actinides
1132
Table 2 Coordination Numbers and Geometries of Actinide Compounds Coordination number
3 4 5
6
7
8
9
10
11
12 14
Complex
Coordination geometry
Probably pyramidal Probably trigonal pyramidal Highly distorted tetrahedron Distorted trigonal pyramidal Octahedral Trans octahedral Cis octahedral Distorted monocapped octahedron Pentagonal bipyramidal Pentagonal bipyramidal Pentagonal bipyramidal Capped trigonal prism Cube Cube Square antiprism Distorted dodecahedron Bicapped trigonal prism Hexagonal bipyramid Tricapped trigonal prism Tricapped trigonal prism Monocapped square antiprism Best described as tram octahedral with four bidentate NO, groups in the equatorial plane 1 :5 :4 geometry Bicapped square antiprism Best described as a monocapped trigonal prism with four bidentate NO, groups occupying four apices Icosahedron Disordered Bicapped hexagonal antiprism Bicapped hexagonal antiprism
Ref. a b C
d e f g h
i J
k I m n 0
P 4 I
4 s
t U
V W
X
V V
Y Z
R. A. Andersen, lnorg. Chem., 1979,18, 1507. R. A. Andersen, A. Zalkin and D. H. Templeton, Inorg. Chem., 1981,Ut, 622. J. C . Reynolds, A. Zalkin, D H. Templeton and N. M. Edelstein, Inorg. Chem., 1977,115,1090. J. G. Reynolds, A. Zalkin,D. H. Templeton, N. M. Edelstein and L. K.Templeton, Inorg. Chem., 1976,15,2498. W. H. Zachariasen, Acta Cry8tallogr.,1948,1,268. * J. F. de Wet and S. F. Darlow, Inorg. Nucl. Chem. Len., 1971,7 , 1041. G . Bombieri, D.Brown and R. Graziani, J. Chem. SOC.,Dalton Tram., 1975,1873. G . Bombieri, E.Forsellini, D. Brown and B. Whittaker, 1. Chem. Soc., Dalton Tram., 1976,735. W. H.Zachariasen, Acta Crystallogr., 1954, 7 , 792K. W. Bagnall, R. L. Beddoes, 0. S. Mills and Li Xing-fu, J. Chem. SOC., Dalton Trans., 1982,1361. G. Bombieri, E.Forsellini, R. Graziani and G. C. Pappalardo, Tramition Mer. Chem., 1979, 4,70. R. A. Penneman, R. R. Ryan and A. Rosenzweig, Snuct. Bonding (Berlin), 1973,13,1. rn R. Countryman and W.S. McDonald, 3 Inorg. Nucl. Chem., 1971,33,2213. D. Brown,J. F.Easey and C. E. F. Rickard, J . Chem. SOC. (A), 1969,1161. G.Bombieri, P. T. Moselcy and D. Brown, J. Chem. SOC., Dalton Trans., 1975, 1520. G. Bombieri and K. W. Bagnall, I . Chem. Soc., Chem. Commun., 1975, 188. W. H. Zachariasen, Acra Crysmllogr., 1948,1, 265. K. Bowmann and 2. Dori, Chem. Commun., 1%8, 636. ' G.Brunton, Acta Crystullogr., 1966,21, 814. V. W.Day and J. L. Hoard, J . Am. Chm. Soc., 1970,92,3626. Mazur-ul-Haque, C. N. Caughlin, F. A. Hart and R. van Nice, Inorg. Chem., 1971,10, 115. ' N. W.Alcock, S. EsperBs, K. W.Bagnall and Wang Hsian-Yun, J . Chem. SOC., Dahon Tmns., 1978,638. * M.N. Akhtar and A. J. Smith, Chem. Commun., 1969,705. T.Ueki, A. Zalkin and D. H. Templeton, Acra Crysrallogr.,1966,20, 836. E.R. Bernstein, W. C. Hamikton, T. A. Keiderling, S. J. La Placa, S. J. Lippard and J. J. Mayerle, Inorg. Chem., 1972,11,3009. R. R. Rietz, A. Zalkin, D. H. Templeton, N. M. Edelstein and L. K. Templeton, Znorg. Chem., 1978,17,658.
I '
'
1
form UCl3.3NH3, which decomposes to UC13.NH3 above 45 "C. UBr3.6NH3 is apparently obtained by treating the tribromide with gaseous ammonia, whereas UBr3-(3-4)NH3 is said to form with liquid ammonia. PuC13 and Pu13 yield products of composition PuCls.ca. 8NH3 and Pu13-ca.9NH3 with either gaseous or liquid ammonia;13these lose ammonia readily to yield the products PuC13.(4.1-4.5)NH3 and Pu13.(5.5-6)NH3 respectively. No structural information is available for any of these products.
The Actinides
1133
(ii) N-Heterocyclic ligands ;pyridine The existence of the only recorded complex, [U(py)6][Cr(NCS)6], is doubtful because uranium(II1) reduces the [Cr(NCS)6]3- ion. l4 (iii) Amides The compound with the bulky ~ilylamide,'~ U(N(SiMe3)2}3, may be a monomer, in which case the molecular geometry is probably pyramidal.
(iv) Nitriles The only recorded compounds are UC13-MeCN, which may be polymeric, and NpC13.4MeCN, for which the 237Np Mossbauer spectrum has been reported. 16a Mossbauer studies indicate16" that products of composition M'NpCl,-xMeCN are mixtures of NpC13-4MeCNand M:NpC15. 40.21.2 Phosphorus ligands
The bis diphosphine complex, [U(BH4)3(dmpe)z] (dmpe = MezPCH2CH2PMe,), has been prepared by treating U(BH4)yTHF with an excess of dmpe in THF. The uranium centre is formally 13-coordinate, with the three BH4 groups tridentate. 16b
40.2.1.3 Oxygen l i g u h
(i) Aqua species, hydroxides and oxides A selection of the known hydrated compounds is given in Table 3. In many cases they are easily dehydrated and the water is presumably held in the lattice. Water does not appear to be coordinated to the metal atom in the hydrated trifluorides, such as PuF3.(0.4-0.75)H20, whereas it is coordinated in PuC13.6Hz0 and the bromides MBr3.6Hz0 (M = U, Np, Pu), which are isostructural with the lanthanide hydrates LnCl3.6H20 and with AmC13-6Hz0, being of the form [MXz(H20)6]+X-. The hydrated plutonium(II1) hexacyanoferrate(II1) , PU[Fe(CN)6].CU. 7Hzo, is black, indicating a considerabk degree of charge transfer. Table 3 Some Hydrates of Actinide(II1) Compounds P~F3.(0.4to 0.75)H20
PuCI3*6HZO M'"Br,.6H20 M1UC1,*5H2O
M;~(so,),.~H~o NaNp(SO,),.xH,O M'Pu(SO&.xEI,O
M"' = U, Np, Pu M' = Rb, NH, MI1' = U, x = 8; Pu, x = 5 or 7 x = 1, 2, 4 or 5 variously with M' = Na, K , Rb, Cs, TI, NH,
(NH4)2U*W3,-9H20
PU~(SO~)~+XH~O PuPO4.0.5H20 HM"'LFe"( CN),].xH,O Pu[Fe '(CN),].ca. 7Hz0 M:'(Go,),.xH2O
M"' = U, x = 9 to 10; Pu,x uncertain
M"'= Np, x = ca. 11; Pu,x
= 1, 2, 3, 6, 9 or 10
The only recorded hydroxide, PU(OH)~.XH~O, rapidly oxidizes to a plutonium(1V) species, but two forms of h Z O 3are known, one with the hexagonal La203 structure and the other cubic; in these the Pu3+ is seven- and six-coordinate respectively. Ternary oxides, such as PuA103, are also known.
The Actinides
1134 (ii) P-Ketoenolates and ethers
B-Ketoenolates have not been isolated for the tripositive elements; in the attempted preparation of P u ( M ~ C O C H C O M ~ oxidation )~, to plutonium(1V) occurred very readi1y.l' Complexes with ethers appear to be restricted to THF solvates; apart from UC13.THF, a useful starting material for uranium(II1) preparative chemistry," the only other examples are solvated organometallic compounds, such as Cp3Np.3THF and Cp,Pu.THF. (iii) Oxoanions as ligands Hydrated sulfato complexes of the type M'M"1(S04)Z.~H20(Table 4) are known for
MI'' = U and Pu, and (NH4)21d2(S04)4.9H20 may be of the same type. K P U ( S O ~ ) ~ - H and ~ Othe corresponding dihydrate are isostructural with the neodymium(II1) analogues and N & P U ( S O ~ ) ~ - ~ is H ~isomorphous O with the corresponding ceriurn(II1) compound. Salts of other complex anions, such as K5M11'(S04)4 (M"' = Np, Pu), are also known. Table 4 Some Actinide(II1) Sulfato complexes MIMIII(
SO,), .XH,O
X-I,
M'=K,M~~'=Pu
x = 2 , M'=Na,M"'=UandM'=K,M"'=Pu x = 4. M' = Rb, M"' = U and M' = Na, NH,,
Rb,Cs or Tl,
MI'' = pu
(iv) Carboxylates and carbonates Uranium(II1) formate, U(HCO,),, isostructural with Np(HC02)3,19 P u ( H C O ~ ) ~and ~'~ Nd(MC02)3, is precipitated when a solution of UCI, in 95% formic acid is treated with a large excess of zinc amalgam. The corresponding uranium(II1 acetate and propionate could not be obtained by this route. N P ( H C O ~ has ) ~ been prepared)9 in the same way as U(HCO& and PU(HCO~)~ by treating Pu(OH)~with 90% formic acid.20a 11H20 10H20, ~ ) ~ - are precipitated on The hydrated oxalates, N P ~ ( Q O ~ ) ~ . C U . and P U ~ ( C ~ O addition of oxalic acid to aqueous solutions containing the tripositive elements. The hydrated oxalato complexes, M ' P U ( C Z O ~ ) ~ . ~(MI H ~=OLi, Na, K, Cs and N&, x = 0.3-3.5) have also been recorded.2ob Very little else is known about the metal(II1) carboxylates. A carbonate, P u ~ ( C O ~has ) ~ also , been recorded. ( v ) Aromatic hydroxyacids
Neptunium(II1) and plutonium(II1) are precipitated from aqueous solution by salicyclic acid; the neptunium salicylate, N P ( C ~ H ~ ( O H ) C O ~ ) ~ . ~can S Hbe ~ Odehydrated , at 50 to 150 "C. ( v i ) N,N-Dimethylformamide and related ligands The DMF and antipyrine (2,3-dimethyl-l-phenylpyrazol-5-one,ap) complexes [UL6][Cr(NCS)6], and the pyrimidone (4-dimethylaminoantipyrine, pym) analogue [U(pym)3][Cr(NCS)6], are probably uranium(1V) However, [U(ap),]Cl, has been obtained by dissolving RbUC14-5H20in ethanol containing the stoichiometric quantity of the ligand, followed by evaporation at low temperature,14 and the corresponding tetraphenylborates, [U(ap)6](BPh4)3and [ U ( p y n ~ ) ~ ] ( B P hare ~ ) ~precipitated , when ethanolic Na(BPh4) is added to a solution of NH4UC&.5HZ0and the ligand in the same solvent, a method also used to obtain the dicarboxylic acid amide [UL4]X3 where L = (CH,),(CONR& (n = 1-4) and Me2C(CH2CONR& with R = Me or Et, and X = BPh4 or PF6.
The Actinides
1135
40.21.4 Sulfur ligands
(i) Sulfides The sulfides M2s3 (M=Np,Pu) exist in three crystal modifications. ThZS3 and M2S3, however, may be metallic in character like the monosulfides MS (M=Th, W, Np, Pu). Plutonium disulfide, PuSZ, is a plutonium(II1) polysurfide and the sulfides of composition M3S5 (M = U, Np, Pu) are mixed phases, M2S3.MS2.Other known sulfides include Pu3S4and Th7S12. Isostructural oxosulfides, NpOzS and Pu202S, are also known. A detailed discussion of actinide sulfides is available.
(ii) N ,N-Diethyldithiocarbamates (dtc) The complexes M"[(dt~)~ (MI" = Np, Pu) are obtained by treating the metal tribromide with Na(dtc) in anhydrous ethanol. P ~ ( d t c is ) ~fairly stable to oxidation, but Np(dtc), and the even less stable U(dtc), are rapidly oxidized to M(dtc 4, so that neither can be isolated. However, the anionic complexes, (NEt4)[M111(dtc)4](M' I = Np, Pu), have been prepared and the geometry about the metal atom is a distorted dodecahedron, best regarded as a planar pentagon of five S atoms with one S atom above and two S atoms below the pentagon.= These salts of the [ M ( d t ~ ) ~ ion ] - are isostructural with the analogous lanthanide complexes, whereas Pu(dtc)3 is not isostructural with any of the lanthanide Ln(dtc)3.
1
40.2.1.5 Selenides and tellurides A variety of selenides and tellurides of composition MX (M = Th, U, Pu; X = Se or Te), MzX3 (M = Th, U , Pu, X = Se; M = Th, X = Te), M3X4(M = U, Np, Pu, X = Se; M = U, Np, X = Te), U3X5 (X = Se, Te) and Th7SeI2have been recorded. These are probably structurally similar to the corresponding sulfides.
40.21.6 Halogens as ligands The known trifluorides, MF3 (M = U, Np, Pu), possess the LaF3-type structure in which the coordination geometry about the metal atom is a fully capped trigonal prism, an interesting example of l l - c ~ o r d i n a t i o nThe . ~ ~ trichlorides, MCI3 (M = U, Np, Pu), adopt the UC13-type structure in which the nine-coordinate metal atom lies at the centre of a tricapped trigonal prism, a structure also found for UBr3 and a-NpBr3. /3-NpBr3 and PuBr3 have the eight-coordinate PuBr,-type structure in which the coordination geometry is a bicapped trigonal prism and this is found also for the triiodides, MI3 (M = U, Np, Pu). Amido chlorides, U(NHz)C1, and U(NH&Cl, obtained by heating UCI3 in NH3 at 450 to 50O0C, are also known; the other trihalides would probably react in a similar manner. Examples of the known halogeno corn lexes are given in Table 5 . NaPuF4 is isostructural with NaNdF4; the geometry about the MIIF centre is a tricapped trigonal prismatic arrangement of fluorine atoms.25 The [M1"C16J3- anions (M = U, Np, Pu) are octahedral and the geometry about the central metal atom in the complex anions of the salts M;MmC1, is a monocapped trigonal prism. Cr stallographic data for M:M1"Cl5 (MI = K, Rb, Cs), CsUC14 and RbU2C17, obtained in MPC1/UC13 phase studies, have been reported.26a
40.2.1.7 Hydrides as Kigands
Hydrides of composition MH2 (M = Th, Np, Pu; dl fluorite structure) and MH3 (M = Pa, U, Np, Pu) have been recorded, as well as phases of intermediate composition, such as NpH2.,. The only recorded borohydride, U(BH4)3, obtained by thermal or photochemical bonding. decomposition of U(BI-L)4,probably involves multiple U-H-B
The Actinides
1136
Table 5 Some Actinide(II1) Halogeno Complexes M'MII'F.,
M:UF, M:UF,
M"' = U, Pu with M' = Na, K M' = Na, K M' = K, Rb, Cs
KPU,F7a
CSUCI,
M' = Rb, NH, M"' = U, M' = K, Rb, Cs, NH, M"' = N P , ~MI = K, Rb, NH4 M"' = Pu, M' = K, Rb = U,"M' = K, Rb, NH,, PPh,, AsFh, M"' = Np,b M' = K, Rb, NH4 M"' = U, Np, Pu X = C l , Br M' = Rb, Cs I. G. Suglobova and D.E.Chirkst, Sou.J . Coord. Chem. (Engl. Trans[.),1981,7,52. bD. G . Karracker and 1. A. Stone, Inorg. Chern., 1980, 19, 3545. J. Drozdzynski, Inorg. Chim. Acra, 1979, 32, L83. a
40.21.8 Mixed donor atom ligands The neutral 2-(dipheny1phosphino)pyridine complex, [U(BH&(Ph2Ppy)2]-OSC6H6, has been prepared by treating U(BH&.THF with the ligand. If each triply bridging BH, group is regarded as occupying one coordination site, the geometry of the formally 13-coordinate uranium atom is close to pentagonal bipyramidal.z6b The only other recorded compound for the +3 oxidation state is the 8-hydroxyquinoline complex, P U ( C ~ H ~ N Oprecipitated )~, when an aqueous solution containing plutonium(II1) is added dropwise, in a stream of nitrogen, to an aqueous solution of the ligand in the presence of reducing agents. It is rapidly oxidized in air.
4.21 9 Multidentate macrocyclic ligands The only examples of complexes with this type of ligand are the crown ether compounds WCbL (L = 15-crown-5,18 benzo-15-cr0wn-5~~ and 18-crown-6"), which precipitate when the ligand is added to a solution of UCL-THF in THF, and U(BH4)3.18-crown-6, prepared by treating the UC13 complex with NaBH4. It appears that the 15-crown-5 ligands provide a better cavity match than 18-crown-6.
40.2.2 The +4 Oxidation State 40.2.2.1 Group IV ligands (2)
Cyanides
Actinide cyanide systems have scarcely been investigated. A product described as Th[Th(CN),] [Th(CN)4?] is reported to be formed when a thorium(1V) hydroxide gel is treated with a 10-20% excess of a solution of stabilized HCN in an autoclave." Uncharacterized cyanide-containing products are obtained when ThC14 or ThBro is treated with an alkali metal cyanide in anhydrous liquid ammonia, a reaction which yields UC13(CN)-4NH3 [v(CN) = 2120cm-'] in the case of UCLn2' UBr4 and U14 appear to react in the same way as UC14. An impure product of approximate composition (Et,N),UCL( CN), has also been recorded. Alkali metal cyanides do not react with UCl, in anhydrous liquid HCN, the only product being a UCb-HCN adduct.
(ii) Ksocyanides, RNC A cyclohexyl isocyanide complex, probably [Th14(C&lNC)4] [v(NC) = 2194 cm-' , Av(NC) = +59 cm-'1 has been obtained by treating Th14 with CJIIINC in n-hexane. The
1137
The Actinides
analogous uranium(1V) complex [v(NC) = 2190 an-', Av(NC) = +55 cm-l was obtained in the same way. Similar complexes appear to be formed with U C 4 and UBr4.
lo
40.222 Nitrogen ligands ( i ) Ammonia and amines
( a ) Ammonia. A selection of the known thorium(1V) , uranium(1V) and plutonium(1V) ammines is given in Table 6. Most of them were obtained from the parent compound or (PuC14.xNH3) from Csz[PuC16] by treatment with gaseous or liquid ammonia. ThBr4.4NH3 (reported31a as (NH&ThBrd(NH2)2) is obtained when (Et4N)z[ThBr6] is treated with liquid ammonia at low temperature. Its nature is uncertain. Although no structural data are available for any of these compounds, two of them, UC&.4NH3 and U3r4.4NH3, are reported to be white solids, which suggests a centrosymmetric structure, possibly cubic. UCl3(CN).4NH3, in contrast, is light green. Hydrated products, formulated earlier as Th(N03)4.3NH3-H20and 2Th(N0)3)4.7NH3.3Hz0, are doubtful; Th(N03)4.4H20and Th(SO4)2 undergo ammonolysis in liquid ammonia, but Th(G04)2is soluble, which may indicate the formation of a complex. Table 6 Some Ammonia Complexes of Actinide(N) Compounds UF,.xNH, MCl,.xNH, UCl3(CN).4NH, MBr,.xNH3 MId.xNH3 Th13(NHz).4NH3 ~Iz(NH?.)z*3~3 U(OR),*xNH, U(OR),.2ROH.NH3 U(RCOZ),.xNH, U(G04)2.3mdW UO(OR),.4ROH*NH, UO(RCO&.XNH,
x = 0.5 (?), 1 or 4.2 to 4.6
M = T h , x = 3 ( ? ) , 6; M = U , x = 1 , 2 , 4 , 5 , 7 . 3 to7.6, 8 , 9 to 10 or 12; M = Pu, x = ca. 5 or cu. 7
M =Th,x =4; M = U , x = 4, 5 to 6 or IO M = T h , x = 7 or 8; M = U , x = 4 to 5 or 10 x = 1, R = Ph, o-ClC6H4,o-, p-MeC6H4, CY-, j?-C,,,H, x = 2, R = rn-, p-ClC6H, R = Ph, p-ClCa,, o-, p-MeC& R = H, x = 2; R = Me, x * 1
R Ph, P-CIC~H, R - H,x = 1 or 2; R = M e , x = 1
( b ) Aliphatic and aromatic amines. Amine complexes are known only for thorium(1V) and uranium(1V) species (Table 7) , prepared either by treating the parent compound with the amine, removing the excess by vacuum evaporation, or by precipitation from a solution of the parent compound in a non-aqueous solvent by addition of a solution of the amine in the same solvent. The thorium atom in [ l l ~ C l ~ ( M e ~ N is )seven-coordinate ~] in a capped octahedral environment, with one Cl in the cap position; three N atoms occupy the capped face and three C1 atoms occupy the uncapped face.31b The IR spectra of Th(N03)4.xarnine.yHZ0 indicate that no ionic nitrate is present when x = 1 and y = 3, or x = y = 2, whereas with x = 3 and y = 4 both coordinated and ionic nitrate groups are present. When x = y = 4 nearly all the nitrate groups are ionic.32No other structural information is available for these compounds. (c) Hydrazines. The few known complexes are listed in Table 8. ThCl4-4PhNHNW2behaves as a 1:4 electrolyte in DMF but this could be due to displacement of all chloride anions and hydrazine molecules by solvent. ( d ) Ethylenediamine and other diamines. Complexes with these ligands (Table 9) are normally prepared from the parent compound and the ligand, either alone or in a non-aqueous solvent, For example, the diaminobenzene complexes, ThC14.2CsHsN2, precipitate when ethereal solutions of the chloride and the ligand are mixed. It is not known whether the water molecules in the hydrated complexes ThBr4.xL.yH20 (Table 9) are bonded to the thorium atom. The N,N,N',N'-tetramethylethylenediamine (tmed) complex, {(CF3)2CHO}4U-(trned), is obtained by treating {(CF3)2CH0}4U.2THF with tmed.33 The majority of the known complexes probably involve eight-coordinate actinide(1V) ions, and it is possible that the 1,Zdiaminobenzene complex of composition 2UCb-5L is of the form [UCl3L4]'[UC1&]-,
The Actinides
1138
Table 7 Some Amine Complexes of Thorium(1V) and Uranium(1V) Compounds ThCI4.4L
ThBr4.4RNH, [ThC&(Me3N)3] ThCI,.3MeC,H4NH, ThC14.2Et3N ThC14.fi-CloH7NH2 Th(acac),-PhNH, Th(NO3),.~B~"NHZ-yH20 Th(NO?h.xMe,NH.yH,O Th(NO,),.xEt,N.H,O Th(C,04)Z.4B~"NH,~2H,0 UCl,-xRNH,
L = RNH,, with R = Me, Et, PS, PhCH,, rn-, p-MeC6H,, 0 - , p-MeOC,H,, p-EtOC6H4, a-,@-C,,H, (naphthyl) L = RR'NH, with R = Ph and RI = Me, Et, PhCH,, Ph L = R,R'NI with R = Me or Et and R' = Ph R = Et, Ph (toluidine)
x = 1, y = 3 ; x = 2 , y = 1; x = 3 or 4, y = 4 IC = 1, y = 2 and x =4, y = 8 x = l or2 x = 1, R = Et," Pr";" x = 2, R = Me," Et," Ph; x = 3 , R = B u t ; x = 4 . R=Pr",aBu" x =2, R = Et; x = 3, R = Me, P f , But
x = l , R=Et;x=2, R=Me
Evidence for the formation of these species was ohvdined from X-ray and thermal desorption studies of the solid phases in the UC14-amine systems (1. Kalnins and G. Gibson. J . InorE. Nucl. Chem., 1958, 7, 55).
a
Table 8 Some Complexes with Hydrazine, N2H4 and Phenylhydrazine, PhNHNH, MI~F,.XN,H,
MIv = Th, x = 1, 1.66; MIv = U, x = 2 , 1.5 or 2
ThC1,.4PhNHNH2 UC14.xNZH4 Th(S04)Z.xN2H4
x=6or7 x = 1.5 or 2
Table 9 Some Complexes of Ethylenediamine and other Diamines with Actinide(1V) Compounds Ethylenediamine, en Th(C$H,NO),-C,H,NO.en C,H,NO = 8-hydroxyquinoline UC14.4en N,N,N',N'-Terramerhyletkyfen~d~arn~~e, tmed U{(CF,),CHO } ,-tmed Dinminoalkanes x = 2, y = 5 , L = 1,2-diaminopropane and 1,Cdiaminobutane ThBr,.xL.yH,O x = 2, y = 2 and x = 4, y = 6, L = 1,4-diaminobutane x = 4, y = 2, L = 1,2-diaminopropane Diarninoarenes L = 1,2-, 1,3- or 1,4-diaminobenzene, 4,4'-diaminoThC14.2L biphenyl (benzidine), 4,4'-diamino-3,3'-dirnethyIbiphenyl (0-tolidine) or 4,4'-diamino-3,3'dimethoxybiphenyl (0-dianisidine) L = 1,8-diaminonaphthalene UCI,*ZL L = 1,Zdiaminobenzene 2UC14*5L L = 1,Zdiaminobenzene UBr4.4L [Th(NOz&zI(NOdz L = 1,Zdiaminobenzene
analogous to the known amide complexes of this stoichiometry. Thorium tetraacetate is soluble in ethylenediamine, but the complex formed has not been identified.
(ii) N-Heterocyclic ligands (a) Pyrrolidine, C4H9N, and B-pyridyl-Lt.-N-methylpyrrolidine (nicotine), ClOHl4N2.The only recorded complexes appear to be ThC4.CloHl4N2 and UC14.3C4HgN.The latter may be an example of seven-coordinate uraniurn(1V).
The Actinides
1139
( b ) Pyridine, C ~ H S Np ,y , and derivatives. Thorium(1V) complexes with pyridines (Table 10) are commonly of the form ThGL4, where X = Cl, Br or NCS and L is pyridine or a variety of substituted pyridines. They are precipitated when a solution of the parent compound in, for example, absolute ethanol, is treated with an excess of the ligand, and are non-electrolytes in n i t r ~ b e n z e n e .The ~ ~ thorium atom is evidently eight-coordinate, and the same applies to Th14.6L, which behaves as [Th12(L),]12 in nitrobenzene, and complexes of the perchlorate, [ThL8](C104)4,formed with pyridine and less sterically demanding substituted pyridines; with 2,4- and 2,6-dimethylpyridine, [ThL6](c104)4are obtained. The coordination numbers of the metal sites in the bis complexes Th(N03)4.2L (L =p y or a substituted pyridine) and Th(C13CC02)4.2py are uncertain. Thorium(1V) and uranium(1V) complexes of composition [M(py)8]3[Cr(NCS)6]4have also been recorded, and bis pyridine complexes U&-2py (X = Cl, Br), which presumably invoIve six-coordinate uranium(IV), have been reported. ( c ) Piperidine, CsH,,N. The piperidine complexes ThC14.4C5H,1Nand UX4-4C5HllN(X = C1, Br) are probably examples of the eight-coordinate metals. The nature of the hydrated compound, Th(N03)4.CsH11N.6H20,is uncertain. Table 10 Some Complexes of N-Heterocyclic Ligands with Actinide(1V) Compounds Pyridine, C,H,N ThCL(2L UX4.2PY ThX4.4L
by) and substituted pyridines L = 2-Me- or 2-H,N-C5H,N X = C1, Br X = CI, Br, NCS; L = py, 2-Me-, 2,4-Me2- and 2,6-Me2-pyridine L = (2-HzN, 3-€IO)C,H,N L = py, 2-Me-, 2,4-Me2- and 2,6-Me,-pyridine L = py, 2-Me-, 2-H2N-, 2,4-Me2- and 2,6-Me,-pyridine
r
L = 2-HzN-, 2,4-Me2- and 2,&Me,-pyridine L = py, 2-Me-C5H4N M=Th,U G H 6 0 , = 2-hydroxybenzaldehyde X = Cl, Br, NCS
L = CgH7N bo-GHTN L = C,H,N or ko-GH,N
( d ) Quinoline and isoquinoline, C9H7N. The quinoline complexes Th&.4C9H7N (X = C1, Br, NCS) (Table 10) are non-electrolytes in nitrobenzene, as is the isoquinoline complex Th(NCS)4-4isoCgH7N, whereas the complex Th4-6C9H,N behaves as a 1:2 electrolyte, [ThI2(GH7N),]I2. The complexes of the perchlorate with quinoline and isoquinoline behave as 1:4 electrolytes, [ThL8](C104)4, and in all of these, the thorium(1V) ion is evidently eight-coordinate. The complexes of both ligands with thorium tetranitrate, Th(N0&-2L, are non-electrolytes in nitrobenzene and may be examples of 10-coordinate thorium(1V). The only recorded uranium(1V) complex with quinoline, UC14C&N, is doubtful. (e) 2,2'-Bipyridyl, bipy. The complexes Thx.2bipy (Table 11) (X = C1, Br,34 NCS35) and Th(N03)4-bipy35are non-electrolytes in nitrobenzene, whereas Th14.3bipy behaves as a 1 :2 e l e ~ t r o l y t e ,[ThIZ(bipy),]I2, ~~ and Th(C104),-4bipy as a 1:4 e l e ~ t r o l y t e The . ~ ~ metal ion is evidently eight-coordinate in these compounds, and in the uranium(1V) complexes, U&.Zbipy (X = C1, Br), except for the nitrate in which it is presumably 10-coordinate. The tetraethylammonium salt of the rather unusual complex anion [U(NCS)5(bipy)z]- is obtained by reaction of (NEt4)4[U(NCS)8]with bipy in MeCN. In the structure of this anion the uranium atom is bonded to nine nitrogen atoms in a highly distorted monocapped square antiprismatic arrangement in which one bipy molecule occupies a position bridging the cap and the near square of the antiprism, while the other bipy molecule spans the opposite edge of the far square of the antiprism (Figure 1). The structure codd also be described in terms of a seven-coordinate pentagonal bipyramidal arrangement in which each bipy molecule occupies an axial position .36a The nature of the other recorded bipy complexes, and of those with 4,4'-dimethyl-2,2'bipyridyl, Th&*1.5C12H12NZ (X= Cl or NO3) is unknown. COC3-KX
1140
The Actinides Table 11 2,2'-BipyridyI (bipy) Complexes with Actinide(1V) Compounds MKV&.2bipy Th14-3bipya Th(NO,),. bipyb Th(C10,),.4bipyb ThCr20,.0.5bipy Th(c8H4F302s)4' bipy UCI,( 0Et)ebipy UCI,( acac),.bipy __ UCl;L.bipy (NEt,)IU(NCS),(biPY )zlc
ThX4*l.5L a
MIv
= Th,X = Cla, Bra, NCSb and M" = U, X = C1, Br
C,H,F,O,S = 4,4,4-trifluoro-1-(2-thienyl) butane-l,3-dione
H,L
= HOC6H4CH=NCH2CH2N=CHC6H4OH
X = C1, NO3; L = 4,4'-dimethyl-2,2'-bipy
A. K. Srivastava, R. K. Aganval, M. Srivastava, V. Kapoor and T. N. Srivastava, 1.Inorg. Nucl. Chem.,
1981.43. 1393. R. K. Agarwal, A. K . Srivastava, M. Srivastava, N. Bhakru and T. N. Srivastava, J . Inorg. Nucl. Chem., 1980,42, 1775. 'R. 0. Wiley, R. B. von Dreele and T. M. Brown,Inorg. Chem., 1980,19,3351. b
w
"
e 1 Perspective view of the structure of the [U(NCS),(bipy),]- aniod6" (reproduced with permission from Inorg. Chem., 1980, 19, 3353, Copyright 1980, American Chemical Society)
c f ) Terpyridyl, c15H11N3. The only recorded complexes are ThC14-2Cl5Hl1N3.8H~0and Tfi(N03)4.1 .5C15HllN3. (g) Imidazole, C3H4N2. Complexes Th&L, some solvated (e.g. X = Cl), have been recorded for L = imidazole or substituted imidazole. Conductivity data for the 2-(2'pyridy1)benzimidazole (C12H9N3) complexes Th;Y4(C12H9N3)2indicate that the chloride complex is a non-electrolyte in MeOH, MeNOz or DMF, whereas the thiocyanate and nitrate are appreciably dissociated in solution. The molar conductivity of Th(C104)4.4C12H9N3is lower than that expected for a I :4 electrolyte. ( h ) Dipyridylethanes and dipyridylumine. Hydrated complexes with 1-(3-pyridyl)-2-(4pyridy1)- and 1,Z-di(Cpyridy1)-ethane, Th(N03)4.L-H20, and di(2-pyridyl)amine, ThClg2L*2Hz0and ~ ~ I ( N O ~ ) ~have * Z Lbeen , reported. Very little is known about them. (i) l,lO-Phenanthroline, phen. [Th(NCS)4(phen)z] (Table 12) is a non-electrolyte in nitrobenzene, whereas Th14-3phen behaves as a 1:2 electrolyte, [Th12(phen)3112.The perchlorate behaves as a 1:4 electrolyte in methyl cyanide, [Th(phen)4](C104)4,so that the thorium ion is evidently eight-coordinate in these complexes. Th(N03)4.2phen, and the 1:1 complexes with the P-diketonates, presumably involve 12- or 10-coordinate thorium(1V). A few complexes with 2,9-dimethyl-l,lO-phenantholine, Th&L, (X = C1 or NO3), are also known. (j) Pyruzine, c4H4N2. The hydrated bis pyrazine complex, T ~ I ( N O ~ ) ~ . ~ C ~ H Yand ~.~H~ the anhydrous tris complex, Th(N0&m3C4bN2, have been recorded. The IR spectrum of the latter suggests that the pyrazine is acting as a monodentate ligand and that the nitrate groups are bidentate, indicating 11-coordination. However, the tris complex behaves as a 1:4 electrolyte in DMF and as a 1 : 2 electrolyte in methyl cyanide or nitromethane. Complexes of UCI4 with pyrazine or pyrazine-2-carboxamide, UC14.2.5L, and with pyrazine-2,3dicarboxamide, UC14.2L, have also been reported.36b
The Actinides
1141
Table l2 1 JWhenanthroline (phen) Complexes with Actinide(1V) Compounds ThX4.phen MIvX4-2phen
X = C1, C&F,O,S (tta) or Cl0H6F3O2(4,4,4trifluoro-l-phenylbutane-l,3-dionate) M'V=ThorU,X=CIorBr;M'V=Th, X = NO, or NCS
Th14.3phen Th(NO3),.2phen.MeCO,Et M Clp(OEt).Zphen UCl,(acac),.phen ThCr20,-0.5phen
MIV=Th or U
( k ) Piperazine, C4HloN2, and its derivatives. Several complexes with thorium(1V) compounds (Table 13) are known. They are probably polymeric, with the ligands coordinated through both nitrogen atoms. Table l3 Complexes of Piperazine, C4Hl$J2 and its Derivatives with Actinide(1V) Compounds Piperazine, C4H1oN2 nC~(C4H10H2)2 Th(No3)4(C4H10N2)z.Ys N-Methylpiperazine, CSHI2N2 ThC14(C5HlZN2)2 ~(N~~(C~HIZN~)X'YS 2-Methylpiperazine, C5H,,N2 ThC14(C5H12N2)2 Th(N03)4(C5H12N2)x.ys N,N'-Dimethylpiperae, c&& ThCI4.1. S C ~ H , ~ N Z Th(N03>4(c6H14NJz.Ys N-Phenylpiperazine, CIoHl4N2 Thc14.1. ~ C , & I H , ~ N ~ Th(NO3)4-2.5C,oH,4N2.S N,N'-Diaminopiperazine, C4HI2N, Th(NCS)4(C,Hi,N3,.4H20
x = 1, y S = MeOH; x = 2, y S = THF or 2EtOH
x = 1, yS = 4MeOH, x = 2, y S = 2EtOH; x = 3, yS = THF x = 1, y S =TEE or 4MeOH x = 2, y S = 2EtOH x = 1, y S = 4MeOH, x =2, yS =THF
S = MeOH, EtOH or THF
(iii) Dialkyl- and diaryl-amides Compounds of the type M(NR2), (M=Th or U) are obtained by reaction of the metal tetrachloride with LiNR, in non-aqueous solvents. They are all highly reactive towards protonated species and they readily undergo COz, COS, CS2 or CSe2 insertion into the M-N bond to form carbamates. When the group R is very bulky, as in U ( N P ~ Z ) ~HU{N(SiMe3)2}338 ,~~ and C1Th{N(SiMe3)2}3,39the compounds are monomeric. The coordination arrangement in the diphenylamide is highly distorted from tetrahedral or other regular whereas in U(NEt,), the compound is a dimer in the solid state and the geometry about each uranium atom is a distorted trigonal bipyramid.40 A partially hydrolyzed derivative of the diphenylamide, [UO(Ph2N)3Li(OEt2)]2,has been isolated as a by-product in the preparation of U(NPh2), and in this compound each five-coordinate uranium atom is bonded to two bridging oxygen atoms and three terminal nitrogen atoms with approximately trigonal bipyramidal geometry; one nitrogen atom and one oxygen atom occupy the axial position^.^' In the preparation of the uranium(1V) alkylamides derived from N,N'-dimethyldiaminoethane, the main product is the trimer, U3(MeNCH2CH2NMe)6;the tetramer, U4(MeNCH2CH2NMe)8,is obtained as a by-product. The central uranium atom in the trimer is octahedrally coordinated to six bridging nitrogen atoms, and the other two uranium atoms are coordinated to three bridging and three terminal nitrogen atoms in a distorted trigonal prismatic array,41whereas in the tetramer the bridging leads to a closed unit in which each uranium atom is bonded to six nitrogen atoms in a highly distorted trigonal prismatic geometry.,'
1142
The Actinides
(iv) Triazenes, thio- and seleno-cyanates
( a ) Triazenes and related compounds. A complex of thorium tetranitrate with 2,4,6tris(dimethy1amino)triazene (C9H1&, hexamethylmelamine), Th(N03)4.&H18N6, and the probably polymeric derivatives of purine and adenine, Th(C5H3N4)2C12, have been re~orded,''~ but little is known about them. (b) Thiocyanates. Anhydrous Th(NCS), has been reported, on the basis of thermogravimetric evidence, to be the product of the thermal decomposition of complexes of the type Th(NCS)4b (e.g. L = 2,6-lutidine N-oxide); it is said to decompose to ThO(NCS)2 above 490 "C." The hydrate, [Th(NCS).,(H20),], is better established; the uranium(1V) analogue is known only as the solvate of composition [U(NCS),(H20),1. (18-~rown-6)~.~.3H~O.MeCOBu in which the structure of the [U(NCS)4(H20)4]molecule is a distorted square antiprism in which the upper square comprises one O(H20) and three N(NCS) atoms and the lower square three O(H20) and one N(NCS) atoms.45 Anionic complexes of the type M!,[M1V(NCS)8]are well known (Table 14). The tetraethylammonium salts of this anion have been reported for MIv = Th, Pa, U, Np and Pu and all have the same crystal structure. The coordination geometry of the anion in (Et,N),[U(NCS),] is a cube, which requires f-orbital participation in the bonding, but the [U(NCS)8]4- anion in the cesium salt is an almost perfect square antiprism. The IR spectra of the dihydrated rubidium and cesium salts of the [M(NCS)*I4- ion (M = Th, U) suggest that the coordination geometry of the anion is dodecahedral in the solid salts, but square antiprismatic in acetone (Crystalline Cs4[Th(NCS),].2H20 is not isostructural with the uranium(1V) analogue.) The hydrated rubidium and cesium salts dehydrate at 60 to 100°C. The other thiocyanato complexes listed in Table 14 require further investigation. Table 14 Actinide(1V) Thiocyanates and Thiocyanato Complexes
( c ) Selenocyanates. The salts (Et,N),[M(NCSe),] (M = Pa, U) are isostructural with the corresponding thiocyanates. Compounds of composition MiTh(NCSe)6-xDMF (MI= Na, x = 3 and K, x = 4.5) and &Th(NCSe)g2DMF have been recorded, but their nature is uncertain. ( v ) Polypyrazol-I-ylborates
The known thorium(1V) and uranium(IV) complexes are listed in Table 15. The attempted preparation of Th(H2BPz& (Pz = C3H3N2)yields the salt KTh(H2BPz&. Although the NMR spectrum of this salt indicates dissociation to Th(H2BPz& and K(H2BPz2)in (CD3)2C0, Th(H2BP~2)4 could not be isolated. Table 15 Actinide(1V) Polypyrazol-1-ylborates ThCl(HBPz,), MIVXzL,
Pz = C3H3N, MIv = Th,L = HBPz, with X = C1 or Br; L = BPz, with X = Br M"' = U , L = H B P z ~H , B ( ~ , ~ - M @ z ) PhZBPq ,, or BPz,. with X = C1
The Actinides
1143
The 'H NMR spectra of the thorium(1V) complexes ThXZ(HBPz& (X= C1, Br) in CD2C12 or (CD&CO (X= Cl) indicate that the HBPz3 ligand is tridentate, in contrast to UC12(HBPz3)2 and UC12(HB{3,5-M@Pz}3)2, in which the ligand appears to be bidentate, a conclusion in agreement with the solid reflectance spectra of the two compounds which are consistent with ~ ) ~ is more six-coordinate uranium(1V). The 'H NMR spectrum of T ~ B I - ~ ( BinP ~CD2C12 complicated and suggests that the proportion of C3H3Nzrings that are bonded to thorium to those that are not bonded is 3 :2, so that the complex may be of the form47 [(PzzBPz2)zTh(pPzzBPz2)Th(PzzBPz2)2]3+ [Th(BPz4)Br6I3-. In contrast, the 'H NMR spectrum of UC12(BPz4)2 is consistent with the presence of one bidentate and one tridentate BPz4 ligand, suggesting seven-coordination. The 13C NMR spectrum of U(HBPz& indicates the presence of two bidentate and two tridentate HPBq ligands, suggesting 10-coordination. Cyclopentadienylthoriurn(1V) and -uranium(IV) pyrazol-1-ylborates of the types (q5-C5HS)MCl2(H,BPz4-,) (M = Th, R = 0; M = U, n = 1 or 2) are also known. (vi) Nitriles The majority of the known complexes are with the tetrahalides (Table 16) and are obtained directly from the halide and the ligand. In most cases they have the composition M&(RCN)4 and the UV-visible spectra of the uraniurn(1V) compounds are consistent with eightcoordination. However, with the more sterically demanding nitrile, Bu'CN, tris complexes, U&(BU'CN)~ (X = C1, Br), are obtained in which the metal centre is presumably sevencoordinate. In the IR spectra of the nitrile complexes the Y(CN) feature is always shifted to higher frequency by 20 to 30 cm-' on complexation, except for the UF4 compounds for which the shifts are 44 to 5 4 m - l . The shifts in v(CN) for the cyanogen chloride complexes, MC14.xNCC1 (Table 16), are +23 to +24cm-'. ThC14.2MeCN and ThOCI2-MeCNhave been obtained respectively by thermal decomposition and by controlled hydrolysis of ThC14.4MeCN. Table 16 Nitrile Complexes with Actinide(IV) Compounds UF,-MeCN-yHF ThC14-2MeCN MCld.4MeCN MBr-4MeCN MT4-4MeCN
ThC1,.4RCN UX4*4RCN U&-3ButCN ThOQ,-MeCN MCI,.xNCCl
y = O or 2 .M = Th, Pa, U, Np M = Th, Fa, U , Np, Pu M = Th, Pa R = Ph, PhCH, X = C1, Br; R = Et, F"',W, Bun, Ph X = C1, Br M = T h , U, N p ; x = 1 or 2
(vii) Oximes and related ligands Very few complexes with oximes have been recorded. A pyridine-Zaidoxime { CsI€,N)CH=NOH} complex, possibly Th(CeHsN20)s, is obtained as a green solid by e maporating an absolute ethanol solution containing Th(NO,), and the ligand, Isatin p-oxime ( :&C(=NOH)C(OH)=lk) and the isomeric &-oxime (k6H4C(0)C(=NOH)NH) yield yellow and red precipitates respectively with ammoniacal solutions of thorium(1V) salts, and phenyl pyruvic acid oxime (PhCH2C(=NOH)C02H) precipitates thorium from aqueous solutions of the tetranitrate, but the composition of these products is unknown. A large number of urea and substituted urea Complexes with actinide(1V) compounds are known; in these the ligands are bonded to the metal via the carbonyl oxygen atom, and these complexes are therefore described in the section dealing with carboxylic acid amide complexes (p. 1164).
4,223 Phosphow und amnic ligands Most of the early reports of phosphine or arsine complexes are erroneous, the products being phosphine oxide or arsine oxide compounds.
1144
The Actinides
(5) Phosphines A complex with trimethylphosphine, UC14(PMe3)3,has been reported, and complexes with M%PCH2CH2PMe2(dmpe)are now well established. ThI,(drn~e)~ separates when a solution of Th14 in CH2C12containing an excess of dmpe is cooled to -20 "C and ThCl,(drn~e)~ is obtained by heating ThC4 with dmpe at 80 "C. T h C L ( d m ~ e )reacts ~ with MeLi at 0°C to form ThMe,(dm~e)~,and this product reacts readily with hydrogen chloride or phenol in toluene to form ThCl,(dm~e)~and ~Th(OPh)4(dmpe)2].C7Hs,which is isostructural with [U(OPh)4(dmpe)2]. This last is obtained either from UMe,(dmpe), and phenol or by direct reaction of UCl, with LiOPh in the presence of dmpe. The uranium(1V) complexes U&(dmpe)2 (X= Br, Me) have also been reported. In the structure of [U(OPh)4(dmpe)z], four phosphorus and four oxygen atoms occupy the A and B sites respectively of a d o d e c a h e d r ~ n .Although ~~ the attempted preparation of complexes of Ph2P(CH2),PPh,(dppe) with ThCl, and Th(N03), led to the diphosphine dioxide complexes, UC14(dppe) has been obtained as a precipitate from a THF solution of the chloride and dppe.
(ii) Arsines The complex PaC14(diars) [diars = o-phenylenebis(dimethylatsine)] is reported to be formed by reaction of the ligand with faC15 in benzene.49 Treatment of UCl5-C3Cl40(C3CbO= trichloroacryloyl chloride) or UCl, with diars yields the analogous UCL,(diars) as a yellow (? oxidized) or blue-green solid respectively.
40.2.2.4 Oxygen ligands
(i) Aqua species, hydroxides and oxides ( a ) Aqua species. Some of the reported hydrates of actinide(IV) compounds are listed in Table 17. A very large number of other hydrates have been recorded, but in many cases it is uncertain whether any or all of the water molecules are coordinated to the actinide metal atom. In the absence of a structure determination, the only evidence for bonded water is a high dehydration temperature. For example, the octahydrated sulfates, M(S0&.8Hz0 (M = Th, U, Pu) lose four molecules of water at about 70 "C and the remaining water can be removed only at temperatures exceeding 400"C,indicating that four molecules of water are bonded to the metal atom, a conclusion confirmed by the determination of the structure of U(S04)2(H20)4. The uranium atoms are surrounded by a square antiprism of oxygen atoms, with each uranium atom bonded to four molecules of water and linked by bridging sulfate groups to other uranium atoms.53 The coordination geometry is similar to that of [U(NCS),(H20),] (p. 1142). Higher coordination numbers have been reported for hydrates of compounds which involve more compact bidentate ligands. The nine-coordinate arrangement in [Th(CF3COCHCOMe)4(H20)] consists of a cap ed square antiprism with the bidentate ligands spanning the edges linking the two square facesP4 and in the hydrated tropolonate, [Th(trop),(HzO)], one trop ligand occupies a slant position on the cap of the square antiprism and the water molecule occupies another vertex of the cap55(Fig. 2). The coordination geometry in the hydrated y-isopropyltropolonate, [ThL4(H20)], is also a singly capped square a n t i p r i ~ m The . ~ ~ same geometry has been reported for the diaqua (oxodiacetato) sulfato thorium(1V) complex, [Th{O(CH2C02)2}(S04)(HZ0)2J~H20. In the polymeric network, each oxodiacetate group is chelated to one thorium atom via the ether oxygen atom and two carboxylate oxygen atoms, and also links two different thorium atoms through the two remaining carboxylate oxygen atoms. The sulfate grou s are also bridging and the polyhedron is completed by the two bonded water molecules.QSb In the hydrated hydroxyacetate, [U(HOCH2C02)4(H20)2], the uranium atom is 10coordinate in a ' bicapped square antiprismatic arrangement in which the eight carboxylate oxygen atoms occupy the corners of the square anti rism and the oxygen atoms from the two water molecules occupy the cap positions (Figure 3)p7 Bicapped square antiprismatic geometry has also been reported5*" for the hydrated pyridine-2,6-dicarboxylate, [Th{C5H3N-2,6(C02)2}2(H20)4], as has that of the oxodiacetate, [Th{O(CH2C02)2}2(H~0)4].6H20.58b
The Actinides
1145
Table 17 Some Hydrates of Thorium(IV), Protactinium(IV), Uranium(IV), Neptuniurn(1V) and Plutonium(1V) Compounds MF4.2.5H20 u[MII(cN)&H,o M(NO&.xH,O M(SO~),.XH~O MF,SO,. 2H20 Th(ClO,),*xH,O Th(HIO,).SH,O M(C204)24HJ' ~ ~ (
M = Th,U , Pu M" = Fe, x = 6; M" = Ru,Os, x = 10 M =Th, x = 2 , 4 or 5; M = N p , x = 2 ; M = Pu, x = 5 M = T h , U , P u , x = & M = T h , P a , U, Np, P u , x = 4 M=Pa,U x = 2, 3, 4, 6 or 8
M = Th,Pa, U, Np, Pu ~
~
~
Bond distances (A) in the coordination polyhedron Th-O(water) 2.608(13) 2.491(14) Th-Wl) TM(12) 2.413(14) TM(21) 2.395(17) 2.362(17) Th--O(W 2.496(14) Th4(31) Th-0 (32) 2.427(13) Th--0(41) 2.417(14) Tha(42) 2.412(15)
~
~
~
~
&
~
~
@
)
Bond angles (") subtended at thorium atoms O( ll)-Th-O( 12) 62.2(0.5) 0(21)--Tk-o(22) 63.1(0.7) O(31j T h 4 ( 3 2 ) 61.8(0.6) O(41) - T h 4 ( 4 2 ) 62.3(0.7)
Figure 2 Perspective view of the [Th(trop),(H,O)] (trop = tropolonate) molecule and of the coordination polyhedron55
Figure 3 The structure of the uranium(1V) glycolate dihydrate molecule, [U(HOCH,C02)4(H,0)J57
~
~
The Actinides
1146
The
coordination
geometry
about
the
11-coordinate
thorium
atom
in
Th(N03)4*5H20([Th(N03)4(H20)3)*2Hz0) is basically a monoca ped trigonal prism in which four of the prism apices are occupied by bidentate nitrate groups.' In the dimeric basic nitrate, [Th2(OH)2(N03)6(H20)6]-2H20, the thorium atoms are bridged by two O H groups, and each thorium atom is also coordinated to three bidentate nitrate groups and three molecules of water. The geometry can be considered as a rather distorted dodecahedron in which the nitrate groups occupy three apices.60 (b) Hydroxides. The structures of the known hydroxides have not been recorded; a few structures have been reported for basic compounds. The structure of Th(OH)2Cr04+H20 is built up of infinite chains, [Th(OH)2]F*, containing two almost parallel rows of OH groups so that each thorium atom is in contact with four OH groups; the Cr04 groups are so packed that each thorium atom is in contact with four oxygen atoms of four different Cr04 groups, making up a square antiprismatic arrangement of oxygen atoms about each thorium atom.6' The structure of Th(OH),S04 is the same.62 (c) Oxides. All the dioxides have the cubic fluorite lattice, with the eight-coordinated metal ion at the centre of a cube of oxygen atoms. A considerable number of ternary oxides, such as &Tho3, BaMIV03 (MIv = Th, Pa, U, Np or Pu), Li8Pu06 and BaCe(or Ti)Pu06, are also known. (ii) Peroxides
Because of the ease of oxidation of protactinium(1V) and uranium(IV), peroxides and peroxo complexes are limited to their higher oxidation states. The compounds M04.nH20 precipitated from dilute acid solutions of neptunium(1V) and plutonium(1V) by hydrogen peroxide appear to be actinide(1V) compounds. Soluble intermediates of the type [Pu(p02)2Pu]4+are formed at low hydrogen peroxide concentrations. The hydrated thorium peroxide sulfate, Th(O2)SO4.3H20, is very stable to heat, and thorium(1V) peroxo compounds of variable composition, approximating to Th(02)1.6(A-)0.50~;5.2.5H20, with A = C1 or NO3, are obtained from aqueous solutions of hydrogen peroxide and the appropriate thorium compound, and in the case of the nitrate the analytical results of one such product are compatible with a hexameric f ~ r m u l a t i o n , ~ ~ T~(02)10(N03)4-10Hz0. Peroxocarboxylato and phenoxo compounds are also examples are Th(RC02)2(02)(R = C5H4N-2- and 2-H2NC&) Th(2-H2NC6H40)&(o2) and Th (2,6-(OzC)&H3N) (02)-H20. ?
(iii) Alcohols, phenols, alkoxides, aryloxides and silyloxides ( a ) Alcohols and phenols. Alcohol solvates of thorium and uranium tetrachlorides, MC4.4ROH (Table IS), are obtained by evaporating solutions of the tetrachloride in the alcohol (M = Th, U) or by treating the hydrated chloride with a mixture of the alcohol and benzene, removing the liberated water by azeotropic distillation (M = Th). ThC14.4Bu"OH and ThCL+.4BuiOH are, however, best prepared by heating ThCI4.4Pr'OH with a mixture of benzene and the butanol, removing the liberated Pr'OH as the benzene azeotrope. The attempted preparation of the butanol (R = Bun, BuSeCor Bu') adducts of UCI4 led to solvolysis. Table l8 Alcohol and Phenol Adducts of Actinide(1V) Compounds MC14.4ROH ThC14.GH70H Th(C,H,NO),.EtOH Th(C&NO),.2ROH Th(GH,NO)(OMe)( CI,CCO,),-MeOH U(CF,COCHCOPh),.Bu"OH Np(OEt),.Et,OH" Pu(OPr'),.Pr'OH
M = Th, R = Me, Et, Pr", P i , Bun, Bu' M = U, R = Me, Et, Pr", PI' C,H70H = o- or rn-cresol
R 2,4-(O&&H,,
'A. K. Solanki and A. M. Bhandari, Radiochem. Radioanal. Len., 1980,43,279.
2,4,6-(OJ'J),C,H,
The Actinides
1147
The bis 2,4-dinitro- and 2,4,6-trinitro-phenol adducts of the 8-hydroxyquinolinate, is heated with the phenol. Th(C9bN0)4-2ROH, are obtained when Th(C&NO),.H,NO The frequencies of the asymmetric and symmetric v(N0) features due to the nitrophenols are almost identical in their IR spectra to those of the free phenols, which may therefore be hydrogen bonded to oxygen atoms of the &€&NO groups. (b) Alkoxides. The tetraalkoxides, M(ORk, are best prepared by reaction of the tetrachlorideg or their alcohol solvates with the sodium or lithium alkoxide in the corresponding alcohol [T~(OPI!)~,M(OR)4 with M = U or Np, R = M e or Et] or in dimethyl cellosolve [U(OR),; R = Pr" or Pr']. PuC14 does not exist, but Pu(OPt), has been obtained by treating a suspension of (pyH)2[PuC16] in benzene and Pr'Oll with ammonia; Th(OPr'), has been prepared from (pyH)z[ThClb] in the same way. An alternative, but expensive, preparative route is by reaction of the dialkylamide [e.g. U(NF&] with the alcohol in ether. The uranium t-butoxide seems oniy to have been obtained by reaction of UCl, and Bu'OH with a solution of KNH, in anhydrous Iiquid ammonia and by dropwise addition of Bu'OH in ether to a solution of the allyl, U(q-GH5)4, in the same solvent at -30°C. Because of the relative ease of preparation of the isopropoxides, M(OPr'), (M = Th, Pu), other alkoxides are commonly prepared from them by reaction with an excess of the appropriate alcohol in boiling benzene. The only recorded fluoroalkoxides are the THF solvates, U{ OC(CF3),),.2THF and U{(OCH(CF3)2)4.2THF(Table 19). Table 19 Actinide(1V) Alkoxides M(Wd
UCIAORl! Mm(Ope)9 Li,U(OMe), NfiO(OEt)6a
M=Th, R = Me, Et, Prn, Pr', Bun, Bu', But, Pent", PentnBo,CMe,Et, CMeEt,, CMe2PP, CMe,Pr', CMeEtPrl, CMeEtPr', CEt, M = U, R = Me, Et, Pr", Pr', Bur M = N p , R = M e , Et M = Pu,R = Pr', But, CMeEt, R = Me, Bu" M = Li, Na
A. K. Solanki and A. M. Bhandari, Radiochem. Radwanaf. Lcff., 1980,43,279.
All the known tetraalkoxides are very easily hydrolyzed by water vapour and the uranium(1V) compounds oxidize rapidly in air, so their preparation must be carried out under nitrogen. Molecular weight determinations (M = Th, U) indicate a considerable degree of polymerization, approximately tetrameric in the case of Th(OR), with R = Pr' or MeEtCH, but the molecular complexity decreases to about 3.4 for R = But, and with R = CEt, and CMeEtPr' the alkoxides are monomers in boiling benzene.65aThe plutonium compound P U ( O C M ~ Eis~ ~ ) ~ volatile at 150"C/O.OStorr, suggesting a low molecular corn lexity. The oxoalkoxides U ~ O ( O B U ) ~and $ ~ ~U 3 0 ( O ~ e 3 ) , 0 g care structurally similar to the molybdenum analogue, M O ~ O ( O C H ~ C Mbut ~ ~ the ) ~ ~U--U , bond distances, 3.576(1) 8, and 3.574(1) A, show that metal-metal bonding does not occur. Anionic alkoxide complexes of the type M'Th2(0Pi)9 (M'= Li, Na) have been mentioned in a reviewM and Li2U(OMe), precipitates when UC4 is added to the stoichiometric quantity of LiOMe in MeOH. Compounds of the type U[A1(OPr')4]4and U[A1(OPr')4]2C12are also known. The dialkoxide dichlorides, U(OR)2C12(R = Me, Bun), are obtained by reaction of UCl, with the stoichiometric quantity of the sodium alkoxide in ether (R = Me) or THF (R = Bun). ( c ) Aryloxides. A few thorium(1V) aryloxides have been recorded (Table 20); the 4,6dinitro-2-aminophenol derivative, Th[OC&12(N02)2(NH2)]4.2H20, is precipitated when aqueous sodium picramate is added to an aqueous solution of thorium tetranitrate. Its IR spectrum suggests that the amino nitrogen atom may be bonded to the thorium atom.67 Unsolvated uranium(1V) tetraaryloxides are not known; for solvates with ammonia, amines and phenols see Tables 6 (p. 1137), 7 (p. 1138) and 18 (p. 1146) respectively. However, U(OPh)2C12is obtained in the same way as U(OMe)zC12 (see above). (d) Silyloxides. The only actinide(1V) compound of this type appears to be U(OSiMeEtz)2Clz,which is precipitated when U C 4 is treated with Li(OSiMeEt,) in a mixture of benzene and dimethyl cellosolve. It oxidizes readily in airam COC3-KK'
1148
The Actinides Table 20 Actinide(1V) Aryloxides -
Th(OR)4+2H,O MCl2(OR)2 MCI,(OR)
4,6-Dinitro-2-aminophenoxide, R = {4,6-(NO2),}(2-H2N)C6H, M = Th,R = Ph, 0-or p-N02C6H,, a-or /3-C,J17 M = U , R=Ph M = Th,R = 0-or m-MeC,H,, (Y- or &C,,H,
(vi) P-Ketoenolates, Tropolonates, Catecholates, Quinones, Ethers, Ketones and Esters (a) p-Ketoenolates. The wide range of known /3-diketone, ketoaldehyde and ketoester complexes is summarized in Table 21. They are commonly prepared by treating a dichloromethane, ethanol, aqueous or aqueous methanol solution of the actinide(1V) with the ligand (or its sodium or thallium salt) followed, when necessary, by addition of ammonia or aqueous alkali to precipitate the complexes ML4. Alternative methods include reaction of the metal tetrachloride with the ligand in the presence of sodium (i.e. reaction with NaL) or reaction of a carboxylate [e.g. U(EtC02)4]with the ligand in dibutyl ether or benzene. Solvent extraction is also a useful preparative method; examples are the extraction of Np(MeCOCHCOMe)4 into benzene by shaking an aqueous solution of neptunium(IV), saturated with the ligand, with benzene at pH4.5 to 5.0, and extraction of P u ( M ~ C O C H C O P ~from ) ~ an aqueous plutonium(1V) solution (at pH8) into a chloroform solution of the ligand. The product is recovered from the extract by evaporation to dryness. The complexes are normally purified by recrystallization from benzene, mixtures of hydrocarbons or methanol and many of them can be sublimed under vacuum. Analogous salicylaldehyde complexes, M( OCJ+4CHO)4 (M = Th, U) and Th(OC6H4CH0)3(N03)-H20,are also known. has C ~been mentioned in the patent A complex of composition P u ( M ~ C O C H C O M ~ ) ~ l i t e r a t ~ r e The . ~ ~ reaction of Th(MeCOCHCOMe)4 in benzene with carboxylic acids, RC02H (R = CF,, CC13, CHC12, 2-C5H4N) leads to partial displacement of the P-diketone, yielding products of the type Th(MeCOCHCOMe)3(RC02).xH20 (x = 4 for R = CF3, CC13 and CHC1, and x = 1 for R = 2-CsI&N). The products of further displacement of the P-diketone appear to contain acetate (e.g. Th(MeCOCHCOMe)(MeC02)2(C5H4NC02).2H20) which presumably results from the decomposition of the acetylacetonate group. The attempted preparation of lithium or thallium salts of the complex anion [U(MeCOCHCOMe)J- have been unsuccessful. Structural information is available for several P-diketonates. ~YI(M ~C OC HC OMexists ~)~ in two crystal forms, the coordination geometry in the a form being principally dodecahedral, with significant distortion to square antiprismatic geometry, while in the j?form the geometry is principally square antiprismatic. It has also been suggested that the a form approximates most closely to a C, bicapped trigonal prism.70 In the benzene solvate, [Th(MeCOCHCOMe)4]-0.5C6&, the geometry is square antipri~matic.~~ Uranium(1V) acetylacetonate also exists in the CY and p forms, isostructural with the thorium analogues, but the protactinium(IV), neptunium(1V) and plutonium(1V) complexes have only been reported as the p form. The IY and p forms of T~(Bu'COCHCOBU')~are isomorphous with the uranium(1V) analogue^.'^ Th(PhCOCHCOPh)4 has been reported as being isomorphous with the corresponding protactinium(IV), uranium(1V) and cerium(1V) complexes; the coordination geometry in the last is a triangular faced dodecahedron, but a more recent p ~ b l i c a t i o n ~ reports ~ the coordination geometry of the uranium(1V) compound as square antiprismatic. The coordination geometry in anhydrous Th(CF3COCHCOMe)4 is a 1111 (D4-422) a n t i p r i ~ m ; the ~ ~ structure of the monohydrate has been discussed earlier (p. 1144). Th[CF3COCHC0(2-C4H3S)l4 is isostructural with the cerium(IV), uranium(1V) and plutonium(1V) analogues. The coordination polyhedron is a distorted dodecahedron in which the four ligands span the two perpendicular trapezoids of the d ~ d e c a h e d r o nIn . ~ the ~ complexes M(n-C3F7COCHCOBut)4,the thorium(1V) , uranium(1V) and neptunium(1V) compounds are isomorphous, but the plutonium compound is not. The coordination geometry about the thorium atom in the salicylaldehyde complex, Th(OC6H4CH0)4,is d ~ d e c a h e d r a lthe ; ~ ~compound is isostructural with U(OC61-L,CHO)4. (b) Tropolonates. The compounds M ( t r ~ p )(trop ~ = GH5O2), have been reported for M = T h , Pa, U, Np and Pu. The thorium complex is precipitated when a slight excess of
The Actinides
1149
Table 21 Actinide(1V) @-Ketoenolates,M(R'C0CR2COR3), R' =Me,
R2=H,
R' = Et, R' = Pr",
R2' CN, RZ=CI, R2 = C,H2Me,, R, = H, R, = H,
R' = P i , R' = But, R' = Ph,
R~ = H, R~= H, R, = H,
R'=PhC&C, R' = CF3,
Rz=Ph, R~= H,
R' = n-C,F,,
Rz= H,
a D.
R3=H, M = U R3 = Me, M = Th,Pa,' U,.Np, Pu R3 = CH,OEt, Et, Prn, Pr', Bun, M = U R3 = But, M = Th, U R3 = CH,CHMe,, M = Th R3 = n-C5H11, n-C6HI3,M = U R3 = n-C17H3,, M = Th R3=Ph, M = T h , U, Pu R3 = p-MeOC,&, M = Th R3 = C,H30 (Zfuryl), M = U R3 = (~-C&,)Fe(~-C,H,), M = Th R3=OEt, M = U R3=Me, M = T h R'=Me,M=U R3=Me,M=Th R 3 = Et , M = T h , U R3=Pr", M = T h , U R3 = CH,CHMe, M = Th R3=Pr', M = T h , U R3 = But, M = Th, U, Np,b Pub R 3 = H , Et, M = U R3 = Prn, M ='I%, U R3=Pr', M = U R3 = CH,Ph, M = Th R3 = Ph, M = Th, Pa,= U, Pu R3 = rn- or p-0,NC&14, p-MeOC,H4, m-BrCp-MeOC,H,), rn-02N(p-MeOC6H,), M = Th R3= C4H,0 (2-furyl), M = Th, U R3 = C,H,NO (3-methylisoxazol-5-yl),M = U R3 = C5H4N(3-pyridyI), M = U R3=OEt, M = T h R3=Me, M = T h , U R 3 = Et , M = U , Pu R3=Pr", M = T h , U R3=Pri, M = U R3 =Bun, But, CH2CHMe2,n-C,H,,, M = U R3=Ph, M = T h , U R3 = C4H3S(Zthienyl), M = Th,U, Np (?), Pu R3=CF3, M = T h , U R3 = OMe, OEt, OBu', M = U R3 = But, M = Th, U,Np, Pu
Brown, B. Whittaker and J. Tacon, J . Chem. SOC.,Dalton Trans., 1975,34.
bE.M.Rubtsov, V. Y. Mischin and V. K.Isupov, Sou. Radiochem. (EngZ. TmnsL),
1981,23,465,
tropolone in methanol is added to a solution of T ~ I ( N O ~ ) ~ .in ~H 33% ~ Oaqueous methanol, and by the reaction of ThC14 with an excess of tropolone in oxygen free dichloromethane. The hydrate, [Th(trop),(H,O)], is obtained from preparations in water or ethanol, and also by recrystallizing Th(trop), from a mixture of ethanol, methyl cyanide and water. Its structure is described on p. 1144. Pa(trop), is rather easily oxidized, but is obtained by reaction of PaCL4 or PaBr, with Li(trop) in oxygen-free dichloromethane. The uranium(1V) compound precipitates when a slurry of U(MeCOz), in tropolone and methyl cyanide is heated under reflux. It is also obtained by reaction of UC14with an excess of tropobne in dichloromethane, ethanol or water; N p ( t r ~ p )is~prepared in the same way from ethanol solution. P u ( i r ~ p precipitates )~ on mixing methanol solutions of tropolone and Pu(N03),; it can be recrystallized from DMF. U(trop)4 is isomorphous with the ceriurn(IV), neptunium(1V) and plutonium(1V) analogues, but not with Th(trop)4. The complexes ML4 (M = Th, U) with HL = a-,6- or y-isopropyl tropolone are also known. Molecular weight determinations (benzene solution) for the thorium compounds indicate that the a compound is close to a dimer, whereas the f3 and y derivatives are approximately trimeric. The corresponding uranium(1V) complexes are all monomers. All three thorium compounds readily form monohydrates and the structure of the hydrated yisopropyltropolonate is described on p. 1144. Uranium(1V) complexes, UL4, are known also for kojic acid and chlorokojic acid, both of which behave as the enolic forms of m-diketones.
1150
The Actinides
Anionic pentakis tropolonato complexes, M'[M'"(frop)5], are known for M'" = Th (MI= Li, Na or K), MIv = Pa or U (MI = Li). The thorium compounds are prepared by heating Th(trop), with H trop and M'OH in a mixture (1 :1:2 by volume) of ethanol, water and methyl cyanide. Li[Pa(trop),] has been obtained from P a ( t r ~ p in ) ~ the presence of Li(trop) in DMF, and Li[ U ( tr~p)~] has been prepared by both of the above methods. Sodium salts, Na[ThLS], are also known for HL = a-and y-isopropyltropolone. ( c ) Catecholates. Complexes formed by catechol and the related compounds resorcinol, phloroglucinol, orcinol and pyrogallol are listed in Table 22. Thorium dichloride catecholate, and the corresponding resorcinolate, phloroglucinolate and orcinolate have been obtained by evaporating an ethereal solution of the components to dryness and heating the residue until the evolution of hydrogen chloride ceased;77when thorium tetrachloride is added to an excess of molten catechol, the product is H2[Th(C6H402)3] .78 The anionic catecholato complex salts, Na4[M(C6H402)4]-21HzO (M = Th, U), separate when a solution of catechol in aqueous sodium hydroxide is added to aqueous solutions of the tetrachlorides. The geometry of the anion is a trigonal faced dodecahedron and the oxygen atoms of the water molecules form a hydrogen-bonded network throughout the crystal.79The other compounds noted in Table 22 were also obtained from aqueous media. In addition to the listed compounds, thorium(1V) bis derivatives of 2,2'-dihydroxybiphenyl or -dinaphthyl and 1,8-dihydroxynaphthalene,T h b , are precipitated from methanolic solutions of the tetrachloride and the diol in the presence of di- or tri-ethylamine as a hydrogen chloride acceptor." Table 22 AcCnide(1V) Catccholates and Related Compounds Catechol. 1.2-dihvdroxvbenzene, C,H,O,
( d ) Quinones. A few thorium(1V) complexes with hydroxyquinones have been recorded. The complex with 2,5-dihydroxybenzo-1,Cquinone (C6H404),Th(c&o4)2, precipitates when an excess of aqueous Th(NO& is added to an aqueous solution of the ammonium salt of the quinone at pH3, 7 or 11. The corresponding 3,6-dichloroquinone (C&Cl2O4) and 3,6diphenylquinone (CI8Hl2O4, polyporic acid) derivatives, Th(C6C1204)2,Th(C6C1204)F2and Th(C18H1004)2 (?>, have also been reported. With 2,3,5,6-tetrahydroxybenzo-l,Cquinone (C6H406)products formulated as [Th2(C6Hz06)](N03)6 and [Th4(C606)](N03)12separate when a mixture of Th(N03)d and the ligand is heated.*Oa A complex with 2-hydroxynaphtho-1,4quinone (CloH6O3), presumably Th(C10H503)4, precipitates from aqueous solutions of thorium(1V) on addition of the quinone and, as precipitation is quantitative, this has some value for the gravimetric determination of thorium in a procedure in which the precipitate is ignited and weighed as Thoz. All of these compounds require further investigation. ( e ) Ethers. In addition to the ether complexes with thorium(1V) and uranium(IV) compounds listed in Table 23, a considerable number of THF solvates of Schiff base complexes with these elements are known [see ref. 12, Thorium (vol. E) and Uranium (vol. El)]. Diethyl ether solvates of imidazole and related complexes of the type ThCl4.2L.nEt,O, where L = imidazole, l-benzylimidazole and 4,5-diphenylimidazole, with n = 14, 8 and 6 respectively, have been recorded but it is uncertain whether any of the ether molecules are coordinated to the metal atom. Otherwise, the only simple ether complexes which have been reported are those of dimethyl and diethyl ether with the tetrahalides, and the dimethoxyethane, THF and
The Actinides
1151
dioxane complexes. These compounds are usually obtained by treating the parent compounds with the appropriate ether. For example, UBr4.2THF is prepared by adding a 10% solution of THF in carbon disulfide to a solution of uranium tetrabromide in the same solvent. In some cases the ether complexes result from the method of preparation of the parent compound. Thus treatment of a solution of uranium tetrachloride in THF with the stoichiometric quantity of thallium acetylacetonate, followed by addition of n-hexane to the supernatant, yields a precipitate of UC12(acac)2.2THF. In the dimethoxyethane (DME) complex, [{(Me3Si)2N)2UC12(DME)],obtained by reaction of UCL, with NaN(SiMe& in DME, the uranium atom is coordinated to two nitrogen, two chlorine and two oxygen (from DME) atoms.'Ob Table 23 Ether Complexes with Actinide(1V) Compounds UX,.RzO MC14.2C,H,,O, UCl,( acac),C,H,,O, ux,.yTHF 3.L-THF ThCl,( HB Pz~) UBr,(HBPq),-THF
R = M e , X = C l ; R = E t , X=CI,Br M = Th,U; C4Hl0O2= MeOCH,CH,OMe X=C1, y = 1 or 3;X=Br, y = 2 Pz = C3H3N2,L = MeCONMe,
UCl~{HB(3,5-M~,pZ)~)~~THF UCl,( acac),.2THF
U(HZBPd4.m
M(HBPz,L.THF U(OR),.2%@' Na4[TWipy))J.aHF UCl,*L m4'Yc4H802
M=Th. U R = CH(CF,),, c(cF,),
L = MeOC,H,OGH,OMe, MeOC2H40C,H40CZH40Me C4H802= dioxane; M = Th, X = Cl, y = 3; M = U, X=C1 or Br, y = 2 or 3
=R.A. Andersen, Inorg. Nucl. Chem. Len., 1979, E, 57.
c f ) Ketones and aldehydes. The only complexes with ketones and aldehydes appear to be those with thorium and uranium tetrahalides. The complexes MCl,.xMeCOMe (M = Th, x = 2 and M = U, x = 31, ThC14-4RCOPh (R = Me or Ph), 2ThBr4-3MeCOPh, ThCl4.2RCHO (R = Me, C6H4(OH)(salicyl) or PhCH=CH) and ThBr4.4PhCH0 are obtained from the components. The acetone solvated lactam complex, ThCl4.4C7HI3NO.MeCOMe(C7HI3NO= 1-azacyclooctan-Zone), is precipitated when ethyl acetate is added to a heated solution made up from thorium tetrachloride in 1:l acetone-nitromethane and an excess of the lactam in acetone. (8)Esters. Ethyl acetate complexes of composition U&-yMeCO,Et (y = 3, X = C1, Br; y = 2, X = Br) are reported to be formed from the components. However, both ethyl and n-propyl acetates react with uranium tetrachloride, yielding the complexes UCl3(NeCO2)-L, where L is the ester. The bis isopropyl acetate complex, ThCL.2MeC02Pr', is obtained when the alkoxide, Th(OP& is heated with the stoichiometric quantity (molar ratio, 1:4) of acetyl chloride in benzene under reflux. With 1:3 and 1:2 molar ratios of the reactants under similar conditions, the products are reported to be ThCl3(OPri)-MeCOzPri and ThClz(OPr')2~0.5MeC02Pr' respectively. The ethyl carbamate complexes, MCl4.xR2NCO2Et(Rz = Mez or MeH, M = Th, x = 3 and M = U, x = 2) are obtained by treating a solution of the tetrachloride in THF with an excess of the ligand, followed by dropwise addition of n-hexane until the solution becomes turbid. ThC1,-3(MeEt)NC02Et is obtained by adding an excess of the ligand to a sus ension of the tetrachloride in benzene. 'H NMR data are available for these compoundssPand their IR spectra indicate that the ligands are bonded to the metal via the carbonyl oxygen atom." Hydrated N-pentylisonicotinate adducts, Th&-yL-zH20 (X = C1, y = 2, z = 8; X = Br, y = 4, z = 9; X = I, y = 2, t = 9) have been obtained by grinding the solid, hydrated tetrahalide with an excess of the ester, but their IR spectra indicate that the ester molecules are not bonded to the metal atom. ( v ) Oxoanions as ligands
( a ) Nifrufes. Hydrated nitrates, M(N03)4*xH20 (M = Th,Np and Pu), and the structure of [Th(N03)4(Hz0)31-2H20,are included in Section 40.2.2.4.i.a (pp. 1145, 1146). Anhydrous
1152
The Actinides
Th(N03)4 is obtained by heating (NO&[Th(NO&] at 150 "C or (No)3[Th(NO3),] at 90 "C under vacuum. Hexanitrato complexes (Table 24) are obtained from moderately concentrated (8 M to 14 M) nitric acid, in the presence of sulfamic acid to inhibit oxidation by nitrite in the case of uranium(1V). The nitrate groups in these compounds are bidentate and the structure of the anion in [Mg(H20)6][Th(N03)6]-2H20 is an irregular icosahedron.82 The corresponding zinc, cobalt and nickel salts are isomorphous with the magnesium salt. The high symmetry of the complex anion is apparent in the almost white appearance of the salts M:[U(N03)6]. The product of composition Np(N03)4.1.2N205,obtained by the reaction of NpC14 with N205, is presumably a- nitrato complex. Table 24 Hexanitrato Actinide(1V) Complexes Mi[M'V(N03)d
M' = NH4, MIv = Th,U, Pu" M' = K, MIv = Pu M' = Rb, MIv = Th,U , Pu M'=Cs, MIV=Th, U, Pu M'=Tl, M'"=Pu t M' = Et,N, MIv = Th,U , Np, Pu MI = Bun, MIv = Pu M' = Me,(PhCH,),N, MIv = Th,U, Np, Pu M' = Me,(PhCH,)N, MIv = Th,U , Np, Pu M' = C,H,NH, MIv = U, Pu(+ca. 14H,O) M' = (PhCH,)C,H,NH, Mw = Th,U, Np, Pu M' = (w-H,N)C5H,NH, MIv = U
MI = GH,NH (quinolinium), MIv = U , Pu M' = NO, NO,, MIv = Th (biPYH*)[U(NO3)61 M"[M'V(N03)6]-8Hz0
M"
M'T~(No,),.xH,o K3H3MTV(NO3),,.~H2O
MIv =Th,x
= Mg or Zn, MIv = Th,U, MI' = Co or Ni, MIv = Th,Pu MI' = Mn, MIv = Th
Pu
M'=Na, x = 8.5; K, x = 6 =4; U, x = 3
* A dihydrate has also been repofled.
In the hydrated compounds (included in Table 24) the water molecules are almost certainly not bonded to the actinide(1V) ion, with the possible exception of the complexes formulated as M1Th(N0&-xH20 (M'=Na or K) and K3H3Th(N03)lo.xH20, the nature of which is uncertain. ( b ) Phosphates. Metaphosphates, M(P03)4 (M = Th, Pa, U, Pu), the hydrated compounds PU(€€PO~)~-XH~O, P U ~ H ( P O ~ ) ~ . Xand H ~POU ~ ( P O ~ ) ~ . X H and ~ Othe , pyrophosphates M(P207) (M = Th, Pa, U, Np, Pu) have been recorded. The structure of the triclinic form of U(PO,), consists of eight-coordinate square antiprisms, UOs, connected by (p4012)~-rings.83a Alkyl phosphates, U{02P(OR)2}4 (R= Me, Et or Bun) and U(O2PH(OR)b4 (R = Me, Et, Pr' or Bu"), and the phenyl derivative, U(03PPh)2, have also been reported.83 Salts of hydrated ( c ) Sulfites. The only simple sulfite appears to be Th(S03)2-4H20. complexes of the type M~MIV(S 03), +Z . ~HZ (Table 0 25) are known for thorium(1V) and uranium(IV), both of which form a series of hydrated salts of what appear to be sulfitooxalato complex anions, Na2nM1V(S03),(Cz04)2.xH20, obtained by dissolving Th(GO& in concentrated aqueous sodium sulfite in varying molar proportions, and then pouring the resulting solution into ethanol to yield a syrup which becomes crystalline on treatment with ethanol.84-85 The product of composition Na2U(S03)(G04)2-5H20 is formulated as Na2[U(S03)(C204)2(H20)2].3.5H20 in the original paper.s4 All these products require further investigation. ( d ) Sulfates. Hydrated sulfates, including the structure of [U(SO~)Z(H~O)~] , have been described in Section 40.2.2.4.i.a (pp. 1144, 1145); some anhydrous sulfates are also known. Fluorosulfates U(S03F)4, UO(SO$)2 and M11U(S03F)6(M" = Mg, Zn) have been obtained by treating U(MeC02)4 or MnU(MeC02)6 with HS03F. The compound U(S03F)4 appears to invohe two mono- and two bi-dentate S03F groups.86Salts of sulfato complex anions of the type M~MrV(S04)2+,~xH20 (n = 1 to 4) are listed in Table 26, together with sulfatooxdates and salts of complex anions derived from them. The structure of the anion in &Th(S04)4.2H20 consists of chains of thorium atoms linked by pairs of bridging sulfate groups, and the coordination geometry about the thorium atom is a
The Actinides
1153
Table 25 Actinide(lV) Sulfites and Sulfito Complexes - .
.
Th(SO3),.4HzO MiTh(S03)3-xH20
M:T~(so~),.xH,o Na,U Na,M
S03)n+2*~H20 (V (SO,),(~O,),-xH,O
..
. - . .
. . . . ... ... .
.
M' = Na, x = 5 ; K, x = 7.5; NH,, x = 4; CN3H6 (guanidinium), x = 12 MI= Na, x = 3 or 6 ; x =5 n = 3,4,5 and 6; x, unspecified MIv = Th; n = 3,4, 5 or 7, x = 5 to 6; n =6, x = 5,6 or 8 n=9,x=6 MIv= U; n = 1, x =5.5; n -2, x -2; n = 3, x = 5; n = 4 , x =4; n = 5 , x =7.5; n = 6 , x = 7 to 8
m,
Ba6Th(S0,)6(~04)2.7H20
Table 26 Actinide(N) Sulfato Complexes Mi[MIv( SO,),] .xH,O M:[MIV( SO,),] *xH,O
MIV=Th;MI= Na, x = 6; K, x = 4; NH,, x = 0 or 5; Rb, x = 0 or 2; Cs, x = 2; TI,x = 4 MIV=U; M'=K or Cs,x =2; NH40r Rb, x=O MIV=Th; Mr=Na, x =4; K,x =2; NH,, x = O or 2; Cs,x = 1 MIv= U; M'=Na, x = 6: K, x = 2; NH,, x = O or 3; Rb, z = 2 ; enH, x = 2 M'~=N~;M'=K,X=~ MIV=Pu; M'=K or NH4, x =2; Rb,x = O , 1 or 2; Cs, x = O M'"=Th; MI= NH, or Cs, x = 3 MIV=U; M'=NH4,x=4 MIV=Pu; M'=Na, x = 1; K,x =O; NH, x = 2 to 4 MIV=Th; M'=NH4,x=2 MXV=U;MI= NH,, x = 3 x = 0, 1 , 2 or 3 x = 0 , 2 , 4 , 8 or 12 M'=NH, or Rb, x = 0 , 2 or 4 x=0,4or6
'M. P.Mefod'eva, N. N. Krot and A. D. Gel'rnan, Russ. J . Znorg. Chem. (E@.
Trunsl.), 1972,17,885,
tricapped trigonal prism.s7 It would be useful to have similar information for the other complex anions. ( e ) Selenites and selenates. Hydrated selenites, M(Se03)2+xH20(M = Th, U, Pu), have been isolated from aqueous solution, and a hydrated thorium selenate, Th(Se04)2-9H20,has been recorded; very little is known about these compounds. c f ) Tellurites and tellurates. Actinide(1V) tellurites,88M(Te03)2 (M = Th,U, Np, Pu), and a basic thorium(1V) tellurate, ThO(TeO4)-xHz0 (x = 4 or S), which may be ThO(&Te06)-yHz0 (y = 2 or 6), have been recorded. There is also evidence for the formation of complexes of composition M:'Th(TeO& (MIr = Ba or Ca) from phase studies.89
(vi) Carboxylates, carbonates and nitroalkanes ( a ) Monocarboxylutes. In addition to the compounds listed in Table 27, which includes basic and mixed carboxylates, a large number of hydroxocarboxylato compounds, such as [Th3(HC02)6(0H)SX].yHz0, where X = NO3, C103, C104or NCS, with y in the range 7 to 16, and anionic derivatives formulated as K2[Th3(HC02)6(0)2(OH)2(NCS)2], have been recorded, but these require further investigation. The formates and acetates, M(RC02)*, are prepared either by reaction of the tetrachloride with the acid or ( M = U ) by reduction of a dioxouranium(V1) compound in non-aqueous media. The product obtained by reaction of the tetrachloride with the carboxylic acid depends on the temperature of the reaction, lower temperatures giving only partial replacement of chloride, so that compounds of the type M(RC02),C14-, are obtained (see Table 27). Other
The Actinides
1154
Table 27 Actinide(1V) Monocarboxylates and Carboxylato Complexes ~
~~
Mw(RC02)4
Mw = Th (also +0.67 or 3H,O), Pa, U and Np, R = H MIv = Th and U, R = Me, CH,Cl, CHCI,, CCI,, CF,, Et, Pr", Pr', 3un, Bu', But, n-C,H,,, PhCH,, Ph, Me,N, Et,N MIv = Th, R = PhOCH,, PhCH-CH, 2M&6H4, 3-0,NC6H,, 3-IC6H4, ((u-C~,H,)CH~ Mrv = U, R = n-C6HI3,MeOCH, (also +lH,O) R = (C4H,S)-2-(thiophene) MIv = Th; R = 3- or 4-H2NC&, 4-(MeO)C,H4 MIv = U; R = (C4H3S)-2 MtV=Th; R = H, x = 1 or 2; R = E t , x = 1 , 2 or 3; R = PhCH,, x = 2 or 3; R = 2-H2NC,H,, x = 2 MIv = U; R = Me, x = 2 or 3
U(MeCO,),Br,
M VO(RC02)2
M'V(OH)3(RC02) M'M'~(RCO~), @Th(RC02)6 M"M1v(RC0,)6.xH20 Cs?Th(HC02), M4Th(HCOz)l, Z n J J(HCO,), KU(OH)(HC02)4.3H20 Na,U(OH)(MeCO,),-H,O M'UO(HCO,),.XH,O Ntb(U0)2(MeC02)5
Mi" = Th, U; R = PhCH=CH, 2-MeGH4, 2-(MeO)C,H, MlV = U; R = H (also hydrates), Me (also +I, 1.5, 2 or 2.5 H,O), CH,Cl (also +1 or 2.5 H,O), CHCl, (also +2H20), CC1, (+lH,O), CH,I (+lH,O) Et (+lH,O), (C,H,S)-2-, 4-(MeO)C6H, MIv = Th, R = Me, x = 0 or 1.5; PhCH,, x = 2; CCI,, x = 2 01 3; Et, x = 4; 2-H0(3,5-I,CeHz), x = 0; 2-IC6H4, x = 0 MIv = U, R = Me, x = 0 MIv = Th, R = H (also +1.3H20), Me (also +I, 2 or 2.5H20), CCI, (+2H,O), PhCH,, Ph,CH, Et, Prn: Bu", (+4H,O), n-C,H,, (+4HzO), Ph, 2-CIC,jH4, 3-IC6H4, 2 3 5-I&,H2, 4-OzNC6Hd (+ 1HzO) M" 2 U, R = H (also +2H,O), CH,CI (+lH,O), CCI, (+3H,O), Et, 2-MeC6H, MIv = Th, Pu; R = Et, PI"" MIv = Th; R = (a-C,,H,)CH, MIv = U; R = PhOCH, Mw = Th, M' = pyH, K, Rb, CS, R = H; MIv = U, M' = Na, R = CH,Cl (+lH,O) R = H,M' = NH,, Rb (2H20), CS; R = Me, M' = NH,, CN3H6(guanidinium; also +ca. 8H,O) R = CC13, M' = Na, K MIV= Th, MI'= Sr, Ba, x = 0 and 2, R = H Mw = U, M" = Mg, Fe, Zn, x = 0, R = Me M' = Na, Rb, Cs
M' =NH,, x = O ; MI= Li, K, NH4, x = 1
R~,U,(OH),(HCO,),(NCS),.XH,O "V.S . Shmdt and V. G.Andryushin, Sou. Radiochem. (Engl. Trawl.), 1982,24,498.
carboxylates are commonly prepared by heating the tetraacetate with the appropriate carboxylic acid, a method used also to obtain compounds of the type M(RC02)2(MeC02)2.In some cases [e+ U0(2-MeCaH4C0&] a basic product precipitates and similar basic compounds are often obtained from aqueous media, although many of the known tetracarboxylates can be obtained anhydrous from aqueous solution. Some authors report the basic species as hydroxocarboxylato compounds, and others report them as hydrated monoxo compounds, and it is uncertain whether such compounds should be regarded as MO(RC02)2.xH20 or M ( O H ) ~( R C O~)~*(X -1)HZO. Anhydrous thorium, protactinium and neptunium tetraformates are isostructural,gObut U(HC02)4 does not possess the same crystal structure and appears to exist in three crystal modifications. Hydrated thorium formate, Th(HC02)4.3H20, has a formato bridged structure in which each thorium atom is surrounded by eight oxygen atoms from eight HC02 groups in a bicapped trigonal prismatic array.g1 Thorium tetraacetate is isomorphous with U(MeC02)4 in which the structure consists of infinite columns parallel to the c axis with acetate groups bridging the uranium atoms, giving rise to square antiprismatic geometry. There are two additional oxygen atoms in neighbouring polyhedra at 2.80A from each uranium atom.92 Although salts of formato complex anions of composition [Th(HC02),+,]"- (n = 1-4, Table
The Actinides
1155
27) have been recorded, no structural data are available for them. Zn2[U(HC02)8]is obtained by reduction of UOzCIzin formic acid by zinc amalgam, but otherwise only basic uraniwn(1V) formato complexes have been reported. Acetato complexes are always M$[M'V(MeC02)6]or M"[M'V(MeC02)6]. The uranates(1V) (M" = Mg, Fe or Zn) are easily obtained by reduction of U02(MeC02)2.2Hz0with the appropriate metal in a mixture of glacial acetic acid and acetic anhydride, but the zinc salt readily loses Zn(MeCO& to yield the tetraacetate. A few trichloroacetato complex salts, M'[Th(C13CC02)6], are also known, and, somewhat surprisingly, a sodium salt formulated as NaU(CHzClC0z)5~Hz0has been reported. Otherwise virtually nothing is known about anionic complexes with other carboxylic acids or with other actinides(1V). (b) Chelating carboxylates. Pyridine-2- and -3-carboxylic acids appear to form simple carboxylates [e.g. U(NCSH4-2-C02)4, Table 281, whereas pyridine-4-carboxylic acid can also act Quinoline-2as a neutral ligand,93 as in the complex Th(N03)4-(NC5&-4-C02H)-H20. carboxylates are also known. It is not clear whether the heterocyclic nitrogen atom is bonded to the metal in these carboxylates. Table 2.8 Actinide(1V) Compounds with Chelating Carboxylates
" G .A. Battiston, G. Sbrignadello, G. Bandoli, D. A. Clementc and G. Thomat. J , Chem. Soc., Dalton Tram., 1979, 1965. bG. Sbrignadello, G. Thomat, G. Battiston, G. de Paoli and L. Magon, Inorg. Chim. Acta, 1976,18, 195. ' C . G. Macarovici and E. Chis, Rev. Roum. Chim., 1917, 22, 657.
The simple hydrated pyridine-2,6-dicarboxylate (pdc), [ T h ( p d ~ ) ~ ( H ~ , 0is) ~precipitated ] from methanol solutions of Th(N03)4.5Hz0 by the acid; the thorium atom is 10-coordinate in a bicapped square antiprismatic arrangement in which the pyridine nitrogen atoms cap the two opposite faces of the antiprism, the corners of which are occupied by two oxygen atoms from each dicarboxylate group and the four oxygen atoms from the water molecu3es.94 The tetrahydrate yields the anhydrous polymer, [Th(pdc),],,, on heating. The corresponding uranium(1V) compound, U@dc)z, is prepared by heating U(MeC02)4 with a solution of the acid in methanol under reflux, and mixed carboxylates, such as U(Hpdc)(MeCO&, are obtained in a similar manner using methanol or ethanol as the solvent. The tetraphenylarsonium salts, (Ph4As)3[M(pdc)3].3H20, have been prepared by adding the metal tetrachloride (M = Th, U) or tetranitrate (M = Th) to an excess of the acid and P U s C 1 in water. In the structure of the anion of the uranium compound, the uranium atom is nine-coordinated with distorted tricapped trigonal prismatic geometry in which the three nitrogen atoms of the pdc groups cap the rectangular faces of the prism.95 The imino-, oxo- and 4,4'-diaminodiphenylmethane-N,N'-diacetateshave been isolated from aqueous solution (Table 28). The central nitrogen or oxygen atoms of these diacetate groups are probably bonded to the metal as found for the pyridine-2,6-dicarboxylates. ( c ) Carbonates. The simple carbonates (Table 29) are not well known, and their preparation usually requires autoclave techniques; for example, Th(C03)2.0.5H20 is obtained% from thorium(1V) hydroxide and carbon dioxide at 1800-3000 atm and 100-150 "C.The best
The Actinides
1156
established carbonato complexes are salts of the [M'V(C03)4]4-and [MrV(CO3)$- anions, but tn-, hexa- and octa-carbonato complexes (Table 29) have also been reported. The two last require confirmation, for the preparation of the hexa- and octa-carbonato plutonates(1V) was by dissolution of Pu(&O& in a solution of the appropriate alkali carbonate, after which the solution was poured into aqueous ethanol, yielding an oil which became crystalline on standing or on treatment with 99% ethanol. The same procedure can also be used to prepare the tetraand penta-carbonato c~mplexes.~' Table 29 Actinide(1V) Carbonates and Carbonato Complexes M'V(C03)2.~Hz0 MXVO( C03).xH,0 Th\OH)2(CO3) .2H20 x M v02*M'VO(COB)*yH20
M" = Th, x = 0.5, 3-4; Pu,x unspecified MIv = Th, n = 2 , 8 ; U , x = O ; Pu,x = 2 MW = Th, x = 1, y = 1.5 or 4; x = 3, y = 1; x = 6, y = 0 M N = Pu, x = 1, y = 0 or 3 M' = NH,, x = 6; CN,H,(guanidinium), x = 0 , 4 MW = Th, M' = CN,H,, x = 0,6; Na, x = 0 , 7 MIv = U, M' = CN,&, x = 4,5; Mi = (CN3HJ3(NH4), x = 4 MIv = Npa, M' = Na, x unspecified MIv = Pu, M' = Na, x = 3; NK, x = 4; K,x unspecified M'"=Th, MT=Na,x = 0 , 3 , 5, 10-11, 12,20; K, x = 10, 13; NH,, x = 3; TI, x = 1 , 2 ; CN3H6,x = 0, 4, 10 Mi = Ca,, Ba,, x = 7; CN &)f(NH4)3r x = 3; [Co(NH,),],, x = 4, 6,6-ld MIv = U, MI= Na, x = 10,12; K,x = 6; CN,H6, x = 4,5 @=(CN&),(NH&, x = 1; ICN&)#W),, x = 2; [Co(NH,),],, x = 4, 5 MIv = Np, M' = K,a x unspecified MIv = Pu, M' = iqa, x = 2,4; K, NH,, x unspecified Mi = [Co(NH3)6]z,x = 5 MI= Na, K, NH,, x unspecified M' = Na, K, NH,, x unspecified M1=Na, x = 8; K,x = 3 MIv
= Th, U
M' = Na, NH, a Yu.
Ya. Kharitonov and A. I. Moskvin,Sov. Radiochem. (Engl. Trawl.), 1973,15, 240.
In the cases of the hydrated salts, some authors include a part of the water in the coordination sphere (e.g. M$[Th(C03)5]-xH20 is sometimes written as Mk[Th(C03)5(H2O)].(x - 1)H20 and (NH4)2[Th(CO3)31'6H20 as (NH4)2[Th(CO3)3(H,O)31. 3Hz0),but there does not appear to be sufficient experimental evidence to support this. The same applies to the formulations of the basic carbonato complexes listed in Table 29. The coordination polyhedron of the anion in (CN3H6)6[Th(C03)5].4H20is an irregular decahexahedrong8and the geometry is a bicapped square antiprism. Replacement of CN3H6by Na, in Na6[Th(C03)5]*12H20,leads only to a slight deformation of the polyhedron.w The water molecules are not bonded to the thorium atom in either salt. The thorium atom in the wbonatofluorothorate(1V), (CN3H&[Th(C03)3F3], is nine-coordinate, with three bidentate carbonate roups and three terminal fluorine atoms making up a monocapped square antiprism.ID (d) Oxalates. A large number of oxalato (Table 30) and mixed oxalato complexes (Table 31) have been recorded. The hydrated oxalates, M ( Q 0 4 ) 2 . ~ H 2 0are , precipitated from aqueous media, the basis of a useful gravimetric method for the elements. The thorium compounds (x = 0, 1, 2 or 6) are isomorphous with their uranium(1V) analogues. The hydrated basic oxalate, U0(G,04)4H20 , precipitates on photoreduction of U02(HC02)2 in the presence of oxalic acid; other hydrates are known and some authors describe them as hydroxo compounds [e.g. U ( O S ) 2 ( ~ 0 4 ) ~ 5 H 2but 0 ] , this requires confirmation.
The Actinides
1157
Table 30 Actinide(1V) Oxalates and Oxalato Complexes
M=Th,x=O, 1 , 2 , 4 , 6 ; U , x = O , 1 , 2 , 3 , 5 , 6 ; N p , F ' u , x = 2 , 6 x = 0, 4, 6 Mw = Th, M' = C N 3 b , x = 6, 8; NH,, x unspecified MIv = Th, U ( N W , = quinoline) Mi = [(PhCH,)N(qH7)]'H',
Th;M' = Na, x = 0, 5.5, 6; K, x = 0, 4;NH,, x = 0, 3, 4.7,6.5, 7; Me,NH,, x = 0, 2, 9; Bu;NH2, x = 0, 4; CN& x = 2 Mi = (CN3H6),(m4), x = 3 MIv = U; MI= K, x = 0, 1,2, 4, 4.5, 5 ; NH,,-. x = 0,, 3,, 5,. 6,7; Cs, x = 3; CN3H6,x = 6 2 MIv = Np; M' = Na, x = 3; K,x. = 4; NH,, x unspecified Mw= Pu; MI= Na. x =5; K. x = 4 MIv = Th;MI' = Sa, x = 11; enH,, x = 2.5 MIv = U, M" = Ca, x = 0, 1, 4,6, 10; Sr, x = 0,4,6; Ba, x = 0, 6, 6.5, 7, 8, 9; Cd, x = 0, 6 , 7; Pb, x = 0, 6, 8; [Pt(NH&], x = 3 MIv = Th, M"' = [Co(enM, x = 22; [Co(tn),], x = 3; tn = H,N(CH,),NH, MIv = U, MI'' =La, x = 22; ICr(urea),], x = 6 to 11 Mw = Pu, M"' = [Cr(urea),], x unspecified MIv =
,
,
,
~,
MIv = Th, M' = NH,, x = 3, 7.5; MIv = Pu, M' = K, x = 4; NH,, x unspecified Mw = Th,MIIT= [Co(NH,),], 1 = 3; [Ct(NH&], X = 20; [Cr(~rea)~], x = 0.5 MIV= U, MI" = [Cr(urea)d, x = 0.5 MIv = Th, M' = H, x = 9;" NH,, x = 2 , 7 MIv = U, M' = H, x = 0, 4, 8; Na, K, x = 8; NH,, x = 0,2,4, 8; CN3&, x = 0 , 1 , 4 MI = Et3NH, x = 0,3; BuZNH,, x = 0 MI = EbNH,, x = 0, 6; Pr,"NHz,x = 0, 8; CN3H6,x = 5 , 8, 12.5 to 13.7 a
Also reported as (H,O),[Th,(~O~),].SH,O.
A few salts of the tris oxalato actinide(1V) anions are known, such as the acid benzylquinolinium compounds (Table 30), but the more usual complexes are the tetraoxalato and pentaoxalato species. The coordination geometry of the 10-coordinatethorium atom in the anion of I(4[Th(&04)4]*4H20is a slightly irregular bicapped square antiprism in an oxalate bridged structure cross-linked into a three-dimensional framework by hydrogen bonding (Figure 4).lo1 The geometry in both crystal modifications of I(4[U(G04)4].4H20is the same as in the thorium compound; in one phase the three bidentate C2O4 groups and a tetradentate bridging GO4 group link the metal atoms in a one dimensional polymeric array and the other phase is isostructurd with the thorium compound.'" It would be useful to have structural information for the [Th(C204)6]'-, [U2(C204),]4- and [Th2(G04)7]6-anions, for it is not clear whether these are genuine complexes or mixtures involving other known oxalato anions. Mixed oxalates and oxalato complexes (Table 31) also require further investigation. The sulfito and sulfato oxalates have been mentioned earlier (p. 1152) and an equally large number of carbonatooxalato species have been recorded,lm-lWsome of which may well be mixtures. In addition to the compounds listed in Table 31, products of the rather unlikely compositions G[U(OH)~~04)2(CO3)3].6H,O and K~~[U~(OH)Z(C~~~)~(CO~)~I'~OH~O have been reported. lo ( e ) Other dicarboxylates. Compounds with dicarboxylic acids other than oxalates or chelating carboxylates have scarcely been investigated (Table 32). In some cases the bis compounds, MIVL, are precipitated from aqueous solution, often as hydrates, a Itreparative method which also gives rise to a variety of basic compounds reported either as M OL.xH20 or M1V(OH)2L-(~ - 1)H20. These are presumably identical compounds, but no structural information is available. A better preparative route, used for example for the phthalates, is by heating the metal tetraacetate with the dicarboxylic acid in ethanol under reflux, a method which could usefully be applied to the preparation of a much wider range of dicarboxylates. The only recorded dicarboxylato complex anions are the malonato compounds, M'[M1V{CH2(C0~)~}~]-xH~0 and Na2[Th{O(CH2C02)3),]~2NaN03. In the structure of the
1158
The Actinides
Figure 4 The five oxalate groups coordinated to one thorium atom in the structure of K4Th(~04)4-4H,0'"'
Table 31 Actinide(1V) Mixed Oxalates and Oxalato Complexes
X = F , y = O ; X=Cl, y = 0 , 2 , 4 or 12 MIv = Th,M' = K,x = 0 MIv= U, M'= NH4, x = 4 M T ~ = T ~ , MI=
K,x = 1, y = 4 . 6
M' = NH4, x = 2, y = 0.5 M: = (CN,H&N&), x = 1, 4 = 1.5,Or 2-3.5, and n = 2, y = 3 MIv = U, M i = (CNsH,),(NH,), x = 1, )' = 2 M~~ = pu, M' = Na, K,x = 1, y unspecified M' =N a ,x=2, y = 3
Mw=Th, x
= 1, y = 6-8 and X =2 , Y = o , 1 or 4 MIv = Pu, x = 2, y unspecified M1=K,x=3,y=6 M~=CCN,H,, x =2, y = 4 or 8 andx =3, y = 14 x = 1, y = 10 to 11 and x = 2, y = 9 to 10.5 or 11 MI= K,x =3, y = 13 or 16 M I = CN,H,, x = 1, y = 6 and x = 3, y = o x = 10, 11, 11.5 or 16 M I ~ = T ~ , M I = K , X = ~ ,y = 8 , 1 2 o r 1 4 a n d x = 4 , y=5or7 MI= CN&, X = 1, y = 8 MIv = U, Mio = (CN3H&(W),, x = 1, y = 4 or 8 Mio = [Cr(urea)&(NH4), x = 1, y = 6
x = O , 1 or 2
M1"=Th,x=1,y=4andx=3,y=2 MIv = U, x = 2, y = 4 x = 8-9 x=Oor2 C4H60, = tartaric acid C6H,07 = citric acid
The Actinides
1159
Tnble 32 Actinide(1V) Dicarboxylates and Dicarboxylate Complexes MIv = Th; R = CH, (malonate), x = 2; (CH,), (adipate), x = 0; (CH,), (sebacate), x = 0; CH=CH (maleate), x = 2; C,H4-1,2-(phthalate), (2,2'-biphenyldicar-
boxylate), x = 0 , 2 ; R = C6H4-2-NHCH,, x = 2 MIv = U; R = (CH,),, x = 0.5; CGH4-1,2-, x = 0
MI= Na, MIV= U, x = 2; MI = K, MlV = Th,x = 0 MIv = Th, R = (CHz), (succinate) or (CH,), (glutarate). x = 8; ((=H2)4,x = 4.5, 5 ;
M'O{ R(CO,),} .xH,O
L
CH,),, x = 4 M' - Th, R - (CH,),, x = 3; C,H,, (camphorate), x = 0,2 MIv = U, R = CH,, x = 6; (CH,),, x = 2, 3; C,H4-1,2-, x = 3 R = (CH,),, x = 4; CH==CH (fumarate), x = 4; C6H,-1,2-, x = 0; C,H,NO, (2'-carboxymaleanilate), x = 2
anion of the latter, the nine-coordinated thorium atom is at the centre of a tricapped trigonal prism, with three tridentate O(CH2C02)2groups bonded to the metal.58b cf) Carbamates. N,N-Dialkylcarbamates, MW(R2NCO2), (MI"= Th, U) are obtained by reaction of the appropriate dialkylamide, M(NR&, with carbon dioxide. A much simpler route is by reaction of Ucl, with R2NH and carbon dioxide in benzene ( R = E t ) or toluene (R = Me). The carbamates precipitate on addition of n-heptane after evaporation to small volume. U(Et2NC02), is a monomer in benzene, and the 'H NMR spectra of the known compounds (Table 27) indicate that the alkyl groups are equivalent. A by-product of the preparation of U(Et2NC0& from the tetrachloride and the amine is a product of composition U402(Et2NC02)12;this is a tetramer in which there are two non-equivalent uranium(1V) sites. In one of them the coordination geometry is a distorted tricapped tri onal prism, and the geometry of the other does not fit any type of regular polyhedron.1k (g) Tetrucurbuxylates. Thorium pyromellitate, [Th(1,2,4,5-C6H2(C02)4)]n,and the 2,3,6,7naphthalenetetracarboxylate, [Th(2,3,6,7-CloH4(C02)4)],,, are both polymers as expected. (h) Nitroalkunes. The only recorded complexes appear to be UC14.xMeN02;U a - M e N 0 2is formed initially when MeNOz is condensed onto UCL at low temperature and the mixture is allowed to warm to room temperature, and UCl,.2.5MeNO2 is obtained from the mixture on standing.
(vii) Aliphatic and uromatic hydroxyacids (a) Aliphatic hydroxyacids. The majority of the known hydroxycarboxylates (Table 33) are precipitated from aqueous solutions of the actinide(1V) ion on addition of the appropriate acid; many of these products are hydrated. Uranium(1V) glycollate, [U(HOCH2C02)4(H20)2], precipitates on photochemical reduction of U02(N03)2in aqueous solution in the presence of an excess of the acid5' and the structure of this compound has been described earlier (p. 1144). The thorium analogue probably has the same molecular structure. The alkali metal salts, such as those of the malato complex ions, have been isolated by dissolving the actinide carboxylate [e.g. (U20CCH2CH(OH)C02)2.H20]in a hot, aqueous solution containing the stoichiometric quantity of the alkali carboxylate, and then evaporating the resulting solution. The hydroxocarboxylato salts of the type MIM'V(OH)2X,-yH20, where X,,represents the hydroxycarboxylate anions, have also been reported as M$MIv0Xn.~H2O (Table 33); these are obviously identical compounds, but it is uncertain which formulation is
1160
The Actinides Table 33 Actinide(1V) Compounds with Aliphatic Hydroxyacids
Glycollates and related carboxylates M1V(HOCH,C0,)4~2H~0 MIv = Th, U Th(OH)(HOCH,C0,)3.H20 ThO(HOCH,CO,),*2H,O Th(PhCH(OH)CO,)4.HzO Th(OH),{PhCH(OH)COz}2 Th$Me,C(OH)CO,},~2H2O MIv = Th, U M' {Ph2C(OH)C02)4 Th{Ph,C(O)C02)* Malates; H3L = HO,CCH,CH(OH)CO,H M'~(HL),.XH,O Mw=Th,x=l t02;U,x=l (ThOH)z(HL)3.4HZO Th,CIZ(HL), NaTh(OH)(HL),.6Hz0 Na2Th(OH)z(HL),-4H20 MI= Na, x = 6; K, NH,, x = 4 M~O(HL),.XH,O MI=Na, K M*[U(HL)~I*HZO MI = Na, K, x = 5 ; NH,, x = 2 Mz[Uz(HL),I.xHzO Tartrates; H,L = HOzC(CHOH),C02H ThL.9H20 x = 0,2, 5 , 18, 22 (the compound is sometimes written ThdOH)4(HzL)4*xH20 as Th3Oz(HZL)& + 2)(HzO) U(H,L),.2H,O UZO(H&)3*SH2O 2U(H,L)z*U02*12H20 KTh(OH)L.10H20 KTh( OH)(HzL),.7H,O K2Th(OH),L.4H,O M:ThOL*4Hz0 MI= K, NH, K,~(OH),(H,L),~~H,O MI= Na, K, x = 8; N K , x = 3 M;T~O(H,L),.XH,O M' = K,x = 3; Mi = [Pt(M3)&, x = 0 M:[U(HzL)z(~O,)zI.xHzO Trihydroxyglutarates; H5L = HO,C( CHOH),C02H Th(H3L)Z* 4H20 Th(OH)(HzL).2H,O x=o, 1 (Th0H)Z(H3L)3*xH20 NaTh(OH)(H3L)2.H20 NaTh(OH),(H,L).H,O Na2Th(0H)2(H3L)2 Mucates; H,L = H0,C(CHOH)4C0,H (ThOH)z(H4L)3.6Hz0 N a n ( OH) (HzL)-2H,0 N a n ( OH) (H4L),.6H,O Na2Th(OH),(H4L),. 10H,O Citrates; H4L = HOC(CH2COzH),(C02H) U~(HL)~*XHZO x unspecified UO(H,J-) [CO(NH~),IZ[~'"~HL),I~.XH~O MIv = Th," x = 0; Pu,'x= 12 &[U(HL)Z(~O~)~*~H@ "M. Hoshi and K. Ueno, Radiochem. Radioanal. Len., 1971,30,145,
correct. The tartrato- and citrato-oxalato compounds are precipitated on dropwise addition of ethanol to a solution of U(C204)2in an aqueous mixture of the hydroxy acid and alkali in the stoichiometric proportions. (b) Aromatic hydroxyacids. Most of the compounds with aromatic hydroxyacids (Table 34) are precipitated from aqueous solution; the composition of the product depends on the pH; for example, with 2-hydroxy-3-naphthoic acid, Th(OCloH6C02)2.4Hz0is reported to precipitate at at pH 4 to 5 , whereas Th(HOC,,H,CO,), is said to precipitate at pH 6 to 7. In both cases the stoichiometric quantities of the reagents were used. It is quite possible that compounds such as Th(OCloH6C0~)2.4H20are really basic species of the type ThO(HOCl&C02)2.3H20. In
The Actinides
1161
addition to the compounds listed in Table 34, 3- and 5-nitro-, 3J-dinitro- and 5-bromosalicylates of composition T h b (all) or ThL(HL)2 (3- and 5-nitro) have been recorded. The hydroxynaphthoate acetates, M'V(HL),(MeC02)4-x, have been obtained by heating the metal tetraacetate with the naphthoic acid in ethanol under reflux. Table 34 Actinide(1V) Compounds with Aromatic Hydroxyacids
MIv= Th,x = 0 ; U , x = 2 , 4 Mw=Th, x = 3 ; U, x=O; Pu, + = O x=lor2
3- and 4-Hydroxybenzoates; H2L = HOC6H4COzH T~(OH)~(HL)~.XH*O Salicylsulfonic acid, HOC6ff3(S03H)COzH U(HOCJ%(SOJCOA Thiodisalicylic acid,
T h ( . ( q 0 H j 2
i4H20
x = 1 ( 3 4 3 0 ) and 3 (4-HO)
l2
COZH
co2 Hydroxynaphthoic aci&; H, L = HOCloH6-2-COzH WxHzO rn(HL), Th(OH),(HL)*2H20 Th(OH)2L*3fiz0 Mw(HL)z(MeCOz)z U(HL)(MeCO2)3 2-Hydroxy-3-naphthoic acid, H,L m*4Hz0
x = 2,4 (1-hydroxy) (1-hydroxy)
(1-hydroxy) (1-hydroxy-4-nitro) M' = Th (1- and 3-hydroxy), U (1-hydroxy) (3-hydroxy)
Th(HL), Th0L*3H20
(viii) Amides and related compounds, carboxylic acid hydrazides, ureas, N-oxides, P-oxides, As-oxides and S-oxides
The majority of the known complexes with neutral oxygen donor ligands have been prepared from the parent compound and the ligand in a non-aqueous solvent of relatively poor donor ability. When thegarent compound does not exist (e.g. PuCI4) or is difficult to obtain anhydrous (e.g. M' (NCS)4), the alternative route is to react salts such as Cs2MIVCkwith the ligand in a non-aqueous solvent, relying on the low solubility of CsCl in such solvents to shift the following equilibrium to the right: CszMCI,+ XLg MCUXf 2CsQlsolid)
In the thiocyanate case, the complexes of the metal tetrachlorides are simply treated with the stoichiometric quantity of KNCS in suitable solvents. In some instances the preparations can be carried out in one step from the metal tetrachloride, potassium thiocyanate and the ligand, but this procedure can make the purification of the final product quite difficult. A number of complexes of the fyg" (q5-CsHS)MX3L,(M = Th, U,Np) have been recorded; these are described elsewhere. ( a ) Amides and related compounds. Complexes with formamide do not appear to have been recorded, but several acetamide complexes are known (Table 35). In the group MQ.6L (M= U, Np, Pu), the uranium compound is ionic and is probably of the form [UC12(MeCON€i2)6]z+C12rather than [U(MeC0NH&l4+Cl4. The complex with the sulfate, U(S04)2.4MeCONH2,has been prepared by heating the sulfate in molten acetamide, and the anionic complex, (NHi)2[U(S04)3(MeCONH2)].xHz0, was obtained from aqueous solution. Relatively few complexes with N-alkylacetamides have been reported. No structural information is available for any of them, but the thorium nitrate complexes,
'
1162
The Actinides
Th(NOs),-2.5MeCONHR (R= Me, Et), and the analogous DMF complex, are probably ionic [Th(N03),L,]'[Th(NO&L]l k e the diethylpropionamide complex, UC14.2.5EtCONEt2 (p. 1164). In the case of the uranium(1V) perchlorate complexes with MeCONHMe (Table 35), it appears that the [U(MeCONHMe)8]4+anion is unstable with respect to loss of ligand, and the corresponding seven-coordinate cation may be formed; this is surprising, for in cone-angle terms (p. 1130), the estimated value of Z caf is well within the stable region. A much wider range of complexes has been reported for N,N-dimethylformamide (DMF) and for N,N-dimethylacetamide (DMA). Structural information is available for four DMF complexes. The mixed oxidation state compound, [U'VCl(DMF)7]2[UV10zC14]3, is formed by treating UC14 in acetone with DMF, and in this compound the coordination geometry of the cation is dodecahedral, and the anion is trans ~ c t a h e d r a l . ' In ~ ~ the complex with the 8-hydroxyquinolinate, [Th(C&H6N0),(DMF)], which separates from a solution of Th(GH6N0),-GH7NO in warm DMF, the coordination polyhedron is a slightly distorted tricapped trigonal prism.lWaIn the complex with the tropolonate, [Th(trop),(DMF)], which is precipitated when Th(trop)4 is heated in anhydrous DMF, the structure of the molecule is a monocapped square antiprism in which one tropolone ligand spans a slant edge of the cap, and the DMF molecule is bonded at another vertex of the ca The structure of the 1-oxo-2-thiopyridinato compound, [ThL4(DMF)],is very similar.' The DMF complex with thorium(1V) perchlorate behaves as a 1 :4 electrolyte in methanol or nitromethane and its IR spectrum suggests that the perchlorate groups are ionic, indicating that the complex is [Th(DMF),](ClO&. The same complex cation is reported for the compounds [M(DMF)s]3[Cr(NCS)6]4(M = Th, U). N , N-Diphenylformamide complexes of composition U&.4HCONPhz (X = C1, Br) have also been recorded. No structural data are available for any of the DMA complexes (Table 35). However, the UV-visible spectrum of 2UC14-SDMA is virtually identical to that of 2UC14-5EtCONEtz, indicating the presence of both six- and seven-coordinate uranium(1V) centres, and it is therefore probable that the DMA complex, as well as the other complexes of composition 2M&.5L (Table 35), is ionic and of the form [MX3L4]+[MX5L]-,even though the conductance of 2UC14-5DMAin nitromethane is low. The thorium atom in ThCL4DMA appears, on the basis of the cone-angle approach to steric crowding (p. 1130), to be very overcrowded, and it is probable that this complex should be written as [ThC13(DMA),]+C1-, even though its conductivity in nitromethane is very low. Repeated dissolution of this complex in THF and re recipitation with a mixture of n-pentane and toluene yields the tris complex, [ThC1,(DMAj3]al!' The conductances of 2Th(NO&.SDMA (in nitromethane) and Th(N03)4-3DMA.3H20(in
w
Table 35 Complexes of Actinide(1V) Compounds with Amides
MIv = U , Np, PU x=0,4
X=CI, R = M e ; X = C I a n d B r , R = E t , P r ' MIv= Th,x = 3, R = Me,Et; M r v = U , x = 2.5, R = Et; MIv = Pu, x = 2, R = Et; HR = Pr, x 6, 7.4-7.68
MIv = Th,U C,H7N0 = 8-hydroxyquinoline MIv = Th,U ; H trop = tropolone x=l, y = 2 ; x = l . S , y = l MI= Na, x = 3; K,x = 4.5
X = C1, MIv = Pa, U, Np, Pu; X = Br, MIv = Pa, U; X = NO3, MIv = Th,U, Np
The Actinides
1163
Table 35 (continued) N,N-Dimethylacetamide, DMA (continued) M~~X,.~DMA
UIZClz.xDMA U13CI-xDMA UClz(HBPz3),.DMA ThC13(HBPz3).DMA.THF U(H,BP%),.DMA Th(HBPz3),.2DMA {U(RC02)3(DMA))20 Other N ,N-Dialkylamides M'~X,G!RCONR;
MrV(NCS),.4RCONR;
IM'V(RC0J3Llz0
X = C1; MIv = Th,y = 1, 2, 3, 4; MIv = Pa, y = 3 X = Br; MIv = Th, y = 2, 4 (acetone. solvate), 5 ; M"' = Pa, y = 5 ; MKv= U, y = 4 (acetone solvate), 5
X = I; MIv = n, y = 6 ; MIv = U, y = 4 X = NCS; M'" =Th, U, y = 4 X = NO,; MIv = Th, y = 3 (+3H,O) X = CIO,; MI"= Th, y = 6 (also +3H20), 8; MIv = U, y = 64aIs0 +3H20) X = CI3CCO2,M' = Th, U,y = 3 X = F,CCO,, MIv = Th,y = 3 X = ClZCHC02,MIv= Th, y = 1 X = ClCH,C02, MIv = Th, y = 1; MIv = U, y = 0.5 x=3,5 x=3,5
PZ = CSH,N,
R = Cl,CH, CF, R = M e ; R = W , X =N O 9, MIV=Pud R ' = P h , X = C I , M w = U ; " X = N 0 3 , MTV=Th R' = cyClo-C&f X = C1, MIv = Th, U" ' PI': X = C1, MIv = U R = Et; R' = Me,A' Et," R=Pr'; R ' = M e , X = C l , Br, M"'=U R'=Pr'," X=C1, Miv=Th, U; X = B r , MIV=U R = Bu'; R' = Me, Pr'," X = C1, MIv = Th,U: X = Br, M'v = U R = Me,CHCH,; R' = Me, X = C1, Br, MIv = U R=Me; R'=Et,X=NO,,MrV=Th,U R' = P f , X = NO,, Mw = Thd, Ud R '= P h , X = C I , M w = U a R = E t ; R ' = M e , X = C l , M'V=U;bX=N03, M'"=Th, U R' = E t , X = Cl,MIv = U;b X = NO3, MIv = Th R = Pr'; R' = Me, X = C1, MIv = Th R = Bu'; R' = Me, X = NO,, MIv = Th R = Ph2CH; R' =Me, X = CI, Br, MIv = U R = Me; R' = cyclo-C,H,,, PI', X = CI MIv = Th" R = Et; R' = Me, Et, W," X = C1, M" = Th R = Pr'; R' = Me, X = C1, MIV = Th; X = NCS, MIv = U' R' = Pr', X = CI, M w = Th" R = But; R' = Me, X = Ct, Br, NCS, MIv = Th; X = NCS (also MeCOMe solvatec), MIv = U R = P h ; R ' =Me , X=C1, Br, MIV=U R = E t ; R '=Me , MN=Th, U' R' = ~ t M~~ , =n ' R = Pr'; R' =Me, Mw = Th, U R=Bu'; R ' = M e , Mw=Th L = MeCONPh,, R = CHCI,, MIv = Th, U L = Bu'CONMe,, R = F,C, Q C , CI,CH, MIv = Th, U
UC14-3MeCON(Me)(Ph) ThC1,.2MeCONPh2,THP
UCl,.ZPhCON(Me)(Ph)-MeCOMe Th(Cl,CHCO,),~MeCONFh,
Th(CF,COCHCOC,H,S),-MeCON(Bu")(Ph) 'A. G. M. AI-Daher and K. W.
Bagnall, J . Less-Common Met., 1984,87,343. bK. W. BagnaIl, R. L. Beddoes, 0. S . Mills and Li Xing-fu, J . Chem. Soc., Dafton Tram., 1982, 1361. K. W. BagnaIl, Li Xing-fu, G . Bombieri and F. Benetollo, J . Chem. Soc., Dalton Truns., 1982, 19. K. W. BagnaIl, 0. Velasquez Lopez and D. Brown, J . Inorg. N u d Chem., 1976,38,1997.
DMA) are, however, consistent with 1: 1 electrolyte behaviour and Th14.6DMA behaves as a 1 :2 electrolyte in nitromethane, presumably ionizing as [Th12(DMA)$+12. The conductivity of Th(C104)4.6DMA in nitromethane is low for a 1 :4 electrolyte, and the [Th(DMA),J4+ion would be very much undercrowded in cone-angle terms (p. 1130), so that one or more C104 groups may be covalently bound to the thorium atom. The IR spectrum of Th(C104)4-8DMA, however, is consistent with presence of ionic perchlorate only, and the [Th(DMA),J4+ cation
1164
The Actinides
would be well within the stable region insofar as steric crowding is concerned (Z caf = 0.80, p. 1130). The 'H NMR spectrum of ThC13(HBPz3).DMA-THF (Pz = C3H3NZ) indicates that the HBPz3 ligand is bidentate, but it uncertain whether the THF molecule is bonded to the thorium atom or not. A selection of a very wide variety of complexes with other N,N-dialkylamides is included in Table 35. The UV-visible spectra and magnetic susceptibility results for many of the uranium(1V) complexes of composition UX& are consistent with octahedral geometry and all of the bis complexes MX& probably adopt this geometry. The complex of composition and the cation is 2UCl4.5EtCONEt2 is ionic, [UC13(EtCONEt2)4]+[UC15(EtCONEt2)]-, pentagonal bipyramidal with the Cl-U-Cl axis normal to the pentagon.' The other known complexes of composition 2 m . 5 L are probably of the same structure. Pentagonal bipyramidal geometry has also been found for the neutral complex' [U(NCS)4(Me2CHCONMe2)3] (Figure 5) and for [ThC14(EtCONEt2)3].111b The other known t i s complexes may have similar coordination geometries. In the complexes M(NCS)4.4RCONR; the metal centres are presumably eight-coordinate; the coordination geometry of [Th(NCS)4(EtCONMe2)4fis a distorted square antiprism.
Figure 5 Perspective view of the molecular structure of [U(NCS),(Me,CHCONMe.J3] viewed down the SCN-U-NCS
axisg
A number of complexes with ligands related to amides, lactams and antipyrine (2,3-dimethyll-phenylpyrazol-5-one), as well as complexes with dicarboxylic acid amides are also known (Table 36), but little structural information is available for them. The diantipyrylmethane complex, U14.4L-2H20,is probably ionic, [UL4]14-2H20. ( b ) Carbuxylic acid hydrazides. Ligands of this type can be regarded as analogous to amides; the few known complexes are listed in Table 37. ( c ) Ureas. A selection of the known complexes with urea and substituted ureas is given in Table 38. The hydrated compounds have been obtained by fusing together the stoichiometric quantities of the components, or by simply grinding them together and finally drying the product to constant mass in a desiccator; alternatively, some have been prepared from aqueous methanol solutions of the components. The anhydrous complexes have been prepared from solutions of the components in non-aqueous solvents, such as ethyl acetate. The complexes with UIC13, U13Cl and UIBr3 were obtained from mixtures of the component tetrahalides (U14+UC14 or UBr4) and urea in the same way as the complexes with the tetrahalides. The product of composition ThBr4-11.6urea is probably a mixture of ThBr4.8urea and free urea; this product, and the majority of the other urea complexes, require further investigation. The IR spectra of all the complexes indicate that the ligands are bonded to the metal atom wia the carbonyl oxygen atom. From the IR spectra of Th(N03)4.2urea-(1or 6)H20 it is clear that ionic nitrate is absent, but the spectra of most of the other urea complexes of thorium tetranitrate show that both covalent and ionic nitrate are present. The systems with substituted ureas are less complicated than those with urea, and most of these compounds have been obtained from non-aqueous solutions of the components.
The Acthides
1165
-
Table 36 Complexes of Actinide(IV) Compounds with Lactams, Antipyrine (2,3-Dimethyl-lphenylpyrazol-5-one, ATP) and its Derivatives, and Dicarboxylic Acid Amides Lactams HN(CH,),CO UC14.4L UBr4.6L [UBr2L61(C104)2
MY,-~L 2UC14*5L
x = 4, 5, 6 x=4,5 x=4 MIv = Th, U; x =4,5,6 L = MeN(CH,),CO
-
Antipyrirae, ATP TW.YATP ThX4.y (4-HZNATP)
[M1VL61dCr(N%14 MI~X,.YL
X = CI,Br, y = 1,3,5,6;X = I," y = 6; X = NCS, y = 2,4; X = NO,, y = 2, 2.5; X = ClO,, y = 6, 7 X = CI, Br, y = 2, 3; X = I," y = 3; X = NCS, y = 1, 2; X = NO,, y = 2,4; X = CIO,, y = 4 MIv = Th, U, L = ATP, 4-Me2N(ATP) (pyrimidone) L = 4,4'-methylenediantipyrine( = diantipyrylmethane) M~~= ~ hx,= I, NCS, NO,, y = 3 MIv = U, X = C1, Br, y = 1;X = I, y = 4 ( +2H20)
Dicarboxylic acid amides, Me,NCOXCONMe, UCl,.L X = CMe, MTVC14. 1.5L MIv = Th, U, X = (CH,),, CH,CMe,CH, MIv = Th, X = CMe,; MIv = U,X = CH2 ThC14*2L X = CH,
'R. K. Agarwal, A. K . Srivastava and T. N. Srivastava, Proc. Nah. Acud. Sci., India, and DTA results included for these and other ATP and 4-H2NATPcomplexes.
Seci. A , 1981, 51, 79; TGA
Table 37 Actinide(lv) Complexes with Carboxylic Acid Hydrazides, RCO N H N SH R' ThC14.2L Th(N03),*2L
R = Ph, R' = CH=CHPh, (4-Me0)(3-HO)C&13 R = Ph, R' = 2-HOC6H4 R = 2-HOC&,, R' = 2-HOC6H,,, (3-MeO)C6H,, (4-Me0)(3-HO)C6H,
ThBr4.3{(Me2N)2CO}behaves as a 1:1 electrolyte in methyl cyanide and the coordination geometry of Lm(NCS),{ (Me2N)2C0}4] is a slightly distorted dodecahedron.''' The bis complexes, M' X,.2L, are probably octahedral, but otherwise structural information is lacking for complexes with these ligands. ( d ) N-Oxides. Most of the known complexes are thorium(1V) compounds (Table 39) because of the oxidizing nature of the ligand. Conductivity data are available for most of these complexes; the complexes with Th(NCS)4 are non-electrolytes in nitrobenzene, as are ThBr4.2(2,6-Me2pyNO) (in methyl cyanide) and the 2,2'-bipyridyl N,N'-dioxide complexes Th(N03)4-L (in nitrobenzene or DMSO) and Thx4.3L (X = C1, Br; in DMSO). However, Th14-4(2,6-MezpyNO)and Th14.4(bipy-N,N'-02) behave as [Th12L4]12in methyl cyanide or nitrobenzene and DMSO or nitrobenzene respectively. Th(N03)4.4(2,6-Me2pyN0)behaves as [Th(N03)2L4](N03)2in methyl cyanide, consistent with its IR spectrum, which indicates the presence of both ionic and covalent nitrate. The conductivities of the complexes with thorium(1V) perchlorate are consistent with 1:4 electrolyte behaviour. The IR spectrum of Th(N03)443pyN0 shows ionic nitrate only, and the complex is presumably [Th(pyNO)8](N03)4, whereas the spectrum of the quinoline N-oxide complex, Th(NO3)4-3L,indicates bidentate nitrate only, and thorium is presumably 1l-coordinate in this compound. The IR spectra of Tl1(N0~)~-2(bipy-NO)and Th(N03)4.(bipy-N,N ' - 0 2 ) also indicate that only bidentate nitrate groups are present, so that in these two complexes thorium is evidently 10-coordinate. The complex with nitrosyl chloride, ThC14.2NOCI, is a nitrosonium salt, (N0)2[ThC16]. (e) P-Oxides. Because of the importance of phosphate esters in the separation of uranium from other actinides by solvent extraction, ligands of this type, as well as phosphine oxides and phosphinate esters (Table 40), have attracted a great deal of research interest and the structures of a number of complexes with these ligands have been reported. ~ also In the structure of WC4{(Me2N),PO},] , the coordination is trans ~ c t a h e d r d , " as found in the structurh4 of [UBr4(Ph3P0)2], whereas in [UC14(Ph3P0)2]the coordination
1166
The Actinides Table 38 Complexes of Actinide(1V) Compounds with Ureas, L = R'KZNCONR3R4
MIv = Th,X = C1, y = 4 and z = 3 or 4; y = 5 , z =6; y =6, z =2; y = 8, z - 0 X = B r , y = 2 , ~ = 6 ; y = 4 , ~ = 4 ; y = 6 , ~ = 2 ;1~1=. 68 ,, ~ = 0 X=I,y=4, z=4;y=6, z=2;y=8, z=O X=NCS,y=4, z = 4 X=NO,,y=2, z = 1 , 2 o r 6 ; y = 3 , z = 1 ; y = 4 , z = 4 ; y = 5 , z = 3 ; y = 6 , z = 0 , 2 o r 4 ; y = 7 , z = 2 . 5 ; y = 8 , 10, z = O ; y = 1 1 , r = 2 . 5 X = HCO,, y = 1.5, z = 0 M'V=U,X=CI,y=3, z = O o r l ; y = 4 , z=Oor3; y=7,z=Oor3 X = B r , y -2, z = U,7 or 9; y = 3, z -0, 5 or 8; y = 6, z = 0 or 2; y = 8, z - 0 X = I , y = 5 , r = 2 ; y = 8 , r=O
geometry is cis octahedral as a result of a ring-ring interaction between two Ph rings in the molecule, one from each of the two adjacent Ph3P0 molecules.'1s The thorium, protactinium, neptunium and plutonium complexes, MC14.2Ph3P0, are isostructural with this form of [UC14(Ph3P0)2],and the complexes MBr4.2Ph3P0 (M = Th, Pa, Np, Pu) are isostructural with the uranium(1V) bromide complex. A second crystal form of [UCL(Ph3PO)z], isostructural with the bromide complex, has recently been r e ~ 0 r t e d . l ' ~ The complex with trimethylphosphine oxide, UC4.6Me3P0, is ionic, [UC1(Me3P0)6]3+C13,and the coordination geometry of the seven-coordinate cation approximates to a singly capped octahedron.1' The coordination geomet in [U(NCS)4(Me3P0)4],'17 [U(NCS)4{(Me2N)3PO}4]1's and [U(NCS)4(O{Me2N)2P0}2)2] TI9 is square antiprismatic, whereas in [ThCI4(0{(Me2N)2PO>2)2]"9 the geometry is dodecahedral. [Th(N03)4(Ph3P0)2]is isostructural with the cerium(1V) analogue, in which the coordination geometry is best described as distorted truns octahedral, with the four bidentate nitrate groups each occupying one corner of the equatorial plane.'" Several apparently non-stoichiometric compounds have been reported for complexes with the actinide(N) nitrates. The complex of composition Th(N03)4.2.67Me3P0 is ionic, [Th(N03)3(Me3PO>4]z[Th(N03)#-, and the 10-coordinate geometry in the cation can be described as a 1 : 5 : 4 arrangement with a planar face (Figure 6).12' The coordination geometry in the cation [Th(N03)3{(Me2N)3PO}4]+of the analogous complex Th(N03)4-2.67(MezN)3P0is described as a 1:6: 3 arrangement.12* Th(N0,)4.2.67Pr:P0 is probably similar in structure to the Me3P0 complex. The products of cornposition Th(N0+2.33 and 3.67Me3P0 appear to be mixtures. It has been suggested that Th(N03)4.3Me3P0 may be ionic, [Th(N03)2(Me3PO)5]z+[Th(N03)5(Me3PO)2]~, but this requires confirmation. The complexes Th(N03)4-4L (L = Me,PO, BuFPO) behave as 1: 1 electrolytes in nitromethane, and these may be of the form [Th(N03)3L4]'N0F. Their IR spectra confirm the presence of both ionic and covalent nitrate groups. Similarly,
The Actinides Table 39 Complexes of Actinide(1V) Compounds with N-Oxides L = pyridine N-oxides, R'RZC5H,N0 ThC1,.2L UC14*2L ThBr4-2L Th14.4L
R' = H and R2 =Ha(also +2H,O), 2-, 3- or 4-Me;" R' R' R' R'
= 2-Me, R2 = 6-Me = R2 = H;b R' = H,R2 = 2-, 3- or 4-COzH = H and R2 = H: 2-, 3- or 4-Me" = H and R2 = H," 2-, 3- or 4-Me;" = 2-Me, RZ= 6-Me (all [ThI,L,]IZ
R' R'= R 2 = H, x
Th(NCS),*xL Th(NO,),.nL Th(C104),.~L-yH20 Th(c13cco2),*2L Th { (0,CCH2)20} ,-3L.3H20 Th(N03),.3CsH11NO L = quinoline N-oxide ThX4.yL
= 4; R' = H, R2=2-, 3- or 4-Me, x =4; R' = 2-Me, R2 = &Me, Y = 2 R' = R2 = H, x = 2 (+MeCO,Et), 8; R1= H, R2 = 2-Me, x = 3; R' = 2-Me, RZ= 6-Me, x = 3,4 R'= H; R 2 = H , x = 8 , 9 , y = O ; Rz=2-Me, 4-Me, x =8, y =O; R2 = 3-Me, x = 8, y = 1or 3; R2 = 4-Cl, 4-NO,, 4-Me0, x = 8, y = 0; R'= 2-Me, R 2 = 6-Me, x = 6 or 8, y = O ~ 1 R Z== H R'= RZ = H C,H,,NO = collidine
X = C1, y = 2 (+2HzO); X = NCS, y = 4; X=NO,, y =3; X = CIO,, y = 6
L = isoquinoline N-oxide Th(NCS),.4L L = 2,2'-bipyridyl N-oxide or 1 ,IO-phenanthroline N-oxide X = CI, Bt, NCS, NO, ThX,.2L x = I, ClO, ThX4.3L L = 2,2'-bipyridyl N,N'-dioxide ThX4*L X = NCS, NO, Thx4.3L X = C1, Br ThX4.4L x = I, c10, L = 1 ,lo-phenanthroline N,N'-dioxide X = C1, Br, NCS, NO3, C10, ThX4-2L Th14.3L "R. K. Agarwal and S. C. Rastogi, Thermochirn.Acta, 1983,63, 363. bA.D. Westland and M. T. H. Tarafdar, Inorg. Chern., 1981, 20, 3992
Table 40 Complexes of Actinide(1V) Compounds with P-Oxides R = M e , a x = 2 , Mrv=Th, U, N p ; x = 3 , MKV=Th,U; x = 6, MIv = Th, Pa, U, Np, Pu R = E t , x = 2 , MIV=Th,U R = Bu", x = 1.5(+6H 0) 2, 3.5,4, 5 , 8, MIv= U R = B u " O , x = 2 , 3, d v A U R = Ph, x = 2, MIv= Th, Pa, U, Np, Pu; x = 3, MrV=Th R = Me,N, x = 2, MIv = Th, Pa, U, Np, Pu R3 = (Me,N),Ph, x = 2, MIv = U R3 = EtzPh, EtPh x = 2, MIV = Th, U R = C l , x = 4 , M"=U ThC13(OH) -2PhSPO UC13(0H).3Bu;P0 UC12(acac)z~2Ph,P0 MKVBr,.xR3P0
ThBr3(OEt).rPh3P0 M1V(NCS)4*xR3P0 U(NCSe),.4R,POC
R = M e , x = 2 , M I V = U ; x = 6 , MIV=Th,U R = Et, Bun, x = 2, Mrv = U R = Ph, x = 2, MIv = Th, Pa, U,Np, Pu; x = 3, MKv= Th R = M e z N , x = 2 , M T V = 7 n , P a , U , N p , P u ; x = 3 ,M w = T h R, = (Me,N),Ph, x = 2, MIv = U x=2,3 R = M e , x = 4 , M I V = U , N p , P u ; x = 6 , M1"=Th R = P h , x = 4 , MIV=Th, U ,Np R = Me,N, x = 4, MIv = Th, U, Np, Pu R = Bun, Me,N
1167
1168
The Actinides Table 40 (continued) R=Me,x=2.33, 2.67,bM'V=Th;x=3,4,5, M'V=Th,bU; x = 3, MIv = Npb R = MeO, x = 3,4, MIv = Th R = Et, x = 2.67, MIv= Th R = Pr", x = 2.67, MIv = l%,U, Npb R = Bun, x = 2, MIv = U; x = 4, MIv = Th R = Bu'O, x = 2, MIv = U; x = 2,2.33, MIv = Th R = Bu'O, x = 3, MIv = Th R = i-C5Hl,, x = 2, MIv = U R = n-CSH1,, x = 2 3, MIv = Th R = Ph,x = 2, MI'= Th, U, N P , PU ~ R = Me,N, x = 2, MIv = Th,b U,b Npb x = 2.67, 3, MIv = Th; x = 4, MIv = Th, U R3= (MeN),Ph, x = 2, MIv = U R3= BU"(BU"O)~P, x = 2.21, 2.67, MIv = Th R3= (MeO)(PhO),, x = 4, MTV= Th
Th(CF,COCHCOR),.L
Th(Nd,),-xL
L = [(Me,N),PO],O MwX4-1.5L MlVX4.2L Th(N03)4.2.SL MIv (C104),.4L L = [(Me,N),PO],NMe Th(C104),.4L L = 1,2,5-Ph,(C,H,PO),
R = Me, x = 6, MIv= U; R = Et, x = 4, MIv =Th; R = P r ",x=4-5, MIV=U; R=Bu", x = 5 (+3H,O), MIV=Th; R = P h , x = 4 , 5 , MIV=l%;x=5-6, MIV=U R = Me,N, x = 6, MIv = Th,x = 5 6 , MIv= U R = Me, L = Ph,PO, (Bu"O),PO, (n-C,H,7)3P0 R = CF,, L = Ph,PO, (n-C,HI,),PO, (&H,F,O),PO, (C,H,I;;O),PO, (Bu"O),PO, (PhO),PO R = 2-C4H3S,L Bu;PO, Bu"(Bd'O)ZPO, (n-CaH17)3PO, Ph,PO, (Bu"O),PO R = Me, CF,, L = (Bu"O),PO R = P h , n = 1,2, MIV=Th, U; n =4, MIV =U R = Ph, n = 1,2, x = 1.5 and n = 2, x = 2; R = MeCH(Et)CH,CH,, n = 1, x = 1.5 R=Et, n=2, x = 3 R = CYCZO-C~HII, rt = 1, x = 1 X = CI, MIv = U, Np," Pu;' X = N03, Mw = Th, U, Np X = C1, M"' = Th; X = NCS, MIv = Th, U, Np MIv = Th, U
1,2,5-triphenylphospholeoxide UCL4.2L L = (EtO),P(O)CH,C(O)NEt, [Th(NO,),LI Z.M. S. Al-Kazzaz, K. W. Bagnall and D. Brown, 3. Inorg. Nucl. Chem., 1973,3S, 1493. bK. W. Bagnall and M. W. Wakerley, J . Chem. SOC., Dalton Trans., 1974, 889. 'V. V . Skopcnko, Yu. L. Zub, V. N. Ymkovich, R . N. Shchelokov, A. V. Rotov and G.T.Bolotova, Ukr. Khim. Zh. (Rum. Ed.), 1984,50, 1011 (Chem. Absn., 1985,102,55 069). a
Th(N03)4.5Me3P0 may be [Th(N03)2(Me3P0)5]2+(N03)2. The geometry of 12-coordinate thorium in -the anion of (PbP)[Th(N03)5(Me3P0)2]is distorted icosahedra1lz1 and the same geometry has been reported for the 12-coordinate thorium atom in the diethyl-N,N'-(diethylin which carbamyl) methylene phosphonate complex, [Th(NO3),( (EtO)2P(0)CH~C(O)NEt2}2], two carbonyl, two phosphoryl and eight nitrate oxygen atoms are bonded to the thorium atom The geometry of the nine-coordinate thorium atom in the thenoyltrifluoroacetonate complex, [Th{CF3COCHCO-(2-C~H~S)},((n-CsH17)3P0}], is a 4,4,4-tricapped trigonal prism (Figure 71.124
The complexes MBr4.2(Me2N)3P0 (M = Th, Pa, U, Np, Pu) are isostructural, and the tris complex, ThBr4.3(Me2N)3P0,behaves as a 1:1 electrolyte in nitromethane.lZ5
The Actinides
Bond length (A)
Th-O(31) Th-0(32) W ( 3 3 ) Th-0(34) Tha(311) Th-0(312) Th--0(321) Th-O(322) M(331) Th--0(332)
2.24(3) 2.38(3) 2.25(3) 2.25(3) 2.71(2) 2.7q3) 2.64(3) 2.64(4) 2.61(3) 2.67(5)
Bond angles (") O(3 1)-Th-0(32) 79.9(11) O(3l ) - T h 4 ( 33) 83.9( 11) 0(31)-Th-0(34) 95.6(10) 0(31)-Th-O(311) 68.2(9) 0(31)-Th-0(312) 113.3(10) 0(31)--Th--o(321) 119.5(10) 0(31)-Th4(322) 72.1(12) 0(31)-Th4(331) 145.0(10) 0(31)-Th4(332) 161.9(13) 0(32)-Th4(33) 72.6(12) 0(32)-Th-0(34) 140.7(11) 0(32)-Th-0(311) 70.6(9) 0(32)-Th-0(312) 77.7( 11) 0(32)-Th-0(321) 136.2(10) 0(32)-Th-0(322) 1 3 6 413) 0(32)-Th4(331) 71.4(11) 0(32)-Th-0(332) 114.3(14) 0(33)-Th-0(34) 146.3(12)
1169
Bond angles (") 0(33)-Th-0(311) 136.8(10) 0(33)-Th-0(312) 142.3(11) 0(33)-Th-0(321) 71.4(10) 0(33)-Th-O(322) 72.0(13) 0(33)-Th-O(332) 110.4(13) 0(34)-Th-O(311) 71.6(9) 0(34)-~h-4(312) 68.3(10) 0(34)--Th4(321) 79.9( 10) 0(34)-Th4(322) 75.9( 12) 0(34fiTh-0(331) 119.3(11) 0(34)-Th-O(332) 66.3(13) 0(311)-Th-0(312) 45.2(8) 0(311)-Th-O(321) 151.2(8) 0(311)-Th-0(322) 124.7(10) 0(311)-Th-0(331) 118.0(9) 0(311)-Th-0(332) 104.9(12) 0(312)-Th-0(312) 120.0(9)
The structure of the [Th(NO,),(PMe,O),]* cation'21
Some interatomic distances
(A) Th-O(ll) Th-0(12) Th-0(9) Th-0(42) Th-O(31) Th-0(21) Th-0(22) Th-0(41) Th--0(32)
2.44(2) 2.44(2) 2.30(2) 2.46(2) 2.45(2) 2.49(2) 2.42(2) 2.37(2) 2.42(2)
Some bond angles (")
O(ll)-Th-0(12) O(21)-Th-0(22) 0(31)-Th4(32) 0(41)-Th4(42) -(11>--c(11) Th4(12)-C(13) Th--0(21)--C(21) Th4(22)4(23) Th 4 ( 3 1 ) - - C ( 3 1 ) Th-O(32)-C(32) Th4(41)4(41) Th-0(42)4(43) Th-0(9)-P
69.5 68.4 67.7 66.3 137.1 140.5 132.8 137.1 135.2 139.2 138.8 142.6 173.1
fiv 7 The coordination geometry of [Th(CF,COCHCO(2-C4H,S))4{ (n-C8Hl,)3PO)]"4
The complexes with thorium(1V) perchlorate, Th(C104)4-4Lbehave as 1:4 electrolytes in nitromethane when L = [(Me2N)2PO]20 or [(Me2N)2P0]2NMe, but with L = Ph3P0 the conductance is consistent with 1:3 electrolyte behaviour in nitromethane, perhaps as [Th(PhsP0)4(02C102)]3+(C104)3. The IR spectrum of Th(C104)4.SBu?P0 shows that both ionic and coordinated perchlorate groups are present, and this compound may have a similar ionic structure.
The Actinides
1170
cf) As-Oxides. The few known complexes with arsine oxides (Table 41) appear to be very similar in composition and behaviour to those formed by the analogous phosphine oxides. The only reported structure is that of the complex [UC14(Et3As0)2],in which the coordination geometry is trans octahedral.126 Table 41 Complexes of Actinide(1V) Compounds with Arsine Oxides M'VC14*xR3A~0 UBr4.xR,As0 UI,.6R,AsO Th(CIO4)4.5Ph,AsO ThL.Ph,AsO.xH,O Pu(N03),.2Ph3As0 L = Ph2As(0)(CH,),As(O)Ph, UCI4*XL
R = Me, x = 2, MIv = IJ;x = 6, MIv = Th,U R = Et, x = 2,4, MIV= U; x = 6 , MIV=Th R = P h , x = 2 , M " = U ; x = 4 , MIV=Th R - Me, Et, x = 2 , 6 R = Ph, x = 2 , 4 R = Me, Et
HZL = CSH3N-2,6-(COzH),;x = 0, 3 x = 1,2 for n = 2 ; x = 1 for n = 4
(g) S-Oxides. The complexes with sulfoxides are usually prepared from the parent compound and the ligand in dry, non-aqueous solvents; the PhzSO complexes of NpC14 and hC14 can, however, be obtained by treating a hot 6 M HCl solution of CszMC16 with the ligand. The complexes with the nitrates have been prepared either by treating the salts Cs2M(N0& with the ligand or by treating the corresponding complex of the metal tetrachloride with silver nitrate in methyl cyanide (e.g. refs. 127, 128). In addition to the large number of complexes with these ligands listed in Table 42, products of composition Th&.yMe2S0.zH20 (X= C1, Br, I, with values of y ranging up to 12) have been recorded, prepared by treating the hydrated parent thorium compound either with the stoichiometric amount of Me2S0 or, in some instances, with an excess of the ligand. These products require further investigation. ThC14.3Me2S0 is isostructural with UCl4-3Me2SO, which is an ionic compound, [UC12(Me2S0)6][UC16].The coordination geometry of the uranium atom in the cation is a distorted dodecahedron. lZY The complex of composition ThC14.5Me2S0 may also be ionic, possibly [ThCl,(DMSO),]Cl, but this requires verification. The IR spectra of the complexes MIVC14-7Me2S0(MIv = 'I% U, Np, , Pu) indicate the presence of both bonded and free ligand, but the structures are not known. Three crystal modifications of Th(N03)4-3Me2S0have been reported, one of which (designated the y form) is isostructural with the analogous complexes of uranium(IV), neptunium(1V) and plutonium(1V). The coordination geometry in the Me2S0 complex of thorium 8-hydroxyquinolinate, Th(&H6N0)4-2Me2S0, is a monocapped square antiprism in which only one MezSO molecule is coordinated to the metal.13' In the structure of [ThC14(Ph2S0)4]the C1 atoms occupy the 3 sites and the sulfoxide 0 atoms the A sites in the dodecahedral c0mp1ex.l~~
(ix) H y droxamates, cupferron and related ligands The known complexes, M'"L4 (Table 43), precipitate from aqueous solutions of actinide(1V) compounds on addition of the ligand. Somewhat surprisingly, one hydroxamic acid, (PhCO)NHOH, behaves as a neutral ligand, and the complex Th(N03)4-2(PhCO)NHOH precipitates from aqueous solution at pH 7. Structural information is available for [T~((BU'CO)(P~')NO)~] and [Th{(t-C5H,,CO)(Pr)NO)4]; the coordination geometry in the former is a distorted cube, whereas in the latter the geometry is an mmmm d0decahedr0n.l~~
40.2.25 Sulfur ligands
(i) Su@des and thiols
( a ) Sulfides. Disulfides, MSz, are known for M = T h , U and Pu, but the plutonium compound is a plutonium(II1) polysulfide (p. 1135). The compounds M2S5 (M = Th, U, Np) and
The Actinides
1171
Table 42 Complexes of Actinide(1V) Compounds with Sulfoxides
R = Me; x = 3, MIv = Th, Pa, U, Np, Pu;x = 5, MIv = Th, Pa, U, Np; x = 6 , M I V = T h ; x = 7 , M'V=Th,U,Np,Pu R = E t ; x = 2 . 5 , M t V = N p , P u ; x = 3 , M r V = T h , U ,N p;x=4, Mw=Th R = Pr"; x = 3, MIv = U R=Bu";x=2, MtV=Th;x=4, M N = U R = B u ' ; x = 2 , M I V = U ; x = 3 , MIV=Th R = But; x = 2, MIv U R n-CSHll, n-C6H13;n-C-,H,,, n-GH,,, x = 2, MIv Th R = Ph, x = 2 (+2H,O), MIv = U; 1 = 3, MIv = U, Np; x = 4, MIv = Th,U, Np R = a-CloH7;x = 3, M'" = Th, U, Np
R = Me; x = 6,8, MIV=Th, U R = E t ; x = 5 , 6 , MtV=U R=Pr";x=7, MrV=U R = B u n ; x = 8 , M"=U R = Bu'; x = 4, MIv = U R = B u t ; x = 2 , MIV=U R = P h ; x = 4 , Mw=Th, U
Th(ClO4),-XR,SO T~(R~COCHCOR~),.BU;SO ThX,.yMe,SO MIv(trop),.Me,SO ThX2.yMe2S0 L = thianthrene 5-oxide,
R = Me; x = 3, MIv= Th, U, Np, Pu; x = 6, MIV=Th, Np, Pu R = Et; x = 3, M'"=Th, U, Np R = Bu", n-CsHll, n-C6H13, n-GH15, n-CsH17, x =2, M'"=Th R = n-C8Hl,; x = 3, MIv = Th R = PhCH,; x = 4, M'" = Th R = fh;x = 3,4, MIv = Th, U, Np, Pu x = 3, MIv = Th, Np R = CY-C~OH~; R=Me,x=6,12;R=Ph,x=6 R' = CF,, Rz = Me, CF,; R' = R2 = &F5 X = HCO,, OSC-,H, (thiotroponate), y = 1; X = C J & N O (8-hydroxyquinolinate),y = 2 Htrop = tropolone; MIv = Th, U X = SO,, y = 4 with 3 or 9H20; X = C,O,, y = 2 with lHZO MIv = Th,U
L = tekthydrothiophene S-oxide m 4 . yL
X = CI, Br, y = 4; X = I, y = 8; X=NCS, y = 2 ; X = NO,, y = 6 ; X = CIO,, y = 10 Table 43 Actinide(1V) Complexes with Hydroxamates, Cupferron and Related Ligands Hydroxamates, HL = (R'CO)(R2)NOH Th(N03),+ZHL R1=Ph, R 2 = H R' =But, f-CSHI1,R2 = P i ; R' = Ph, ThL4 2-02NPh, R2= Ph; R' = 3-02NPh, Rz= 3MeC,H, R' = But, R2 = Pr' UL4 Cu ferron, NH4L = NH,[(ONLPhNO] M' LA M' = Th. U, Pu NeocGpferron, NH4L = NH,[(ON)( 1-CiOH7NO)] MI~L, MV=Th,U 3-Phenylcupferron, W L = NH,[(ON)(C,H,Ph)NO] mI-4 2-Phenyl-3-toluenesulfonylcupferron, NH4L = NHJC,,H,,N,O,S) ThL4
e
MS3 (M = U, Np) are actinide(1V) polysulfides; the geometry of 10-coordinate thorium in Th2S5, which is isomorphous with U2S5 and NpzSs, is derived from a square-based antiprism surmounted by a c a r e d , ap roxirnately pentagonal array of S atoms.133Oxide sulfides, such as the compounds M OS (M -Np, Pu), Pu202S3 [(Pu")~(O~-),S~-(S-S)~-] and KO& (M= Np, Pu; possibly 2M02.MzS3)are also known. (b) Thiols. The only recorded compounds appear to be U(SR), with R = E t or Bun,
PV-
coc3-LL
1172
The Actinides
prepared by reaction of U(NEt2), with the thiol in ether; both compounds are oxidized very easily and they inflame in air.
( i i ) Thioethers
A complex with 1,2-dimethylthioethane, UCl4-2(MeSCH2CH2SMe),has been recorded.
(iii) Monothio- and dithio-carbamates, and thiohydroxamates (u) Monothiocurburnares. The thorium(1V) and uranium(IV) compounds, M'v(R2NCOS)4
(MI"= Th, R = Me, Et; MIv = U, R = Et) have been obtained by insertion of COS into the M-N bond of the dialkylarnides, M(NR2)4. The 'H NMR spectrum of Th(Et,NCOS), indicates that rotation about the (Et2)N-C bond is slow on the NMR time scale, and the two ethyl groups are not equivalent. (b) Dithiocarbamates. The compounds MIV(R2NCS2), (R = Me, MIv = Th; R = Et, MIv = Th, U, Np, Pu) have been prepared by reaction of MrvC14 or M:MIVC16 with the alkali metal dithiocarbamate in non-aqueous solvents, and by CS2 insertion into the M-N bond of the dialkylamides (MIV=Th, U only). A much simpler route is by reaction of UCl, with the dialkylamine and CS2 in toluene. U(Et2NCS2)4is a monomer in benzene and is very sensitive to air oxidation in solution. It is isomorphous with Th(Et2NCS2),, in which the coordination geometry is close to an ideal d0decahedr0n.l~~ The 'H NMR spectra of Th(R2NCS2)4 (R =Me, Et) indicate that the two alkyl groups are equivalent. Mixed complexes with Schiff bases, M'V(Et2NCS2)*(2-OC6H4CH=NCH~CH~N=CHC6H402) (MIv = Th, U), have also been recorded. (c) Thiohydroxamates. The acids R1C(S)R2NOH (R1 = Ph or 4-MeC6H4, R2 = Me) yield precipitates of composition Th(R1C(S)R2N0)4.H20 when aqueous thorium tetrachloride is added to an aqueous solution of the sodium h y d r 0 ~ a m a t e . l ~ ~
(iv) Dithiolates
The maleonitriledithiolate complex, (Ph4As),[U(cis-SZC2(CN)2)4], precipitates when PbAsCl is added to a solution containing UCl, and Naz-(cis-SzCz(CN)2); it oxidizes on exposure to air. Uranium tetraiodide reacts with toluene-3,Cdithiol in hexane under reflux to yield a product of composition U214(C7H6S2)-C7H8S2, which may be a uranium(II1) compound and is possibly a sulfur-bridged dimer. A product of somewhat similar composition, U2C14(S2C4F6), is obtained when UC14 is heated with bis(trifluoromethy1)-1,Zdithiete, whereas the reaction of U14 with S2C4F6in ethylcyclohexane yields a product of composition136U3F31(S2C4F6)2. These last two products are probably dithiolates, but they require further investigation.
(v ) Thiocarboxamides
The only recorded complex is the product of composition UF4.SC3N2H2which results from the reaction of UF6 with dithio~xamide.'~~
(vi) Thioureas
A few complexes of composition ThC14.xL (L = SC(NH2)2, x = 2, 8; L = SC(NH2)(2I ClC&)NH, x = 4 and L=S&NHCH2CH2NH, x = S ) have been reported. They are 1 : 1 electrolytes in methanol and although there is no detectable shift in v(CS) in their IR spectra as compared with the free ligand, the ligands do not appear to be bonded to the metal via the thiourea nitrogen atoms.
The Actinides
1173
(wii) Dithiophosphinates Thorium compounds of composition Th(S2PR2)4(R = Me, Et, Pr', Ph, C6Hll,OEt and OPr') have been reported. In the structures of the compounds with R = M e and R = C a l l , the thorium atoms are dodecahedrally surrounded by eight sulfur atoms. 13*
40.2.2.6 Selenium and tellurium ligands ( i ) Selenides and tellurides Diselenides, MSe2, and ditellurides, MTez (M = Th, U, Pu), are known but the oxidation state of the metal in these compounds is uncertain. Other selenides and tellurides [cg. Th2Ses, Use3, MTe3 (M = Th, U) and UTe5] are presumably analogous to the actinide(IV) polysulfides (p. 1135). A more detailed description of these systems is a ~ a i 1 a b l e . l ~ ~
(ii) Diselenocarbamates The thorium(1V) and uranium(1V) compounds, MIV(Et2NCSe2)4,have been prepared by insertion of CSe2 into the M-N bond of the diethylamides, M(NEQ4, and (M=U)by reaction of EtzNHz(EtzNCSe2) with U C 4 in methanol under nitrogen. The 'H NMR spectrum of the thorium(1V) compound indicates that the two ethyl groups of the ligand are equivalent.
(iii) Phosphine selenides The complex ThCL+.2Ph3PSeseparates when ThC14 is heated with the stoichiometric quantity of Ph3PSe in benzene under reflux. It is a non-electrolyte in DMF.
40.2.27 Halogens as Iigands The known halides, and a selection of the halogeno complexes derived from them, are listed in Table 44. (i) Tetrahalides and oxohalides These compounds are halogen bridged polymers in the solid state. All of the tetrafluorides, MF4 (M = Th-Pu), have the same structure in which the MIv centre is surrounded by eight F atoms in a slightly distorted square antiprismatic array. In the hydrated neptunium compound, Np3Fl2-H2O,the water molecule is not bonded to the metal atom, and three kinds of fluorine polyhedra surround the neptunium atoms. These are a tricapped trigonal prism with two vacant apices, a bicapped trigonal prism and a square antiprism,lm The coordination geometry in solid ThC14 is d ~ d e c a h e d r a l 'and ~ ~ the other tetrachlorides, MCL (M = Pa, U, Np), have the same structure. Two of the tetrabromides, B-ThBr4 and Pa&, are isostructural with the tetrachlorides, whereas the coordination geometry in Th14 is a distorted square antiprism.'" Several oxohalides, MOX2, have been recorded (Table 44); the structure of PaOClz consists of an infinite polymer in which the Pa atoms are seven-, eight- and ni11e-co0rdinate.l~~
(ii) Halogeno complexes A detailed account of the very wide range of fluoro complexes is available.lU The structures of the main types of fluoro complex are well established. In LiUF5 the U atom is surrounded by nine F atoms in a tricapped trigonal prismatic array, with adjacent prisms sharing edges and The andogous Th, Pa, Np and Pu compounds adopt the same structure. The coordination geometry about the metal(1V)' centres in the compounds KM$vF9 (MIv = ThPU)'~ and in the structure of NhU3F13, which is isostructural with other M1MiVFI3(MI = NH4, Rb; MIv = Th, U, Np), is also a tricapped trigonal prism.147
The Actinides
1174
Table 44 Actinide(IV) Halides and Halogeno Complexes M'"X, M'"OX, MIMI~F,
M'M;"VE, M'M:~F,,
M;M'"F,
M I ~ M ~ ~ F
M:M'~F, M:M'"F~ M:M:~F,, M;[M'VCI,] MI1[UCI,] Mi[M'VBr,]
M:[MI,I
X = F, MIv = Th, Pa, U, Np, Pu X = C1, Br, MIv = Th, Pa, U, Np X = I, MIv = Th, Pa, U x=F,M~=T~ X = CI,Br, MIv = Th, Pa, U, Np X = I, MIv = Th, Pa MI= Li, MIv = Th, Pa, U, Np, Pu M' = Na, K, Rb, MTv= U M' = Cs,MI" = Th, U, Pu M' = NH,, MIv = Th,U, Np," Pu M' = Li, MIv = Th; M' = Na, MIv = Th, U M'=K, MIV=Th, U, Np, Pu; M'=Rb, MiV=Th, U; M ' = G , M'"=U MI= Li, MIv = Th, U,N P u ; ~ M' = K, MIV= Th, U; M p I Rb, Cs, MIv = Th; MI = NH,. MIv = U, Np," PU M' = Na, MIv = Th, U, Np, Pu M' = K, MIv = Th, U, Np MI= Rb, MIv = Th, Pa, U, Np, Pu MI= Cs,MIv = Th, U, Pu M' = NH,, MIv = U, Np," Pu M' = Et,N, MIv = Pa, U = Ca,Sr, MIv = Th, U, Np, Pu ~ MT' M" = Ba, Pb, MIv = Th, U , Np MI' = Cd, Eu MIv = Th M"=Co, M X L = U Np , (+3H20) MI = Li, MI" = Th, U; M' = Na, K, M" = Th, Pa, U; M' = Rb, Cs, MIv = Th, U; M' = NH,, MIv = Th MI= Li, MIv = U, N P , ~PU M' = NH,, MIv = Th, Pa, U, Np, Pu M' = Na, K, MIv = Th, Pa, U, Np, Pu M' = Rb, MIv = Th, Pa, U, Np, Pu MI = NH,, MIv = U, Np, PU MIv = Th, U; M' = Li-Cs, Me,N, Et,N MI" = Pa, N . M' - Cs, Me,N, Et,N MIv = Pu; My'= N i C s , Me,N, Et4N M" = Ca, Sr, Br MIv = Th, Pa; M' = Me,N, Et4N MW = U; M' = Na-Cs, Me,N, Et4N MIv = Np; M' = Cs, Et,N Mw = Pu; M' = Et,N MIv = Th,' U;' M' = Et,N, Me,PhN MIv = Pa;' MI= Et,N, Me,PhN, Me,PhAs
H. Abazli, 5 . JovE and M. Pagks, C. R. Hebd. Seances Acad. Sci., Sw.C , 1979,288,157. bJ. Jov6 and A. Cousson, Radiochim. Acta, 1977,24, 73. D. Brown, P. Lidster, B. Whittaker and N. Edelstein, Inorg. Chem., 1976,15, 511. a
The structures of the compounds Rb2MIVF6(MIV=U, Np, Pu) consist of chains of M1"Fs units with triangular faced dodecahedral geometry (K2ZrF6 type)'& whereas in 8-Na2UF6 the coordination geometry in the UF, chains is a tricapped trigonal prism,149as in the structure of f11-K2UFh.150 The compounds M"M"'F6 (MI' = Ca, Sr, Ba, Pb, Cd, Eu; Table 44) adopt the LaF3 structure.151 In contrast, the nine-coordination geometry about the Np atom in the [NpF8(H20)]chains in CoNpF6.3H20consists of singly capped square antiprisms sharing two F atoms, with the water molecule occupying the cap position. The uranium compound is analogous.lS2 In the structures of compounds of the type M$UF7 the seven F atoms are statistically distributed over fluorite lattice sites.153 The nine-coordinate thorium atom in (NH&ThFs is surrounded by a distorted tricapped trigonal prismatic array of fluorine atoms, with the prisms sharing edges to form chains, whereas the uranium(1V) compound contains discrete dodecahedrally coordinated [UF8I4- ions. The protactinium(IV), neptunium(1V) and plutonium(1V) analogues are isostructural with the uranium compound. 154 Compounds of the type M$MYF,, (Table 44) are isostructural with Na7Zr6F31rin which the basic coordination geometry about the Zr atom is approximately square antiprismatic, and six antiprisms share corners to form an octahedral cavity which encloses the additional F atorn.lS5 The complex anions formed with the other halogens are all of the form [MX$--, with
The Actinides
1175
octahedral geometry. For example, CSz[NpBr6] is isomorphous with Cs2[UBr4,(face-centred cubic, Kz[PtC16]type).158The IR and Raman spectra of the salts M:[MIVCl6] (M = Th, Pa, U, Np, Pu; M' variously Cs, Et4N, ButN, Ph4As), (Me&)zIPaBr6] and (Et4N)2[MIV&] (MN= Th, Pa, U; X = Br, I) have been r e ~ 0 r t e d .The l ~ ~ far IR spectra of the compounds M:[NpC16] (MI = Cs, Me4N, E t a ) have also been p~b1ished.l~'
40.228 Hydrogen and hydrides as ligands The only examples of corn ounds of this type are the borohydrides, MIV(BH& (MIv = ThPu) and MIV(MeBH& (M"=Th, U, Np). These compounds are conveniently prepared by reaction, for example, of the metal tetrafluoride with AI(BH4)3 in a sealed tube, followed by vacuum sublimation of the product.lS9The neptunium and plutonium compounds are liquids at room temperature and are more volatile than the thorium, protactinium or uranium analogues. In the structures of Np(BH4I4 and P u ( B H ~the ) ~ four BH4 groups are arranged tetrahedrally around the metal atom and three hydrogen atoms from each BH4 group are bonded to the metal, making it 12-~oordinate,'~~ whereas in the polymeric uranium(1V) compound the uranium atoms are each surrounded by six BH, groups, two of which provide three hydrogen atoms each as above, and the other four provide two hydrogen atoms to each of two uranium atoms, making a 14-coordinate arrangement which could be regarded as a distorted bicapped hexagonal antiprism. In the structures of the methylborohydrides, M'V(MeBH3)4(M'V= 'I%, U, Np) the MeBH3 units are tetrahedrally coordinated to the metal via triple hydrogen bridges. 16' 40.2.29 Mired donor atom ligands
( i ) Schiff base ligands
There is a wide variety of complexes with neutral Schiff bases derived from benzaldehyde, as well as mono- and di-basic ligands derived from salicylaldehyde. A selection of these is given in Table 45, from which it can be seen that the mono- and di-basic derivatives can also act as neutral ligands. The range of complexes can be further extended by utilizing substituted salicylaldehydes, an example163 being the compounds MIVL (MIv = Th, U) formed with (5-But)(2-HOC&)CH=NCH~CHZN=CH( ChH30H-2)(5-But). The coordination geometry in the complex [Th(2-OC6H4CH=NC6H4cHc6~o-2),1is a distorted square antiprism in which the corners of each square face are occupied by the two N and two 0 atoms of one ligand.'64 In the structure of [Th{(3-Me0)(2O)C4H,CH=N(CH2)zN=CHC6H3(2-O)(3-OMe)}2]-py, the pyridine molecule is not bonded to the metal atom, and the coordination geometry is dodecahedral, with the two Schiff base ligands each occupying one of the trapezia which make up the polyhedron; the imine nitrogen atoms occupy the dodecahedral A sites and the phenolic oxygen atoms the B sites.165 ( i i ) Amino acids
The majority of the aminocarboxylates listed in Table 46 have been prepared by precipitation from aqueous solution, a procedure which frequently yields partially hydrolyzed products. No structural information is available for any of these compounds. The iminodiacetates, derived from HN(CH2C0zH)z, are discussed on p. 1155. (iii) Complexones
The simple thorium(1V) and uranium(1V) compounds of the type ,[M'VL]-xHzO(Table 47) separate from aqueous solutions of the actinide(1V) salts on addition of the acid, or on concentrating the resulting aqueous solution. The salts of the complex anions (e.g. M:Hz[U2L3]) are obtained from concentrated solutions of UL in aqueous M;H,L. The pentaacetic acid compound, ( N H ~ ) ~ [ U ~ ] - ~ isH precipitated ZO, when ethanol is added to a solution of U(&04)2.6Hz0 in the aqueous acid after neutralizing with ammonia and evaporation to small volume.
The Actinides
1176
Table 45 Some Actinide(1V) Schiff Base Complexes Neutral ligands: L = C6H,CH=NR MIVC14.2L MN = Th, U, R = Ph; MIv = Th, R = 4-MeC6H4 Th(N03),.2L R = 3-(Me,N)C6H4 Monobasic ligands: HL = 2-HOC6H4CH=NR MIVCI4.2HL MIv = Th, R = 2-, 3- or 4-MeC6&; MIv = U, R = Ph, 4-CIC6H4,4-O,NC6H4 ThX4.yHL R = 4-MeC6H4;X I, NO,, y 2; X = NCS, y = 3; R - Ph, X = NO,, y = 2 ThCI,L, R = Ph, 4-MeC,H4 MI~L, MW = Th, R = Me, Et, Ph; MIv = U,R = Me, Pr"
-
-
Dibasic ligands:H2L = 2-HOC6H4CH=NXN=CHC6H4-2-OH ThA4.2H2L A = C1, Br, I, NCS, NO,; X = CH,CH, UCI4'HzL X = CHZCH,, CHZCHZCH,, 1,2-C6H4 MI~LCI, MIv = Th, U; X = CH2CH, M'VL, MIv = Th, U; X = CH,CH,; Mw = Th, X = 1,2-C6H4; MIv = U, X = CH,CH,CH, HZL = 2-HOC,H,CH=NR UCI4.2H,L R = 3- or 4-HOC6H, ThLA, A = CI, NO,; R = 2-HOC,H4, CH(CH,CHMe,)CO,H Th(HL),A,.y H20a A=C1,y=4;A-I,NCS,C104,y=2; A=N03,y-3; R e CeH4-2-COzH Thl, R = 2-HOC6H4,CH(CH,CHMe,)CO,H UL" R = 2-HOC6H4, C6H4COzH Mohanta and K. C. Dash, 1. Indian Chem. Soc., 1980, 57, 26; the water molecules are only held weakly and the compounds dehydrate at 120°C. bA. D. Westland and M. T. H. Tarafdar Inorg. Chem., 1981, 20, 3992.
* H.
Table 46 Actinide(1V) Compounds with Amino Acids ~~
HL = aminoacetic acid (glycine), H,NCH,CO,H [ThL(OH)(H,O),I(NO,)Z [ULCl,(H,O),I~2~*0 x = 1.5 or 2 UL4.xH20 HL = N-phenylglycine, PhNHCH,CO,H UL&12.4H20 H,L = phenylglycine4-carboxylic acid, H02CC,H4NHC02H Th~(OH).2H,O HL = a-and b-aminopropionic acid (u-and B-alanine), MeCH(NH2)C02H and H,NCH,CH,CO,H ThL,(C104)2~2H,0'(?)inadequately characterized H,L = aminosuccinic (aspartic) acid, H0,CCH,CH(NH2)C02H [ThL(HzO)3I(NOdz
[T~L(HL)(H,o)I(NO,).~H,O
H,L = aminoglutaric (glutamic) acid, H02C(CH2),CH(NH2)C02H [ThL(H,O) SI(NO,) 2 [~L(HL)(H,O)I(NO,).HZ0 HL = aminobenzoic acid, H2NC&C02H ThLZ(OH), 2-amino
THL(ow3 UOL, Ul,(OH),.2H20 UL,Cl,b UL3(MeCO,), 3-and 4-amino ThL(0)(OH).4H,O 4-amino H,L = p-aminosalicylate, (H2N)(HO)C6H3C0,H Th(HL)(OH),*3H2O HL = 2-(N-phenylamino)benzoic acid, 2-(PhNH)C6H4C02H ThL4 HL = 5-iodo-2-aminobenzoicacid, (SI), (2-H2N)C6H,C0,H n = 0 , 1o r 2 ThL4-n (OH), HL = 3-amino-2-naphthoic acid, 2-H2NCloHb-3-C02H ThL4 'M. M. Mostafa, T. A. El-Awady and M. A. Girges, J . Less-Common Met., 1980, 70,59. bA. D. Westland and M. T. H. Tarafdar, Inorg. Chem., 1981, ZO, 3992.
1177
The Actinides Table 47 Actinide(1V) Complexes with Complexones Tricarboxylic acids Nitrilotriacetic acid, H,L = N(CH,CO,H), (NH&[ThWWO Tetracarboxylic acids Eth lenediaminetetraacetic acid, H4L = (HOzCCH2),NCHzCHzN(CHzC02H), M' YL*xH,O MrV=Th,x=2,3,9;Mrv=U,x=0,2,2.5-3.5 U3L(OH)4*xH20 M' = K, x = 9-18; NH,, x = 8-16 M~Hz[U,~3I*x~zO MI' = Ca, x = 8, 12; Sr, Ba, x = 4, 12, 18 M3 IUzL,I-xH,O M [ThL(Om].4H20 M' = Na, NH, 1,4-Diaminohexane-N,N,N', "-tetraacetic acid, H,L = ~O*C~H,~*~(CH*),N(CHZCOZH)Z MI L.xHZO M'"=Th, x = 4.5; MIv= U,x = 0 1,2-Diaminocyclohexae-iV,N',N',N'-tetraacetic acid, H4L = (H~zCCHz)zN(C,H,,)~(~~z~~zH)z ThL.x HzO x=o,1 NH,[ThL( OH)] .5 HzO Pentacarboxylic acids Diethylenetriaminepentaaceticacid, H,L = ~
~
~
z
H[ThL].H,O Na[ThL].4H20 (NH4)hIUL21*xH2O
~
~
~
,
~
,
~
~
~
~
z
~
z
~
~
x=0,4
In addition to the complexes listed in Table 47, the compounds N,N'-di(2-hydroxypheny1)ethylenediamine-N,N'-diaceticacid and N'-(2-hydroxyethyl)ethylenediamine-N,N,N'triacetic acid behave as tetrabasic ligands, forming ThL.xH20 (x = 6 and 2 respectively).
(iv) Ligands containing N , 0 donor sites
-
HexafluoroacetonyIpyruzoZides. These ligands, derived from hexafluoroacetone and pyrazole or 3-methylpyrazole , form complexes of the type M'V[OC(CF3)2NCHCRCH=N]4 (R = H, Me; MIv = Th, U, Np, Pu) which are prepared by adding a methanol solution of (CF3)zC0 to the solid obtained when, for example, solid (Et4N)2UC16is added to a toluene solution of potassium pyrazolide. The four compounds with R = H are isostructural, and in the thorium complex the eight-coordinate metal atom lies at the centre of a distorted square antiprismatic array of four N and four 0 atoms. In benzene solution, however, the molecular weights of the complexes with R = H indicate that they are dimers, whereas those with R = Me are monomers.166 ( b ) 8-Hydroxyquinolines. 8-Hydroxyquinoline can act as a neutral ligand, forming complexes of the type ThCL.xGH7NO ( x = 2, 3, 4 or 6), but the majority of the known compounds are of composition M'"L4 (HL = 8-hydroxyquinoline or the 5,7-dibromo ligand, MIv = Th, U, Np, Pu; 5-chloro- and 5-chloro-7-iodo-8-hydroxyquinoline, MIv = Np, Pu; 5,7-dichloro-8hydroxyquinoline, MIv = Th, Np, Pu; 2-methyl-8-hydroxyquinoline,MIv = Th, Np). Many other substituted 8-hydroxyquinoline complexes of this type are known for MIv = Th. , m ost of them are precipitated on addition of the ligand to aqueous solutions of actinide(1V) compounds. In some cases (for example, HL = 8-hydroxyquinoline, MIv = Th, U and the 5,7-dibromoligand, MIv = Th) the product obtained from aqueous solution has the approximate composition M I V 4 . H L , but detailed studies have shown that the composition is variable, an example being Th(CgH$r10)4.xC9H7N0 with x = O S , 0.8 or close to unity. A hydrate, Th(C9H6N0)4.2H20, has aIso been recorded. Chloro and hydroxo compounds, ThC1(GH6NO), and Thz(GH6N0)7(0H).xH20( x = 0 or 4) are known, and a variety of mixed 8-hydroxyquinolinate/trichloroacetatespecies, obtained by heating a suspension of Th(&HaO), with the appropriate quantity of trichloroacetic acid in benzene, have been recorded. ( c ) 2,6-Diacetylpy ridine bis(p-rnethoxybenzoyl hydrazone) ( = !I& The)coordination . geometry in the complex of composition [UL].2C6&-2H20 is a pseudo bicafxed square antiprism in which the two N302donor ligands are chelated to the uranium atom. (a)
~
~
z
The Actinides
1178
40.2210 Multidentate macrocyclic ligands (i) Phthalocyanine The bis phthalocyanine complexes, M E V P(MIV=Th, ~ Pa, U, Np, Pu), can be prepared by heating a tetrahalide, usually the iodide (M = Th, Pa, U), "ora triiodide (M = Np, Pu) with an excess of o-phthalodinitrile alone at 240-250 "C or, better, in 1-chloronaphthalene (see, e.g. ref. 168). The thorium compound has also been prepared16' by heating the metal powder with the dinitrile at about 270 "C. An iodine adduct, U P Q - I ~is, also known. In the structure of UP%, the nitrogen atoms of the pyrrole rings of the two Pc ligands form a distorted prism in which the Pc ring systems are rotated by about 37" from the prismatic conf igur ati~n.'The ~~ structure of ThPQ has also been published.169 The five complexes (M = Th, Pa, U, N , Pu) are isostructural. The bonding in UP@has been investigated by X-ray PE spectroscopy. 17P
(ii) Crown ethers and cryptands The structures of some of the known crown ether compounds (Table 48) have been reported; the dicyclohexyl-18-crown-6compound of composition 3UC14-2Lis ionic, [UC13L]2[UC16],with the uranium atom in the cation bonded to all six oxygen atoms of the crown ether and to three chlorine atoms."' The 18-crown-6 complex of the same composition is probably of this form, for on recrystallization from nitromethane the mixed oxidation state complex [UC1,L]2[UOzC13(OH)(H,0)]-MeNOz is obtained, in which the nine-coordinate uranium atoms in the cations are bonded to six oxygen atoms of the crown ether and to three chlorine atoms in an irregular polyhedron, while the anions adopt the pentagonal bipyramidal geometry commonly found for dioxouranium(V1) species.173However, in the uranium(1V) thiocyanate compound with 18-crown-6, the ether (L) is not bonded to the metal atom and the structure of this product, [U(NCS)~(H20)4].1.5L.3H20.MeCOBui, is described on p. 1142.
Table 48 Some Complexes of Actinide(1V) Compounds with Crown Ethers and Cryptands L = 12-crown-4= CBH1604 ThC1,-L L = 15-crown-5= C,,H,,O, and benzo-15-crown-5= Cl4Hz0O5 [n(N0&(Hz0)313'5L L = 18-crown-6= ClzHz40, Th(NO3),.L.xHZO x = l or3 2Th(NO,),.3L.2HzO U&-L X = CI, CF,CO, 3UCI4'2L
IUC~,l,[UO,C~,(OH)(HzO)I.M~NO*
L = dibenzo-18-crown-6= C2,HXO6 3Th(NO3).,.5L.3HZO L = dicyclohexyl-18-crown4= G,H& Th(NO3),*L.3H2O Th(N0,),.2L.HN03 3UC14.2L = [UC13L],[UCI,] L = dibenzo-24-crown-8= C,H,,O, Th(N03),.L.3HZO L = dicyclohexyl-24-crown-8= G H , O , Th(NO3),.2L.HN03 L = kryptofix (2,2,2), 4,7,13,16,21,24-hexaoxa-1,l0-diazabicyclo[8.8,S]hexa~sane= Cl,H,,N20, Th(N03)4.L*4H,O UX,.L X = C1, NCS
'H NMR spectroscopy of the complexes of UCl, with 18-crown-6 and dicyclohexyl-18-crown6, and of U(CF3C02)4with these two ligands and with the cryptands 12.1.11, [2.2.1], [2.2.2] and [2.2.2.B] in nitromethane or methyl cyanide indicate that the metal atom is bonded to the oxygen atoms of the ligand.172
The Actinides
40.2.3
1179
The +5 Oxidation State
40.2.3.1 Nitrogen ligands ( i ) Ammonia and amines
( a ) Ammonia. The only recorded ammine appears to be the alkoxide compound, U(OCH2CF3)5.(6-12)NH3, described as a green liquid. It is obtained by the reaction of UCls with CF3CH20Hin the presence of an excess of ammonia. (6) Aliphatic amines. Complexes of the trifluoroethoxide, U(OCH2CF3)5.2RNH2(R = Pr", P?), U ( O C H ~ C F ~ ) Y X R ~(xN=H3, R = Me and x = 2, R = Pr") and U(OCHZCF3)5-2Me3N are prepared by treating an ethereal solution of the alkoxide with an excess of the amine in ether, followed by vacuum evaporation of the mixture below 40°C and, finally, vacuum distillation of the green liquid products. No other actinide(V) complexes with amines appear to have been reported. (c) Dihydroazzrine (ethyleneimine, CZHSN).The complex U(OCH2CF3)5-3C2H5N,a volatile green liquid, is obtained in the same way as the amine complexes (see above). ( i i ) N-Heterocyclic ligands
( a ) Pyridine, C5HsN, p y , and derivatives. Complexes of uranium pentachloride, UCls.xL (x = 2, L = py or 2-HSC5H4Nand x = 3, L = py) and of the t-butoxide, U ( 0 B ~ ~ ) ~ -have py,
been recorded. Very little is known about them. (b) Quinoline and rkoquinoline, C9H7N. Quinoline and isoquinoline complexes of uranium pentachloride, UcI5.xL (x = 2 and 3) have been recorded, but it is not known whether the uranium atoms are seven (E = 2) and eight-coordinate (x = 3), or whether the compounds are ionic (e.g. [UC14L]+Cl-). ( c ) 2,2'-Bipyridyl, 6ipy. The only known complex, UCl,.bipy, behaves as a 1:1 electrolyte in n i t r ~ m e t h a n e 'and ~ ~ is evidently [UC14(bipy)]+C1-. ( d ) I , IO-Phenanthroline, phen. An anionic complex of composition (EtJV)2UOC15(phen)2 has been r e ~ 0 r d e d . lIt~ ~is probable that some of the chloride ions are displaced from the coordination sphere. ( e ) Pyrazole, substituted pyrazoles and 1,8-naphthyridine. Complexes of composition UCl5-1.5L (L = pyrazole or 3,S-dimethylpyrazole) and UC15-2L (L = 1-phenylpyrazole or 1,8-naphthyridine) have been obtained by treating a solution of UCls in SOClz with the appropriate ligand.176 (f) Pyrazine, c&&, phthalazine, CSHSN2, and phenazine, Cl2Hl8N2.Complexes of UC15 with these ligands (UC15-2C4HJV2,UC15.2CsH& and UCl5-CI2H8N2)are precipitated from benzene solutions of the trichloroacryloyl chloride compound, UC15C3CL,0, and the ligand. An anionic product of composition (EtJV)2UOC15.C12H& has also been reported, but the reaction of salts of the [UOC15]2-ion with phenazine in nitromethane appears to yield a salt of a free radical cation, (C1~l&N2)H2C1.177 (iii) Thiocyanates
A salt of a dioxoneptunium(V) thiacyanato complex, C S ~ { N ~ O ~ ( N C S has ) ~been ] , recorded; the coordination geometry of the complex anion is presumably pentagonal bipyramidal. (iv) Nitriles
Reaction of UF5 with Me3SiC1 or UC15 in methyl cyanide yields a complex with the mixed halide, UC12F3-MeCN.178 Complexes with the pentabromides, MVBr5.xMeCN(MV= Pa, x = 3; MV= U, x = 2 to 3) and with an oxonitrate, Pa20(N03)8-2MeCN,are also known.
40.2.3.2 Phosphonrs and arsenic ligan& The only recorded compound appears to be the complex with o-phenylenebis(dimethy1arsine), PaC15.~CsH4(AsMe~)~ (1< x < 2)." coc3-LL'
The Actinides
1180
40.23.3 Oxygen ligands (i) Aqua species, hydroxides and oxides ( a ) Aqua species. A selection of the reported hydrates is given in Table 49. The structure of one of them, K3Np02(C03)2-xH20 ( x d 2 ) , has been reported to consist of layers of [KNp02(C03)2]2-ions with K+ ions in between the but in a later paper"' the MVO2 group in the compounds M:Mv02(C03)2.xHz0 (MV= Np or Pu with M' = Na, K or Rb) is reported to be coordinated to six oxygen atoms from three bidentate carbonate groups to form a distorted hexagonal bipyrarnid. In most cases it is not known whether any or all of the water molecules are bonded to the actinide(V) metal ion in these compounds. In the hydrated carbonates, KMv02(C03)*xH20and K3MV02(C03)2-~H20 (MV= Np, Pu) the water is considered to be zeolite type.179a Table 49 Some Hydrates of Protactinium(V), Uranium(V), Neptunium(V) and Plutonium(V) Compounds PaF,*xH,O (Et4N),(UOF&2H20 NpOF3.2HZO Np02(C104).xH,O M I M ~ O , ( CO,) .XH,O
x=l,2 x=3,1
M'=Na,MV=Np,x=0.5,1,2,3,3.50r4;MV=Pu, x unspecified M'=K, Rb, M V = N p , Pu M'=NH4,MV=Np,Pu(x=3)
M V = N p , Pu ( x S 2 ) 2<x,]~-layer structure.251 The phosphate of composition U02H(P03)3 is obtained252 by heating U02(N03)2.6H20with 85% H3P04 at 340-380 "C and the salt NaU02(P03)3crystallizes2" from 85% H3Po4at 360 "C. The pyrophosphates MlU02Pz07 (M' = Na, K, Rb, Cs) have been prepared by heating U02(N03)2.6Hz0 with (NH&HP04 in the presence of the alkali metal pyrophosphate or nitrate.254The rubidium and cesium compounds are isostructural and in the latter, layers of UO2P2O$- groups are held together by cesium ions. The UO$+ion is coordinated by four oxgyen atoms from four P207groups in the equatorial positions of a tetragonal b i ~ y r a r n i d . ~ ~ ~ ( c ) Arsenates. A number of hydrated arsenates are included in Table 63 (p. 1192). Products of composition U02HAs04-4H20, U02(H2As04)2.H20,(U02)3(A~04)2, (U02)2A~207 and U02(As03)2have also been reported.z6 (d) Sulfites. Hydrated dioxouranium(V1) sulfite is precipitated when aqueous UOz(N03)2 is treated with sodium thiosulfate;257 it is completely dehydrated at 130-150 "C. The sulfito complex, [CO(NH~)~]~[UO~(SO~)~]~*~~H~O, is precipitated from aqueous and a basic compound of composition bipyH[(U02S03)2(0H)(H20)2]has also been reported.259 ( e ) Sulfates. Dioxouranium(V1) sulfate hydrates, UO2SO4-xHZ0(x = 1, 2,2.5, 3 or 4, and the fluorosulfate, U02(S03F)2r have been recorded; the last is obtained86 by treating U02(MeC02)2 with HS03F. Salts of sulfato complex anions, such M:U02(S04)2.2Hz0 (Mv' = U, Pu), (MI = K, NI-L,), (NH&U02(S04)3-4Hz0 and (NH4)2(MV102)2(S04)3.5H20 have been recorded. Other known salts include M1'U02(S04)2-~H20 (MI' = Ni, Zn, x = 4; M" = CU, x = 3), Mu(UOz)3(S04)5*~H20(MI' = MII, x = 5; M = Cd, x = 2; M = Hg, x = 3), C O ~ U O ~ ( S O ~ ) ~ - Ce(U02)2(S04)4.3H20261 H~O, and the two isomorphous hexammine (Mv' = U, Np), all of which are cobalt(II1) salts,262{[CO(NH~)~]HS~~)~[M~~O~(SO~)~]~XH~O obtained from aqueous solution. The coordination geometry about the uranium atom in (NH4)2U02(S04)2.2H20is pentagonal bi yramidal, with sulfate groups joining the polyhedra to make up a layered structure.' The structure of &U0z(S04)3 involves the anion [(S04)2U02(S04)2UO~(S04)~]8-, in which each UOz group is bonded to five oxygen atoms from four non-equivalent sulfate groups in the equatorial plane: of a pentagonal bipyramidem The structure of MgU02(S04)2.11H~0 consists of infinite layers of composition [UOZ(SO~),(H,O)]$- in which and the the coordination geometry about the uranium atoms is also pentagonal bi~yramidal,'~~" same coordination geometry has been found for the metal atom in NH4U02S04F,in which the equatorial plane is made up from three 0 atoms from three tridentate sulfate groups together with two bridging fluorine atoms.265b Crystallographic data for the hydrated salts M"U02(S04).5H20 (MI' = Mg, Fe, Co, Ni, Cu, Zn, Mn, Cd) are also available.266 c f ) Selenates. Hydrated dioxouranium(V1) selenate , UOzSeO4.4H20, is not isostructural with U02S04.4H20; its structure consists of an infinite strip, [U02Se04(H20)2]03, with the equatorial positions of the pentagonal bipyramidal arrangement of oxygen atoms surounding each uranium atom occupied by the oxygen atoms from three monodentate Se04 groups and two water molecules.267Hydrated salts of the selenato complex anion, M1'U02(Se04)2.6H20 (MI' = Mg, Co, Zn), are also known.268 ( g ) Tellurites. Compounds of composition UTeO5(UO3.TeO2) and UTe3O9(UO3-3TeO2) are obtained by heating U 0 3 with the appropriate quantity of Te02. The structure of UTeOs
1198
The Actinides
consists of infinite chains, [U05]l"-', in which pentagonal bipyramidal U 0 7 groups share edges,269 whereas UTe309 involves eight-coordinate uranium in which the UO;' group is surrounded by six oxygen atoms in a flattened octahedral array.270The structure of the lead compound, Pb2U02(Te03)3,prepared by hydrothermal treatment of a stoichiometric mixture of PbO, U02(MeC02)2.2H20and Te 02 at 230°C, consists of U 0 7 pentagonal bipyramids in sheets, [UOz(Te03)3]",., connected by Pb atoms.271
(vi) Carboxylates, carbonates and nitroalkanes ( a ) Monocarboxylates. Most of the available information concerns dioxouranium(V1) cornpounds and very few neptunium(V1) or plutonium(V1) analogues have been reported (Table 69). The hydrated formate, U02(HC02)2-H20,precipitates when solid U02(N03)Z.xHz0 is added to anhydrous formic acid and when aqueous U02(N03)2is heated with an excess of the acid. The structure of the compound consists of an infinite chain of seven-coordinate (pentagonal bipyramidal) uranium units linked by bridging HC02 groups and by hydrogen bonds involving the water molecule^."^ The monohydrated neptunium(V1) and plutonium(V1) formates are isostructural with the uranium(V1) compound.273 The anhydrous formate is obtained by washing the hydrate with dry methanol or by heating it at 105-170°C. The basic formate, U02(OH)(HC02).H20,is obtained by hydrolysis of U02(HC02)2.H20;the structure again consists of a pentagonal bipyramidal arrangement of oxygen atoms about the uranium atom with the uranium atoms doubly bridged by OH groups and linked to the two second nearest neighbouring uranium atoms via HC02 groups. The pentagonal plane is completed by the water oxygen atom.274 In the structure of Na[U02(HC02)3].H20the coordination arrangement is also pentagonal bipyramidal, with the bipyramids connected through two types of bridging bidentate HC02 groups and linked with the Na+, H20 layers by a third type of bridging bidentate HC02 The tetraformato complex salt, (NH4)2U02(HC02)4,is obtained when a solution of U02(HC02)2 in aqueous HC02H and HC02NH4 is evaporated and then cooled; the pentagonal bipyramidal coordination geometry is made up of two oxygen atoms from one bidentate HC02 rou , and the remaining three HCOZgroups bridge U and N atoms, making a layer structure.E76e; strontium salt, SrUOz(HC02)4.1.38H20,obtained when a hot solution of Sr(HC02)2-2H20and U02(HC02)2,Hz0in aqueous formic acid is cooled slowly from 80 "C, shows a similar arrangement in which the HCOz groups bridge either two U atoms or one Sr and one U atom.277 The structure of the 2,6-dihydroxybenzoate, [U02{(H0)2C6H3C02}2(H20)2].8H20 consists of an irregular hexagonal bipyramidal arrangement with two bidentate carboxylate groups and two H20 molecules in trans positions in the equatorial plane.278b Five crystal modifications of anhydrous U02(MeC02)2have been reported. The structure of the dihydrate consists of chains of pentagonal bipyramidal coordination polyhedra in which half of the MeC02 groups link adjacent bipyramids and the remainder are chelated to individual uranium atoms; one water molecule is bonded to each uranium atom.278a The coordination polyhedron of the anion in Na[U02(MeC02)3] is a hexagonal bipyramid with three bidentate MeC02 grou s in the equatorial plane"' and this has also been found for the anion in Na[Np02(MeC02)3]a ! Hexagonal bipyramidal geometry has also been reported for the anthracene-9-(or 10)-carboxylate, [U02(C14H,CO&( H20)2].3C4Hs02; the dioxane molecules are not bonded to the uranium atom, and the six equatorial positions of the bipyramid are occupied by four oxygen atoms from two bidentate carboxylate groups and two oxygen atoms from the water molecules.280The IR spectra of Na[MV102(MeC0z)3](Mv' = U, Np or Pu) have also been reported.249 The coordination geometry in the crotonate, [U02(C4H502)2(H202)], is similar to that of the anthracene carboxylate, with the oxygen atoms of two bidentate carboxylate grou s and two water molecules coordinated to the UOg+ ion.281 The precipitation of salts M'M'fU02(RCOz)3]3 (Table 69) has been used for the gravimetric determination of alkali metals (particularly M' = Li and Na). For this purpose an anhydrous product is desirable, but in many cases hydrates of varying composition are obtained. Salts of basic species, such as T1U02(MeC02)(OH)2,and of mixed thiocyanato/carboxylato anions, such as C S ~ ( U O ~ ) ~ ( M ~ C O & ( Nhave C S ) ~been , reported, but require further investigation, A variety of halo- and mercapto-propionates, U02(RC0&, are also known. (b) Chelating carboxylates. Furan-2- and thiophene-2-carboxylates are included in Table 70 because the heterocyclic oxygen or sulfur atoms may be bonded to the metal atom. The
The Actinides
1199
Table 69 Dioxoactinide(V1) Monocarboxylates and Carboxylato Complexes R = H (x = 0, 1 or 2), Me, Et, Pr" ( x = 0 or 2), CF, ( x = 0, 1 or 1S), CCl, (x = 0, I! 2 or 3), CH,CI (x = 0 or l), PhCH, (x = 0), Pr', Bun, Me,CHCH, (x = 2), Me3C n-CiiHz3, n-C&zn n-CisH,i, n-C17H,s ( x = 0)) PhCH=CH (x = 0 nr 3), P h W ( x = 3), 1-Cl,H7 (x = 3), Ph ( x = 0, 1 or 2), 2-Cl0H, (x = 1) Mv' = Np, Pu C,,H,CO,H = anthracend(or 10)-carboxylic acid; C4H,0 = dioxane R = H (x = 1);R = M e (x = 0 or 2)
R = H, Mv'= U, M' = Na or TI (+H,O), Cs R = Me, Mvl = U, M' = H, Li (+2 or 5HZO),Na, K. Rb, CS, TI, Ag, NH4, NEt4 Mw = Np, Pu, M'= Na, Cs R=CH,CI, Mvl= U , M I = Na (+1 or 2H,O), Rb, Cs R=CHCI,, MV'=U, M'=Na R = CCI,, M" = U, M' = Li, Na, K, Rb, Cs, TI, NH, R = PhCH,, M" = U, MI = Na R = Et, Mv' = U, MI = Li (also +ZH,O), Na (also +H,O), K, Rb, Cs, TI, NH ( a h +2H,O) R = Pr", Mv' = U, M'= Na, K, Rb, Cs, TI, NH, R = PhCH=CH, Mv' = 11, M' = NH4 R = Ph, Mv' = U, M' = H, Na (also +2H,O), NH,, AsPh, R = Me, M''
= Be, x = 0 or 2; Mg, x = 0, 6, 7,8 or 12; Ca, x = 0 or 6; Sr, x = 0,2 or 6; Ba, x = 0 , 2 , 3 , 6 or 10; Zn, x = O , 7 or 8; Mn, r=O or 9; Ni, x = O or 7 R = CCI,, MI' = Be, Mg, Ni, x = 0 R = Et, MI' = Ca, x = 6; Sr, x = 4; Ba, x = 3; Zn, x = O or 6; Mn, Co,x=O or 7; Ni, x = O or 8 R = Pr", MI' = Mg, Mn, Ni, Co, x = 0; Mg, Mn, x = 6; Ni, Co, x = 7 MI' = Sr, Ba M' = Li, MI' = Mg, Zn, Cd, Hg, Mn, Ni, Co, Fe, Cu MI = Na, M" = Be, Mg,a Ca," Zn," Cd," Hg," Mn," Ni," CO? Fe, Cu" M' = K, MI' = Be. Zn M' = Rb. NH,, MI' = Be Mr=Na. M"=Zn. x = 8 : Mn. x = U nr 6; Ni. Co, x = 7 M' = NH,, M" = M g , Zn[ x = 8; Ni, x = 7; Ca, Mn, Co, x =6; Sr, x =4; Ba, K = 3 R = H , M'=NH, R = Me, M' = NMe,, NEt, R = H, MI' = Sr, x = 1.38 R = Me, MI' = Cd, Mn, x = 6
R =Me, M"' = La, x = 2 or 5 ; Nd, x = 3 R = Et, M"' = La, Ce, Nd, x = 2; La, Nd, Eu, x = 3 Hydrated.
pyridine-2-carboxylates, MV'Oz(NCsH4C02-2)2(MV1= U, Np, Pu), precipitate from aqueous solutions of the actinide(V1) in the presence of the acid, but at lower pH and higher NC5H4COzH-2concentrations the acid salts HMV'0,(NC5H&02-2), are obtained. In the case of the N-oxopyridine-2-carboxylates,MV'0z(ONC5H4C02-2)z, the primary products are hydrated, but are easily dehydrated at 120 "C. Similarly, the hydrated pyridine-3-carboxylates lose water at 160°C (U) or 130°C (Pu). In addition to the compounds listed in Table 70, 6-methylpyridine-2-carboxylatesof composition UOzL, HU02L3 and U02L(OH).3Hz0 (dehydrated at 95 "C) are known (L = 6-MeC5H3COZ-2). The pyridine-2,6-dicarboxylategroups in the polymeric compound [U02(C7H3N04)(H20)],, are terdentate to each uranium atom and each of the U02(C7H3N04)units is linked to an adjacent unit through one of the carboxylate group oxygen atoms; the slightly distorted pentagonal bipyramidal coordination of the uranium atom is completed by the oxygen atom of the coordinated water molecule.28z The coordination geometry of the anion in ( A S P ~ ~ ) ~ [ U O ~ ( C ~ H ~isN an O ~irregular ) ~ ] - ~hexagonal H ~ O bipyramid with four oxygen and
The Actinides
1200
Table 70 Dioxoactinide(V1) Compounds with Chelating Carboxylates Furan-2- and thiophene-2-carboxylates U 0 2 ( R C 0 2 ) 2 . ~ Hz0 (Mw02)2(RC02)3(0H)'xH20 M'[MwOz(RC02)3]~~Hz0
R = OC4H3, SC4H3,x = 0 or 1 Mv' = U, R = OC4H3, SC4H3, x = 0 Mv' = U, Np, Pu, R = OC4H,, SC4H,, x = 4 Mvl = U, Np, Pu, R = OC4H3,SC4H,, M' = NH, !definite) M - Np, Pu, R = OC,H,, SC,H3, M' = Na (x indefinite)
5:
Pyridine-2-carboxylates MV'$2(NCsH,-2-C02), HM 02(NC,H4-2-COZ), MTUOZ(NCSH,-2-C02)3
Mv'= U (also +H,O),Np, Pu M"' = U, Np, Pu MI = Na, AsPh4(+3H20)
Pyridine-3-carboxylates MV10Z(NCsH4-3-C0J2~~Hz0
M~~ = U, NP, PU, x = 0,2
N-Oxopyridine-2-carboxylates
MV'02(ONCsH4-2-C02)2-~H20
M~ = U, NP, PU, x
=o, 2
Pyridine-2,6-dicarboxylates(acid = C,H,N04) [UO2(GH3NO.d], [UO~(~H~NOL~)(H~O)~~ UOz(C,H3N04).2HzO
(AsPh,)2[U02(C,H,NO,)zl.xH,O
x = O,2,
5 or 6
N-Oxopyridine-2,6-dicurboxylates"
IUOZ{ONC,H,(COZ)Z}(H,OM,.~H~O [UO2{ONC,H3(COz)z}In
[UO2{ONC,H3(CO2)2}(H2O)l,.H2O Pyridine-2,3-dicarboxyIates(quinolinates) U02(GH3N04)' X H20
x=O, 1 o r 2
Quinoline carboxylates (acid = C,,H,N02) UOz(CioH&'J02)2~xHzO
x=Oor2
Oxydiacetates (acid = O(CH2COzH),)
x=2or3
"S. Degetto, L. Baracco, M. Faggin and E. Celon, .I. Inorg. Nucl. Chem., 1981,43, 2413.
two nitrogen atoms of the two terdentate pyridine-2,6-dicarboxylategroups in the equatorial plane.283 In the structure of the dimeric N-oxopyridine-2,6-dicarboxylate,[U02(ONC5H3(COZ)2)(H20)2]2.2H20,the two units of the dimer are bridged by carboxylate group oxygen atoms and the coordination geometry is pentagonal bipyramidal, with one oxygen atom each from a monodentate carboxylate group, a bridging group and the N-oxide, and two from the water molecules, all in the equatorial planeaZs4 Both carboxylate groups in the iminodiacetate, U02{HN(CH2C02)2H}2,are ionized, with the extra proton linked to the imino nitrogen atom. The iminodiacetate groups bridge UO;+ ions in infinite chains, with one COz group of each ligand monodentate and one bidentate to two adjacent uranium atoms, making up an irregular hexagonal bipyramid about each uranium atom.285In the polymeric iminodiacetate, [UOz{HN(CH2COz)2}],,each UO;' ion is bonded to two carboxylate oxygen atoms, the imino nitrogen atom of one HN(CH2C02)zgroup and oxygen atoms from two other dicarboxylate ligands to form an irregular pentagonal bipyramid.286The polymeric oxydiacetate, [UOz{O(CH2C02)2}],, has the same coordination geometry, with the ether oxygen atom in place of the imino nitrogen atom.287In the anion of the oxydiacetato complex, Na2[UOz{O(CH2C0z)z}2]~2H20, each O(CH2C02 group is terdentate, making up an irregular hexagonal bipyramid about the uranium atom.
b*
The Actinides
1201
(c) Carbonates. The main types of carbonato complex are listed in Table 71; in addition, peroxocarbonato and fluorocarbonato complexes, such as &&[U02(02)(C03)2] and K3[U02F3(C03)], are also known. The coordination geometry about the uranium atom in U02C03 is approximately hexagonal bipyramidal, with two bidentate and two monodentate carbonate groups in the equatorial plane.289 The tricarbonato complex anion in (NH4)4[U02(C03)3]is a distorted hexagonal bipyramid with three bidentate carbonate groups in the equatorial planez9* and the same geometry has been reported for the anion in I(4[uo2(co3)31.291 Table 7l Dioxoactinide(V1) Carbonates and Carbonato Complexes MV'OzC03~~Hz0 (UOZ),(CO~),(OH)~.~H,O M:[UOz(COdzI M1'[UOz(COJ~].xH~O M:[MV102(Co3)31 M~'[U02(C03)3].~H,0 MmfV'Oz),(C03)sI
Mv' = U,Pu,x = 0; Mw = W, x = 1 , 2 . 2 or 2.5 The mineral sharpite M' = Li (hydrated), Na, K, Rb, Cs, TI (hydrated), NH,, CN3H6, (CH*)a,HZ MI' = Ca, x = 10; Ba, x = 4 Mv' = U , M' = Li (hydrated), K, Rb, Cs, TI, NH, Mv' = Np, M' = K, NHs,b Ag Mv' = Pu, M' = Li,' Na,' K,' NH, M" = Mg, x = 16, 18 or 20; Ca, x = 0,4, 8, 9 or 10; Sr, x = 9; Ba, x = 5 or 6 ; Pb, Mn,x = 0; Ni, x = 6; Co, x = 2; Cu,x = 1 Mv'= U, MI= Li (hydrated), Na, NH, (hydrated) M"' = Pu, M' = Rb," CS,"NH, (+2H20)
Ba,[(Puo2)Z(Co,),(H,o),l"
NH4MV1O2( CO,)( OH).3Hz0
Mv' = U,
Pu
Ba[PuOz(C03)(OH)(H20)3],~8H20a Ba3[(Pu02)z(C03),(0H)(H*0),I,'6H,0a The assignment of water molecules within the coordination sphere requires confirmation. Ya. Kharitonov and A. I. Moskvin, Sou. Radiochem. (Engl. Transl.), 1973, 15,240. 'J . D. Navratil, US report RFP-1760,1971 (Chem. Abstr., 1972,77, 66876). a
( d ) Oxalates. Only one water molecule is bonded to the metal in u O 2 ~ 0 4 - 3 H 2 0 the ; pentagonal bipyramidal coordination geometry is made up of four oxygen atoms from two independent G O 4 groups and one from the water molecule in the equatorial plane. Each C204 group is tetradentate, bridging two UOg+ ions.292 In addition to the salts of the type M1[UOz(Cz04)2]listed in Table 72, a wide range of hydrates are also known; some have been reported in the form M#.JOz(c204)2(H20),]~yHz0 without ade uate justification. The coordination geometry in the polymeric anion [U02(C204)2;J,"- of the ammonium salt is approximately pentagonal bipyramidal, with one bidentate G O 4 group and one GO4 group which is bidentate to one uranium atom and monodentate to a second uranium atom.293 The oxalate groups in the anion of (NH4)4[U02(C204)3] are all bidentate, giving rise to distorted hexagonal bipyramidal coordination geometry,B4 whereas in the polymeric anion of (NH4)Z[(U02)2(G04)3] the coordination geometry is pentagonar bipyramidal, with one quadridentate CZO4 group coordinated to two uranium atoms and the other bidentate to one and unidentate to a second uranium atom, forming infinite double chains,295 [ ( ~ 0 4 ) U 0 2 ( ~ 0 4 ) U 0 2 ( C 2 0 4 ) ] $In '-. &[(U02)2(~04)5]~10H20, the coordination geometry is again pentagonal bipyramidal, with two oxygen atoms each from two bidentate GO4groups and one from the bridging GO4group in the equatorial plane.296 A number of basic oxalates, such as (U02)2C204(OH)2-2H20,and salts of basic oxalato complex ions of the types M;U02(G04)2(OH) and M~(U02)2(C204)4(OH) have also been reported, as well as salts of a wide range of peroxo-, halogeno-, sulfito-, sulfato-, selenito-, selenato-, thiocyanato- and carbonato-oxalato complex anions: all need further investigation. (e) Other dicarboxylates. A number of peroxomalonato compounds, such as I(4[(U02)2{CH2(C02)2}3(02)(Hz0)4] , have been recorded in addition to the compounds listed in Table 73. The anion in (NH4)2[U02{CH2(C02)2}2]*HZ0 is polymeric, with one malonate group bidentate to one uranium atom and the second terdentate, with two oxygen atoms bonded to one uranium atom and one to an adjacent uranium atom, producing infinite chains in which the coordination geometry about the uranium atom is approximately pentagonal bipyramidaLZw The coordination geometry in the polymeric anions of the salts
1202
The Actinides Table 72 Dioxoactinide(V1) Oxalates ~ M"' = U , N P , Pub x=Oor 1 M' = Li, Na, K, Rb, CS,NH,, Tl, CN3H6 MI' = Sr, Ba MI= H(hydrated), NH,, TI,NBuY M' = Li, Na, K, Rb, Cs, NH,, Tl MI= Na, K, Rb, Cs, NH4, CN,H, ~~~~
Hydrates are also known. h M . P. Mefod'eva, M. S. Grigor'ev, T.V. Afonos'eva and E. B. Kryukov, Sou. Radiochem. (E@. Transl.), 1981, 23, 565. The neptunium(V1) and plutonium(V1) compounds are isostructural with their uranium(V1) analogues.
M"[U02{CH2(C02)2}2]-3H20 (MI' = Sr, Ba) is similar, except that the terdentate malonate group in these salts has a boat conformation whereas a chair conformation is adopted in the ammonium salt.298The coordination geometry in the succinate, [U02{CzH4(C02)2}(H20)Jn is an irregular pentagonal bipyramid, with four carboxylate oxygen atoms from four different succinate groups and one from the water molecule in the equatorial Table 73 Dioxouranium(V1) Dicarhoxylates Malonates
UO,{CH,(CO,)z~ M~[UO,{CH,(CO,),},], MI= Li, Na, K, Rb, Cs, NH,
M11[UO~{CH,(C0,)z}2]~3H,0, M" = SI,Ba Succinates Fumarate and maleate
UO,{~H,(CO,),}~xH,O,x = 1, 2 or 3 C~[UOZ{CZH~(COZ)Z}ZI UOz{cis- or trans-CH=CH( CO,),}
Glutarate
~o,{(cH,),(coZ),}
(NH4),[U0,{cis-CH=CH(C0,)2},] ~'[(UOz),~(CH~)3(COz)z~~l, M' = Li, Na, K Benzenedicarboxylatet;
~ o A ( c H z ) & o z ) d.LiH{(CHz)~(C02)2}.4HzO UO,{C,H,(CO,),} 1,2-, 1,3- and 1,4-dicarboxylates; hydrates are also known
In the glutarate of composition U02(C5H604).Li(C5H704).4H20 , one glutarate group bridges the UOg+ ions in infinite chains and the other bridges UOz+ and Li+ ions, the coordination geometry about the uranium atom being hexagonal b i p ~ r a m i d a l ;the ~ ~ same coordination geometry has been reported for the hydrated fumarate, U02(C4H204)(H20)2, in which fumarate rou s bridge the UOz+ ion in chains, and the water molecules are trans in the hexagonal ring.3 (f) Carbamates. The only recorded compound of this type appears to be U02(Et2NC02)2, which is precipitated when the uranium(1V) carbamate, U(Et2NC02)4,is oxidized in heptane. ( g ) Hexacarboxylates. A mellitate (benzene-t,2,3,4,5,6-hexacarboxylate), (UO&( C6(C02)6}*12H20, has been recorded. ( h ) Nitroalkanes and nitroarenes. The nitromethane solvate, U O Z ( C ~ O ~ ) ~ . ~ Mis ~obNO~, tained by treating the anhydrous perchlorate with nitromethane, either alone or in solution in carbon tetrachloride, followed by evaporation of the resulting solution in the latter case. The C104 groups may be covalent and bidentate in this product. Complexes of the corresponding nitrate, U02(N03)2.2RN02(R = Me, Ph) crystallize from saturated solutions of U02(N03)2 in RNOz. The complex U02(N03)2-MeN02has also been reported, and in this the MeNOz molecule may be bidentate.
g p
(vii) Aliphatic and aromatic hydroxyacids ( a ) Aliphatic hydroxyucids. A considerable number of dioxouranium(V1) compounds with aliphatic hydroxyacids are known (Table 74); alkali metal and barium salts of tartrato complexes, such as Na2[U02(C4H406)2]-2H20, have also been recorded. The glycolate,
The Actinides
1203
U02(HOCH2C02)2,is obtained on cooling a warm aqueous solution of the acid which has been saturated with U 0 3 . The polymeric structure consists of infinite chains of pentagonal bipyramidal UOz(0)s units with two sets of glycolate groups, one tridentate, which is bonded to one uranium atom and bridges two other uranium atoms, and the other bidentate, which bridges two uranium atoms.302Salts of the type M'[U02(HOCH2C02)3] are not known and very little information is available for the other compounds listed in Table 74. Table 74 Some Dioxoactinide(V1) Compounds with Aliphatic Hydroxyacids
R = HOCH,, PhCHOH (also +2H,O), Ph2C(OH)(+2H,O), MeCWOH (also +2 or 5HzO), MeC(0H)Ph (+2H20), HOCHzCHPh, 2-HOC,H4CHzCHz 1-Hydroxycaprylate M' = Na, K, NH,, pyH M' = Na, Cs Mesoxalate Malate; x = 0, 1 or 3 Malate Malate; x = 0 or 2 Tartrate; x = 1 or 4 Tartrate Dihydroxymaleate Trihydroxyglutarate;x = 1 or 1.5 C6H807= citric acid, HOC(CH,COzH)2COzH Citrate Citrate
(b) Aromatic hydronyacids. In addition to the hydroxybenzoates listed in Table 75, a number of compounds derived from substituted hydroxybenzoic acids have been recorded; a 2,3,4trihydroxybenzoate (pyrogallate) and salts of complex anions derived from 3,4,5-trihydroxybenzoic acid (gallic acid) are also known. Table 75
,
Some Dioxouranium(V1) Compounds with Aromatic Hydroxyacids
The salicylato nitrato complex, [{UO2(2-HO~H4CO2)(N0~)) { (4-Me2N)C5H4N}2]2,is obtained when the aminopyridine is added to a mixture of U02(N03)2.6Hz0 and 2HOC6H4C02Hin methanol. In this compound the pyridine molecules are not coordinated to the uranium atoms, but are hydrogen bonded to the phenolic oxygen atom of the salicylate group. The hexagonal bipyramidal arrangement of oxygen atoms about each uranium atom in the dimer is made up of one carboxylate oxygen from each of the two salicylate groups which bridge the two uranium atoms, and the second carboxylate oxygen atom of one salicylate group and the phenolic oxygen atom of the other, together with two oxygen atoms from the bidentate nitrate In the structure of the p-aminosalicylato complex anion in Na[U02(2-HO-4H2NC6H3CO2),].6H20, the coordination geometry is again hexagonal bipyramidal, with the six carboxylate oxygen atoms from three nearly planar aminosalicylate groups in the equatorial plane.304 COC3-MM
The Actinides
1204
(viii) Amides, carboxylic acid hydrazides, ureas, N-oxides, P-oxides, As-oxides and S-oxides
( a ) Amides and related compounds. The normal preparative route for complexes with these ligands (Table 76) is from the parent compound, usually hydrated, and an excess of the ligand in a non-aqueous solvent; in most cases the complex crystallizes on addition of a small amount of ether or a hydrocarbon to the solution. In some instances the complexes have been obtained from aqueous solution. The complexes with the peroxide chloride, formulated as C l ~ U 0 2 ( p z 02)U02L2C1 (L=DMF or DMA), in which the bridging 0;- group appears to act as a tetradentate ligand, are obtained by treating the complexes UOzCl& with H202in aqueous ethanol. 30s Table 76 Amide Complexes of Dioxouranium(V1) Compounds [UO,F,(L)In UO,Cl,.xL
UO,Br,-xL
L = HCONMe,(DMF), MeCONMq(DMA), MeCONH, = 1, L = Me,NCO(R)CONMe, with R = CMe, or CH,C(Me),CH, x = 1.5, L = Me,NCO(CH,),CONMe, with n = 1 or 3 x = 2, L = MeCONH,(+H,O); MeCONHR with R = Pr', p-H,NC,H, or p-EtOC,H,; HCONMe,," MeCONR, with R = Me, Pr", Pr' or n-C,H,,; RCONMe, with R = Pr', Me,CHCH, or Me$ x = 3, L = MeCONHCp-EtOCA), MeCONHEt, HCONMe, x = 2 , L=DMA x=3, L=DMF x = 4, L = MeCONH(p-EtOC&)
X
L = DMF," DMA" x = 1, L = R:NCOCH,CONR$ with R' = Bu" or Bu' and
R2= Me(CH,Ph) x = 2, L = DMF, HCON(Me)(CH,Ph), MeCONH,,
MeCONHCp-EtOC,H,), MeCONR, (with R = Et, Pr', n-C,H,,, n-C,,H,,, n-C,,Hz or Ph), MeCONEt(MeC,H,), RCONMe, (with R = Pr', Bun, Me,CHCH, or Me&), PPCONBu;, Me3CCONBu; x = 3, L = MeCONHPh L = DMF, DMA or MeCONEhd L = MeCONH, L = DMF, x = 1 or 2; L = DMA,b MeCONPr;, x = 1 L = MeCONPri x = 1, L = MeCONH, x = 2, L = MeCONH(p-EtOC,H,) x = 3, L = DMF L = HCONH,, x = 1, y = 2; L = MeCONH,, x = 1, y = 1.5 or x = 1.5, y = 0.5; L = DMF, x = 1, y unspecified L=MeCONH,, x = 1, y = 2 ; x = 2, y = O or x =3, y = 1; L=DMF,x=2, y = O L = MeCONH,, DMF L = H,NCOCH,CONH,C L = MeCONH; L = MeCONH,," D M F x = 1, L = DMF; x = 2, L = HCON(CH,Ph)(Me) L = CF,COCHC0(2-C,H,S) L = OC,H,CH=NCH,CH,N=CHC,H,O R. N. Shchelokov, I. M. Orlova and A. V. Sergeev, Dokl. Chem. (Engl. Truml.), 1980,250/5, 573. Mistryukov, Yu. N. Mikhailov, I. A. Yuranov, V. V. Kolesnik and K. M. Dunaeva, Sou. J . Coord. Chem. (EngL Truml.) 1983, 9, 163. 'K. A. Avduevskaya, N. B. Ragulina, 1. A. Rozanov, Yu. N. Mikhailov, A. S. Kanishcheva and T. G. Grevtseva, R u s . J . Inorg Chem. (Engl. Trawl.), 1981, 26, 546. d A . M. Hounslow, S. F. Lincoln, P. A. Marshall and E. H. Williams, Ausr. J . Chem., 1981, 34, 2543.
a
bV. E.
In the polymeric acetamide complex of the chromate, [U0zCr04(MeCONH2)2]n, the structure consists of chains of (UOz)Os pentagonal bipyramidal coordination polyhedra in which the equatorial plane is made up from three oxygen atoms from [CrO4I2- tetrahedra and two from the acetamide molecules.306Similarly, in the dimeric DMA complex with the acetate, [U02(MeC02)2(DMA)]2, the structure consists of two pentagonal bipyramidal polyhedra
The Actinides
1205
bridged by two bidentate acetate groups, the other two acetate groups each chelating a uranium atom.307 Several complexes with ligands related to amides, such as lactams, lactones and antipyrine (atp) have also been recorded (Table 77); apart from preparative and IR spectroscopic information, little is known about them. The precipitation of U02(NCS)2.3atp from aqueous acid in the presence of thiocyanate has been used as a method for the determination of uranium in minerals. Table 77 Some Complexes of Lactams, Lactones and Antipyrine (atp) with Dioxouranium(V1) Ccmpounds Lactam and lactones UOZ(Et2NCS2),,L L = MeN CH ) CO
uo,x,
fT
yL
'
L = O(CH&CO X = F, C1, Br, y unspecified; X = NO,, y = 2; X = OSSO,, y = 1 ( +2HzO)
Anfipyridine (2,3-dimethyl-1-phenylpyrazol-5-one) UO,F,~atp~ZH,O Probably polymeric UO,X,.3atp X = NCS, NCSe UOz(C104)2.5atp U0,S03-xatpa*b x = 1.5 (also +SH,O) or 2 (+H,O) UO2SO,.3atp.xH,O x = 0 or 3 U02C,0,.3atp Yu. Tsivadze, A. N. Smirnov, G. T. Bolotova, N. A. Goluhkova and R. N. Shchelokov, Russ. J . Inorg. Chern. (Engl. T ~ m l . )1979, , 24, 904. bR. N. Shchelokov, G. T. Bolotova and N. A. Golubkova, Sou. J . Coord. Chern. (Engl. T r m l . ) , 1977, 3, 810. *A.
( b ) Carboxylic acid hydrazides. The IR spectra of many of the known complexes with this type of ligand (Table 78) suggest that the hydrazides are bidentate, with bonding to the metal atom via a carbonyl oxygen and a hydrazine nitrogen atom in the case of the ligands RCONHNHp. Table 78 Some Complexes of Carboxylic Acid Hydrazides with Dioxouranium(V1) Compounds U02X2.2RCONHNH2 UOzX,.3RCONHNH, UO,Xz.3RCONHNHz U0,(N03),~2RCONHNHCOMe
X = C1, NO,; R = p-MeOC,H,, p-O,NC,H, (all hydrated)
0-,m-
and
X = NO,, R = Ph (also +2H20), R = 2-HOC,H4 (+2H,O) X=CI,R=Ph;X=NO,, R=Ph X = C1, NO,; R = Me, o-, m- and p-HOC,H,, 0-,m- and p-MeOC,H., (all hydrated) R = Me (+2Hz0), Ph
( c ) Ureas. A very large number of urea complexes with dioxouranium(V1) compounds are known, a selection of which is given in Table 79. In all of them the urea, or substituted urea, molecules are bonded to the metal atom through the carbonyl oxygen atom. The complex UO2Fz.3urea is obtained by treating U02F2.2H20 with urea in aqueous solution, and the dimeric complex, [U02F2(urea)2]2, in which two pentagonal bipyramidal [UO,F,(urea),] groups share a common F(1)-F(1') edge, is obtained in the same way by slow crystallization from dilute aqueous solutions.308 The coordination geometry in the complex [U02(urea)4(H20)](N03)2is again pentagonal bipyramidal, with four urea and one water molecule oxygen atoms in the equatorial plane309and the structure of the dimeric basic iodide, [U0z(0H)(urea)3]214,is made u of two pentagonal bipyramidal U 0 7 units bridged along one edge by two hydroxyl groups?''The same coordination geometry has been reported for the complex with the ph~sphite,~"[U0~(HP03)(NH2CONMe2)(HzO)] and for the cation in the complex [U02(MeC02)(urea)3]+ [U02(MeC02),]- ( = W02(MeC02)2.1.5urea), in which two oxygen atoms from the chelating acetate group and three from the urea molecules lie in the equatorial plane.307 The structures of three urea complexes of dioxouranium(V1) sulfate have also been reported. In the bis complex, UO2SO4.2urea, the structure consists of infinite ribbons of OUO groups
1206
The Actinides Table 79 Some Urea Complexes of Dioxouraniurn(V1) Compounds L = urea, OC(NH,j, UO,Xz~yL~~H,O
[UO,XL3] [u ozx31C [UO,(OH)L,I,I, u 0,X.y L L = MeHNCONHMe UO,X,.yL
U0,X.yL L = Me,NCONH, UO,(HPO, j-l,.l-lzo L = Mc,NCONMe, U02X,.yL U0,X.yL (uo,),(o,)cI,4L" L = H,NCONHE t u o,xz.y L U0,X.yL L = EtNHCONHEt UO,X,.yL
U0,X.yL
X = F , y = 2 , z = O (dimer) and z = 1; y =3, z = O X=C1, y = 2 or 3, z = 1; y = 4, z = O X=NCS,y=2, z = O o r 1 ; y = 3 , z = O X=NO,, y = z = 1; y = 2 or 3, z = O ; y = 4 , z = 1 or 2; ):=5,z=Oorl;y=6,z=O X= C 1O 4,y=5, z = o X = MeCO,, y = I, 1.5, 2 , 4 or 6, z = O X = EtCO, x=so,, y=2 X = SO,, y = 2 , 3 or 4 X=C204,y=10r3
X = C1, y = 2 , 4 or 5 X=NCS, y = 2 X=NO,, y =4,5 or 6 X = CIO,, y = 5 or 6 x = so,, y = 3 X=C,O,,y=l
X = C1, NCS, NO,, MeCO,," y = 2 X=C104,y=5 X = SO,, CrO,, y = 2 X=G04,y = 1 X=Cl,y=3 X = N 0 3 , y = 2 , 3 or 5 x = so4,y = 3 X=C,04, y = 1 or 3
x = c1, y = 3 x = NO,,
y =2
X = SO4, y = 2 or 3 X = C,O,, y = 1 or 3
'V. 1. Spitsyn. K. M . Dunaeva. V. V. Kolesnik and I . A. Yuranov. Rws. J. Inorg. Chem. (Engl. Trartsl.), 1982, 27, 473. bR.N. Shcheiokov, I. M. Orlova and A. V. Sergeev. Sou. J . Coord. Chem. (EngL Trans[.), 1981. 7 , 441. ' V . I. Spitsyn, V . V . Kolesnik, V . E. Mistryukov, Yu. N. Mikhailov and K. M. Dunaeva, Dokl. Chem., 1983, 2681273,362,
coordinated to two urea oxygen atoms and to three oxygen atoms from tridentate sulfate groups, making an approximately pentagonal bipyramidal arrangement about each uranium atom,312and in U02S04.3ureathe polymeric structure is similar, with three urea oxygen atoms and one oxygen each from two bridging bidentate sulfate The same coordination geometry has been found for [U02S04(urea)4]in which the four oxygen atoms from the urea molecules and one from a monodentate sulfate group lie in the equatorial plane.314 ( d ) N-Oxides. A selection of the known Me3N0 and pyNO complexes with dioxouranium(V1) compounds is included in Table 80; in addition, a large number of pyridine N-oxide complexes with 0-diketonates, UO&.pyNO, and many complexes with substituted pyridine N-oxides, quinoline and isoquinoline N-oxides, 2,2'-bipyridyl N-oxide and 2,Z'bipyridyl N , "-dioxide have also been recorded. Several complexes with p-nitroso N , N dimethyl and -diethyladine are also known; they are precipitated when an ethanol solution of U02X2(X = C1, Br, NO3 or MeC02) or UOzX (X = SO4 or Cz04)is treated with the ligand in the same solvent; their IR spectra indicate that the ligand is bonded to the uranium atom via the nitroso oxygen atom only. The uranium atom in [U02(Et2NCS2)2(Me3NO) J is seven-coordinate, with four sulfur and one oxygen atom from the amine oxide in the equatorial plane of a pentagonal bipyramid3I5
The Actinides
1207
Table 80 Some Complexes of N-Oxides with Dioxouranium(V1) Compounds
UO,X,.yL
L = Me,NO; X = NO,, y = 1 or 4
x = CIO,,
y =4 X = Et,NCS, or Et,NCSe,, y = 1 L - C,H,NO; X=C1, y = 2 or 3 X = NCS, y = 3 X = NO,, y = 2 or 3 X=CI04,y=5 X = acac, trop or 2-OC6H,CH0, y = 1 [UOZ(Me,NO)4I[J3P~,l, WO,X,yL
L = C,H,NO; X = SO4,y = 2
X = NC,H,-2,6-(CO&, y = 2 X = O(CH,CO,),, y = I (polymer) or 2 L = 2-MeC.jH4NO; X ONC,H,-2,&(CO&, y = 2" * S. Degetto, L. Baram, M. Faggin and E. W o n , J. Inorg. Nucl. Chem., 1981,43,2413.
and the coordination geometry in [UO{O(CH2C02)z}(pyNO)z] is probably the same, with three oxygen atoms from the O(CH2C02)2 group, including the ether oxygen atom, and the two N-oxide oxygen atoms bonded to the U 0 2 group.28* (e) P-Oxides. In addition to the complexes listed in Table 81, several 1:l complexes of UOz(NO& with bis(dialkylphosphinyl)alkanes, RzP(0)(CHZ),P(O)RZ, and many complexes of phosphine oxides, phosphate and phosphonate esters, (RO)3P0 and (RO),R'PO, with dioxouranium(VI) p-diketonates and Schiff base compounds have been recorded. In the complexes [MWO2Cl2L],trans octahedral geometry has been reported for Mv' = U with L=(MezN)3P0316or Ph3P0317 and for MV'=Np with Ph3P0;318 in the complexes [MV'02(N03)2(Ph3PO)2](Mv' = U or Np), the coordination geometry is hexagonal bipyramidal, with the bidentate nitrate groups trans,318 as found also for the structure of [U02(N03)2{(EtO)3PO)z].319 The cation [U02{Me2N)3P0}4]2+in the perchlorate3" and in the polyiodide, [2u0&4](13)2, obtained by atmospheric oxidation of a mixture of U14 and (Me2N)3P0,3 exhibits trans octahedral geometry. In the dimeric compound [UOz(MeC02)2(Ph,PO)]2, which crystallizes when a cold acetone solution of UOZ(MeCOz)z.2Hz0containing the stoichiometric quantity of Ph3P0 is stirred for 20-30 minutes, the coordination geometry about the uranium atom is pentagonal bipyramidal with two bridging and two chelating MeC02 The same coordination geometry has been reported for monomeric [U02(MeCSS)2(Ph3PO)],which is precipitated from methanol solutions of the dithioacetate and Ph3P0,323a for the dithiophosphinate complex, [U02(SzPR2)2(Me3P0)]3Wb, for the complex [U02{CF3COCHC0(2-C4H3S)2){ ( P Z - C ~ H ~ ~ ) ~ PO}]324 and for the a and P forms of the complex [U02(CF3COCHCOCF3)2{(MeO),PO}], which differ in that the P-diketonate groups in the a form are tilted by 22.5" to the plane of the pentagon in a boat conformation325whereas the pentagon is nearly planar in the B form.326 cf) As-Oxides. Complexes with these ligands (Table 82) are usually obtained by treating the parent compound with the ligand in a non-aqueous solvent. The complexes [U02(MeCOz)z(Ph3AsO)]zand [U02(MeC02)2(Ph3AsO)2]are isomorphous with their Ph3P0 analogues. The coordination geometry in the dithio- and diseleno-carbamate compounds, [U02(EtzNCX2)(Ph3AsO)]is pentagonal bipyramidal, with four ~ u l f u f "or ~ four selenium328 and one oxygen atom in the equatorial plane. As expected, the coordination geometry in [U02(N03)2(Ph3As0)2]is hexagonal bipyramidal, as in the phosphine oxide analogue; the U-O(As) bond length is shorter than the U-O(P) bond length in the latter.329 (g) S-Oxides. In addition to the compounds listed in Table 83, a large number of R,SO (R = Me, Bun, i-C5HI1, n-C6HI3, PhCH2 and Ph) complexes with dioxouraniurn(V1) fidiketonates and Me2S0 complexes with analogous Schiff base compounds are known and a few complexes of dioxouranium(V1) compounds with thioxane S-oxide, thianthrene 5-oxide7 substituted sulfoxidothiophenes (RC4H3SO) and thiourea dioxide ((H2N)2CS02) have also been reported. The structure of the polymeric complex [U02F2(Me2SO)], consists of zigzag chains of uranium atoms linked by di-p-fluor0 bridges; the coordination geometry about each uranium
The Actinides
1208
Table 81 Some Complexes of P-Oxides with Dioxoactinide(V1) Compounds; L = R,PO
u 0,xz.y L
NpOZX2'2L U0,X.yL U0,XyyL
PUO,X,L
R = Me; X=C1, Br, NO,, y = 2; X = ClO,, y = 4 R = Et; X = C1, Br, NO,, y = 2 R = Pr"; X = NO,, y = 2; X = ClO,, y = 4 R = Bun; X = F, y = 4/3; X = Cl, Br, NO,, CIO,, y = 2; X = I, NCS, y = 3; X = I, ClO,, y = 4; X = MeCO,, y = 1.5; X = Et,NCS,, (q-C,H,)Fe(q-C5H4C02), (CO,)Cr(q-PhCO,), y = 1 R = n-C,H,,: X = F,y = 1.5 or 3; X = CI, NO3, y = 2; X = MeCO,, CF,COCHCO-2-C4H,S, CF,COCHCOPh, y = 1; X CF,COCHCO-2-C,H,S, y = 3 R = Me,N; X = F, Ci, Br, NCS, N,, NO,, N(CN),,b y = 2; X = NCS, y = 3; X = Clod, y = 4 or 5 ; X=13,y=4 X = MeCO, (dimer), Et,NCS, Et,NCSe,, y = 1 R = Ph; X = Cl,Br, I, NCS, NO , MeCO,, y = 2; X = C104, y = 4; X = N(CN),,'y = 3; X = MeCO, (dimer), MeCOS, M e a , , PhCS,, Et,NCS,, EtzNCSe,, S,Ph, (with R = Me, OMe, OEt, OPr", OPr', OB?", Ph), trop, X = R,PS," (with R = OMe, OEt, OPr", O h ' , OBu", Me or Ph), y = 1 R = Ph; X = C1, NO, R=Bu"; X = SO4, y =2.5 or 3; X=C,04, y = 1 R = Me,N; X = SO3,y = 1;' X = SO,, y = 2; X = ONCSH3-2,6-(C02)z,'y = 1.5 R = OMe; X = CF,COCHCOCF,, y = 1; X = ClO,, y = 5 R - OEt; X=NO,, y =2; X = ClO,, y - 5 R = OBu"; X = Q, Br, NCS, NO3, C104, y = 2; X = NCS, y = 3; X = CF,COCHCOR' with R' =Me, CF,, Ph, 2-C4H3S, y = 1 R = OBu"; X = CF,COCHC0(2-C4H3S)d
aI. Haiduc and M. Curtui, Synth. React. Inorg. Mefol-Org. Chem., 1976, 6, 125. bE.L. Ivanova, V. V. Skopenko and Kh. Keller, Ukr. Khim. Zh. (Rurs. Ed.), 1981, 47, 1024 (Chem. Abstr., 1982, %, 45 232). S. Degetto, L. Baracco, M. Faggin and E. Celon, J . Inorg. Nucl. Chem., 1981, 43, 2413. J. P. Shukla, V. K. Manchanda and M. S. Subramanian, J. Radioanal. Chem., 1976, 29, 61. CR.N. Shchelokov, G. T. Bototova and N. A. Golubkova, Sou. J, Coord. Chem. (Engl. Tram[.), 1977,3, 810.
Table 82 Some Complexes of As-Oxides with Dioxouranium(V1) Compounds
R = Ph, X = MeCO, (dimer), MeCSS, PhCSS, Et,NCSS, Et,NCSeSe, trop (C,H,O;, tropolonate), FhCOCHCSPh UO,X,.2R,AsO R = n-C,H,,, X = CI R = Ph, X = Cl, NCS, NO,, MeCO, U O ~ X , . ~ R ~ A S O R = Ph, X = C104
UO,X,.R,AsO
atom is pentagonal bipyramidal, with four fluorine and one oxygen atom in the equatorial plane.330The same coordination geometry has been reported33' for [UOz(MezS0)5](C104)2and €or the complex with the ph~sphite,~" [UOz(HP03)(MezSO)(H20)].
(ix) Hydroxamates, cupferron and related ligands A selection of the known dioxouranium(V1) complexes with this type of ligand is given in Table 84. These compounds are precipitated from aqueous solution, usually after the addition of alkali. The coordination geometry of the anion in the ammonium salt of the cupferron complex, NH4[UOZ{(ON)N(Ph)0)3], is close to hexagonal bipyra~nidal.~~'
1209
The Actinides Table 83 Some Complexes of S-Oxides with Dioxouranium(VI) Compounds UO2X,.yR,SO
y = 1; R = Me, X = F (polymer), MeCO,, trop y = 2; R = Me, Et, Bun, n-CJIH,,, X = NO, R = n-C,H,,, X = CI R = Ph, X = C1, Br, NCS, NO,, MeCO, y = 3; R = Me, X = C1, Br, NCS, NCSe R = P h , X=C1, Br y = 4; R = Me, X = Br, NO,, CIO, R = PhCH,, X = ClO, R = Ph, X = I, C10, y=4.5;R=Me,X=Br y = 5 ; R = Me, X = NO,, CIO, R = Ph, X = C10, X = SO3(+0.5HzO), SO4,CrO,, ONCSH3-2,6-(C0J2 (polymer)" "
UOZX.2Me2S0 3UO2(&0,).5M&SO (UO,),(OZ)Cl2~4Me2SOb
"S. Degetto, L. Baracco, M. Faggin and E. Celon, J. Inorg. Nucl. Chem., 1981, 43, 2413. "R. N. Shchelokov, I. M. Orlova and A. V. Sergeev, Sou. J. Coord. Chem. (Engl. Trunsl.), 1981,7,441
Table 84 Some Dioxouranium(V1) Complexes with Hydroxamates, Cupferron and Related Ligands Hydroxamates; HL = (RCO)(R')NON R = Ph, R' = H (also +lH,O), Ph (also +2H,O), 2-MeC6H, UO2L ( + l HzO or HL) R = 2-HOC6H4,R' = H or Me (+2H20) R = 2-C4H,0, PhCH=CH (+ 1H20), 3- or 4-0,NC6H,, R' = Ph H,L = PhN(OH)C(O)(CH,),C(O)N(OH)Ph UO~L'XH~O n-3 x=3.n=4 x=2. UO,(HL),.xH,O n =4, x = 5; n = 5 , x = 2 (UOz)zL(HJ42 n=2 Cupferron { (ON)N(Ph)O-) M'[U02{ ONN(Ph)O) M' = Na, K, Rb, Cs or NH, N-Nitroso-N-cycloherylhydroxylamine, C6H,,N,0z NH4[UO2(C&i iN20d31
40.2.4.4 Sulfur ligands
( i ) Monothiocarbamates, dithiocarbamates and xanthates
( a ) Monothiocarbamates. The diethylammonium salt, (Et2NHz)[U0~(0Et)(Et2NCOS)~], precipitates when a saturated solution of U02C1,.3H20 in ethanol is added to the solution obtained by passing COS through ethanolic diethylamine at 0 "C. The coordination geometry in the anion is pentagonal bi yramidal, with two sulfur and three oxygen atoms in the rather irregular pentagonal plane!' The anion structure is the same in the di-n-propylammonium salt of the [U02(OEt)(Pr;NCOS)2]- ion, prepared in a similar In the structure of an analogous compound, (Pr~NH2)2[U02(Pr~NCOS)z(S2)], the uranium atom is at the centre of an irregular hexagonal bipyramid, with the equatorial plane positions occupied by the (S-S)'group bonded sideways on, together with two oxygen and two sulfur atoms from the monothiocarbamate groups.335 ( b ) Dithiocarbamates. The existence of the compounds reported as U02(R2NCS2)2(R = Et, Pr", But) and U02(RHNCS2)2(R = Et, Bu') is dubious and these are more likely to be salts of the anion [U0z(R~NCS2)3]-.A number of salts of composition M1[U02(Et2NCS2)3].xH20 [M1=Na (x = 2 , 3 or 6 ) , K (x = 1), Rb, Cs, NH4 and NMe4 (all x =O)] and (R'R''NH2)[UO2(R'R"NCSZ)3], with R' = Me and R" Me, Pri, CH2CHMe2, CH(CHM& CH2CH2CHMe2, CH(Me)CMe3 and CHMeCHzCMezOH, or R' = R ' = CH2CHMe2, Bun or CHzCH20H, have been reported. The coordination geometry in the anion of (NMe4)[U02(Et2NCS2)3]is approximately hexagonal bipyramidal, with six sulfur atoms in the non-planar hexagon.336
The Actinides
1210
(c) Xanthates. The salts K[U02(ROCS&] (R = Et, Pr') are precipitated when an excess of the potassium alkyl xanthate is added to an aqueous solution of U02S04.Early reports of the formation of xanthates of the type U02(ROCS2)2from aqueous solution appear to be doubtful. (ii) Dithiolates
The cis maleonitriledithiolate salts (NR4)2[U02(~i~-C4N2S2)2] (R = Et, Pr") and the corresponding isomaleonitrile compounds (NR&[U02{ (CN)2C=CS2)2] (R = Et , Pr") have been recorded.
(iii) Thioureas A number of complexes of dioxouranium(V1) compounds with thioureas have been reported (Table 85) but little is known about them. In addition to those listed in the table, a number of complexes with dioxouranium(V1) nitrate formulated as [U02Lx(N03)](N03), with x = 4 and L = o-chlorophenyl thiourea, x = 3 and L = 0- or p-hydroxyphenyl thiourea, and x = 2 and L = o-methoxy henylthiourea, as well as [UO,L(NO,),] with L = benzylthiourea, have also been recorded.Y37 Table 85 Some Complexes of Thioureas with Dioxouranium(V1) Compounds
L = SC(NH2)2 UO,X,.yL UO2S0,-yL L = SC(NHMe), U02(NCS),.3L L = SC(NH,)(NHPh) UO2(MeC0,),.2L L = SC(NMe,), UO,X,.yL U02(NCS)(N03).2L
X=C1, y = 2; X = NO,, y = 2 or 4;" X=MeCO,, y = 1, 2 or 4 y=lor2
X = Cl,b NO3: y = 2; X = NCS, y
=3
"A. S. R. Muny and M. B. Adi, Proc. Nucl. Chem. Rodiochem. Symp., 1980 (published 1981). p. 357, (Chem. Absfr., 1982, 96, 134 7%), A. 0. Baghlaf and K. W. Bagnall, Bull. Fuc. Sci. King Abdul Aziz Uniu., 1980, 4, 143.
(iu) Dithiophosphinates Dithiophosphinates, U02(R2PS2)2,have been obtained by ether5' (R = Me) or benzene5' (R = Ph) extraction of a mixture of concentrated aqueous solutions of U02(N03)2 and Na2(R2PS2),followed by concentration of the extract, or (R = p-tolyl, p-C1C6H4)by heating a methanol solution of the ligand under reflux with the calculated quantity of5' U02(MeC02)2. These last, and the phenyl analogue, are monomers in benzene." Alcohol adducts of the type U02(SzPR2)2.R10Hare obtained when UOzClz in R'OH (R'= Me, Et, Pr') is treated with a salt of the dithiophosphinic acid, R2PS2H ( R = M e , Et, Pr', OMe, OEt, OPr', Ph or ~ y c l o - C ~ HIn ~ ~the ) . presence of an excess of chloride ion, the adducted R'OH is displaced and salts of the type Et4N[U02(S2PR2)2C1]are formed. The coordination geometry about the uranium atom in U02(S2PR2)2-R'OH (R =P h , cyclo-C6HII; R1=Et) and in the anion [U02(S2PR2)2C1]- (R = Me, Ph) is pentagonal bipyramidal with four S atoms from two bidentate SzPRz ligands in the plane together with either one 0 atom from coordinated EtOH or one C1 atom, as in the structure of [U02S2PMe2(Me3PO)](p. 1207).323b
40.24.5 Halogens as ligands
(i) Hexahalides Octahedral hexafluorides, MF6 (M = U, Np and Pu), are usually prepared by fluorination of lower fluorides, MF4, at relatively high temperatures (e.g. M = Np, at ca. 500 "C). They are low
The Actinides
1211
melting, volatile solids. The only known hexachloride is UCk, prepared by chlorination of UC4, for example in SbC1,. This is also octahedral.338 (ii) Oxohalides
The oxotetrafluorides, Mw0F4, are prepared by hydrolysis of the hexafluoride in HF with the stoichiometric quantity of water or, more elegantly, by using the calculated quantity of silica wool to provide the water by reaction with the solvent (Mv1=U,339 Np340and PuM1). The coordination geometry about the metal atom in a-UOF4 is close to pentagonal bipyramidal, with an oxygen and a fluorine atom in the axial positions.339 NpOF4340and PUOF:~~are isostructural with B-uoF4. The coordination geometry in P-UOFd is also pentagonal bipyramidal, but with the two non-bridging axial fluorine atoms and a non-bridging equatorial oxygen atom together with four equatorial bridging fluorine atoms.342 Complexes of the composition UOF4-xSbF5(x = 1, 2 or 3) are also known. In these the structure consists of a fluorine-bridged network of UOF4 and SbF5 molecules in which each uranium atom is surrounded by a pentagonal bipyramidal array of one oxygen and six fluorine atoms.343The dioxodihalides, MV10zX2,are known for X = F (MW= U, Np, Pu), X = C1 IMv' = U, Pu (+6H20)] and X = Br (Mv' = U, hydrated).
(iii) Halo complexes Heptafluorouranates(VI), M'UF7 (M1=Na, K, Rb, Cs or NF,), have been prepared by reaction of M'F with UF6 in G F I 6 ;the NF; compound is obtained from UF, and NF4-HF2in HF.344Octafluorouranates(VI), Mi[UF8] are obtained by heating M1UF7 (MI = Na, K, Rb or Cs) at 100°C (M'=Na) to 210°C (MI= C S ) . The ~~~ U-F bond lengths in Na2[UFs] are all Oxidation of (N&)4UxVFs with XeF, at 55°C yields a product of composition (NH4)4UF10which appears346bto be an example of uranium(V1) with coordination number greater than eight. The compounds M:UF9 (M' = K or Rb) are prepared by fluorination of M:UF7 at 400 (K) to 500 "C (Rb).347 (iu) Oxohalogerzo complexes
A few salts of complex anions derived from monoxoactinide(V1) halides are known; (Ph4P)[UOClS] is obtained when a solution of (Ph4P)2[U02C14]in thionyl chloride is heated under reflux for a few minutes and then cooled. The U - C l bond length trans to the oxygen atom is shorter than the equatorial U-Cl bond lengths. The chloro complex reacts with HF at low temperature and with HBr in dichloromethane to form (Ph4P)[UOF5]and (PbP)[UOClBr4] respectively.348 The more common complexes are derived from dioxoactinide(V1) dihalides. The structure of the aquated anion in (enH2)[U0,F4(H20)] has been described earlier (p. 1192) and a number of hydrated salts of the type M'Mw02F3-xHZ0 (MVI= U, MI = an organic base and x = 0-6; Mv' = Pu, M' = Na, K, Rb, Cs, NH4, C,H7NH (quinoliniurn), x = l), MiPu02F4 (M' = Na, K, Rb, NH,), M"U02F4.4W20 (M" = Zn, Cd, Cu, Mn,Co,Ni), M1(MV102)2F5*~H20 (Mw = U, MI = an organic base, x = 0-6; Mv' = Pu, MI = Cs, x = 3) and M1(U0z)3F7.~H20M' = an organic base) have been reported. The more usual fluoro complex anion is [MVO2F5I3+ (M"' = U, Np, Pu), for which several alkali metal salts are known. The otassium salts for the three elements are isostructural; in K3[UOzF5]349aand K3[Np02F5f49b the coordination geometry of the anion is pentagonal bipyramidal. The IR spectra of K3[MV102F5](Mw = U, Np, Pu) have also been reported.249 The compounds CszNH4(U02)2F7,350Na3(U02)2F7*6H20351 and Ni3{ (U02)2F7}2-18H20352 each involve infinite chains, [(U02)2F7]~:"-, formed by UO2F5 pentagonal bipyramids joined by a common edge arid a common apex, with three bridging and two terminal fluorine atoms in the equatorial plane. Chloro complexes are normally of composition M:[Mvx02C14](Mw = U, Np, Pu, with M' = Cs, NEt4; M w = U, with M' = Li or Na (+6H20), K (+2H20), N&, 1,lO-phenH, PPh,, PBu:; M"' = Pu, M' = NMe4, NPr,", Et3NH), MU[U02CL&xH20 (MI' = Be, x = 4; Mg, Ca, Ba or VO, x = 2; Sr, x = 1).The coordination geometry of the anion in (l,10-phenH)2[U02a] is
i
b
The Actinides
1212
a flattened square b $ ~ r a m i d .The ~ ~ ~IR spectra of Cs2[MV1O2CI4](Mv' = U, Np and Pu) have also been re orted. Products of composition M"'[UO2ClS]-xH20 (M"' = Y, x = 3 or La, x = 2), MI [U02C16]*XH20 (MIv = Ce, x = 5 or Th, X = 4), Mn3[U02C18].6H20, Al2[Uo2Cl8].6H20 and a number of salts which appear to contain the anion [(U02)2C1,]3have been reported,3s4 but these could be salts of the UOZ' ion. For example, Ce[UO2CI6].5H20is more likely to be [U02(H20)5][CeC~]. Relatively few salts of bromo and iodo complex anions, M: U02&], have been reported (X = Br, MI = PPh,Bu", PPh3(CH2Ph),HPPh3, HPEt3, HPPr??5' m u $ ;X = I, M' = PPh3Bun). is again a flattened square The coordination geometry of the anion in (NBU?)~[UO~B~,] bi~yramid.,'~
s
40.2.4.6 Mixed donor atom ligands
(i) Schiff bases
A very large number of dioxouranium(V1) compounds with this type of ligand have been recorded (see ref. 12, Uranium, vol. E2), examples of which are given in Table 86; these include some instances in which the mono- and di-basic ligands behave as neutral donors. Structure determinations indicate that the usual coordination geometry is approximately pentagonal bipyramidai; examples are [U02(2-OC,H4CH=N(CH2>2NH(CH2)2NMe2)(N03)] in which four equatorial positions are occupied by one oxygen and three nitrogen atoms of the tetradentate Schiff base and the fifth by an oxygen atom of the monodentate nitrate [U02(2-OC6H4CH=N(CH2)2NMe2)2], in which the equatorial sites are occupied by two oxygen and three nitrogen atoms from the two potentially tridentate ligands;358 [U02(2OC6H4CH=N(CH2)2NH(CH2)2N3CHC6H40-2)],with two oxygen and three nitrogen atoms of the ligand in the puckered equatorial plane;359and [UO2(2-0GH4N=CHCH=NC6H40-2)(H20)] with two oxygen and two nitrogen atoms from the ligand and one oxygen atom from the water molecule in the near planar pentagon.3ma Table 86 Some Dioxouranium(V1) Schiff Base Complexes .. -
UO,(MeCO,),~LH UOZ(MeCO&.LH2 UO,X,*2LH
u0,xL UW2 U0,L W,L(H,O)I
LH" = 2-HOC,H4CH=NR and 2-HOC,&CH=NR with R = Et, Pr", Bu" and Ph LH," = 2-HOC6H,CH=N(A)OH and 2-HOC,oH,CH=N(A)OH with A = CH,CHMe or (CH,), LH = 2-HOC6H4CH=NR with X = C1 o r NO, and R = Et, Pr", Bu",(CH,),NHPh LH = 2-HOC6H4CH=NR with R = (CH2),NH(CH,),NMe, and X = F, C1, Br, NCS, NO,, MeCO, or trop (tropolonate) LH = 3-Me, 2-HOC6H,CH=NHCOPh and 2-HOC6H,CH=N(CH,),NMe, LH, = 2-HOC,H4CH=N(CH,),NH(CH&N&HC,H,-2-OH LH, = 2-HOC6H4N=CHCH=NC,Hc2-OH
'R. G.Vijay and J. P. Tandon, Monatsh. Chem,, 1979,210,889.
(ii) Amino acids Dioxouranium(V1) compounds with aminoacetic acid (glycine), U02(H2NCH2C02)2and U02(H2NCH2C02)(0H)+2H20, and similar derivatives of a and P-aminopropionic acid (aand p-alanine) , U02(MeC€INH2C02)2.4H20, U02(MeCHNH2C02)(C104).2H20and U02(H2NCH2CH2C02)2-xH20(x = 0 or 4), have been recorded. In the glycine compound [U02(02CCH2NH3)4](N03)2 all of the glycine ligands are in the zwitterionic form and act as oxygen donor ligands; two are bidentate and two unidentate, making up the equatorial plane of a hexagonal bipyramidal arrangement of oxygen atoms around the uranium atom.3mb L-Arginine [H2NC(=NH)NH(CH&CHNH2C02H ( = HX)] compounds of the type UOzXY.zH20 (Y = NO3, z = 2; Y = C104 or MeC02, z = 3) are also known.3h12-, 3- and 4-H2NC6€&C02Hform compounds of composition UO&; an acid salt of the 2-amino acid ( = HL), U02L*HL*2H20,and a mixed oxalato salt, Na2[U02(G04)(L)2]-2H20, have been recorded.
The Actinides
1213
(iii) Complexones Nitrilotriacetic acid, N(CH2C02H),( = H3L), forms the complexes UOz(HL).xHzO ( x = 0, 2 or 5 ) and (U02)3(L)2.xH2O ( x = 3 or lo), and the analogous phosphorus tripropionic acid, P(CH2CH2C0zH)3(= H3L), forms the salt of the complex anion, Na[UO2L]-H20. The hydrates are precipitated from aqueous solution. Similarly, ethylehediamine-N,N,N', N'-tetraacetic acid, (H~zCCHZ)ZNCHZCHZN(CH~CO~H)~ ( = H4edta), yields compounds of composition U02(H2edta).xHz0 ( x = 0, 1 or 2), (UOz)z(edta)~xH20(x = 0, 2, 3 or 4) and salts of the complex anion, such as M;[UO2(edta)].xH2O (MI = K, x unspecified; M' = NH4, x = 0, 2 or 3), [C0(NH~)~]~[UO~(edta)l3.22H~0 and Na2[U02(H3edta)4].Mixed peroxo-edta compounds of composition [(U02)2(W2edta)( 0 2)]*4Hz0, [(UO,),(&edta)( 02)2].9H20, [(UOz)2(H4edta)2(02)2],10H~Oand Na2[(UOz)2(edta)(02)].7Hz0 have also been recorded. Hydrated compounds of the type UO2(H2L)*xH20have been reported for 1,Z-diaminopropane-N,N,N',N'-tetraacetic acid ( x unspecified), trans diaminocyclohexane-N,lV,N',N'tetraacetic acid (x = 0 or 3; (U0z)2L.xHz0,with x = 0 or 6 also known), 2,2'-diaminodiethyl ether-N,N,N',N'-tetraacetic acid ( x = 3), and 2,2'-diaminodiethyl sulfide-N,N,N',N'-tetraacetic acid (x = 0 or 4); the hydrates are precipitated from aqueous media, but in the last instance a mixture of ethanol and acetone was added to induce precipitation. The analogous di(2-aminoethoxy)ethane-N,N,N',N'-tetraacetic acid (H4L) forms the compounds [(U02)zL(H20)2] and Na[UQ@L)(H20)]-
(iv) Ligands containing N, 0 and S,0 donor sites ( u ) 8-Hydroxyquinolznes. Neptunium(V1) and plutonium(V1) are reduced to lower oxidation states by 8-hydroxyquinoline or its derivatives362and only dioxouranium(V1) compounds are known; these are of composition U 0 2 L , UO&.HL and M1[U02L3] (Table 87). In addition, 1:1 adducts with the pyridine-2-carboxylate, U02(NC5&-2-C02)z.HL, are formed by 8-hydroxyquinoline and the 2-methyl derivative. Further work on the wide range of substituted 8-hydroxyquinolates is desirable, for the existence of some of the reported solvates, UO,L.HL, is uncertain. Table 87 Dioxouranium(V1) Complexes with 8-Hydroxyquinoline and Substituted 8-Hydroxy quinoiines U0&2
U0,LfHL
MU[U0,L3] a
HL = 8-hydroxyquinoline,"2-, 5- or 7-1nethy1,~ 5-acetyl, k h l o r o , hitro-, 5-pheny1, 7-?-butyl-,2,7-dimethyl-,b 5-chioro-7-iod0, 5,7-dibrom0-~and 5,7-dichloro-8-hydroxyquinolineb HL = 8-hydroxyquinoline: 2- or 5-methyl-, h i t r o - , 5-phenyl-, 5-acetyl-, 5-chloro-, 5-iodo-, 7-flUOrO-, 7-phenyl-, 7-(cu-anilinobenzyl)-, 7-[m](m- or p-nitroanilinobenzylj-, 7-@-carboxyphenylaminobenzy1)-,5,7-dichloro-, 5,7-dibromoand 5,7-diiodo-8-hydroxyquinoIine HL = 8-hydroxyquinoline
Mono-,tri- and tctra-hydtatc also known. Monohydratc also known. Monc- and di-hydrate also known.
In the complex [U02L(HL)]CHC13 (HL = 8-hydroxyquinoline), the additional molecule of HL is coordinated to the uranium atom through the phenolic oxygen atom; the coordination geometry is approximately pentagonal b i ~ y r a m i d a l . ~ ~ ( b ) Ligunds with S,O donor sites. The complex with S(cyclo-C6HloOH)z,[U02C12L], is obtained by adding an excess of the ligand in ethyl acetate to a solution of U02C12-3Hz0in the same solvent. The ligand is terdentate in this compound, with the two oxygen atoms from the hydroxyl groups and the sulfur atom, together with the two chlorine atoms, in the equatorial positions of a distorted pentagonal bipyramid.m Thiovanol (HSCH2CH(OH)CH20H= C~H&OZ)corn lexes, [ U O ~ ( C ~ H ~ S O Z ) ~ ( H and ~ O [U02(C3H+02)Cl(H20)3]-H20, )~] have been recordedF5 and these may involve bonding to sulfur as well as oxygen, but the assignment of the water molecules to the coordination sphere requires confirmation. An 8-mercaptoquinolinate, U02(C9H$JS)2, the sulfur analogue of the %hydroxy uinolinate, has been prepared by reaction of U02(N03)z.6H20with Ca(C9&NS)2 in benzene!&
The Actinides
1214
40.24.7 Multidentate macrocyclic ligands
(i) Phthalocyanine and superpkthalocyanine The dioxouranium(V1) phthalocyanine complex, U02(C32HldN8).C6114(CN)2(C32HlgN8= phthalocyanine), has been reported to be formed by reaction of U02C12 with ophthalodinitrile, C6H4(CN)z, but this reaction in DMF has been shown to yield the cyclopentakis(2-iminoisoindoline) (CmHZ2Nl0)derivative, [UOZ(C40H20N10)],in which the coordination geometry about the uranium atom can be described as a compressed pentagonal bipyramid, with five nitrogen atom$ of the C40H20N10ligand in the irregularly distorted pentagonal girdle.367 This compound reacts with di- and tri-valent metal halides with contraction of the ring and formation of the corresponding metal phthalocyanine complex.368 Complexes with 2,3,9,10 ,16,17,23,24,30,31-decaalkylsuperphthalocyanines, (4 ,5-R2)5PcU02 (R = Me, Bu"), and the analogous pentamethylphthalocyanine complex, (4-Me)5PcU02, have been reported. In these the uranium atom is bonded to five nitrogen atoms of the superphthalocyaninate
(ii) Crown ethers and kryptands A number of crown ether and kryptand solvates of dioxouranium(V1) compounds have been recorded (Table 88). The IR and '€3 NMR spectra of many of these products suggest that the crown ether is not bonded to the metal atom, and an X-ray study has shown that the 18-crown-6 solvate, U02(NO&.L.4H20, consists of layers of hexagonal bipyramidal [UO2(N0&(H,O)z] units with the crown ether lying between the uranium atoms.37oHowever, IR evidence suggests that the ether in the hydrated and anhydrous 15-crown-5, 18-crown-6 and dicyclo-hexyl-18-crown-6 solvates of U02C12 may be bonded to the The constitution of the salt Na2U02C14.2-benzo-15-crown-5 has been shown to be [Na(bemo-l5-crown5)12[U02C14],with the anion Table 88 Crown Ether and Kryptand Solvates of Dioxouranium(V1)Compounds
L = 12-crown-4; 1,4,7,1O-tetraoxacyclododecane X = C1,y = O or 2;X=NO,, y = 2 U02X2.L.yH,0 L = 15-crown-5; 1,4,7,lO,13-pentaoxacyclopentadecane UO,X,*L.yH,O X = C1,y = 3 ; X=NO,, y = 2 x = 0 or ca. 4 (UO,F,),.L.xH,O (uo2c1,)2~3L-2Hc1 L = benzo-15-crown-5; 2,3,5,6,8,9,11,12-octahydrobenzo[b]-1,4,7,10,13pentaoxacyclopentadecin
U02X2*L*yH20 X = CL,y =2; X = NO,, y = 2 or 5; X = Clod, y = 7 L = 18-crown-6; 1,4,7,10,13,16-hexaoxacyclooctadecane X = C1,y = 2 or 3; X=NO,, y =2,3, 4 or 5; U02X2.L.yH20 X=MeCO,, x = 3 or 4 (UO2C12)2*L 2,3,5,6,11,12,14,1S-octahydrodibenzo[b, k]-l,4,7,10,13,16L = dibenzo-18-crown-6; hexaoxacyclooctadecin UOzX2.L.yHZO X = CI, NO3, y =2; X=C104,y =6 L = dicyclohexyl-18-crown-6; icosahydrodibenzo[b,k]-1,4,7,10,13,16hexaoxacyclooctadecin
U02(N03)2*L*2H20 (U02CI2)2.L 8.8.81-hexacosane L = kryptofix(2,2,2);4,7,13,16,21,24-hexaoxa-l ,10-diazabicyclo[ U02X2.L*yH,0 X = C1,y = 2;X = NCS,y = 1; X = NO,, y = 4; X = MeC02,y = 0 or 3
40.2.5
The +7 Oxidation State
Salts of oxoderivatives of the types [MOJ, [MOSl3- and [MO6I5- have been reported for ne tunium(VI1) and lutonium(VI1). Compounds of composition M:MV'I05 (Mvx' = Np, MP= K, Rb or Cs; M' = Pu, M' = Rb or Cs) have been prepared by heating the alkali metal peroxides with the actinide dioxide at 320 to 355 "C (Np) or 250 "C (Pu). Hydrated compounds, M$'(NpO&xHzO [M" = Ca, Sr, Ba ( x = 3)] have been prepared from aqueous alkaline
The Actinides
1215
solutions of neptunium(VII), as have the hydrated salts M"'Np05-xH20 [M"' = [&(NH&], [Co(en),] (x = 5) and [Pt(NH3)sCl] ( x = l)].The IR spectra of Ba3(MV1'05)2.~H20 (MV"= x = 3; Pu, x = 3), [CO(NH3)6]MV1105*~H20 (Mv" = Np, x = 5 ; Pu, x = 3), [Pt(NH3)5C1]Mw105.~H20 (Mv" = Np, x = 1; Pu373,x = 3) suggest that the oxoanion structure consists of infinite chains of [Np06]F:"-1 Structural investigations of compounds of the general composition M'Np05-xHz0 indicate that they are really M:[Np04(0H)2]-(x - l)HzO; the coordination geometry of the neptunium(VI1) centre in the anion is described as tetragonal bipyramidal, with the two OH groups in the axial positions (MI= Na,375x = 5; K,376x = 3). The anion reserves the same coordination geometry in Na3[Np04(0H)2]-~H20 (x = 0,377a 23"b and 43F"); both hydrates lose all their water at 373 K. Ba3(Np0& has also been prepared by heating Np03.H20 with barium eroxide in oxygen at 500 "C. : (MI = R, Rb,Cs) are prepared in the same way as the Compounds of corn osition M1Np04 hydrated salts MI3 (Np05)yxHzO but with concentrated alkaline solutions of nept~nium(VII).~'~ The ammonium salt, NH4NpO4.nH2O(a = 1.5 to 3) has been prepared by anodic oxidation of neptunium(V1) in (NH&C03 solution; it is isostructural with M1Np04.2.5H20 (MI= Li or Na).3 The compounds Li5Mv"06 (Mwl = Np, Pu) have been obtained by heating the actinide dioxide with lithium oxide in oxygen at ca. 400"C(Np), and the analogous Ba&N !'pO6 (MI = Li or Na) by heating Np03.H20 with the alkali metal peroxide in oxygen above 400 "C. The [NpO6I5- anion appears to be octahedral. The IR spectrum of Li,NpO2(OH), has been reported; it was prepared by dissolving Np02(OH)3,3H20in aqueous lithium hydroxide and then evaporating the solution.3m In the structure of the compound L ~ [ C O ( N H ~ ) ~ ] [ N ~ ~ ~ ~ (precipitated O H ) ~ ~ .from ~H~O a, lithium hydroxide solution of neptunium(VI1) by allowing aqueous [co(NH&]Cl, to diffuse into it, there are two independent neptunium centres, each coordinated by an octahedron of oxygen atoms; the octahedra share corners to form a chain.381
93
P
40.3 THE TRANSPLUTOMUM ELEMENTS 40.3.1 The +2 Oxidation State The americium halides A m 2 ( X = C1, and 1383) have been obtained by heating americium metal with the appropriate mercury(I1) halide; they are isostructural with the correspondin europium dihalides. The californium(I1) compounds CfX2 [X = Cl,3R4B P 5 and I (dimorphic3 8 p)I, as well as the einsteinium analogues EsXz (X= C1, Br, I),387 have been prepared by hydrogen reduction of the trihalides at high temperatures. CfBrz has also been by heating CfzO3 in HBr at 500-625 "C, and there is also tracer level evidencem for the formation of FmC12. The known monoxides and monosulfides, such as A m 0 and AmS, are probably semimetallic in character.
40.3.2 The +3 Oxidation State Complexes involving group IV or group V ligands do not appear to have been recorded.
40.3.2.1 Oxygen lig& (i) Aqua species, hydroxides and oxides ( a ) Aqua species. A selection of the known hydrates is given in Table 89. The hydrated xenate(VIII), An1@e0~)~.40H~o, has also been rep~rted.~"The hydrated trichlorides, MCl3.6HZ0(M = A m or Bk), and tribromides, MBr3.6H20 (M = Am or Cf), are isostructural with the hydrated lanthanide chlorides, LnC13.6H20, and involve aqua ions, such as390 [ A ~ I C ~ ~ ( O H ~ )In ~ ]the + Chydrated ~. salicylate, Am(GH50&.H20, each metal atom is linked to six different salicylate groups and is surrounded by nine oxygen atoms, eight from the salicylate groups and one from the water molecule; two salicylate groups are bidentate, one via its carboxylate group and the other via its carboxylate and phenolic groups, and the other four are monodentate via the carboxylate
1216
The Actinides Table 89 Some Hydrates of Transplutonium Actinide(II1) Compounds ~~
~~~
~~
M"'X3.6Hz0 M\"(S04),.xH20 M'Am(S04),-xHz0 K3Am(S04),.H20 Am,(CO,),~xHZO NaAm(C0,),.4H20 Na3Am(C03),.3H,0 M;~'(C,O,),XH,O M"'P04~0.5H20 [Am(GH,O,),(%O)I
~
X = CI, M"' = Am, Bk; X = Br, MI" = Am, Cf M"'= Am, x = 5 ; MI'' = Cf, x = 12 M'=Na, x = 1; K, x =2; Rb, Cs, TI,x = 4 x=4or5 M"'=Am,x=OS, 1, 3,4, 7, 9or 11 M1"=Cm, x = 1,2, 3 , 4 , 5, 6, 7 , 8 , 9 or 10 MI" = Am, Cm C,H,O, = salicyclic acid
( b ) Hydroxides. The hydroxides M(OH)3 are known for M = A m , Cm, Bk and Cf; A I ~ ( Q H is) ~isostructural with Nd(OH)3.392 ( c ) Oxides. Three crystal modifications have been reported for both Am203 and Cm203, two of which (the A and C types) have been found for BkzO3 and the A and B types have been reported for Cf203. Ternary oxides, such as LiAmOz, obtained by heating AmOz with LizO in hydrogen at 600 "C, M"(Am02)2 (MI' = Sr or Ba) and M"'A103 (Mi" = Am or Cm) have been reported. (ii) P-Ketoenofates The hydrated americium compounds, AmL3-xH20 (HL = MeCOCH2COMe, x = 1; CF3CQCH2COPh and CF3COCH2C0(2-C4H3S), x = 3), are precipitated from aqueous media393and A~(B u' C OC HC OB U~)~ is precipitated from an aqueous ethanol solution of the components. It is isomorphous with the raseodymium and neodymium chelates, in which (Pr) the metal atoms are se~e n-coordina t ejThe ~ berkelium compounds, Bk(CF3COCHCOR)3 (R = CF3, But and 2-C4H3S),have been obtained by a solvent extraction method395and the (M = Am,3963397 Cm397and Cf39h,397) has also sublimation behaviour of M(BU~COCHCOBU~)~ been reported. Compounds of composition CSM(CF~COCHCOCF~)~*XH~O have been isolated for M = Am ( x = 0 or 1) and Cm ( x = l), and there is evidence for the formation of analogous compounds with M = Bk, Cf or Es. In the structure of the anhydrous americium compound, the metal atom is surrounded by a dodecahedral arrangement of oxygen atoms.398
(iii) Oxoanions as ligunds ( a ) Phosphates. Hydrated phosphates, MP04.0.5H20 (M = Am, Cm), are obtained from aqueous solution and can be dehydrated at about 200 "C. ( b ) Sulfates. Hydrated sulfates and sulfato complexes are included in Table 89. Anhydrous Am2(S04)3is obtained by heating the pentahydrate at 500 "C; anhydrous sulfato complexes of composition JSAm(S04)* and M;Am2(S0& (MI=K, Cs or Tl) are also known. The oxosulfates, M:''OzSO4 (MI"= Cm or Cf), have been reported; the curium compound is obtained by calcining the Cm3' form of Dowex 50W-X8resin in air at 900 "C. Cm2(S04)3may be formed at 760 to 870°C in this process399and the californium compound is obtained by heating Cf2(S04)3~12H20 in air at 700 to 800 0C.400 (iv) Carboxylates and carbonates
( a ) Monocarboxylutes. The only recorded compound appears to be the formate, Arn(HCO&, prepared by evaporating a solution of Am(OH)3 in concentrated formic acid at 50 "C. ( b ) Carbonates. Hydrated americium carbonate, Am2(C0&.4Hz0, is precipitated from aqueous solutions containing americium(II1) by NaHC03. It decomposes to AmzO(C03)2,and then to Am20z(C03)on heating. The anhydrous carbonates, M!*r(C03)3 (MI" = Am or Crn) are formed in the radiolytic decomposition of the oxalate (Am) or by heating the anhydrous
The Actinides
1217
oxalate at 360°C (Cm); Cm20(C03)2 and Cmz02(C03) are obtained at 420 and 510°C respectively. (c) Oxalates. The hydrated oxalate, Amz(Cz04)3.xHz0( x = 7, 9, 10 or l l ) , is precipitated from aqueous solutions containing americium(II1) by oxalic acid and the anhydrous oxalate is obtained by heating the hydrate above 240 "C. The corresponding berkelium and californium compounds are precipitated from aqueous nitric acid solutions of Bk"' or Cf"' by oxalic acid.401aThe oxalato complexes, MAm(C204)2.xHz0 (M = N&, x = 5, M = Na, Cs), have been prepared from Am2(C204)3-10H20and Mz&O, in neutral solution.401bThe potassium salts (x = 2.5, 3.5 and 5) have been obtained in the same way.401C
( v ) Aliphatic and aromatic hydroxyacids
The only recorded compounds appear to be the citrates, Am(C6H507).xH20 and
[Co(NH3)6][Am(GH~07)z]~xHZ0,40z and the hydrated salicylate, Am(C7H503)3.H20, the structure of which is described on p. 1215.
(vi) P-Oxide3
The only complexes known (Table 90) are those of P-ketocnolates with ( ~ Z - C ~ H ~ , )and ~PO (Bu"O),PO, prepared by solvent cxtraction from aqueous solutions of the actinides(II1) with a mixture of the P-ketoenol and the P-oxide. There is also solution chemistry evidence for the formationm3 of AI~[CF~COCHC~(~-C~H~S)]~-~(B~"~)~PO. Am[CF3COCHCOCF3)3. ~ ( B u " O ) ~ PisO volatile at 175 OC404and a gas chromatography study of this complex and the curium analogue has been reported.405 Table 90 Complexes of Transplutonium Actinide(II1) B-Ketoenolates with P-Oxides M~~~L,.x(~-c~H,,)~Po M'[~L,.x(BU"O),PO
HL = CF,COCH,COR, with R = Me, CF, or But, MIT1= Am or Cm, x unspecified" HL = CF,COCH,COR, with R = Me or Bu', M"' = Am or Cm, x unspecified" HL = CF,COCH,COCF,, M'" = A m , x = 2;a,b Cm, x unspecified;a Bk, x = 1'
=A. V. Davydov, B. F. Myasoedov, S. S. Travnikov and E. V. Fedoseev, Sou. Radiochem. (Engi.
Transl.), 1978, 20, 217. b A . V . Davydov, B. F. Myasoedov and S. S. Travnikov, Dokl. Chem. (Engl. Transl.), 1975,220/5, 672. 'E. V. Fedoseev. L. A. Ivanova, S. S. Travnikov, A. V. Davydov and B. F. Myasoedov, Sou. Radiochem. (Engl. Traml.),1983, 25, 343.
40.3.2.2 Sulfur ligands
Compounds with sulfur containing ligands do not appear to have been reported, but the sulfides M:"S3 (MI'' = Am, Cm, Bk or Cf) and AmS1.9[probably an americium(II1) polysulfide] structure, and the y form the are known. Am2S3is polymorphic, the a form having the E-&& Th3P4 structure which is also adopted by the other sesquisulfides. Oxysulfides, M;"O2S (M"' = Am, Cm or Cf), are also known; Cf202Sis obtained by heating Cfz(S04)3.xH20(x = 0 or 12) in hydrogen or in a vacuum at 800 "C.
40.3.23 Selenium and tellurium ligands The only recorded compounds appear to be americium selenide and telluride, Am3&.
1218
The Actinides
43.24 Halogens as ligands (i) Trihalides The trifluorides, M1I1F3(Table 91), possess the 11-coordinate LaF3structure (MI*'= Am, Cm and one form of BkF3), and a second modification of BkF3 has the eight-coordinate high temperature LaF3-type structure, found also for CfF3 and EsF3.& The trichlorides, MIuCl3, have the nine-coordinate UC13-type structure (M"' = Am, Cm, Bk and one form of CfC13), and the second modification of CfC13 has the eight-coordinate PuBr3 structure. EsC13 has the UC1,-type structure at 425 "C. The structures of the tribromides, MU113r, (MI'' = Am, Cm and Bk), are PuBr3-type, and cv-Am13 has this structure also. However, P-Am13 and the other triiodides M11113(MI"= Cm, Bk and Cf) have the six-coordinate Bi13 structure. The structure of the hydrated trichlorides and tribromides, MX3-6H20,has been discussed earlier (p. 1215). Table 91 Transplutonium Actinide(II1) Halides and Halogeno Complexes M'ITX3 M"'0X M'AmX4 K,AmF,' &AmF," ""m,F;',' Cs,NaM C1,
X = F, CI, Br or I, M"' = Am, Cm, Bk, Cf and Es" X = F, MIn = Cf; X = CI, M"' = A m , Cm, Bk, Cf and Es; X = Br, MI1'= Cm, Bk and Cf; X = I, MI" = Am,b Bk and Cf X = F, M' = Na, K,F Rb;' X = C1, M' = Cs (also +4H,O)
M'" = Am,"*eCm' and Bk".'
(Et,N),LiAmCl, MiAmX,
X = Cl, M' = Cs, Et,N and Ph,PN; X = Br, M' = Ph,PH
"For EsF,, see D. D. Ensor, J. R. Peterson, R. G. Haire and J. P. Young, J. Inorg. N u d Chern., 1981,43, 2425. bR.G. Haire, J. P. Young and J. R . Peterson, J. Less-Common Met., 1983, 93, 339. J . Jove and M. Pages, Inorg. NwZ. Chern. Len., 1977, W, 329. dL.R. Morss, M. Siegal, L. Stenger and N. Edelstein, Inorg. Chern., 1970, 9, 1771. OM. E. Hendricks, E. R. Jones, Jr., J. A. Stone and D. G . Karracker, J . Chem. Phys., 1974, 60,2095.
(ii) Halogeno complexes The fluoro complexes M1AmF4 (M1=K or Rb) are isostructural with the analogous plutonium compound^‘"'^ and the anion in the salts Cs2Na[MmC16] is octahedral (e.g. MI" = Bk),408
40.3.25 Hydrides as ligandr The only recorded compounds are the hydrides AmH3 and x
(M = Am, Cm or Bk,
< 1).
40.3.26 Mixed donor atom ligands
Compounds of composition AmL3, where HL is 8-hydroxyquinoline, 5-chloro- or 5,7dichloro-8-hydroxyquinoline, are precipitated when an aqueous solution containing americium(II1) is added to an aqueous, or aqueous dioxane, solution of the ligand in the pH range 5 . 1-6.5.409 There is solvent extraction evidence for the californium(II1) 5,7-dichloro-8hydroxyquinoline complex.410
40.3.27 Multidentate macrocyclic ligands The bis phthalocyanine (Pc) complex, Amp%, has been obtained by heating Am13 with a-phthalodinitrile in l-~hloronaphthalene~~~ or from americium(II1) acetate and o phthalodinitrile; it is frobably a sandwich compound similar to those obtained with the tripositive lanthanides. l2
The Actinides
1219
40,3.3 The +4 Oxidation State 40.3.3.1 Oxygen ligands
(i) Oxides All the reported dioxides, MIV02(MI" = Am, Cm, Bk and Cf), possess the fluorite structure. Ternary oxides of the types M;AmIVO3 (MI = Li or Na), M11Am'v03 (M" = Sr or Ba) and Li8MIVO6(MIv = Am or Cm) have been recorded; they are obtained by heating the dioxides with the alkali or alkaline earth metal oxide at high temperatures under vacuum or in nitrogen.
(ii) p-Ketoenolates The berkelium(1V) complexes BkL4 [HL = CF3COCH2COBut39sand CF3COCH2CO(2CqH3S)395,413] are formed by solvent extraction of aqueous BkW solutions with the p-ketoenol.
40.3.3.2 Halogens as ligands
( i ) Tetrahalides
.
The tetrafluorides M'"F4 (MI" = Am, Cm,Bk and Cf) are obtained by fluorination of lower oxidation state compounds; for example, CfF4 has been prepared"" by heatihg (3% Cf203or CfC13.xH20 in fluorine at 400-450 "C under pressure (3 atm). All have the eight-coordinate UF4 structure (p. 1173); for crystallographic data see refs. 415 and 416.
(ii) Halogeno complexes The fluoro complexes LiM1"F5 (MLv= Am or Cm) and Rb2MIVF6(MIv = Am or Cm) have the LiUFs and Rb2UF6structures (p. 1173) respectively. Compounds of composition M!MLvF31 (MI = Na, MIv = Am, Cm, Bk or Cf and MI = K, MIv = Am, Cm or Bk) are also known (see p. 1174). this compound would provide a Cs2BkC16is not isomorphous with Cs,PuCI, or CszCeC16;40p useful starting material for the preparation of oxygen donor complexes of BkCI4, which are at present unknown, by the methods used to prepare PuCL complexes of this type (p. 1161).
40.3.4 The +5 Oxidation State There is tracer scale evidence"ss for the formation of the CfO,' ion during ozonization of
249Bkand subsequent fl decay to 249Cf.However, the only compounds isolated in this oxidation state are all americium(V) species.
40.3.4.1 Oxygen ligands
(i) Aqua species, hydroxides and oxides ( a ) Aqua species. Hydrated americium(V) carbonato and oxalato complexes are discussed below in the sections on carbonates and oxalates. The hydrated hydroxide, AmO2(0H)*ca.2.3H20, is precipitated from aqueous solution by alkali. It decomposes to AmOz on heating.41s (6) Oxides. Pentoxides, MYO5, are unknown, but ternary oxides, such as Li3AmV04, Li7Am06 and Na3Am04, are obtained by heating AmOz with the alkali metal oxide (e.g. Li7Am06 in N2/02 at 900 "C; this decomposes to Li3Am04 at 1000"C).Crystallographic data have been reported for these corn pound^.^'^
The Actinides
1220
(ii) Carboxylates and carbonates (a) Monocarboxylates. The acetato complex salt, Cs2Am02(MeC0&, is isostructural with the analogous neptunium(V) and plutonium(V) compounds (p- 1183)lS6 and its vibrational spectrum has also been reported. (b) Carbonates. The hydrated carbonato complexes KAmO2CO3.xH20 and K3Am02(C03)2.~H20 have been obtained by electrochemical reduction of americium(VI) solutions in the presence of carbonate. The carbonato complex salts M:Am02(C03)2-xH20 are isostructural with the analogous neptunium(V) compoundsls0 (p. 1180). The vibrational spectrum of CsAmOzC03 has also been r e ~ 0 r t e d . l ~ ~ (c) Oxalates. The hydrated oxalato complex salts M1Am02(&0,).xH20 (MI = K or Cs) are precipitated from aqueous solution .41M
40.3.4.2 Halogens as ligands
The chlorocomplex salt, Cs3 AmOzC14], is isostructural with the analogous neptunium(V) and plutonium(V) compounds.266 Its vibrational spectrum has also been reported.lS5
40.3.5
The +6 Oxidation State
40.3.5.1 Oxygen ligands (i) Aqua species and oxdex
( a ) Aqua species. Hydrated phosphato and arsenato complex salts, M'AmOzXO4-yH20 (MI= K, Rb, Cs or NH4 and X = P or As, with y in the range 0 to 3) are precipitated from americium(V1) solutions421at pH 3.5. NH4ArnO2PO4.3H20is isomorphous with the analogous dioxouranium(V1) and neptunium(V1) compounds422 (Table 63, p. 1192). The hydrated is isostructura1262with the corsulfatocomplex salt, ([Co(NH3)6]HS04)2Am02(S04)3~xHz0, responding dioxouranium(V1) and neptunium(V1) compounds (p. 1197). ( b ) Oxides. Am03 is unknown, but ternary oxides of the type M$Am04 (M'=K, Rb or Cs), MiAmos (MI = Li or Na), M a m 0 6 (MI = Li or Na) and Ba3Am06have been recorded, prepared by heating AmOz with the metal hydroxide or oxide in oxygen (e.g. M:Am04).423
(ii) Oxoanions as ligands The nitratocomplexes M'Am02(N03)3 (MI = Rb or Cs4") are precipitated from nitric acid solutions of americium(VI). The IR spectrum of RbAm0z(N03)3 has been reported.249 Hydrated phosphato, arsenato and sulfato complex salts are included under 'Aqua species' in the preceding section. (iii) Carboxylates and carbonates ( a ) Monocarboxylates. The
IR spectrum of NaAmOz(MeCO& has been reported.249 ( b ) Chelating carboxylates. The pyridine- and N-oxopyridine-2-carboxylates,Am0& and €€Am02L3,have been obtained from aqueous solutions of americium(VI) and the acid HL; the N-oxide 2-carboxylate is formed as the dihydrate which is dehydrated at 100 (c) Carbonates. The carbonato complex salts, I V & ~ ~ O ~ ( C O (MI= ~ ) ~Cs or NH4), are obtained on addition of M$C03 to a bicarbonate solution containing ameri~ium(VI).~'~ 40.3.5.2 Halogens as ligands
AmOzFz has been prepared by the reaction of NaArn02(MeC02)3with anhydrous HF and The IR spectrum of fluorine at -196 "C. It is isostructural with U02F2, Np02Fz and PuOZFZ.~'~ CszAm02C14has been r e p ~ r e t d " and ~ the corresponding rubidium salt has been mentioned in a The cubic phase of CszAmOzC1442sis probably a mixed oxidation state compound,
Cs7(AmV02)(Amw02)zCllz.2W
The Actinides
1221
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The Actinides
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1226
The Actinides
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The Actinides 365. 366. 367. 368. 369. 370. 371. 372. 373. 374. 375.
1227
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The Actinides
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36.1 Molvbdenum: The Element and Aq6eous Solution Chemistry A. GEOFFREY SYKES University of Newcastle upon Tyne, UK 36.1.1 THE ELEMENT
1229
36.1.2 AQUEOUS SOLUTION CHEMISTRY
1229 1229 I230 1234 1243 d249‘ 1256
36.1.2.1 36.1.2.2 36.1.2.3 36.1.2.4 36.1.2.5 36. I.2.6
General inlroduction Oxidation Stare I i Oxidation Srate Ii1 Oxidation Stare Oxidation State V Oxidation State VI
1261
36.1.3 REFERENCES
36.1.1 THE ELEMENT Molybdenum occurs chiefly as molybdenite, MoS,, but also as molybdates PbMoO, and MgMo04. The largest known deposits are in Colorado (USA), but it is also found in Canada and Chile. The natural abundance in the earth’s crust (-1.2 p.p.m.) is about the same as that of tungsten, but is much less than of chromium (122 p.p.m.). The name in fact originates from the Greek rnolybdos meaning lead. The Swedish chemist Scheele (1778) produced the oxide of a new element from (black) MoS2, thereby distinguishing the element from graphite with which it had been confused. The metal was isolated by Hjelm in Sweden three to four years later by heating the oxide with charcoal. In the procedure now used to isolate the metal, the MoS2 component in ores is concentrated by flotation methods. The concentrate is then converted by roasting into Moo3, which, after purification, is reduced with hydrogen. Reduction with carbon is avoided since carbides rather than the metal are obtained. The chief use of molybdenum is in steels. The oxides and sulfides have some applications as catalysts. Molybdenum is the only element in the second and third transition series which appears to have a major role as a trace metal in enzymes. Several aspects of molybdenum chemistry have been widely studied in order to gain a better understanding of the biological relevance. Molybdenum is one of the few elements which currently has its own series of international conferences.’ Molybdenum and tungsten are similar chemically, although there are differences which it is difficult to explain. There is much less similarity in comparisons with chromium. In addition to the variety of oxidation states there is a wide range of stereochemistries, and the chemistry is amongst the most complex of the transition elements. 36.1.2 AQUEOUS SOLUTION CHEMISTRY 36.1.2.1 General introduction Molybdenum has an extensive aqueous solution chemistry for oxidation states I1 through VI. It is unique in having aqua or aqua/oxo ions for all five states in acidic solution (pH < 2). These are well defined in all but the Mo”’ case, the study of which is complicated by the existence of rapid equilibria involving protonated/deprotonated monomer/dimer (and higher) forms. The VI state is without question the most stable and in contrast to Cr”’ is only the mildest of oxidants. Compounds which have contributed to the development of the aqueous solution chemistry, including the aqua ions themselves, are considered under Section 36.1.2. It is only since 1971 that the aqua forms of oxidation state 11-V ions have been identified, and
1229
1230
Molybdenum
preparative as well as structural features defined.2 The chemistry of aqua ions generally has a somewhat elevated position, since they are often regarded as a point of reference or prototype for the behaviour of a particular oxidation state. Since they are difficult to crystallize, structures of derivative complexes are relevant. The simple aqua ions of oxidation states II-V are indicated in Table 1. The quite different structures of adjacent oxidation states are to be noted, which gives rise to an interesting and varied redox chemistry. All but [ M O ( H ~ O ) ~are ]~+ diamagnetic and metal-metal bonding is a significant contributing feature. Table 1 Summary of Aqua Tons of Molybdenum at pH < 2
Mo-Mo
Description
Moi'
bonding
Quadruple None Triple Single Single None
Mo"'
Moi" Mo:" Mo;
Mo;'
Colour Red Pale yellow Green Red Yellow Colourless
The three most often used 'lead in' compounds for the chemistry described are sodium molybdate, Na2[Mo04]-2H20,molybdenum hexacarbonyl, [Mo(CO),], and potassium hexachloromolybdate, K3[MoC16]. Synthesis of 11 and I11 state complexes generally requires rigorous 02-free techniques, using a range of methods from those involving Schlenk apparatus, to the use of N2 or Ar gas, syringes, Teflon tubing and/or stainless steel needles, and rubber seals. In some cases solutions of IV and V state complexes must also be stored O2 free. Perchlorate cannot be used with I1 and I11 aqua ions, and also appears to oxidize some trimeric MoIVions on leaving overnight. Instead weakly coordinating, redox inactive and strongly acidic trifluoromethanesulfonic acid, CF3S03H (abbreviated HTFMS or triflate), or p-toluenesulfonic acid, C6H4(Me)S03H(abbreviated HPTS), finds wide usage. Methanesulfonic acid, MeS03H, has also been used.
36.1.2.2
Oxidation State 11
The aqueous chemistry of the I1 state is dominated by dimeric complexes. The latter constitute the largest group of compounds of any element containing quadrupule bonds, a subject extensively covered in the Cotton and Walton text.3 Soon after the recognition of the quadruple bond in [Re2C1 in 1964 the structure of [Mo2(02CMe),], Figure 1, was published The [M0zC18l4- complex, Figure 2, is obtained by treating (Mo-Mo distance 2.09 [ M O ~ ( O ~ C M in ~ ) ~aqueous ] KCl solution with 12MHCl at 0°C when crystals of &[MozC18]-2H~0 are obtained. Similarly the bromo analogue (NH4)4[M~zBr8]has been isolated. There are no bridging ligands and in both cases the Mo-Mo distance remains short (2.14 A), consistent with retention of a quadruple metal-metal bond.5 For comparison, typical M-M distances for quadruply bonded dimeric d4 complexes are 2.21 A (for [W2(02CCF3)4]),6 2.19 A (for [ T C ~ ( O ~ C C M ~ & C and ~ ~2.2 ])A ~ (for K2[Re2C18].2H20).374b The d4-d4 technetium complex is the only Tc:" example for which there is structural information, and the ease with which [T%Cl8I3- rather than [Tc2Cl8I2- is obtained is at present puzzling. The range of metal-metal distances in Moi' complexes (2.04-2.18 A) is much smaller than that observed for Cr:' (1.83-2.54 A).3 Examples of monomeric Mol' complexes are much less common. It has been demonstrated that the [Mo(CN),]'- ion has a pentagonal bipyramidal structure. Other examples are nitrile (i. e. isocyanide) complexes [Mo(CNR),]'+, which has a capped trigonal prismatic structure, and the diarsine complex [Mo(diar~)~X ~]. The acetate complex [ M O ~ ( O ~ C Mis~obtained )~] by heating [Mo(CO)~]with glacial acetic acid or a mixture of the acid and its anhydride.8 Yields are low (15-20%), but can be improved (-80%) when diglyme is used as solvent.' The low yields are the result of the formation of trinuclear Mo'" complexes of the type [Mo~X,(O,CM~),(H~O)~]~~, where the Mo3X2 unit is a trigonal bipyramid with capping groups X either 0 or CMe (alkylidene).
J!
'-
Aqueous Solution Chemistry
1231
t
b Figure 1 The structure of the MoY complex [Mo,(O,CM~),]~~
Figwe 2 The structure of the Mo:' complex [Mo2C1,l4-
A listing of structural data of compounds containing M M oO quadruple bonds with no bridging ligands (16 examples), with carboxylato bridges (23 examples), and a number of other compounds (25) up to 3981 has appeared.* A further listing of 67 structures from 1981 to 1984 has been reported." The chemistry of this type of compound is therefore well established. As far as the bonding between two d4 metals is concerned,2 the eight electrons fill four bonding orbitals in a configuration u2n4d2.The order of energies of the molecular orbitals beginning with the most stable is (J < a 6 , the p-acetato Moi'l-edta complex gives a maroon-red Mol" V-edta complex.h2 The crystal structure has indicated a tetramer, &[M0~0~(0H)~(edta)~].16H~O (4), in which the four Moatoms are in the same plane.63 The number of oxidizing equivalents per Mo has been determined by titration, and indicates an average oxidation state of 3.5. The complex is diamagnetic and all bridging groups are symmetrically disposed (equal bond lengths) to adjacent Mo atoms. The short M-Mo distance of 2.41 8, bridged by the p-oxo-p-hydroxo ligands is consistent with metal-metal bonding. These two bridges can be distinguished from each other, the p o x 0 M-0 bonds (1.93 A) being shorter that the p-hydroxo M e 0 bonds (2.08 A). The single p-oxo bridges have Mo-0 bond lengths of 1.90 A and an MoOMo angle of 164". It is possible to comment on the mechanism of formation. The products of the reaction of azide are both the M o " ~ , ~tetramer " , ~ ,and ~ MoY dimer. Quantitative formation of N2 and NH3 is observed, consistent with equations (13) and (14). A maximum yield (37%) of Mo"I,III,IV,IV is observed for a 1: 1ratio of reactants. Competition between Mo;" and N; for an intermediate is implied. Since azide is capable of a two-equivalent change we tentatively suggest that this intermediate is an M o F dimer. Reaction Scheme 2 is proposed. No reaction is observed at pH 4.7 (the pK, for HN3 is 4.68) in the presence of free acetate, but reaction does occur slowly when no acetate is present. Displacement of acetate would therefore seem to favour reaction. As the pH is increased to 8 (even in the presence of acetate) the rate increases markedly. This corresponds to acid dissociation of one of the p-hydroxo ligands determined spectrophotometrically as pK, 7.73 at 0 "C (equation 15). Furthermore no reaction of the bis(thiocyanat0) Mo;" complex is observed with NP at pH 8. It is concluded that coordination of Ny precedes reaction.
/OH Mo \Mo \OH/
0 M 'o
' 'OH'
e Mo
f H'
Controlled O2oxidation of the MO"'~"'~'~~'~ tetramer yields a product which titrates for Mo'" and from an incomplete X-ray crystal study appears to be basically the same tetrameric structure, except that all the bridging ligands are now p-oxo. This Mo"' tetramer is also an intense red colour, Figure 10. The reduction potential for the couple involving the two tetramers is +0.07 V. Rapid reactions of both tetramers with O2 are of interest in that there is no site for binding unless the coordination number increases. Mild oxidants such as [Co(NH3)&1I2+ react rapidly (tin < 1min) with both tetramers to give the MoY dimer. As already mentioned under Mo" the reaction of &[M%Cls] with 2 M H3P04 in air for 24 h yields a dimeric Mo"' product, The structures of CS~[MO~(HPO~)~(HZO)Z], which has axial HzO molecules, and (pyH)3[Mo;3(HP04)4C1],in which there are infinite chains with shared axial
Aqueous Solution Chemistry
3 241
X (nm)
Figure 10 UV-visible spectra ( E values per tetramer) for the M O " ' ~ " ' ~complex ~ ~ ~ ' ~[Mo 04(OH)2(edta)2]4- (-). and the related Moiv complex, [Mo,0,(edta),14- (- - -) in H,0"
chloride ligands, have been reported (Figure ll)." The Mo-Mo distance of 2.23A is consistent with a triple bond (a27c4).Interestingly the positions of the protons can be identified as attached to outer 0 atoms of the p-HP04 ligands, outer P-0 distances being 1.48 A as compared to 1.54 8, for P-OH.
Figure 11 The structure of the Mo;" complex [MO~(HPO,),(C~)]~-, where the chlorides are bridging"
Binuclear complexes of Mol" have been prepared from paramagnetic mononuclear hdogeno complexes of the type LMoX3, where L = 1,4,7-triazacyclononane,sometimes referred to as 9-aneN3.64 Hydrolysis of LMoC13 in aqueous solution containing acetate or HCO; yields [LMO(OH)~(O~CM~)M~L]I~-H~O and [LMo(OH)~(CO~)MOL 12.H20. The acetato-bridged complex has been shown to have an Mo-Mo distance of 2.41 , which compares with 2.43 8, for [M0,(0H)~(O,CMe)(edta)].~~ On treating the p-carbonato complex with acids, products as indicated in Scheme 3 are obtained. A feature of the structure of the dichloro complex (Figure 12) is the anti (sometimes referred to as trans) dichloro arrangement. The MO,(OH)~ring is planar and the Mo-Mo distance 2.50A. On oxidation, e.g. with Cloy, the corresponding red-purple Mo; complex with anti terminal oxo ligands (and planar MozOz ring) has been observed for the first time, and the structure verified.= Cleavage of a bridge in triply bridged complexes to give nnri rather than syn structures has been observed previously with [(NH3)3Co(OH)3Co(NH3)3]3*.66 A comparison of different metal-metal bond distances is made in Figure 13, indicating the influence of M-M bonding.@ All di-p-hydroxo Moii' complexes are diamagnetic. Kinetic studies on the reaction of [L(H20)Mo(OH)2Mo(H20)L]4+with C1- (0.11 M-'s - I at 25 "C) and C10; (a redox process) (2.0 X M-' s-l at 21 "C) give rate constants independent of [H+] in the range 0.05-1.0M. It is proposed that the C10; reaction is substitution controlled. This reaction proceeds Moil'-* anfi-Moz (red-purple) +syn-MoY (yellow). No intermediates were identified. The reaction with NO; has also been studied (again strictly air-free), when quantitative formation of the red-purple unri-[LMo~O~]*+ complex and nitrite is observed (equation 16).67
8,
Mot1+ 2NO;
-
MoY + 2NO;
(16)
Molybdenum
1242
L
H
J
0
MO;" (green)
[9]aneN3
n H
144
Mo:" (green)
MO?' (green)
MoY (yellow)
Mor (red) Scheme 3 C
C
c C
Figure 12 The structure of the Mo;" complex [MO~L~CI,(OH),]~~, whew L triazacycIononanea*
IS
the [9]ancN, ligand 1,4,7-
The formation of NO; was demonstrated by carrying out the reaction in acid solution in the presence of sulfanilic acid and a-naphthylamine, neither of which reacts with the Mo complexes. The formation of the known red azo dye is diagnostic of NO; formation. The stoichiometry was confirmed by determining the N2 evolved (equation 17). H,NS03H + NO;
HSO;
+ N2+ H 2 0
(17)
At 25°C the kinetics give a rate law (equation 18), with k =O.lOM-ls-', and activation parameters AH# = 69 kJ mol-' and AS# = -33 J mol-' K-l, I = 0.50 M (CF3S0,Na). By using 90% '*O-enriched NO; it was demonstrated by IR that the terminal oxo ligands of the antz-Mo~O$+product were labelled and that reaction is therefore by 0 atom transfer. The increase in k with [H+] is possibly related to the need to transfer an 0 atom as in C10; reactions. -d[Mo:'']/dt
= d[Moy]/dt = k[Mo:"][NO;]
(18)
A less extensive study of the reaction of [ M O ( H ~ O ) ~with ] ~ +NO: has also been reported.68 As in the above study rate constants are consistent with a substitution-controlled process. By adjusting conditions, aqueous HCl solutions of Na2[Mo0,] can be reduced electrolytically to give [Mo2C19I3- rather than [ M o C ~ ~ ]An ~ -alternative ,~~ procedure" yielding a purer product is by electrolytic oxidation of [Mo2C18J4-.The W"' complex [W2C19]3- is known, but [WClSl3is more difficult to prepare. The [M2X9I3-(X- = CI-,Br-) complexes have three bridging halide ions in a cofacial bioctahedral arrangement, which is also present in [Cr2C19]3-.
Aqueous Solution Chemistry
1243
H
A
cw r e j r H
planar
C
D
0 H Mo"'-Mo"'
planar
2 5 0 8, (triple M-M)
E
planar MoV-Mo''
2.56 8, (single M-M)
F lU2.6" 0 1.98 77.6' M 0 o e ? O
planar
MoV-MoV 2.56
A
(single M-M)
G
Cr"'-Cr"' -re
M o ~ ' - M o ~ ' 3.03 8,
H
2.90
A
13 A comparison of M-M
Mo"'-Mo"I
Me 2.47 8, (triple M-M)
distances and bond angles in bridged binuclear complexesa"
However, the strength of interaction between two d3 ions increases markedly in the series, Cr, Mo, W. There is no M-M bond in the case of Cr, and the Cr atoms actually repel each other. The magnetic and spectroscopic properties are essentially those of two unperturbed d3ions. In the case of W2C12- the M-M interaction is very strong (2.45 with no unpaired electrons. The M-M distances in the case of [Mo2Cl9I3-(2.67 A) and [Mo2Br9I3-(2.78 A) indicate intermediate b e h a v i ~ u r In . ~ ~the latter a temperature dependent magnetism has been observed suggesting that the Mo-Mo interaction is weak enough to allow some unpairing of electrons. The reaction of [MO~(O~CM?)~] and RbCl or CsCl in deoxygenated 12MHC1 at -60°C gives green-yellow product^,'^ initially assigned formulae Rb3[MozCls] or Cs3[Mo2C18]. Until they were demonstrated to be diamagnetic they were thought to be mixed-valent 11,111 complexes. Tritium labelling experiments have demonstrated the presence of an H atom, and the complexes are therefore reformulated as [Moz&HI3- (X = C1- and Br-) molybdenum(II1) species. Consistent with M-H-Mo hydrido bridging, IR spectroscopy gives v(M*H-Mo) at 1260 The structure has been verified crystallographically for the pyridinium salt, Figure 14.73
36.1.2.4
Oxidation State Iv
Souchay and colleagues were the first to report a red Mow aqua ion.74 Since that time preparative procedures have changed very little and generally involve heating Mow as Na2[Mo04].2H20, or Mov as the aqua dimer MozOq+,with KswoCb] in 2 M HFTS for 1-2 h at 80-90°C.2 The product can be stored in this form and purified by Dowex cation-exchange chromatography as required. Elution can be carried .. out with 2 M acid, but in perchloric acid COCI-NN*
Molybdenum
1244
Figure 14 The structure of the Mo;" complex [Mo,C~,(CI,)(H)]~-~~
solutions accelerated conversion to yellow Mo,V occurs overnight. This process has not been studied and may have features similar to the reaction of [Mo H20)6]3+with C10;. (Figure 15), the thioX-Ray crystal structures of the oxalate, ~z[Mo304(~04)3(HzO)3]~H207 cyanate, [Mefi]4[Mo304(NCS)8(HzO)]~6 and the N-methyliminatodiacetate complex, Na2[Mo304((02CCH2)2NMe)3]~7Hz0,'7 have indicated trinuclear structures. The M0304 core has an incomplete cube structure with one corner unoccupied. In the case of the edta complex Na4[Mo304)2(edta)3].4Hz0, there are two trimeric Mo304 units per molecule, and the three edta ligands coordinate via two carboxylates and the imine group to an Mo of each Mo,Ot' cluster, givin three brid ing edta ligands. This results in a prolate spheroid shape.78In much ~ ~ Sreported, H~O but full structural earlier workk the Mol' complex K ~ M o ~ O ~ ( C Z O ~ )was implications were not realized at the time.
5
oa
-0
Figure 15 The structure of the Mo:" complex [Mo304(~0,),(H,0),]*"
The spectrum of the aqua Mo30a+, peak at 505 nm (E = 63 M-' cm-' per Mo), is shown in Figure 16. Substituents often have little effect, and equilibration with 4MHCl followed by ion-exchange chromatography provides evidence for at least two chloro products, with visible spectra difficult to distinguish from that of the aqua ion. The d o u r originates therefore from the central Mo30i+ core. With thiocyanate, on the other hand, changes in absorbance at 340 nm are observed, and substitution processes can be monitored. There is no evidence in the aqueous solution chemistry for the existence of stable monomer and dimer forms, which are transients only in redox processes. Thus [ M o ( H ~ O ) ~ is ] ~oxidized + to the MoY aqua dimer, and with a deficiency of oxidant unreacted [MO(H,O),]~+and Mo: are the only identifiable Mo species present. Reports from a number of groups in the period 1973-1980 referring to the red aqua Mo'" ion as dimeric or monomeric are now known to be incorrect. The Mo30i+ core has a coordination number of nine. Evidence for retention of the Mo30if unit in the aqueous solution has come from "0 studies, also directed at measuring the rate of The precipitating agent used was tetramethylammonium thiocyanoxygen-atom exchange.-' ate which gives [Mefi]5[Mo304(NCS)9] within 2min at 0°C. With l8O enrichment of the solvent the amount of label transferred to the solid after minimal contact times was lo-fold) excess, a statistically corrected rate constant is obtained assuming three equivalent sites on the Mo302+ion, and equilibration rate constants k,can be expressed as in equation (19). With [MO~O.+(H~O)~]~+ in excess the reaction is constramed by the low [NCS-1 to form only Mo~O,(NCS)~+,and the rate law in equation (20) applies.
1246
Molybdenum
A comparison was made using the different anions, p-toluenesulfonate, trifluoromethanesulfonate and perchlorate ( I = 2.0 M). At 25 "C forward and backward rate constants at [H+]= 2.0 M are kl = 0.78, 1.02, 2.13 M-l s-l and l b k - l = 1.57, 1.94, 2.18 s-l, for these three anions respectively. Rate constants kl and k - l have [H+] dependences of the kind a + b[H+]-l, with b dominant for [H+] = 0.8-2.0 M. In common with other conjugate-base pathways, b can be assigned an Id mechanism. The large contribution from such a path for a low electron population metal (here d2) is believed to be in some part due to the steric crowding around each MoxVresulting from Mo--Mo bonding. Thus with the inclusion of Mo-Mo bonding each Mo is eight-coordinate. Solutions containing higher complexes obtained M) with NCS- (5 x M) give, on ion exchange, bands by equilibrating Mo30:+ (1.3 x that in order of elution analyze (Mo and NCS-) for M0304(Na)$+, Mo~O~(NCS)~' and Mo30i+. A fraction not held on the column at [H+] = 0 . 3 M is believed to contain Mo3O4(NCS): as well as free NCS-. Two kinetic stages are observed in the aquation of Mo3O4(NCS):+ in 2 MHPTS, which are assigned to k-l (here 1.7 x s-'), and k-2 (0.24 X 10-3 s-1). The red aqua Mo30j' ion is reduced by amalgamated Zn under air-free conditions to a green Mo$ aqua ion.84The spectrum, Figure 16, is different from that of the green Moi" aqua ion previously discussed. It is held more strongly on a Dowex column than Mo3O:+, which suggests that it may have a fully protonated core Mo3(OH):+. This is an interesting point which requires O )be ~ ] 'similarly reduced.= Both confirmation. The oxalato complex [ M o ~ O ~ ( C ~ O ~ ) ~ ( H ~can nine-electron Mo;" complexes exhibit EPR spectra. On labelling the core with "0 atoms the complete redox cycle can be accomplished with as high as 97% retention of the labelling.= In one experiment the MoilTion was allowed to exchange with solvent for two months at room temperature, and no (f3%) exchange was detected.g1 Rapid (7MHCI. The closely related [MoOCLJ and [MoOCL&O]- ions are also known, and chloro and iodo analogues
Molybdenum
1250 0
11 /
-MO-
'I have been obtained.'" Other preparative routes in addition to the reduction of [MoO4I2include the oxidation of [Mq(O,CMe),] in aqueous HX (X-= halide), or by dissolving M o a 5 in an appropriate acid. In aqueous solution, at lower HX concentrations, orange-yellow M%OZ+ is obtained. Elution of Mo,O$+ from Dowex cation-exchange columns with 0.52.0 M HC104 gives the orange-yellow aqua ion [Moz04(H20)$'+. Monomeric oxo-Mo" complexes have a characteristic EPR spectrum with a strong centra1 band, and EPR provides the most direct and definitive means of distinguishing this form."' The 4d' electron is believed to be located in an orbital in the plane perpendicular to the M-0 bond. The bond trans to the terminal oxo ligand is relatively weak and five- as well as six-coordinate complexes are known. The structure of K2[MoOC15]gives four M d l bonds at 2.40 A (mean) and one trum ,to the oxo at 2.95 A.'" The difference is similar to that for the V02' aqua ion (0.18A) in hydrated vanadyl sulfate. A bigger trans effect is expected in the case of the nitrido complex tMoN(C1) ]3-.113 The length of the M-0 bond is shorter in five-coordinate complexes, and is 1.61 in ( A s P ~ ~ ) [ M o O C LOn ] . ~addition ~~ of a sixth ligand as in K2[MoOC15] the M& bond is -1.47A. The presence of the terminal oxo results in doming as in (AsPh4)[MoOC14(HzO)](Figure 21),115 and this is also a feature of Mo204 type structures with an O,MoOb angle between terminal and bridging oxo ligands of -105". The stabilization of [MoO(SR)~]-type complexes using small pe tides containing cysteine residues, and with bulky aromatic thiulate ligands has been reported. ?16
Figure 21 The structure of the Mo" complex [MOOC~(H~O)]-"~
Typical UV-visible spectra of M%Oi+ complexes are shown in Figure 22. Crystal structures have been reported for the compounds (pyH)z[Mo204C14(H20)2].Hz0, (pyH)3[Mo~04(HC02)(NCS)4J.2H20 and the p-acetato analogue (Figures 23 and 24).'12 The complexes are diama netic and M+Mo distances are short (2.56-2.58A8, consistent with metal-metal b~nding!'~ Structure data for [ M o ~ O ~ ( N C S ) ~ have ] ~ - indicated a difference in the cis and trans Mo-N bonds of O.15A.'l8 However, for the oxalate complex [ M O ~ O ~ ( G O ~ ) ~ ( H ~it Ohas > ~been ] ~ - demonstrated , that the waters are coordinated cis to each terminal oxo, with the oxalate cis and trans to the same oxo. No difference in the oxalate M o - 0 distances is detected in this and the corresponding di-p-sulfido complex [ M O ~ O ~ S ~ ( G O ~ ) ~ ( (14) H ~ .l19 O)~ In] a~ similar bipyridyl complex (E) the Mo-N bond length cram to the terminal oxo is 2.32& while that in the cis position is 2.25A,12* giving an intermediate elongation effect. The absence of a trans influence in the case of oxalate suggests it has a low trans susceptibility as compared with bipyridyl and other ligands. A more extensive list of compounds would be of interest to explain this effect. It has been noted that the 13CN M R spectrum of [ M o ~ O ~ ( C ~ O ~ ) ~ ( H in ~ Owater ) ~ ] " at - pH 4 gives only one sharp peak at 168p. .m. at 34 "C,which suggests that in solution there is some rapid exchange process taking place. L 1 8
Aqueous Solution Chemistry
1251
A (nm)
r (
Figure 22 UV-visible s ctra E values per dimer) of the Mov aqua dimer [MO~O,(H,O),]~'in 0.5 M HClO (-), and of [Mo,O,(edta)] (- - -) at pH 5 (acetate buffer), and (E per Mo) for the polymeric MoV ion (- . -)Bbv13a 0
,
0
Fignre 23 The structure of the Mo; complex [Mo,O,C~~(H,O),]~-'12
Figme 24 The structure of the MoY complex [Mo~O,(HCO,)(NCS),]~-11*
04)
Asin the case of the Moii'-edta complex the edta ligand in [Mo2O,(edta)]*- is coordinated to the binuclear core in a 'basket' like conformation. Studies on complexes of MqOq+ having optically active amine-carboxylate ligands such as propylenediaminetetraacetate have been reported. 12' Oxidation of the Mo:i' complex [L(H20)Mo(OH)2Mo(H20)(L)]4+, where L is the cyclic triamine 1,4,7-triazacyclononane ligand, with (for example) C10: gives the red-purple coloured anti-[Mo204L] complex (Ama 525 nm) , which undergoes irreversible acid-catalyzed isomerization to the yellow syn-[Mo204L2]complex (Figure 25).& If the 1,5,9-triazacyclododecaneligand L' is used the unti-[MozOJ4] form is likewise obtained, and in contrast to the behaviour observed with L, is stable for at least two days in 1M HClO, at 25 0C.65Base-catalyzed isomerization is observed in both cases. Structures of the anti forms have been determined. For
--
1252
Molybdenum
the L complex the short Mo-Mo distance (2.55A) is essentially the same as that for the yellow isomer (2.56 A).'= The diamagnetism in both cases is consistent with metal-metal bonding. The four-membered Moz02ring in the anti form is planar whereas for the syn isomer it is puckered with an OtMoOb angle of 109". Kinetic studies on the acid-catalyzed isomerization of the L complex (red-purple- yellow) give a dependence (equation 22), which is consistent with H+-induced reaction (equations 23-25).
syn-[LM0,0,H]~+
ocid cotol zed isomerizoYion
syn-LMqO,]
+H+'
(25)
-
ye1low
Figure 25 The structures and isomerization of nnti- to syn-[Mo,0,LJ2+, L = 1,4,7-triaza~yclononane~~
A mechanism involving protonation at a terminal oxo ligand followed by addition of a trans H20 has been proposed.64b At 21 "CK, = 2.3 M and k = 2.4 X M-l s-'. Theoretical calculations on s 0 - and anti-EM~zS~(S~CzH~)~l~indicate that the M-Mo bond is stronger in the syn isomer.' Y At present these are the only examples of the anti form. The preparations of di-p-sulfido and p-oxo-p-sulfido complexes of Mov with edta, cysteine and cysteine-ester ligands have been described.124 Formulae with cysteine coordinated are shown in (16)and (17). To prepare (16)hydrogen sulfide is bubbled slowly through a solution of Na2Mo04-2H20in HzO for -2 h and after removal of excess H,S and adjustment of the pH, the appropriate ligand is added. To prepare (17) H2S is bubbled slowly through a solution of MoC15 in 3 M HC1 for 2 h. Both types of complex are an orange-yellow colour. The ligands can be removed by addition of acid, e.g. 1-2 M HC1, followed by gel filtration chromatography to give the aqua ion analogues of [ M O ~ O ~ ( H ~ O Absorption ) ~ ] ~ + . bands in the UV-visible for Mo2O2SZf (277, 311, 340 and 463 nm) are at lower wavelengths than the corresponding bands for M@O$+ (280, 324, 365 and 493nm). The edta and cysteine complexes undergo electrochemical reduction in a single four-electron step to Mo"' dimers in aqueous (buffered solutions. Although the ease of reduction and electrochemical reversibility of the Mo~/Mo$' couples increase with insertion of Sz- into the bridging positions, the MolI1 dimers become increasingly unstable for the S2--bridged complexes. With 1,l-dithiolate ligands such as N,N-diethyldithiocarbamate and diisopropyldithiophosphinate bridging and terminal oxo positions of the MozO:* structure have been replaced to yield all the products from Mo204(dtc)2to MozS4(dt~)2.~~ Using IR spectroscopy and "0 for l 6 0 replacement, it has been demonstrated that substitution of 0'- by S2- at the bridging positions is fairly facile. Much more extreme conditions using P4Sl0 ?ofor example are required to effect terminal oxo substitution.
1
Aqueous Solution Chemistry
1253
The second main category of dimeric MoV complexes having a single p-oxo bridge can also give syn and anti (rotamer) forms. Structures of ~ ~ ~ - [ M O ~ O ~ ( N C S ) ~and ( QantiO~)~]~[ M O ~ O ~ ( N C S ) ~ ( H C Ohave ~ ) ~ ]been ~ - determined (Figures 25 and 27), and indicate a linear Mo-0-Mo bridge with M-Ob distances 1.85 and 1.86 A, and Mo-0, of 1.68 and 1.67 A respectively.'12 Other examples of this type are with bidentate thio hos hate in [Mo203{(S2POEt)2>4]and [Mo203(LL)4], where LL represents 0-thiopyridine.' Examples of the syn structure (18) are with bidentate sulfur-donor ligands in [Mo203(S2COEt),] and [ M o ~ O ~ ( S ~ C N PIn ~ )the ~ ] .first of these (0-ethyldithiocarbonate)the M-Mo bond is 3.72 A, and the Mo-0, bond is 1.65 All known y-oxo-bridged dimers are diamagnetic or exhibit only a small practically temperature independent paramagnetism.
Y P
n 0
U
Engore 26 The Structure of the Mo: complex [Mo~O~(NCS),(C@,),]~-'12
Figure 27 The structure of the Mo; complex (MO~O~(NCS)~(HCO~),]~-
0
0
II/
-M*O-M*
I'
II/
I '
(18) 1
It is possibIe to use IR spectroscopy as a means of distinguishing between Mo&+ and Mo20$+ complexes.125No interconversion of these two forms has been studied, and no aqua ion having the Mo20Sf structure has yet been identified. The staggered conformation of terminal oxo ligands observed for MoV' in Mo,O$' is not observed for Mov. However, in the tetraphenylporphyrin complex [M o ~O~(T P Pthe )~]linear Mo-0-Mo structure is retained, and the terminal oxo ligands lie trans to the y-oxo group giving the opposed Ot--Mo-O~-MO--O, structure. In a recent comprehensive study of the coordination of polypyrazolylborate, HB(pz), , to MoV a number of complexes have been prepared.lW First by reacting HB(pz),MoC13 with Oz the mononuclear complex [HB(PZ)~MOOC~~] is obtained (Figure 28). A pair of geometric isomers, [ M o ~ O ~ C ~ ~ ( H ( B ~and Z ) &[Mo~O~(HB ] (P Z )~)~], can be isolated from the reaction of MoC& with HB(pz), in dilute HCl. By further reaction of the latter with MeOH the zigzag tetramer [Mo~O~(OM~)~(HOM~)~(HB~Z~)]~ is obtained (Figure 29). Crystal structures of all but the Mo2024+ complex have been determined. The M02O3 geometric forms provide an example of distortional isomerism with terminal M e 0 distances of 1.67 and 1.78A respectively. Both are however yellow-brown in colour, unusual for MozOf+,which is generally purple with an intense band at -525nm. The latter was thought to be independent of the ligands present, a belief which it seems has now to be modified.
Molybdenum
1254
Figure 28 The structure of the hydrotripyrazolylborate,HBpz;, MoV complex, [MOOCI,(HB~Z,)]*~~
Figure 29 Representation of the tetrameric zigzag chain structure as found in [Mo,(HBpz,),(O),(p OMe),(HOMe),]'30
Complexes reported as mixed-valence tetramers Mov,v7w,v', with formulae [Mo406C14(0Prn)6] and [M0408Cb(OEt)4]~-,1313132 have been reformulated as [MO4O6Cl4( OPr")4(HOPr)2] and [MO~O~C~~(OE~)~(HOE~)~]~-,~~~ indicating the importance in this chemistry of distinguishing between a coordinated alkoxide and coordinated alcohol. Both are accordingly tetrameric Mo", with close structure analogies to the above zigzag com lex. The latter occurs also as a central unit in the [ M O ~ O ~ ~ ( O complex.133 P ~ ' ) ~ ~ ] A cyclic Mo octamer has been r e ~ 0 r t e d . l ~ ~ One example of a distorted cubane (MOO), complex of Mo", [Mo40s(MezPOS),],having four terminal oxo ligands has been reported (Figure 30).'35
p.
9" 0
Figme 30 The structure of the Mo' tetramer core in [ M o ~ O ~ ( M ~ ~ P Oshowing S ) ~ ] the Mo,04 cubic arangement with terminal oxo ligands attached to each Mo' 135
The structure of an amorphous orange-brown polymeric Mov form obtained from aqueous solutions has not been determined.136It has a distinctive peak at 318 nm ( E 3300 M-l cm-' per Mo) and is stable in H 2 0 at pH-6. On adjustment of [H+] to 0.17-0.50, I = 0.50 M (H'/LiC104), aquation to MozOi+ proceeds by rapid protonation followed by a single rate-determining step. At 25 "C the protonation gives an acid dissociation constant K , = 0.13 M, and k = 5.5 x M-ls- for the kinetic step. The rate of "0 exchange of 02-in [MoOCl5l2- has been studied under conditions of high Cl- (-12M), and found to be slow, with a half-life of 1-2h." One of the reaction paths is believed to involve formation of [Mo0Cl4(H2O)]- as an intermediate. In dichloromethane the substitution of C1-, Br-, NO; and NOT for OPPh3 (which is trum to the oxo group) in
Aqueous Solution Chemistry
1255
[ M o O C ~ ~ ( O P P is~ ~consistent )~] with a dissociative (D)mechanism (equation 26). Rate constants obtained are kl=42.0, 41.6, 51.8 and 40.3s-l and k21k-l=27, 8, 47 and 31 re~pective1y.l~' Thus in a non-coordinating solvent kl is seen to be relatively small.
Measurement of H20 solvent "0 exchange into aqua Mo20i+ has been possible, using a procedure in which camplexing with edta was followed by precipitation of the [Pt(en)#+ salt.81 The two types of oxo ligand were shown to have quite different exchange rates. Using vibrational spectroscopy it was demonstrated that the fast exchanging 0 atoms are those in the terminal positions. At 0°C in 0.50MH+, these two Oatoms exchange with a half-life of -4 min. The other two 0 atoms have a half-life of -70 h at 40 "C in 0.5 M H+ and -3 years at 0 "C in 1M H+. The rate law for the latter exchange has an [H+I2dependence consistent with a mechanism involving Mo202 ring opening. An involvement of other ligand positions in the exchange of terminal oxo ligands is strongly implicated by the observation that [Mo2O4(edta)I2- gives lo3 times slower exchange of bridging 0 atoms.8F Kinetic studies on the 1:l complexing of NCS- for H20 on 6M~04(H20)6]2+,138 [ M o ~ O ~ ( C ~ O ~ ) ( H ~ O ) and ~ ] ' - di-p-sulfido [M%02S2(CZ04)2(H20)2]2- ' (the latter two at pH values of 3.0-4.5) have been reported. Formation rate constants (lop4k) are respectively 2.9, 0.50 and 21 M-'s-l at 25 "Cywith I at either 2.0 (first study) or 1.0 M in perchlorate. With pyridine as the incoming ligand the rate constant of 0.50 decreases to 0.30M-'s-'. The mechanistic interpretation has to take into account the observation that the oxalate of [ M o ~ O ~ ( G O ~ ) ~ ( Hmay ~ O )be ~ ]undergoing ~rapid r e ~ r a n g e m e n tTherefore .~~ substitution is not necessarily confined to a basal plane site, and as in the case of [VO(H20),]", rapid substitution at an axial position (equilibration process K) is believed to be followed by the slower exchange to a position in the plane of the complex. The not too dissimilar rate constants for [Mo,O,(H,O),]~+ and [ M o ~ O ~ ( ~ O , ) ~ ( H ~ O )are , ] "noted. With the di-p-sulfido complex a faster rate is observed and the p-sulfido ligands produce a labilizing effect. The kinetics of the multistage equilibration reaction of [ M O ~ O ~ ( H ~ Owith ) ~ ] edta ~ + to give [Mo204(edta)j2- have been studied, [Hf] = 0.5-2.0 M, and are observed to be much slower (tin < 1min). From kinetic studies the oxidation of aqua Mo20$+ with [IrC16]2-, [Fe(phenI3l3+ and [(NH3)5Co(02)Co(NH3)5]5+ proceed by competing pathways.'"''. 41 The first is oxidant independent, and is the result of the dimer undergoing rate-determining change to a singly bridged form, or to monomeric Mov, which then reacts rapidly with the oxidant. Interestingly, this path has an [H+]-' de endence suggesting involvement of a conjugate-base form of Mo2O$+, and gives k = 3 X lO-'M s-'. This contrasts with the [HCIl2 dependence (not at constant ionic strength) indicated for the exchange of bridging oxo A second rate law term, first order in oxidant as well as [Mo20$+],and representing direct attack of the oxidant on the dimer, is observed. This has a dependence on [H+]-l of the form a b[H+]-', where the contribution from b is presumably favoured because of the increase in extent of hydrolysis accompanying the MoV+MoV' change. The reactions with IT, NO, and [PtCkI2- as oxidants are very much slower. In contrast to the above study the [IrCl6I2- oxidation of Mo,04(edta)12- gives evidence for an Movpw intermediate.143In the presence of edta the MoV,I1 is presumably held together for a sufficiently long time for it to become kinetically significant. Thus on addition of the product [IrCl6I3- the reverse reaction is seen to be effective, and there is a decrease in the rate of reaction, which can be quantified in terms of the mechanism shown in equations (27)-(29).
+
Mo;
--
+ Ir"' e Mov," + Ir"' MoV*"'
M0"$1r'~
MoV+MoV1 Movl+KIf'
The reaction of Mo,O:' with NO; in tartrate buffer (which is capable of coordinating to the Mo;) at pH2.2-3.5 has also been studied.144 The kinetics give a half-order dependence on
Molybdenum
1256
[MozO$+] consistent with a mechanism shown in equations (30) and (31). From EPR measurements it has been confirmed that mononuclear MoV is present. MoY MoV+NO;
e
2MOV
products
Pulse radiolysis studies on [Moz04(edta)]'-, [Mo20z&(edta)]'-, [ M o ~ ~ ~ ( c ~ and s)~]~[Moz04(G04)2(H20)~]'- at pH-6, in which the MoY is reduced with e, and Zn+ to the MO'~,"mixed-valence form have been r e ~ 0 r t e d . lThe ~ ~ latter absorbs in the visible range ( E 300-500 M-' cm-' per dimer). In the absence of O2 the decay is relatively slow (tlR-510 s), and does not result in M o ' ~production. With O2 present there is rapid (-lo8 M-'s-') oxidation back to Mo:. Inner-sphere paths are observed in the C?' reduction of 10-4-10-3M solutions of Mo?, ~ ] ~25+ "C, , I = 2.0 M (NaPTS).'& In [ M O ~ O ~ ( H ~ O )and ~]~+ of, MoiV, [ M o ~ O ~ ( H ~ O ) at 1.9 M HPTS and with [Cr"'] in a greater than ten-fold excess of Mo:, the reaction proceeds via a grey-green Cr-containing intermediate (-1 min), to give a product with the Mo:" spectrum (-24 h). The rate law for formation of the intermediate is of the form k1[C?+]'[Mo:][H+], with kl = 9.1 x lo3M-3 s-'. Decay of the intermediate is independent of C? and can be M-'s-' is believed to expressed as k2[intermediate][H+]. The rate constant k2 = 2.0 X correspond to a process involving loss of Cr"I. With a Cfi' :Mo; ratio of reactants of 2: 1 evidence for two intermediates is obtained (-1 min), one (green in colour) giving a Cr:Mo ratio of 1:1, and the other the same as that generated in the reaction of excess Cr"+ with Mo; giving a Cr:Mo ratio of 2 : l . Some 30% of the Mo remains as M o : . Over longer periods (-22 h) 60-70% overall conversion to Mo;" is observed, and 24-30% of the Mo is present as MoY. In separate experiments, with C P in a greater than ten-fold excess, MoiV is reduced to Mo'",m3'v in a two-stage process, complete within 1min, and then through to Mo:" (-40 min). Reduction of MoZ to either MoY' or Mo:" is observed therefore, depending on whether a single addition of excess CP' or successive additions of C?' are made.
36.1.2.6
Oxidation State VI
The solution chemistry of the Mow state has been an area of intense research activity for several decades. It is dominated by the isopolymolybdate and heteropolymolybdate forms,147 brief mention of which is included here. Such studies remain an active area of research with applications in the area of industrial catalysis.' More than 65 elements from all groups of the Periodic Table (except the rare gases) are implicated as heteroatoms in such structure^.'^^ In addition to the text by Pope,14' a review of isopolyanions has appeared.'& An overview on the hydrolysis of cations is also r e 1 e ~ a n t . lAt ~ ~pH > 7 Mov' exists as the monomeric tetrahedral [MoO4I2-ions. Polymers are generated by conversion of tetrahedral Mow to octahedral forms in which there is edge, corner and (occasionally) face sharing of coordinated 0'- ions between adjacent Mo atoms. In the absence of heteropolyanions protonation to give [HMoO,]-, more precisely [Mo03(OH)]- (pK, 3.47 in 1M HCl), is followed by a second protonation to give [HZMoO4](pK, 3.7 in 1M NaC1). The latter at least (possibly both) is believed to be octahedral and is sometimes referred to as [MO(OH)6] although other formulations have been suggested.1s0The incidence of protonation at pH 7 triggers polymerization. Polymeric forms play a dominant role in the chemistry of Mow from pH 7 down to 2.15' At pH 2 to 1(depending on the concentration) break down of polymers to give dimeric and monomeric octahedral forms occurs, where the latter are often referred to as [Mo(OH)~].The first protonation constant for [Mo(OH)~]has been determined.''lbSingly and doubly charged cationic species are present in such s o l ~ t i o n s , ' ~ ' ~ ,and ' ~ ~ formulae which have been suggested include ~ is - [ M oO~ (H~0)~]' and+[Mo(OH),(H~O)~]'+.The cis-dioxo structure is found in a number of mononuclear coordination complexes, including ci s -[M ~O~(E t ~d t c)~] (Et2dtc = N,N-diethyldithi~carbamate),'~~ ~is-[MoO~(tox)~] (tox = thiooxine or 8-mercaptoquinolate),154 h-cis[Moo2{(2R)-cy~OMe}~] (cysOMe = l-cysteine methyl ester)lS5and A-ci~-[Mo~(S)-penOMe)~] (penOMe = penicillamine methyl ester).lS6Two cis 0x0s are also resent in the oxalato p o x 0 complex [(HzO)(C204)02MoOMo02(~04)(H20)]2(Figure 731) ' . Facial trioxo coordination is on the other hand present in [Mo03dien] (dien = diethylenet~iamine)'~~ and the ethylenediaminetetraacetate complex [O3Mo(edta)MoO3I4-lS9 (Figure 32). Features of both structures are the Moo3 bond angles (-106") which are approaching those of a regular tetrahedron,
-
Aqueous Solution Chemhtry
1257
and the Mo-O(oxo) bond lengths of -1.74 A which indicate a bond order of two. The bond angles N-Mo-N (75") for the dien complex, and N-MP-0 (73") and 0 - M A (75') for the edta complex also suggest that the structures should be regarded as pseudotetrahedral. From Raman studiedm it has been concluded that [Mo03dien] is largely dissociated in water (equation 32), and reforms rapidly and quantitatively as a solid on addition of alcohol at pH -7. Thus although it is necessary to replace an oxo ligand on [MOO,]'- the reaction occurs rapidly. [MoO,dien] + H,O e MOO^]^- + dienHi+
(32)
F m e 31 The structure of the Mo"' complex [M~Os(C@4)2(H20)2]2lS7 n
Figure 32 The structure of the Mow complex [O,Mo(edta)M00,]~-
'*'
The polymeric forms obtained on decreasing the pH from 7 have been extensively investigated. For solutions with Mo"' concentrations greater than M, and pH in the range 3.0 to 5.5, the predominant form present is the heptamolybdate(V1) ion, [M*OMl6-, sometimes referred to as paramolybdate (equation 33). Commercially available ammonium molybdate, (NH..&[MO~O~~]-~H~O, is obtained by crystallization of a solution of Moo3 in aqueous NH3. In spite of numerous attempts, no intermediate has been unambiguously characterized in the aqueous conversion of [MoO4I2- to [ M o ~ O ~ ~ ]At ~ - - p. H < 4 the /3-[Mo8O%l4- octamolybdate form is obtained (equation 34). Representations of ( M o 7 0 ~ ] ~ and [ M O ~ O ~ ~are ] ~ shown in Figure 33. The existence of significant amounts of the [MogOZ6I4ion has been controversial, but recent Raman and X-ray scattering161 studies would seem to provide confirmation of its existence. Previously the interpretation of EMF data (pH 2-3) has been uncertain, because equally acceptable fits are given by a series of protonated [Mo7OaI6- forms, as by a mixture of protonated [M*OaI6- and [ M O ~ O ~ ~From ] ~ - .one set of stability constant determinations, Figure 34 has been obtained, indicating the distribution of different species. The &tetrahedral [Mo2O7r- ion has recently been identified in Mg[Mo207] and in the double salt K2[Mo207],KCl.162*1These structures are to be contrasted with the crystalline product (N&)2[Mo207], obtained from hot aqueous ammonium molybdate solution after some hours, which consists of infinite chains of tetrahedral Moo4 and octahedral Moo6 units.'"
7[Mo04I2-+ 8H' [MoTO~]~+ 4HzO ~[MOCI]~+ 12H+ e WO,O,,]~--+ 6H20
1258
Molybdenum
Figore 33 The structures of (a) heptarnolybdate, [ M o , ~ , , ] ~ ~and , (b) octamolybdate, [ M O ~ O ~ ](one ~ - MOO, octahedron hidden)
3
Fqure 34 Distribution of Mov' forms with pH for a solution of 5 X
M molybdate at 25 "C, Z = 1.0 M (NaCI), as calculated by A. Nagasawa and R. Iwata (unpublished work) from data in ref. 151
Solutions of Mow acidified to H+/[MoO4I2--l.8 contain one or more very large polymol2:date structures. Earlier measurements based on ultracentrifugation and EMF -studies, through to a recent structure report,'% are consistent with a formula [ M O & ~ I Z ( H ~ O ) ~ ~The ] ~ - .reaction can be summarized as in equation (35). There are two seven-coordinate MeV' atoms in this structure. In all isopoly and heteropoly structures the metal ion does not lie at the centre of its polyhedron, but is displaced towards the exterior of the structure and towards a vortex or edge of its own polyhedron. Structures appear to be governed by electrostatic and radius-ratio principles as observed for extended ionic lattices. The MoV' tetrahedral radius is 0.55 A (0.56 A for W), the octahedral radius 0.73 A (0.74 8, for W), and the radius of 02-is 1.40 A.167 36[MoO4I2-+ 64H'
e
Mo,,Oh
+ 32H,O
(35)
Important differences are observed in comparing the behaviour of MoV' and WW. Equilibria involving [MoO4I2- and polymolybdates in aqueous solution are established rapidly and are complete in a matter of minutes, whereas those for Wv' can take several weeks. The popnions of tungsten are made up of W 0 6 octahedra, but in other respects the behaviour of W solutions is quite different, and structurally the polyanions in one series do not have precise counterparts in the other. Known structures of isopolytungstates from aqueous solution include [w4016]8-? [W4019]~-,[W10032]4-,[H2W12042]10-and [ H ~ W , ~ O N ] ~ - . ~ ~ ' A number of other isopolymolybdate anions including [ M O ~ O ~C~ E] ~ - [,M O ~ O ~and ~]~[Mo5017HI3-can be stabilized in non-aqueous or mixed solvents.'@ The [Mo207]*-ion is also obtained fromacetonitrile solution. Tetrahedral coordination is retained in the latter .169 There
Aqueous Solution Chemistry
1259
are two distorted tetrahedra in a - [ M 0 ~ 0 ~ above ~ ] ~ - and below a crown of six octahedra formed by edge sharing. A tetrahedron spanning a ring of four edge- and face-shared octahedra is also believed to be present in [ M O ~ O ~ ~ H The ] ~ - a: . ~and ~ ' /3 structures can coexist in solution, and a cation dependent isomerization of a: into /3 has been ob~erved."~ Isopolymolybdates are generally colourless, and heteropoly forms can be coloured if another transition metal ion is present. Some other heteropolyanions can be coloured, for example that obtained by addition of ammonium molybdate to phosphoric acid to give ammonium 12-rnolybdophosphate, [ N H & [ P M O ~ ~ Owhich ~ ] , is yellow. The [ M O ~ O ~anion ~ ] ~ -is unique for an isopolyanion in being yellow (Amax 325 nm) and having a structure with a monocoordinated terminal oxo group rather than Mooz units. This ion is electrochemically reduced to the mixed valence [Mo6OIYl3-and [Mo6OI9l4-forms in DMF. Incorporation of the Mo' state in blue-red and ellow mixed-valence species formulated as [ M O ~ M O ~ ' O ~[ M ~ ]o~Y- M , o~O~~]~and [MO~ M O$O,~H]~respectively has also been reported. The reduction of addenda atoms in heteropolyanions results in the formation of heteropoly blues, and gives rise to a vast chemistry. The added electrons are delocalized according to varying timescales over certain atoms and/or regions of the structure. Because the surfaces of polyanions have some similarities to those of metal oxides, it is thought they may have some relevance in the area of heterogeneous catalysis. As a result a whole new area concerned with the synthesis and study of organic and organometallic derivatives of polyanions is being investigated. 173 A kinetic investigation of the l80 exchange between water and molybdate [MoO4I2- at pH > 11, [OH-] = 3 X 10-'-0.15 M,1 = 1.00 M (NaC104), has been carried 0 ~ t . l 'The ~ rate law (equation 36) indicates paths involving reaction with H20 and OH- (kl = 0.33 s-', kOH= 2.22M-'s-' at 25 "C). The corresponding kl values for [CrO4]+ (3.2 x s-') and [WO4I2(0.44 s-l) provide one of the few kinetic comparisons for Cr, Mo and W. The difference in rate constants is attributable to the larger enthalpy of activation in the case of chromate, reflecting tighter CrV1-O bonds. It has to be borne in mind that there are other contributing paths for chromate, the full rate law having been determined as equation (37). The molybdate exchange cannot readily be studied by l80labelling at pH 11, because protonation is effective and much faster rates are ~bserved."~
+
Rate = k,[MoO:-] k,,[MoO:-][OH-]
Rate = k,[CrO:-]
+ k,[HCrO;] + k,[H+][HCrO;j
f
k,[HCrO;]
(35)
+ kS[HCrO;l2
(37)
Fast monomer-dimer equilibria involving Mo"' in 0.2-0.3 M HC104, I = 3.0 M (LiC104), have been studied by the temperature-jump method. 176 A major pathway involves rapid dimerization of a monomer form, referred to as [HMo03]', which might alternatively be written [MOO~(OH)(H~O)~]+ or a related form. At 25 "C, I = 3.0 M [LiC104], rate constants are kf = 1.71 x IO5 M-l s-' , and k b = 3 . 2 x 103s-l. Information has been obtained by the stopped-flow method for the interconversion of tetrahedral chromate and dichromate ions.177 At pH2-4 hydrogen chromate, [HCr04]-, is specified as the reactant. From this and other similar studies on the substitution reactions of [HCr04]- there is evidence for a dissociative process. 17' Further information concerning precise formulae of MoV1 in acidic solutions is required to enable more extensive comparisons. Temperature-jump studies of MoV' solutions at pH 5.50-6.75 have been reported for the monomer-he tamer and heptarner-octamer interconversions which are complete within a few milliseconds.'P Molybdate as [HMo04]- (here assumed to be tetrahedral) reacts with bidentate 8-h droxyquinoline'80 and catechol,18' to give octahedral products in net 4- 6 conversions.'84: Table 4 summarizes the kinetic data, where different degrees of protonation of the ligating groups are indicated. For the reaction with edta a path assigned to the reaction of [H2edtaI2- with [HMoOJ gives a rate constant of 2.3 x lo5M-' s-l.lX2A reaction in which the tetrahedral geometry of [MOO,]'- is retained is the rapid stopped-flow equilibration of molybdate with [ C O ( N H ~ ) ~ ( H ~ O (pK, ) ] ~6.3) + at pH 7-8 (equation 38).lX3 [CO(NH&(H,O)]~'+ [MOO,]*-
e [Co(NH,),(OMoO,)]'
+ HZO
(38)
Substitution is at the Mo"' centre with retention of the Cu-0 bond. The first-order [H+]-dependent term is consistent with [HMo04]- as a reactant, and the [H+]-independent term is assigned to the reaction of [HMo04]- with [ C O ( N H ~ ) ~ O HAn ] ~ +interesting . situation arises in the reaction of [MoO4I2- with [Cr(edta)(H20)]- at ~ H 7 . 3 - 8 . 7 . l ~ As~ in the case of
1260
Molybdenum Table 4 Second-order Rate Constants for Reactions of [HMoOJ Ligand
ionic strength
Coordination change at MO~'
0.20 0.20 0.20 0.20
4 +,, 6 4+6 4+6 4+6
0.10 0.10 1 .o 1.o
4-6 4-6
H-oxine H-oxine-SO; oxineoxine-SO:H-catecholH,edta
[c0(N H ~ ) ~ H + ~ o ~ [Co(NH,)@Hl +
[Cr(edta)H,O]HS-
4+4 4+ 4 4+ 4 4+ 4
1 .o 0.50
at 25 "C
k (M-* s-l) 4.5 x 106 3.9 x 106
1.5 X 10'
4.0 x io7 1.9 x 108 2.3 x 3.2 x 6.6 x 3.1 x 1.3 X
105 io5 lo4 104 lo6
H-oxine = 8-hydroxyquinolinc. H-oxine-SO; = 8-hydroxyquinoline-5-sulfonic acid. H-catechol- = product of first acid dissociation of catechol.
[ C O ( N H ~ ) ~ ( H ~ Othere ) ] ~ + ,are two terms for complex formation in the rate law, one of which can be assigned to the reaction of [Cr(edta)(H20)]- with [MOO,]'- (rate constant 21 M-' s-'). The second, which is first order in [H+], corresponds to replacement of the H 2 0 of [Cr(edta)H20]- by [HMo04]- (3.1 X lo4M-l s-'). Whereas the first is believed to correspond to replacement of the H 2 0 (labilized by the presence of the carboxylate) at the Cr"', the latter as in other cases corresponds to substitution at the MeV' centre. There are proton ambiguities however because the first reaction could also be assigned to the reaction of [Cr(edta)OHI2( ~ K ~ 7 . 4with ) [HMo041-. In all these examples the interpretation requires that there are pathways in which there is protonation of [MOO,]'- prior to complexation. Protonation labilizes at least one oxo ligand and may itself induce a 4-6 change in coordination to give a more labile species. Consistent with this, rates are much more rapid than for H2"0 exchange with [MoO4I2-. Tetrahedral sulfido or thiolate complexes are known for a number of do transition metal ions including MeV'. lS5 Preparative procedures involve the reaction of solutions of oxyanions with H2S. In addition to [MoS4l2- the mixed oxo/sulfido complexes [Mo0S3l2-, [ M O O ~ S ~and ]~[MoO,SI2- are well characterized. Although the latter is difficult to obtain in a pure crystalline state, X-ray crystal structure information on (NH&[MoS4] ( M o - 4 = 2.17 A),1x6 and Cs2[MoOS3](Mo-S = 2.18 A; M e 0 = 1.79 A)187have been reported, and the M o - 0 distance is slightly longer than in Mov' corn lexes K2[Mo04(1.76 A), [Mo03(dien)] (1.74 A) and [ M o ~ O ~ ( C ~ O ~ ) ~ ( H(~M O )M ~ ],*1.69 ). Bridged Mo p-sulfido complexes do not appear to be formed and there are no counterparts in the polymeric (and heteropoly) species found from [Mo04d2-. Kinetic studies on the interconversion of [Mo0,S4-J2- forms have been carried out.' The 1:1 equilibration of H2S with [MoO4I2- can be expressed as in equation (39) *
K
[MoOJ-
'
+ H' + HS- e [Mo0,S12- + H@ kni
(39)
*lo
There are few quantitative studies with H2S as a reactant, and some difficulties were experienced in choosing conditions appropriate to the study. At pH 9.210.2 (0.25 M NH3/NH: buffer), I = 0.50 M (NaCl), plots of equilibration rate constants k,, against [MOO:-] are in accord with equation (40). keq= k,[MoO:-]+ kb
(40)
The formation constant kf is dependent on [H+] giving k = 4.0 X lo9M-2 s-l, and kb for the reverse reaction is 6.5 X s-'. Interpretation of k in terms of a reaction between [HMo04]and HS- is preferred rather than the alternative of [MOO,]'- with H S , and on this basis a second-order rate constant of 1.3 X 10' M-' s-l is obtained, of similar ma nitude to those listed in Table 4. Other formation rate constants for the different [MOO,S,-~] ions have to be left in the form of the experimental third-order rate constants since the relevant protonation constants are not known. For the rate law k[MoO,S%_,][HS-][H'], k values range from 4.0 x lo9 MP2s- for the reaction of [MoO4I2- to less than 1.6 x 106MT2s-' for [MoOS3I2-. Rate constants for aquation of [MoO3SI2-(6.5 x s-l), [MoOS3]'- (-5 x lO-'s-l) and
f-
Aqueous Solution Chemistry
1261
Figure 35 The structure of the Mo"' peroxo complex, [MO(~,).,]~'m
Figure 36 The structure of the Mow peroxo/hydroperoxo complex [(0,)20Mo(OOH),MoO(O~)2]z~ '%
[MoS4I2- (1.6 x s-l) indicate a trend to smaller values as more sulfide ligands are introduced. The presence of sulfide does not appear to give a labilizing effect. Different crystalline phases can be separated from aqueous solutions of potassium molybdate(V1) following reaction with varying amounts of hydrogen peroxide, pH 4-8. 189 Crystal structure determinations of [Zn(NH3)4][Mo(02)4],lW K2[0{ MOO(02)2(H20)}2]. 2H20,lW&[M040~2(0&]'~ and K ~ [ M o ~ 0 2 ~ .8H201Y3 ( 0 ~ ) ~ ] have for example been reported. In all cases the Of-is bound sidewa s to the metal, with 0-0 distances varying from 1.38 to 1.55 A, but generally around 1.48 which is the expected value for peroxide. The peroxo ligands in the dark red [Mo(02)4I2- compound (0-4 distance 1.55A) are positioned tetrahedrally about the Mow (Fi ure 35). A Cr" analogue [Cr(02)4]3-, having a similar configuration (0-0 distance 1.48 ), is known. An interesting variation is the occurrence of where the hydroperoxide as a bridging ligand in (pyH)z[(Oz)zOMo(OOH)2M00(02)2],194 bridge has a pendant -O(OH)arrangement (Figure 36). Only one other example of this structure is known in the Coin cornplex [(en)zCo(NHz,02H)Co(en)2]4+.19' The complex K2[MoO(02)2(C20,)] is monomeric, and to be regarded as five- or seven-coordinate depending whether the 0:- is assigned one or two coordination positions.
2,
x
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1264
Molybdenum
136. F. A. Armstrong and A . G. Sykes, Polyhedron, 1982,1, 109. 137. C. D. Garner, M. R. Hyde, F. E. Mabbs and V. I. Routledge, J. Chem. Soc., Dalton Trans., 1975, 1175; i ~ 1198. 138. (a) Y. Sasaki, R. S. Taylor and A. G. Sykes, J. Chem. SOC., Dalton Trans., 1975, 3%; (b) Y. Sasaki and A. G . Sykes, J. Chem. Soc., Dalton Trans., 1974, 1468. 139. F. A. Armstrong, T. Shibahara and A. G. Sykes, Znorg. Chem., 1978, 17, 189. 140. R. G. Cayley, R. S. Taylor, R. K. Wharton and A. G. Sykes, Inorg. Chem., 1977,16, 1377. 141. Y. Sasaki, Bull. Chem. SOC. Jpn., 1977, 50, 1939. 142. R. K. Murmann, Znorg. Chem., 1980,19, 1765. 143. R. K. Wharton, J. F. Ojo and A. G. Sykes, J . Chem. SOC., Dalton Trans., 1975, 1526. 144. E. P. Guyman and J. T. Spence, J . P h y . Chem., 1966,70, 1964. 145. I. D. Rush and B. H.Bielski, Znorg. Chem., 1985, 24, 3895. 146. E. F. Hills and A. G. Sykes, J. Chem. Soc., Dalton Trans., in press. 147. M. T. Pope, ‘Heteropoly and Isopoly Oxornetalates’, Springer-Verlag, Berlin, 1983, pp. 1-180. 148. K.-H. Tytko and 0.Glemser, A d v . Inorg. Chem. Radiochem., 1976,19,239-316. 149. C. F. Baes and R. E. Mesmer, ‘The Hydrolysis of Cations’, Wiley-Interscience, New York, 1976. 150. R. R. Vold and R. L. Vold, J. Magn. Reson., 1975, 19, 365. 151. (a) J. J. Cruywagen, Znorg. Chem., 1980, 19,552; (b) J. J. Cruywagen, J. B. B. Heyns and E. F. C. H. Rohwer, J. Znorg. Nucl. Chem., 1976,38,2033; 1978, 40, 53. 152. L. Krumenacker, Ann. Chim. (Paris), 1972, 7 , 425; Bull. SOC. Chim. Fr., 1974, 362, 2820. 153. J. M. Berg and K. 0. Hodgson, Znorg. Chem., 1980, 19, 2180. 154. K. Yamanouchi and J. H. Enemark, Inorg. Chem., 1979, 18, 1626. 155. T. Buchanan, M. Minelli, M. T. Ashby, T. J. King, J. H. Enemark and C. D. Garner, Znorg. Chem., 1984, 23, 495. 156. I. Buchanan, C. D. Garner and W. Clegg, J. Chem. SOC., Dalton Trans., 1984, 1333. 157. F. A. Cotton, S. M. Morehouse and J . S. Wood, Inorg. Chem., 1964,3, 1603. 158. F. A. Cotton and R. C. Elder, Inorg. Chem., 1964, 3, 397. 159. J . J . Park, M. D. Glick and J. L. Hoard, J. Am. Chem. Soc., 1969, 91, 301. 160. R. S. Taylor, P. Gans, P. F. Knowles and A. G. Sykes, J. Chem. Soc., Dalton Trans., 1972,24. 161. G. Johansson, L. Pettersson and N. Ingri, Acta Chem. Scand., Ser. A, 1979,33,305. 162. K. Stadnicka, J. Haber and R. Koziowski, Acta Crystallogr., Sect. B , 1977, 33, 3859. 163. H. J. Bechner, N. J. Brockmeyer and U. Prigge, J . Chem. Res. ( S ) , 1978, 117; (M),1978, 1670. 164. A. W. Armour, M. G. B. Drew and P. C. H. Mitchell, J. Chem. Soc., Dalton Trans., 1975, 1493. 165. (a) J. Aveston, E. W. h a c k e r and L. S. Johnson, Znorg. Chem., 1964,3, 735; (b) Y. Sasaki and L. G. Sillen, Ark. Kemi, 1968, 29, 253. 166. B. Krebs and I. Paulat-Boeschen, Acta Crystallogr., Sect. B, 1982,38, 1710. 167. Ref. 147, p. 18. 168. M. Filowitz, W. G. Klernperer and W. Shum,J . Am. Chem. SOE.,1978,100,2580. 169. V. W. Day, M. F. Fredrick, W. G. Klernperer and W. Shum, J. Am. Chem. & E . , 1977, 99, 6146. 170. B. Krebs and I. Paulat-Boschen, Acta Crysfallogr., Sect. B, 1976, 32, 1697. 171. W. G. Klemperer and W. J. Shum, J . Am. Chem. SOC.,1976,98, 8291. 172. S.Ostrowetsky, Bull. SOC. China. Fr., 1964, 1003. 173. Ref. 147, chapter 6, pp. 101-117. 174. H.von Felton, B. Wernli, H. Gamsjager and P. Baertschi, J. Chem. Soc., Dalton Trans., 1978, 496. 175. A. Okumura, M. Kitani, Y. Toyomi and N. Okazaki, Bull. Chem. SOC. Jpn., 1980, 53, 3143. 176. J. F. Ojo, R. S. Taylor and A. G. Sykes, J. Chem. Soc., Dalton Trans., 1975,500. 177. J. R. Pladziewicz and J. H. Espenson, Inorg. Chem., 171, 10, 634. 178. A. Haim, Znorg. Chem., 1972,11, 3147. 179. D. S. Honig and K. Kustin, Znorg. Chem., 1972, 11,65. 180. P. F. Knowles and H. Diebler, Trans. Faraday SOC., 1968, 64, 977. 181. K. Kustin and S.-T. Liu, J. Am. Chem. Soc., 1973, 95, 2487. 182. D. S. Honig and K. Kustin. J. A m . Chem. SOC.,1973,95, 5525. 183. R. S . Taylor, Inorg. Chem., 1977, 16, 116. 184. Y. Sulfab, R. S. Taylor and A. G. Sykes, Inorg. Chem., 1976,15,23#. 185. M. A. Harmer and A. G. Sykes, in ‘Molybdenum Chemistry of Biological Significance’,ed. W. E. Newton and S . Otsuka, Plenum, New York, 1980, p. 401. 186. H. Shafer , G. Shafer and A. Weiss, 2.Naturforsch., Teil B, 1964, 23, 76. 187. B. Kreks, A. Muller and E. Kindler, Z. Naturforsch., Teil B , 1970, 25, 222. 188. M. A. Harmer and A. G. Sykes, Znorg. Chem., 1980,19, 2881. 189. R. Stomberg and L. Tysberg, Acta Chem. Scand., 1969,23, 314. 1W. R. Stomberg, Acta Chem. Scand., 1969, 23, 2755. 191. R. Stomberg, Acta Chem. Scand., 1968, 22, 1076. 192. R. Stomberg and L. Trysberg, Acra Chem. Scand., 1970,24, 2678. 193. I. Larking and R. Stomberg, Acta C k m . Scand., 1972, 26, 3708. 194. A. Mitschler, J. M. Le Carpentier and R. Weiss, Chem. Commun., 1968, 1260. 195. U. Tbewalt and R. E. Marsh,J. Am. Chem. Soc., 1967,89,6364. 196. R. Stomberg, Acta Chem. Scund., 1970, 24,2024.
,
36.2 Molybdenum(O), Molybdenum(1) and Molybdenum(II) G. JEFFERY LEIGH and RAYMOND L. RICHARDS A FRC Unit of Nitrogen Fixation, University of Sussex, Brighton, UK 36.2.1 INTRODUCTION
1265
36.2.2 MOLYBDENUM(0) 36.2.2.1 Carbon Ligands 36.2.2.1.1 Carbon monoxide 36.2.2.1.2 Isocyanide ligands 36.2.2.2 Dinitrogen Complexes 36.2.2.3 Nitrogen Monoxide Complexes 36.2.2.3.1 Complexes of the type [MoX,(NO),] 36.2.2.3.2 Complexes of the type [MoX,(NOhL,] 36.2.2.3.3 Other dinitrosyl complexes 36.2.2.3.4 Mononitrosy1 complexes 36.2.2.4 Nitrogen Donor Ligands 36.2.2.5 Phosphorus Donor Ligands 36.2.2.6 Miscellaneous Compounds 36.2.2.6.1 Complexes with dithiolene ligands 36.2.2.6.2 Complexes of CO,, COS and SO,
1266 1266 1266 1266 1267 1271 1271 1212 1274 1275 1276 1277 1277 1217 1277
36.2.3 MOLYBDENUM(1) 36.2.3.1 Carbon Ligands 36.2.3.1.1 Carbon monoxide 36.2.3. I.2 Isocyanide ligands 36.2.3.2 Dinitrogen Complexes 36.2.3.3 Nitrogen Monoxide Complexes 36.2.3.4 Nitrogen Donor Ligands 36.2.3.5 Phosphorus Donor Ligands
1278 1278 1278 1278 1278 1279 1280 1280
36.2.4 MOLYBDENUM(I1) 36.2.4.1 Carbon Ligatd 36.2.4.1.1 Carbon monoxide 36.2.4.1.2 Isocyanide ligands 36.2.4.1.3 Cyanide ligands 36.2.4.2 Complexes of Group 15and Group 16 Ligands 36.2.4.2.1 Complexes of the type [MoX,L,] 36.2.4.2.2 Complexes of the type [MoH2P5] 36.2.4.2.3 Nitrosyl complexes 36.2.4.2.4 Diazenido complexes 36.2.4.3 Miscellaneous
1280 1280 1280 1283 1283 1283 1283 1285 1285 1292 1296
36.2.5 REFERENCES
1296
36.2.1 INTRODUCTION This survey of the lower oxidation states of molybdenum concentrates on those compounds which have a minimum of metal-carbon bonds. This is because there is an extensive literature of low-valent carbonyls, arene, alkene, alkyne and other organometallic complexes of molybdenum which has been thoroughly reviewed in ‘Comprehensive Organometallic Chemistry’. We therefore exclude compounds of alkenes, alkynes and higher organic entities. Nevertheless, reference is made to significant compounds which contain molybdenum-carbon bonds where it is judged appropriate. Coverage of complexes containing molybdenummolybdenum bonds is also kept to a minimum because these compounds are reviewed in Chapter 36.3. In certain cases the nature of the ligands involved makes definition of the metal oxidation state difficult, e.g. NO, N2R, dithiolenes, etc. For simplification, therefore, arbitrary assignment of ligand charge and therefore metal oxidation state has been made in the text, e.g. NO is
1265
1266
Molybdenum
defined as being NO' in its compounds and N2R is arbitrarily regarded as N'R-. Hydride is regarded as H-. Complexes of formal oxidation state below zero, e.g. the anion [MO(CO)~]'-, are not reviewed here because of the high number of Mo-C bonds, as explained above.
36.2.2
MOLYBDENUM(0)
36.2.2.1
Carbon Ligands
X221.1 Carbon monoxide
There is an extensive chemistry of Moo formally based upon substitution of [Mo(CO)~]by various ligands, without oxidation of the metal. These include hydride; nitrogen donors such as bipy, phen, NR3, RCN, NCS-; oxygen donors such as OH-, THF, RCO;; sulfur donors such as RzS, Me2PhPS; selenium and tellurium donors such as RzSe and R2Te; and halogens. In general compounds of the type [MO(CO)~L],[Mo(CO),L,], [Mo(CO)~L,] and [Mo(CO)~L,] result from direct substitution reactions using [Mo(CO)~],which are summarized in 'Comprehensive Organometallic Chemistry' (COMC) .la A recent significant development has been the application of 95Mo NMR to the study of these carbonyl compounds. Ranges of chemical shifts (p.p.m. relative to [MOO,]'-) which have been determined are: [Mo(CO),] -1858; [Mo(CO),L] (L = N, P, As or Sb donor) -1350 to -1870; [ M O ( C O ) ~-1100 ~ ] to -1600; [Mo(C0),L3] (here L may also be an arene or related ligand) -1100 to -1750; [Mo(CO),L,] -1400 to -1800.' Alternative routes to carbonyl compounds are also known, e.g. by direct reduction of a higher oxidation state compound in presence of CO or by displacement of a more labile ligand by CO. Reaction (1) illustrates these routes.'" Mixed substituted complexes can also be obtained, e.g. [MO(CO)~(PR~)~(CNM~)~].'~ [MoCl,(dppe),]
+ Mg 9cis-[Mo(CO),(dppe),]
T
1
Nz
truns-[Mo(N2),(dppe),]
A
truns-[Mo(CO),(dppe),]
+ 2N2
A series of monocarbonyl complexes is known of which the five-coordinate complex [M~(CO)(dppe)~] may be regarded as the parent. It has an almost square pyramidal structure and adds ligands L to give the series [Mo(CO)(L)(dppe),] (L = Nz,DMF, C2H4,4-ClC&CN, PhCN, C5H5N, Me2NCH0, CN-, SCN- or N3).3a Further analogues of this series, ~U~-[MO(CO){P~P(CH~CH~PP~~)~} (PMe2Ph)'] and [Mo(CO)(PF,),(dppe)], have also been ~ r e p a r e d .These ~ compounds are notable for their low values of v ( C 0 ) for mononuclear compounds, in the range 1690-1820 cm-'.
36.221.2 Isocyanide ligands Molybdenum carbonyl-isocyanide Complexes may be obtained as in Section 36.2.2.1.1. In most respects isocyanides behave in a very similar way to carbon monoxide, with which they are isoelectronic. As for the carbonyls, the isocyanide complexes may be regarded as derivatives of [Mo(CNR)~]and have been reviewed comprehensiveIy.l">b Isocyanides are rather more reactive than CO at low-valent molybdenum centres and may react with nucleophiles, e.g. R'NHz, to give carbene complexes,la e.g. equation (2), or with electrophiles to give carbyne comple xe ~, e.g. '~~ (3). [Mo(CNR),(NO)]+
+ R ' N H 2 = [Mo{C(NHR)(NHR')) (NO)(CNR),]+
truns-[Mo(CNMe),(dppe)J
*
truns-[Mo{CN(R)Me)(CNMe)(dppe),]t
(2)
(3)
R- = W + or Me'
Simple oxidation/reduction has also been studied for a wide range of isocyanide complexes. Again those complexes containing mainly molybdenum-carbon bonds have been reviewed in
MoZybdenurn(O), (I)and (11)
1267
COMC and here we mereIy give as an example the series trans-[Mo(CNR)~(dppe)~] (R = alkyi or aryl). They are not reducible under normal conditions, but they can be reversibly oxidized electro~hemically~~~ by one electron to give the series [M~(CNR)~(dppe)~l+ (see Section 36.2.3).42' Complexes with aryl substituents are less easily oxidized than those with alkyl substituents, as might be expected and these compounds are notable for their low Y(NC) values, 1850-1920 an-*.The X-ray structure of tran~-[Mo(CNR)~(dppe)~] confirms the trans geometry of the series.'
36.2.2.2
Dinitrogen Complexes
These complexes are reviewed in a variety of article^^.^ and are collected in Table 1. In general they are prepared by the following methods. (A) Reduction of a high-valent molybdenum species in presence of dinitrogen, e.g. reaction (4). M O ~ N Z 4pMe2ph
'
[MoCI,(THF),I
cis-[Mo(N2>2(PMeZPh)41
(4)
(B) Displacement of a ligand from a preformed complex by dinitrogen, free or coordinated, e.g. reactions (5) and (6).6-s trans-[Mo(Nz),(PPr;Ph),] [Mo&(Ph,PCH,CHzPPhz)2
+ Nz
+ N,
-
__*
mer-[Mo(N,),(PPcPh),l+
PPrYPh
(5)
~~u~~-[Mo(N,),(P~~PCH~CH~PP~,)~] + 2H2 (6)
(C) Displacement of dinitrogen by a ligand, e.g., reaction (71.' tram-[MO(NZ)I ( Ph*PCH2CH,PPh2)2] + RCN
-----*
-
tr~m-{Mo( Nz) (RCN)(Ph2PCHzCHZPPh2)zI
(7)
(D) Displacement of a coligand by a second coligand, e.g. reaction (8)." truns-[Mo(N,),(PPhzMe),]
+ GH,N
[MO(N,)~(PP~~M~),(C,H~N)] + PPh,Me
(8)
The binding of dinitrogen to the metal centres in these mononuclear complexes is end-on to the molybdenum as shown, for example, by the X-ray structures of trans[ M O ( N ~ ) ~ ( P ~ Z P C H ~ C H(1)l' ~ P Pand ~ ~ )rner-[M~(N~)~(PPr;Ph)~] ~] (2). (Data for other structurally characterized complexes are in Table 1).8 Characteristic of this bondin mode for dinitrogen are slight elongation of the N=N bond on binding (by about 0.01-0.02 from that of 1.09768, in free N2) and a v(NZ) value some 300-400 cm-' less than the 2331 cm-' value of free N2 in its Raman spectrum.
1
N
111
11;
Ph,
N
N
Ph,
11:
N
The majority of complexes contain tertiary phosphines or arsines as coligands but there is also a class which has m-arene coligands which is included in Table 1 for completion. Despite the obvious relevance of dinitrogen complexes to the chemistry of the molybdenumsulfur site of nitrogenase: dinitrogen complexes of molybdenum with sulfur ligation have proved to be very elusive. They are confined to the compounds coc3-00
1268
Molybdenum
r-
id vr
3
m 3
I4133
3 3 3 3
I I
2
2
10
2
I I
I
3 3
I l l
x I
I
I
I I I I
VI m .e
I
3I
I
I I I I
v! 3 m
2
2m
$
I
I l l 1
I
I
I
I
m I
m Q
Q
Q
4
h
W h
S
W
3
Y
i
P
I
1269
Molybdenum(O), ( I ) and ( [ I )
I
I
I
E
Molybdenum
1270
x I
m
0
: e
.t
I
I
3 9
~],but treatment of this complex with MeNC evolves C02 to gve trans-[Mo(CNMe)2(dppe)2].MReaction of COS with [ M o ( N ~ ) ~ ( P M ~gives ~ ) ~ ]the complex
1278
Molybdenum
[Mo(S2CO)(CO)2(PMe3)2].The disproportionation of COz and COS to give carbonato species commonly occurs at Moa centres.43
(8)
Several SO2 com lexes of molybdenum have been prepared which are assigned both the q2-(S-O)- and q (S-bound)-bonding modes. Generally the metal coordination site has a mainly carbon environment and we give here only the examples f~c-[Mo(CO)~(dmpe)( qzSO,)]; mer-[Mo(CO)~(dmpe)(~l-SOz)], rner,tr~ns-[Mo(CO)~(PR~)(q~-SO~)] and trans[Mo(Co)~(dppe)(PR3)(q'-SO~)j (PR3= PMe3, PPhzMe or PPG) and fuc-[Mo(CO)3 (MeCN)2(qZ-S02)].They are generally prepared by displacement of a labile ligand, such as by MeCN, from a precursor complex, e.g. [Mo(CO)~(M~CN)~]
P
36.2.3 MOLYBDENUM(]) 36.23.1 Carbon Ligands
36.23.1.1 Carbon monoxide In this section we mention the paramagnetic monocationic complexes [M~(CO),(dppe)~]+ and [M~(CO)~(bipy)~]+, both of which are produced by oxidation of the parent Moo complexes. In the former case, the oxidants used include [MeC6H4N2][BF4],46NO[PF6], Iz and AgIla,47 and a variety of analogues [Mo(CO)~L]' has been obtained (L= dppm, dmpe, d i m , etc.). Electrochemical oxidation has also been used, with other measurements, to show that a rapid cis + trans isomerization follows oxidation and an explanation for this phenomenon has been proposed on the basis of extended Huckel molecular orbital calculations, the stereochemical change being dependent on the number of valence electrons and the nature of the coligand n-donor or n-acceptor capacity. Similar studies have been made upon the compounds [MO(CO)~L$ (L = bipy or hen).^^
3623.1.2 Isocyanide ligands The isocyanide analogues of the above carbonyls, rrans-[M~(CNR),(dppe)~](R = alkyl or aryl) undergo a reversible one-electron oxidation in solution4" and oxidation with I2 or Ag' salts allows isolation of the paramagnetic cation [Mo(CNC&,Me),(dppe),]+ (pen=2.01 BM). With other isocyanides, substitution at the metal centre occurs on further oxidation.
36.2.3.2
Dinitrogen Complexes
Oxidation of tr~ns-[Mo(N~)~(dppe),] with I2 in methanol allows isolation of the unstable cationic complex tr~ns-[Mo(N~)~(dppe)~]I~.~ This cation has also been prepared electrochemically and the rate of its decomposition by loss of N2 studied.6 A range of dinitrogen complexes shows reversible one-electron oxidation in solution, but in general the monocations are too unstable to be isolated for molybdenum.6 Species of the type [MoX(N,)(dppe),] (X = C1, Br or I) are thought to be intermediates in the alkylation reactions of coordinated dinitrogen in truns-[Mo(N~)2(dppe)~] and their isolation for X = C1 or Br has been claimed. The analogue [M o C I(N~)(P M ~~)~] was also thought to exist but subsequent studies have indicated that the properties attributed to [MoX(N2)L4](L = PMe3 or L2 = dppe) can be reproduced by an equimolar mixture of [M o X~L and ~] [ M O ( N ~ ) ~and L ~ ]at
Molybdenum(O), ( I ) and (11)
1279
best it appears that [MoX(Nz)L4] very rapidly disproportionates in s~lution.~'However, the analogue [ M O ( S C N ) ( N ~ ) ( ~ Pis~stable ~ ) ~ ] at room temperature."
36.2.3.3 Nitrogen Monoxide Complexes These complexes are collected in Table 4. The pale green paramagnetic complexes [MoCl,(NO)L] (Table 4) result from reaction3' of alkyl phosphines with the red solution obtained from MoC15 and NO in CH2C12.Treatment of [MOC~~(NO)(PM~P~~)~(PM~P~~O)] with [NEt4]C1 gives3' the green paramagnetic salt [NEt4][MoC13(NO)(PMePh&]. Other complexes in this series are generally obtained by oxidation of the formally Moo nitrosyl (Section 36.2.2.3) or reduction of the formally Mo" nitrosyl (Section 36.2.4.2). Table 4 Nitrosyl Complexes of Molybdenum(1) Complex
Y(No)a(m-')
1585 1600 1678
1665 1600 [MoBr2(NO)(ttacn)] [Mo(NCO)(S,CNR2)( OPPh,] R = M e or Et [Mo(ttp) (NO)(MeOH)] -2C&
1565 1640
'
1540
-
~ ~ f f 1 . BM 91 pcs1.93BM p.ff2.1 BM k v ) 1.99 pCE1.8BM (g,) 1.99 p,1.9BM (gay>1.97 pes 1.83 BM
[Mo(SCN),(NO)]'-
1676
2 2
2 3 4
pes 1.71 BM
5
(g) 1.968 (A95*97Mo)-99.66 2.02
6 798 6 7,8
1652 1685
1610 1624 1585
1 1
L
1645- 1650
1700 1796
Ref
Other data
(Ev> 1.98
7 8
1 ~ ~ ~ 1 . 8 9 9 d(N0) 1.158 8, X-Ray structure disordered 9 pcrr1.96BM { g a v ) 2.00 9
~~
'Solid state spectra (Nujoi or KBr). bSquare pyramidal structure. 'Trigonal pyramidal structure. dX-ray structure-NO trans to MeOH, disorder precludes accurate d(N0). ttacn = N,N',N"-trimethyl-l,4,7-triazacyclononane;ttp = mcso-tetra-p-~olylporphinate. 1. F. King and G. S. Leigh, 1. Chem. SOC.,Dalton Tram., 1977, 429. 2. R. Bhattacharyya, G.P. Bhattacharjee, A. K. Mitra and A. B. Chatter@, J . Chem. Soc., Dalrqn Trans., 1984. 487* 3. G.Backes-Dahmana and K. Wieghardt, Inorg. Chem., 1985, 24, 4044. 4. K. H. AI-Obaidi and T. 3. Ai-Hassani, Transition Met. C h m . , 1978, 3, 15. 5. T.Diebold, M. Schappacher,B. Chevrier and R. Weiss, J . Cbem. SOC., Chem. Cummun., 1979,693. 6. J. A.McCleverty, Chem. Soc. Rev., 1983, U , 331. 7. P. B. Hitchcock, C. T. Kan and R. L. Richards, J . Chem. Soc., D a I m Trans., 1982,79. 8. S. Clamp, N. G . Connelly, G. E. Taylor and T. S. huttit, J . Ckm. SOC., DaNon Trans., 1980,2162. 9. A. Miiller, W. Eltzner, S. Sarkar, H. BiSgge, P. J. Aymonio, N. Mohan, U. Seyer and P. Subramanian, 2. Anorg. A&. Chon.,1983,503, 22 and referencestherein.
The compound [MO(NO)(NH~O)(NCS)~(~~~~)] on heating under reflux in acetophenone or on photolysis at 80 "C gives the five-coordinate, paramagnetic isomeric pair [M0(NCS)~(N0)(phen)l (Table 4); a square pyramidal analogue [Mo(NCS),(NO)(bipy)] was also obtained, In all compounds the NCS is N-b ~n d ed . ~' The reaction of HN03 with [M0(Hacn)(C0)~] (Hacn = N,N',N"-trimethyl-l,4,7triazacyclononane) gives the air- and water-stable complex cation [MO(CO)~(NO)(H~C~)]+ which on bromination and reduction gives the paramagnetic complex [MoBrz(NO)(Hacn)]
Molybdenum
1280
(Table 4). The complexes [MoX2(NO)(Hacn)]+ can be reduced electrochemically by one The complexes electron to give the formally Mo' species in [Mo(S2CNRz)z(NO)(OPPh3)] (R = Me or Et), obtained by treatment of [Mo(S~CNR~)~(CO)~] The compound with NO, are formulated solely on the basis of analysis and IR spectro~copy.~~ [Mo(ttp)(NO)(MeOH)]-2C6& is obtained upon reduction of [Mo(ttp)ClZ]by zinc followed by chromatography on A1203 with CH2C12 containing traces of MeOH (ttp = meso-tetra-ptolylpo hinate). Its X-ray structure confirms octahedral coordination with NO trans to MeOH.T3 Electrochemical reduction of [Mo { Me2pzb}NO12] (Mezpzb= tris(3,5-dimethylpyrazolyl)borate) gives the green, paramagnetic monoanion, which dissociates I- to give the uncharged, A range of pyrazolylborate-Mo" complexes can also green, paramagnetic [M0(Me~pzb)N0].~~ be reduced electrochemically to give the Mo' species in solution, e.g. the compounds [Mo{Me2pzb}XY(NO)] (X, Y = anions) and the bimetallic series {Mo}NHRNH{Mo} ((Mo} = Mo(Me2pzb)I(NO); R = C&I4R'C6&; R' = CH2, CZ&, SO2, 0) and {Mo}NHC&Y (Y = NH2 or OH)." The magenta, paramagnetic complex [M0{Me~pzb~(N0)1~Li(OEt)~] was also isolated by reduction of [Mo{Me2pzb}12(NO)]in ether with LiPh. An alternative procedure for the production of Mo' nitrosyl species is to oxidize Moo complexes. Thus oxidation33 (electrochemical, Ag', Fe"' or Cu") of the series trans[M~X(NO)(dppe)~l (X = F, C1, Br, I, OH, SPh, CN, H, NCO, NCS or N3) gives the violet or brown , monocationic series [MoX(NO)( d ~ p e ) ~ ] + . The green-brown, monocation ~ 0 ( M e ~ p z b } ( N 0 ) ( M e C N ) is ~ ]obtained + by oxidation of the uncharged parent compound with Ag' in MeCN54and the dications [Mo(NO)(RCN)(dppe)2I2+ are similarly obtained from the monocationic precursors33(Table 4). Treatment of [Mo(C0)6] with an excess of NO[PF6] in MeCN gives55 [Mo(NO)(M~CN)~]~+. The complex anions [Mo(CN),(NO)l3- and [M o C ~(NO)(OH~)]~have been prepared by oxidation of [Mo(CN)~(NO)]~-with air (in conc. HC1 in the latter case) and have been structurally characterized as the expected octahedral complexes, with N O trans to HzO in the A compound formulated as [Mo(NCS),(N0)I2- has also been prepared56 by treatment of [MoO4Iz- with NH20H in presence of SCN-.
36.2.3.4
Nitrogen Donor Ligands
The one member of this class is the cation [M~(bipy)~]+ obtained by reduction of MoC15 with Mg in THE:in presence of bipy, or by oxidation of [ M ~ ( b i p y ) ~ ] . ~
363.3.5
Phosphorus Donor Ligands
The green, paramagnetic cation \M~(dmpe)~]+ is obtained by oxidation of [M~(drnpe)~] with iodine or methyl iodide in toluene.
'
36.2.4
36.2.4,l
MOLYBDENUM(II)
Carbon Ligands
362411 Carbon monoxide Here in general we exclude compounds with more than two M earlier.
M bonds as discussed
(i) Complexes of the type [ M o X ~ ( C O ) ~and L ~ []M O X ( C U ) ~ L ~ ] + These complexes (in which X = anionic, L = uncharged ligand) form the largest class of Mo" carbonyls and examples are shown in Table 5 with pertinent physical properties. We have excluded complexes of the type [{MoX~(CO)~}~], [MoX(CO)&]+ and related compounds for the reasons ciied earlier and for the same reasons [MoX3(CO),]-, [MoX(CO)~L]-and related anions are excluded. They are reviewed in COMC.la Complexes of the type [M OX~(C O)~L ~]
MoZybdenurn(U), (I) rand (ZI)
1281
where L is not a carbon ligand are prepared generally by two main routes: oxidation and substitution of a zero-valent carbonyl derivative with halogen (method A), e.g. reaction (9);la9I9 and reduction of a high-valent molybdenum compound in presence of CO and another ligand if necessary (method B), e.g. reaction (10).1a7'9 Simple displacement of CO by another ligand may also be used (method C), e.g. reaction (11).
-
[Mo(CO),(diars)] + Iz -[M~I,(CO)~(diars)] [MoC&(PPh&]+ AlEtClZ + CO + PPh3 [(MoBr,(CO),},]
+ 2dpam
-.-$
+ 2CO
(9)
[MoClZ(CO),(PPh3)z]
(10)
[M~Br~(CO),(dparn)~] + 2CO
(11)
Table 5 Dicarbonyl Complexes of Molybdenum(I1) Compound
Preparation"
~ ( c O (cm-I)" )
See text
1920-1950 1820-1860
A A A, B A A See text
1934,1842 1934,1834 1920,1795 2010,1920 1950,1880 1908,1768 1925-1945
D
Ref.
Other data X = NCO, mer-PMe,, CO caps one face of octahedron Cap O / c T p Cap 0 V, distorted octahedron -
-
TP for R = Pr'
1
2 2 2 2,3 2,3 4
5,6
1855- 1875
D D C, D c,D C A, J3 A A, B A A
1940-1850 CTP for L = C,H$ R = Me, L = PPh3 1955,1875 1933,1832 1929,1833 4:3 piano-stool, 9'and ql-OzCH 1940,1855 CapO 1940,1865 Ca pO 1940,1865 Cap O / C P 1940,1870 1940,1865 Cap 0 1890
-
4,5,6 5
7 7 2 2 2 2 2 2
A A, B See text
1945,1889
CTP
7
circa 1880
Ca pO
8
A, B A, B A, B
1965,1875 1960,1890 1965,1905
-
2,3 2,3
1988,1898 1988f 4,1908 f 10
-
10 10
A A
I
2
9
Solid sf.ae spectra (Nujol, KBr), unJes: otherwise stated. E btp = N-n-butylthiopicolinid~. 6(%0) 390 p.p.m. (R = Et). L =range of ligands, see text. tnphos = bis(o-dipheny!phosphinophenyl)phenylphosphine. Cap 0 = capped octahedron;CTP = capped trigonal prism, TP = trigonal prism. 1. E. Carmona, K. Doppert, J. M. Marin, L. Poveda, L. Sanchez and R. Sanchez-Delgado,Inorg. Chem., 1984,23,530. 2. M. G. B. Drew, h g . Inorg. Gem., 1977,23,67 and references therein. 3. R. Colton, Coord. Chem Rev., 1971, 6, 269 and references therein. 4. M. H. Chisholm, J. F. Hdmann and R. L. Kelly, 1. Am. Chem. SOL, 1979,101,7615. 5 . G. J. Chen, R.0.YettonandJ. W. McDonald, Inorg. Chim. Acm, 1977,22,249. 6. C. G . Young and J. H. Enemark, Aust. J . Chem., 1986,39, 997 and references therein. 7. D. C. Bromer, P. B.Winston, T. L. Tonker and J. L. Templetra, lnorg. Chem., 1986,25,2883. 8. S. Datta, B. Dezube, J. K. Kouba and S. S. Wreford, J . Am. Chem. Soc., 1978, 100,4404. 9. M. R. Snow and F.L.Wimmer, Aust. J . Chem., 1976,29,2349. 10. J. A. Connor, E. J. James, N. El Murr and C. Overton, 1. Chem. Soc., Ddron Tram., 1984,255.
*See text.
The coordination number for these complexes may be six or seven and extensive X-ray structural studies have been carried out, particularly on the seven-coordinate complexes. The geometry and some other details of a representative selection of these compounds are shown in one potentially chelating iigand is Table 5. In some cases, e.g. [M~(CO)~Cl~(dparn)~],
Molybdenum
1282
monodentate to maintain seven-c~ordination.~~ Sulfur coordination may also be introduced at the metal centre by use of displacement metathesis by e.g. the dithiocarbamato ligand, see reaction (12) (method D).59 ~[(MoCl,(CO),},]
+ 2Na[Me,NCS2]
-
[Mo(CO),(Me,NCS,),]'
e ~ M O { C O ) , ( M ~ ~ N C S+~C)O~ ] (12)
The reversible loss of CO from [ M O ( C O ) ~ ( M ~ ~ N CisS ~ a )property ~] shown by other Mo" carbonyls. For example, the complexes [ M O X ~ ( C O ) ~ ( P R(X ~ )=~ C1 ] or Br) also lose one mole of CQ reversibly to give blue, diamagnetic [ M O X ~ ( C O ) ~ ( P Rwhich ~ ) ~ , (for X = Br) has been structurally characterized as a 'badly distorted octahedron' .59* A common geometry for the seven-coordinate complexes is the capped octahedron, with a CO ligand in the capping is a position, although there are variations (Table 5 ) , in particular [M~Cl(CO)~(diars)~]+ capped trigonal prism@ .' A further series of dithiocarbamato complexes has been obtained from [MoC~~(CO)~(PM (obtained ~ ~ ) ~ ] from [ M o C ~ ~ ( P M and ~ ~ )CO ~ ] directly)61by treatment with Na[S2CNRz]. These are the dicarbonyls [MO(S~CNR~)~(CO)~(PM~~)] and the monocarbonyls [MO(S,CNR~)~(CO)(PM~~)~], the latter being obtained when a slight excess of PMe3 is used in the reaction of [MoC12(C0)2(PMe3)3] with Na[S2CNRz].61 The analogues [Mo(&CNE~Z)Z(CO)~] (L = PMezPh or PMePh2; = dppe) have also been synthesized.6z These monocarbonyl compounds are notable for their very low value of v(C0) (17601735cm-') implying a very electron-rich metal centre in these complexes.62They transform readily into the dicarbonyl complexes with CO. The structures of the [Mo(SCNR,),(CO),(PR,)] series appear to be analogous to that determined for [W(SZCNE~~)~(CO)~(PP~~)], namely a tetragonal base (4:3 'piano stool') geometry.61 As is often observed for dithiocarbamate complexes, the alkyl groups of the SzCNR2 ligands show 'H NMR equivalence at room temperature. The fluxional process required for this does not appear to involve phosphine dissociation.61 The series [ M O ( S ~ C N E ~ ~ ) ~ ( C Oderived ) ~ L ] from [M O(S ~C NE ~~)(C O)~] has been investigated by y5M0 NMR spectroscopy. The resonance region (310 to -430 p.p.m. relative to [Moo4]'-) has shown that all the resonances are shielded relative to [Mo(S2CNEt&(CO),] (390 p.p.m.) and the dependence of nuclear shielding upon L is NHZNHSO2C6H4Mc < C1- < OPPh3< p pyrazine == py < NH2NHCOPh < NH2Nh4e2< p-NH2NHMe~] described above. They have Mo-lH chemical shifts in the range -4 to -5 p.p.m. (rel. SiMe4).82 The diamagnetic dithiolate complexes frans-[Mo(SR)z(diphos)2]are reversibly oxidized to the Mo"' species and the E$ values for R = C 6 w - 4 (X = Me, H, NH2, OMe, F, C1) show a linear dependence upon the Hammett u function for X, as do the 31Pchemical shifts of these compounds.81 The hydride compound ~rans-[MoH~(dppe)~] was claimed from reaction of trum[ M ~ ( N ~ ) ~ ( d p p with e ) ~ ] H2, but some doubt exists about its formulation because its low solubility prevents accurate '€3 NMR integrati~n.'~Preparation of a more soluble analogue with (2-MeC6H4)2PCH2CH2P(C$14Me-2)Z allowed accurate integration and demonstrated the presence of four hydrides per Mo. By analogy, the above dihydride is probably [Mo&(dppe)2], although one could not exclude the existence of it and of [{trans-M0H2(dppe)~}~(y-dppe)] which also appeared to be formed by reaction of truns-[Mo(N&(dppe)2] with H2.83 A number of other hydride complexes have been prepared; the purple , diamagnetic hydride [MoH(BH,)(PM~~)~] has been obtained by reaction of N a B a with [MoC13(PMe3),]. The essentially trans octahedral structure of this molecule has been determined by X-rays as shown in (9); the hydride, the molybdenum and the boron atom of the bidentate BH, group lie precisely on the intersection of two mirror planes in the molecule.s6
(9) P = PMe,
In a related reaction, treatment of [ M o C ~ ~ ( P Mwith ~ ~ )L ~ i]A l b gives the bridged hydride aluminohydride complex [(Me,P)4HMo(p-H)2A1H(p-H)2A1H(p-H)zMoH(PMe,)4], which has been shown by NMR to be non-rigid in solution. The central core of the structure is shown in (10).85 The carboxylato hydrides [MoH(02CR)(PMe3),] ( R = H , Et or CF,) are obtained by treatment of [MoH2(PMe&] (see Section 36.2.4.2.2)with C 0 2 (R = H) or RC02H (R = Me or CF3)86or by reaction of [Mo(&H.,)~(PM~~)~] with C 0 2 followed by Hz (R = Et).87 The X-ray structure of [MoH(02CH)(PMe&] is essentially pentagonal bipyrarnidal, with a chelating
MoZybdenurn(O), ( I ) and (11)
1285
formate tram to hydride (11). The interaction of [ M o H ~ ( P M ~ with ~ ) ~ ]PhNCO gives a moderate yield of the analogous, red complex [MoH{P~NC(H)O}(PM~~)~], formally by the insertion of the isocyanate into the Mo-H bond.86 ' H
\I
(11) P = PMe,
The phosphite analogue, [MOH(O~C C F ~)(P (OM ~~} is ~], obtained by treatment of [Mo{P(OM~)~}~] (see Section 36.2.2.5) with CF3C02)I; it has G(MoH) at -4.6 p.p.m. (rel. SiMe4). It has a similar solid-state structure to [M OH(O~C H)(P M ~~)~], but is fluxional in solution with non-pairwise exchange of phosphorus-donor atoms being the dominant process.@ 3624.22 Complexes of the type [MoHzP~](P =tertiary phosphine or phosphite)
The reduction of (MoCh(THF)2] by magnesium in presence of PMe3 in THF under dihydrogen gives a high yield of the yellow, air-sensitive hydride [ M o H ~ ( P M ~ ~Its ) ~ crystal ]. structure contains two crystallographically independent molecules which have essentially pentagonal bipyramidal goemetry, equal within the limits of experimental error. The Mo--P bond lengths vary in the structure. The axial Mo-P distances are very similar (2.424-2.428 I&) but one unique Mc-P(equatoria1) bond in each molecule, which lies between two hydrogens, is some 0.6-0.7 8, shorter than the other two Mo-P(equatoria1) bonds, probably because the long bond is almost trans to hydride.89The hydride resonance is a sextet at -5.23 p.p.m. (rel. SiMe4) indicative of fluxionality at ambient temperature. [ M o H ~ ( P M ~reacts ~ ) ~ ] with COz and RC02H (R = Me or CF,) to give the monohydrides [M o H(C O~C R )(P M ~~)~] as described in Section 36.2.4.1.* Photolysis of [ M O{P(OM~)~)~] (Section 36.2.2.5) in presence of dihydrogen gives the off-white hydride [MoH~{P(OM~)~}~]. The hydride resonance is indicative of fluxionality in solution [sextet at -5.8 p.p.m. (rel. SiMe4)] down to -90 "C, and its structure is most Likely a pentagonal bipyramid, analogous to that of [MOH~(P M ~, )~] above. The analogous cation [MoH{P(OM~)~}~]+, is obtained in low yield by treatment of [Mo{P(OMe)3}6] with acids; the cation [Mo{P(OM~)~}{P(OM~)~}~]+ was also obtained by the same route. These products are not interconvertible and the isolation of the one or the other depends upon as yet undefined kinetic factors.88 3624.23 Nitrosyl complexes
The largest group of molybdenum(I1) complexes is constituted by the nitrosyls (oxidation states being assigned on the somewhat arbitrary assumption that nitrogen monoxide, as a ligand, is NO+). Tables 6 and 7 give a selection of the compounds discussed. The most important molybdenum nitrosyl, used in many studies as a starting material or as a catalyst, particularly for alkene metathesis, is {MOC~~(NO))~.*~' It is produced- in a reasonably pure state by reaction of NOCl with [Mo(C0)&12], the chief contaminant probably being {MoC12(NO),}n or &[MozCls] .93 The greenish-black hygroscopic, air-sensitive solid has v(N0) at 1590crn-l. It reacts with acid species to eliminate chloride, for example with 1,2-MeSC&14SLi it generates [Mo(NO)(SC~&SM~),],~ and forms adducts with neutral
1286
Molybdenum Table 6 Nitrosyl Complexes of Molybdenum(I1)(ExcludingPyrazolylborates) Complex
NO) (cm-')
1590 (Nujog;'1739, 1851 (IU3r)' 1655(KBr)
,
1710 (Nujol),'1705 (CH,CL,),Z 1710 (Nuj01)~ 1705 (Nujol),'1706 (CH2CIz)2 1740 (KBr)? 1740 (NU~OI)~ . - , 1707 (CHzC12)2 1713 (CH2C1$, 1712(CH2Cl2): 1700 (CH,Cl,)', 1692(CH2C12) 1692 (CH,CI$* 1725 (Nuioll 1700(cH,CQ~ 1720 (CH2Cli)4 1695 (Nujol) 1701 (Nujol)' 1630 (Nujo1)6 1660 (Nujo1)6 1660 (?)' 1700 ( K B I ) ~ 1690(KBr)8 1675 (KBr)' 910
1630 (CHzC12)" 710,12
?I2 ?'2
710
1565, 1545 (Br); 1695, 1640(Cl);1710 (I) (all KBr)', 1660,1665 (KBr)', 1660 (KBr)2 1640 (Me);1635 (Et);1630(PS); 1625 (Bun)(all CHC111'4 1634 (NCO);1630 (N3)i1641(NCS)(all KBr)" 1648 (Me2SO);1651 (py);1645 (NH,); 1661 (N,H,/2) (all KBr)" 1648 (KBI)'~ 1626 (KBr)15 1633 (KBr)I5 cu. 1635
1633 (KBr)"
[Mo(NO)CI,( O,PCl,)]'-
[Mo(NO)(CN)5I2~~o(NO)(H,")&l2[Mo(NO)(pyridine-2,6-di~arboxylate)(H,~O)L]~
1695, 1715; 1690 (aU Nuj01)~ 1692 (Nuj01)'~ 1676 (Nujo1),161670(Nujol): 1670 (N~jol),~ 1679 (KBr)' 1680(Nujol)16 cu. 1700 (OPMGPhor OPEtPh,); cu. 1675 (PPh,or PM%Ph) (all Nujol)6 1688 (Nujol)" 1645 (KBr)," 1650 (KBr)8 1590 (Cl);1620(N3)(all mr)" 1633 (H,O), 1660 (CN),1665 (N3), 1680 (NCS)(all KBr)" 1650 (KBr)lg 1625 (KBr)" 1690(KBr)" 1690 (KBr)19 1658 (KBr)2D 1655;1657;1647(KBr)m 1657;1652;1645 (KBr)m 1650 (KBr)m 1615 (?)" 1630 (?)" 1671 (KBr)"
1. R.Davis, B. F.G. Johnson and K. H. AI-Obaidi, 1. Chem. Soc., Dulton Tram., 1972, 508. 2. R.Taube and K. Seyferth, 2. Anorg. Allg. Chem., 19l7, 437,213. 3. D.Sellmann, L. Zapf, J. Kcller and M.Moll, J. Orgummet. Chem., 1985,289,71. 4. L. Ben- and J. Kohan, lnorg. Chim. A m , 1%2,65, L17; L. Ben-, J. Kohan, B. Mohai and L. Marko,J . Organomet. Chem., 1974,70, 421. 5. K. Dehnicke, A. Lickett and F. Weller, Z.Anorg. AI&. Chem., 1981,474,83.
Molybdenurn(O), ( I ) and (IZ)
1287
References to Table 6 (continued) F. King and G. J. Leigh, J. Chem. Soc., Dalton Tram., 1911, 429. P. Chaudhuri, K. Wieghardt, Y. H. Tsay and H. Kriiger, Znorg. Chem., 1984,23,427. R. Bhattacharya, R. Bhattacharjee and A. M. Saha, J . Znorg. Nucl. Chem., 1985, 4, 583. J. Beck and J. Strue, Z. Natrrtforsch, Teil B, 1985, 40, 891. P. J. Blower, P. T. Bishop, J. R. Dilworth, T. C. Hsieh, J. Hutchinson, T. Nicholson and J. Zubieta, Znorg. Ckim. Acra, 1985, 101, 63. 11. S. Sarkar and P. Subramanian, J. Inorg. Nud. Chem., 1981, 43,202. 12. P. T. Bishop, J. R. Mworth, J . Hutchinson and J. A. Zubieta, Inorg. Chim. Acta, 1984, 84, L15. 13. G. Backes-Dahmann and K. Wieghardt, Znorg. Chem., 1985, 2 4 , M . 14. B. F. G. Johnson, K.H. AJ-Obaidi and J. A. McCleverty,J. C h m . Soc. (A), 1969, 1668. 15. J. A. Broomhead and J. R . Budge, A w t . J. Chem., 1979,32, 1187. 16. S. Sarkar and A. Miiller, Angew. Chem , Znt. Ed. Engl., 1977.16, 183. 17. A. Liebelt, F. Weller and K. Dehnicke, 2. Anorg. Allg. Chem., IW, 480, 13. 18. K. Wieghardt, G. Backes-Dahmann,W. Swiridoff and J. Weiss, Inorg. Chem., 1983, 22, 1221. 19. K. Wieghardt, W. Holzbach and €3. Prikner, Angew. Chem., Znt. Ed. Engl., 1979, ls, 548; K. Wieghardt, W. Roltbach, 3. Nuber and J. Weiss, Chem. Ber., 1980, LU,629. 20. R. Bhattacharya and G. P. Bhattacharjee, J . Chem. Sac., Dalton T m . ,1983, 1593. 21. K. Wieghardt, W. Holzbach and J. Weiss,Z . Naturforsch., Teil B, 1982,37,680; K. Weighardt and W. Holzbach, Angew. Chem., Znt. Ed. Engl., 1979,18, 549. 22. H. Adams, N. A. Bailey, A. S. Drane and J. A. McCleverty, Polyhedron, 1983, 2, 465. 6. 7. 8. 9. 10.
Table 7 Molybdenum(I1) Nitrosyls with F'yrazolylborates Complex [Mo(NO)(Me,Pzb)X*I [Mo(No)(pzb)x2l [Mo(NO)(4-X-Me,pzb)X2] [Mo(NO)(Pzb)Br(oR)l [Mo(NO)(Me,pzb)X(OEt)]
[Mo( N 0)(4-Cl-Mq~b)Cl(OR)] [Mo(NO)(Me-$zb)X(OPr')] [Mo(NO)(Mwzb)Br{OR)] [Mo(NO)(Megzb)l(OR)]
[MO(N0)~MWZb)(0R),l
[Mo(NO)(Megzb)(OR)(OR')] [Mo(NO)(Me2pzb)X(OAr)]
[Mo(NO)(Me$zb)(OPh)(OR)] [Mo(NO)(Mwb)X(NHMe)]
Y(NO)(cm-')
1688 (F) (KBr);' 1702 (Cl) (KBr);' 1702 (Br) (KBr); 1700 (I) (KBr);' 1718,1723(CI) (CH2C12)3 1700,1710(Cl); 1700,1710(Br); 1656,1700(I) (all KBr)' 1705 (Cl) (KBr)? 1722 (Br) (CHCI,)' 1677 (Me);' 1677 (Et)' (all CHC1,) 1666 (F) (KBr);' 1676 (Cl); 1673 (Br); 1678 (I) (all CHC1,)4 1704 (Et); 1682(Pi)(all CHC1,)4 1674 (Cl); 1674 (Br);1673 (I) (all CHC13)4 1676 (Bu'); 1697(GH,) (all CHCLJ4 1688 (CH,CH,OMe); 1675 (CH,CH,CI); 1673 (CH,CH,OH); 1670 (CH,CH,CH,OH); 1666 [CH,(CH2)2CH,OH]; 1672 [CH,(CH,),CH,OH] ; 1668 [CHz(a2)4CH20H] (all KBr);' (exact values not quoted for R = CH,OH or CH,CH,CH,Br, ca. 1675); 1665 (H); 1675 (c,&lI); 1675 (CH ,W CH ,O H ) (all KBr)6 1640 (H); 1640 (Me); 1640 (Et); 1640 (P+); 1640 (But) (all KBr)6 7640 (Et, W); 1640 (Pi,Bu') (all KBr)6 1673 (Ph); 1672 (4-C&Me); 1686 (CC&4CN) (all KBr)' 1656 (Me];1655 (Et).1656 (Ph)(all KBr)' 1640 (F); 1643(C1);'1654 (Br);6 1640,(9;' 1625 (OMeV6 1628 (OEt): 1637 (OPr'); 1650 (OPh) (all KBr) 1649 (Cl);f1655(I);8 1633 (OMe)? 1630 (OEt);6 1631 (OW)6 (all KBr) 1630 (I);' 1635 (OMe);6 . ,. 1630 (OEt);6 . . 1637 (O*l6 (an K B ~ ) 1630 (OEt; n); 1638 (OPf, i) (all KBr)' 1646 IH, Bu"); 1661 (H, Bu'); 1658 (H, CH,Ph); l a(H, C,I-&); 1655 (H, GH,); 1665 ( C M d (all KBr) 1630 gH,H); 1615 (H, Pri); 1625 (H, CH,Ph) (all KBr) 1625 (H, CH,Ph); 1625 (H, C,H,,) (all KBr)6 1638 (H, hi):1633 (H,&Hi,) (aKBr)6 1672 ((21)"; 1659 (Br)'; 1638 (I)' (all CHCl,) Mean, R = Me, Et or Bun, 1646'; 1654 (Ph); 1653 (=&Me) (all KBr)'
Molybdenum
1288
Table 7 (continued) ~
~~
Complex
v ( N 0 )(an-')
[Mo(NO)(Me,pzb)I(NHAr)]
1659 (Ph); 1668 ( 4 - W E t ) ; 1647 (4-CJ140Me); 1654 (4-C&oEt); 1659 (4-Cd34F); 1661 (4C6H4Cl); 1660 (4-CABr); 1657 (4-C,H,I); 1646 (4C6H4C0,Me); 1666 (4-GH4N0,); 1666 (4C,H4CN); 1654 (C,J-I,) (all KBr)' 1677 (Et); 1669 (Bun;1685 (CI6HS3); 1672 (C6H11); 1672 $CH,Ph); 1689 {Ph); 1692 (4-C,H4Me) (all
[Mo(No)(Me,Pzb)I(sR)l
KBr) [Mo(NO)(Megzb)X(SBu")] [Mo(NO) (Megzb)X(SPh)]
[Mo(NO)(Me,pzb)Cl( SR)]
[Mo(NO)(Me,pzb)I{hydrazide(l
[Mo(NO)(Me,pzb)Br{hydrazide(l
- )}]
- ))]
1653 (SBu");' 1625, 1636 (NHMe): 1625 (NHEt);' 1634 (NHPr"): 1649 (OMe)? 1643 (OEt);9 1655 (Cl);' 1665 (Br)' (all KBr) 1681 (SPh); 1624, 1635 ( m e ) ; 1630 (NHEt); 1652 (NHPr"); 1637 GWBu"); 1658 (OMe); 1653 (OEt) (all KBr) 1662 (Bun); 1671 (CJZl1); 1671 (CH,Ph); 1684 (Ph) (dl KBr)' 1635 (NHNH,); 1635 (NHNHMe); 1634 (NHNMe,); I630 ( " H F ' h ) ; 1639 ( " H C $ 5 ) ; 1640 (NHNHCJ14Me-4); 1636 (NHNMePh); 1654 (NHNHCSNH,); 1652 ("HCO2MS); 1623 (NIWHCOPh); 1658 (NHN= CMel) (all KBr)'' 1615 (NHNH,); 1615 (NHNHMe); 1615 (NHNHPh); 1630("=CMe,) (all KBr)" 1663 (?)" 1675 (N, 2); 1670 (0,l);1668(S,1) (all KBr)" 1684 (KBr)" 1620 (Et, H); 1632 (Ph, H); 1628 $4-CJ14Me, H); 1682 [CH,(CH,),CH,] (all KBr)' 1638 (KBr)I3 1710 (?)'" ~~
_________~~ ~
1. A. S. Drane and J. A. Mccleverty, Polyhedron, 1983, 2, 53. 2. J. A. McCleverty, Chem. SOC.Rev.,1983,12,331. 3. S.Trohenko, Inorg. Chem., 1969, 8,2675. 4. 3. A. McCleverty, D. Seddon, N. A. Bailey and N. W. Walker, J. C h . Soc., Dalton T r m . , 1976,898. 5. G. Denti, J. A. McCleverty and A. Wodaruyk, J. Chem. SOC.,Dalton Trom., 1981,2UZ1, 6. J. A. McCleverty, E. A. Rae,I. Wdochowicz, N. A. Bailey and J, M.A. Smith, J . Chem. Soc., Dalton Trans.,
1982,951. 7. J. A. McCleverty, G, Denti, S. J. Reynolds, A. S. Drane, N. El Mum, A. E. Rae, N. A. Badley, H. Adam and J. M. A. Smith,J . Chem. Soc., Daltott Tram., 1983, 81. 8. J. A. McCleverty, E. A. Rae, I. Wdochowia, N. A. Bailey and J. M. A. Smith, J . Chem. SOC., Dalton Trans.,
1982,429. 9. J. A. McCleverty, A. S. Drane, N. A. Bailey and J. M. A. Smith,J. Chem SOC., Dalton Trans., 1983,91. 10. J. A. McCleverty, A. E. Rae, I. Wolochowicz,N. A. Bailey and J. M. A. Smith, J . Chem. SOC., Dalton Tram., 1983, 71. 11. H. Adams, N. A. Bailey, D. Giankanco, J. A. McCleverty, J. M. A. Smith and A. Wlodarcyzk, J. Chem. SOC., Dalton Tram., 1983, 2287. 12. H. Adams, N. A. Bailey, A. S. Dram and J. A. McCleverty, Polyhedron, 1983, 2,465. 13. N. Al Obaidi, K. P. Brown, A. J. Edwards, S. A. Hollins, C. J. Jones, J. A. McCleverty and B. D. Neaves, 1. Chem. Soc.. Chem. Commun., 1984,690. 14. M. E. Deane and F. J. Lalor, J . Organornet. Chem., 1973,57, C61.
donors, sometimes with oxidation. Thus Ph3P and also OPPh3 react to give [MoCl3(N0)(OPPh3),] and bipy reacts to generate [MoCI,(NO)(bipy)] .w The former complex is also attainable from [Mo(C0)4(PPh&], or [Mo(C0)3(PPh3)3] with NOCl.89 An alternative synthesis of { MoCl3(N0)}, (described as brown-black) is from MoC15 and NO. The temperature and reaction time need to be controlled to avoid further reductive nitrosylation to the green Moo species (MOCI,(NO)~},. Addition of pyridine to such a reaction mixture yields [MoC13(NO)py2], and similarly OPPh3 yields [MoCI~(NO)(OPP~~)~] .89 It is puzzling that the two sources for {MoC13(NO)}, quoted give different products with different IR spectra, yet the OPPh3 complexes appear the same from either source, whereas the bipy complexes are different. It is also pertinent that MoC1, and NO react in benzene rather than in dichloromethane to give a red solid. This is not {MoC~,(NO)}~,and may even be [MoCls(NO)], and has v(N0) at 2000 However, it is a good source of nitrosyl complexes such as [MoCl5(N0)I2-.
Molybdenum(O), ( I ) and (11)
The complex [ M O ( N O ) ( S & H ~ S M ~ is ) ~ probably ]~~ seven-coordinate, although its NMR spectrum in CDzCl2 is more complex at 0 "C than one might expect. It reacts with nucleophiles, such as N2&, PMe3 and OH-.94 The adducts of the general formula [MoC13(NO)L](see Table 6) are all probably obtainable by reaction of {MoC&(NO)}, with the neutral ligand, L a mHowever, reductive nitrosylation followed by treatment with can be used directly. Thus, reaction of NOCl with [M o , (O~C M ~)~] OPPh3 gives [ M o C ~ @ I O ) ( O P P ~dire~tly.'~ ~)~] Alternative preparations have been reported.95 Molybdenum(I1) chloride and NOCl react in POC13 to yield [MOC~~(NO)(POCI~)~], which loses one POcl3 in solution in CH2C12 to generate [{MOC~~(NO)(POC~~))~].~~ The dinuclear species has chloro bridges, with chlorides in the axial positions and the molecules, NO ligands and remaining chlorides bound to give similar octahedral coordination about the molybdenum atoms, but all tram with respect to the pair of metal atoms. The MONO system is approximately linear (178"), with Mo-N = 1.81 and N-0 = 1.08 The mononuclear complex reacts with chloride to generate [MoCl,(NO)]- and [MoCl5(N0)I2-. All the complexes [MoC13(NO)L] are presumably six-coordinate and octahedral. CIearly, this is not a limiting coordination number, since seven-coordination is allowed by electroncounting rules for molybdenum(II), and is observed among nitrosyls (cf. [Mo(NO)(SC&SMe),]). Specific examples based on this chloro system include [ M o C ~ , ( N O ) ( P M ~ ~ Pand ~ ) ~[]{MOC~~(NO))~(P~~PCH~CH~PP~~)~], prepared by variation of the routes discussed Com lete replacement of chloride to give complexes based upon {Mo(NO)X3), (X= CN,97+9N3, 99 SRlm or S2CNR2) has also been observed. For example, the compound [Mo(CO)~L](L = 1,4,7-triazacyclononane) reacts with nitric acid to yield [Mo(CO)~(NO)L]+ which contains MOO, but inclusion of KCN in the reaction mixture causes formation of [Mo(CN),(NO)L]." Probably all the nitrogens of the cyclononane are coordinated, making this complex seven-coordinate. Reaction of molybdate [Mo04]*- at pH 8 in the presence of KCN and L (L = bipy or phen) with hydroxylamine can give rise to [MO(CN)~(NO)L].H~O, though the precise products are a function of reaction conditions.99 In an unusual reaction This [M~(CO)~(terpy)] and MeSiN3 in nitromethane produce yellow [M~(NO)(N~)~(terpy)]. has a pentagonal bipyramidal structure with NO and a bent azide in axial positions. Mo-N is 1.777(4) and N--0 f.196(5) A, and LMoNO is practically linear [176.2(4)"].w The reaction of SR- with {MoCl,(NO)}, does not normally stop with [Mo(NO)(SR),]. Exceptions to this are [Mo(NO)(SC,jH$Me)3] cited aboveP4and [Mo(NO)(NH)(SCt,H&)3], which has a thiolate ligand with exceptional steric requirements.'" This is produced by reaction of P$3CJ12S- with 'Mo(NO)(NH20)'. lo' This is a genuinely five-coordinate, trigonal bipyramid with a thiolate in the trigond plane and NO trans to NH3. Again LMoNO is close to 180" [176.1(3)"] with M-N = 1.78 3 8, and N-0 not reported. In contrast Me3C6HzS- gives rise to [Mo(NO)(SC,H,M~~)~]->' BUS- produces [MoC~(NO)(SBU')~]-,and PhS- yields [MO ( NO) ( SP~ ) ~]-.'The ~ last complex has a trigonal bipyramidal structure (note that [MoC&(NO)]- is essentially square yramidal), a linear NO occupying an axial position, Mo-N = 1.766(6), N - 0 = 1.188(8) and LMoNO = 178.5(5)0.'M The steric influences at work are obvious, though not the precise way in which they operate. We now discuss some compounds which may be formally considered to be based on {MoC13(NO)},, though they include a collection of miscellaneous species not easy to categorize. A complex described as [M~(NO)(acac)~(hia)][acac = 1,3-pentadionate, hia = hydroximinoacetylacetone (sic)J with v(N0) at 1635 cm-' (CH2C12) has been mentioned."'> The complex [LMo(CO)~](L = N,N',N''-trimethyltriazacyclononane) reacts with nitric acid to give [LMo(C0)2(NO)]+, and this with HBr gives rise to [LMoBr2(NO)]+, isolated as the hexafhorophosphate. The dichloride and diiodide were obtained similarly. The former reacts with ethanol to produce [LMoCI(NO)(OE~)]+.~~*~* The electrochemistry of the species has been reported, and the structures and NMR spectra rationalized. Several of these complexes are present in two isomeric forms. A comparison is made between L and tris(pyrazoly1)borate as a ligand." Finally, {MoC13(NO)}, reacts with the Schiff base H2salen to give [Mo(NO)Cl(salen)], which has not been characterized in detail.89y'04The reaction is probably general for Schiff bases. A range of complexes can be synthesized from [Mo(CO),(Schiff base and NO in benzene. They analyze for [Mo(NO)2(N02)2(Schiffbase)] and may contain Mohl. The values of v(NO), two bands in the ranges 1770-1785 and 1650-1670cm-l, would be consistent with this. The seven-coordinate complexes [Mo(NO)(&CNR2)3] (R = Me, Et, Pr" or Bun) were originally prepared from {MoCl3(N0)}, and the appropriate Na(S2CNR2),105'07 although
r
1
I
1289
1290
Molybdenum
irradiation of [Mo(N0)@2CNR&] also leads to [Mo(SzCNR&] .lo7 They are volatile, and have NMR spectra consistent with the structures established by X-ray analysis.10s110 The compound with R = Bun is pentagonal bipyramidal, with NO in the axial position, LMoNO = 173.2(07)", Mo-N = 1.731(8) and N-O = 1.154(9) A. A detailed discussion of the structure has been provided,lWand the 95Mo NMR spectra have been reported for the complexes with R = Me, and R = Et.'" The structure of [Mo(NO)(GO~)~] has been reported. It is similar."' (X = C1, N3, NCS Compounds formally related to these are [Mo(NO)(NCO)(S2CNEt2),X]or NCO), [Mo(NO)(NCO)(S~CNE~~)~B] (B = NH3, MeSO or py) and [{Mo(NO)(NCO)(S2CNEtz)2}2N2~J.f13~114 These are also all reckoned to be seven-coordinate species, with one of two possible structures admitted as likely on the basis of NMR and IR data. The general relationships are indicated in Scheme 1. These compounds are of interest in the context of nitrogen fixation, and the observation that the azido ligand in [Mo(NO)(N3)(S2CNEt2),(M&SO)]reacts with acid to give ammonia (via a nitrido complex) and Nzis particularly interesting in view of the reduction of azide by nitrogenase. A reasonably comprehensive list of compounds is given in Table 6. The dithiocarbamato complex [Mo(NO)(S,CNR,),] has a homologue based upon NS rather than NO. The preparation (for R = Me) is not related to that of the nitrosyl, but is by reaction of propylene sulfide or S on [ M o N ( S ~ C N M ~ ~ ) ~ ] .However, " ~ , " ~ its structure is very similar, pentagonal bipyramidal with NS in an axial position, and LMoNS = 172.0(7)", Mo-N = 1.738(11) and N-S = 1.592(11) A. These complexes are part of a series of approximately isostructural compounds [MoX(S2CNR2),](X= N, 0, NS or NO) .'" Thionitrosyl compounds with R2= Et, or (CH,), have also been prepared. A band assignable to v(NS) could not be identified in the IR spectra, though it is characteristically found in other complexes at ca. 1160cm-lT116
[Mo(NO)(NCO)(X)(S,CNEt,),].. Scheme 1
The 95MoNMR spectra of [Mo(NS)(S2CNR2),] (R = Me or Et) have been reported."' The complex {MoC13(NO)}, is capable, formally, of adding chloride ions to form [MoCL(NO)]- and [MoCl5(N0)I2-. The six-coordinate species seems to crystallize as the cesium salt, and the five-coordinate as the tetraphenylphosphonium salt , from similar reaction mixtures which use &[Mo(CN),(N0)].2H2O as the starting material. There is an alternative (The bromo anaroute to the tetraphenylarsonium salts starting from [MoC~~(NO)(POC~~)~].% logues have also been reported."') An alternative preparation from MoC15 + NO seems rather more easy to The complex [Et4N][MoC14(NO)] reacts with tertiary phosphine and phosphine oxides (L) to give adducts [EbN][MoCL+(NO)L](see Table 6). The other adduct of [MoCb(NO)]- which has been described is [ M O C ~ ~ ( N O ) ( O ~ P C ~ obtained ~ ) ] ~ - , by reaction of [AsPh4](0ZPCl2) with [MoCb(NO)]-. This mildly air-sensitive complex has a roughly octahedral structure, with the 02PC12 binding throu h one oxygen only, LMoNO being 176.8(10)", Mo-N = 1.701(13) and N-0 = 1.182(11) .'19 A hint that [Mo(CN),(NO)]- can be synthesized"* has not been amplified. The complex [MoCIS(NO)]'- has had its structure determined as the tetraphenylarsonium salt. It has an octahedral structure, with LMoNO = 173.9(25)", M-N = 1.751(32) and It is apparently a rather inert complex, said not to be of much value for N - O = 112.0(38) synthetic It can be prepared from &[Mo(CN)~(NO)].~H~O,'~* MoC15 and NO,31" (MOCI~(NO)},~~ or [NEt4J[Mo2C19]and nitrosyl chloride.93Finally, the ion [MO(NO)(CN)~]~has been prepared by the action of cyanide on the complex material obtained from the reaction of sodium molybdate with hydroxylammonium chloride.w,'O1 Some interesting nitrosyls, formally of molybdenum(II), have been obtained using this complex. These contain protonated
x
Molybdenum(O), (Z) rand (ZZ)
1291
nitrosyl as well as NO. Thus HCI reacts to give [MOC~~(NO)(H~NO)]~+, and sodium azide produces [MO(NO)(H~NO)(N,)~]~-.~~~ The latter, as the tetraphenylphosphonium salt, has been shown to contain seven-coordinate molybdenum, with pentagonal bipyramidal coordination. The hydroxylamido(1- ligand is side-on, and the axial NO has LMoNO = 172.4(5)”, Mo-N=1.761(8) and N-O= 1.21(1)A.101 The structure is very similar to that of [MO(NO>(H~NO)(NCS)~]”-,’~~ and can be compared with [Mo(NO)(H2NO),(bipy)]+, which has two side-bonded hydroxylamido groups? [ M o ( N O ) ( H , N O ) ( ~ ~ ~ ~ )which , ] ~ + , has one,lZ1 and [MO(NO)(H~NO)(H~~)(~~~~~)]~+, which also has one.122 All these structures contain seven-coordinate pentagonal bipyramidal molybdenum, with linear, axial NO. The nitrosyl complexes containing hydroxylamine derivatives include [Mo(NO)(H,NO)(dipic)(L)]”(dipic = pyridine-2,6-dicarboxylate, II = 0 or 1, L = H20, CN, N3 or SCN),121 [Mo~NO)~NH20H)(H,NO)(terpY)I7’2~[Mo(NO)(H,NO)(phen),I2+, [Mo(NO)(H2NO)2 (terpy)]+ and [Mo(NO)(HzNO)(pic2] (pic = pyridine-Zcarboxylate). The molybdatelhydroxylamine reaction which gives rise to the species which is the source of these materials is clearly complex, and its constitution depends upon the pH of its preparation. For example, reaction in water at pH4-4.5 can give rise to [Mo(NO)(H2NO)(NCS),]”-, [Mo(NO)(HZNO)(NCS),L] (L = bipy or phen) and [Mo(NO)(HzNO)(S2CNEtz)z].Ia At higher pH the products are similar, but formally Mo’ or Moo. Possibly hydroxylamine derivatives could also be used, to give, for example, [Mo(NO)(Et,CNO)(NCS)4I2- lZo and [Mo(NO)(MezCNO)(NCS)4]2-lZ4 which have actually been synthesized from the H2N0 precursor and a ketone. The structure of the latter has been shown to contain side-on hydroxylaminate. 112~120 They again fit the usual pattern, with axial NO. However, MeHNOH and molybdate do not give rise to a homologue, but to [MoOz(MeHNO)~],”’ so that the reactivity pattern seems to depend upon the particular hydroxylamine used. The H2NO- ligand can be deprotonated to give HN02- complexes. Thus, [Mo(N0)~H2N0)(terpy)(H20)]”~ and CN- yield a complex [Mo(NO)(HNO)(CN)(terpy)].12 The deprotonation is reversible and the proposed structure has been confirmed by X-ray analysis. In this case, too, the basic seven-coordinate pentagonal bipyramidal structure is observed, with NHO side-on, and NO axial, LMoNO = 178.0(7)”, Mo-N = 1.802(7) and N-0 = 1.209(9) A.1zz Tris(pyrazoly1)borate has a formal resemblance to cyclopentadienide, in that it can behave as a six-electron donor and occupy three sites in a coordination sphere. Consequently, one might expect a similarity between complexes containing the Mo(pzb)(NO) group and those containing the Mo(Cp)(NO) group [pzb = tris(pyrazolyl)borate(l - ), cp = cyclopentadienide(1- )]. This is indeed the case, and an extensive chemistry has developed for complexes of the types [Mo(NO)@zb)X2] and [Mo(NO)(pzb)XY] (X and Y = anionic ligands). Much of this chemistry develo ed from an attempt to synthesize [(Mo(N0)(pzb)X2},], analogous to [{Mo(NO)(C~)X~}~].~ In this work, the differences between tris(pyrazoly1)borate and tris(3,5dimethylpyrazoly1)borate(Me2pzb) also became evident. The compounds [Mo(NO)(Me2pzb)X2] (listed in Table 7) are obtained by the general reaction of [M0(C0)~(NO)(Me~pzb)l with Xz.This is not always a straightforward reaction, and is sometimes accompanied by halogenation of the tris(pyrazoly1)borate. The Me2pzb derivatives tend to be monomeric, whereas those based on pzb itself are dimeric. 125~126 The dihalides react with alcohols leading to the formation of an enormous range of alkoxides and even aryloxides IMo(NO)(Mezpzb)X(OR)] as detailed in Table 7. The crystal structure of [Mo(NO)(Me,pzb)CI(OPt)] has been determined,’= and the molybdenum shows normal octahedral six-coordination, albeit with a very short Mo-0 bond. Reaction of [Mo(NO)(Mezpzb)I(OR)] with silver acetate in the presence of the alcohol ROH leads to the bislalkoxides) [Mo(NO)(Me,pzb)(OR),j. The mixed alkoxides were similarly prepared.lZ6The structures of several dialkoxides (Et, Pr’; Pr’, Pr’; Et, Et) have been investigated in detail in view of the rather bulky character of the ligands at molybdenum. The reaction of the dihalides with alcohols is paralleled by their reactions with other organic species containing acid hydrogens, such as phenols, amines and thiols. The resulting compounds are all detailed in Table 7. Representative structures include [MO(NO)(M~~~Z~)Z(NHC&~OM~)] and [MO(NO)(M~~~~~)I(NHC~H~M~)]~~~ which are octahedral and very similar, with linear LMoNO systems Mo-N = 1.754(11) and 1.78(2), N-0 = 1.211(14) and 1.16(3) A, respectively].1zJ The thiolato complex [ M o ( N O ) ( M ~ ~ ~ Z ~ ) I ( S &isHalso ~ ~ )distorted ] octahedral with a rather short Mo-S (2.32 A); LMoNO = 176(3)”, M-N = 1.74(4) and N--0 = 1.18(5) A.128
B
1292
Molybdenum
The compounds with NHNHz are unusual and rare examples of hydrazide(1- ) as a ligand, and have end-on, rather than side-on, hydrazide coordination. The structures are otherwise as expected with LMoNO = 174(3) and 174.8(9)", Mo-N = 1.70(3) and 1.80(2), and N-0 = 1.24(4) and 1.17(3)A for X, Y = I , NHNMePh and X, Y = I , NHNMe2 in [Mo(NO)(Me,pzb)XY], respectively.129So far, the rich hydrazinelhydrazide chemistry of the cyclopentadienide homologues'30 has not been reproduced. The NO stretching frequencies quoted in Table 7 fall into classes as shown [Mo(NO)(Me2pzb)XY]: X, Y = I, SR (mean 1679 f 5) > SR, SR (1675) > I, NHR (1667) > SR, OR (1656)>0R, OR (1646)>SR, NHR (16352>OR, NHR (1632) (all cm-l), generally consistent with the donor capacities of X, Y.12 For a discussion of the reactions of the cyclopentadienyl analogues of some of these pyrazolyborate complexes, see the references ~it~d.54,131 Whereas these reactions of dihalides with proton-active compounds seem reasonably simple, [Mo(NO)(Me2pzb)Iz] reacts with MezCO, MeEtCO or diacetone alcohol in complex fashion, affording some [Mo(NO)(Me2pzb)I(OEt)] but also some [ (Mo(NO)(M~~~Z~)I}~O] presumably via a reduced species such as the paramagnetic M0(NO)(Me~pzb)1.'~~ The possible reaction course has been discussed, and it has been established that the dinuclear species has a slightly bent MoOMo system [171.0(15)"], with linear LMoNO (176(3)", mean), Mo-N = 1.74(3) and N-0 = 1.21(4) A (mean values). The two molybdenum coordination shells are eclipsed with respect to each other.'33 Less usual reactions of [Mo(NO)(Mezpzb)12] are with 1,2-C6&(XH)(YH) where X and Y bind replaceable (acid) protons. In general, only one group loses its proton, giving [Mo(NO)(Me2pzb)(XC6H4YH)I] for X = N, Y = N; X = N, Y = 0 ; X = N, Y = S and X = S , Y = S. Why this should be isn't clear. Even more, for X = 0, Y = 0, the product is [Mo(NO)(3,5-Me2C,N2H,),(OC6H4~)]I3in which the catechol is still monodentate though dianionic, and from which a proton has been lost. The mechanism is again unclear, though the structure is une qui~oca l.'The ~~ molybdenum is still six-coordinate, with LMoNO = 173.3(12)", Mo-N = 1.771(13) and N - O = 1.188(18) A.134 The Mo" diiodide reacts with heterocyclic compounds to give [Mo(NO)(Me2pzb)&]+ (Z=pyridine, etc.), and with sodium amides to give diamido species, as detailed in Table 7. The product containing pyrollidides has the usual octahedral structure with LMoNO approaching linear, and N-0 = 1.163 A.135 Although 1,2-C6J&(XH)(YH) (see above) do not form chelate or bridging (dinuclear) species, dinuclear species are formed if the groups XH and YH are far enough apart to enable the two centres to react with different metal ions quite independently. Thus 4H2NC&14CH2C61&NHz-4 forms [Mo(NO)I(Mezpzb)(NHC&CH2C6H4NH2)] and thence [{MO(NO)(M~~~~~)I)~NHC~H~CH~C~€&NH]. The species so formed, together with their reduction potentials, are reported in Table 8.136The two metal ions may be the same or different. Apart from those species listed in Table 8, [Mo(NO)(Me2pzb)I{NHCd&Pd(PPh3)zI}] and [MO(NO)(M~~~Z~)I{NH(CH~)~PP~~}~H~I~] have also been 1nenti0ned.l~~ The electrochemical properties of these species have been studied, and it is apparent that there is no electrochemical influence of one metal atom upon the other if there are CH2 groups in the bridge, However, if the bridge contains S, SO2 or 0, which allow transmission of electronic effects, two overlapping reduction waves are observed, suggesting some interaction. Finally, where the bridge is C6H4,the interaction is very strong. There is a linear correlation between EIl2 and the Harnmett IJ pura constants for compounds containing appropriate para-substituted groups. This study is part of a wider discussion of the electrochemistry of these Mo" species.13' In particular, [Mo(NO)(Me2pzb)I,] is reduced reversibly in a range of solvents, S, to generate [Mo(NO)(Me2pzb)I(S)], in a one-electron process. The product is itself redox active. The Eln value is +0.22 V (THF) whereas the corresponding dichloride is reversibly reduced at -0.09 V, under similar conditions. The reduction product is substantially more stable than {Mo(NO)(Me2pzb)12]-, and shows little sign of the production of free chloride. The only product isolated from the reduction of the diiodide is [MO(NO)(M~~~~~)I~(L~(OE~~)~
X2424 Diazenido complexes The principal interest of diazenido complexes of rnolybdenum(I1) is that they are formed by the alkylation or protonation of coordinated dinitrogen. They thus represent a unique group of
MoZybdenurn(O), ( I ) and (11)
1293
Table 8 Reduction Properties of Some Bimetallic and Monometallic Mo" Species
MoNHC&NH, MoNHC&NHMo MoNHC&NH W MoNHC,&,OH
-0.98, - 1.52 (Ep/red) -0.48 -0.74, -1.66 (Ep/red)
MoNHC,H.,OMo MoNHC&I,OW MON HC&I,XC,H,NHM' X=CH,,M'=H X=CH2, M = M o X = (CH,),, M' = H X = (CH,),, M'= Mo X=SO,, M ' = H X = SO,, M'= Mo X = 0, M ' = H X = 0 ,M' = Mo X = 0,M' =W
-0.15, -1.16 -0.28, -1.60 (Ep/red)
-1.01 (Ep/red)
-0.85 -0.84 (two-electron process) -0.85 -0.84 -0.54 -0.49,-0.60 -0.86 -0.88 (two-electron process) -0.81, -1.37
For oneelectron process unless otherwise stated, Mo = Mo(NO)(Me,pzb)I, W = W(NO)(Me,pzb)I. See G. Denti, C. J. Jones, J. A. McCleverty, B. D. Neaves and S. I. Reynolds, J . Chern. Soc., Chern. Commun., 1983, 474; and T. N. Brigs, C. J. Jones, J. A. McCleverty, B. D. Neaves, N. El Mum and H . M. Colquhoun, I . Chsm. Soc., Dalton Tranr., 1985, 1249 for a complete discussion.
compounds , since well-defined species from the reactions of coordinated dinitrogen which contain dinitrogen hydride residues are uncommon. The general formula of the majority is [M~(N~R)X(diphosphine)~] (R = H or alkyl, X = halide, diphosphine usually dppe). They are listed in Table 9. The base members of the series [M~(N~H)X(dppe)~l (X = F or Br) were obtained initially by treatment of [Mo(N2H2)X(dppe),]+ (themselves obtained from reaction of [M~(N,),(dppe)~] with HX) with exactly one mole of NEt3.138If an excess of base is used, reaction under N2 regenerates the parent [M0(N2)~(dppe)~]. They react with acid, primarily to regenerate the hydrazide(2 - ) complexes, and kinetic studies suggest that they are also formed as intermediates during the conversions of coordinated dinitrogen to hydrazide(2 M*N2
e
M-NZH
(13)
5 M-NZH,
Their proton base strengths are less than that of the dinitrogen complex, but greater than that of the hydrazido(2 - ) c ~ m p l e x e s .Their ' ~ ~ structures are likely to be similar to those of the alkyldiazenido complexes discussed below , that is trans octahedral with a linear MoNN system and NNH = 120", but definite evidence is lacking. The alkyldiazenido complexes [MoX(N,R)(dppe),] are generally synthesized by reaction of alkyl halides with [ M ~ ( N ~ ) ~ ( d p p eThe ) ~ ] .reaction mechanism has been elucidated in some detail,I4" and is believed to involve a step in common with substitution reactions of the bis(dinitrogen) species, namely rate-controlling reversible thermal loss of N2 (equation 14). The full reaction sequence is set out in equation (15). [ M 4 N d d d p ~ e ) ~e l [ M ~ ( N ~ ) ( d p p e ) ~N2 l+ [MO(N2)z(dPPe)21
-
(14)
IWN*)(dPPe)21 Rx-, [ M o ( N z ) ( ~ ) ( d P P e ) 2 1
[Mo(N,)X(dppe),l+
R. (15)
The radical R. may then attack the solvent, as it does with THF, affording RH and a solvent radical. If R. is unreactive, it may eventually dimerize, yielding Rz (e.g. for R = PhCH2) and the principal complex product is [ M ~ X ~ ( d p p e ) ~If) ~R. ) . is unstable, then it can decay (e.g. BrCHzCHZ*yields &and € [M~Brz(dppe)~]). i, Finally, and in the most significant case, R. (or the solvent radical produced by it) can attack the remaining coordinated N2 to produce [Mo(N,R)X(dppe),]. The compounds obtained by this route are all listed in Table 9. In general, reactivity of alkyl halides falls in the sequence RJ > RBr > RC1, but certain activated
1294
Molybdenum Table 9 Diazenido Complexes of Molybdenum(1I)
v(N=N) (cm-')
Complex
Not assigned (F); 1940(Br) (Nujol)' 1540 (Br);' 1535 (Br)? 1542 (I)4 (all KBr); 1550 (I (?),' not assigned in ref. 6 1560-1540: 1555-1505' (all KBr) 1560-1515 (KBr)' 1560-1515 (KBr)7 1540-1500 (KBr)7 1535-1500 (KBr)7 1530, 1516 (Br) (Csl);' 1529, 1511 (I) (KBr)? 535 (CNB (Nujol);R 1529 (CNS) (KBr);3 1525 (SCN) (Nujol); 1523 (N,) (KBr);3 not assigned (OMe)' 1540 (KBr)'" 1510 (KBr)" 1527 (KBr)" 1525 IKBr)3 1550 (I) (KBr);4 1550 (I) ( ? ) ; 5 not assigned (1): 1475 (OH) (CSI) Between 1516 and 1535, not assigned3 1535 [CSI):~not assigned in ref. 12 Not assigned" Not assigned (Me): 1330 (Et) (Nujol);, 1356 (Ph) (KBr)13 Not assignedI4 1632 (KBr)" 1345 (?)16 ca. 1508 (KBr) (for X = C1, Br or I)I7 1622, 1570 (CHzC12)'s 1632, 1561 (4-C6H4Me); 1628, 1565 (4-C,H4F); 1621, 1564 (3-C6H4F);1623, 1560 (Ph) (all CH,Cl,)" 1623,1562 (4-CJIH,Me);1625, 1567 (4-C,H4F); 1628, 1656 (sic) (3-C,H4F); 1633, 1561 (Ph) (all CHzC12)'s 1545 (4-C,H4Me, Cl); 1546 (4-C6H,Me, Br); 1550 (4-C6H,F, Cl); 1543 (4-C,H4F, Br); 1553 (4-C H F ' I) (all CH,Cl,)'s 1546 (Cl); 1547 (Br) (all CH,CI,)'86 1559 (Ph); 1568 (CCa4F); 1538 (3-C,H,F); 1530 (4C,H4NOz); 1562 (2-C,H4Me); 1579 (2,5-C6H,Me,) (solid state Raman);" IR values for v(N2) are ca. 1620'" 1569 (Ph); 1582 (4-CbH4F); 1554 (3-C,H,F); 1528 (4 C,H NOz); (solid state Raman);" IR values for v(N,) cu. 1620" 1534 (bipy); 1535 (phen) (all CH2C1z)'8 1530,1485 (4-C6&Me); 1532,1486(4-C,H4F) (all CH,C1,)21 probably Ar = Ph only, 1642 (?)" ~~
~
\
L
CsH,, = cyclohexyl. dppb = bis(dipheny1phosphino)butane. E pzpzb = tetra(pyrazoly1)borate. 1. J. Chatt, A. J. Pearrnan and R. L. Richards, 1. Chem. Soc., Dalfon Trans., 1976, 1520. 2. J . Chatt, A. A. Diamantis, G . A. Heath, N. E. Hooper and G. J. Leigh, J. Chem. Soc., Dalton Trans., 1977, 688; A. A. Diarnantis, J.,Chatt, G. J. Leigh and G. A. Heath, J. Organomel. Chem., 1975, 84, C11. 3. G. E. Bossard, D. C. Busby, M. Chang, T. A. George and S. D. A. Iskc, J. Am. Chem. Soc., 1980,102, 1001. 4. D. C. Busby, T. A. George, S. D. A. Iske and S. D. Wagncr, Inorg. Chem., 1981,ZO, 22. 5. P. Brant and R. D. Feltham, J. Organomer. Chem., 1976, UO,C53. 6. V. W.Day, T, A. George and S. D. A. Iske. J . Am. Chem. Soc., 1975, 97, 4127. 7. W. Hursain, G. J. Leigh, H. Mohd.-Ali and C. J. Pickett, J. Chem. Soc., Dalfon Trms., 1986, 1473. 8. J. Chatt, G.J. Leigh, H. Neukomm, C. J. Pickett and D. R. Stanley, J . Chem. SOL, Dalron Tram., 1980, 121. 9. D. C. Busby, C. D.Fendrick and T. A. Georgc, Proc. Third Int. Conf. Chem. and Uses Molybdenum, 1979,290. 10. G. E. Bossard, T. A. George, R. K. Lester, R. C. Tisdale and R. L. Turcotte, Inorg. Chem.,1985, 24, 1129. 11. P. C. Bevan, J. Chatt, A. A. Diamantis, R. A. Head, G. A. Heath and G. J. Leigh, J. Chem. SOC., Dalton Trans., 1977, 1711; A. A. Diamantis, J. Chatt, G. A. Heath and G. J. Leigh, J. Chem. Soc., Chem. Commun., 1975, 27; P. C. Bevan, J. Chatt, R. A. Head, P. B. Hitchcock and G. J . Leigh, J. Chem. Soc., Chem. Commun., 1976,509. 12. V.Day, T. A. George and S. D. A. Iske, J . Organornet. Chem., 1976, lU, C55. 13. M. Sato, T. Kodama, M. Hidai and Y.Uchida, J. Organornet. Chem., 1978, 152, 239; T. Tatsurni, M. Hidai and Y.Uchida, Inorg. Chem., 1975, 14, 2530. 14. K. Iwanami, Y.Mizobe, T. Takahachi, T. Kodarna, Y. Uchida and M. Hidai, Bull. Chem. SOC. Jpn., 1981, 54, 1773. 15. G. Butler, J. Chatt, W. Hussain, G . J. Leigh and D. L. Hughes, Inorg. Chim. A m , 1978, 30, L287. 16. H. M. Colquhoun, Transition Mef. Chem., 1981, 6, 57. 17. D. C. Busby and T. A. George, J. Organornet. Chem., 1976,118, C16. 18. D. Condon, M. E. Deane, F. J. Lalor, N. G. Connelly and A. C. Lewis, J. Chem. SOC.,Dalton Tram., 1977, 925. 19. D. Sutton, Can. J . Chem., 1974,52,2634. 20. S . Trofimenko, Inorg. Chem., 1969,8, 2675. 21. D. Condon, G. Ferguson, F. J . Lalor, M. Parvez and T. Spalding, Znorg. Chem., 1982,21, 188. 22. M. E. Deane and F. J. Lalor, J. Organornet. Chem., 1973, 57, C61.
a
Molybdenum((?), ( I ) and (IZ)
1295
chlorides form diazenido complexes although most simply give [M~Cl,(d$pe)~]. This mechanism has received support from studies involving chiral phosphines. The rarer acylation reactions are believed to follow the same pattern.I4" An alternative reaction route for electron-rich dinitrogen complexes, such as [Mo(N2)(SCN)(dppe),]giving [Mo(N,Bu")(SCN)(dppe),], has been de~cribed,~' as has a rather restricted reaction sequence for [ M ~ ( N ~ ) ~ ( d p p and e ) ~ ](CF3C0),O yielding [MO(N~COCF~)(O~CCF~)(~~~~)~]. This is believed to involve non-radical nucleophilic reaction of the coordinated N2.14, The general reactions of the compounds are much as expected. They react with strong reducing a ents to give ammonia and amines, with complete destruction of the complexes. 14s14sj The halides are labile and can be substituted.'" The compounds react with acids to form alkylhydrazido c o r n p l e x e ~ , and ' ~ ~ with alkyl halides to form dialkylhydrazido(2 - ) complexes, in reactions which are typical SN2 r e a c t i ~ n s , though '~~ electrophilic attack (viz. by Me+) is also a p~ssibility.'~' The structures of some of these complexes have been determined. They all have a trans configuration, with octahedral coordination around the molybdenum, with a linear MoNN system, and LNNC ca. 120". For [MoI(N2C Hll)(dppe)2], LMoNN = 176(1)", LNNC = 142(2)", Mo-N = 1.95(1) and N-N = 0.91(1) This last value is almost certainly an artefact of the poor quality of the crystals used, and a value of ca. 1.15 hi might be expected. In [M~Cl(N~CO~Et)(dppe)~l, LMoNN = 178.9(5)", LNNC = 117.0(6)", Mo-N = 1.732(5) and N-N = 1.382(10) A.152The acyldiazenido complex [MoCl(N,COPh)(dppe)d has LMoNN = 172.1(6)", LNNC = 116.7(7)", Mo-N = 1.813(7) and N-N = 1.255(10) A.931 XPS data on [M~I(I%Me)(dppe)~] and [M01(N~C,H,,)(dppe)~]have been reported, and in the latter case suggest the presence of two ty es of nitrogen, as expected, but for the former the two nitrogens give rise to only one signal.' 4 ~ 1 5 5I5N NMR spectroscopy has been carried out on [MoBr(N2Et)(dppe)2]. The other diazenido complexes of Mo" are generally carbonyl species and almost all are phenyldiazenido complexes. None is obtained via coordinated dinitrogen. The complex [Mo(C0),(N2C6H4F)(bipy)]BF4 was prepared by reaction of (p-FC6E€&)(BF4) with [M0(C0)4(bipy)].'~~ This is the only compound of the class characterized, although the preparative route is suggested to be quite general for aryldiazonium salts. The complex shows two IR bands assigned to v(CO), which were used as a basis for ascribing afac conformation to the ~ a r b o n y 1 s . The l ~ ~ presence of two IR bands associated with v(N=N) was explained by mixing of N-N and phenyl-ring vibrations, which finds support from l5N-labelling studies. 157 Dicarbonyl complexes [MO(CO)~(N,A~)(L-L)(PP~~}]' (L-L = phen or bipy, Ar = aryl) can be prepared by reaction of the tricarbonyls with PPh3, but the preferred route appears to involve the reaction of [Mo(CO)~(L-L)(PP~~)] with the appropriate diazonium salt at -78 "C. The compounds so prepared are listed in Table 9. No discussion of structure is provided. These dicarbonyls react with triphenylphosphineiminium salts to lose phosphine and generate another series of dicarbonyls. [Mo(CO),(N,Ar)X(L-L)] (X = C1, Br or I). Again, no structural discussion is provided. Finally [Mo(CO),(N,Ar)(L-L)(PPh,)]+ react with PPh3 in acetone at reflux to yield [Mo(CO)(N,AR)(L-L)(PPh,),]+ . Preparations and characterizations of all these compounds are fully described.' The structures (not fully discussed) are presumably the same as those of their nitrosyl analogues (classified under Moo). The last considerable group of diazenido complexes contain tris(pyrazoly1)borate as a ligand. The complexes [M0(C0)~(N~Ar)(pzb)] and [M~(CO)~(N~Ar)(pzpzb)] (pzpzb = B(Pz)~,where first obtained by the reaction of the appropriate pz = pyrazolyl) were tricarbonyl(pyrazolylborato)molybdenum(l - ) with a diazonium salt. 15* The values of v(C0) follow the trends expected from the electronic properties of the pyrazolylborate and arenediazenide groups. Where appropriate, the I?!? NMR spectra were discussed, but no specific values of Y(N=N) were reported. These compounds are remarkably stable, and [Mo(CO),(N2Ph)(pzb)] is unaffected by hot 70% sulfuric However, halogens produce very stable {Mo(N2Ph)(pzb)X}. (X = C1, Br or I).'59 Later Raman studies, involving labelling with 2H and/or "N enabled assignments of v(N=N) to be made with more precision. The band assigned to v(N=N), or predominantly to (N=N),occurs in the region of 1550 cm-' and is strongly Raman active.lm The crystal structure of [M0(C0)~(N~Ph)(pzb)l shows the expected octahedral coordination around molybdenum, with a 'singly bent' LNNPh and Mo-N = 1.83 A.161 The pyrazolylborate dicarbonyls react with PPh3 in refluxing xylene to generate monocarbonyls [Mo(CO)(N2Ar)(pzb)(PPh3)], and with (4-MeC6H4S)z to produce [Mo(NZAr)(pzb)(SC6HMe)2J.162 They also undergo oxidative additions with NOCl in CH2C12, J~~
P
Molybdenum
1296
to produce [Mo(Cl)(N2Ph)(pzb)(NO)], for example.la The chloride seems inert to Grignard reagents and to halide abstractions.
36.2.4.3
Miscellaneous
The five compounds in this group are unrelated, and even open to question to some degree. Thus, reaction between and H2S in the presence of cyanide yields [Mo&(CN),]~which has a double sulfide bridgela and hence is formally Mo"'. However, if there is significant S-S interaction, this could be formulated as Mo". In the presence of 02,this plus ] ~ - , has a single Mu-S-Mo bridging system, the first [Mo(CN)~]~yields [ M O ~ S ( C N ) ~ ~which of its kind reported, with short Mo-S [2.172(5) A3 and a linear MoSM0[169,5(3)'].'~ The reaction of [MoCl,(bipy)] with Me3SiN3 results in a mixture of [MoN(N,)(bipy)] (isolated and characterized by X-ray structural analysis) and an insoluble material which analyzes for MoC12(N2)(bipy),and possesses no IR bands assignable to v(Mo=N), v(N3) or d(N3) [nor, apparently, to v(N2)]. It has peff= 2.2 BM. If confirmed, it would be the first stable Mo" dinitrogen complex. 165 Carbon dioxide reacts with [ M O ( N ~ ) ~ ( P M ~to~produce P ~ ) ~ ] not a carbon dioxide complex, but a carbonato-carbonyl, whose formation involves reductive disproportionation of CO, (equation 16). The structure of the Mo" product [{MO(CO)(PM~,P~),}~(~--C~~)~] has been determined by X-ray structure analysis.'66
+
4c0, 4e-
-
2co;-+2co
(16)
Nitrobenzene reacts with [ M O ( C O ) ~ ( S ~ C N Eto~ yield ~ ) ~ ] [Mo(ONP~)O(S,CNE~~)~] which is reduced by PPh3 giving [ M O ( O N P ~ ) ( S ~ C N Eand ~ ~ ) ~OPPh3. ] This is regarded as a q2nitrosodurene complex on the basis of NMR evidence. If PhNO is regarded as a neutral ligand, this material contains Mol'. It does not isomerize to [MOO(NF'~)(S~CNE~~)~], a known compound, either thermally or ph~tolytically.~~' The complex [ M O ( C O ) ~ ( S ~ C N Ealso ~ ~ ) reacts ~] with acetylenes, losing one CO, to be replaced by acetylene, and with diazenes, yielding [Mo(RN2R')(S2CNEtz)2] and [ M O ( R N ~ R ' ) ~ ( S ~ C N E(R-= ~ ~ ~R'] = C0,Et). Both complexes hydrolyze to give Et02CNHNHC02Et.'6 There is, of course, an extensive organometallic chemistry, some of it of MolL,based on the Mo(S2CNEt2)2moiety. Only the little discussed above is appropriate to this chapter.
36.2.5
REFERENCES
1. (a) G. Wilkinson, F. G. A. Stone and E. W. Abel (ed.), 'Comprehensive Organometallic Chemistry', Pergamon, Oxford, 1982, vol. 6, p. 1079; (b) D. A. Bohling, K. R. Mrun, S. Erjer, T. Gennelt, M. J. Warner and R. A. Walton, Znorg. Chim. Acta, 1985, 97, W1 and references therein. 2. A. F. Masters, G. E. Bossard, T. A. George, R. T. C. Brownlee, M. J. O'Connor and A. G. Wedd, Znorg. Chem., 1983,22, 908. 3 . (a) M. Sato, T. Tatsumi, T. Kodama, M. Hidai and Y. Uchida, 1. Am. Chem. Soc., 1978, 100,4447; (b) T. A. George and R. C. Tisdale, Polyhedron, 1986, 5,297 and references therein. 4, A. J. L. Pombeiro and R. L. Richards, J. Chem. SOC., Dolton Trans., 1980, 492; A, J. L. Pombeiro and R. L. Richards, Transition Mer. Chem., 1980, 5, 55. 5. J. Chatt, C. M. Elson, A. J. L. Pombeiro, R. L. Richards and G. H. D. Royston, J. Chem. SOC., Dalton Trans., 1978, 165. 6. J. Chatt, J. R. Dilworth and R. L. Richards, Chem. Rev., 1978, 711, 589; J. Chatt and R. L. Richards, J. Organomet. Chem., 1982, 239, 65 and references therein. 7. E. Carmona, A. Galindo, J. L. Atwood, L. G. Cavada and R. D. Rogers, J. Organomet. Chem., 1984,277,403. 8. S . N. Anderson, D. L. Hughes and R. L. Richards, J. Chem. SOC., Dalton Trans., 1986, 1591. 9. T. Tatsumi, M. Hidai and Y.Uchida, Znorg. Chem., 1975, 14, 2530. 10. R. H. Morns and J. M. Ressner, J. Ckem. SOC., Chem. Commun., 1983,909. 11. Y. Uchida, T. Uchida, M. Hidai and T. Kodama, Acta Crystullogr,, Sect. B, 1975, 31, 1197. 12. M. Aresta and A. Sacco, Gazz. Chim. Ital., 1972, 102, 755. 13. R. H. Morris, J. M. Ressner, J. F. Sawyer and M. Shiralian, J . Ana. Chem. Soc., 1984, 106, 3683. 14. S. Donovan-Mtunzi, M. Hughes, G. J. Leigh, J. Mason, H. Mohd.-Ali and R. L. Richards, J. Organomet. Chem., 1983,246, C1. 15. G. J. Leigh and C. J. Pickett, J. Chem. SOC., Dalton Trans., 1977, 1797. 16. J . Chatt, C. T. Kan, G. J. Leigh, C. J. Pickett and D. R. Stanley, J. Ckem. SOL, Dalton Trans., 1980, 2032. 17. M. L. H. Green and W. Silverthorn, I. Chem. SOC.,Dalton Trans., 1973,301.
Molybdenurn(O), ( I ) and (ZZ)
1297
18. J. H. Enemark and R. D.Feltham, Coord. Chem. Rev., 1974, W, 339; D. Ballivet-Tkatchenko, C. Bremard, F. Abraham and G . Nowogrocki, J . Chem. SOC., Dalton Tram., 1983, 1137; C. Schumacher, F. Weller and K. Dehnicke, 2.A m r g . Alig. Chem., 1984,508, 79. 19. E. I. Stiefel, Prog. Inorg. Chem., 1976, 22, 1 and references therein. 20. N. G. Connelly, Inorg. Chim. Acta, Rev., 1972, 6, 48. 21. W. B. Hughes and E. A. Zuech, Inorg. Chem., 1973,12,471. 22. M. Minelli, J. L. Hubbard and J. H. Enemark, Inorg. Chem., 1984, 23, 970. 23. B. F. G. Johnson, J. Chem. SOC., Dalton Trans., 1974,475. 24. M. W. Anker, R. Colton and I. B. Tomkins, Aust. J. Chem., 1968,21, 1149. 25. R. D. Feltham, W. Silverthorn and G. McPherson, Inorg. Chem., 1969, 8,344. 26. M. Green and S. H. Taylor, J. Chem. SOC., DaZton Trans., 1972, 2629. 27. N. G. Connelly, J. Locke, J. A. McCleverty, D. A. Phipps and B. Ratcliff, Inorg. Chem., 1970, 9, 278. 28. B. F. G . Johnson, A. Khair, C. G. Savory and R. H. Walter, J. 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1298
Molybdenum
L. Pombeiro and R. L. Richards, Transition Met. Chem., 1981, 6,255. 7 5 . S. N. Anderson, D. L. Hughes and R. L. Richards, Transition Met. Chem., 1985, 10, 29. 76. T. AI-Salih and C. J. Pickett, J. Chem. SOC., Dalton Trans., 1985, 1255. 77. G. Pelizzi and G. Prediere, Gazz. Chim. IraL, 1982, 112, 381. 78. T. A. George and D. B. Howell, Inorg. Chem., 1984,23, 1503. 79. 3. Lewis, R. S. Nyholm and P. W. Smith, J . Chem. SOC., 1962, 2592. 80. G. E. Bossard and T. A. George, Inorg. Chim. Acta, 1981, 54, L241. 81. D . C. Povey, R. L. Richards and C. Shortman, Polyhedron, 1986,5, 369. 82. R. L. Richards and C. Shortman, 3. Organornet. Chem., 1985, 286, C3. 83. M. Hidai, K. Tominari and Y. Uchida, J . Am. Chem. Soc., 1972,94,110; L. J. Archer and T. A. George, Inorg. Chem., 1979,18,2079. PA. J. L. Atwood, E. Carmona-Guzman, W. E. Hunter and G. Wilkinson, J. Chem. Soc., Dalton Trans., 1980, 467. 85. A. R. Barron, J. E. Salt and G. Wilkinson, J . Chem. Soc., Dalton Trans , 1986, 1329. 86. M. E. Hursthouse, D. Lyons, M. Thornton-Pett and G. Wilkinson, J . c'hem. Soc., Dalton Trans., 1984, 695. 87. R. Alvarez, E. Carmona, D. 1. Cole-Hamilton, A. Galindo, E. Gutierrez-Puebla. A. Monge, M. L. Poveda and C. Ruiz, J . Am. Chem. Soc., 1985, 107, 5529. 88. H. W. Choi and E. L. Muetterties, J . Am. Chem. SOC., 1982,104, 153. 89. R . Taube and K. Seyferth, 2. Anorg. A&. Chem., 1977,437, 213. 90. K. Seyferth and R. Taube, J. Mol. CataL, 1985, 28, 53. 91. D. Schnurpfeil, G. Lauterback, K. Seyferth and R. Taube, J . Prakt. Chem., 1984, 321, 1025. 92. R. Davis, B. F. G. Johnson and K. H. AI-Obaidi, J . Chem. SOC., Dalton Trans., 1972, 508. 93. K. E. Voes, J. D. Hudman and J. Kleinberg, Inorg. Chim. Acta, 1976,20,79. 94. D. Sellmann, L. Zapf, J. Keller and M. Moll, J . Organomet. Chem., 1985, 289, 71. 95. L. Bencze and J . Kohan, Inorg. Chim. Actn, 1982, 65, L17; L. Bencze, J. Kohan, B. Mohai and L. Marko, J . Organornet. Chem., 1974,70, 421. 96. K. Dehnicke, A. Lickett and F. Weller, Z . Anurg. Allg. Chem., 1981, 474, 83. 97. P. Chaudhuri, K. Wieghardt, Y. H. Tsai and H. Kruger, Inorg. Chem., 1984,23,427. 98. R. Bhattacharya, R. Bhattacharjee and A. M. Saha, Polyhedron, 1985,4, 583. 99. J. Beck and J. Strahle, Z . Nafurforsch., Teil B, 1985, 48, 891. 100. P. J. Blower, P. T. Bishop, J. R. Dilworth, T. C. Hsieh, J. Hutchinson, T. Nicholson and J. Zubieta, Inorg. Chem., 1985,101,63. 101. K. Wieghardt, G. Backes-Dahmann, W. Swiridoff and J. Weiss, inorg. Chem., 1983, 22, 1221. 102. P. T. Bishop, J. R. Dilworth, J . Hutchinson and J. A. Zubieta, Inorg. Chim.Actn, 1984, 84, L15. 103. S . Sarkar and P. Subramanian, J. Inorg. Nucl. Chem., 1981, 43, 202. 104. S. C. Tripathi, S. C. Srivastava, A. K. Shrimal and 0. P. Singh, Transition Met. Chem., 1984, 9,478. 105. B. F. G . Johnson and K. H. Ai-Obaidi, Chem. Commun., 1968, 876. 106. B. F. G . Johnson, K. H. AI-Obaidi and I. A. McCleverty, J . Chem. SOC. ( A ) , 1969, 1688. 107. 8 . F. G. Johnson, A. K. Khair, G. G. Savory, R. H.Walter, K. H. AI-Obaidi and T. Y. Al-Hussain, Transirion Met. Chern., 1978, 3, 81. 108. T. F. Brennan and I. Bernal, Chem. Comrnun., 1970, 130. 104. T. F. Brennan and 1. Bernal, Inorg. China. Acta, 1973, 7, 283. 110. R. Davis, M. N. S. Hill, C. E. Holloway, B. F. G. Johnson and K. H. Al-Obaidi, J . Chem. SOC. ( A ) , 1971,994. 111. F. Minelli, C. G. Young and J. H. Enemark, Inorg. Chem., 1985, 24, 1111. 112 A. Muller, S. Sarkar, N, Mohan and R. G. Bhattacharya, Inorg. Chin. Acta, 1980, 45, L245. 113. J. A. Broomhead and J. R. Budge, Aust. J . Chem., 1979,32, 1187. 114. J. A. Broomhead, J. R. Budge, W. D. Grumley, T. R. Norman and M. Sterns, A u t . J . Chem., 1976,29, 2175. 115. J. Chatt and J. R. Dilworth, Proc. Int. Conf. Coord. Chem., l&h, 1979, 4.9; J. Chatt and J. R. Dilworth, J . Chem. SOC., Chem. Commun., 1974,517. 116. M. W. Bishop, J. Chatt and J. R. Dilworth, J . Chem. SOC., Dalton Trans., 1979, 1. 117. M. B. Hursthouse and M. Motevalli, J . Chem. SOC., Dalton Trans., 1979, 1362. 118. S. Sarkar and A. Muller, Angew. Chern., Int. Ed. Engl., 1977, 16, 183. 119. A. Liebelt, F. Weller and K. Dehnicke, 2. Anorg. Allg. Chem., 1980, 480, 13. 120. A. Muller, W. Eltzner, S . Sarkar, H . Bregge, P. J. Aymonino, N. Mohan, A. Seyer and P. Subramanian, 2. Anorg. A&. Chem., 1983, 503, 22. 121. K . Wieghardt, W. Holzback. J Weiss, 3.Nuber and B. Prikner, Angew. Chem., Int. Ed. Engl., 1979, l B , 548; K. Wieghardt, W. Holzback, B. Nuber and J. Weiss, Chem. Rer., 1980, 113,629. 122. K. Wieghardt, W. Holzback and J . Weiss, Z . Naturforsch., Ted R, 1982, 37, 680; K. Wieghardt and W. Holzback, Angew. Chem., In!. Ed. Engl., 1979,18, 549. 123. R. Bhattacharya and G. P. Bhattacharjee, J . Chem. Sac., Dalton Tram., 1983, 1593. 124. A. Muller and N. Mohan, 2.Anorg. Allg. Chem., 1981, 480, 157. 125. J. A. McCleverty, D. Seddon, N. A. Bailey and N. W. Walker, J. Chem. Soc., Dalton Trans., 1976, 898. 126. M. E. Deane and F. J. Lalor, J. Organornet. Chem., 1974, 67, C19. 127. J. A. McCleverty, G. Denti, S. J . Reynolds, A. S. Drane, N. El Murr, A. E. Rae, N. A. Bailey, H. Adams and J. M. A. Smith, J . Chem. SOC., Dalton Trans., 1983, 81. 128. J. A. McCleverty, A. S. Drane, N. A. Bailey and J. M. A. Smith, J. Chem. Soc., Dalton Trans., 1983, 91. 129. J . A. McCleverty, A. E. Rae, I. Wolochowicz, N. A. Bailey and J. M. A. Smith, J . Chem. Soc., Dalton Trans., 74. A. J.
1983, 71. 130. P. D. Frisch, M. M. Hunt, W. G . Kita, J . A. McClevcrty, A. E. Rae, D. Seddon, D. Swann and J. Williams, J . Chem. Soc., Dalton Trans., 1979, 1819. 131. See, for example, T. A. James and J. A . McCleverty, J. Chem. SOC. ( A ) , 1971, 1068; J . A. McCleverty and J . A. Williams, Transition Met. Chem., 1976, 1, 288. 132. H. Adams, N. A. Bailey, G. Denti, J. A. McCleverty, J . M. A. Smith and A. WIodarczyk, J. Chem. SOC., Chem. Commun., 1981,348.
MoZybdenurn(O), ( I ) and (11)
1299
133. H. Adams, N. A. Bailey, D. Gianfranco, J. A. McCleverty, J. M. A. Smith and A. Wlodarczyk, J. C h m . SOC., Dalton Tram., 1983, 2287. 134. H. Adams, N. A. Bailey, A. S. Drane and J. A. McCleverty, Polyhedron, 1983,2,465. 135. K. H. Al-Obaidi, K. P. Brown, A. J. Edwards, S. A. Hollins, C. J. Jones, J. A. McCleverty and B. D. Neaves, J. Chem. SOC.,Chem. Commun., 1984,690. 136. G. Denti, C. J. Jones, 3. A. McCleverty, B. D. Neaves and S. J . Reynolds, J. Chem. SOC., Chem. Comrnun., 1983, 474. 137. T. N. Briggs, C. J. Jones, J. A. McCleverty, B. D. Neaves, N. El-Murr and H. N. Colquhoun, J. Chem. Soc., Dalton Trans., 1985, 1248. 138. J. Chatt, A. J. Pearman and R. L. Richards, J. Chem. SOC., D a h n Trans., 1976, 1520. 139. R. A. Henderson, J. Chem. Soc., Dalton Trans., 1982, 917; R. A. Henderson, J . Chem. Soc., Dalton Tram., 1984,2259; R . A. Henderson and J. Lane, J. Chem. Soc., Dalton Tram., 1986, 2155. 140. J. Chatt, R. A. Head, G. J . Leigh and C. J. Pickett, J . Chem. SOC., Dolton Trans., 3978, 1638. 141, G. E. Bossard, T. A. George, R. K. Lester, R. C. Tisdale and R. L. Turcotte, Inorg. Chem., 1985, 24, 1129. 142. H. M. Colquhoun, Trumdion Met. Chern., 1981, 6 , 57. 143. P. C. Bevan, J. Chatt, G. J. Leigh and E. G. Leelamani, I. Organornet. Chem., 1977,139, C59. 144. G . E. Bossard, D. C. Busby, M. Chang, T. A. George and S . D. A. Iske, J . Am. Chem. SOC.,1980, 102, 1001. 145. D. C. Busby and T. A. George, Inorg. Chim. Acta, 1978, 24, L273. 146. V. Day, T. A. George and S. D. A. Iske, J . Organomet. Chern., 1976, 112, C55. 147. D. C. Busby, C. D. Fendrick and T. A. George, Proc. Third Int. Con5 Chem. and Uses Molybdenum, 1979,290. 148. J. Chatt, A. A. Diamantis, G. A. Heath, N. E. Hooper and G. J. Leigh, J . Chem. Soc., Dalton Trans., 1977, 688; J. Chatt, W. Hussain, G. J. Leigh, H. Neukomm, C. J. Pickett and D . A. Rankin, J . Chern. SOC., Chem. Comrnun., 1980, 1024. 149. W. Hussain, G. J. Leigh, H. Mohd-Ali and C. J. Pickett, J . Chem. Soc., Dalton Trans., 1986, 1473. 150. G. E. Bossard and T. A. George, Inorg. Chim. Acta, 1981, 54, L239. 151. V. W. Day, T. A. George and S. D. A. Iske, J . Am. Chem. Soc., 1975,97, 4127. 152. G. Butler, J. Chatt, W. Hussain, G. J. Leigh and D. L. Hughes, Inorg. Chim. Acta, 1978, 30,L287. 153. M. Sato, T. Kodama, M. Hidai and Y. Uchida, J. Orgunomex. Chon., 1978, 152,239. 154. P. Brant and R. D. Feltham, J. Less-Common Met., 1977,54, 81. 155. P. Brant and R. D. Feltham, J. Organomet. Chem., 1976, l20,C53. 156. J. R. Dilworth, C. T. Kan,R. L. Richards, J. Mason and I. A. Stenhouse, J. Organomet. Chem., 1980,201, C24. 157. D. Condon, M. E. Deane, F. J. Lalor, N. G. Connelly and A. C. Lewis, J . Chem. SOC., Dalton Trans., 1977, 925. 158. S . Trofimenko, Inorg. Chern., 1969, 8, 2675. 159. M. E. Deane and F. J. Lalor, J. Oranomet. Chem., 1974, 67. C19. 160.. D. Sutton, Can. J. Chem., 1974,52;2634. 161,. G. Avitabile, D. Ganis and M. Nemiroff, Acta Crystallogr., Sect. B , 15'1, 27, 725. 162.. D. Condon, G . Ferguson, F. J. Lalor, M. Parvez and T. Spalding, Inorg. Chem., 1982,21, 188. 163. M. E. Deane and F. J. Lalor, J. Organomet. Chem., 1973,57,C61. 164. M. G. B. Drew, P. C. H. Mitchell and C. F. Pygall, Angew. Chem., Int. Ed. Engl., 1976,15,784. 165. E. Schweda and J. Stdhle, Z. Naturfoixh., Teil B, 1980, 35, 1146. 166. J. Chatt, M. Kubuta, G. J. Leigh, F. C. March, R. Mason and D. J. Yarrow, J. Chem. Soc., Chem. Comrnun., 1974, 1033. 167. E. A. Maatta and R. A. D. Wentworth, Inorg. Chem., 1980, 19, 2597. 168. J. W. Macdonald, W. E. Newton, C. T. C. Creedy and J . L. Corbin, J . Organomet. Chem., 1975, 92, C25. ~~
*
coc3-PP
36.3 Molybdenum Cornplexes Containing One or More C. DAVID GARNER University of Manchester, UK 36.3.1 INTRODUCTION
1301
36.3.2 DIMETALLIC COMPLEXES 36.3.2.1 Complexes Containing a Quadruple Metal-Metal Bond 36.3.2.1.1 Synthesis and structure 36.3.2.1.2 Properties of M A 0 quadruple bonds 36.3.2.1.3 Reactions of compounds containing an Mo-Mo quadruple bond 36.3.2.2 Complexes Containing a Triple Metal-Metal Bond 36.3.2.2.1 Synthesis and structure of [Mo2L6](L = CH,SiMe,, NMe,, OR) molecules 36.3.2.2.2 Substitution and adduct formation of [Mo2L6]molecules 36.3.2.2.3 Properties of M o 4 0 triple bonds 36.3.2.2.4 Reactions of Mo-Mo triple bonds 36.3.2.2.5 Dicy clopenladkny ltetracarbonyldimolybdenurn 36.3.2.2.6 Halide and related complexes 36.3.2.3 Complexes Containing a Double Metal-Metal Bond 36.3.2.4 Complexes Containing a Single Metal-Metal Bond
1302 1302 1302 1308 1309 1310 1310 1311 1314
36.3.3 METAL CLUSTERS 36.3.3.1 Trimeric Clusters 36.3.3.2 Tetrameric Clusters 36.3.3.3 Hexameric Clusiers and Related Extended Arrays
1317 1317 1319 1321
36.3.4 REFERENCES
1322
36.3.1
1315 1316 1316 1316 1317
INTRODUCTION
Molybdenum displays a remarkable propensity to form metal-metal bonds, in all of its oxidation states-even MoV’ if one allows the close (ca. 2.7A) approach of metal atoms in complexes of [M0S4]’- (see Section 36.6.1) to be considered as a direct metal-metal interaction. As with other second row transition metals, this tendency to form metal-metal bonds can be rationalized in terms of the effective covalent overlap that can be achieved using the 4d orbitals, especially for the metal in its lower oxidation states. As a clear indication of this, the heat of formation’ of atomic Mo(g) is 658 kJ mo1-’, the bond dissociation energy of Mo2(g) is2 404 f20 IrJ molp1 and the M M oO separation 1.93 A.’ The electronic structure of this molecule has been discussed in some detail and potential energy curves ~al cu l at edSimple .~ MO considerations lead to the conclusion that the maximum Mo-Mo bond order will occur in Mo2, which could be considered to possess the configuration a , ( ~ ) ’ u ~ ( ( d ) ~ ~ ~ ( d ) ~ ~ ~ ( d corresponding to a sextuple bond; however, correlation effects mean that such a simple description is inappropriate. These effects are also important in molybdenum compounds which involve a close approach of two metal atoms and, hence, there has been vigorous discussion concerning the nature , strength and the interpretation of spectroscopic properties of complexes involving multiple Mo-Mo bonds. The formal bond order for Mo-Mo interactions in compounds ranges from 4 to ,(~C~C),(R'NC)~]. This is a novel conversion, thus: (i) an L4MoMoL4compound is converted to an L,Mo(~-L)~L, derivative; (ii) the multiple bond is not cleaved by isocyanide ligands; (iii) the unbridged $n4 triple bond is converted to an edge-bridged d3-d3 dimer with a formal u2n2S2configuration. This change in electronic structure is accompanied by a significant increase in the Mo-Mo distance, from 2.237(1) to 2.508(2) 8, (R' =But).'"
1314
Molybdenum
> / F
fit
C\
C
Figure 8 Structure of [ M O ~ F ~ ( O B U ~ ) ~ ] ' ~ ~
-
[MO~(OBU')~] reacts with PF, to produce [MO~F~(OBU')~] (Figure 8) This molecule consists of two MOB+centres linked by four bridging fluorides'95 which may be cleaved by PMe3 to yield [Mo2F2(O B U ' ) ~ ( P M ~196 ~)~]. Thiolato derivatives of M g + were initially elusive,197but the use of 2,4,6-alkyl-substituted aryl thiolates has permitted the isolation of several such compounds. 1, ~ - [ M o ~ C ~ ~ ( N M ~ ~ reacts with LiSR (R = Me, But) to produce ~ , ~ - [ M o ( S R ) ~ ( N Mand ~ ~this ) ~ ]compound '~ reacts with RSH (R = 2,4,6-Me3C6H2)to produce [ M O ~ ( S R ) ~ ][Mo2L4 . ' ~ ~ (L= NMe2, OPr', OBu', CHzCMe3) react with this thiol at room temperature to form the corresponding [MO~L(SR)~] derivatives, but at 80 "C complete substitution is achieved.200 An alternative route to [Mo2(SR),] derivatives has been developed which involves the reaction of MoC14 with RSNa (R = 2,4,6-Me3C6H2,2,4,6-P?~C&) in 1,2-dimetho~yethane,~' Metathesis between ~ , ~ - [ M O ~ C I ~ ( N and M [(THF)3Li][M'(M'Me3)3] ~~)~] (M' = Si, Sn) yields 1,2-[Mm{M'(M'Me3)3}2(NMe2)4], compounds which involve a branched chain of 10 (WMo=MoM$ metal atoms. The compounds possess high barriers to rotation about the Mo-N bonds (67-80 kJ mol-') as a consequence of the severe steric crowding about the MO;' centre.202
363.22.3 Properties of Mo-Mo
triple bonds
Thermochemical measurements have been made in an attempt to establish D(Mo=Mo). The interpretation of the data is dependent upon assumptions of the relevant metal-ligand bond energies and, in view of these uncertainties, ranges of values have been quoted; 592 f 196 kJ mol-' for [MoZ(NMe&] and 310-395 kJ mol-' for [Mo2(OPri)6].203MO calculations have indicated that the strength of the Mo-Mo triple bond is affected significantly by the nature of the attached li ands and x donors, such as NHz, stabilize this bond.'64 The &c4 d e s ~ r i p t i o nof~ ~ the~ electronic ~'~ structure of the Mo-Mo triple bond in [Mo2L] for molecules with molecules and their relatives has been reinforced by PES L = CH2SiMe3, NMe2 and OR (R=Pr', But, CH2CMe3). The observed spectra have been interpreted successfully by calculations on simpler analogues. The electronic absorption spectra of these molecules have been described and the lowest energy absorption assigned to promotion of an M e M o n electron into an orbital of e symmetry, which is an admixture of the MEMO 6* and x * orbitah.'* Unlike their Mo$+ counterparts, the v(Mo-Mo) stretching mode of Mag+ complexes is not a distinctive feature of their Raman spectra. Rather, as shown by a study of [Mo2(NMe&] and [MR(N(CD&}~] ,16' this mode is strongly coupled with other internal vibrations. Considerable attention has been paid to the dynamic behaviour of Mag+ complexes in solution and 'HNMR spectroscopy has been an essential probe. These properties have been reviewed7.'' and emphasis given to the point that the barrier to rotation about the tri le bond is determined only by steric factors associated with ligand-ligand interaction^.^'^ f'Mo NMR
Complexes Containing Metal-Metal Bonds
1315
spectroscopic studies of [MozL4](L = NMe2, OPr', OBu', OCH2CMe3, CHzSiMe3) have been accomplished. The resonances all occur in a very deshielded portion of the known chemical shift range; for [ M ~ ~ ( c H ~ s i Mresonance e ~ ) ~ ] occurs at 3624 p.p.m. and for the other molecules between 2430 and 2645 p.p.m.M6 363.224 Reactions of Mo-MO
triple bonds
Compounds containing an Mo-Mo triple bond have been shown to exhibit novel r e a ~ t i v i t y ~ , ~with , ' ~ * a' ~versatility which goes well beyond the Lewis base association, ligand substitution and COz insertion reactions described in Section 36.3.2.2.2. These compounds undergo reductive elimination and oxidative addition reactions which result in a change in the bond; the conversion of ( M ~ M O } ~ + + { M O = M O } ~ has + been order of the Mo-Mo mentioned in Section 36.3.2.1.1 and further information concerning the products of the transformations {MO=MO}~++ {MO=MO}~~ or {MO-MO)~'+ is provided in Sections 36.3.2.3 and 36.3.2.4, respectively. These reactions, and the interesting /3 hydrogen effects have no observed in the reactions of alcohols with 1, ~-[M o , R , (NM ~~)~] precedence in the chemistry of mono-metal centres. Oxidative additions to compounds containing an Me-Mo triple bond can (equation 3) give products containing an Mo-Mo double bond.*75-190,m Also, [MO~(OCH~BU')~] reacts with PhCHBr, in hexane/pyridine to form [Mo2(OCH2But)J3r2(py)]which contains an Mo-Mo double bond.m8 The direct addition of halogens (X2= C12,Br2, 1,) to [MOz(OP?)6] gives [Mo2(OPri)&] which contains a pair of octahedrall coordinated molybdenum atoms linked by two p-OPr' groups and a single MCF-Mo bond2'; and [Mo,(OR),] compounds react with tetrachloroquinone to produce [M02(OR)6(02C6C1&] which contains an Mo; centre."
Compounds containing an Mo-Mo triple bond have been shown to produce a variety of interesting products in reactions with molecules containing a C-0, C-N or C-C triple bond.10*194These may be considered to be oxidative addition reactions, in which electron density is removed from the Mo-Mo triple bond to form metal-ligand bonds. A solution of [Mo,(OBu'),] in a hydrocarbon solvent reacts with CO; in the first instance, reversible addition to [ M O ~ ( O B U ~ ) ~ ( occurs C O ) ~ and, eventually, [Mo(CO)~] and [Mo(OBu'),] are formed.209 Carbonylation of [ M O ~ ( O P ~proceeds ')~] in a similar manner and, in the presence of pyridine, [ M ~ ~ ( o P r ~ ) ~ ( p y ) ~ (isC oformed.210 )] [MO~(OR )~] compounds form simple adducts with dimethyl- and diethyl-cyanamide (RiNCN), the process being reversible for R = Bu and R' = Me and Et, in which an electronic redistribution occurs forming {RiNCN}'- and an M&+ Aryl-substituted diazomethanes react with [Mo,(OR)~] compounds and are moieties in compounds such as [Moz(OPr')6(NzCPh2)2(py)] .212 reduced to { Ar2CN2)'Alkynes react with [MO2(0R)6], or their adducts [Mo2(OR)&], in hydrocarbon solvents at 25 "C to give a wide variety of products. Careful control of the conditions is required to isolate compounds such as I M O , ( O P ~ ' ) ~ ( ~ ~ ) ~ ( Y since - C ~ Hthe ~ ) ]alkyne adducts are labile towards C-C coupling. This coupling can lead to alkyne polymerization, but the interesting derivative [MO~(OCH~BU')~(~-C~&)(~~)] has been isolated and structurally characteri~ed."~ Mo-Mo triple bonds may be cleaved by reactions with molecules containing multiple bonds and several interesting products have been isolated and characterized. [Mo2(0P&] reacts with NO (1:2) to form 1,2-[M0~(0Pr')~(N0),],in which the Mo-Mo distance of 3.335(2)A is spanned by two p-alkoxy groups. Therefore, the formation of the two Mo-NO tri le bonds O ) ~the ]~'~ results in the loss of the Mo-Mo triple bond?14 Reactions of 1 , ~ - [ M O ~ ( O P ~ ' ) ~ ( N and formation of the mixed-metal dimer [ C ~ M O ( O P ~ ' ) ~ ( Nhave O ) ~been ] ~ ~ ~described. Hydrocarbon solutions of [MO~(OBU')~] react with aryl azides and molecular oxygen at room temperature according to equations (4) and ( 5 ) , re~pectively.~'~ [Mo,(OBU')~(NC~H,),]has a structure which resembles that of [M02(OPri)6(N0)2],with an Mo-Mo separation of 3.247(1) 8, bridged by two NGHs ligands. Although (equation 5 ) O2 will cleave Mo-Mo triple bonds, interesting oligomers have been isolated in other reactions. Thus, [Mq(OR)6] (R = Pr', CH2CMe3)react initially, subsequently [M060~0(OPr')~2],and finally with O2 to form [MO,O(OR)~~] [M002(0Pr')2].~~'[M@(OBu')6] reacts with Ph2CN2with cleavage of the Mo-Mo triple bond
1316
Molybdenum
and formation of [MO(OBU')~(N,CP~~>]"~ and, in a similar manner, [ M o ~ ( O P ~ 'reacts ) ~ ] with bipy to produce [ M ~ ( O P r ' ) ~ ( b i p y ) ~ ] . ~ ~ ~ [Mo2(OBut)6]+ 4ArN3
--
+ 4N2+ 2Bu'OH 2[Mo0,(OBut)2] + 2Bu'OH
[Mo,(OBu'),(NAr),]
[M02(0BUt)6]+ 202
(4) (5)
36.3225 DicyclopentadienyltetmEarbonyldimolybdenum [CpzMoz(C0)4] is an important member of a group of cyclopentadienyl compounds and, like its chromium and tungsten analogues, this species is considered to involve a metal-metal triple bond.73m These and related compounds manifest a rich and novel but this falls outside the field of this review.
363.22 6 Halide and related compleres '
'
[MoZX9l3- (X = C1, Br) anions are readily prepared by a variety of methods, including: electrolytic reduction of a solution of Moo3 in aqueous HX,222thermally induced reactions of rnolybdenum(II1) halides with alkali metal halides,223con ro ortion reactions of carbonyl (Section 36.3.2.1.3) the halide anions of molybdenum with molybdenum halides$ :nd oxidation of complexes containing an M-Mo quadruple bond in the presence of X-. These [MoagI3- anions consist of two Mo& octahedra sharing a face and, in their Cs+ salt, the Mo-Mo separation is 2.655(11) and 2.816(9) 8, for X = C1 and Br, respectively.225Although the potential for formation of an Mo-Mo triple bond exists in these anions, this is not redized; thus, as compared to their tungsten counterparts, the molybdenum atoms are shifted but little from the centre of the halide octahedron towards each other and the anions display effective magnetic moments at room temperature of 0.6 (Cs3[Mo2C19]) and 0.8 BM ( C S ~ [ M O ~ B ~The ~ ] )substitution .~'~ of p-H for p-Xin these anions has a significant effect upon the Mo-Mo interaction. The [Mo2&HI3- anions are diamagnetic and involve Mo-Mo separations which are some 0.3 (X=C1) or 0.4A (X=Br)227,USshorter and, in contrast to [MQ&]~- (X= C1, Br), the Mo-Mo distance in [Mo2XSHI3- (X = Cl, Br, I) anions is little affected by a change in X.229Initially, these anions were incorrectly formulated as [Mo2XsI3species, but the presence of the p-hydrido ligand was demonstrated by isotopic labelling, vibrational spectroscopy, and its location determined by X-ray crystallography and neutron diffraction; the Mo-H bonds in [NMe4]3[Mo2C18H]are 1.73(1) and 1.823(7) The heteronuclear anion [MoWClsHl3-, prepared by the reaction of [MoW(02CMe),] with HCI, has the [Mo2ClsHI3- structure and involves an Mo-W distance of 2.445(3) [NBu4][Mo2Br6]has been obtained from the reaction of [Mo2(0zCMe)4]with HBr in MeOH in the presence of [NBu4]+ and this anion considered to possess a strong Mo-Mo interaction.u2 The synthesis, redox roperties and reactivity towards phosphines of [NBu&[Mo2Br6]have been investigated.g3 The formation of [Mo~(HYO~)~]'(Y = P, As) anions has been described in Section 36.3.2.1.3; these anions contain an ( M O ~ } ~centre + spanned by four (p-02Y02H)ligands in an eclipsed conformation and involve M e M o separations of 2.228(5)1s9 and 2.265(1) respectively, consistent with the existence of a triple bond (Table 2).
36.3.2.3
Complexes Containing a Dooble Metal-Metal Bond
Mooz adopts a distorted rutile structure in which the metal atoms are drawn together in pairs some 2.511 8, apart,u4 a distance (vide supra) which is consistent with the presence of an Mo-Mo double bond. [ M O ( N M ~ ~reacts ) ~ ] readily with alcohols (ROH) to form the corresponding [Mo(OR),], compound and, for R=CHMez and PI!, it has been established that the diamagnetic, is centrosymmetric, with an [Mo2(OR),], dimer is formed.uS (Pr'0)3Mo(p-OPri)2Mo(OPr')3] M-Mo separation of 2.523(1) spanned by two bridging isopropoxide ligands which form M A bonds of length 1.958(3) and 2.111(3) Thermochemical measurements for [M%(OPr')6] and [MO~(OP~')~] have led to a value in the ran e 837-962kJmol-' being suggested for the Mo==Mo bond strength in the latter
A
Complexes Containing Metal-Metal Bonds
1317
Reactions of [Mo&] compounds containing an Mo-Mo triple bond can lead to the of corn ounds with an Mo-Mo double bond (Section formation 36.3.2.2.4).7*8*10*164*174*'751 90,197~198*207-m,213 Thus, as an alternative to the above procedure, [ M O ~ ( O P ~ may ' ) ~ ] be prepared by the oxidative addition of Pr'OOPr' to [ M O ~ ( O P ~ ' )and ~],~~~ to form [ M O ~ ( O R ) ~ ( O ~ C P ~ ) ~ ] . ~ ~ [Mo2(OR)6] (R = Pr', But) reacts with PhCOr02CPh [M02(OBUt)6(CO)] separates as dark purple crystals when [Mo~(OBU')~] is reacted with CO and the reaction mixture is cooled to -15 "C. Molecules of this compound possess a structure involvin an Mo-Mo separation of 2.489(1)A spanned by one p-CO and two p-OBu' groups.' Carbonylation of [Mo2(OPr')6] proceeds to give [Mo(CO)~],but in the presence of pyridine the adduct [(Pr'O),(py)Mo(p-OPr')2(p-CO)Mo(py)( OPr')2] can be isolated; this molecule involves an Mo-Mo separation of 2.486(2)A consistent with the presence of a double bond.210 TRis bond order is also considered to occur in the related compound [ M O ~ ( O P ~ ' ) ~ ( ~ Y ) ~ ( ~ - Cin~ H which Z ) ] , the M-Mo separation is 2.554(1) [MO,(OR)6] compounds form 1:1 adducts with dialkylcyanamides, RiNCN, and a crystal structure determination for R = But and R' = Me has shown that the Me2NCN ligand spans the MEMO bond of distance 2.449(1) [ M O ~ ( O C H ~ B U ~ and ) ~ ( ~PhCHBr2 ~)~] react to form [Mo2(OCH2But),&r2(py)]which has a distorted confacial (p-Br, bioctahedral strucbond of length 2.534(1) Anrn8 [MO*(NMe2)6] and [ M O ~ ( S B U ~ ) ~ ( N M ~ ~ ) ~ ] ture with an M-Mo react with ButSH to form [(HNM~~)(BU~S)~MO(~-S)~MO(SBU')~(HNM~~)] which involves an Mo-Mo separation of 2.730(1) 8,.lW,lg8 [ M R ( O A ~ ) ~ ( C H Z S ~ M ~(OAr ~ ) ~ ]= 2,6dimethylphenoxide) undergoes smooth loss of one equivalent of MeSi in the presence of pyridine to form [MO&-H)(~-CS~M~,)(CH~S~M~,),(OA~),(~~)~] which is considered to involve an { M o ~ } ~core + and an Mo==Mo bond of length 2.380(2) A.'" The variation in the length of Mo=Mo bonds is a demonstration of the general observation that there is no strict correlation between the bond order and the length of Mo-Mo bonds;7 the molybdenum atoms in these compounds will have different electronic structures leading to different types of double bond (e.g. a 2 ~ 2 1or 7 4n262236), a difference augmented by the particular stereochemical demands of the bridging ligands. With this pers ective, it may be possible to view the formally Mo'" dimers [Mo2S2(p-S2CNR2)2(SCNR2)2]L as involving an Mo-Mo double bond, even though the metal-metal separation of 2.705A is longer than usual.
P
36.3.2.4
Complexes Containing a Single Metal-Metal Bond
The vast majority of these compounds are to be found in Section 36.4.3.5 which describes the chemistry of systems containing an (Mo2}'*+ core. Several other complexes containing an Mo-Mo single bond have been prepared by the reaction of dimeric complexes possessing a quadruple (Section 36.3.2.1.3)7,161,162 or triple (Section 36.3.2.2.4)7,8,10,164 Mo-Mo bond. Also [M02(OPri)6] reacts With XZ (X=Cl,Br,I) to produce [Mo2(OPr')&]; an X-ray crystallographic study of the chloro and bromo derivatives has shown that both compounds have central Mo20& moieties with virtual D2hsymmetry and Mo-Mo separations of 2.731(1) and 2.739(1) A, respecti~ely.~' [M~~(Opri)~(N~cPh~)~(py)], formed by addition of diphenyldiazomethane to [M (OPf)6] in the presence of pyridine, involves an Mo-Mo single bond of length 2 . 6 6 2 ( 1 3 spanned by three (p-OPr') ligands.212 The M-Mo separation in [M~,(OCH~Bu~)~(py)(p-c,f&)] of 2.69(1) 8, approaches that of an Mo-Mo single bond, but there are alternatives to this description of the Mo-Mo intera~tion.'~~ Asimilar situation pertains in [(Me2NCS2)2Mo(p-S$p-EtCkCEt)Mo(S2CNMe2)(SCNMe2)], in which the M-Ma separation is 2.647(1) A. [ C ~ ~ M O ~ ( C O )and ~ ] ~ fGM02(CO)6]6 * (G = guaiazulene) involve Mo-Mo bonds of length 3.235(1) and 3.267(6) A, respectively, and are generally considered to involve an Mo-Mo single bond.
36.3.3 METAL CLUSTERS
36.3.3.1
Trimeric (%stem
Despite the proposala9 in 1929 that molybdenum(IV) complexes such as Mo304(G04)$+ ~ + it was some 50 ears later that the significance of trimeric contains an { M o ~ O ~ }core, molybdenum clusters was clearly appreciated.8 Three types of Mo3 cluster have been
1318
Molybdenum
Figure 9 Structural arrangements recognized for Mo, clusters (0= Mo, 0= ligand donor atom)'
recognized (Figure 9), involving differing patterns of ligation to an equilateral triangular array of molybdenum atoms. The common types are A and B and a fundamental understanding of the metal-metal bonding in these species has been achieved.'"O The bicapped M03X17 clusters (A) involve two orbitals per metal being used for metal-metal bonding and, therefore, a maximum of six electrons can be accommodated in the Mo-Mo bonding orbitals, thereby forming a metal-metal single bond over each edge of the triangle. Such a centre may be designated as {M03}l2+. Several compounds exist with an oxidized version of this centre, viz. {Mo3)13+or (Mo3}14+,which have M e M o bond orders of 2 or$, respectively, and the relative length of the Mo-Mo bonds (typically between 2.75 and 2.80 ) are consistent with this MO picture. An important reaction in the context of these clusters is the formation of the [MO,O,(CM~)~-,(~~CM~)~(H~O)~]~+ (n = 0,2, m = 2; n = 1, m = 1) ions in the reaction of MeC02H/(MeC0)20 mixtures with [ M O ( C O ) ~ ] .Although ~ ~ - ~ ~ ~the initial reportx1 was incorrect in some details, the successful isolation and characterization of these species was important as it stimulated further investigations which led to the synthesis of several [ M O ~ ( ~ ~ - X ) Z ( O ~derivatives, C R ) ~ ~ ] in which X is an oxygen atom or an alkylidene The presence of one p3-alkylidene ligand maintains the electron count of six electrons in the Mo-Mo bonding orbitals, as in [ M o ~ O ( C M ~ ) ( ~ ~ C M ~ ) ~ ( H ~ O ~ ] + but the presence of two such ligands stabilizes the {Mo3}14+ core, as in [Mo3(CMe)Z(OZCMe)6(HZo)~]'+The monocapped M03X13 clusters (B) have an electronic structure which can accommodate six M-Mo bonding electrons,m but one or two more electrons can be accommodated in an orbital that has little if any M-M antibonding character. Therefore, in addition to the {Mo~}"+ clusters, other type B systems based on {M03)11+ and { M O ~ ) cores ' ~ ~ exist. Type B clusters are known for a wide variety of ligands and those containing { M o ~ O ~ }and ~ + the corresponding partial or completely substituted sulfur cores have special importance; details of the synthesis, substitution and redox chemistry of many of these systems are described in Section 36.1.4 with particular attention being given to the trimeric species involved in the aqueous chemistry of The 95MoNMR chemical shift of the Mo'" aqua ion in acidic media and those of the Mom complexes containing ~ x a l a t e , * ~ 1,2-diaminoethanetetra',~~~ acetate252 and N-methyliminatodiacetate, whose solid state structures are based on an { M O ~ O ~ }core, ~ + fall in the relatively narrow range 990-1162 p,p.m, and this observation serves as a useful spectroscopic probe for the presence of this core in solutions of molybdenum complexes.253Thiocyanate ligands bind readily to the { M O ~ O ~core ) ~ +and kinetic studies have indicated that there are three equivalent sites on this cation to which these ligands have The synthesis and characterization of complexes involving ( M O ~ ~ , S ~ - , (n } ~=+3-0) cores have been and, as the number of sulfur atoms in the core increases, so does the Mo-Mo separation, from ca. 2.5 A in { M o ~ O ~ }centres ~ + to ca. 2.75 8, in { M o ~ S ~ } ~ + and electrochemical studies centres. [MO,S~(CN)~]~has been structurally have identified the quasi-reversible one-electron oxidation of this anion. [M03(p3-S)(S&]2(Figure 10, Section 36.6.1) is also based on an {M03)l2+ core, but involves a ( p - S 2 ) ligand spanning each edge of the triangle and possesses one terminal disulfide ligand on each cores ) ~ } ~have + been shown to bind chloride261,262and molybdenum.260 { M O ~ ( ~ ~ - S ) ( ~ - & dialkyldi~xo'~~ and dialkyldithiophosphinate ligands;'@ in respect to the last ligation, the reaction of [Mo(CO)~]with R2P(SeSrP(S)R2 (R = Et, Pr) affords [Mo3S7(R2PS2),][R2PS2] and reaction of this cation with PPh, leads to sulfur abstraction and formation of the [Mo3S4( R2PS2),] compound. 26s As indicated above, type 3 clusters can accommodate one or two additional electrons, beyond the six needed to form the three Mo-Mo bonds of the triangular array. This redox versatility is clearly manifest in mixed-metal oxide phases of molybdenum. Examples include
Complexes Containing Meta I- Metal Bonds
1319
M2M0308 (M=Mg, Mn, Fe, Co, Ni, Zn, Cd),266 LiZnzM308 and Zn3M030267 which, respectively, involve Mo3013 units with six ({M03}12+),seven ({MOg}"+) and eight ({MOg}lO+) electrons in the metal-based orbitals; La3Mo4Si01:~~a ears to involve six electrons in these orbitals. [MO~(~~-O)(~-C~)~(~--O~CM~)~(H~O)~]~+, 0 3 P~-O)(PTCI)~(P02CMe)2C1#-?61,270 [M03(p33-0)(p2-X)3(p-O~CR)~Cl~]-, (X = C1, R %le!61 X = Br, R = H271)and [MO~(~~-CM~)~(~~-B~)~(~-O~CM~)~(H~O)~]+ 272 are all based on an {M03)l0+core. (R = Pr', Type C (M03X11) clusters occur for [Mo~(~~-O)(~~-OR)(~-OR)~(OR)~] CH2CMe3).218,273,274 Th ese species may be considered to involve an {M03}l2+ centre and involve Mo-Mo separations of ca. 2.53 A; their electronic structure has been described and related to their electronic These species are obtained in the simple conproportionation reactions between the [Mo2(OR),] and [MoO(OR),] derivatives and this route has permitted the synthesis of the trianguZo-Mo2W cluster.u4 The related systems [Mo&-S)~(~C1)3Ck]3- and [M03(p&(p-C1)~{ SZP(OEt),},] involve an {MOj}lO+ centre and M e M o bonds of ca. 2.6A276and a compound containing an ( M ~ ~ ( p ~ - s ) ( p -core S ) ~appended } by diethoxythiophosphinate ligands has been shown to involve a triangular arrangement with two Mo-Mo separations of 2.808(1) and 2.839(1) A and one longer distance of 3.337(1) A.276The complex [ M o ~ S ~ ( N ~ Mhas ~ ~been )~]~ synthesized by the reaction of [ M O ~ S ~ ( N ~ M ~with ~)~PP~~] LiSPh in MeCN. X-Ray structure analysis of the [PPh$ salt has revealed the anion to be a linear molybdenum trimer with two distinct coordination sites. The central metal atom enjoys pseudooctahedral geometry, through coordination of the N atoms of two end-on, linearly bound hydrazido(2-) groups and to four bridging sulfido groups and crystallographic symmetry which gives a rigorously planar Mo2S4 unit, with trans hydrazido-groups. The terminal molybdenum atoms are tetrahedrally coordinated to two brid ing and two terminal sulfido groups. The unusual Mo-N and N-N bond lengths of 2.13(1)% and 1.16 2 A, respectiveIy, suggest that the -"MeZ ligands are best described as isodiazene groups." (7) Infinite chains of Mo3 triangles occur in a variety of solid phases of molybdenum chalcogenides, including MMo3S3 (M = K, Rb, Cs) and TlzM%Se6; the electronic structure of the latter and related systems has been in~estigated.'~~
'
36.3.3.2
Tetrameric Clusters
Complexes involving four molybdenum atoms are known for several types of systems including pairs of ( M o ~ O ~ ) {MO~(NR)~S~}'+ ~+, and related systems (Section 36.4.3.5.4) and [Mo&~(NO)~(CN),]*-(Section 36.6.4). This section will describe moieties in which a framework of metal-metal bonds links all the molybdenum atoms. [ M O ~ I ~has ~ ] ~been synthesized by the attack of HI on [ M o ~ ( O ~ C Mand ~ ) ~by] ~the~ ~I2 oxidation of [ M O ( C O ) ~ I , ] - the ; ~ ~structure of this entity involves a distorted M o ~tetrahedron and may be viewed as containing an { M o ~ I ~ }fragment ~+ of an [Mo4IsI4+cluster (Section 36.3.3.3) with an iodide bound as a terminal ligand to each molybdenum. [ M O ~ I ~reacts ~]~with diphos to produce [M~J~(diphos)~] .279 [ M o ~ C ~ ~ ( P P ~ , ) ~ ( M ~ O a Htypical ) , ] , quadruply bonded Mo" dimer (Section 36.3.2.1), undergoes an interesting condensation reaction upon dissolution in benzene; thus, it loses MeOH to form {MoC12(PPh3)}, which reacts with PR; (R' = Et, Bun) to form the corresponding, diamagnetic, tetramer [ M O ~ C ~ ~ ( P R(Figure ; ) ~ ] 10). The Mo-Mo distances for the unbridged and bridged edges of the M04 rectangle are 2.21113) and 2.901(2) 8, respectively, consistent with their formulation as M-Mo single and triple bonds respectively. Thus, these systems are molybdenum analogues of the cyclobutadiynes.281 Alternative procedures may be used to prepare these and analogous tetramers; treatment of &[MOzCls] with the stoichiometric amount of PR; in refluxing MeOH, or reaction of [ M O ~ ( O ~ C Mwith ~ ) ~ ]AlC13 and PR; (2:4:4) in THF, produces these systems and other derivatives corresponding to the general formula [Mo4X8L4](X= C1, Br, I; L = neutral donor ligand, e.g. PR;, PPh3 MeOH, THF,RCN) have been isolated.282Also, [M o ~C ~, { P (OM ~)~} is ~] formed when [Mo2C14{P(OMe)3)4]is dissolved in CH2C12, or when &[MqCl,] reacts with P(OMe)3 in MeOHnB3The relationship of these tetrameric structures to P-MoCl, has been considered and the reaction of [Mo~CI,(PR;)~]with [Mo(CO)~CI,] or [Mo(CO),] in refluxing chlorobenzene shown to lead to phosphine extraction and the formation of the condensation products [Mo4Cl,(PR3)&, for which the formulation x = 2 is favoured.284[Mo~X~(OW)~](X = C1, Br), which have a structure similar to that of [ M o ~ F ~ ( O B U(Figure ~ ) ~ ] 8), react with MeCOX to form [MO&(OP~')~];an alternative preparative procedure is the reaction of [ M O ~ ( O P ~ and ' ) ~ ] MeCOX (1:2). The chloride derivative has a structure based on a square of
1320
Molybdenum
molybdenum atoms, some 2.37A apart, with eight p-OPr' groups and four terminal chloride ligands. This system may be viewed as a molybdenum analogue of cyclobutadiene. However, the bromide involves a butterfly arrangement of molybdenum atoms with five Mo-Mo distances of ca. 2.5 A and one of 3.1 A; the bromide li ands are alI terminal and two of the OPr' groups are terminal, two p- and three p3-bridging.4 5 ,
Figure 10 Sticture of [MO.,C~~(PE~~).,]~'
Ba1.13M08016 contains two types of Mo4Ol6 subunits which differ most significantly in their
Mo-Mo distances and this difference can be correlated with the number of electrons invoived in cluster b ~ n d i n g . [MON(OBU')~] ~ ~ ~ , ~ ~ ~ reacts with [M0~(0Pfi)~] in hexane/Pr'OH to form [MO~(~~-N)~(~-OP~')~(OP~')~~~, the structure of which is shown in Figure 11 and involves The aquated contiguous Mo-Mo separations of 2.918(1), 2.552(1) and 2.918Cl) { M o ~ O ion ~ } has ~ ~ been claimedzs6 and considered to possess a cubane-like structure, as identified in its sulfur analogue (vide supra).
P
OPri
dOPr' Pigwe 11 Structure of [ M O ~ ( N ) ~ ( O P ~ ) ~ ~ ] ~ ~
Mod clusters have been identified in man solid phases of molybdenum chalco enides the including P~MO&S,"~MoSX (X = C1, Br, 1)"' and GaMo4(YY') (Y, Y' = S, Se, Te)!w physical properties of these systems are of particular significance. Tetrahedral arrays of
Complexes Containing Metal-Metal Bonds
1321
molybdenum atoms occur in {M04S4}"+ cubane-like clusters which have been identified with a variety of ligands appended to the cluster. The number of electrons involved in the metal-metal bonding framework of these systems is variable and the most extensive, homologous, series is [(P~'CSH~)~MO~S~]"+ (n = 0, 1,2). These complexes are related by reversible, one-electron, redox reactions and oxidation causes only a slight decrease in the The electronic structure of these volume of the MO4 tetrahedra (Mo-Mo=2.9-2.8A). systems has been discussed with reference to the MO scheme originally presented for [Cp4Fe4S4]and the PES data recorded for the neutral compound. The 12 electrons available for metal-metal bonding in [(P+C5H4)4M~4S4] are assigned to al, e and t2 Mo-Mo bonding orbitals of the Td cluster, with the al and e orbitals being strongly bonding, but the t2 orbital is less stable as manifest by its relatively low ionization potential.291 [ M O ~ S ~ ( C N ) also ~~J~possesses a regular M O 4 tetrahedron with 12 electrons in the Mo-Mo bonding orbitals and involves Mo-Mo separations of 2.855(1) A.259,2g2 An {MOqS4}s+ cluster has been prepared as ~ ~ and ] ~ - [Mo4S4(edta),]'- 293 have been the aquated ion and the complexes [ M o ~ S ~ ( N C S ) 256 isolated; the latter complex approximates to S, symmetry with Mo-Mo distances which average 2.8088,, possesses one unpaired electron, and is capable of undergoing a quasireversible one-electron oxidation or reduction. (M04S4}6+ has been prepared as the aquated species and crystallized as its [ M O ~ S ~ ( N C S ) salt; ~ ~ ] ~the - central cluster possesses 10 electrons for metal-metal bonding and the observed change in structure from Td to C3" symmetry is consistent with a Jahn-Teller type distortion removing the degeneracy of the t2 orbital to give a filled e and an empty a2 orbital. The M o ~ unit is a triangular pyramid with slant edges of length 2.791(1) A and basal edges of 2.869(1) [MO&(S2CNEt&] also involves an {Mo4S4}'+ cubane-like cluster in which two of the dithiocarbamate ligands each form a bridge between the two molybdenum atoms on the opposite sides of the cube, the remaining four each coordinate to one molybdenum atom in a bidentate mode; the M e M o distances range from 2.732 5) 8, for two Mo atoms bridged via the same dithiocarbamate, to an average value of 2.861(6) for the other four M-Mo distances.29sThe cubane-like {M04!&}6+ aquated ion is converted by aerial oxidation to the triangular {MO'S~}~+ aquated species (Section 36.3.3.1).294
8,
36.3.3.3 Hexameric Clusters and Related Extended Arrays The halides of molybdenum(I1) were first described in 2848296 and the structural characterization of the cluster species accomplished a century later for [Mo&ls(OH)4].14H20 and [ N H & [ M O ~ C ~ ~ ~ ] - HIn~ Oidealized .~~' cubic s mmetry, the [Mo6Cll4I2-ion consists of an octahedron of metal atoms (Mo-Mo is ca. 2.6 ), ligated by eight p3 and six terminal chlorine atoms. This { M o & } ~ ~(X= C1, Br, I) moiety has been demonstrated as a constituent of a large number of compounds,298 including the dihalides M o a l 2 which are isomorphous and possess a central M o a s unit ligated by four p and two terminal halides.2w The reduction of higher molybdenum halides by aluminum in the appropriate N a A W ( X = C1,Br) melt provides a convenient and safe synthesis of the corresponding molybdenum(I1) chloride or bromide in good yield.300The coordination chemistry of these (MO&}~+ species is well developed and includes the ligation of other halides, carboxylates, thiocyanates, alkoxide ions and a wide variety of neutral donor molecules at the terminal position^.^^**^' The bonding in these clusters has been defined and spectroscopic studies have shown that the [MC&.+]~(X = C1, Br) ions are luminescent; these clusters undergo a one-electron oxidation and reduction and303electrogenerated [Mo6Cll4In-(n = 1 or 3) ions react with electroactive donors or acceptors, respective1 to produce the luminescent state of 'boand [Mo5Cll3l2-305 may be considered as fragments of the The ions {Mo,&}~+ cluster; the former involves a distorted tetrahedron, and the latter a square-based pyramid, of metal atoms. The mixed-metal halide cluster cations {Ta5MoC112)3+and { T a J ~ l o ~ C l ~have ~ } ~ been + obtained by reduction of TaC15-MoC15 mixtures with aluminum in fused NaAlCL-AlCl3 at 325 "C and these cations are considered to be molybdenum-substituted derivatives of (Ta6Cll2}"+clusters.306 Substitution of sulfide and selenide into {Mo,&}~+ (3X= C1, Br, I) centres has been achieved in a host of solid phases and cores such as {Mo6C17S}+,{MogBr&}, {Mo&Y2} (Y= S,Se), 290,307 and MMo6(S1--xSex)8(M=La, Yb, Sm, Ca, Sr, Ba, Eu, Sn, 0 r6s31, {Mo4c&} have been characterized+ Chevrel phases, i. e. ternary molybdenum chalcogenides, M,Mo& (typically M = Pb, Sn or a rare earth element; X = S or Se) are of particular interest because of their physical properties-high superconducting critical temperatures, high critical
w
k538
1322
Molybdenum
fields and coexistence of magnetic order and superconductivity. Considerable flexibility in composition exists for these phases since, in addition to the variations indicated above, the ratio of Mo :X can be increased from the ideal 6 :8 value. The fundamental structural unit in the Chevrel phases is an Mo6 cluster and these units are coupled together to give linear arrays of the general composition M03nX3n+2(n = 2, 3, 4 . . . 03; X = S, Se, Te).309A combined MO and crystal orbital analysis310has been presented for these systems and for NaM0406, which is dso a metallic, infinite-chain polymer, derived from the condensation of Mo6 ~ c t a h e d r a . ~ ’ ~ Oxygen-donor ligands will also support discrete Mo6 clusters; thus, Mo6Cl12when refluxed with NaOMe in MeOH forms Naz[{Mo&18} (OMe)6] and eventually Na2[{Mo6(OMe)8}{ OMe}6].312
36.3.4
REFERENCES
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1328
Molybdenum
297. C. Brosset, Ark. Kemi, Mineral. Geol., 1945, MA, no. 7; 1947, SA,no. 19; P. A. Vaughan, Proc. Natl. Acad. Sci. USA, 1950, 36, 461. 298. J. H. Canterford and R. Colton, ‘Halides of the Second and Third Row Transition Metals’, Wiley, New York, 1968, pp. 99-106; D. L. Kepert, ‘The Early Transition Metals’, Academic, London, 1972, pp. 351-356 and references therein. 299. H. Schafer, H.-G. Von Schnering, 5 . Tillack, F. Kuhnen, H. Wohrie and H. Baumann, Z. Anorg. A&. Chem., 1%7, 353, 281. 300. W. C. Dorman and R. E . McCarley, Inorg. Chem., 1974, 13, 491. 301. G.Holste and H. Schafer, Z. Anorg. A @ . Chem., 1972, 391, 263; P. C. Hedy,D. L. Kepert, D. Taylor and A. H. White, J . Chem. SOC.,Dalton Trans., 1973,646. 302. F. A. Cotton and T. E. Haas, Inorg. Chem., 1964,3, 10. 303, H, Schafer, H. Plautz and H. Baurnan, Z.Anorg. Allg. Chern., 1973, 401, 63; D. L. Kepert, R, E, Marshall and D. Taylor, J . Chem. SOC., Dalton Trans., 1974, 506. 304. A. W. Maverick and H. B. Gray, J . Am. Chem. SOC., 1981, 103, 1298; A. W. Maverick, J. S. Najdzionek, D. MacKenzie, D. G. Nocera and H. B. Gray, J. Am. Chem. SOC., 1983,105, 1878; D. G. Nocera and H. B. Gray, J. Am. Chem. SOC., 1984, 106, 824; R. D. Mussell and D. G. Nocera, Polyhedron, 1986, 5 , 47. 305. K. Jodden, H. G. von Schnering and H. Schafer, Angew. Chem., Znt. Ed. Engl., 1975,14, 570. 306. J. L. Meyer and R. E. McCarley, Inorg. Chem., 1978, 17, 1867. 307. J. B. Michel, Energy Res. Abstr., 1980, 5 , 16250 (Chem. Abstr., 1980, 93, 196803); J. B. Michel and R. E. McCarley, Znorg. Chem., 1982, 21, 1864; C. Perrin and M. Sergent, J. Chem. Res. ( S ) , 1983, 38; C. Perrin, M. Potel and M. Sergent, Acta Crystallogr., Sect. C, 1983,39, 415. 308. J.-M. Tarascon, D. C. Johnson and M. 3. Sienko, Inorg. Chem., 1983, 22, 3769; D. C. Johnson, J.-M. Tarason and M. J. Sienko, Inorg. Chem., 1983, 22, 3773; 1985, 24,2598; 1985, 24,2808. 309. R. Chevrel, M. Hirrien and M. Sergent, Polyhedron, 1986, 5 , 87 and references therein. 310. T. Hughbanks and R. Hoffman, J. Am. Chem. Soc., 1983,105, 1150; 1983, 105,3528. 311. C. C. Torardi and R. E. McCartey, J. Am. Chem. Soc., 1979,101, 1963. 312. P.Nannelli and B. P. Block, Inorg. Chem., 1968, 7 , 2423; M. H. Chisholm, J. A. Heppert and J. C.Huffman, Polyhedron, 1984, 3, 475.
36.4 Molybdenum(lll), (IV) and (V) C. DAVID GARNER and JOHN M. CHARNOCK University of Manchester, UK 36.4.1 MOLYBDENUM(III) 36 4.1.1 Introduction 36.4.1.2 Monomeric Complexes 36.4.1.2.1 Halide complexes 36.4.1.2.2 Complexm wiih oxygen and sulfiur donor ligands 36.4.1.2.3 Complexes with nitrogen donor ligands 36.4.1.2.4 Cyanide complexes 36.4.1.2.5 Magnetic and electronic properties 36.4.1.3 Dimeric and Polymeric Complexes
1329 1329 1330 1330 1331 1331 1332 1332 1332
36.4.2 MOLYBDENUM(IV) 36.4.2.1 Introduction 36.4.2.2 Monomeric Complexes with Oxo Groups 36.4.2.2.1 General comments 36.4.2.2.2 Complexes of oxygen and sulfur donor ligands 36.4.2.2.3 Complexes of phosphine ligands 36.4.2.2.4 Complexes of nitrogen donor ligands 36.4.2.2.5 Cyanide and isocyanide complexes 36.4.2.2.6 Spectroscopicstudies 36.4.2.3 Complexes lnploluing Multiple Molybdenum-Nitrogen Bonds 36.4.2.4 Other Monomeric Complexes 36.4.2.4,l Halide complexes 36.4.2.4.2 Dithiocarbamate and dithiocarboxylate ligands 36.4.2.4.3 Eight-coordinatecomplexes with other bidentate ligands 36.4.2.4.4 Alkoxy and thiolate ligands 36.4.2.4.5 Dialkylarnide and other nitrogen donor ligands 36.4.2.4.6 Cyanide and isocyanide complexes 36.4.2.4.7 Magnetic and electronic properties 36.4.2.5 Cyclopentadienyl Complexes 36.4.2.6 Dimeric and Polymeric Complexes
1333 1333 1335 1335 1337 1331 1338 1338 1339 1340
36.4.3 MQLYBDENUM(V) 36.4.3.1 Introduction 36.4.3.2 Monomeric Complexes with an Oxo Group 36.4.3.2.1 General comments 36.4.3.2.2 Halide complexes 36.4.3.2,3 Complexes wiih sulfur and selenium donor ligands 34.4.3.2.4 Complexes with nitrogen donor ligands 36.4.3.2.5 Spectroscopic studies 36.4.3.3 Complexes Involving Multiple Molybdenum-Nitrogen B o d 36.4.3.4 Other Monomeric Complexes 36.4.3.4.1 Halide complexes 36.4.3.4.2 Complexes of sulfur donor ligands 36.4.3.4.3 Cyanide complexes 96.4.3.4.4 Spectroscopic studies 36.4.3.5 Dimeric and Polymeric Complexes 36.4.3.5.1 General comments 36.4.3.5.2 Dimeric complexes with one bridging ligand 36.4.3.5.3 Dimeric complexes with two bridging ligandr 36.4.3.5.4 Oligomeric complexes based on the {MoZOJ2' core 36.4.3.5.5 Dimeric complexes with three bridging ligands
1347 1347 1348 1348 1348 1350 1351 1352 1353 1353 1353 1354 1354 1355 1355 1355
36.4.4 REFERENCES
1366
1342 1342 1343 1343 1343 1344 1345 1345 1346 1346
1355 1358 1363 1364
36.4.1 MOLYBDENUM(II1) 36.4.1.1 Introduction Molybdenum(II1) is a relatively rare oxidation state and is easily oxidized to more stable states by air and other mild oxidants. However, recent work has shown the existence of many Mo"' 1329
Molybdenum
1330
compounds in both aqueous and nonaqueous systems. Also, Mo'" may be involved in catalytic and biological reactions of molybdenum, such as dinitrogen reduction, but there is no firm evidence for this. This section omits the aqueous chemistry of Mo"' (see Section 36.1.2), Mo-Mo triple bonds, including [Mo2X9I3- (see Chapter 36.3), [CpMo(SR),] and related compounds, and M-S clusters (see Section 36.6.5). Molybdenum trihalides may be prepared either by reduction of higher halides or by direct combination of the elements.' They all adopt extended lattice structures: M o F ~has a rhombohedral structure similar to VF3 and CrF$ MoC13 exists in two polymorphic forms in which Mo atoms occupy octahedral holes in cubic (a)or hexagonal (p) close-packed arrays of c h l o r i n e ~ ;MoBr3 ~ and Mo13 have structures containing polymeric strings made up of face-sharing MoX, ~ c t a h e d r aThe . ~ magnetic moments of the trihalides given in Table l5show an increase from C1 to I, indicative of antiferromagnetic coupling between adjacent Mo atoms which decreases with the increase in the metal-metal distance. Table 1 Magnetic Moments of Molybdenum(II1) Complexes Ref.
0.7 1.2 1.4 0.77 1.17 3.79 3.79 3.65 3.69 3.82 3.53 1.73 a
Room temperature.
a
1
a
1
a
1 6
293 293 300 300 300 293 302 b 298
6 37 37 34 15 23 28 35
Temperature not specified.
Compounds of the stoichiometry Mo(SR)~can be prepared by oxidation of Moo complexes using thiols or disulfides6 Their spectroscopic properties and low magnetic moments, ranging from 0.77 to 1.17 BM (Table 11, indicate a polymeric structure with considerable metal-metal interaction. 36.4.1.2
Monomeric Complexes
364.1.21 Halide complexes
'
K3[MoF6] can be prepared by reacting K3[MoC16] or Moo3 with KF or KHF,; the anion contains Mo octahedrally coordinated with Mo-F bond lengths of 2.00(2) A.7M$[MOC&]and M$[MoBr6](M' = Li+, Kf, NHZ) have been obtained by electrolytic reduction of Moo3 in HCl or HBr solution respectively, followed by addition of the appropriate alkali metal halide. Other products isolated from these reactions include salts of the [ M O C ~ ~ ( M ~ O )and ]~[MOB~,(H,O)]~-anions.' Crystal structure determinations of salts containing these ions all show that the Mo"' possesses simple octahedral coordination with typical average bond lengths of, for example, 2.452(3) A for Mo-Cl in [Ph2HP(CH2)2PHPh2]3[MoC16]2.12H20 ,9 2.591(1) A for Mo-Br in [LH]2[H502][MoBr6](L = 2-arnin0-4,6-dihydroxypyrimidine),'~ and 2.155(3) 8, or 2.185(3) A for Mo-0 in the hydrolysis products [NH4]z[MoC15(Hz0)]and [m]2[MoBr5(H20)],res ectively." Other monosubstituted halide anions, such as the nitrile complexes [MoC15(NCR)f-,12 can be prepared by ligand substitution from [Mo&I3- species. [NEt3H][MoCL(THF)2], obtained from the reduction of [MoCIQHF)~]in the presence of NEt3, involves octahedral stereochemistry around the Mo, with the THF molecules adopting a trans geometry. This complex can be used as a starting material in the synthesis of other disubstituted anions.13 A PIS geometry has also been established for the anion of {SMe3][MoBr4(SMe2)2], formed in the reaction of MoBr4 with MezS.I4 A large number of neutral compIexes of the form [MoX3L3](X = halide; L = neutral ligand) are known. These are easily prepared by direct combination of MoX, and L, or by substitution
Molybdenum(ZII), ( W )and ( V )
1331
reactions of [Mo&I3-- They can also be formed by a disproportionation reaction of the corresponding Mo" complex [Mo(CO),X2], with L acting as the solvent,15 or by reduction of the appropriate MoIV complex [Mo&L2]. This last reaction has been used to prepare, in hi h yield, [MoC~~(THF)~], which is a useful starting material for the synthesis of other Mo411 compounds .I6 Generally, these complexes adopt a meridional configuration, even for the tridentate macrocyclic polythiaether 1,4,7,10,13,16-hexathiacyclooctadecane,which forms polymeric chains." However, the facial isomer is formed with the tridentate ligand 1,4,7trimethyl-l,4,7-triazacyclononane.1sCrystal structure determinations have confirmed the meridional geometry of [ M ~ C l ~ ( p y )and ~ ] [M~X~(pic)~].O.Spic (X = C1, Br; pic = 4methylpyridine) ,I' Mo"' halide complexes formed with chelating diazadienes or diamines (N-N) include [MoC13(CNMe)(N-N)] species; with aromatic chelates such as 1,lo-phenanthroline or 2,2'-bipyridine, salts containing the cation cis-[MoCl,(N-N),]+ have been i s ~l at ed . ~' Compounds of stoichiometry [MoX3L4] are known for X=C1, Br; L = R C N and X=B r; = 2,6-lutidine,I5 but it is not clear whether these involve seven-coordinate Mo'". The reaction of potassium hydrotris(3,5-dimethyl-l-pyrazolyl)borate, K[HB(3,5-Me2pz)3], with [MoC13(THF)3Jgives the yellow crystalline compound [K(THF)3][HB(3,5-Me2pz)3MoC13]. Its magnetic susceptibility (3.69 BM), electronic spectrum and ESR spectrum are consistent with a d3, S = $ ground state. An X-ray diffraction study of [NEt4][HB(3,5Mezpz)3MoC13].MeCN has revealed a monomeric anion with near octahedral facial coordination about the metal. This compound undergoes two one-electron oxidations: MO'"/MO'~at +0.49 V and Mo1"/MoV at +1.55 V (us+ SCE). As anticipated from the value of the former couple, [H3(3,5-Me2pz)3MoC13]is readily reduced to the Mo"' anion by mild reductants such as sulfide and alcohols. The stabilization of the Mo"' by the polypyrazolylborate has been attributed to the interaction of the Mo d, orbitals with the ligand x * orbitals. This view receives support from the preparation of [(HBpz3)MoCi2(pyrazole)]by reacting MoC15 with potassium hydrotris(1-pyrazolyl)borate, K H13pz3], in aqueous HC1 without the addition of a reducing agent. The structure of this Mo' derivative has been determined and shown to be similar to that of [HB(3,5-Me2pz)3MoC13],with replacement of a chloride by a pyrazole group.21 [NBu4][MoBr4(dppe)] has been obtained as a minor product of the reaction of [NBU,]~[MO~B~~] with dppe in MezCO and its crystal structure has been determined; the Mo atom displays a distorted octahedral coordination geometry.=
36.41.2.2 Compleres with oxygen and sulfur donor ligands
[M~(acac)~] is conveniently prepared from [MoCl6I3- and Na(acac) or by oxidation of [Mo(CO)~]with H a ~ a c ;the ~ ~molecule has an octahedral geometry of approximately D3 symmetry, with Mo-0 bonds of average length 2.04(1) A.24Several other tris(P-diketonates) of Mo"' are known and, with an unsymmetrical ligand, facial and meridional isomers have been i~olated.'~Oxidative decarbonylation of [Mo(CO),] has also been used to prepare the 0,s-bound analogue tris(monothiadibenzoylmethanato)molybdenum(III).26 A complex of the stoichiometry [Mo(HL)L] is formed in the reaction between the tridentate O,S,N ligand salicylidenethiosemicarbazone (H2L), and [Mo(NCS)~]~-.~' Dithiophosphate'6*2*and dithiocarbamate29 ligands, (W)-, form pseudooctahedral complexes [ M O $ S S ) ~ with ] Mol"; in [MO{S~P(OM~),}~], the Mo-S average distance is 2.507(2)A.l The Mo'" complex [ M o C ~ ( S ~ C N E ~can ~ > ~be] reduced by SO, to give [Mo(S2CNEt2)3(q2-S02)], which has an ei ht-coordinate structure, with the SOz bonded through the sulfur and one oxygen aotm!' Mixed thiolate diphosphine complexes trans[M~(SR)~(diphos)~]+ can be oxidized by FeC13 or CuC12 to give [M~(SR)~(diphos)~]+, which has been isolated as the [FeC14]- or [BPh4]- salt.31
36.4.1.23 Complexes with nitrogen donor ligands
A variety of salts containing the [MO(NCs)$ ion are known, in which the ligand is bonded through nitrogen with almost linear Mo-N-C arrangement. In K3[Mo(NCS)6].H20.MeCOZH the average M o - N distance is 2.09 A selenocyanate complex, [NEt4]3[Mo(NCSe),j, has also been isolated.33 Substitution reactions between [MO(NCS)~]~and bidentate nitrogen
Molybdenum
1332
ligands, such as 1,2-diaminoethane, give various products containing one, two or three chelates per r n o l y b d e n ~ m . ~ ~ 364.12.4 Cyanide complexes
The cyanide ligand, when present in excess, replaces chloride in [MoCl6I3- to give the seven-coordinate complex [Mo(CN),I4-. In aqueous solution a pentagonal bipyramidal geometry is adopted, but in the solid state spectroscopic studies indicate a lower symmetry for K4[Mo(CN)7].2H20.3s The crystal structure of NaK3[Mo(CN),]-2H20 shows a relatively undistorted entagonal bipyramidal coordination sphere, with average Mo-C distances of 2.160(3) A.3t?
36.41.25 Magnetic and electronic properties
All the monomeric molybdenum(II1) complexes are paramagnetic, and in Table 1' the magnetic moments of some typical examples are given. For six-coordinate compounds the values lie between 3.53 and 3.86 BM, with most in the region 3.7 to 3.8 BM as predicted for octahedral d3 complexes.37 The value of 1.73BM in K4[Mo(CN),].2H20 is consistent with a spin-doublet ground state d3 sy~tern.~' ESR spectroscopy has been used to examine molybdenum(II1) complexes with C1, 0, S and N donor ligands.3a At temperatures between 5 and 80K,these compounds show broad axial or rhombic signals centred at g = 2 and 4. For [M~(acac)~] the estimated A values are 10f 1 ( x , y ) and 12.2f0.5 ( 2 ) s . Complexes with sulfur atoms which are part of an extended x system have typical ESR spectra, with all g values near 2. Electronic absorption spectra of Mo"' species can be interpreted using a ligand field approach for a d3 ion with octahedral ~ymmetry.~'Three spin-allowed transitions should be seen, corresponding to excitation from the ground state 4A2g to 4T2g, 4T1g (F) and 4T1g(P) states. Bands due to spin-forbidden transitions to 2Eg,2Tlg.and 2T2gstates can also be detected because of spin-orbit coupling. Table 2 lists the absorption bands observed in some typical complexes.s For compounds of the form [MoX3L3], the lower symmetry leads to a loss of degeneracy within the T states and this has been observed in some systems, and used to demonstrate the formation of meridional rather than facial isomers. l5 Table 2 Electronic Absorption Spectra of Molybdenum(II1) Complexes (transitions in cm-', molar extinction coefficients in parentheses) Complex
4A2,+2Eg,2Tl,
4A2g+2Tzs
4Azs+4Tzs
4Azs+"TI* (F)
Ref.
(MoC1,I3[MoCI,(H,O)]~[MOCl3(PY),l [MO(%PFZ)~I
9500 (1.4) 8500 (4.0) 8800 (7.0)
14 800(2.0) 15 300 (3.1) 14 500 (7.0)
19 400 (23) 19 700(29)
24 200 (38) 25 500 (51)
20 020 (205)
24 310 (99)
39 39 39 28
Phosphorescence has only been observed in [MoX3L3]complexes with L = urea or thiourea. The emission lies between 9000 and 10 000 cm-', and has been assigned to the 'Eg+ 'Azg tran~ition.~"
36.4.1.3 Dimeric and Polymeric Complexes The best characterized Mo"' dimers are those with a Mo-Mo triple bond alone between the metal atoms or bridged by halides, as in [Mo2X9I3-, complexes which are discussed in Section 36.3.2.2.6. Substitution of three halides by neutral ligands gives complexes of the type [Mo2&L3], which probably retain the halide bridges. One example is [ M C ~ C ~ ~ ( T Hwhich F ) ~ ] ,is formed from the monomeric [MoC&(THF)~]in solution.41 The reactions between K,[MoC&,] and various thiols yield dimeric or polymeric complexes of uncertain stoichiometry, thought to be [ M O ~ C ~ ~ ( S R ) ~ ( Hor , O [) M ~ ]O ~ C I ~ ( S R ) ~ ( H ~with O)~], magnetic moments which indicate considerable metal-metal i n t e r a ~ t i o n Crystal .~~ structures
Molybdenum(ZZZ), (Iv)rand (V)
1333
have been determined for the chloride-bridged compounds JPPh4][(MezS)C12Mo(pC1)3MoC12(SMe2)] and [(Me2S)C12Mo(p-SMez)(p-C1)2MoC12(SMe,>4 and of the bromidebridged compound [LBrMo(p-Br)(p-OH)MoBrL]Br2.4H20(L = 1,4,7-tria~acyclononane).~ In the chloride complexes, the structure around the Mo atoms is confacial bioctahedral, with three bridging atoms. For the bromide, each molybdenum is octahedrally coordinated and the two centres share a common edge. The short Mo-Mo distances in these compounds indicate strong metal-metal bonding. Oxo-bridged dimers such as [MozO,(H20)5(DMF)] and [Mo~O~(DMSO)~] can be obtained by heating [Mo(CO),] in wet DMF or DMSO.45Other dimeric species have been characterized with a variety of 0 and N donor ligands, including [Mo2O2Cl2(HzO),LY](L = bipy, 4,4'-bipy, pyrrole, pyridazine, py) ,46 [MO~O(C Z O~)L @I~O)~] (HL = phenylalanine) ,47 and [MOZOZ(HL)~]~(H2L = cysteine) .48 Low magnetic moments of these and analogous complexes, of between 0.4 and 0.6 BM, provide clear evidence for spin pairing between the metal centres. Molybdenum( 111) complexes of 1,4,7-triazacyclononane (L) dimerized by hydroxide bridges can be prepared by hydrolysis of the corresponding [LMoC13] c o m p l e ~ e s .Crystal ~ ~ , ~ ~structure determinations on [LClMo(p-OH)2MoC1L]Iz and [ L ~ ~ O ( ~ - O H ) ~ ( ~ - ~ ~ C M have ~)M~L]I~revealed a distorted octahedral geometry around the Mo atoms in both cases, with short Mo-Mo distances thought to indicate a formal bond order of three.49 In the complex K[M~~(p-OH)~(p-O&Me)(edta)], the hexadentate ethyienediaminetetraacetate also bridges the Mo-Mo bond via the two N atoms (see 9, Section 36.1.2).51Edta is thought to bridge in a similar manner in the oxo-bridged complexes MH[M~~(p-O)~(edta)l.nH~O (M = Li, Na, K, NH3 .52 A mixed oxidation state tetrameric molybdenum(II1,IV) complex N ~ [ M ~ ~ O ~ ( o H ) ~ ( e12H20 d t a ) ]has * been structurally characterized, and shows essentially two dimeric species connected via oxo bridges (see 4, Section 36.1.2). With the optically active ligand (R)-propylenediaminetetraacetate [(I?)-pdta], strong peaks are observed in the CD spectrum of [ M o ~ ( ~ - O H ) ~ ( ~ - O ~(R)ydta}]-, C M ~ ) { indicating that the four coordinated carboxylate oxygens arrange asymmetrically. Dithiocarbamate compounds of Mo"' can be prepared by oxidative decarbonylation of [Mo(CO),], giving complexes of the form [ M ~ ~ ( R ~ d.54 t cSpectroscopic )~] evidence is consistent with the presence of two bridging R2dtc ligands. The crystal structure of [(Et2NCS&Mo(pCSC(S)S)(p-S3C2NEt2)Mo(S2CNEt2)], a product of the reaction between CS, and [ M o ( C O ) ~ ( S ~ C N E ~has ~ ) been ~ ] , determined; the bridging ligands appear to be formed by the insertion of CS into CSz and EtzNCS; .55 Mixed oxidation state molybdenum(II1,VI) complexes [PPh4]z[S2Mo(p-S)2MoBr,(SMe2)] and [PPh4][S2Mo(p-S)zMoBrz(NO)2], in which the Mo atoms are linked by sulfido bridges, have been prepared by the reaction between [PPh&[MoS4] and MoBr4 or [ M O B ~ ~ ( N OA) disulfide ~ ~ . ~ ~ bridge and a sulfur dioxide bridge are found in the [ M O ~ ( ~ ~ - S ~ ) ( ~ ~ - S O ~anion, ) ( C Nthe ) ~ structure ]~of which has been determined as its [PFh4]+ and K+ salts; in the former the Mo-Mo separation is 2.730(4) A, whereas the latter involves two different anions with Mo-Mo distances of 2.691(1) and 2.974(1) A, re~pectively.~~ The salts & [ M O ~ ( C N ) ~ ~ S ] . ~and H ~ &.68[Mo04]0 O ~ ~ , ~ ~ 34[Mo2(CN)l~S]-5.32H2058 have been isolated. The crystal structure of the latter has shown that the anion involves the two metal atoms, each in a pentagonal bipyramidal environment, bridged by an axial sulfur atom (Mo-S-Mo ca. 170"). The diamagnetic nature of the compounds and their electronic spectra are consistent with S(p,) -+Mo(d,) bonding over the Me-S bonds of length ca. 2.17 8.
36.4.2 36.4.2.1
MOLYBDENUM(IV) Introduction
The coordination chemistry of Mo'" is diverse and involves an extensive range of monomeric and trimeric systems; the latter are described in Chapter 36.3 together with the few dimeric systems which involve a Mo-Mo double bond. Although there are many examples of monomeric oxo complexes, unlike the higher oxidation states, MolV forms a comparable range of non-oxo complexes and exhibits a greater versatility in coordination number, with examples of four to eight, inclusive, being well established. The most important binary systems which (formally) involve Mo'" are the chalcogenides MoSz and MoSe; some comments on these systems are included in Section 36.6.3. The synthesis and properties of molybdenum(1V) halides have been thoroughly investigated.'@'
1334
Molybdenum
Fluorination of [Mo(CO)~]at -75 "Cforms the olive green material Mo2F9,which decomposes on heating to the volatile MoFS and the light green, involatile MoF4; reduction of MoF5 with elemental silicon (4: 1) provides an alternative route to MoF4. The Raman spectrum of MoF, suggests that the material possesses a structure similar to that of NbF4.61MoCl, exists in three forms. cu-MoCL,, conveniently prepared6' by the prolonged refluxing of MoClS and &C14 in CCL, is isomorphous with NbC& and comprises infinite chains of MoC16 octahedra sharing opposite edges over alternating long and short Mo-Mo distances, with magnetic exchange occurring down these chains leading to a room temperature effective magnetic moment of 0.85BM. P-MoC14 is obtained by reduction of MoC15 by molybdenum metal under carefully controlled condition^;^ the structure consists of cyclic (MoCl& aggregates in which each mol bdenum is octahedrally coordinated by two terminal and four bridging chlorines (Figure 1). B The closest approach between the Mo atoms (3.67A) does not lead to any significant spin-pairing (peaca. 2.4 BM). A third form of MoC1, has been obtained by treatment of Mooz in a stream of CCl, v a ~ o u rMoBr4 . ~ ~ may be synthesized by the reaction between MoBr3 and warm (60 "C) liquid bromine.65 The tetrahalides are reactive, for example to air and water, and may be used as starting materials for the synthesis of MoIV complexes.
Figure 1 The structure of the (MoCL), molecule in B-MoCL,~~
MoOClz has been prepared by various chemical transport reactions1 and MoSClz and MoSeCl, have been d e ~ c r i b e d Mooz . ~ ~ may be prepared by reduction of Moo3 by H a , the compound adopts a distorted rutile structure with Mo- --Mo separations of 2.5106(5) and 3.1118(5) the former leading to spin pairing and the absence of a permanent magnetic moment on the metal centres.@ Mononuclear Mo'" oxo complexes have special relevance to the molybdenum centres in the molybdenum-containing enzymes. [MoO(SzCNR2),] complexes undergo addition reactions with a wide variety of unsaturated molecule^.^^^^ These were once herd to be of relevance to the activation of the substrates of nitrogenase prior to reduction; however, the nature of the molybdenum centre in these [MoO(S,CNR,)~] molecules has not been demonstrated to have any significant similarity to the catalytic site within the Fe-M-S cluster of the nitrogenases (see Section 36.6.2) and, therefore, detailed comparisons are unwarranted. [MoO(S2CNR&] and a selection of other MoIV complexes are special in respect of participating in reversible oxygen atom transfer, Moo2++ XO +MOO$.++ X, which have particular relevance to the catalyses effected at the molybdenum centres of oxomolybdoenzymes such as xanthine oxidase, sulfite oxidase or nitrate reductase." The forward reaction was first observed in 1972 for [ M O O ( S~ CNR ~)~] complexes8' and X = Ph3P and this reaction has been developed and used for the preparation of Moo2+ complexes8z-84and the mechanism defined.85.86A complicating feature of this reaction is the dimerization (equation 1) which is prevented in the oxomolybdoenzymes by structural constraints within the macromolecular assembly. Macrocyclic ligands have been employed in chemical studies to prevent such dimerization and, thereby, have allowed the development of more realistic chemical analogues of the oxomolybdoenzymes. These ligands include the anions of the molecules (Figure 2)"4,87-91and these studies represent elegant developments in the coordination chemistry of molybdenum. [MoOz(SzCNR,)z] + [MoO(S2CNR,),]
[MoLh(SG%)J
(1)
0 MeN
NM-
LJ
WNHCH2
N, N'-Dimethyl-N,N'-bis 2'-
N,N'-Bis-(3' ,l",l"-dimethylethyl-2'-
aHNuNH Hn0s" "so
hydroxyphenylmethyl)-l,2-diaminoethane
mercaptoethyL1,Zdiaminoethane
S
WS
N, N'-Bis-2'-mercaptophenyl1,2-diaminoethane
S,S'-Bis-2' -mercaptophenyl1,Z-dimercaptoethane
*
Y H H Yb P h Ph 2,6-Bis(2,2-diphenyl-2-hydroxoethanyl)pyridine 2,6-Bis(2,2-diphenyi-2-mercaptoethanyl)pyridine
Ph
Y =0 Y =S
Ph
Figure 2 Precursors of macrocyclic ligands which allow redox reactions for monomeric molybdenum centres
36.4.2.2
Monomeric Complexes with Oxo Groups
36.4.2.2.1 General comments
Oxo complexes dominate the chemistry of molybdenum in its higher oxidation ~ t a t e s , ~ * ~ ~ - ~ , ~ ~ and play a major role in the catalytic activity of molybdenum in biological systems (see Section 36.6.7). MoIV complexes have been characterized containing the monooxo {MOO}'* and the trans-dioxomolybdenum(1V) { MoOp} cores, but no compound has been isolated analogous to the cis-dioxomolybdenum(V1) species described in Section 36.5. Crystal structure determinations have been accomplished for a number of MolV complexes, and some typical examples are listed in Table 3." Table 3 Molybdenum-Oxygen Bond Lengths of some Oxornolybdenurn(1V) Complexesso
Compound
M o a , (4
ReF
1.665
100 103 107 108
1.667 1.803 1.676 2.69 1.834 1.698 1.668 1.636 a
hd = 1,5,9,13-tetrathiacycIohexadecane. Green isomer.
111 120 121 121 126
Blue isomer.
The nature of the bonding of oxo groups to molybdenum has been discussed in several reviews.80592793 There are three components to molybdenum-oxygen terminal bonds (Mo-OJ, comprising a CJ interaction along the bond axis ( z axis) and two mutually perpendicular n components, each formed by the overlap of an oxgyen p n orbital (px or p,) with the appropriate molybdenum d, orbital (dxz or dyz). Normally, each component is considered to
1336
Molybdenum
involve donation of an electron pair from 02-to a vacant orbital of the metal, and the formal bond order varies according to the degree of n interaction, Correlations have been observed between the bond order of a Mo-0, bond and its length and force constant, with a high bond order corresponding to shorter and stronger bonds.yk96 Generally, the M o - 0 , bond lengths in monooxomolybdenum(1V) compounds are -1.67 A (Table 3), consistent with a triple bond: for the trans-dioxo species a longer bond length of -1.83 A (Table 3) is found, indicative of a bond order between one and two. In monooxomolybdenum(1V) centres the Mo-0, bonding dominates the ligand field at the molybdenum, so that the two valence electrons of the metal are spin-paired in an orbital which is in the plane ( x y ) perpendicular to the M o - 0 , bond, and the complex has the (dxy)2 configuration. The dioxo anion rrans-[M~O~(CN),]~also possesses this c~nfiguration;'~the two d electrons are in an orbital which is nonbonding with respect to the oxo groups and the CT orbitals of the cyano groups and, hence, at a lower energy than the other d orbitals. This situation may be further stabilized by back-bonding from (d,y)2into the x* orbitals of the CNligands. Similarly, ligand field stabilization may explain the formation of tran~-[MoO~(CO)~] in the photooxidation of [ M O ( C O ) ~ ] . ~ The coordination sphere of molybdenum in the oxomolybdenum enzymes is believed to contain oxo and sulfur ligands?' and, therefore, interest has focussed on oxo complexes of MoIV with S-donor ligands as possible models for the reduced state of these systems. A number of oxomolybdenum(1V) complexes contain a five-coordinate Moos4 coordination sphere, includin the dithiocarbamate species [MoO(SZCNR2),].These can be prepared by reduction of the Mo dimers [ M o ~ O ~ ( S ~ C Nusing R ~ ) ~zinc ] dust or thiophenol, or by reduction of aqueous solutions of Na2[MoOr] and Na(S2CNR2) with d i t h i ~ n i t e An . ~ ~ alternative route is by the reaction of PPh3 with the corresponding MeV' complex, [ M O O ~ ( S ~ C N R in ~ ) which ~ ] , ~ ~one atom of oxygen is transferred from the molybdenum to the phosphorus in a reaction which may be analogous to the role played by the molybdenum in xanthine oxidase and other related enzymes@ .' The reverse reaction, involving abstraction of an oxygen atom from a substrate molecule by the monooxo MoIV complex to give the cis-dioxo MoV' complex, has been observed with dioxygen or pyridine N - ~ x i d e the ; ~ ~claim that this also occurs with Ph3P0 appears to be erroneous. A kinetic study of the forward and reverse oxygen atom transfer reactions with [ M o O ( S ~ C N E ~has ~ ) ~shown ] that the dominant factor in determining reaction rate is the breaking of the bond to oxygen in the substrate.85 The act of oxygen atom transfer has been suggested to involve a transition state comprising one molecule of each reactant, and the donation of the lone pair of electrons on phosphorus into the M-0 n* orbital, thus weakening the M o - 0 bond and forming the P-0 bond to produce the molybdenum(1V) complex [M00(Et~dtc)~(Ph,PO)]. Isomerization followed by the ioss of Ph3P0 would produce the reaction products.s6 The MolV complexes can also react with one of the oxo groups of [ M o O ~ ( S ~ C N Rto~ )give ~ ] the oxo-bridged Mo" dimer [ M o ~ O ~ ( S ~ C N(equation R ~ ) ~ ] 1). The crystal structure of [MOO(S~CNP~;)~] has shown that the molecules possess a square pyramidal geometry, with the molybdenum displaced out of the plane of the four sulfur atoms in the direction of the axial oxo ligand;lm the M o - 0 , distance is 1.695A. Complexes of this type are coordinatively unsaturated and react with ligands, such as pyridine, phosphines, alkynes and alkenes, to form 1:l adducts. Spectroscopic evidence has indicated that the unsaturated molecules add to produce a seven-coordinate c~mplex,'~and this has been confirmed by a cr stal structure determination of the tetracyanoethylene (TCNE) adduct of [MOO(S~CNP~;)~]which shows distorted pentagonal bipyramidal coordination around the molybdenum. The equatorial plane contains the two ethene carbons of TCNE and three of the sulfur atoms; the fourth sulfur occupies an axial position trans to the oxo ligand, and the Mo-0, bond length is 1.682(4) A. Although the oxidation state of the molybdenum is difficult to define in these compounds, IR spectra of some eth ne adducts suggests that they are complexes of MoV1,formed by oxidative addition,76the dY electrons of the molybdenum being donated into the n* orbital of the multiple bond. Kinetic studies of these addition reactions indicate that they proceed via a monocapped trigonal prismatic intermediate, followed by intramolecular rearrangement [MOO(S~CNR~)~] forms 1: 1 adducts with molecules containing nitrogen-nitrogen double moiety is thought to be sideways bonded to the molybdenum via bonds in which the -N=Nboth nitrogen atoms. In contrast, an end-on nitrogen-molybdenum u bond is formed in the reaction between molecules of the type [ M O O ~ ( S ~ C N Rand ~ ) ~R'NNH, ] to produce the adducts .71 These complexes may be of interest as models for the intermediates [(R'NN)Mo(S~CNR~)~] formed during reduction of N2 by the nitrogenases;5 the adduct [Mo0(S2CNEt2),(R'NNR')]
3.
yZ
.77778
Molybdenum(ZZZ), (IV)and ( V )
1337
(Rr=C02Et) can be hydrolyzed in moist CHC1, to produce R'NHNHR' and [Mo02(S2CNEt2)],showing that the N=N bond is activated by adduct formation.69The crystal structure of the adduct [MOO(P~CONNCOP~)(S~CNM~~)~]-C~H~C~ has been determined." The stereochemistry about the molybdenum is that of a distorted pentagonal bipyramid; the sulfur atoms of the dithiocarbamate ligands and a nitrogen atom of the benzoyldiazene ligand occupy the equatorial positions and the axial sites are occupied by the terminal oxo group and an oxygen atom of the diazene, which thus forms a five-me,mbered chelate ring. Studies of MoIV/MoV' redox reactions involving [MOO(SCNR2),] complexes are complicated by the formation of the MoV dimer, [ M O ~ O ~ ( S ~ C N(equation R ~ ) ~ ] 1). In biological systems this type of dimerization is prevented by structutal constraints exercised by the enzyme, and attempts have been made to emulate this by preparing model compounds with chelates, some of which introduce steric constraints at the metal. Complexes with polydentate ligands of the form [MoOLL'] (L = salicylaldehyde-2-hydroxyani1,salicylaldehyde-2-mercaptoanil; L'= bipy , phen) and [MoOL] (L = N,N'-bis(2-mercapto-2-methylpropyl)-l,Zdiaminoethane) have been in~estigated.'~ They undergo electrochemical one-electron oxidation or reduction reactions, but can not be easily oxidized to MoV' species. [MoO(PPh,Et)(dttd)] (H2dttd = S,S'-bis-2'mercaptophenyl-l,2-dimercaptoethane, Figure 2) prepared by reduction of the MoVKcomplex [MoOz(dttd)] with PPh2Et, loses the phosphine in DMF/[Et4N]Cl solution to form [MoOCl(dttd)]-. This anion undergoes a reversible one-electron oxidation to produce [MoOCl(dttd)] but is not oxidized further to a Mo"' species.w The ligand 2,6-bis(2,2-diphenyl-2-mercaptoethanyl)pyridine(Figure 2) (LN(SH),) has been used to prepare the MolV complex [MoO(LNS2)(DMF)] and the complementary dioxo MoV' complex [MoO,(LNS,)]; in both cases, the ligand acts as a tridentate N,S,S donor. The MolV complex can be oxidized electrochemically to a Mov species but not to MoV1. However, [MoO(LNS2)(DMF)J reacts with DMSO to abstract an oxygen atom, forming [Mo02(LNS2)]: the reverse reaction can be accomplished using Ph3P, generating Ph3P0 and [MoO(LNS,)(DMF)].~~These reactions produce a catalytic system for the oxidation of P h g by DMSO, and provide an interesting model for the reactions catalyzed by the oxomolybdoenzymes. 364.222 Compleres of oqgen and sulfur donor Iigands
Reduction of [NH4]2[MoOC15] with hydrazine, followed by extraction of the product with glacial acetic or malonic acid, gives the MoIV complexes [Mo0(02CMe),(H,0)] or [MO O ( O ~CCH~ CO~)(H~O)~], respectively."' Spectroscopic evidence has indicated that the molybdenum in these species is coordinated to six oxygen atoms. Other oxomolybdenum(1V) complexes with oxygen donor ligands include [MoO(CN)~(OH)]~and [MOO(CN)~(OH~)]*(see Section 36.4.2.2.4), [MoO(dppe),(OH)]+ and the aqua ions Mo02+(aq) and MoO(OH)+(aq) (see Section 36.1.4). [M00(dppe)~(OH)]+contains a linear O = M e O H group, with the two bidentate dppe ligands completing the octahedral coordination sphere. An analogous sulfur-bound complex has been isolated, using the macrocyclic tetrathioether +.Io3 This cation contains a 1,5,9,13-tetrathiocyc~ohexadecane(hd) to form and an M a - O t distance of linear O=Mo-SH group, with a M w S 1.667(3)A. The thioether acts as a tetradentate ligand to form a square planar arrangement around the molybdenum. Difluorodithiophosphate forms the five-coordinate complex, [MoO(S2PF2),], horn which the (S2PF2)- ligands are easily dis laced by pyridine to give [MOO(~~)~](S~PF & . ~bidentate S,O ligands, (SCH2C02)', (SCH(Me3)C02)2L and The (SCH(CH2C02H)C02)2- ( S O ) , also form five-coordinate complexes of composition [MoO(S-O)(H20)2), which have been isolated as solids. A cysteine complex of Moo2+ has been identified in solution, but not characterized as a solid.'04 With the S,P-donor ligand (SCH2CH2PPh2)-, the coordination geometry around the molybdenum in [MoO(SCH2CHzPPh2)2]has been shown to be intermediate between trigonal bipyramidal and square pyramidal. lo5 36.4.2.23 Complaes of phosphine ligands
In ethanol solution, tertiary phosphines react with [MoC14(NCEt)2] to yield mer[ M o O C ~ ~ ( P R ~which ) ~ ] , can undergo exchange reactions to give mer-[MoOX2(PR3)3J(X = Br,
1338
Molybdenum
I, NCS, NC0).lo6 Some of these complexes are green, with v(M-0) stretching frequencies below 946cm-', and others blue-green, with v(M-0) above 946cm-'. In the case of [ M O O C ~ ~ ( P M ~ both ~ P ~ a) ~blue ] , isomer and a less stable green isomer have been isolated. Crystal structure determinations of green cis,rner-[MoOC12(PEtzPh)3]'07 and blue &,mer[ M O O C ~ ~ ( P M ~ ~have P ~ )shown ~ ] ' ~ that ~ both complexes have an octahedral geometry with notable differences in particular bond lengths and angles. Therefore, it is considered that a balance exists between metal-ligand attraction and ligand-ligand repulsion which, in this case, is very sensitive to the nature of the ligands and results in two possible distortional isomers of the same structural unit. Alternative routes to [MoOC12(PR3)3]complexes have been developed and these include the reaction of the phosphine with Na2[Mo0,] in EtOH in the presence of HC1 and Zn-Hg reduction of [ M O O C ~ ~ ( P Rin~THF."" )~] The reaction of an excess of a diphosphine (PP), such as dppe, with [MoOCI;(PR~)~] yields [MOOC~(PP)~]CI; the cation has been isolated as [BF,]-, [BPh4]-, [NCSI- and [NCSeI~ a l t s . ' When ~ ~ , ~a ~stoichiometric ~ (1 :1) amount of (PP) is present, the intermediate, mixed phosphine complex [MoOC1~(PP)(PR3)]can be prepared. lW A crystal structure determination of [M00Cl(dppe)~][ZnCl~(OCMe~)] has shown that the cation contains octahedral M O ' ~ . ' ~ ~ Moo2+ complexes with other bidentate ligands have also been prepared, and these include [MoOClzL(PMePhz)] (L = bipy, phen) and [MoOL'(PMePh2)] (H2L' = 8-hydroxy- or 8rnercaptoquinoline) . All, except the mercaptoquinoline system, undergo an essentially reversible one-electron oxidation.'" 36.42.2.4 Complexes of nitrogen donor ligan&
Complexes of MorV with tetraphenylporphyrin (TPP) or tetratolylporphyrin (TTP) ligands are generally obtained by reduction of the corresponding Mov species. Zn-Hg reduction of [MoOCl(TPP)].HCl yields [MoU(TPP)], the structure of which has been determined.'13 The MoIV atom is in a square pyramidal geometry, slightly displaced from the plane formed by the four N ligands, in the direction of the apical oxo group. The M-0 and Mo-N bond lengths are 1.656(6) and 2.110(6) A, respectively. ESR and electronic spectral data suggest that, in the reduction of [MoVOJ3r(TPP)]by 0, to give [MotVO(TPP)],an intermediate dioxygen complex [MO'~O(TPP)(O,)]~ is formed, which is stable in solution at -72 'C.'l4 The photochemical reduction of [MoVOX(TPP)] (X = NCS, F, C1, Br) to [MorVOX(TPP)]- at 7 7 K has also been investigated. On warming the solution, X- is released to give the [MO'~O(TPP)]complex.'15 The reaction of [MoO(P)] (P = TPP, TTP) with HC1 in benzene yields the corresponding dichloro complex, [MoC12(P)], and these are the only MoIV complexes with porphyrin ligands which do not contain an oxo group.'13 [MoO(Pc)] (Pc = phthalocyanate dianion) can be prepared from the reaction between phthalodinitrile and [MO(C0)6]116 or [NH4]2[Mo7024]-4H20.117 A crystal structure determination has shown that this complex possesses the same structure as [MoO(TPP)], with a M o - 0 bond length of 1.668(4) A. The reaction between MoX3 (X = Cl, Br), aniline and 2,6-diacetylpyridine in butanol yields [M~O{py(anil)~}X~], where py(anil)2 is a tridentate Schiff base ligand formed in These complexes are isomorphous with [VO{py(anil)2}X2] and, therefore, are considered to have octahedral coordination of the molybdenum. 36.4.225 Cyanide and isocyanide complexes
The photolysis of aqueous solutions of K4[Mo(CN),] gives compounds containing the tran~-[MoO~(CN)~]~ion.'19 These are easily protonated to given trans-[M~o(OH)(CN)~]~and trans-[MoO(OHz)(CN),]2-, and crystal structure determinations have been accomplished for compounds of each of these anions. In NaK3[Mo02(CN)4j, the Mo-0, bond length is 1.834(9)A,120which is significantly longer than found in MooZt complexes (Table 3). In proceeding from [cr(en),][M00(0H)(CN)~]to [Pt(en),][MoO(OH,)(CN),], the Mo-0, bond length decreases from 1.698(7) to 1.668(5) A, whilst the Mo-0 distance for the protonated oxygen increases from 2.077(7) to 2.271(4) A.121In the trures-[M00(OH~)(CN)~]~anion, a significant degree of distortional isomerism can occur, and compounds of this anion have been isolated from the reduction of aqueous solutions of Na2[Mo04] in the presence of CN-, in
Molybdenum(III), (N) and (V)
1339
which the Mo-0, distance varies between 1.60(2) A, for the [AsPh4]+ salt, and 1.72(2) A,for the [PPha]+ salt.122 The only ligands, apart from cyanide, which are known to stabilize a dioxomolybdenum(IV) group are carbonyls, in the complex trans-[MoO2(C0),], which is formed in the photooxidation of [Mo(CO)~].~* The strong n-acceptor capacity of both CN- and CO removes electron density from the filled dxyorbitals of the molybdenum, thereby increasing the (formal) positive charge on the metal and increasing the ci and JT donation from the two terminal oxo groups. Protonation of an oxo group reduces this donation, resulting in the relative bond Iengths Mo-0 < Mo-OH < Mo-OHz with the concomitant decrease in the length of the bond to the trans-oxoligand. Monooxo complexes containing the [MoO(CN),I3- anion have also been isolated from reactions involving the reduction of [MoO4I2- or the photolysis of [Mo(CN)~]~-."~ The crystal structure of [PPh4]3[MoO(CN)5].7H20shows octahedral coordination around the Mo , with the Mo-0, bond length being 1.705(4) A.1zz The reaction between [MoO2(CN),I4- and 1,lO-phenanthroline yields [MoO(CN)3(phen)]-. A crystal structure determination of Na[M~O(CN)~(phen)l .2phen-MeOH.H20 has shown that one nitrogen of the phen ligand is trans to the oxo group, with the other nitrogen and the three CN- ligands completing the octahedral coordination sphere of the Mo.IX The Mo-N distance trans to M o- 0, is 2.363(7) A, as compared to a distance of 2.173(8) A for the bond trans to a CN- ligand, showing the large structural trans influence of the oxo group. Mol" isocyanide complexes of the form [MoOX(CNR)~]Y(R = alkyl; X = C1, Br; Y = C1, Br, Br12, 13, PF6) have been prepared by several synthetic routes.'25 These complexes contain a trans 0-Mo-X arrangement, and in [ M o O C ~ ( C N M ~ ) ~the ] ( I ~Mo-0, ) distance is 1.64(4),126 a length typical of the multiple terminal oxornolybdenum(1V) bond (Table 3). 36.4.2.2.6 Spectroscopic studies
Oxomolybdenum(1V) complexes have been studied by a wide variety of spectroscopic techniques, including NMR, vibrational, electronic and electrochemical measurements. Each of these provides information about different aspects of the compounds, and can be used in conjunction to illustrate various aspects of the bonding. A survey of 38 complexes of Moo2+ has been made using 95MoNMR spectroscopy and some typical chemical shifts and line widths are given in Table 4.12' The compounds display a large chemical shift range and highly variable line widths: the most deshielded resonance occurs for [MoOCl,(bipy)(PPh2Me)], with 6 3160 p.p.m. (referenced to [MOO$-] of Oly.p.m.), and the most shielded resonance is that of [MoOCI(CNM~)~]+, which is 1035p.p.m. 0 NMR spectra have been recorded for [ M O O ( S ~ C N E ~and ~ ) ~[ M ] o O ( S ~ P E ~ ~and ) ~ ]compared , with spectra of oxomolybdenum(V) and (VI) compounds. The " 0 chemical shifts vary with the strength and multiplicity of the Mo-0, bond, but changing the oxidation state of molybdenum produces a bigger change, so comparisons between M o - 0 bonds using "0 NMR data can only be made within a particular oxidation state.12* Table 4 9sM0 NMR data'= Compound
[MoO(S,~Et2)21
[MoOC1,(PE$Ph),]" [MoOC1,( bipy)(PPh,Me)] tMOOCWPPe~2l[ B P U K,[MOO,(CN)~]*~H,O KdMoO(W(CN)4I [MoOCl(CNMe),][BPh,]
Chemical shift (p.p.rn.)
2400
2200 3160 1260 1220
1444 1035
Line width (E)
2700 1200 800
1780 240 240 330
*Green isomer.
Anomalies in the '€3 and 31P NMR spectra of complexes mer-[MoOX2(PMePh),] (X= C1,Br) have been interpreted as being due to second order effects caused by the small difference in chemical shifts between the two trans phosphines and the unique phosphine trans to C1.lZY The vibrational spectra of oxomolybdenum(1V) complexes all show a band due to Mo-0, COC3-QQ.
Molybdenum
1340
stretching at cu. 900-1000cm-l. isotopic substitution has been used to make definitive ~ ~ ~some values are given in assignment of this vibration for a series of c o r n p l e x e ~ ,and Table 5.5 The frequency of the vibration is a sensitive indicator of the strength and formal order of the bond, and can distinguish between distortional isomers such as the green and blue forms of [ M O O C ~ ~ ( P M ~'06 ~P~)~]. Table 5 Molybdenum-Oxygen Stretching Frequencies'
Compound
v(Mo-0,) (cm-')
Ref.
97s 940
100
954
948
850" 900 980
946 a
Green isomer.
Blue isomer.
107 108
111 121 121 122 125
Asymmetric stretch.
Electronic absorption spectra have been recorded for a large number of oxomolybdenum(1V) species but, in addition to d-d transitions, there is the possibility of two-electron excitations, charge transfer bands and splitting of degenerate energy levels due to low symmetry. These factors make it difficult to interpret or even to compare ~ p e c t r a . ~ Electrochemistry has been used to investigate the redox properties of MoIV oxo complexes, particularly those used to model some aspects of molybdenum enzyme chemistry, such as complexes with a {Moos4} [MoO(S,NCR2),] complexes undergo a reversible one-electron oxidation to the Mov species but cannot be further oxidized to Mo"'. Complexes of the type [MoOLL'] (L = salicylaldehyde-2-hydroxyanil, salicylaldehyde-2-mercaptoanil; L' = bipy, phen) can also be reversibly oxidized to the Mov species, and reduced to the Mo"' complex .84 Other complexes which undergo reversible one-electron oxidation include [MoOClz(bipy)(PMePhz)], [MoOClz(phen)(PMePh2)] and [MoOL(PMePhz)] (HzL = 8hydroxyquinoline). l2 36.4.2.3
Complexes Involving Multiple Molybdenum-Nitrogen Bonds
The chemistry of molybdenum with nitrogen ligands has been extensively studied, with particular attention being paid to those species which may serve as models of the nitrogen fixation process catalyzed by the nitrogenases. Complexes of MoIV which contain multiple Mo-N bonds are known with diazenide (NNR)-, hydrazide (NNRR')'- or (NHNRR')- , nitride (N)3-, imide (NR)'- and azide (N3)- ligands, and interest has focussed on reactions involving changes in the: oxidation state of the molybdenum and the related reaction of the coordinated ligand. Arenediazenide ligands, such as (NNPh)-, are electronically analogous to NO and, therefore, can act as one- or three-electron donors. Thus, in mononuclear complexes they can coordinate to form linear Mo-N=N groups, in which the ligand can be regarded as (NNAr)+ with the ligand acting as a three-electron donor, or to form bent Mo-N=N groups, with (NNAr)- acting as a one-electron d0n0r.l~' The degree of multiple bonding in the Mo-N bond is reflected in the Mo-N-N interbond angle and the closer the angle is to 180" the higher is the bond order. Diazenide complexes can be obtained by reacting a diazonium salt with a metal complex containing suitable leaving groups, such as carbonyls.132[MO(SC~H~P~~-~,~,~)~(CO),] reacts with [NNPh][BF4]in MeCN to give [MO(NNP~)(SC~H~P+~-~,~,~)~(NCM~)], in which the NNPh and MeCN ligands occupy the axial sites of a trigonal bipyramid with the sterically hindered thiolates eq~atoria 1.l~~ The M-NNPh bond length is 1.78(1) A,with a bond angle of 171(1)O, suggesting a considerable amount of M-N multiple bonding. This was the first reported example of a molybdenum-thiolate-diazenide complex and its existence supports the suggestion that bulky ortho-substituted arenethiolate ligands stabilize the binding of n-acceptor molecules to the molybdenum and, thereby, may provide interesting models for metal-sulfur
Molybdenum(ZZZ), (N) and (V)
1341
sites that will interact with dinitrogen. Diazenide complexes can also be formed by reduction of hydrazide compounds. The MoV' complex [Mo(NNHP~)~L,&H~NNHP~ (H2L= 2,3butanedithiol) reacts with PhSH in MeOH, in the presence of NEt,, to give [NHE~~][Mo~(NNP~)~(SP~)~] In this dimer each MoIV atom is octahedrally coordinated to two terminal diazenides, one terminal thiolate and three bridging thiolates. The Mo-Mo distance is 3.527(1)& which is one of the largest separations identified in any simple molybdenum dimer. For molybdenum dithiocarbamates, several compounds are known with both diazenide the and hydrazide ligands attached. In [Mo(NNCO~M~)(NHNHCO~M~)(S~CNM~~)~] overall geometry corresponds to that of a distorted pentagonal bipyramid, with the NNC02Me group in one axial site and one sulfur atom from a S2CNMezgroup in the other. Two of the equatorial sites are occupied by the nitrogen atoms of the side-on hydrazide ligand, and the remaining three by sulfur atoms of the dithiocarbamates. The Mo-N distances of the hydrazide are 2.13(1) and 2.11(1) A, whilst the Mo-N distance of the diazenide is 1.71(1) A, this last value indicating considerable multiple bonding.135The reaction of alkyldithiocarbazates, NH2NHC(S)SR', with [ M o O ~ ( S ~ C N R (R, ~ ) ~R' ] = Me, Et) yields mixed diazenidohydrazido complexes of composition [Mo{NNC(S)SR'} {NH2NC(S)SR'}{ S2CNR2}2].A structure determination of the complex R = Me, R' = Et has identified pentagonal bipyramidal coordination at the molybdenum with the axial positions occupied by the diazenide ligand and one sulfur atom of a dithiocarbamate group. The hydrazide ligand is bound as a N,S chelate, via the NH2 nitrogen and the unsubstituted sulfur atom, giving a five-membered ring. The other three equatorial positions are occupied by dithiocarbamate sulfur atoms. The Mo-N distance to the diazenide ligand is 1.77(1) A, and to the hydrazide ligand is 2.250(8) Treatment of this complex with HCl in MeOH leads to the formation of the dimer [(Me2NCSz)(O)Mo(pNNC(S)SEt)2Mo(S2CNM~)],in which the bridging ligands are coordinated to both molybdenum atoms, via the terminal nitrogen, and are also bonded via the unsubstituted sulfur atoms to the non-oxo bound m01ybdenurn.l~~ The N-N distances in the brid in li ands are longer than found for bridging diazenido(1-) ligands in other compounds' * and, hence, can be regarded as hydrazido(3-) groups with the formal oxidation state of each molybdenum being V. The hydrazido(2-) complexes [MoBr(NNRR')(dppe)$ (R, R' = H, alkyl) can be prepared in a two-stage process by the reaction of RBr with [Mo(N2)2(dppe)~] to give [MoBr(NNR)(dppe)z], which reacts with R'Br to give [MOB~(NNRR')(~~~~)~]B~.'~~ Twoelectron reduction of these complexes, followed by reaction with HBr, yields the imidomolybdenum(1V) species [M~Br(NH)dppe)~]Br, which can be converted by treatment with base to the nitrido complex [ M ~ B r ( N ) ( d p p e ) ~This ] , ~ ~series of reactions converts a terminally bonded dinitrogen ligand into a nitrido group, thereby breaking the N-N bond and forming an amine; the sequence provides a model of a possible mechanism for the fixation of dinitrogen. A crystal structure determination of [M0Br(NH)(dppe)~]Br.Me0H has shown that the cation possesses octahedral geometry, with the bromide trans to the imido ligand. The Mo-N distance is 1.73(2) A, typical of a multiple Mo-N bond.'41 The reaction of Me3Si(N3)with [ M ~ ( N ~ ) ~ ( d p pyields e ) ~ ] the nitrido azido complex [M~(N)(N~)(dppe)~]. The crystal structure arrangement, with an unusually long Mo-nitrido of this compound shows a trans N-Mo-N3 distance of 1.79(2)1k and a Mo-azide distance of 2.20(2).4. The Mo-N-N azide angle is 167.1(11y, suggesting some JT interaction in the metal-nitrogen bond.14' Complexes of mdybdenum(1V) with 4-tolylimido ligands, (Ntol)2-, have been prepared by E ~ ~ ) with ~ ] PPh3 to form reduction of Mow and MoV species. Thus, [ M o O ( N ~ O ~ ) ( S ~ C Nreacts [M O( N ~O~ ) $S~C NE~)~], which is easily oxidized by O2 or DMSO to reform the starting . rnaterial.l4 ,143 These oxygen transfer reactions are analogous to those of the cis[MoO~(S~CNR~)~]/[MOO(S~CNR~)~] systems (see Section 36.4.2.2.1) and form a catalytic cycle for the oxidation of tertiary phosphines by O2 or DMSO. Reduction of [M~(Ntol)Cl~(PMe~)~l by Na-Hg in the presence of PMe3 yields [ M O ( N ~ ~ ~ ) C ~ ~ (The P Mgeometry ~ ~ ) ~ ]of. this ~~~ complex is pseudooctahedral, with the phosphines in a meridional confi uration and the chlorine atoms in mutually cis positions. The Mo-N distance is 1.739(2) with an almost linear Mo-N-C arrangement. 15N NMR spectroscopy has been used to characterize hydrazido, imido, and nitrido complexes of M o ' ~ , and to monitor the reaction of [MOI~('~N'~NH~)(PM~~P~)~] with HI in THF to give l5NH3.lUIn the imido complexes, coupling between the 'H and 15Nin the (NHl2ligand was observed, confirming that the proton remains on the nitrogen, a fact which was not clear from other techniques.
g g g
2,
1342 36.4.2.4
Molybdenum Other Monomeric Complexes
364 24.1 Halide complexes The simplest monomeric molybdenum(1V) halide complexes are the [Mo&I2- anions. Na2[MoF6]was first pre ared by reacting MoF6 with NaI in liquid SOz, and was obtained as a water sensitive powder." A crystal structure determination has been carried out on Liz[MOF,& showing octahedral coordination with Mo-F distances between 1.927(2) and 1.945(2) A.' Potassium salts of [MoC&]~-can be prepared by fusing MoCl, with KC1 at 20O0C, or by reacting [MoC15] with IC1 and KC1 at 150 0C.147MoCIS or [NE4][MoCl6]dissolved in either of the room temperature ionic liquids AlC13/N-n-butylpyridiniumchioride or AlCl,/l-methyl-3ethylimidazoliurn chloride gives [MoCl6I2- in solution as a stable entity.14' No full crystallographic determination has been reported for an [MoCl6Iz- salt, but interpretation of the powder diffraction pattern of K2[MoC16] has given an M 4 distance of 2.31(5) Hi." Hexabromomolybdates, M2[MoBr6] (M = Rb, Cs), have been prepared by reacting MoBr3, MBr and IBr in a sealed tube at 200 Molybdenum(1V) tetrahalides react easily with a variety of ligands to form complexes of stoichiometry [MoX&] (X = F, C1, Br; L = two neutral monodentate ligands or a neutral P bidentate li and . Examples of these complexes are known with N d o n ~ r , ' ~ 523 . ~ ~ As ~ . donorlSOand 0 donor ligands.15' Complexes of this type can also be prepared by reduction of MoC& with an excess of the ligand,151 reaction between PR, and [MO~$NCM~)~CL,],'" or by oxidation of [Mo(CO)~(PR&]with Xz (R = Me, Et, Ph; X = C1, Br).' An alternative and convenient synthesis is via [MoCL(OEt,),], which can be prepared from MoCls and Et20 in the presence of alkenes which act as C1 acceptor$" the E t 2 0 ligands of this complex are easily exchanged to give [MoC14k]. Vibrational studies have shown that the [Mo&(PR&] complexes adopt a trans octahedral configuration but, as expected in view of the span of the bidentate ligand, [MoC14(dppe)]is obtained as the cis isomer.lM Spectroscopic studies have indicated that [MoC14L] (L = pyrazine, quinoxaline, 4,4'-bipyridyl or k = 4,4'bipyridyl) complexes contain a cis-[MoC14N2] chromophore. In contrast, for [ N R & [ M o C ~ ~ ( S ~ Xprepared ~ ) ~ ] , from [MoC14] and [NR4][SnX3](R = H, Me, Et; X = Cl, Br), vibrational spectroscopic evidence favours a trans structure. 155 Oxidation of [ M O ( C O ) ~ ( P M ~ ~ with P ~ ) ~X2 ] (X = C1, Br) gives the seven-coordinate complexes [ M O & ( P M ~ ~ P ~ )The ~ ] . structure ~'~ of these is monocapped octahedral, the capped face consisting of three phosphorus atoms, while the cap and remaining three vertices are occupied by the halogen. A five-coordinate complex [MoF4(NEt2Ph)]has been isolated, but not structurally characterized.156 The reaction ofn[NR4][LMo(C0)3] with SOCl, yields [LMoCl,] (L = (HB(pyrazoly1)z)-, {H3(3,5-Mezpyrazolyl)3}-), in which the tris(pyrazoly1)borate anion acts as a tridentate N donor ligand. An analogous compound is obtained from the oxidation of [M~(nane)(CO)~] (nane = 1,4,7-trimethyl-l,4,7-triazacyclononane)by Br2, which yields the cationic complex [MoBr3(nane)]+.This has been isolated as the [PF6]- salt and a crystal structure determination accomplished; the geometry around the molybdenum is octahedral, with a facial configuration of the ligands and an average Ma-& distance of 2.427(4) A.'* Anionic ligands can replace chloride in MoC14, giving complexes of the form [MoCI~LQ], where L is a uninegative bidentate ligand. Thus, heating MoCL or MoCh with a /3-diketone, HL, yields the corresponding monomeric [MoC12L]complex,lS7Analogous complexes with the anions of 8-hydroxyquinoline and N-substituted salicylidenimines can be prepared by the direct substitution of chloride in [M' oC ~~(NC M ~)~], or of (acac)- in [ M ~ C l ~ ( a c a c ) ~ ] . ~ ~ * Although two bands are observed in the M d l stretching region of the IR spectra of these complexes, implying a cis g e ~ m e t r y , l ~ ~the - ~ ~crystal ~ structure of [MoC12(Nmethylsalicylaldimate)~]shows this complex to be the all trans isomer, with M o - C l , M-0 and Mo-N bond lengths of 2.388(2), 1.953(6), and 2.137(8) A, respectively.lm Complexes with tetradentate ligands L (L = (salen)'-, (acacen) -), of the form [MoC12L], or [ M ~ C l ~ ( p ywith ) ~ ] H2L.15' The reaction have been prepared by reaction of [MoCI,(NCM~)~] of HC1 with [MoO(TPP)] (TPP = dianion of 5,10,15,20-tetra-4-tolylporphyrin) forms [MoCl,(TPP)]; the structure of the latter involves a planar porphyrin ring with the trans chlorides at distances from the molybdenum of 2.347(4) and 2.276(4) A, although they appear to be chemically eq~iva1ent.l'~ Seven-coordinate compounds [MoClL3] (L = anion of picolinic acid or a derivative of 8-quinolinol) have been prepared by prolonged reaction between [Mo02Cl2] and HL; these
F 4
MoZybdenurn(ZZZ),( W )and (V)
1343
complexes have been characterized by spectroscopic and magnetic measurements but no structural conclusions were possible 16' A
3642.42 Dithiocarbamate and dithiocarboxylate ligands
Eight-coordinate dithiocarbamate complexes, [Mo(S2CNRR')4], have been prepared by several different methods including: substitution reactions between MoC14 and (S2CNRR')salts;'" oxidative decarbonylation of [Mo(CO),] with K(S2CNRR'),163and of [ M O ( C O ) ~ ]or '~ [ M o ( ~ ~ - C , & ) ( C O with ) ~ ~ ~(RR'NCS2)2. ~ The crystal structure of [Mo(S2CNEt2),] has shown that the molecule possesses a distorted square antiprismatic (or triangular dodecahedral) geometry around the central molybdenum atom. 165 The reaction between MoC15 and potassium pyrrole-N-carbodithioate in aqueous solution yields [Mo{&C(NC4H4)}4], which also has dodecahedral coordination about the metal, with the two different sulfur sites at average bond lengths of 2.541(6) and 2.507(6) Dithiocarbamate ligands are also known to form seven-coordinate mixed-ligand complexes with MoIV. The complex [Mo(SH)(S2CNEt2),].THF has been isolated as a product of the reaction between [ M O ( C O ) ~ ( S ~ C N Eand ~ ~ ) ~[F~,(CP)~S,] ] .167 The coordination around the molybdenum is pentagonal bipyramidal, with the axial sites occupied by the (SH)- ligand and one sulfur of a dithiocarbamate ligand. Complexes [MoX(S~CNR~)~L]Y and [ M O X ~ ( S ~ C N R ~(R2 )~L =]Me2, Et,, (CH2)5;X = CI,,Br; Y = C1, PF6, BF4, BPh4; L = $(dppe), PPh2Me, PMe2Ph, PEtzPh) have been prepared by the reduction of the corresponding [MoOX2(S2CNR&] complex with the phosphine. The crystal structures of [MOC~(S~CNE~~)~(PM~P~&][BF~] and [MoCl(S,CNEt2),(dppe)][PF6] both show pentagonal bipyramidal coordination around the molybdenum but, in the former, both axial positions are occupied by phosphorus atoms and, in the latter, the chlorine atom and one phosphorus of the dppe are in the axial positions.16* The reaction between [MoC14(NCPr),] and dithiocarboxylic acids is a general route to the preparation of eight-coordinate [Mo(S2CR),] cornp1e~es.l~~ The crystal structure of [ M O ( S ~ C P ~reveals ) ~ ] these compounds to be isostructural with the dithiocarbamates, with a dodecahedral coordination around the molybdenum and average Mo-S distances of 2.475( I) and 2.543(1)A to the two different sulfur sites.17* Cyclic voltammetry has shown that in [M~(acda)~] (Hacda = 2-aminocyclopent-1-ene-1-dithiocarboxylic acid) the MoIVcan be reversibly oxidized and reversibly reduced in one-electron processes. The cyclopentadienedithiocarboxylate dianion, (Cs&CS2)2-, reacts with [MoCl,] to give the six-coordinate complex [MO(S~C C SH~)~]~-, which has been isoIated as its [NEt4]+ salt. This has been characterized by spectroscopic measurements but the structure has not been determined.17'
w.'"
'"
36.4.2.4.3 Eight-coordinate complexes with other bidentate ligands
In addition to dithiocarbamate and dithiocarboxylate compounds, Mo'" forms eightcoordinate complexes With several other chelating ligands. For example, [Mo(CO),] reacts with {PhCOCHC(Ph)S)2 to form [MO{OC(P~)CHC(PII)S}~],in which the monothio-@-diketonate ligands are bound to the metal via oxygen and sulfur atoms to form six-membered rings.173 Compounds have also been prepared and studied with ligands containing both a heterocyclic nitrogen and anionic oxygen donor, such as 8-quinolinolate or picolate derivatives.174~'75 These are prepared by stepwise substitution of chlorides in MoC14 and, by isolating intermediate partially substituted species, it is possible to prepare mixed-ligand [MoL,L;-,] complexes, where L and L' are anionic bidentate N,O-donor ligand^."^
36424.4 Alkoxy and thiolate ligands
Alkoxy complexes of stoichiometry [Mo(OR),] can be prepared from the reaction of [ M O ( N M ~ ~ with ) ~ ] alcohols. With bulky groups (R= But, CH2But) the complexes are
1344
Molybdenum
monomeric, but for R =Pri, Et or Me polymeric species are formed.176The reaction of F O ( N M ~ , ) ~with ] 1-adamant01 (Hado) yields [M~(ado)~(N€€Me~)], The structure of this compound is trigonal bipyramidal, with the amine ligand and one alkoxy group occupying the axial sites.177 Phenoxy complexes can also be prepared by reacting [Mo(NM~,)~] with phenols: the stoichiometry of the products obtained with a series of alkylated phenols was found to be dependent on the size of the substituent. [Mo(NMe2k] reacts with 4-methylphenol (Hmp) to yield [M0(mp)~(NHMe2)2]but 2,6-diisopropylphenol (Hdpp) only substitutes three amido ligands to give [MO(~~~)~(NM~~)(NHM~~)~].'~~ Trialkylsilanols also react with [ M O ( N M ~ ~to) ~ ] Me, give the six-coordinate species [ M o ( O S ~ R ~ ) ~ ( N H M(R ~ ~=) ~ ] , Et). The crystal structure of [Mo(OSiMe&(NHMez 2 shows a Pans octahedral geometry, with an average Mc-0 bond length of 1.951(4) A.17 21 [Mo(SBU')~]was the first monomeric four-coordinate thiolate complex of Mol" to be reported. The compound was prepared by treating MoC14 with Li(SBut) and characterized by X-ray crystallography; the arrangement of the four sulfur atoms around the molybdenum has approximately D2d symmetry, with the Mo-S distance 2.235(3) This complex is very labile and readily undergoes ligand exchange reactions with BuWC, CO and PMe2Ph to yield cis-[Mo(SBut)2(CNBu')4], [ M O ~ ( ~ - S B U ~ ) ~ ( Cand ~)~], [MO~(~-S)~(SB~')~(PM~~P~ respectively.180 The reaction between [Mo(SBu'),] and the peptide ligands Ac-Cys-OH, Z-Ala-Cys-OMe and Z-Cys-Ala-Ala-Cys-OMe (Z = carbobenzoxy) (in which the free amino groups at the N terminus is blocked to enforce coordination via the cysteine sulfur atoms) give [Mo(Ac-Cys-OH)4], [Mo(SB~~)~(Z-Ala-Cys-0Me)~l and iMo(Z-Cy~-Ala-Ala-Cys-OMe)~], respectively, Reaction of [Mo(SBut)4] with [Fe4S4(SPt)41- has been followed by CD spectroscopy, and the formation of a Mo-Fe cluster observed.lSZ The bulky thiolate ligand 2,4,6-triisopropylbenzenethiolate(tipt) forms the stable complex [M~(tipt)~], which gives monoadducts with alkynes, nitriles, CO and other small ligands and, unlike (Mo(SBu')4], the molybdenum does not appear to be reduced in these reactions."' The reaction of MoC15 with (HSCH2CH2)2S2(H2mes) yields [M ~(m es )~], in which the (mes)2- dianions act as tridentate S donor ligands. The molecule possesses trigonal prismatic geometry with average Mo-S bond distances of 2.427(8) and 2.361(9) 8,for the thioether and mercapto sulfurs, respectively. The complex undergoes a reversible one-electron oxidation and a reversible one-electron reduction;183an attempt to oxidize the complex with yielded the adduct [{MO(~~~)~}~A~][PF~].$DMF. In this latter compound the molybdenum atoms retain a distorted trigonal prismatic coordination, and one of the mercapto sulfur atoms of each (mes)'- ligand is also coordinated to Ag, forming a severely distorted tetrahedron.lM The reaction between [MoCL(PPh&] and (HSCH2CH2)2PPh gives [Mo{ (SCH2CH2)2P}2].The structure of this compound is analogous to that of [Mo(me~)~], with the ligands acting as tridentate P,S,S donors in a trams octahedral configuration; the average M o S distance is 2.348(9) A.155 Addition of CS2 to [PPh4]2[(S4)2MoS]leads to insertion of the CS2 into the Mo-S bonds and formation of perthiocarbonate ligands, (CS4)2-. The crystal structure of [PPh4I2[(CS4)zMoS] shows that the anion possesses a square pyramidal geometry, with an apical sulfido ligand at a M 0 - S distance 2.126(3) 8, and two nearly planar CSZ- ligands in a trans arrangement with average M o S distances of 2.327(3) and 2.383(3) 8, for the persulfido and sulfide S atoms, respectively.lS6 Molybdenum(1V) complexes with dithiolenes, of general formula [ M o ( S ~ ( ~ R ~ ) adopt ~]~-, trigonal prismatic coordination. These compounds are discussed in Section 36.6.6. 364,245 Dialkylamide and other nitrogen donor ligands
The tetrakis(dialky1amido) complexes [Mo(NR~)~] (R = Me, Et) have been synthesized by the reaction of MoC15 with Li(NR2) and spectroscopic evidence suggests a monomeric distorted tetrahedral geometry for the MoN4 core. These compounds are highly reactive and the methyl derivative is readily converted into [ M o ( S ~ C N M ~ by ~ ) ~insertion ] of CS2 into the Mo-N bonds,lS7and reactions with alcohols, phenols and silanols yield alkoxy, phenoxy or silanoxy c o r n p l e ~ e s ~(see ~ " ~Section ~ ~ 36.4.2.4.4). The reaction of [MOCL(NCM~)~] with R(SCN) in MeCN solution yields [Mo(NCS)~]'- and this anion has been isolated as its [NBm]' and [AsPh4]+salts.188Vibrational spectroscopy has indicated N-bonded rather than S-bonded thiocyanate ligands. The adduct [Mo(NCS),(bipy)] has also been obtained.189
MoZybdenurn(ZIl), (W)and (V)
1345
3642.4.6 Cyanide and isocyanide complexes
The octacyanomolybdate(1V) anion [Mo(CN),I4- has long been known and characterized ~ ~ ~com ' lex is readily obtained and many aspects of its chemistry have been r e v i e ~ e d . ' ~This from the addition of cyanide ions to aqueous solutions of Mo'" or Mog compounds. A simple preparation is by the reaction between molybdate VI , K[BH4] and CN- in the presence of acetic acid, followed by precipitation with ethanol.lL d e crystal structure of several complexes containing the [MO(CN)~]~-anion have been reported. In K4[Mo(CN),]-2H20,193 [C~-€,NO,],[MO(CN)~]~~~ and Rb4[Mo(CN)8]*3HZ0'95dodecahedral coordination has been found, but all the M M bond lengths are equal, at 2.163(7) However, the molybdenum atoms in [NH&[MO((JN)~]*O.~H,O exist in two different coordination geometries, dodecahedral and square antiprismatic.195 The structure of H4[Mo(CN),].4HCl.12H,O has not been completely determined, but the compound is isomorphous with the tungsten analogue, which is known to have a square antiprismatic geometry about the 1neta1.l'~ [NHEt3]2[H30]2[Mo(CN)8] contains anions with a coordination polyhedron which is half-way between a dodecahedron and a bicapped trigonal prism.'97 Energy differences between the various topologies of [Mo(cN),l4- are small (see ref. 8, pp. 11-22), and rapid interconversion between them may be expected in solution, with crystal packing forces determining the more stable arrangement in the solid state. Vibrational spectroscopy has been used to investigate the structure of aqueous solutions of [Mo(CN)8I4-. Raman spectra of K4[Mo(CN)8]-2H20in the region 500-200 cm-l are identical for solid and solution species and the number of totall s mmetric modes is consistent with dodecahedral rather than square antiprismatic geometry.K Y3C NMR spectroscopy detects only one sharp carbon resonance even at temperatures of -16O"C, which may be due to the formation of square prismatic species, dodecahedral species with negligible differences in chemical shift between the two carbon sites, or rapid interconversion between the two.'99 Polarized UV absorption spectra of [Mo(CN),I4- in the temperature range 4.2-300 K have been recorded and show dodecahedral geometry, with appreciably less distortion from idealized geometry at low temperatures than at room temperature.m Salts of stoichiometry h@e[Mo(CN),] (M = Cs, Rb, K, N&) can be prepared by treating Li4[Mo(CN)8] with FeCb and MC1, and the corresponding M2Fe[Mo(CN)6] complexes are obtained using FeC12. There is weak interaction between the [Mo(CN),I4- and the Fe"' or Fe" atoms, and JR spectroscopy has indicated the presence of M A N - F e bridges.201,202 Evidence for the formation of Co2[Mo(CN),], on mixing CoClz and &[MO(CN)~]solutions, has been obtained from radiometric titrationsm and, in a series of complexes M2[Mo(CN)8].xH20 (M = Mn-Zn) , magnetic studies have demonstrated weak electronic interactions between the Mot" and M" centres.204 Photolysis of aqueous solutions of [Mo(CN)~]~- produces [MoO~(CN),]~-, or [MOO(OH~)(CN)~]~species, depending upon the pH.'19 In liquid [MoO(OH)(CN)~]~NH3, photolysis of [N&14[Mo(CN),] yields the six-coordinate [MO(CN),(NH~)~] complex.205 The eight-coordinate complexes K2[Mo(CN)6L] (L = bipy, phen) can be prepared via substitution reactions of [Mo(CN),I4-. O6 A number of isocyanide complexes, [Mo(CN),(CNR)~] (R= Me, Pr, But, allyl, NCPhz), have been made by the action of RI on Ag4[Mo(CN),]. The crystal structure of [Mo(CN)~(CNM~),]shows a dodecahedral geometry, with the CN- ligands at 2.177(8) A and the MeNC ligands at 2.148(8)A from the Both 13C NMR and vibrational spectroscopy indicate that only one isomer is present in solution. Treatment of an aqueous solution of K5[Mo(CN)7]with acetic, hydrochloric br ascorbic acid, .*OS This has been or with hydrogen sulfide gives the hydride complex K,[MOH(CN)~].~H~O characterized by 'H and I3C NMR and by vibrational spectroscopy.
''
36424.7 Magnetic and electronic properties
The magnetic moments of some non-oxo molybdenum(1V) compounds are listed in Table 6.5 The six-coordinate complexes are all paramagnetic, with moments ranging between 1.9 and 2.8 BM. The temperature dependence of the magnetic susceptibility of these complexes is consistent with a configuration derived from a 'Tu ground state split by spin-orbit coupling and/or a ligand field of low symmetry, Electronic absorption spectra have been recorded for many six-coordinate species but assignments have generally been tentative.'" Seven-coordinate
Moly bdenurn
1346
Table 6 Magnetic Moments of some Non-oxo Molybdenum(1V) Complexes5
2.28 2.48 2.61 2.63 2.61 2.70 0.68 Diamagnetic Diamagnetic Diamagnetic
300
3.02
a
a
300 297 292 294 a
Diamagnetic
147 149 106 159 159 161 164 173 179 187 188 207
“Temperature not reported.
species, such as [MoCl(pic)3],are also paramagnetic with magnetic moments slightly below the spin-only value for a d2 system because of spin-orbit coupling. Most of the eight-coordinate complexes are diamagnetic; although some compounds have been reported to possess a magnetic moment, this may be due to impurities.’ The dodecahedral geometry results in a d orbital splitting pattern that leaves the d, level lowest and this is thought to contain the two electrons in [Mo(CN),I4- and [Mo(L-L),] c o m p l e x e ~ . ’This ~~ interpretation is in agreement with X-ray PES studies and CNDOIINDO MO calculations.209 The four-coordinate complexes [Mo(SBu‘),] and [M o (NR ~)~] are also diamagnetic. The UV-PE spectrum of [Mo(SBut),] exhibits a low ionization potential at 6.8 eV which has been assigned to the ionization of electrons with predominant molybdenum 4d,z character, on the basis of discrete variational-X, MO calculations on the model compounds [Mo(SH)~]and [Mo(SM~)~].’~’ For [Mo(NR~)~] (R=Me, Et), the UV-PE spectrum contains a low energy ionization at 5.3 eV which has been attributed to ionization from the molybdenum 4d,~-~z orbital, This assignment was based on Fenske-Hall calculations on [Mo(NMe2),].’ll
36.4.2.5
Cyclopentadienyl Complexes
Complexes of the general formulae [Cp2MoLL’]2+,[Cp2MoLX]+and [Cp2MoXY] (L, L’ = neutral ligand; X, Y = uninegative ligand) are known for a wide variety of ligands (ref. 212 and refs. therein). The structure of [Cp2MoC12]is a typical example of these complexes. The cyclopentadiene rings are staggered with respect to each other and lie on either side of the MoCl, plane; the angle between normals of the two Cp lanes is cu. 135” and the overall geometry about the metal is that of a distorted tetrahedr~n!’~ Other compounds are known in which a second metal atom is coordinated to the Cp2Mo centre via bridges formed by the other ligands, such as the hydrides in [Cp2M~H,]214 or the thiols in [Cp2M~(SR)2]215 complexes, but there appears to be little direct metal-metal interaction in these systems. Generally, in the course of chemical reactions the two Cp ligands remain coordinated and unchanged while the other two sites are reactive.
36.4.2.6
Dimeric and Polymeric Complexes
Molybdenum(1V) forms a variety of dimeric and polymeric complexes with oxido, hydroxido and carboxylato bridges in aqueous solution, which are discussed in Section 36.1.4. Oxide bridges have also been found in non-aqueous systems. The reaction between [MoCl,(TI’P)] (TPP = dianion of tetra-p -tolylporphyrin) and N-phenylhydroxylamine yields [{MOCI(TPP)}~O],in which each Mo is octahedrally coodinated and is displaced 0.08 8, from the N4plane towards the bridging oxygen. The complex is aramagnetic (p = 2.82 BM per Mo) with no magnetic interaction between the metal centres.” A similar structure has been found in ethoxooxidobis(l,5,9,13-tetrathiacyclohexadecane)-~-oxidodimolybdenum(IV)trifluoromethylsulfonate hydrate: the cation contains a linear O M o O M d unit , with each macrocycle
P
Molybdenum(III), ( W )and (V)
1347
formin a lanar coordination around a molybdenum atom and staggered 43” relative to each other.’ The oxo-bridged mixed oxidation state MoIV,Mov compound [{MO(S~CNE~,)~>~O][BF~] has been prepared by the reaction of [MoO(SzCNEt2),][BF4] and PPh3. The cation contains two pentagonal bipyramidal units linked by an almost linear Mo-0-Mo bridge, with the oxygen occupying an axial site in each coordination sphere. The complex is paramagnetic ( p = 2.17BM per Mo) and is the first example of a dimolybdenum oxo-bridged species with no terminal oxo or sulfido ligands.218 The dimeric complexes [Mo203(S2CNR2),] (R = alkyl) have been identified spectroscopically and isolated as intermediates in the oxidation of [M02(S2CNR&] to MoV and Mow compounds. They have low magnetic moments (1.29BM per Mo), suggesting the presence of a significant metal-metal i n t e r a ~ t i o n . ~ ~ Molybdenum sulfides react with CN- in aqueous solution to form a wide variety of oligomeric cyanothiomolybdate anions, which contain MozS, Mo2S2, Mo& and Mo4S4cores59 (see Section 36.1.4).These types of complexes are formed under conditions thought to occur in prebiotic times on this planet and, thus, they could be important in the evolution of molybdenum enzymes. Sulfur bridges are also important in other non-oxido dimolybdenum(1V) species. In [ { ( H S ) M O ( ~ ~ ) } ~ ] [ C F ~ (hd SO= ~ ]1,5,9,13-hetrathiacyclo~ hexadecane) there is an MoSMoS ring, in which each sulfur atom belongs to an intact macrocycle coordinated in the endo form.’19 An Mo2S2 ring is also found in the sulfidobridged complex [(PrzNCS2)(Pr2NCS)Mo(y-S),Mo( SCNPr2)(S,CNPrz)], which also contains bidentate dithiocarbamates and thiocarboxamido ligands bound to the molybdenums in involve Mo-Mo a side-on manner, and distances of 2.70712) with the formation of a metal-metal bond.=’ A triply-bridging sulfur atom is found in [Mo~S~(CP)~]+, in which there is a triangular cluster of Mo atoms triply-bridged by one sulfur atom above the centre of the triangle, with three doubly-bridging sulfur atoms below.221 [NEt4]2[C13Mo(p-S2)(p-Cl)2MoC13] contains a dimolybdenum(1V) centre bridged by one disulfide and two chloride ligands; the Mo-Mo distance is 2.763(2)w and the low magnetic moment (1.40BM per Mo) indicates formation of a metal-metal bond, leaving one unpaired electron per Mo.” Disulfide bridges are also found in [AsPh4]2[MozBr6(Sz)z(SM~)2],which has been obtained from [ M ~ B r ~ ( s M e , and ) ~ l [AsPh&(S7) .% Chlorides act as bridging ligands in [{ M O C ~ ~ ( F ~ C ~ and P ~ ) [PPh4]2[Mc&llo }~] The [MozClz0]’- anion contains two bridging CI groups, with a Mo-Mo distance of 3 . 8 O k and, therefore, there is no direct metal-metal interaction.= The bromide-bridged anion [(NO)2Br,Mo(y-Br)pMoBr,(N0)2]2-also contains a long Mo-Mo distance (3.978 A)-56
F P
36.4.3 MOLYBDENUM(V) 36.4.3.1 Introduction Molybdenum(V) chemistry is dominated by oxo complexes: many of these exist as dimers, but monomeric species can be isolated from strongly acidic solutions or under nonaqueous conditions. Non-oxo compounds are also known, both as monomers and as dimers or polymers with halide or sulfur bridges. ESR spectroscopy has been used extensively to investigate the properties of monomeric Mov systems, and has shown the participation of this oxidation state in the reactions of the oxomolybdoenzymes (see Section 36.6.7). Of the binary halide compounds of MoV, only fluoride and chloride species have been isolated and characterized. [{MoF~}~] can be prepared by the reduction of MoF6 with Si powder in the presence of dry HF; it is also a product of the reactions between (MoF6] and [Mo(CO)~]or Mo, and of the direct combination of Mo with Fa. The crystal structure shows formation of a tetramer, in which the four molybdenum atoms form a square plane bridged by four fluorines. Each molybdenum is octahedrally coordinated to four terminal fluorines with an average Mo-F bond length of 1.78(8)& and to two bridging fluorines at a distance of 2.06(4)A.224 Monomeric [MoF,] can be prepared by condensation of [MoF5] vapour in an argon matrix at liquid hydrogen temperature, or by UV photolysis of [MoF,] in an argon matrix. Vibrational spectra of [MoF,] formed by the condensation route are consistent with a D3h trigonal bipyramidal structure, but the IR spectrum of the photolysis product has been interpreted assuming a CW square pyramidal geometry.= In the liquid and vapour states [MoF5] exists as a monomer: the electronic spectrum of liquid [MoF5] suggests a trigonal bipyramidal shape but 19F NMR data are consistent with a square pyramidal structure.n6
1348
Molybdenum
In the solid state MoC15 forms dimers of edge-sharing MoC16 octahedra, with the two bridging chlorides involving significantly longer Mo-Cl bonds (2.5311) A) than the terminal chlorides (2.24(1) A).227The long Mo-Mo distance of 3.84(2) A and magnetic moment of 1.64 BM per Mo are consistent with the absence of significant metal-metal interaction. In the gas phase monomeric [MoCl,] is formed, which has been shown by electron diffraction and Raman studies to have a trigonal bipyramidal structure.228 The reaction of MoF3 with CIz at 120°C yields MoF3C12. Magnetic measurements indicate antiferromagnetic exchange between Mo" centres in a polymeric structure and mass spectroscopic data show the presence of [MoFCl,], [MoC15], (n = 0-6) and [ M o ~ F ~ ~ (rn - ~=C0-7) ~ ~in] the vapour above solid M O F ~ C ~ ~ . ~ ~ MoOCI,(s) exists in two structural forms; the monoclinic form contains a terminal oxo group and the tetragonal form involves bridging M-0-Mo units. The former is obtained by the reduction of MoOCl, with Al at 130 "C,the thermal decomposition of MoOCL,, or by reacting SOz(liq) or Sb03 with MoCls.23eu2 The structure is based on an infinite zigzag chain of molybdenum atoms, each possessing two pairs of p2-C1 atoms and cis terminal oxygen and chlorine atoms in a distorted octahedral environment; the length of the Mo-C1 bond (2.81 A) tram to the oxo group is significantly longer than those cis to this The room temperature magnetic moment of this material is cu. 1.65 BM. The latter form of MoOC13 is obtained by reduction of MoOCl, with HI in SOz(liql at -23 "C;the compound is isomorphous with Nb0C13;230MoOBr, also adopts this structure. 33 MoCl, reacts with Sb2Y3(Y = S, Se) at an elevated temperature, or with Sb2S3 in CSz solution at room temperature, to produce MoYC13.66
36.4.3.2 Monomeric Complexes with an Oxo Group 36.43.2.1 General comments
Molybdenum(V) forms many mononuclear complexes containing one oxo group but no analogues have been reported of the cis-dioxomolybdenum(V1) or transdioxomolybdenum(1V) species. The 0, group possesses a multiple bond see Section 36.4.2.2.1) and dominates the ligand field at the molybdenum, so that the 4d electron is located in an orbital which is in the plane ( x y ) perpendicular to the oxo group Most monomeric Mo" oxo complexes adopt a square pyramidal or octahedral geometry, and their electronic configuration is (dXy)'.
I
(z).80,92393
36.4.3.2.2 Halide complexes
A large number of compounds are known containing the anions [MoQX5I2- (X= F, Cl, Br). These complexes are stable in acidic solution, but magnetic measurements show that as the pH is increased towards neutrality the magnetic moment decreases, indicative of dimer formation, which allows an antiferromagnetic metal-metal interaction.= Various other equilibria exist in solution (see Section 36.1.5) and these produce, inter aliu, the monomeric species [Moo&]and [Mo0&(H20)]- (X= C1, Br), which have been characterized by ESR spectroscopy.235 Crystal structures have been obtained for several salts containing the six-coordinate [MOOX~]~and the five-coordinate [Moo&]- anions. In K2[MoOF5]-H20the molybdenum is octahedrally coordinated; the Mo-0, distance is 1.66(2) A. The length of the four cis Mo-F bonds (1.88(3)A) is significantly shorter than that of the trans Mo-F (2.02(1)A)236 and the molybdenum is displaced out of the equatorial plane in the direction of the oxo ligand so that it is located close to the centre of the donor atom array; these are common features of all oxomolybdenum complexes. In KZ[MoOCl5], the Mo-0, distance is 1.67A and the bond ~ lengths to the cis and trans chlorides are 2.40 and 2.63& r e s p e c t i ~ e l y , ~and [AsPb][MoOCL,(H20)]involves the following bond lengths: M A = 1.672(15), Mo-OH2 = 2.393(15) and Mo--Cl= 2.359(3) A shorter Mo--O, bond is found in [AsPh,][MoOCL], distance is which has a square pyramidal geometry with the oxo group axial; the M-0, 1.610t1018, and the M e 1 distances are 2.333(3) Crvstal structure determinations for relateb 'com lexes include: [MoO&(H20)]- ' {X = C1, -Br, I),24o [MoOB~,]-~~'and FMOOBrs,Z-.~z Complxes containing [Moo&]- anions (X = C1, Br) are coordinatively unsaturated and
MoZybdenurn(ZZZ), (IV)apld (V)
1349
react readily with neural or anionic ligands to give octahedral species. The mixed halide complex [NEt&[MoOCl,Br] can be obtained by treatin4 [NEt4][MoOCl,] with [NEt,]Br; spectroscopic data indicate that the trans isomer is formed. Complexes of the form [LHz][MoOX5] (L = bidentate N donor ligand; X = C1, Br) can be isolated from the reaction between Moo3 in hot HX and HI, followed by addition of L. These compounds lose HC1 when dissolved in dry solvents, yielding [MoOX3L], which form as the mer or fac isomers depending on the conditions. With the ligand 2,2’-biquinolyl (biquin) an intermediate complex, [biquinH][MoOCL], has also been isolated. Monodentate ligands L’ form the compounds [L’W]z[MoOC15],which can lose HCI to yield [ M O O C ~ ~ GFor ] . ~the ligand 8-hydroxyquinoline and its derivatives, IR spectra indicate that there is hydrogen bonding between [La]’and [ M O O C ~ ~in ] ~[LH]Z[MoOC15].245 Molybdenum halides and oxyhalides, including [MozCllo],[Mo2O2Cl6]and [N&]2[MoOC15J, are useful starting materials for the preparation of monomeric MOV oxo compounds. A large number of these corn ounds are of the form [MoOCl&,] (n = 1,2) and commonly involve 0 or S donor ligands.‘3247 [MqOZClb] reacts with THF to give [ M O O C ~ ~ ( T H F )from ~],~~~~ which the loosely bound THF molecules are easily displaced by a wide variety of ligands, L, yielding [MOOC&]. Many neutral ligands react directly with [MozOZCl6]to give [MoOCl3L,1 or [MoOCLL] species; [MoZC110] can also form oxo complexes by abstracting oxygen from the ligand or solvent. Equilibria exist in solution between the five- and six-coordinate products; for the oxygen-donor ligands Ph3P0, (MezN)3P0 or THF, the only product formed is the corresponding [MoOCI~L]compound, but for sulfur-donor ligands Ph3PS, (MeN)2CS or Mez& the five-coordinate complex [MoOC13L] predominate^.'^^ [MoOC13L] (L = Ph,PO; P(NMe2),0) possess an octahedral coordination around the molybdenum, with the chloride ligands adopting a mer configuration and the phosphine oxides mutually cis. In the Ph3P0 adduct the Mo-0, bond length is 1.662(13)& and the Mo-0 distances to the phosphine oxide ligands trans to the oxo group and trans to chloride are 2.136(11) and 2.065(10)& respectively.250This compound reacts with halide ions and the rate-determining step is the loss of the Ph3P0 ligand trans to the oxo group; subsequent to halide substitution, exchange of the substituted halide with the other Ph3P0 molecule occurs.251 [MoOC13(SPPh3)] adopts a geometry intermediate between square pyramidal, with an axial oxo group, and tri onal bipyramidal, with sulfur and chloride ligands axial. The Mo-0, bond length is 1.647(3) !& .252 Molybdenum(V) forms a wide range of compLxes with bidentate and polydentate ligands. The reaction of [py€%jz[MoOXS] (X = C1, Br) with acetylacetone yields [pyH][MoOX3(acac)] which, on heating, is converted to [MoOXz(acac)(py)]; [Mo2Cllo reacts with Hacac and other B-diketones, HL, to give the disubstituted complex [ M o O C I L ] . ~In~ ~all of these compounds, spectroscopic evidence suggests that the diketonate acts as a bidentate ligand to form an octahedral mononuclear complex. Dithiocarbamates, R2dtc, form mononuclear [M00X(R~dtc)~] (X = F, Cl,Br, I) products in reactions involving the reduction of Moo3 by NzH4.HX.254An analogous complex, [MoOCl(dttd)] (H,dtdd = S,S’-bis(2‘-mercaptophenyl)1,2-mercaptoethane, Figure 2; Section 36.4.2.2.1), is formed in the reaction between [NH4],[MoOC15] and H,dttd. This complex undergoes a reversible one-electron reduction to [MoOCl(dttd)]- but is not easily oxidized to an Movl species.g0 Interest in the nature of the molybdenum coordination sphere in the oxo-molybdenum enzymes has led to investigation of complexes formed by MoV with Schiff bases and related ligands. [MoOC13(THF)z] has been shown to react with a variety of Schiff bases (HzL), including salenHz and its substituted derivatives, in the presence of base, with LizL or L(SiMe3)2 to give the corresponding MoOLCl] complex, the configuration of which was assigned from ESR and IR data.? The anionic complex [MoOCl2L]- (HzL= N-2hydroxysalicylidenimine) has been prepared from [pyH]2[MoOC15]and HzL, and the crystal structure of its [AsPb]+ salt has been determined. The molybdenum atom in the anion is in a distorted octahedral environment in which the planar tridentate Schiff base occupies meridional positions, with the nitrogen atom truns to the terminal oxo group; the Mo-0, bond length is 1.673(3) A.256ESR spectroscopy has been used to investigate equilibria in a solution of [MoOCl(salpn)] (salpnHz= N,N’-bis(salicylaldehydo)-l,3-diaminopropane); the results showed that considerable dimer formation occurred, with the dominant monomeric species being [MoOCl(salpn)].=’ The reactions of [MoOC13(THF)2] with substituted quinolines yield a number of difierent types of complexes containing one or more quinoline ligands. The crystal structure of [H2ox][Mo0Cl3(ox)](Hox = 8-hydroxyquinoline) has been determined; the hydroxyquinolate acts as a bidentate 0 , N ligand, with the oxygen bound trans to the oxo group in an octahedral
I
1350
Molybdenum [Moz02C16]reacts with 8-mercaptoquinoline, H(tox), to give [MoOClz(tox)Q]
(0= solvent) in which tox is bidentate, [M00Cl(tox)~Q]in which one tox is bidentate and the other monodentate, and [ M ~ O ( t o x ) in ~ ] which two tox ligands are bidentate and the third is monodentate. [MoOCl(to~)~] has been prepared by the reaction of [MoC~~(DMF)~] and 8,8'-diquinolyl disulfide and its crystal structure determined. The molybdenum atom is in a distorted octahedral environment, with two bidentate tox ligands; the oxo and chloride ligands occupy cis positions, and the sulfur atoms of the tox ligands are trans to each other; the Mo-Ot bond length is 1.716(4) A."' The reduction of nitrate and nitrite by a number of monomeric oxomolybdenum(V) complexes, including [ M o O C I ~ ( O P P ~ ~[MoOC13L] )~], (L = bipy, phen), and [MoOClG] These reactions have relevance to the reduction of (L' = ox,tox), has been nitrate and the competitive inhibition by nitrite which OCCUTS at the molybdenum centre of the nitrate reductases. The molybdenum centres of the Moo3+ complexes reduce nitrate in a one-electron step with concomitant transfer of an oxygen atom, producing NOz and the corresponding cis-MoO$+ complex. The initial substitution of nitrate occurs at the labile position trans to the oxo group, but the redox reaction proceeds only after ligand rearrangement so that the nitrate is coordinated by an oxygen atom cis to the oxo ligand.'@ Nitrate is also reduced to NO2 by tripeptide (Gly-Gly-Met or Gly-Gly-His) molybdenum(V) complexes formed in the reaction of [NH4]2[MoOC15]with the tripeptides immobilized via the terminal glycine to a polystyrene matrix. Analysis shows a molybdenum to tripeptide ratio of 1:1, but it is unclear how these potentially tridentate ligands bind to the molybdenum and which other ligands are present in the coordination sphere.z63 Molybdenum(V) chloro complexes of a cysteinyl-containing peptide have been prepared by reaction of woOCl3(THF),] with the peptide aIone and in the presence of Et,N. Although the compound produced under the former conditions is capable of reducing nitrate, this is not the case for the latter compound, perhaps because the chelation of the MoV prevents nitrate from ligating at a site cis to the oxo The properties of [MoOCl(TPP)] (K2TPP= meso-tetraphenylporphyrin) and related complexes are covered in Section 36.4.3.2.4.
3643.23 Complexes with sulfur and selenium donor ligands
Complexes of the type [MoO(SR)~]- are important, both in their own right as interesting chemical species and as systems which rovide information of relevance to the molybdenum (Ar= Ph, 4-MeC&, C6C15) and centres of the oxomolybdoenzymes.2 4 9 , ~ 8[MoO(SA~)~][MoO(SePh),]- have been prepared by the reaction of the corresponding thiol, or selenol, with [MoOC13(THF)2], [pyH]2[MoOC&]or [pyH][MoOBr,] in MeCN containing Et3N and isolated as their quaternary ammonium salt. Their alkanethiolato analogues, [MoO(SR)~]- (R = Et, CHzPh) have been prepared in solution at -60°C. [MoO(SPh),]- has a square pyramidal structure with Mo-0, = 1.669(9) A and Mo-S = 2.403(5) A. The spectrosco ic and magnetic properties of these species have been investigated in considerable detailz6' and chemically reversible one-electron reduction has been observed; oxidation leads to the formation of the dimer [ M o ~ O ~ ( X R )(X ~ Q=~S, Se; Q = solvent) and RXXR.= An alternative synthesis to [MoO(SPh)J, which has also been used to prepare [MoO{S(CH&S}Z]- (n = 2, 3), involves the reaction of the Mom butanediolato complex, [MOO~{M~CH(O)CH(OH)M~}~]. Z[MeCH(OH)CH(OH)Me] with the appropriate thiolato anion in MeOH. The crystal structure of [PPh4][MoO{S(CH2)3S}2] has been determined and the anion (Figure 3) has a square pyramidal geometry with Mo-0 = 1.667(8) 8, and M o - S = 2.389(4) A. Although [MoO(SPh),]- reacts with R2NNHz (Rz=Me2 or (CH2)4 in refluxin5 MeOH to produce reacts with HN3 in [MoO(NNR2)(SPh)3]-, it is inert to HN3; however, [MOO(S(CH~)~S}~] MeOH to produce [Mo2O2(pN3){S(CH2)3S}3],-.267[MoOC14]- in MeCN containing Et3N reacts with the sterically hindered thiol, 2,4,6-PrjC&S- (Htipt) to produce [ M ~ O ( t i p t ) ~ ] - . ~ ~ Oxomolybdenum(V) centres can form five- and seven-coordinate complexes with bidentate carbodithioate ligands. The reaction between Na2(C5&CS2)-THF and [Mo20ZCls]in the presence of [NEt4]Br gives [NEt4][MoO(S2CC5H&]. ESR spectra of this complex in solution Reductions and frozen solution indicate a d' monomer with square pyramidal C4, of [MoOzL] with HL' (HL' = 2-aminocyclopent-l-ene-l-carbodithoicacid and its N-alkylated derivatives) yield the mixed ligand compounds [MoObL']. The magnetic moment values (ca. 1-60BM) and ESR spectra are consistent with the (dxy)lground state.z69[ M O O ( N C S ) ( R ~ ~ ~ C ) ~ ]
MoZybdenurn(ZZI), ( W )and ( V )
1351
no
Figure 3 Stmcture of [MoO(SCH,CH,CH,S),]-
in its [PPh$ saltzM
(Rzdtc = substituted dithiocarbamate) have been prepared and IR spectra show the thiocyanates to be N-bonded. ESR, electronic spectra and magnetic moment values (1.69 BM) confirm the monomeric nature of the complex. Cyclic voltammetry has been used to demonstrate reversible one-electron reduction to the corresponding Mow species.”’ Interesting redox behaviour is exhibited by the complexes [MoOLI- where L is a tetradentate S, S‘,N,N‘ aromatic aminothiol group (N7N’-bis-2’-rnercaptophenyl-l,2diaminoethane (Figure 2) and its 1,2-diamino-1,Zdimethylethane analogue). In these complexes the amino groups are deprotonated, giving the ligand dithiolene characteristics. The compounds undergo both one-electron oxidation and reduction at a platinum electrode in DMF and show no tendency to form dimeric species due to the geometrical restraint of the ligand L.271The electrochemistry of a series of Moo3’ complexes, including MoOCILz (L = 8mercaptoquinoline) and MoOClL (L = N,N’-dimethyl-N,N’-bis-2’-mercaptoethyl-l,2diaminoethane (Figure 2) and N,N‘-bis-2-mercapto-2-methylpro1-1,2-diaminoethme), has been reported; the complexes undergo a facile reduction to an MJ’ species but oxidation was not observed.87 36.4.3.24 Complixes with nitrogen donor ligands
Solutions containing [MOOCI~]~react with thio anic or selenocyanic acid to produce [MoO(NCS)5I2- and [MOO(NC S ~)~]~-, respectively.’ Magnetic, vibrational and electronic spectra indicate that in both anions the ligands are N-bonded, forming monomeric complexes species. with close similarities to the corresponding halide [MoOX~]~In HCl solution, in the presence of SnC12, Mo’ forms complexes J M O O ( H ~ ) ~with ] + the dioximes (Hzd) of cyclohexane-l,2-dione and cyclopentane-1,2-dione.” Several compounds of tetradentate Schiff bases255,274 with oxomolybdenum(V) have been prepared and the crystal structure of trans-[MoO(salen)(MeOH)]Br has been determined.274The Mo-0, distance is 1.666(10) A, and there is dose interaction in the crystal between each cation and an attendant bromide anion. Tetraphenylporphyrin, HzTPP, reacts with [Mo(CO),] in refluxing decalin to give [MoO(TPP)(OH)]. Derivatives of the type [MoO(TF’P)X] are known with a wide variety of uninegative ligands X, including C1, OzH, F, Br, BF,, NCO, N3 and NCS.275Vibrational, electronic and ESR spectroscopy confirm the presence of one unpaired electron and a central Mo-0, group with a tram octahedral geometry of CdVsymmetry. The cyclic voltammograms of [MoO(TPP)X] (X = MeO, AcO, Cl) show a reversible one-electron reduction to a MoIV species, whereas that for the related corrolate complex, [MoO(mec)] (mec = trianion of 2,3,17,18-tetramethy1-7,8,12,13-tetraethylcorrole) shows both one-electron reduction and oxidation processes. In addition to one-electron reduction, [MoO(TPP)(OMe)\ exhibits other redox couples, involving dissociation of (0Me)- and formation of Mo”’ and Mo complexes.z76 [MoO(TPP)Br] can be chemically reduced by superoxide to give [MoO(TPP)], via Mov and MoIV superoxide complexes, which are stable at low temperature^."^ In the presence of superoxide, the substitution reaction of [MoO(TPP)Br] with MeOH to give [MoO(TPP)(OMe)] proceeds via an MoIV species rather than via [MoO(TPP)(MeOH)]+, Photochemical reduction of which is the pathway in the absence of s ~p ero x i d e. ~~’ [MoO(TpP)(OMe)] yields [MoO(TPP)], which reacts with 0 2 in MeOH to re-form [MoO(TPP)(OMe)] and produce H202, thus giving a catalytic route for the photoassisted reduction of molecular oxygen to hydrogen per~xide.”~
Molybdenum
1352 3643.25 Spectroscopic studies
A comprehensive tabulation of such data is available in ref. 5 and selected details will be presented here. Monomeric oxomolybdenum(V) complexes are paramagnetic, with magnetic moments in the range 1.65-1.73 BM (Table 7) due to the single unpaired electron. This makes Mov complexes accessible to investigation by ESR spectroscopy, and many studies have been reported, both with well-defined coordination compounds and with biological SyStemS.131,238,250,252,255,265,28~284 A monomeric Mo" complex in solution which possesses sufficient thermal motion to average the molecular anisotropy on the ESR timescale has a characteristic ESR spectrum. This consists of a strong central line, due to resonance of molecules with even molybdenum isotopes (Z=O), flanked by six hyperfine lines due to interaction between the unpaired electron and the 95Mo and "Mo nuclei (Z=a). When molecular tumbling does not occur,for example in solids and frozen solutions, or is slow on the ESR timescale, the spectrum becomes more complex, displaying major features due to the principal g values. Monomeric MoV centres with axial symmetry should manifest two principal resonances, g , , (gzz) and gl (gm,gyy), whereas in systems of lower symmetry three principal resonances should be observed, each associated with six hyperfine linesm A low-symmetry ligand field allows extensive d orbital mixing, which generally results in the principal molecular g values being displaced from the metal-ligand axes. ESR can provide a fingerprint of a MoV centre in a coordination complex or an enzyme, and Table 8 lists the ESR parameters recorded for some typical complexes. ESR spectra of MoV complexes can be grouped into two types according to the degree of g a n i ~ o t r o p y One . ~ ~ group is typical of octahedral and the other of square pyramidal complexes and, using this criterion, the Mov centres of the oxomolybdoenzymes would appear to be square pyramidal; this correlation is based on circumstantial evidence and, so far, no definitive structural prediction has been derived from ESR data alone. Table 7 Magnetic Moments of some Oxomolybdenum(V) Complexes'
1.73 1.67 1.73 1.65 1.70 1.71 1.72
303 294 a 293 79 297 293
243 272 248 254 275 279 265
&dt = 4-morpholinodithiocarboxylate. TPP = dianion
Table 8 ESR Data for some Oxomolybdenum(V) Complexes
Compound ~~
g valuesa
Medium ~
~~
~
DMF
[NH4I,[MoOCl,I [ A S P ~[MOOCI~(H,O)]~ ~] [AsPh4][MoOCI4]' trum-[M~OCl(acac)~]
DMF
cis-[M~OCl(ox),]~
DMF
[MoOCl(S2CNEt2),r [MoO(S~CNE~~)J [MoOClz(Pz)l'
GH8
'For axial systems in q e sequence, g, gry, gzz. A z z . 'Single crystal. Hox = 8-hydroxyqu1nohe.
~
A valuesb
Ref.
34.0,34.0,74.5 45.5,45.5,72.8 47.9,47.9,72.8 33.4,36.2,77.6 46.5,9.1,74.2 24.1,27.5,61.4 56.9,33.6,27.1 38.1,18.1,71.4
280 238 239 280 280 281 281 282
~~
1.938,1.938,1.970 1.935,1.935,1.970 1.950,1.950,1.967 1.950,1.940,1.927 1.939,1.953,1.970 1.945,1.958,1.984 1.970,1.978,1.985 1.934,1.941,1.971
~ 1 0 - ~ ~ for - ' axial , systems, the sequence is A ~ A,,~ , pz = h y d r o t r i s ( 3 , 5 - ~ e ~ ~ l p y r ~ l y ~ ) ~ ~ t e .
The use of ESR spectroscopy to probe MoV centres in biological systems has stimulated attempts to reproduce the ESR spectra of molybdoenzymes using well-defined chemical systems. The similarity between ESR parameters of species formed by reacting Na2[Mo04] with S-donor ligands and those of xanthine oxidase imply that in the enzyme (cysteiny1)S ligation of molybdenum may occur.284However, no well-characterized MoV complex has an ESR spectrum which reproduces all the aspects of one observed for the MoV centre of an oxomolybdenum enzyme. A particular problem is the absence of superhyperfme coupling constants for well-defined Mov systems; a notable exception is the observation of "0
Molybdenum(ZlI), (W) and (V)
1353
superhyperhe coupling in 170-enriched [98M~O(SPh)4]-,a complex which exhibits exceptionally narrow ESR linewidths.265 , symmetry which is According to molecular orbital calculations, €or a ligand field of C dominated by a strong molybdenum-oxygen interaction along the z axis, the d orbitals transform accordin to a1 ( d , ~ )bl , ( d , ~ - ~ ze) ,(d,,, dyz)and b2 (d,,) representations, in order of decreasing energy!= The ground state configuration of a Mov complex is 2B2(dxy)', and three d-d transitions are predicted: '&42E, 2B2+ 2Bl and 2B2+ 'A1. Several interpretations of the electronic spectra of Moo3+ complexes have been proposed.238,2s288The lowest energy band, which appears in the near IR region of the spectrum between 600 and 850nm and (at low temperature) often manifests vibronic coupling with v ( M M t ) of ca. 900 crn-l, is due to the 2B2+ZE promotion. The other two d-d bands are often obscured by intense ligand to metal charge transfer bands,265,288 but the '&+2Bl transition is generally responsible for the second absorption band which is usually observed between 400 and 450nm. In the case of [AsPh4][MoOC14],the 2B2+2A1 promotion has been observed as a shoulder at ca. 380 nm on a intense Cl-, Mo charge transfer band.288 The vibrational spectra of oxomolybdenum(V) complexes display an intense band in the range 940 to 1020cm-' due to Y(MO-O,);~,~ the higher frequencies, corresponding to the shorter molybdenum-oxygen bonds, are found in square pyramidal complexes, in which there is no ligand trans to the oxo group.
36.4.3.3 Complexes Involving Multiple Molybdenum-Nitrogen Bonds Monomeric non-oxo molybdenum(V) complexes have been isolated with nitride (N3-), imide (NR2-) and amide (NRR'-) ligands, which may act as models for intermediates in the degradation of dinitrogen in nitrogenase. However, no molybdenum(V) compounds are yet known which correspond to the diazenide or hydrazide analogues of the molybdenum(1V) complexes described in Section 36.4.2.3. [PPh3MeI2[MoNCl4]can be prepared by reduction of [PPh3Me][MoNC4] or MoNCb with [PPh3Me]I. A crystal structure determination has shown the presence of a square pyramidal coordination around the molybdenum in the anion, with an axial nitride ligand at a bond length of 1.634(6) [ M O N C ~ ~ ( P P may ~ ~ ) ~be ] obtained b treating [MoC14(THF),] or [MoC~~(THF)~] with Me3SiN3, followed by addition of PPh3?'The ESR spectrum of this latter compound at 20°C shows superhyperfine coupling to two phosphorus nuclei which are equivalent on the ESR timescale. The analogous complexes [MoNCl2(bipy)lZw and [MoNBr2(bipy)lZg1 have also been prepared and characterized. The IR spectrum of all of these nitrido complexes shows a strong absorption in the range 950-1050 cm-', which is assigned to the stretching of a molybdenum-nitrogen triple bond. Imido ligands bind to molybdenum in a manner equivalent to the binding of oxo groups, and several compounds have been synthesized which are analogues of known oxo species. 4-Tolylimido (Nto12-) complexes of molybdenum(V) can be prepared from the reduction of molybdenum(V1) compounds; for example, reaction of [Mo(Ntol)CL,(THF)] with tertiary phosphines, L, yields [Mo(Ntol)C13b]. These complexes display magnetic moments close to the spin-only value for one unpaired electron and show strong ESR signals at room temperature. The structure of [Mo(Ntol)Cl3(PEtPh2),] displays a meridiond arrangement of chlorine atoms and trans phosphines in octahedral geometry; the molybdenum-nitrogen bond length is 1.72S(6) A.292The reaction of [Mo (C O)~C ~~] with [N&][S2P(OEt),] and RN3 )~)~~ ESR . (R = Ph, 4-tolyl) gives [Mo(NR){S2P(OEt),},1 and [ M O ( N R ) C ~ { S ~ P ( O E ~The spectra of these compounds show that the unpaired electron is coupled to I4N, P and 35,37C1 nuclei and an unambiguous determination of the superhyperfine coupling constants was achieved using 15N labelling.293ESR and IR spectroscopy have been used to monitor the reactions of [Mo~C~IO] and [MoOC14(DMF)]- with PhNH2. The reactions proceed to give [Mo(NPh)(NHPh)SI2- via [Mo(NPh)Cl,_,(NHPh), (DW)] (n = 0-3) intermediates.294
36.4.3.4 Other Monomeric Complexes
3643.41 Halide compleres The hexahalide ions [MoFJ and [MoC16]- form well-characterized compounds which are unstable towards hydrolysis. MoF6 readily accepts electrons to form [MoF6]- ions and will
Molybdenum
1354
oxidize Ag metal or I2 in MeCN at ambient temperatures, p l d i n g the molybdenum(V) salts [Ag(NCMe)4][MoF6]2 or [I(NCMe)2][MoF6], respectively.' Simple salts of [MoC16]- are readdy prepared, for example reaction of [MozCllo] with [NEt4]C1 gives [NEt4][MoC16].2" [MoC16]- is also obtained in a KCl-MoC15 melt.297 The reactions of various ligands with [MozCllo] in dry solvents provide a general route for the preparation of non-oxo chloromolybdenum(V) complexes. [ M O ~ Creacts ~ ~ ~with ] ICN, to produce [MoC~S(NCI)]in which the added molecule is N-b~nded,~',and with Y(CN)2 (Y = S, Se) in CS2 to give [MoCl5{U(CN),),], in which the new ligand is coordinated through its Y atom.299The reactions of [M%Cllo] or [MoOC14(DMF)]- with thiols, RSH, in py have been monitored by ESR and IR spectroscopy; [MoSCl,-,(SR),(py)]- ( n = 0-3) and [MoS(SR)~]~were suggested as possible products, in a reaction exactly analogous to that using PhNH, instead of RSH (see Section 36.4.3.3).294The dimer [Mo2S2C16]dissolves in MeCN to form [MoSC13(NCMe),], which reacts with pyridine and HC1 to give [pyH][MoSCl,(py)], Direct reaction of [Mo2S2C16]with pyridine gives the seven-coordinate compound [ M ~ S C l ~ ( p y ) ~ l . The chloro alkoxide complexes [MoCl,(OR),]- (R = Me, Et) have been prepared by low-temperature reaction of Mo2Cll0] with ROH, and have been isolated as their [NMe4]+, [pyH]+ and [quinH]+ salts." The vibrational, electronic and ESR spectra of these materials are consistent with a trans octahedral structure for these anions. [MoCL(NPPrPh2)(OPPrPh2)] has been obtained by oxidation of [ M O C ~ ~ ( P P ~with P ~ ~tolyl-4-sulfonyl )~] azide.301 An eight-coordinate complex [ M ~ C l ~ ( d i a r s ) has ~ ] I ~been prepared, and the crystal structure determined; the ligands adopt a dodecahedral geometry around the molybdenum, with arsenic atoms on the A sites, and chlorine atoms on the B sites.302
d
3643.4.2 Complexes of sulfir donor ligands
Compounds containing the [Mo(S2CNEt2),]+ cation can be prepared by the reaction of [Mo202C16]with [Et2NH][SzCNEt2],303 or by oxidation of the corresponding Mo'" or Mo"' c o m p l e ~ e sCrystal . ~ ~ ~structures ~~~ have been determined for salts with various anions, all of which display the same geometry for the cation. Crystals of [ M o ( S ~ C N E ~ ~are ) ~ monoclinic, ]I~ with dodecahedral coordination of sulfur atoms around the molybdenum; the mean Mo-S distances to the two sites are 2.545(2) and 2.500(2)A.305The electrochemistry of several eight-coordinate complexes, including the dithiocarbamate, [Mo&CNEt,),]+, dithiolenes, [ M o ( S ~ C ~ R ~ )and ~ ] ~the - , thioxanthate, [Mo(S2CSBuf),]+, has been investigated.3m All the systems exhibit a reversible one-electron reduction and, with the exception of the thioxanthate complex, a reversible one-electron oxidation. Reaction of [ M O ~ O ~ ( & C N R(R ~ )= ~ ]Et, P i ) with thiol-containing ligands 2 -NHR 'w4 S H or 1,2-(HS)&& yields the monomeric complexes [Mo(S&NR2)(NRC6H&] or [ M O ( S ~ C N R ~ ) ( S C ~ H ~respectively.307 S)~], A determination of the crystal structure of [MO(S~CNE~Z)(NHC~H~S)~] has shown a coordination geometry at the molybdenum midway between that of a trigonal prism and an octahedron, in which the two nitrogen atoms are mutually trans. 308 [NEt4][MoO(S-4-tolyl)41 reacts with N-(2-hydroxybenzyl)-2-mercaptoaniline (H3hbma) to yield [NEt4][Mo(hbma)2], the anions of which possess a highly distorted, all-trans, octahedral geometry. Cyclic voltammetry has been used to demonstrate reversible one-electron oxidation and reduction processes for this complex.309 364.3.4.3 Cyanide complexes
The octacyanide ion [Mo(CN)$- can be re ared by oxidation of [Mo(CN),I4-. Like the Molv anion (Section 35.4.2.4.6) [Mo(CN),F- \as been structurally characterized in more than one geometry. In [NB&],[Mo(CN),] the anion possesses a near-re ular, DU triangular4 However, the eight dodecahedral structure, with an average Mo-C distance of 2.12(2) cyano groups in C S ~ [ M O ( C N ) ~ ] . ~are H ~arranged O in a 4,4'-bicapped trigonal rism around the Furthermore, molybdenum, with an average Mo-C distance of 2.17(2) A."' K 3 [ M o ( ~ ) 8 ] . H 2 0is isomorphous with K3[W(CN),].H20, which has square antiprismatic geometry, although a full structure was not determined for the molybdenum analogue as it decomposed under X-ray i r r a d i a t i ~ n . ~The ~ ' ESR spectrum of &[MO(CN)~] in glycerine solution indicates that all the cyanides are equivalent, consistent with a square antiprismatic structure, or with stereochemical non-rigidity on the ESR t i r n e ~ c a l e . ~ ~ ~
Molybdenum (IZZj, ( W )and (V)
1355
i43.4.4 Spectroscopic studies
The magnetic moments of monomeric non-oxo molybdenum(V) complexes typically lie :tween 1.55 and 1.73BM,*corresponding to the spin-only value for one unpaired electron. he ESR spectra of these complexes usually show one strong fine with six hyperfine lines due y5,97M~ coupling; a typical example is [Mo(S2CNEt&]13 with isotropic g and A values of 9794(1) and 37.1(1) x cm-l, r e s p e c t i ~ e l yI. M ~ ~O ( S ~ C N E ~ ~ ) ( N H C ~displays H ~ S ) ~super] rperfine splitting for two equivalent nitrogen and two equivalent hydrogen atoms, with A dues deduced from spectral simulation of 2.4 x lov4cm-' and 7.4 x low4cm-', ~pectively.~'~ These couplings are the same order of magnitude as those observed for iperhyperfine splittings in the spectra of the molybdenum oxidases, demonstrating the issibility that these may be caused by coordinated N-H groups. [NEt4][M~(hbma)z](H3hbme = N-(2-hydroxybenzyl)-2-mercaptoaniline) exhibits an axial SR spectrum, with gi > g , , and A,,> A , . Superhyperfine coupling to the two nitrogens was it detected over a temperature range from room temperature to 77K.309The single crystal -band ESR spectrum of the DZdanion of [NBu4I3[Mo(CN),] has been determined at room mperature. The sense of the anisotropy in g values ( g , ,>gJ contradicts redictions based on mple crystal field theory, and possible explanations have been proposed. Q14 The electronic absorption spectrum of the [MoClJ ion in solidified KC1-MoCl5 melt shows 1 absor tion band at 24 100 cm-l, assigned to the (tZg)'+ (eg)' transition of an octahedral j m ~ 1 e x . PIn~ ~[NEt4][MoC16]this band is split by symmetry lowering (as suggested from ESR ita obtained for this and/or spin-orbit coupling.296Other, more intense, bands at gher energies in [IvIoC16]- complexes are due to ligand-to-metal charge transfer transitions. ow energy absorption bands in [MoC14(0Rj2] (R = Me, Et) complexes can be interpreted assuming a truns octahedral ~tructure.~"The UV-vis s ectrum has been reported for [Mo(S2CNEtj4]13,but no assignments have been proposed. 3 2
36.4.3.5 Dimeric and Polymeric Complexes 36.4.3.5.1 General comments Dimeric species play a major role in the chemistry of moiybdenum(V), and of particular importance are compounds containing one or more oxygen bridges. Examples are also known of complexes bridged by sulfur, selenium or nitrogen, and halide bridges are found in [{ M O F ~ } ~ and ] ~ ~ [' { M O C I ~ ) ~ ] Many . ~ " ~ dimeric compounds are diamagnetic due to interaction between the two molybdenum(V) atoms, either directly and/or via the bridging atoms, which lead to spin-pairing of the two unpaired electrons. However, in some (MO")~systems spinpairing does not occur or is incomplete and ESR signals are observed characteristic of monomeric species, with one electron associated with each molybdenum.
36.4.3.5.2 Dimeric eomplenes with one bridging ligand Almost all of these compounds are based upon an { M O ~ O ~ core } ~ + and a representative selection of these compounds is given in Table 9. In these compounds, each molybdenum is coordinated to a terminal oxo ligand with a linear, or near-linear, M o -GM o bridge. The terminal oxo groups are usually cis to the oxo bridge, directed either cis (syn) or trans (anti) to each other across the bridge, and no complexes are known in which the two terminal oxo ligands are cis to the M-O-Mo bridge in a skew arrangement, forming a dihedral angle of 90". Bonding considerations for these { M o ~ O ~ }complexes ~+ show that the molybdenum dxy orbitals, which are nonbonding in mononuclear oxo complexes, are in proper alignment with the p x orbital on the oxygen bridging atom to form three-centre delocalized molecular orbitals, producing a bonding, a nonbonding and an antibonding level.5 This interaction is maximized at dihedral angles between the terminal oxo ligands of 0" (syn) and 180"(anti), but cannot occur at 90" (skew). Two electrons from the oxygen and one from each molybdenum fill the bonding and nonbonding orbitals, to give diamagnetic compounds. In the skew arrangement each molybdenum dxy orbital would interact with a different oxygen p orbital, giving, for each Mo-p-0 n bond, one bonding and one antibonding level occupied by one molybdenum and two oxygen electrons; therefore, the electronic structure of the centre would involve the two
1356
Molybdenum
degenerate antibonding orbitals each containing one unpaired electron, to give a paramagnetic complex with an S = 1 ground state. Although no crystal structures are known with the skew geometry, it is possible that in solution rotation about the MM -o bonds may occur, giving rise to paramagnetic species. Bending of the Mo-O-Mo bridge would also reduce the MM -o ~d interaction and lead to paramagnetism. Table 9 A Representative Selection of Dimeric Molybdenum(V) Complexes Containing a {MO,O,)~+Core Ref. 316 318 317 3 19 344 100,320,321 323 324,325 326 327,328 329 330 33 1 332 333 334 335 336 337 338 339 340-343 337 a
Hfacac = 1,I ,1,5,5,5-hexafluoropentane-2,4-dione. W(0NN) = RC,H,C(O)NHNC(Ms)C(Me)NOH, 2-H2NNHC(O)C,HplC(Me)C(Me)NOH.
Rarely, a linear O=M+O-Mo=O arrangement is adopted, in which the molybdenum dxy orbitals are orthogonal to the oxygen pn orbitals and, therefore, the system is paramagnetic. A typical example of a complex with a normal { M O ~ O ~core ) ~ +in the anti arrangment is [MO~O~CL,(DME)~], prepared by refluxing [Mo2ClI0] with DME. Each molybdenum is octahedrally coordinated to a bridging oxo group, a terminal oxo group, two chloride ligands and the two oxygen atoms of a bidentate dimethoxyethane ligand; one oxygen of the DME is trans to the oxo bridge.316 The crystal structure of [Mo203(acac)2(facac)z] (Hfacac = 1,1,1,5,5,5-hexafluoropentane-2,4-dione) displays a similar geometry, with the terminal oxo groups cis to a linear Mo-0-Mo bridge in an anti conformation, and the diketonates completing the octahedral coordination spheres of the m~lybdenurn.~’~ Magnetic susceptibility measurements of [ M ~ ~ O ~ ( a c abetween c ) ~ ] 77 and 292 K show this material to be paramagnetic, with a temperature dependent magnetic moment suggesting some direct or indirect interaction between the molybdenums. The paramagnetism may be due to the presence of a bent M-O-Mo bridge, which would be consistent with the IR spectrum of this compound.318 There are a large number of well-characterized complexes with 1,l-dithiolate ligands which contain an {M0203}~+ core; these include compounds of dithiocarbamate (S2CNR2)-, trithiocarbonate (S2CSR)-, dithiocarboxylate (S2CR)- and dithiophosphate (S2P(OR)&ligand^^^%^^* (see Table 9). All of these complexes are diamagnetic, or have very low magnetic moments, and crystal structure determinations have shown examples of syn and anti conformations of the { M o ~ O ~ core; ) ~ + the oxo grou s in [Mo~O,{S~P(OE~)~}~] adopt the anti configuration?28 but in both [Moz03(SzCOEt)4]~25 and [Mo,O,(S~CNE~,)~]~‘@ the syn configuration is preferred. The length of the Mo-0, bonds is significantly shorter than that of Mo-Ob bonds (e.g. in [ M O ~ O ~ ( S ~ C Othe E ~ distances )~] are 1.65(2) and 1.86(2) A, respectively) and the longest Mo-S bonds are those trans to the terminal oxo ligands. The xanthate complexes, [ M O ~ O ~ ( S ~ C O (R R )= ~ ]alkyl), react with dialkylamines, alcohols or hydrogen
Molybdenum(ZZZ), (W) and ( V )
1357
sulfide to lose xanthate and produce [MO~O&(S~COR)~], which contains two sulfido bridges.324 The dithiocarbamates [ M o ~ O ~ ( S ~ C N disproportionate R~)~] in solution to generate an equilibrium with Mow and Mo"' species (equation 1, Section 36.4.2.2.1), and can easily be prepared by reacting [ M O O ~ ( S ~ C N Rwith ~ ) ~ ][MoO(S2CNEt,),] .70 The mixed MoV-MoV1 dimer [Mo203(S2CNEt&j- has been prepared and structurally characterized in a material ako containing [Mo203(S2CNEt2)4] and (probably) [H502]+.322 The anion is paramagnetic, and ESR spectroscopy has shown the unpaired electron to be localized on one molybdenum, rather than over the whole molecule. Complexes with an { M O ~ Q ) ~ core + which contain five-coordinate molybdenum atoms have been isolated with tridentate sulfur donor li ands. These complexes have the form [Mo203L] (L = S(C2H4S)$-, O(C2H4S)%-,MeN(C2H,S)2-), and crystallize with the terminal oxo groups in ~ ~ ,coordination ~~' geometry around each molybdenum is that of a the anti c o n f i g ~ r a t i o n . ~The distorted trigonal bipyramid, with the axial positions occupied by the bridging oxygen and the central sulfur, oxygen330or nitrogen331atom of the tridentate ligand. With the tetradentate L, (R= H, Me) the molecules ligands { SCH2CH2N(Me)CH2CHRN(Me)CH2CH2S}2-, [Mo2O3L] gossess an anti configuration and the molybdenum atoms are octahedrally coordinated. 32 Structure determinations of the isothiocyanate Complexes &[ M O ~ O ~ ( N C S ) ~ ] and -~H~O~~~ [~YH]~[MO~O~(NCS)~(CO~)~]~~~ have shown that they adopt the anti configuration, with N-bound isothiocyanates. A report has suggested that [ M ~ ~ O ~ ( o x i n e(Hoxine )~] = 8hydroxyquinoline) can be isolated in four isomeric forms, one of which is strongly paramagnetic ( p = 1.83BM).3'"'However, a later 'H NMR study of this compound in DMSO showed no evidence of such isomer formation, and indicated that all the N atoms of the oxine ligands are coordinated Puns ,to an oxo group.341The electrochemistry of [ M ~ ~ O ~ ( o x i nine )DMSO ~] has shown that this molecule can be reduced in two reversible one-electron steps, but oxidation to an MeV'-MoV dimer causes d e c o m p o ~ i t i o n An . ~ ~ electrochemical ~ study comparing MoV dimers with one, two or three bridging ligands has shown that the monooxo-bridged species are reduced by successive one-electron processes, rather than by a two-electron process which is observed for dioxo-bridged complexes.34s Assignments have been made of both terminal and bridging v(Mo-0) stretching modes in the IR spectra of complexes containing the {Mo,O~}~+c 0 r e . ~ ~In~ the 3 ~ Mo-0, ~ region (900-1000cm-') one band should be observed for the anti and two for the syn conformer; however, only one band has been found in all cases, which may be due in the syn complexes to overlap of the two bands or to a low intensity of the antisymmetric mode. A distinctive feature of the electronic absorption spectra of { M O ~ O ~ complexes }~+ is an intense band at ca. 19 000 cm-', which is not appreciably ligand dependent and appears to be characteristic of a linear Mo-0-Mo bridge, as it is not present in the monomers or dimers with two bridging ligands.s The 1 7 0 NMR chemical shifts of the oxo ligands of molybdenum(V) and (IV) complexes have been used to compare relative M A n-bond strengths. 12' A complex closely related to those possessing an {Mo,O~}~+ core is [Mo,O(N~O~)~(S~CN (to1 E ~=~4-tolyl), )~] prepared by reducing [MoO(N~O~)(S~CNE~,)~] .14' IR and 170NMR studies have indicated an oxo-bridged structure for the dimer; there is an intense absorption at 18 760 cm-' in the electronic spectrum, which does not obey the Lambert-Beer law due to a disproportionation reaction which sets up an equilibrium with MoIV and MeV' species. ~ + in which the two The only complexes which are known to contain an { M O ~ O ~ }core bridge are [Mo,O,(TPP),] and terminal oxo groups are trans to the Mo--O-Mo [MO,O~(OEP)~] with tetradentate porphyrin ligands (TPP = tetraphenylporphyrin, OEP = octaethylporphyrin) .347.m In [ M O ~ O ~ ( T P Pthe ) ~ ] Mo-0, and Mo-Ob bond lengths of 1.707(3) and 1.936(3) A, respectively, are sli htly longer than the corresponding distances observed in the other ( M 0 2 - 0 ~ ) ~ ~ presumably because of the mutual trans effect of the oxo ligands. A complex containing a mixed MoV-MoV' oxidation state (Mo205}+core can be obtained by reduction of [ M ~ ~ O , ( n a n e ) ~ ] [ B (nane r ~ ] ~= N,N',N"-trimethyl-l,4,7-triazacyclononane). In bridge, and each molybdenum is also the Mo"' dimer there is a linear Mo-0-Mo coordinated to two oxo ligands cis to the oxo bridge in an anti conformation, but the structure of the MoV-MoV1 species is not known. An MoV-MoV dimer of this system can be produced electrochemically, but not by chemical The reduction of [ M O O ( S ~ C N E ~ ~ ) ~ ] [ B F ~ ] results in the formation of [Mo20(S2CNEt2)#3F4], an MO"-MO'~ dimer which contains a linear Mo-0-Mo bridge. Each molybdenum is seven-coordinate, and the equal Mo--0 bond
5
system^,'^^^^^^"
1358
Molybdenum
lengths and equivalent environments suggest that the unpaired electron is likely to be delocalized.'*' Two types of complex are known with only one bridging ligand which is not an oxo group. In [Mo202&(C204)] ( X = F , C1, Br) the oxalate is bidentate to each molybdenum; the coordination geometry around the molybdenum is square gyramidal, with axial oxo ligands, and the room temperature magnetic moments of 1.67BM er molybdenum indicate that the interaction between the two molybdenum atoms is weak.3 The reaction of [MOCI,(THF)~] with 4-C61%(N02)(N3) and [Et2NH2][S2P(OEt),] yields [Mo2(NCd4N){S,P(OEt),},], which is bridged by the diimide, (4-NC$14N)4-, ligand. Each molybdenum atom is in a distorted octahedral environment, comprising an imido nitrogen and five sulfur atoms from one monodentate and two bidentate dithiophosphate ligands.351
9
364.3.5.3 Dimeric complexes with two bridging ligands
The majority of these complexes contain an (MO,Y,Y~-,}~+(Y = 0; Y' = S, Se; n = 4-0) cure, and such systems constitute an important class of molybdenum c o r n p o ~ n d s ; ~ Table , ~ ~ 10 ,~~ provides a reasonably comprehensive summary of the known complexes and shows the extensive range possible, given the versatility in the nature of both the core and the appended ligands. These cores consist of two molybdenum atoms, each coordinated to one terminal and two bridging 0, S or Se atoms and involve a relatively close approach (2.5-2.9 A) of the two metals. Related dimeric systems with one (or two) terminal imido (RNZ-) ligands in place of the Y , atom(s) are known. Also, Mo" dimers, in which two Mo = Y,groups are linked by two bridging ligands of another type, notably Cl-, [S2I2-, [SO4]'-, have been characterized and are included in this section, as are complexes containing an {Mo?Y,}~+ (Y = 0, S) core, the bridging of which is supplemented by an additional polydentute ligand. Compounds involving oligomeric aggregates formed by the association of { M O ~ Y ~(Y } ~=+0, S) cores are described in Section 36.4.3.5.4. The first structural characterization of a compound containing a (M0204}~+ core was achieved352for B ~ [ M O ~ O ~ ( C ~ ~ ~ ) ~ (the H ~anion O ) ~of]which - ~ H(Figure ~ O , 4) consists of two distorted octahedra sharing an 0-0 edge. The anion possesses a C2 axis, with the two terminal ligands cis to the oxo bridges to give the syn Conformation, and each molybdenum centre has structural features identified earlier for Moo3+ and { M O ~ O ~ moieties. }~+ The shortest Mo-0 bond is that involving the terminal oxo group (1.70(3)A) and the bonds to the bridging oxygen atoms (l.S8(3) and 1.93(3) A) are shorter than those to the oxalato and aqua oxygens cis to the M-0, groups (2.14(4) and 2.22(4)& respectively), but there is no structural trans effect (Mo-O(oxalate) trans to M-0, = 2.11(3) A). Each molybdenum lies out of the equatorial plane defined by the four donor atoms cis to M-0, and this results in a rough equality in the 0---0distances at ca. 2.78 A, close to the van der Waals separation for these atoms.353The atoms of the Mo202bridge are not coplanar and there is a dihedral angle of 152" between the two Moo2 planes. The terminal oxo groups of the { M O ~ O ~core } ~ +point away from each other; if they were not so aligned, their separation would be the same as the Mo-Mo separation (2.541 A) and much closer than the van der Waals separation. This short M e M o approach, and the essentially diamagnetic nature of the compound imply a direct M w M o bonding interaction, and the bent bridge permits a closer approach of the Mo atoms without unduly enlarging the Ob-Mo-Ob, and/or contracting the Mo-Oh-Mo, angles. The qualitative description of the Mo-Mo bonding follows from the electronic structure of MOO'+ centres. Each molybdenum centre can be considered to possess a (dXY)' configuration and the geometry of the bridge allows a direct overlap of the two dXYorbitals with the resultant bonding level accommodating the two electrons .5,354 [ M ~ ~ O ~ ( d a n e ) ~ ] [ C(dane l O ~ ]=~1,5,9-triazacyclododecane)has been obtained both as a } ~ + with the anti purple and a yellow material. The purple form contains5' an { M O ~ O ~ core conformation in which the four-membered Mo202 ring is planar with an Mo-Mo separation of 2.586(1) A. This separation, and the diamagnetic nature of the compound, are consistent with the formation of an Mo-Mo bond, as in a syn structure, by overlap of the molybdenum dxy orbitals. The cation of the yellow form of this compound is considered to possess an (Mq04}2+ core with the syn conformation by analogy with I M 0 ~ 0 ~ ( n a n e ) ~ (nane ] ~ + 1,4,7triazacyclononane), which has been to exist in both syn (cis)and anti (trans) conformers, with the immediate environment at molybdenum almost identical in the two forms and the M e M o separation being 2.555(1) 8, and 2.561(1) A, respectively. t r ~ n s - [ M o ~ O ~ ( n a n eis) ~ ] ~ + 5
MoEybdenurn(ZZZ), ( W )and
(v)
1359
Table IO Dimeric MoIybdenum(V) Complexes Containing an {Mo2Y,Y~-,J2+ (Y = 0; Y' = S, Se; n = 4-0) Core Complex
Bridging
Re5 336,354 384 372 352,402 384 318 403 344 344 344 383 383 360-363 362-365 361-363,365,366 363,365,367 362,363,365,368-371 404 405 373 324 406 357,382,406 399 329 358 331 329,389 329,390 329,391 392 393 394 401 407 408 333 385 386 333 329 355 50 343 343 343 387 409 409 375 376 374 377 378 379,380 380,381 410 359,411
a
Hdmmpd= N,N'-dimethyl-N,N'-~(2-mercaptoethyl)propylenediamineI Hmpr = 2-mercaptopyrimidine. nane = 1,4,7-triazacyclononane. dane = 1,5,9-triazacyclododecane. e HoKine = 8-hydroxyquinoline and substituted derivatives.
1360
Molybdenum
Figure 4 Structure of the anion of Ba(Mo20,(~04),(H,0)~~5H203s2
irreversibly converted into ~ i S - [ M o ~ O ~ ( n a n evia ) ~ ]acid-base ~+ catalysis and reasons for the relative thermodynamic stabilities have been attributed to the increased, local, O,---Ob nonbonding distances in the latter as compared to the Other factors may be important in determining the most stable conformation of (Mo204)'+ and related complexes and the effects of Mo-Mo bonding357and steric interactions among coordinated have been considered. The series of dithiocarbarnato complexes [ M O ~ O , S ~ - ~ ( S ~ C N(nR=~ 4-0; )~] R = demonstrate two important properties of these systems. Firstly, it is the y-oxo groups which are substituted initially and, secondly, this substitution leads to an increase in the Mo-Mo separation, from 2.580( 1) A in [ M O ~ O Z ( ~ - O ) ~ ( S ~ C N through E ~ ~ ) ~ ]2.673(3) 8, in [Mo202(p0)( p -S)(S2CNPr2)2],364 to 2.820(1) , 2.826(3) and 2.814( 1) 8, in [M02O2(P - S ) ~S2CNHz)2] ( ,366 [ M O ~ O S ( ~ - S ) ~ ( S ~ Cand N E[~M~ O ) ~~]S~~~( ~ - S ) ~ ( S ~,369 CN respectively. E~~)~] Molybdenum(V) dimers with { M O ~ Y , , Y ~ - ~(Y } ~=+ 0, S; n = 4-0) may involve both molybdenum atoms with five-coordination or six-coordination (not including the Mo-Mo interaction). Examples of five-coordinate complexes include [ A S P ~ ~ ] ~ [ M OF2 ~O~C~ [ M O ~ O . S ~ - , ( S ~ C N R ~ ~(n ] = 4-0, R = H, alkyl),3*371 [Mo& {S2P(OEt),} 21?73 [Moz02S2(Szj(S302)]-,374 and [ M O ~ O . S ~ - ~ ( S ~ ) ~ - (n ~ (= S 2, ~ ) x~ = ] ~0,- 1; n = 1, x = 1; n = 0, x = 0, 1, 2).3 5-3R1 In these compounds the coordination around each molybdenum atom is square pyramidal, with the Mo-Xt (X= 0,S) groups axial, cis to the two bridging ligands and in the syn conformation. The Mo2Xz bridging unit is not planar, but involves a dihedral angle between the two MoX2 planes of 150-160'. The 1,2-dithiolate complex [NEt4]2[MozS4(SC2H4S)2] has been isolated and structurally characterized in both syn and anti ~ ~ n f o r r n a t i o n The s . ~ ~syn ~ form has a geometry similar to that of the corresponding dithiocarbamate complexes, with an Mo-Mo distance of 2.863(2) A; in the anti conformation the Mo2S2bridge is planar, and the Mo-Mo distance is 2.87&(2)A. In the five-coordinate species with an {MOZY~}~+ ( Y = O , S ) core there is a vacant coordination site trans to each terminal oxo or sulfido ligand. These can be filled to give species in which each molybdenum is six-coordinate; for example, recrystallizing [ M O ~ O ~ ( O S C P ~ ) ~ ] from pyridine yields [MoZO4(OSCPh)2(py)2].383 The crystal structures of [Mo~O~X~C~~(H (X ~=O0, ) ~S)] ~show octahedrally coordinated molybdenum atoms, with two bridging oxides or sulfides and syn terminal oxo ligands; the water molecules are rrans to the oxo g r o ~ p s .Other ~ ~ ~complexes , ~ ~ ~ with similar geometries include [ M ~ ~ O ~ C l ~ ( b,385 ipy)~] [M0~0~(O~PH~)~(bipy)~]~~~ and [ M O ~ O ~ ( N C S ).387 ~]~ Also, [Mo203S{SzP(OPr')2}2] reacts with various ligands to form 1 : l adducts, in which the addended molecule is located in the 'molecular crevice' formed by the phosphorodithioato ligands388 and forms a third bridging group. Complexes of molybdenum(V) with biologically relevant ligands, such as L-cysteine, have been investi ated as possible models for the molybdenum sites in enzymes. In [ M q 0 4 ( ~ cy~teinate)~]'-. the {Mo204}2Ccore possesses the syn conformation and each (cysteinate)2acts as a tridentate O,S,N donor ligand giving octahedral coordination at each molybdenum with a carboxylato oxygen atom trans to the terminal oxo group.389A similar geometry is found
Molybdenurn(ZII), (N) and (V)
1361
in [M0~0~S~(~-cysteinate)2]~-, which contains two bridging sulfide atoms.390In the corresponding derivatives of the methyl and ethyl cysteinate esters, (Me-Cys)- and (Et-Cys)-, a carboxylate oxygen does not bind to molybdenum and [M0~0,(Et-Cys)~] and [Mo202S2(MeCYS)~] involve five-coordinate metal atoms;391.392 in both cases the syn conformation is adopted and the geometry around each molybdenum is perhaps best described as distorted trigonal bipyramidal, with one bridging oxygen and the nitrogen of the ester in the axial positions. [ M ~ ~ O ~ ( ~ - h i s t i d i n and a t e )[~M ] ~ ~ O ~ S ~ ( ~ - h i s t i d i nhave a t e )structures ~] closely resembling those of the corresponding (L-cysteinate)'- complexes, with the amino acids acting as tridentate O,N,N donors and with the carboxylate oxygen trans to the terminal oxo IR and Raman studies of molybdenum(V) dimers with { M o ~ O ~ } ~{Mo203S12+ +, and { cores have established that in complexes with syn geometry two bands are observed in the region 900-1000 cm-' which can be assigned to molybdenum-terminal oxygen stretching. 130*395 Vibrations associated with the bridging framework give rise to weaker bands at lower frequencies, which are often obscured by vibrations of other ligands. 95MoNMR spectra have been measured for a number of oxo and sulfido-bridged spin-paired dimers; the majority of complexes examined to date involve the s y n - { M 0 ~ 0 ~ Y ~(Y } ~=+0,S) core and possess a chemical shift in the range 320-982 p.p.m.396,397The chemical shifts are sensitive to the electronic environment of the molybdenum; for example the yellow s y n - [ M ~ ~ O ~ ( n a nand e ) ~the ] purple a n t i - [ M ~ ~ O ~ ( n a n(nane e ) ~ ] = 1,4,7-triazacyclononarie) are readily distinguished by 95MoNMR spectroscopy, having chemical shifts of 586 and 342 p.p.m., respectively. Furthermore, their interconversion and its catalysis by acid are readily monitored by this techniq~e.~' CD spectra have been recorded for complexes containing { M O O ~ X Y } cores ~ + (X,Y = 0, S) liganded by optically active chelates, such as (S)-cysteinate and (S)-histidinate. These spectra manifest two distinctive peaks with opposite signs at ca. 26 000 and 33 000 cm-l, which can be related to the nature of the asymmetric distortion around the Mo-Mo axis.3Y*Unlike the { M O ~ O ~ }dimers, ~+ those containing an { M O ~ O ~core } ~ + do not exhibit any characteristic absorption in their electronic spectra, although the compounds are generally yellow/orange in colour. The electrochemistry of selected systems containing an {MOZY~}~+ (Y = 0, S) core has been extensively investigated. For the series of dithiocarbamato complexes [ M o ~ O ~ - ~ S ~ ( S ~ C N R ~ ) ~ ] (n = 0-4; R = alkyl) the y-dioxo species are reduced by a two-electron process, whereas all other species are reduced by two successive one-electron processes. The substitution of oxy en by sulfur enhances the ease of reduction of the binuclear centre.346*361*362.~371 [ M O ~ O ~ S ~ ( S ~ C isN H reduced ~ ) ~ ] irreversibly in a two-electron process, with dissociation of the dithiocarbamate^.^^^ Both syn and anti isomers of [NEt4]2[Mo2S4(SC2H4S)2] undergo a reversible one-electron reduction in DMSO; they also exhibit two irreversible oxidation processes, each of which appear to involve two electrons per dimer.382 Cyclic voltammetric studies of [NEt4]2[Mo2S4(SC6H5)4], [NEt4]2[Mo2S4(2-SC6&NH)2] and [Mo2S4{SC(Me)2CH2NHMe}2]have shown that reversible one-electron reductions occur with complexes of the bidentate ligands, but that the thiophenolate complex is irreversibly reduced and decomposes.399The cysteinato complexes [MozOs-,S, (~-cysteinate)~]~(n = 0-2) undergo electrochemical reduction in a single four-electron step to Mo'" dimers in aqueous buffers. The ease of reduction and electrochemical reversibility of the Mo~/Mo~''couple increase with insertion of sulfur into the bridge system. However, the corresponding ethyl cysteinate complexes [M0204-~S,(Et-Cys)z] (n = 0-2) are reduced by two successive one-electron transfers to Moiv species.400[M0204(glycinate)~(H~O)~] is reduced in a two-electron transfer to give monomeric MoIV species, which can be electrochemically oxidized to M V monomers that dimerize to re-form the initial complex. Proton-assisted oxidation of an aqueous solution of [Mo204(glycinate)2(H~0)2] yields the MoV/MoV1 dimer [M0204(~lycinate)~( OH)(H20)2], which can be electrochemically reduced in a one-electron ste to a Mo2 dimer, which is further reduced in a two-electron step to monomeric MolV species4 ~+ = The references in Table 10 should be consulted for other studies on { M O ~ Y ~ }(Y 0,S, Se) systems. Dimeric complexes, analogous to those containing an {M02Y4}2f (Y = 0,S) core, but with one or two terminal NR2- ligands have been reported. (Me3CN)2Sreacts with [{CpMo(C0)3}21 to form [ { C ~ M O ( N C M ~ ~ ) ~in} which ~ S ~ ] ,the central Mo2S2 moiety is planar with an Mo-Mo separation of 2.920(1) A."' [Mo&(Ntol)4{ S2P(OEt)2}4] reacts with (S2CNBu2)- to produce [ M O ~ ( N ~ O ~ ) & ( S ~ C N B which U~)~ has ] , a structure very similar to that of [Mo202S2(S2CNBu&]; the imido ligands are cis to the two sulfide bridges, in the syn conformation, and each molybdenum is five-coordinate with an approximately square
r
1362
Molybdenum
pyramidal geometry. The Mo-Mo distance is 2.807(2) A.413The acid-assisted hydrolysis of [ M O ~ S ~ ( N ~ O ~ ) ~ { S ~ Pto( Ogive E ~ [Mo202S2{ ) ~ } ~ ] SZP(OEt),},], proceeds via the binuclear interwhich E ~ ) contains ~ } ~ ] , one terminal oxo group and one terminal mediate [ M O ~ O ( N ~ O ~ ) S ~ ( S ~ P ( O 4-tolylimido group, on different molybdenum atoms, and adopts a syn conformation, with an Mo-Mo distance of 2.812(1) A crystal structure determination of [pyH]4[Mo203(S04)(NCS)6] has shown the two molybdenum atoms to be bridged by one oxo and the sulfato ligand. The terminal oxygens are syn and each molybdenum has a distorted octahedral Persulfide bridges have been identified in several dimeric MaV complexes. In [NI&]2[Mo2(S2)h]each molybdenum is coordinated by four SZ- ligands in a distorted dodecahedron, with two bridging and two terminal groups. The complex is diamagnetic, and has an Mo-Mo distance of 2.827(2) A.416A similar geometry is found in [PPh3Me]2[Mo2(S2)2Cls], in which each molybdenum is coordinated to two bridging persulfides and four terminal chlorides; the Mo-Mo distance is 2.857(1) A.417 [ M O ~ O ~ C ~ ~ ( Ocontains P C ~ ~ two ) ~ ] chloride bridges; each molybdenum is also coordinated to two terminal chlorides, a terminal oxo ligand and a phosphate oxide in an octahedral geometry. The oxo groups are cis to the chloride bridges, in an anti conformation, and are trans to the OPC13 ligands.418[Mo2Cl2L8]Cls(L = H2N(CH2),NH2, n = 2-6, 8) can be prepared by reacting [Mo2Cl10]with the appropriate diamine under an inert atmosphere. These complexes are diamagnetic, and IR data suggest that they possess a dimeric structure with two bridging chlorides and monodentate diamines. In [Mo2C12L4]Cls(L = H2N(CH2),NH2, n = 9,lO) the diamines appear to be bidentak4” Asymmetric dinuclear molybdenum(V) complexes with two nitrogen bridges are formed on reaction of [Mo02(S2CNEt&] with the hydrazines PhC(Y)NHNH2 (Y = 0, S) in methanol. A structure determination of [(Et2NCS2)Mo{OC(Ph)NN}2MoO(S2CNEt2)]has shown that one molybdenum is six-coordinate, bound in a trigonal prismatic geometr to two sulfur atoms, two oxygen atoms, and two bridging nitrogen atoms of the (OC(Ph)NN)’-; the other molybdenum is five-coordinate, bound to the two bridging nitrogens, two sulfurs and an oxo ligand which occupies the axial position of a square pyramid.420 These dimers undergo a reversible one-electron reduction. [Mo2Cll0] reacts with thioureas to give [MO~CL+{RNHC(S)NHR’}~]C~ (R = pyridyl, 5-nitropyridyl, 4-methylpyridyl, 6-methylpyridyl; R’ = Ph, 2-MeC6&, 4M e a ) . Two thioureas are believed to be bridging, coordinated to one molybdenum via the sulfur atom and to the other via the pyridyl nitrogen atom; the other two thioureas are bidentate N,S donor ligands, and two chlorides on each molybdenum complete an octahedral coordination sphere.421 Complexes based on an { M o ~ Y ~ }(Y ~ += O , S ) core may involve an additional ligand bridging the two MoV atoms. (Section 36.4.3.5.5). However, many of these complexes share characteristics with the dimeric complexes having only two bridging ligands, especially if the two sites trans to the terminal ligands are occupied by different atoms of a polydentate ligand. [ M O ~ O ~ S { S ~ P ( O P ~reacts ’ ) ~ } ~with ] pyridine or pyridazine (pydz) to give a 1:1 adduct. The structure of these two adducts are similar, in that the basic core of the parent compound is maintained, but there are major differences in the manner of the bonding of the py and pydz molecules; the py molecule is weakly bonded to both molybdenums through nitrogen with M-N distances of 2.97(2) and 2.93(1) A, whereas each nitrogen atom of the pydz molecule coordinates to a different molybdenum atom with Mo-N distances of 2.59(2) and 2.58(1) Bidentate heterocyclic ligands bipy, phen, 5-N02phen and 2,9-Me2phen (L.J react with [M~,O,(ab),(py)~] (Hab = picolinic acid, 8-hydroxyquinoline) to give the corresponding [M0~0.&)(ab)~] derivative, in which the bidentate ligand bridges the ( M O ~ O ~core. } ~ + The products are diamagnetic, indicating retention of the metal-metal interaction.422 Crystal structure determinations of [pyH]3[Mo204(02CR)(NCS)4] (R = H, Me) have revealed N-bonded thiocyanates, with the molybdenum atoms bridged by two oxo groups and a bidentate carboxylate, coordinated to each molybdenum through a different ox gen atom, trans to the terminal oxo groups. The Mo-Mo distances are 2.566(1) and 2.560(1) for the formate , ~ ~ ~ structure has been found for the and acetate complexes, r e ~ p e c t i v e l y . ~A~ ~similar [ M O ~ O ~ ( O E ~ ) ( O ~ C C ~ H ~anion, O H ) Cin~which ~ ] ~ - the bridging ligands are one oxo group, the ethoxide group and the salicylate; the Mo-Mo bond length is 2.646 A.424 The potentially six-coordinate ligands (edta)4- and ( ~ d t a ) ~ (H4edta =N,N,N’,N‘tetrakis(2-ethanoic acid)-l,2-diaminoethane, H4pdta = N , N ,Nf,N’-tetrakis(2-ethanoicacid)1,2-diaminopropane) add to dimeric molybdenum(V) centres to form complexes such as [Mo202Y2(edta)12- and [MoOzY2(pdta)l2- (Y = 0,S).42i429 The structure of
f
kfolybdenurn(ZII), (IV)and ( V )
1363
Cs2[Mo202S2(edta)]shows a syn { M O O ~ S ~core } ~ +with sulfide bridges; the nitrogen atoms of the (edta)4- are coordinated trans to the terminal oxo groups, and the octahedral coordination spheres of the moIybdenums are completed by oxygen atoms of the carboxylate The structures of Na2[Mo204-,S,(R-pdta)] ( n = 0-2) are exactly analogous to this.427The low magnetic moments (ca. 0.5 BM) of [Mo204(edta)12- and [Mo202S2(edta)]’- indicate a strong interaction between the molybdenum atoms. 28 The kinetics of formation of [Mo2O4(edta)I2-in aqueous solution have been investigated, and a detailed mechanism for dissociation proposed.429 Cyclic voltammetry has been used to study the electrochemistry of [ M ~ ~ O ~ X ~ ( e d t(X a )= ] ~0, - S) in aqueous buffers; the complexes undergo reduction in a single four-electron step to molybdenum(II1) dimeric species, but no oxidation process was observed.400 The reaction of [PPh4I2[MoS4] with the mineral realgar, As4S4, yields [PPh4]2[Mo202S2(A~4S1Z)j. The structure of the anion consists of a sulfido-bridged syn{Mo202S2}2+core and a tetradentate S-bonded ( A S ~ S ~chelate. ~ ) ~ - Each molybdenum is in a square pyramidal geometry with the terminal oxo groups in the axial positions. The ( A s ~ S ~ ~ ) ~ ligand contains a 1,5-(As2S8)eight-membered ring, which is broken in the reaction between [PP~&[MoZO~SZ(AS~S~~)J and [PPh4]2[Mo02S2], yielding [ P P ~ $ [ ( M o ~ O ~ S ~ ) ~ ( A S ~In S &this ]. complex two {Mo202S2} units are linked by two (S2AsSAsS2)- ligands to give a symmetrical 16-membered macrocycle .430 +
36.43.5.4 Oligomeric complaes based on an {M&OP)2+and related cores
Oligomers may be formed by ligands linking two or more { M o ~ O ~ }units ~ + together; alternatively ligands may join other molybdenum centres to an { M o ~ O ~ }core. ~+ The anions of ~ ~ [ { ~ ~ ~ ~ ~ ~ ( ~ and 2 ~ K6[ ~ {M0203S(C204)2)2(C204)]. ) 2 } ~ ( ~ 2 ~ 4 ) ~ ~ 10H20 consist of two {M0203S(C204)2}~- dimers bridged by a quadridentate oxalate ligand, in which each carboxylate group brid es one dimeric unit. The Mo-0 bond lengths to the quadridentate oxalates (av. 2.303(4) are longer than those to the bidentate, terminal oxalates (av. 2.106(4) A), due to the trans influence of the terminal oxo ligands.431 A similar structure is adopted by the anion of &[(Mo2O4(O2CCH2C02)~}2(0~CCH~CO2)]~ 4H20 in which one malonate dianion forms a bridge between two dimolybdenum species.432 Methoxide bridges between { M O ~ O ~ ) ~dimeric + units are found in [{Mo2O4(PMe,)},(OMe),1, in which four dimers are joined in an eight-membered Mo ring by four pairs of bridging (0Me)- ligands. In each dimer one molybdenum is square-pyramidally bound to a terminal oxo group, two bridging oxo groups, and two methoxides; the other molybdenum has an additional phosphine ligand, and is in an octahedra1 coordination (pz = pyrazolyl) contains a zigzag geometry (Figure 5).433 [ (Mo~O~(HB~Z~)(M~OH)}~(OM~)~] arrangement of four molybdenum atoms composed of two dimeric units, joined by two methoxide bridges. Each dimer consists of one molybdenum octahedrally bonded to a tridentate tris(pyrazoly1)borate ligand, a terminal oxygen, and two bridging oxygens; the other molybdenum is also octahedrally coordinated to two bridging oxygens, one terminal methanol trans to a terminal oxo group, and to the two bridging m e t h o x i d e ~ In . ~ ~[ M ~ ~~o~Cl~(oPr)~], propoxide bridges are present in two { M 0 ~ 0 ~ ( 0 P r ) }cores ~ + (with Mo-Mo separations of 2.669(2) A); these two cores are linked by two propoxide bridges and by two bridging oxygens of the {Mo2O3(0Pr)).’+ units (Figure 6). This complex formally possesses two Mov and two ~ ~ - a similar structure, but involves a shorter MoV1 centre^.^ [ M 0 ~ 0 & 1 ~ ( 0 E t ) ~adopts Mo-Mo separation (2.587(3) A).43 [ M o ~ ( O P ~in ) ~pyridine ] is oxidized by O2 to yield, as a minor product, [M0~0,(OPr’)~(py)~], which has been characterized by X-ray crystallography. The complex contains four terminal, two p3- and two p-oxo grou s, two terminal and two OS)~] p-bridging OPr‘ ligands, plus two localized Mo-Mo single bonds.4R [ M O ~ O ~ ( M ~ ~ Palso involves the pairing of two M O ~ O ~ cores } ~ + and partial substitution of the Me2POS ligands by Me2P02 has been achieved.638 [MO~O~(CO~)(OH)~(MO(CO)(PM~~)~)~] involves a central {M0204}~+unit in which each molybdenum is also bound to an {Mo 1(CO)(PMe3)2}2+group via two bridging hydroxides, with the carbonate ligand brid ing all four metal atoms. The two outer Mo” atoms are seven-coordinate and the Mo $ atoms are six-coordinate, with an M-Mo distance of 2.5522(9) A.439Another tetranuclear arrangement occurs in [ M 0 ~ 0 ~ ( 0 ~ C M e ) ~ { M o O cwith l}~], each of two {MoOCl)” units coordinated to one molybdenum of the { M O ~ O ~core ) ~ +through two acetates, to the other through one acetate, and directly bonded to one of the oxygen bridges (Figure 7). The Mo-Mo distance in the central core is 2.609(3) A.44oMo6010(OP~)12, an intermediate in the reaction between [MO~(OP~’)~] and O9 consists of a serpentine chain of
1)
COC3-RR
~
Molybdenum
1364
0
Figure
5 Representation
of
the
tetrameric
assembly
of
dimeric
molybdenum(V)
centres
in
..
Figure 6 Structure of [Mo,O,C~,(OP~),]~~~
Figure 7 Representation of the structure of [Mo,0,(0,CMe),{MoOC1)2]440
molybdenum atoms, supported by p-oxo and p-OPr' ligands, in which two MoV-MoV interactions (2.585(1) A) occur.441 The tetranuclear complexes [{MoZY(NR)S~[S~P(OEt)2]~)z] (Y = 0, NR; R = Ph, 4-tolyl) contain two {MoY(NR)S~}~+ units linked by weak coordination of the bridging sulfides of one unit to the molybdenums of the other unit, in positions tram to the terminal oxo or imido l i g a n d ~ . ~ lAddition ~ , ~ l ~ of CF3C02H to solutions of [{Mo2(Ntol)zS2[SzP(OEt)~]z}2] leads to formation of [Moz(Ntol)zS(SH){S~P(OEt)z}z(OzCCF3)], in which one of the bridging sulfides is protonated, and the trifluoroacetate bridges the dimolybdenum unit in the positions trans to the imido ligands. In solution two conformers have been detected by 'H NMR spectroscopy, thought to be due to different orientations of the S-H bond.438 Treatment of [Mo,(Ntol),S(SH) {S2P(OEt),),(O2CCF3)] with triethylamine, followed by addition of MeBr, , which has an yields the methylated complex [M~,(N~o~),S(SM~J}S~P(OE~)~}~(O~CCF~)] identical structure to the (SH)--bridged compound. 36.4.3.5.5 Dimeric compleres with three bridging ligands
One of the products of the reaction between thiophenol and [Mo2O4(S2CNEt2)2]is [Mo203(SPh)~(SzCNEt2)2]. A crystal structure determination has shown that this dimer is
Molybdenum(ZII),
(N) and ( V )
1365
bridged by two thiols and an oxo group, and that each molybdenum has distorted octahedral coordination (Figure 8). One of the bridging thiols is trans to the two syn terminal oxo groups, and the average M o 4 bond lengths to this thiol is ca. 0.2 8, longer than that to the cis ligand. The Mo-Mo distance is 2.678(9) A. Prolonged exposure of [ M O ~ O ~ ( S P ~ ) ~ ( S ~toC N E ~ ~ ) ~ ~ CHC13 gives [ M o ~ O ~ C I ( S P ~ ) ~ ( S ~ C Nin E ~which ~ ) ~ ]the + , three bridging ligands are a chloride and two thiophenolate anions. The chloride is trans to the terminal oxo froups, the two thiophenolate ligands are equivalent and the Mo-Mo distance is 2.822(2) 8,." The reaction of [NBu4][MoO(SPh)4] with HC1 yields [ N B U ~ ] [ M O Z O ~ C ~ ~ which ( S P ~ )has ~ ] , been characterized by X-ray crystallography. The structure of the anion is analogous to that of [Mo~O~CI(SP~)~(S~CNE~~)~]+, with one chloride and two thiolate bridges, the chloride being trans to each terminal oxo ligand. However, the Mo-Mo distance of [ M O ~ O ~ C ~ , ( S Pis~ ) ~ ] 2.915(1), which is significantly longer than that of the related cation. The reduction of [ M O O ~ ( S ~ C N Mby ~ ~thiophenol )~] yields [Mo2O,(SPh),( S2CNMe&] , which is isostructural with its diethyldithiocarbamate analogue, and involves an Mo-Mo distance of 2.649(1) A.-
Figure 8 Structure of [Mo~O~(SP~),I~C~E~,M"~
The reaction between [MoOC~~(THF)~] and alkanethiols in the presence of base yields [Mo202(SR),]- anions, which have been isolated as their tetraalkylammonium salts. Physical data indicate that thepe binuclear monoanions contain three bridging thiolates. The arylthiolato anions [MoO(SAr),]- (Ar= Ph, 4-tolyl) react in the presence of an alkoxy or amido ligand, Z, to form [Mo20z(SAr)&], in which the Z group and two thiolates act as bridging ligand^.^^^^ A rich electrochemistr of Mo202(YI$),Z]- (Y = S , Se) has been demonstrated and stepwise reduction of these Moz centres to Mo' and Mokv species has been observed.266The reaction between [MoOC~~(THF)~] and Na(S-4-tolyl) in methanol, followed by addition of [NEtIBr, yields [NEt4][Mo20z(S-4-tolyl)6(0Me)], which has been structurally characterized. The anions I involve an Mo-Mo separation of 2.919(5) A, with two syn terminal oxo groups, four terminal and two bridging thiolate ligands, and a bridging methoxide ligand trans to each of the terminal oxygen^."^ Interconversion of [MoO(SR)~]-(R = aryl, alkyl) and [MOZO~(SR)~Z](Z = OMe, OEt, SCH2Ph) anions has been shown to occur via redox processes involving both the molybdenum and the thiolate redox centres.266,448 Several examples are known of triply bridged dimers in which one ligand provides two of the bridging atoms. The reaction of [M~~O~(py),(oxine)~] (Hoxine = 8-hydroxyquinoline) with 2-hydroxyethanethiol gives [Mo203(SCH2CH20)(oxine)21,the X-ray structure of which shows an MoY unit bridged by one oxo group and by the sulfur and oxygen atoms of the (SCH2CH20)'- ligand. The two syn terminal oxo groups are trans to the oxygen atom of the mercaptoethanolate, with the nitrogen atoms of the oxines trans to the bridging sulfur.449A similar structure is found in [MO~O~S(SCH~CH~O)(&CNE~~)~], in which the three bridging atoms are the sulfide ligand and the oxygen and sulfur atoms of the (SCH2CH20)- ligand; the bridging oxygen is trans to each of the terminal oxo groups.45o Reactions of 2-hydroxyethanethiol and various bases with [MoOC14(H20)]- or
0
7"
1366
Molybdenum
[MOOC~~(THF)~] yield many products, including the triply bridged dimers [ M O ~ O ~ ( S C H ~ C H ~ O ) ~ (X X ~=YY] -= C1; X2= SCH2CH20, Y = C1, SCH2CH20H). These complexes involve bridging atoms which are the oxygen and sulfur of one (SCH2CH20)2ligand and the oxygen atom of another, the sulfur atom of which is coordinated to one of the molybdenum atoms in a terminal position. The terminal oxo groups are trans to the oxygen atom of the doubly bridging ligand. In each case the Mo-Mo distance is 2.734(6) A.””.”” The reaction of [ M O ~ O ~ ( S C H ~ C H ~ Owith ) ~ Cexcess ~ ~ ] - 2-hydroxyethanethiol and base, B, yields [BH]2[Mo203(SCH2CH20)3](B = piperidine, morpholine, pyrrolidine) . The bridging atoms of the anion are one oxo group and the oxygen and sulfur of a (SCH2CHz0)2- ligand; each molybdenum atom is also coordinated to a terminal oxo group and a bidentate (SCHzCH20)’ligand, The Mo-Mo separation is 2.676(1)
Figure 9 Structure of [MO,(NNHP~)(NNP~)(SCEI,CH,S),(SCH,CH~SH)]~451
Treating [M0(NNHPh)~(butane-2,3-dithiolate)~] with dimercaptoethane and trimethylamine in methanol gives [NEt3H]2[Mo2(NNPh)(NNHPh)(SCH2CHzS)3(SCH2CH2SH)], which contains terminal and bridging bidentate thiolate ligands and a bridging monodentate thiolate ligand (Figure 9). The Mo-Mo distance is 2.837(2)Ai, which is longer than in triply bridged molybdenum(V) dimers involving one or more bridging oxygen^.^^' The anion [MOO(SCH~CH~CH~S)~]reacts with HN3 in methanol to give [MO~O~(N~)(SCH~CH~CH~S)~]-. The crystal structure of this dimeric anion shows that the three bridging atoms are provided by a nitrogen atom of the azide, and by sulfur atoms of two different dithiolates, which are both chelated to the same molybdenum; the third dithiolate ligand is chelated to the other molybdenum. Two terminal oxo groups trans to the bridging azide complete octahedral coordination spheres for each molybdenum, and the Mo-Mo distance is 2.893(3) A.452
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Doub, Jr. and D. T. Sawyer, Znorg. Chem., 1975,14,2110. 362. W. E. Newton, G. J.-J. Chen and J . W. McDonald, J . Am. Chem. SOC.,1976,98,5387. 363. F. A. Schultz, V. R. Ott, D. R. Rolison, D. C. Bravafd, J. W.McDonald and W. E. Newton, Inorg. Chem., 1978,17, 1758. 364. 5. Dirand-Colin, L. Ricard and R. Weiss, Inorg. Chim. Acta, 1976,18,L21. 365. K. Musha, Y.Ohashi, S. Yamazaki, S. Toda and S. Tanaka, Nippon Kagaku Knkhi, 1983, 653 (Chem. A h t r . , 1983,99, 15 475); K. Musha, S. Yarnazalri and S. Toda, Nippon Kagaku Kutrhi, 1984,702 (Chem. A h t r . , 1984, 101,47 630). 366. R. Winograd, B. Spivack and 2.Doh, Cryst. Struct. Commun., 1976,5,373; N. C. Howlader, G. P. Haight, Jr., T. W.Hambley, M. R. Snow and G. A. Lawrance, Inorg. Chem., 1984,23,1811.
Molybdenum(ZZI), (W>and (v)
1373
367. W. E. Newton, J. W. McDonald, K. Yamanouchi and J. H.Enemark, Inorg. Chem., 1979,18,1621. 368. B. Spivak, 2.Don and E.I. Stiefel, Inorg. Nucl. Chem. Lett, 1975, 11, 501. 369. J. Huneke and J. H. Enemark, Znorg. Chem., 1978,17,3698. 370. A. Muller, R. G. Bhattacharyya, N. Mohan and B. Pfefferkom, 2.Anorg. Allg. Chem., 1979, 454, 118. 371. K. S . Nagaraja and M. R. Udupa, Polyhedron, 1985, 4, 649. 372. K. J. Moynihan, P. M. Boorman, J. M. Ball, V. 0. Patel and K. A. Kerr, Acta CrystalZogr., Sect. B, 1982, 38, 2258. 373. S. Lu, M. Shang, W.Cheng, M. He and J. Huang, Fenzi Kexueyu Huuxue Yanjiu, 1984,4,435 (Chem. Abstr., 1985,102, 16 516). 374. A. Muller, W. Reinsch-Vogell, E. Krickemeyer and H. Bogge, Angew. Chem., Int. Ed. Engl., 1982,21,7%. 375. W, Clegg, N. Mohan, A. Miiller, A. Neumann, W. Rimer and G. M. Sheldrick, Inorg. Chem., 19S0, 19,2066; W. Clegg, G. M. Sheldrick, C. D. Garner and G. Christou, Acta Crystallogr., Sect. B, 1980, 36, 2784. 376. X. Xin, N. L. Morris, G. B. Jameson and M. T. Pope, Inorg. Chem., 1985, 24,3482. 377. L. Huang, B. Wang and X. Wu, Huuxue Tongbao, 1984, 15 (Chem. Ash.., 1985,102, 124459). 378. W.-H. Pan, M. A. Harmer, T. R. Halbert and E. I. Stiefel, J. Am. Chem. Soc., 1984,106,459. 379. W. Clegg, G. Christou, C. D. darner and G. M.Sheldrick, Inorg. Chem., 1981,20, 1562. 380. M. Draganjac, E. 0.Simhon, L. T. Chan, M. Kanatzidis, €4. C. Baenziger and D. Coucouvanis, Znurg. Chem., 1982,21,3321. 381. S . A. Cohen and E. I. Stiefel, Inorg. Chem., 1985,24,4657. 382. G. Bunzey, J. H. Enemark, J. K. Howie and D. T. Sawyer, J . Am. Chem. SOC.,1977,9!3,4168; G. Bunzey and J. H. Enemark, Itwrg. Chem., 1978, 17, 682. 383. M. Rakowski DuBois, Inorg. Chem., 1978, 17, 2405. 384. K. Mennemann and R. Mattes, J. Chem. Res. ( S ) , 1979, 100. 385. J. Beck, W. Hiller, E. Schweda and J. Striihle, Z. Naturforsch., Teil B, 1984,39, 1110. 386. B. M. Gatehouse, E. K. Nunn, J. E. Guerchais and R. Kergoat, Inorg. Nucl. Chem. Lert., 1976,12, 23. 387. B. 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Chem. SOC.Jpn., 1980, 53, 1288. 399. K. F. Miller, A. E. Bruce,J. L. Corbin, S. Wherland and E. I. Stiefel, J. Am. Chem. SOC., 1980,102,5102. 400. V. R. Ott, D. S. Sweiter and F. A. Schultz, Inorg. Chem., 1977,16,2538. 401. M. Chaudhury, J. Chem. SOC., Dalton Trans., 1983, 857. 402. M. C. Chakravorti and D. Bandyapadhyay, Synth. React. Inorg. Met.-Org. Chem., 1984,14,443. 403. L. M. Charney and F. A. Schultz, Inorg. Chem., 1980,19, 1527. 404. K. S. Nagaraja and M.R. Udupa, Transition Met. Chem. (Weinhim, Ger.), 1983, 8, 191. 405. P. J. Baricelli, M. G. B. Drew and P. C. H. Mitchell, Acta Crystullogr., Sect. C, 1983,39, 843. 406. T. C. Hsieh, K. Gebreyes and J. Zubieta, Transition Met. Chem. (Weinheim, Ger.), 1985, 10, 81. 407. X. Jin, B. Bai and Y . Tang,Jiegou Huuxue, 1984, 3, 139 (Chem. Abstr., 1985,103,63 853). 408. F. A. Cotton and W. H. Ilsley, Inorg. Chim. Acta, 1982, 59, 213. 409. T. Shibahara, H. Kuroya, K. Matsumoto and S. Ooi, Bull. Chem. SOC. Jpn., 1983,56,2945. 410. D. L. 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SOC.Jpn., 1981,54,2457.
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Molybdenum
H. Kobayashi, I. Tujikawa, T. Shibahara and N. Uryu, Bull. Chem. SOC.Jpn., 1983, 56, 108. Y. Sasaki and A. G. Sykes, J . Chem. Soc., Dalton Trans., 1974, 1468. G. A, Zank, T. B. Rauchfuss and S. R.Wilson, J. Am. Chem. Soc., 1984,106,7621. T. Shibahara, S. Ooi and H. Kuroya, Bull. Chem. SOC.Jpn., 1982, 55, 3742. T. Shibahara and H. Kuroya, Inorg. Chim. Acta, 1981, 54, L75. D. J. Darensbourg, R. L. Grey and T. Delord, Inorg. Chim. Acta, 1985,98, L39. S. A. Koch and S . Lincoln, Inorg. Chem., 1982, 21, 2904. J. A. Beaver and M. G. B. Drew, I . Chem. Soc., Dalton Trans., 1973, 1376. M. F. Belicchi, G. G. Fava and C. Pelizzi, J. Chern. SOC., Dalton Tram., 1983, 65. 437. M. H.Chisholm, J. C. Huffman, C. C. Kirkpatrick, J. Leonelli and K.Folting, b. Am. Chem. SOC., 1981,103, 6093. 438. R. Maltes and K. Muhlsiepen, 2. Narurforsch., Teil B, 1980, 35, 265. 439. E. Carmona, F. Gonzalez, M.L. Poveda, J. M. Marin, J. L. Atwood and R. D. Rogers, J . Am. Chem. Soc., 1983, 105,3365. 440. B. Kamenar, B. Korpar-Colig and M. Penavic, J . Chem. Soc., Dalton Tram., 1981,311. 441. M. H. Chisholm, K. Folting, J. C. Hu€fman and C. C. Kirkpatrick, J. Chew. Soc., Chem. Commun., 1982, 189. 442. M. E. Noble, J. C. Huffman and R.A. D. Wentworth, Znorg. Chem., 1983,22,1756; M. E. Noble, K. Folting, J. C. Huffman and R. A. D. Wentworth, Inorg. Chem., 1984,23,631. 443. K. Yamanouchi, J. H. Enemark, J. W. McDonald and W. E. Newton, J. Am. Chem. Soc., 1977,99,3529. 444. J. R. Dilworth, B. D. Neaves, P. Dahlstrom, J. Hyde and J. A. Zubieta, Transition Met. Chem. (Weinheim, Ger.), 1982, 7 , 257. 445. I. W. Boyd, I. G. Dance, A. E. Landers and A. G. Wedd, Inorg. Chem., 1979,18, 1875. 446. I. G . Dance and A. E. Landers, Inorg. Chem., 1979,18,3487. 447. I. Buchanan, W. Clegg, C. D. Garner and G. M. Sheldrick, Inorg. Chem., 1983,22,3657. 448. 3. R. Bradbury, A. G. Wedd and A. M. Bond, J . Chem. SOC., Chem. C o m w . , 1979,1022. 449. J. I. Gelder, J. fl. Enernark, G. Wolterrnan, A. Boston and G. P. Haight, J. Am. Chem. Soc., 1975,97, 1616. 450. J. T. Huneke, K. Yamanouchi and J. A. Enemark, Znorg. Chem., 1978, 17,3695. 451. T.-C. Hsieh, K. Gebreyes and J. Zubieta, 1. Chem. SOC.,Chem. Commun., 1984, 1172. 452. P. T. Bishop, J. R. Dilworth, J. Hutchinson and J. A. Zubieta, J . Chepa. SOC.,Chem. Commun., 1982, 1052.
428. 429. 430. 431. 432. 433. 434, 435. 436.
36.5 Molybdenum(V1) EDWARD 1. STIEFEL Exxon Research and Engineering Company, Annandale, NJ, USA ~~
36.5.1 INTRODUCTION
1375
36.5.2 MOLYBDATE AND RELATED SPECIES 36.5.2.1 Structure of Moa--containing Compounds 36.5.2.2 Thio and Seleno Deriuariues of Moo:36.5.2.3 Spectra and Bonding in the Moot- and Related Ions 36.5.2.4 Reactions of Moo:-
1376 1376 1377 1378 1379
36.5.3 COMPLEXES CONTAMING THE MOO, CORE
1379
36.5.4 COMPLEXES WITH THE MOO:+ CORES 36.5.4.1 Structures Containing cis MOO:+ Cores 36.5.4.2 Spectra and Characterizationof Complexes with MOO$+Complexes 36.5.4.3 Preparation and Scope of Mo@? Core Complexes 36.5.4.4 Reactiuiv of Complexes with the MOO:+ Core
1380 1381 1386 1388 1390
36.5.5 COMPLEXES WlTH MOO4* CORES
1392
36.5.6 NITRIDO COMPLEXES
1394
36.5.7 IMIDO
AND
ARYL- AND ALKYL,-IMIDO COMPLEXES
1396
36.5.8 HYDRAZID0(2-) COMPLEXES
1397
36.5.9 PEROXO COMPLEXES 36.5.9.1 Mononuclear Peroxo Complexes 36.5.9.2 Dinuclear Peroxo Complexes 36.5.9.3 Spectral Indicaiors 36.5.9.4 Reactivity Toward Organic Molecules
1398 1398 1402 1403 1403
36.5.10 HYDROXYLAMIDO(l-) AND HYDROXYLAMIDO(2-) COMPLEXES
1404
e BONDS
1405
36.5.12 DINUCLEAR COMPLEXES OF Mow
1408
36.5.13 COMPLSXES LACKING OXO-TYPE LIGANDS
1412
36.5.14 REFERENCES
1414
36.5.11 COMPOUNDS
36.5.1
WITH M
INTRODUCTION
The chemistry of hexavalent molybdenum is dominated by the oxo ligand and its analogs. This highest of Mo oxidation states requires for its stabilization the presence of ligands that are not only good a donors but also good n donors. Thereby, sufficient charge density can be placed on the Mo to avoid violating the Pauling Electroneutrality Principle.' This requires ligands with filled p orbitals that are not otherwise engaged in bonding with other atoms. Formally, this engenders a situation in which a multiple bond is formed between the Mo and its donor ligand. About 10 years ago' the oxo ligand was virtually alone in stabilizing large numbers of formally Mow complexes. This situation has changed dramatically in recent years. It is now clearly recognized that the s a d 0 ligand, S2-, can, in a s i m c a n t number of cases, isomorphously substitute for the oxo ligand, 02-.Moreover, in certain cases selenido (Se"), peroxido (OZ-), persulfido (SZ-), imido (NR2-), nitrido (N3-), alkylcarbido (RC3-, equivalent to alkylidyne), hydrazido(2-) (Rz"2-) and hydroxylamido (RzNO-) ions can form complexes with strict analogs in oxo chemistry. The oxo chemistry stands as a guidepost from which one can extrapolate to make predictions of the structural chemistry of its analog ligands. Mononuclear examples are dominant in the simple coordination chemistry of molybdenum(VI). This is due in part to the absence of d electrons in Mow complexes, which precludes the formation of MoV1-Mow metal-metal bonds. However, note must be taken of the very large class of homo- and hetero-polymolybdates which contain polynuclear Mo. These
1375
Molybdenum
1376
are discussed elsewhere in this treatise. A substantial number of dinuclear complexes of MoV1 are also known and these are discussed in this chapter. In presenting the chemistry of Mom we first discuss complexes in which oxo groups provide the sole n-donating ligands. We then discuss complexes in which sulfido, selenido, imido, nitndo, alkylcarbido, peroxido, disulfido, hydrazido and hydroxylamido can substitute in a direct way for oxo. Within the mononuclear class we make distinctions between complexes with four, three, two and one oxo ligands. Within the dinuclear class we distinguish between singly, doubly and triply bridged species. Finally, we discuss examples of non-oxo MoV' complexes. Aspects of MoV1chemistry have been considered in recent reviews.%'
36.53 MOLYBDATE AND RELATED SPECIES The tetrahedral molybdate ion is the key antecedent for much Mow chemistry. This ion serves as the starting point for many of the preparative schemes in the Chemistry of Mow and, directly or indirectly, often for low oxidation state chemistry as well. The molybdate ion is found naturally in alkaline solutions such as sea water (pH = 8.0-8.5). It is the form in which Mo is often added to culture media used for the growth of microorganisms. In solutions of moderate concentrations the mononuclear ion is only present above pH = 7.Below pH = 7, the equilibria in equations (1) and (2) have considerable effect and the heptamolybdate ion predominates at moderate concentrations in aqueous solution. Detailed treatment of equilibria in aqueous Mo"' solutions has been presenteda6Although the chemistry of polyoxoanions of Mo is discussed elsewhere in this volume, it is worth pointing out that the six-coordinated octahedral MOO;' and Moo4+ units can be viewed as the mononuclear building blocks of these polynuclear anions.' For example , PMolz02; has a central phosphate surrounded by an M o ~ ~unit O in ~ ~which each Mow has a single terminal oxo in its coordination s here. Similarly, Mo4012CH2020H3-can be pictured as an Mo4OI2ring containing dioxo Mo' with each Mo additionally bridged to all others by the OH- and CH20$- anions.7
+ 8H* e Mo,O& + 4H20 Mo7Oz + MOO:- + 4H' I Mo,OZ + 2H20 7Mo0:-
(1)
(2)
At low concentrations hexavalent molybdenum in dilute acidic solutions is likely mononuclear.' At higher acidity the ion Mo02(H20)$+ is probably present.' In special situations the dimolybdate ion, Mo208-, is formed whose structure is analogous to that of the better known dichromate ion, Cr20:-. (Dinuclear MeV' ions are discussed separately below.)
36.5.2.1
Structure of Moo;--containing Compounds
Molybdate, MOO%-,is isolated in the form of salts of monovalent, divalent and trivalent cations. The salts of the simple monovalent cations are usually water soluble while salts with larger cations, e.g. N-propylammonium, N-ethylpyridinium and tetra-n-butyl ammonium may also have solubility in non-aqueous solvent^.^ The salts of di- and tri-valent cations are generally insoluble and form three-dimensional structures in the solid state. As discussed below, although many of these maintain the MOO:- structural unit, some salts which stoichiometrically contain MOO:- have octahedral six-coordinate MeV'. In K2M004,10discrete MOO:- ions have an M o - 0 distance of 1.76%iand the 0-Mo-0 bond angles of a regular tetrahedron. This distance is among the longest for terminal oxo molybdenum linkages, henceforth designated Mo-0,. The (NH4)2Mo04salt is isomorphous to K&'f004.10 The MOO$- ion can act as a ligand to many metal ions. In some cases, as with higher valent metal ions containing labile coordination spheres, all or most of the original ligands around the second metal can be replaced by the 0-donor MOO:- leading to three-dimensional solid state structures. Recently studied exam les, among many,11*12include F ~ z ( M ~ O ~NiMo04,15 )~,~~,~~ MgMo0.,.Hz0:6 KDy(MoO,),,Q Tb2(Mo04)2>8 Me S ~ M O O ~ ;MnMo0,.H20m ~ and C ~ T ~ ( M O OIn~ )the ~ . last ~ ~ six compounds the MOO:- ion maintains an approximately tetrahedral four-coordination and performs a bridging role. However, in other cases these polymeric structures have six coordinate Mow, e.g. NiMoO4.lS
Molybdenum(VI)
1377
In cases in which the full coordination sphere of the second metal is not accessible to substitution, MOO$- can serve as a mono- or bi-dentate ligand. Examples of monodentate MOO$-include CO(NH~)~MOO: 22 and Cr(edta)MoO:-.l9 An example of bidentate MOO:- is A rare example of bridging MOO$- in a non-polymeric structure involves the CO(NH~)~MOO,‘. Dm symmetry K(Mo04)K unit which is isolated in a matrix of frozen N2.23X-Ray structural studies are notably lacking for compounds containing MOO:- as a ligand. One exception is the homopolymolybdate ion M O ~ O ~ ~ ( M Owhich O ~ ) ~contains two monodentate MOOS- u n h X The tetranuclear complex anion M O ~ O ~ ( O M ~ ) ~ ( N N Palso ~ ) $contains bridging bidentate MOO:- ligands.= The structure, shown in Figure 1, is roughly centrosymmetric in its inner coordination sphere. In this complex two Mo(NNPh)%+units are bridged by two transmethoxy groups and by two MOO!- groups trans to each other.% The non-molybdate Mo-Mo distance of 3.46A indicates the absence of metal-metal bonding. The terminal M e , distance of 1.72 com ares with 1.8OA for the bridging Mo-Ob. The near-tetrahedral angles !show that two Moo4 -like ions are ~ r e s e n t .This ~ ~ brief discussion of MOO:- as a ligand contrasts with the extensive structural chemistry of MoS$- as a ligand discussed elsewhere in this volume.
Figure 1 The struchue of Mo,O,(OMe),(NNPh):-
in its HNEti- salt (unlabelled atoms are carbon)=
36.5.2.2 Thio and Seleno Derivatives of MOOSThere are a significant number of strict analogs of MOO$- that contain oxo-analog li an& in place of 0 while maintaining a tetrahedral or pseudotetrahedral structure about Mod For example, the treatment of basic solutions of molybdate with sulfide leads successively to thio substituted anions according to Scheme 1. MOO:-
- - - Mo03SZ-
~00,s;-
Mo03S2-
MoS:-
Scheme 1
Although the chemistry of these conversions had been known for many years, recent thermodynamic and kinetic work” has provided quantitative information. Similarly, treatment with Se2- leads to MoSei- 23-30 while treatment with H202 gives the, Mo(0,)z- ion discussed below. The compound MoOZCIZis also well known and, although containing six-coordinate Mo in the solid state, has a four-coordinate approximate tetrahedral structure in solution or in the gas phase. Tetrathiomolybdate has been grepared with a number of organic cations including NEtZ ,31 NC&; (pyrrolidinium) ,3 *34 N2C*r1 (piperazinium), NC5H& ( p i p e r i d i n i ~ m ) ~ , ~ ~ and N4C&113 (monoprotonated hexamethylene imine) These salts are useful for structural .33732
Molybdenum
1378
and preparative studies. The structure of the MoSi- ion was determined in its tetraethylamangle of 107.5(9)" are monium salt.35 The average M o 4 distance of 2.177(6) and S-M& found in a near perfect tetrahedral arrangement of sulfur atoms about the molybdenum. The ion MOOS:- in CszMoOS336 has M o - 0 = 1.78 and M o S = 2.18 A, values that are distances' found in MOO$- and MoSZ-, indistinguishable from the M M and Mo-S respectively. Further, the angles in MOOS:- are all within 1" of the 109.5" tetrahedral angle. This identity indicates that M o - 0 and Mo-S bonds are similar in their electronic and steric effects on the Mo"' center (barring, of course, fortuitous cancellation of steric and electronic factors). Reaction of Mo20;- with (Me3Si)*Sleads to the series Mo03-,S,(OSiMe3)- (x = 0-3) with the x = 3 member of the series cleanly isolated. The anions are tetrahedral, derived from MOO:-. MoOs(OSiMe3)- has M-0 = 1.881(7), 0-5 = 1.631(8) and Si-0-Mo = 146.6(4)' and Mo-S averaging 2.154.37 Clearly OSi(Me); can substitute for oxo in an isostructural series.37
36.53.3
Spectra and Bonding in the MOO$- and Related Ions
In the UV region MOO:- displays two band systems, one in the range 42000-46000cm-1 and a second, more intense system, in the range 46000-53000cm-*. In single crystal studies (e.g. MOO:- in &SO4) both bands show vibrational structure.38 The progression observed with v1 = 780 cm-l corresponds to v(Mo-0,) for the excited state compared to y1 = 897 cm-' for the ground state. The bonding in the 4d0 tetrahedral molybdate and related systems has been treated by molecular orbital appro ache^.^',^,^' An approximate energy level diagram in Td symmetry is shown in Figure 2. The oxo (or thio or seleno) ligands are involved in both CT and n bonding with Mo. The difference between the 3tz and 2e* levels represents the At value for the system. Since there are no d electrons, this splitting is not discernible from d-d spectral transitions. It can, however, be estimated from the difference in energy of charge-transfer bands or X-ray absorptions, provided they can be properly assigned. Mo
4t 2
S (or 0 or Be)
Figure 2 Qualitative energy level diagram for Tdions such as MOO:- and MoS:-
The assignment of charge-transfer spectra in tetrahedral oxoanions and their tetrathio derivatives has been a controversial area. Electronic spectra have been studied in some The assignments of 'charge-transfer' bands for MoSi- are likely to be t1-+ 2e*(v1), The difference between vz and v1 is the At 2t2+2e*(v2), tl+3t;(v3) and 2f2+41:(v3).41 value. Assignments differ somewhat for other ions. The assignments are supported by detailed electronic spectral studies,44valence photoelectron s e ~ t r o s c o p yand ~ ~MCD data.46 Interestingly, for MoOf the theoretical studies4? show that little 0- Mo charge transfer occurs in going from the ground to excited states. It is nevertheless clear that the configuration
Molybdenum(VI)
1379
changes from 4d0 to 4 8 due to excitation of an electron from mostly ligand-based into predominantly metal-based levels. However, electronic rearrangement in the remaining levels makes the net charge transfer negligible. It is therefore more appropriate to call these electron-transfer transitions as a guide to the configurational nature of the transition, since they do not necessarily reflect large net redistribution of charge. The IR and Raman spectra of the tetrahedral molybdates, tetrathiomolybdates and tetraselenomolybdates have also received stud^.^,^^^ For the Td, tetrahedral dianion MOO$-, the two stretching modes (Al and F2) are Raman active and one (F2) is IR active as well. Bending modes are of E and FZsymmetry, and while both are Raman active, only F2 is IR active. Bands at 897 and 837 ern-' have been assigned as stretches, whereas those at 325 and 317cm-' are assigned to bends. Detailed vibrational analyses of MOO$- have included 92Mo/100Moisotopic substitution e f f e ~ t s l ~ and , ~ ~polarized IR measurements.l7 Studies on Mo03S2- and MOO&- reveal the ~(Mo-0) bands expected for C3" and C, structures, re~pectively.~~ When MOO$- coordinates to a metal ion, as in [Co(NH3)5Mo04]Clor [CO(NH,)~MOO~]NO,, the IR and Raman spectra of the MOO$- grouping are perturbed.22 An instructive example considers the coordination isomers [ C O ( N H ~ ) ~ C ~ ] Mand O O[ ~C O ( N H ~ ) ~ M O O ~ ] C ~ . ~ ~ 36.5.2.4
Reactions of MOO:-
In alkaline solutions the exchange reaction of equation (3) occurs in hydroxide-independent and hydroxide-dependent paths.54 Rate constants, activation parameters and isotope effects have been reported.54 Although proton transfer is implicated in the rate-determining step of the OH- -independent path and an association involving MOO$- and OH- is likely for the OH--dependent pathway, no firm mechanistic conclusions can be drawn.
+ 180H, -+
MOO:-
Mol*O@-
+ 160Hz
(3)
The kinetics of reactions of MOO$- leading to the binuclear complexes CO(NH~)~OMOO; and Cr(edta)OMoO:- have been studied around pH = 7.55Interestingly, the reaction mechanisms for the formation of these two species differ. The Co complex is formed by equation (4) where Mo-0 bond breakage occurs. The Cr complex is formed as in equation (5) where the uncoordinated carboxylate of the edta4- ligand apparently plays a key role in labilizing the H,O ligand on the otherwise normally inert CrJIKlinkage.
+
CO(NH~)~OH:*Mo0Z2-
Cr(edta)H,O*-
+ MOO:-
--
Co(NH,),OMoO;
Cr(edta)MoO:-
+ H,O*
+ H,O*
(4)
(5)
These species are of interest as they represent examples of Mo bridged to a first transition row element. This is a suspected occurrence in nitrogenase where Mo bridged to Fe (through 0 or S ) is implicated.'
36.5.3
COMPLEXES CONTAINING THE M003 CORE
The Moo3 structural unit is found in a few monomeric oxo Mom complexes. The complex Mo03(dien) has a six-coordinate octahedral s t r u c t ~ r e ~with ~ , ~ the ' cis-trioxo arrangement, an Mo-0 distance averaging 1.74A, 0-Mo-0 angles averaging 106" and Mo-N averaging 2.32 A. Mo03(dien) shows little stability in s ~ l u t i o n . ~ A ' , ~similar ~ six-coordinate monomeric structure with M 4 , = 1.74 A is found in the M00,(nta)~- ion in K3[Mo03(nta)]-H20.59 Here, as shown in Figure 3, one of the nta3- carboxylate arms is uncoordinated. Interestingly, the NMR spectrum shows that the coordinated and free carboxylate arms of nta3- exchange rapidly.60 A related nominally dinuclear complex is the long studied edta complex found in Na4(Mo03)2edta+!H20.61 Here the two Moo3 groups are connected only by a - N C H 2 C H ~ bridge. A six-coordinate structure with Mo--0 = 1.74 8, makes the Moo3 coordination similar to that in the nta3- complex. Several amino acid complexes appear by spectroscopic criteria to have Moo3 core structures.62
1380
Molybdenum
x W
Figure 3 The structure of MoO,(nta),- in K3[MoO3(nta)]-HzOs9
The Moo3 unit is characterized by vibrational bands at -890 and &40cm-1.57,63," 170and g5MoNMR spectra may also be used to identify the Moo3 core. In fact, 170NMR is sensitive to the nature of the group trans to oxob6 as is also the case in MOO%+ complexes.65 In addition to the above structurally characterized complexes there are other cases where the presence of the Moo3 unit seems possible on stoichiometric grounds. Reaction of K2Mo04
roi
with O==C(CF2)3C==0(perfluoro lutaric anhydride) leads to Naz(Mo03[02C(CFz)3C02]) which is likely to be polynuclear.6.g A band at 945cm-' due to M M , and bands at lower energy possibly attributable to M A b make the presence of the Moo3 unit in this complex unlikely. Likewise, the stoichiometry M00~(Hars)'-,~' where Hars is alizarin red 4 (9,lOdihydro-3,4-dihydroxy-9,10-dioxo-2-anthracene sulfonic acid), does not necessarily implicate a cir-Mo03 core. Interestingly the Mo enzyme formate dehydrogenase shows an EXAFS pattern that has been interpreted in terms of approximately three Mo-0 = 1.74 A distances indicating the likely presence of an Moo3 unit bound to non-sulfur ligands.69 Of late there are also reports of molybdate-like monoanions, namely ClMoO,, R3SiOMoOF and MeMoO;, which contain an Moo3 grouping and a single monodentate monoanonic ligand. ClMoO; is prepared7' as the Fh4Asf or Bh4P' salts by reacting Moo3 with [Ph&]Cl or stretching vibrations occur at 930 and 894 ~ m - l . ~ ' [PbPICl, respectively. The Mo-0 R3SiOMoO; (with R = Ph or t-C4H9)is prepared7' by the reaction of equation (6). The crystal structure of the NBu$+ salt of Ph3SiOMo0F shows the expected tetrahedral (C,) structure and v(M0-0) stretching frequencies are identified at 906 and 884~m-'.~' The complex anion MeMoO; prepared from Mo02Me3r(bipy) by base hydrolysis has only been characterized in solution.72 It shows a single 'HNMR peak at 0.611 p.p.m, downfield from TMS." All of the XMoO: ions are colorless. 2R3SiOH+ MozO:2R,SiOMoO; + 2H20 (6)
-
36.5.4 COMPLEXES WITH THE MOO:+ CORE STRUCTURE Complexes containing the MOO$+core are by far the most common in Mow chemistry. These complexes are usually six-coordinate although some four-coordinate compounds are known. The latter are tetrahedral or pseudotetrahedral and related to MOO$-. Examples include Moo2$-, Mo02Se$-, Mo02Br2and Mo02(R2NO),. An example of a five-coordinate complex now exists.73 The vast majority of M o a f core complexes are six-coordinate and mostly of distorted octahedral structure. Some non-octahedral complexes are known. In the octahedral structure the two Mo-Ot bonds are invariably cis to each other. The strong u- and n-donor nature of the oxo ligands makes it favorable for them to avoid competing for the same p and d orbitals. If the oxo groups were trans they would be forced to share two d orbitals and one p orbital. By residing on adjacent coordination sites they are forced to share only a single d orbital.
Molybdenum(VZ)
1381
Repulsion between the short M-0, bonds and between these and other bonds determines remaining details of geomet Several articles have addressed structural trends in oxomolybdenum(V1) complexes. An MOO$+unit is likely to be present in the oxidized form of sulfite oxidase and in the desulfo form of xanthine dehydrogenase.20 The reactivity of the MOO$+ unit has been thoroughly studied, especially with regard to oxygen atom transfer reactions.
T?'i-?*
36.5.4.1
Structures Containing cis MOO:+ Cores
The MOO:+ unit has by far received the most attention in the structural chemistry of Mow. There have been over 50 X-ray crystallographic structure determinations of six-coordinate complexes containing this structural unit. All but five of these determinations have revealed a near-octahedral structure with ck dioxo groups. The structural results in Table 1 clearly show the narrow range within which both the Mo-0, distances and the 0,-Mo-0, angles lie. The five exceptions shown at the bottom of the table are complexes in which a sterically demanding substituted amine thiolate ligand or a porphyrin ligand forbids the molybdenum from having the otherwise highly favored cis-octahedral structure, In addition to the mononuclear structures listed in Table 1, there are several dinuclear complexes discussed below and many polymeric structures (see e.g. ref. 79) which also have MOO$+cores with dimensions similar to those found in the mononuclear complexes. Table 1 StNctural Studies on MOO:+ Complexes" Complex
Disunce Mo-0
(A)
1.695(8), 1.697(8) 1.71(2), I.62(2) 1.72(2), 1.&(2) 1.66,1.70 MoO,(oxine), 1.71 MoO,(Et,dtc), 1.703(2) MoO,(Pedtc), 1.695(5) 1.696(5) 1.77 K2[Mo02(02C6H4)21*2H20 1.75(4),1.80(4) MoO,[H( OCH,CH,) 3N] 1.77(4), 1.83(4) 1.643(17), 1.826(18) MoO,(bipy)Br, 1.68(2), 1.73(1) K~[MoO~F,I*H~O 1.695(1), 1.673(1) MoO,C12[OPPh3], 1.69(1), 1.73(1) MoOzBr,[OPPh3], 1.68(1) MoO,Cl,(DhIF), 1.708(9) (Cs),[MoO,(HmW 1.63(4), 1.73(2) MoOZ(OCHZCH20CH2CHZO) 1.723(3),1.694(3) MoOZ[C,H3N-2,6( CH.,$T&S)J(C~H&30) 1.702(3), 1.708(3) MoOz[C6H3N-2,6(CH,CPh@)J [Me,SO] 1.688(7) MoO,(OC&CHNMe), Moo,( uramil-N,N-diacetata)4iT20 1.689(5) M o o , [ O C , ~ C H N C ~ ~ e ~ C ~ C ~ ~ O 1.714(4) ] 1.701(4) 1.695(3) MoO,Cl,(phen) 1.68(1), 1.68(1) MoO,(NCS),(HMPA), 1.67(1), 1.69(1) 1.686(8) MO02C12(HMPA)Z 1.686(8) 1.667(11) MoO,C1,(MeOCH,CH,OMe)z 1.673(10) 1.7U(10), 1.729(10) MoO,(HOCH,CHZO), 1.704(10), 1.737(11) 1.687(2) MoO,CI,[(P~O),P(O)CH,C(O)~~] 1.677(2) MoO,C~,(H~O),~~[~&N~Q-] 1.701(8) MOO,[OC,J&NCHMCCH~NC&O] 1.709(3) 1.710(3) 1.682(8) Mo0,[OC6H4CHNMe12 1.706(5) MOO,[SC(C&-p -Me)NMeOI2 1.713(5)
MoOz(PhCOCHCOPh)2
MoO,(acac),
Angle 0-Mo-0
(")
Ref.
104.8(4) 111.1(9) 104.3(8) 105 104 105.8(1) 105.7(1)
92 1%
102
199 200
103.3 95Q) 103.17(7) 103.2(5) 102.2(7) 104.5(6) lOl(3) 106.0(2) 105.4(2) 107.7(4) 106.6 103.3
201 202 74 74 203 204 205 176 79 206 207 208
106.8(2) 101.4(5) 100.9(5) 102.46(46)
209
105.0(5)
36
107.6(5) 104.6(5 ) 10218( 1)
213
103.0(5) 103.2(1)
35
107.7(4) 103.7(2)
216 22
197 198
80 296
210 211
214
215
1382
Molybdenum Table 1 (continued)
Complex
Distance Mo-0
(A)
Angle 0-Mo-0
MoO,[ONMeC(N-Bu')O], MOO,[OCMeN(C6H4p-OEt)O] MOO,[ OCMeN(Ph)O],
1.726 1.703(2) 1.704(3) 1.692(3) 1.671(3) MoO,C1,(9,l0-phenanthroquinone) 1.671(3) Mo02[OC6H,(Buc)CHNCH,CH,NCH~H3(Bd)O] 1.706(10), 1.718(9) 1.703(9),1.708(10) 1.706(10), 1.718(9) M o O ~ ( P ~ ) ~ H ~ N C W , C HPr')O] ~NC~~~( 1.70 M~o2(~c6&~H&.)," 1.714(4) Moo&-CysOMe), 1.711(3), 1.720(4) MoO,[(S)-PenOMe], 1.714(4), 1.703(3) 1.693(9),1.684(7) MOO,[SCH2CH,NMeCH2CH,NMeCHzCH2Sl 1.677(9), 1.750(10) MoO,[SCH,CH,NMeCH,CHzCH~NMeCH~CH~S] 1.704(6) 1.708(3),1.715(3) MOO,[ SC6H,NHCH~CH,NHC&S] 1.721(3), 1.718(3) MOO,[SCH2CH,NHCH,CH,SCH,CH2S] M ~ O ~ S ~ ~1.69(2), ~1.70(2) ~ 1.705(5), 1.699(2) MOO,[(SCH~CH&NCH~CH&HZNMG]
Mo02[(SCH,CH2),NCH,CH2SMe]
a
~
(")
Ref.
103.94(11) 105.2(2)
212 154 154
104.8(2)
129
109.2(6) 109.2(5 ) 109.2 103.5(2) 108.1(3) 107.0(2) 107.6(2) 109.1(5) 107.8(7) 107.8(3) 109.9(1) 106.0(1) 111 ~.0(1) ~ 101.6(2)
190 94 109 217 218 219 219 219 219 219 ~81 220
221
1.694(5), 1.694(5) 1.709(6), 1.703(6) 1.688(5), 1.705(2) 1.714(2), 1.710(2) 1.705(3), 1.705(3) 1.711(5), 1.723(5)
101.8(2) 100.8(2) 100.1(2) 109.8(1) 106.7(2) 122.2(3)
1.731(1), 1.727 1.720(3) 1.69(6), 1.696(6) 1.709(9) 1.744(9)
120.8(1) 122.0(2) 110.5(2) 95.1(4)
220
221 85 85
84 94 84 84
79 91
The ligand is 2-o-hydroxyphenylbenzimidazole.
Representative structures are shown in Figures 4-9. In addition to the characteristic M o - 0 distance and 0-Mo-0 angle, there are several other features that the octahedral complexes in Table 1 have in common. The bond trans to the Mo-0 linkage experiences a structural rrm influence and is significantly longer than corresponding bonds to the same donor type that are cis to the oxo. For example in the complex M ~ O ~ ( E t ~ d t cshown ) ~ * * in Figure 4, which has two-fold symmetry, the McF-!~ distance trans to Mo-0 is 2.639 A, whereas that cis to oxo is 2,450 A. This effect is clearly attributable to the competition for the (T and n orbitals along the shared axis between the oxo group and the sulfur atom tram to it. The complex Mo02L with L = -SC&SCHZCH2SC6H4S- has the typical distorted octahedral structure" with the two thioether donors of the tetradentate ligand lying trans to M-0, at Mo-S = 2.69 and the two thiolate donors tram to each other with Mo-S = 2.41 A. The longest known thioether M H distance is 2.79 8, foundz3in the complex of the tripodal ligand N(CH2CH2S)2(CH2CH2SMe)2shown in Figure 5 . The dimensions of this complex closely parallel those found by EXAFS for sulfite oxidase.= In complexes where there is a choice, the general rule is that the weaker n-bonding donor atoms are found trans to M o - 0 , since they do not compete for the empty p and d orbitals along the same axis.74,80~83y&1,85 For example in Mo02C12[OPPh,], the n-donating chloride ligands are found trans to each other and cis to the M A , bonds. Likewise, in Mo02[(SCHzCH&NCH2CHJW~]the thiolate sulfur atoms are found trans to each other and c h to the terminal oxo groups.41After this preference has been exercised the trends in bond angles can be rationalized qualitatively in terms of interligand repulsions in the Mo coordination ~ p h e r e . ~ ~ . ~ ~ ~ ~ , ~ ~ There are several complexes now known to possess a cis dioxo structure and to be nonsctahedral in nature. In a number of these the coordination number about Mo is lower.
~
Molybdenum (VI)
1383
Figure 4 The structure of Mo0,(Et,dtc)280
Figore 5 The structure of MOO,[N(CH~CH~S)~CH~CH~SM~]~~
For example, four-coordinate MOO&- and Mo02C12 have tetrahedral structures related to MOO:-. In some cases there ma be formal six-coordination as in the complexes of deprotonated hydroxylamine ligands.s9 These complexes have, in addition to two oxo ligands, two deprotonated hydroxylamine ligands (RzNO-) in the Mo coordination sphere. The hydroxylamine(-H) ligands can be considered as bidentate N,O donors, in which case the complexes are formalIy six-coordinate. Alternatively, the complexes may be considered as four-coordinate with the hydroxylamine(-H) ligands occupying two of the tetrahedral sites about the Mo atom. If the center of the N-0 bond is taken as representing the donor site for the ligand, a geometry very near to tetrahedral is indeed obtained. The monodentate diatomic ligand formulation seems a structurally more consistent way of thinking about the deprotonated hydroxylamine ligands. As illustrated in Figure 6, if the structure of MoOz[EtzNO]z is considered to be six-coordinate then it is distinctly nonangle of 113.7' is significantly larger than the angles found in octahedral. The 0 - M o - 0 the octahedral complexes in Table 1. The N and 0 atoms of the ligands and the M o atom lie in a plane with the two oxo ligands roughly equidistant above and below that plane. The structure superficially resembles the skew trapezoidal bipyramid structure that is described more fully below. However, in the present case the trapezoid defined by the N202 ligand set does not contain the Mo atom within its boundary. The structure is therefore distinctly different from the skew trapezoidal structure discussed below and is really best represented as being tetrahedral with hydroxylamine(-H) ligands occupying two tetrahedral binding sites. Additional complexes containing hydroxylamine(-H) ligands are discussed separately below.
B
Molybdenum
1384
1 C
0
0
0
Figure 6 The structure of MOO,[E~NO],~
There is one example73of a five-coordinated Mo"' complex wherein the atypical coordination number is enforced by sterically bulky ligands. This ~trncture'~shown in Figure 7 is describable as a trigonal bipyramid in which two S-donor atoms of the tridentate ligand occupy the apical sites. The oxo groups and pyridine nitro en of the tridentate ligand define the trigonal plane. Interestingly, the oxygen a n a l ~ g ~ ~of, ~this ' sulfur donor ligand forms a conventional near-octahedral complex with MeOH or DMSO as the sixth ligand donor. Apparently, the difference in steric bulk and/or donor ability of two sulfur donors compared to two oxygen donors in the tridentate ligand is sufficient to cause the observed change in coordination tendency. n
Figure 7 The structure of MoO2[NCSH3(CPh2S),](unlabelled atom are carbon)73
There are two types of non-octahedral unequivocally six-coordinate formally Mow complexes. The first type involves a complex of the ligand tetra@-t~lyl)porphyrin.~lHere the structure shown in Figure 8 reveals the molybdenum atom to possess a distorted trigonal prismatic coordination in which the two oxo groups are found on the same side of a highly deformed porphyrin 'plane'. The 0-Mo-0 angle of 95.1" is the smallest known for any high resolution crystallographic determination of a cis-MoO$+ core. The simple alternative of having the oxo groups on opposite sides of the porphyrin ring is apparently not viable as this geometry is only known and expected for the 4d2 configuration of Mow. The Mo atom in MoO,(ttp) is fully 1.095A above the mean plane of the saddle-shaped porphyrin ligand. That the structure is stable, despite the fact that the small O,-Mo-O, angle must entail a significant oxo-oxo repulsion, is a testimony to the proclivity of Mow to adopt the cis dioxo configuration.
Molybdenum(VI)
1385
Figure 8 The structure of MoO,(ttp) (unlabtlled atoms are carbon)g1
The second type of six-coordinate complex in which a distinctly non-octahedral structure is adopted involves N-substituted cysteamine ligands. When N is not substituted, a complex such as MoO2[NH2CMe2CH2SI2forms and adopts the expected octahedral structures5 shown in Figure 9. In contrast, as illustrated in Figure 10, a skew trapezoidal bipyramidal structurew is found to be p r e ~ e n t ~ , for ’ ~ ,M ~ ~O O ~ [ M ~ ~ N C H ~ C M The ~ ~two S]~ N-. and two S-donor atoms form an approximate plane in which the Mo atom also lies. The oxo donors lie equidistant above and below this plane. The non-octahedral nature of the complex is seen by looking for the atom that is expected to be trans to oxo in the octahedral structure. Indeed, the largest angle about the oxo ligand is 122’ and this is the 0 - M e 0 angle. This is the largest O,-Mo-O, angle which has been found in oxo Mow complexes and it raises the interesting question as to whether these complexes are best formulated as containing Mow. Indeed, it has been suggested that the observed short S - - S contact of -2.74A may represent the partial formation of a sulfur-sulfur (i.e. disulfide) bond. If a full disulfide bond had been formed, the complex would be formulated as one of MoIVwith oxidized ligands. Such a complex wouid be
Figure 9 The structure of MOO,[NH,CM~~CH,S]~~
Molybdenum
1386
expected to have a trans dioxo structure with 0,-Mo-0, = 180". However, the observed 'bond' length may indicate partial reduction of Mo. This would be consistent with the observed angle being increased from the roughly 105" expected to the larger value actually observed.
W
Figore 10 The structure of M O O ~ [ M ~ ~ N C H ~ C M ~ ~ S ] ~ ~
Clearly there is a universal tendency for dioxo Mo"' complexes to adopt a cis configuration and there is a strong disposition toward octahedral structures. The distortions within these structures are clearly attributable to interligand repulsions. However, recent work reveals that appropriately rigid or sterically demanding ligands can enforce lower coordination numbers or non-octahedral coordination geometry on the Mo s here.73 In molybdoenzymes there is strong evidence for the presence of an MOO$+ While octahedral coordination must be considered as a candidate structure for these enzymes, non-octahedral structures and lower coordination numbers cannot be eliminated. Indeed, proteins are capable of acting as extraordinarily sterically demanding ligands. 36.5.4.2
Spectra and Characterization of Complexes with MOO$+Cores
The MOO$+group manifests itself in the infrared and/or Raman spectrum in the form of an intense two-band pattern corresponding to the symmetric and asymmetric Mo-0 stretching vibrations. Values for representative complexes are shown in Table 2 which also displays the lowest visible absorption bands for those complexes where these are available. The infrared (or Raman) absorptions are by far the most characteristic feature by which to identify the presence of the MOO:+ unit. Several studies9s98 of IR and Raman spectroscopy have concentrated on systematics of the MOO$+core vibrations. Substitution of "0 for l60is useful in confirming assignments and calculating forces constants.'6398The frequency shifts between complexes are dependent upon the donor ability of the remaining li ands, the geometry of the complex and the participation of the Mo-0, linkage in H-bonding.' Often the IR spectrum along with elemental analysis data is sufficient to identify the MOO$+core. Proton and carbon NMR data (see, for example, refs. 99-102) are often useful in a complementary way in establishing the disposition of the remaining non-oxo ligands. NMR can also be useful to establish the liability or fluxionality of the coordinated ligands. 100*lM ' ' 0 NMR spectroscopy of oxomolybdenum complexes can ive valuable structural information.10s1os Since the sensitivity of the quadrupolar I = 2 $0 nucleus is inherently low, enrichment with "0 is required to obtain signals in acceptable time periods. Apparently, 1 7 0 can be incorporated by exchange into already formed MOO&+ core c o m p l e ~ e sThe . ~ ~ rates of exchange, although not quantitatively measured, appear to be qualitatively different for M+O, groups in somewhat different ligand environments, i.e. with different trans groups.65In certain skew-trapezoidal bipyramidal structureslM "0 NMR was used to support the maintenance in solution of the structure found by single crystal X-ray diffraction in the solid.84The Mo-0, resonance occurs at 850-1000p.p.m. downfield from H 2 0 making it readily distinguishable from bridging (-400) and ligand oxygen "MoNMR has been applied to MOO$+core complexes. The chemical shifts range over A comprehensive review of WOO p.p.m. and display potentially useful ligand %MoNMR summarizes key trends."' Detailed electronic spectral studies for MoOzC12 and Mo02Br2 have been performed (ref.
P
Molybdenum (VZ 3
1381
TnMe 2 Vibrational and Electronic Spectra of MOO;* Core Complexes
Complex MoO,Cl[OPPh,], MoO,Br,[ OPPh,], MOO,Cl,(blPY) Mo02Cl,(DMF)2 (C~H~)MOO~C~ (NEt4)Mo0,CI,( acac) MoO,(acac), MoO,(oxine), MoO,(N-MeSal), MoO,(salen) MoO,(Cys-OMe), Na,[MoO,(Cys),]~DMF MoO,(Me,dtc),
2
IR or Raman ( R v(Mo-0,) (cm- ) 947,905 944,903 935,906 939,905 920,887 931,897 935,905 925 (R), 898 (R) 926.899 922,904 920,885 912,884 922,892 909,875 905,877 909,877 925,890 925,890 891,866
907,872 MoO,(SCMe,CH,NM%),
893,879
MOO,(SC,H4SCH2C&SC&S) MOO,[(SCH,CH,)zNCH,CH,NMe,l
910,885 921,893
MOO,[(SCH,CH,),NCH,CH,SMe]
921,891
A (nm)
3M 270 370 250 242 351 306 377 290 252 380 395
375 371 354 272 246 230 355 260 228 327 278 391 325 287 236
366 350 326
286 248 910,877 910 (R), 880 (R) MoO,(CysOMe), 912,884 930,910 Mo0,(OC6H4NHCH2CH2NHC&O) Mo0,[OC,H4CHNHC(0)~H4N~H20 922,898 932,908 Mo0,[OC,H4CHNHC(0)C,HJV]DMF 910,894 MOO,[OC6H4CHNHC(O)C,HdN]DMSO Moo,[ ON (Et)COMe], 935,900 930,900 Mo02Cl(9-ane) 925,890 (NEt3JMoO@%I 964,925 NaZ[MoOzC14]
E (cm-')
31 200 37 000
E
(M-' cm-')
3200 22 OOO
27 OOO
R@. 74 74 222 223 224 225 225 225 226
39 100 41 400 28 500 32 700 26 500 34 500 39 700 26 320 25 320 26 700 26 950 28 250 36 760 40 650 43 480 28 170 38 460 43 860 30 500 35 970
5300 12 OOO 3770 6300 15 OOO 3700 1020
25 600 30 800 34 800 42 400 27 300 28 600 30 600 35
2930 sh 4430 9530
sh
137 138 227 227 228 226 228 228 229
3270
229
4Ooo
106 227
4200 4400
5100 5600
7200 8OOO 3200 3980
106 106
124 65
65
ooo
mooo
96
MoO,(Et,dtc),
351 322
28 500 31 100
6130 4570
274
36 500
19 OOO
417 227 189 97 97 97 153 192 157
1W 230
Na,[Mo02F4] . H 2 0
948,912 951 (R), 920 (R) 960,922 1007,987 1009,987 994,972
MoO,Br, (gas)
991,969
231 310 226 306 259 225 296 252
235 972,950
32 300 33 200 32 680 38 610 44440 33 780 39 680 42 500
5OOO 7000
230 157 157 157 118
157 118
157
1388
Molybdenum
118 and refs. therein) and interpreted by SCF-Xa-SW cal~ulations.~'~ However, the electronic spectra of the six-coordinate species, which are the major group discussed in this section, have not received much attention. EXAFS studies12' have shown that the MOO$+ core is readily identifiable in a variety of known compounds. These have been used12' to calibrate the EXAFS technique which was subsequently used to identify this core in sulfite oxidase and in desulfo xanthine oxidase.82
36.5.4.3
Preparation and Scope of MOO$+Core Complexes
The method of choice for the preparation of many dioxo Mo" complexes involves the use of MoO,(acac), which is easily prepared from MOO$- and acacH by adjusting the pH.121,122 Thermodynamic data for the formation of M ~ O ~ ( a c ahave c ) ~ been presented. '23 The reaction of equation (7)lU is typical and proceeds readily in THF. Alcohols, especially MeOH, are also much used in related syntheses with base (often a tertiary amine) sometimes used to help ionize the incoming ligand. In the absence of base, the free acacH (b.p. = 139 "C) formed can be readily distilled under partial vacuum from the reaction solution along with the solvent.
r-
MoO,(acac), 4
-
cs '1 SH HS LHz
MoO,L
+ 2acacH
(7)
Alternative preparative approaches, while not systematically directed toward MOO$+ complexes can nevertheless be useful. For example, oxidation of Mo(C0)4(bipy) with BrZ in &H50H-CH2C12 yields Mo02Br,(bi y) .26 The corresponding dichloride can be prepared by ligand exchange in boiling acetone." The complexes Mo02X(9-ane) (X = C1, Br; 9-ane = 1,4,7-trimethyl-l,4,7-triazanonane) are also prepared by oxidation of the low valent complex M0(C0)~(9-ane)with the appropriate halogen and subsequent h y d r~l y s i s . '~~ M 0 0 ~ ( R ~ d t complexes c)~ can be prepared from the M o ' ~complexes, M 0 0 ( R ~ d t c ) As ~ . the latter are often formed from the former, the route is not syntheticall useful. However, since the oxygen atom donor can be DMSO (forming dimethyl ~ u l f i d e ) ~ ~route ~ ~ ~is~reminiscent ~the of the enzymic reduction of biotin sulfoxide.' Other oxidants capable of effectin the conversion include N-oxides such as pyridine N-oxide or N-hexylmorpholine N-oxide12' and oxygen128*129 but not NZO13' as originally ~ 1 a i m e d . l ~ ~ Certain classes of ligand deserve special mention as their MOO:+ complexes have received particular attention. The I,l-dithiolate ligands, especially N,N-disubstituted dithiocarbamates, have been very thoroughly s t ~ d i e d . ' ~ ' - 'Evidence ~~ for the existence of the dithiophosphate complex M o O ~ [ S ~ P ( O E in ~ ) ~solution ]~ has been ~b t ai n ed . ~' This and a number of other dithiolate complexes may be unstable toward internal redox in which the S ligand serves as the reductant to produce a lower valent M-S complex. A related ligand is the ferrocenecarbodithioate ligand which behaves similarly to dithiocarbamates in forming an isolable series of MoIV, MoV and Mow c ~ m p l e x e s . ' ~The ~,~~~ M O O ~ ( S ~ C C ~ H ~ Fcomplex ~C~H~ shows ) ~ v(M-0) at 920 and 889cm-' and gives a quasireversible reduction wave with El,*of -0.89 V us. SCE attributable to Mo reduction and a quasireversible oxidation wave at +0.95 V vs. SCE attributable to ligand (ferrocene) ~xidation.'~~,~~~ The interesting ligand (1)'36has two potential chelating modes upon ionization. Depending on its tautomeric state, either N,S or S,S coordination allows it to act as a bidentate chelating ligand. Apparently, it acts as a 2-aminocyclopent-l-ene-l-carbodithioate, i.e. as a 1,ldithiolate, to form M 0 0 2 L complexes.'36
Molybdenum(VI)
1389
Complexes containing Schiff base chelating ligands have been extensively studied. 137-*44 Here the ligands include bidentate, tridentate and tetradentate representatives where 0 or S donors supplement the imine nitrogen. Examples are listed in Tables 2 and 3. Sample ligands include (2), (3) and (4).
The bidentate ligands (L) form MOO& complexes while the tetradentate ligands form complexes such as Mo02(Salen). The tridentate ligands (L') form complexes Mo02L'(Y) where Y can be a variety of 0-, N- or S-donor ligands, often the solvent from which the complex is isolated. Extensive studies have probed the effect of substitution on the aromatic ring, substitution for the hydrogen of the Schiff base carbon, and the length of the group pendent (or bridging) on N, the Schiff base n i t r ~ g e n . ~ -Mixed ~ ~ $ -Fe, ~ ~Mo ~ complexes with Schiff base ligands have been reportedla but not structurally characterized. The complex anion, MoOz(cat)Z- (where cat = catecholate = benzene-1,Zdiolate has been extensively studied as the starting point for a series of complexes in the MO"', Mol and Mo"' oxidation states.149*'51,150 Mo02(cat)s- in slightly acidic aqueous solutions undergoes sequential electrochemical one-electron and two-electron reductions leading to penta- and tri-valent Mo. 149 In slightly alkaline solution four oxidation states are electrochemically ac~essible.'~' Upon reduction the number of oxo groups per Mo decreases leading to reactivit and lability which may be related to the function of oxomolybdenum centers in enzymes.' Y Complexes containing 3,5-di-fert-butylcatecholatebehave in a somewhat different fashion15' involving an isolated dinuclear Mom species. The biologically active compounds glycine,62hydroxamate ,153 p h e n a ~ e t i n and ' ~ ~ acetanilid154 each form M o 0 2 L complexes. While these complexes help define the ways in which these organic molecules may serve as ligands, they have, at present, no obvious biological relevance. Several heterocyclic N-oxides also form MOO$+complexes. 155 Neutral complexes are known of the form Mo02X2 where X=F,156 C1,lls Br,lI8 I,157 OSiBu;,l5' R2N08-' and rne~ity1.l~'These corn lexes contain the MOO:+ core and two monodentate iigands. The halo are mononuclear in the gas phase and dior poly-nuclear as solids or in solution. The - - O S ~ B U \and ~ ~ ~-C&Me259 complexes are apparently mononuclear. The siloxy compound'58 is made by the direct reaction of tris(tbutylsilanol) with Moo3, presumably according to equation (8).
J
2Bu;SiOH
+ MOO,
-
M 0 0 ~ [ 0 S i B u ~+]HzO ~
(8)
The mesityl complex159 is prepared by the reaction of the Grignard reagent and Mo02C12(THF)2in THF presumably according to Scheme 2.
Scheme 2
These complexes likely have approximate tetrahedral structures and resemble MOO:- rather than the largely octahedral Moo2+complexes discussed in this section. The series of complexes M002X2 ( X = F , C1, Br) can be used to prepare Mo02XzL, complexes where L is py2, bipy, or other ligands. Examples of these complexes, which are formally adducts of M002X2, are given in Table 1. The structurally characterized complexes have the a-donor halides trans to each other and cis to the Mo-0, bonds.
1390 36S.4.4
Molybdenum
Reactivity of Complexes with the MOO$+Core
The dioxo core is susceptible to redox reactions which lead to reduction of the Mo center in conjunction with oxo remova1,131,1M),161,162,163,164 Schematically, the core reaction in equation (9) gives the Mo"' complex. The reactant and product of this reaction, however, can combine with comproportionation as in equation (10) to give the dinuclear Mo" core. IV
VI
Iv VI Moo2++ MOO;'
-
v Mo,Oi'
A general treatment has been published of oxygen atom transfer kinetics for com lexes wherein the dimerization/comproportionation reaction is a complicating feature. This treatment critically summarizes all pertinent earlier information. The assumption that dimerization is very fast with respect to oxygen atom transfer is born out by the agreement of the observed and calculated kinetic parameters. The oxo removal reaction can be effected by a variety of trialkyl and triaryl group V reactants. For example, the reactivity of M ~ O , ( E t ~ d t cwith ) ~ regard to oxo removal is reported'& to qualitatively follow the order Ph3P >> Et3N = Ph3As > Ph3Sb > Ph3Bi> Ph&. For phosphines, the rates of reaction follow the expected order of nu~leophilicity~~~ PEt3 > PEtzPh > PEtPhz > PPh,. As far as the li and is concerned, toward oxidation of either PPh3 or PhNHNHPh, the order of reactivity is16B S2CNEt2> thiooxinate > Cys-OMe. The oxidizing ability of MOO$' complexes is enhanced by the presence of S-donor ligands.'63 The reaction of M 0 0 ~ ( R ~ d t c(R) ~= Et, Pr" and Bu') with PPh3 and other phosphines has been thoroughly studied.167The reaction is first order in both complex and phosphine which, along with the large negative entropy of activation, fits a simple bimolecular rnechani~m.'~~ Similar studies have been performed on MoO,(C~S-OE~)~ with similar results. The general equation (ll),(with the previously mentioned comproportionation as a complication) is clearly well e s t a b l i ~ h e d . ~It ~ 'has , ~ ~potential ~ analytical applications for the determination of PPh3 by monitoring the intensely colored M 0 ~ 0 ~ ( E t ~ d formed t c ) ~ in the comproportionation of equation (12).
--
Mov'O,L + PR, OPR3+ MoNOL, MoOz[Etzdtc]z + MoO[EtZdtc]z MozOJEtzdtcIz
Kinetics of the reaction of certain MoOZLcomplexes (L = S-containing salicylaldehyde Schiff base) with PEtPh, also show the reactions to be first order in complex and in phosphine,'4s147.168 The rate constants depend upon the substituent on the aromatic (salicylaldehyde derived) ring and correlate with reduction potentials of the complexes.145 The kinetics of the equilibrium reaction (13) was studied by concentration-jump relaxation techniques1689169 for L = SzCNEtz, SSeCNEtz, SezCNEt2, S2P(OEt), and S2PPhz. The diethyl diselenocarbarnate complex is found to be the most highly dissociated and the rate constant for its dissociation is the highest. The diphenyldithiophosphinate complex is the most stable and slowest to dissociate. 169~168
The porphyrin complex MoO,(tpp) also reacts with phosphines to form phosphine oxideslm or with secondary alcohols to produce ket011es.l~' The latter reaction appears to occur by one-electron (radical) steps showing the versatile reactivity of the Mo-oxo-porphyrin grouping.171 A one-electron oxidation of Mo02($p) forms the radical cation which readily undergoes oxygen atom transfer to yield the Mo complex M00(tpp)+.'~' The reaction has some features in common with those of cytochrome P45O.l7' Reaction of Mo02(MeHN0)z with heterocumulenes such as alkyl or aryl isocyanate
1391
Molybdenum( VZ)
yieldsz6by equation (14) an adduct with a five-membered chelating ring. Related 'heterocumulenes' such as SCN- and CS2 yield similar structures (see below). MoO,[MeHNO],
+ R NC O
-
,O+
MoOf
C
,NHR
\kO/'\Me
1,
(14)
Reaction of MoO~(Et2dtc)~ with C6H11NC0 forms the dinuclear M 0 ~ 0 ~ ( E t ~ dcomplex. tc)~ However, MoOz(Et2dtc)z or M ~ O ~ ( a c a yields c ) ~ with ArNSO the oxo arylimido complexes MoO(NAr)(EtZdtc):! and MoO(NR)(acac)z, respectively.130 The former compound has also been prepared by the reaction of MoO(Etzdtc)z with ArN3.173 The reactions163 of equations (15) and (16) ilhstrate the substitution of a hydrazide(2-) ligand for an oxo ligand. Similar reactivity is seen for the complex MoOz(SCH2CH2NMeCH2CH2NMeCHZCHzS) = MoOzL which yields MoO(N2Rz)L and M O ( N ~ ~ ) ~ LInterestingly, .~'~ the related complex, Mo02(SCH2CH2NMeCH2CH2CH2NMeCH2CH2S)= Mo02L' reacts only with benzoylhydrazines to give MoO(PhCONNH),L', a possible seven-coordinate Moo4+ core complex. 174
Thermodynamic considerations indicate that the oxygen atom transfer reactions between Mo02(R2dtc)2 and enzyme substrates should be largely irre~ersib1e.l~~ For example, the hypothetical reaction of equation (17) has A H = -28.5 f 5.5 kcal mol-' (= -119 f 23.0 kJ mol-'). The reaction should have a small AS and hence should have a negative AG and proceed spontane~usly.~~' There are, however, complicating factors.
+
Mo02(EtZdtc), MeC HO
-
MoO(Et,dtc),
+ Me C0,H
(17)
The formal transfer of an oxygen atom is one way of describing the function of the Mo site in molybd~enzymes.~ The formation of dinuclear reduction products is a complication that causes difficulty in trying to model the mononuclear This difficylty can be overcome by the use of sterically demanding ligands that prevent the formation of the dinuclear c 0 m p l e x , 7 ~ For ~ ~ example, ~ ~ ~ ~ the ~ ~cycle ~ ~ ~shown * ~ ~in ~Scheme 3 can be effected without dimerization. Further, in this case DMSO and the enzyme substrate biotin sulfoxide, can serve as the oxo donor to form the Mo"' dioxo complex during the catalytic The MoV' complex involved is discussed structurally above (Figure 7). Ph, ,Ph
Ph
Ph
Ph
Ph
-
'i (l Ph Ph Mov'
MeSMe
A
[biotin] [ s 2 2 e
MOT"
1
Scheme 3
MOO,(C~S-OE~)~ and other dioxo-MoV' complexes have also been studied as reactants toward enzyme substrate^."^^'^^,^^'^^^^ The original report of aldehyde oxidation to a carbox lic acid mimicking the action of the enzyme aldehyde oxidase17*has not been substantiated, 17Zl81 Ligand, solvent and photochemical reactions are occurring179 rather than the simple oxo transfer reaction postulated for the enzyme. Aldehydes react with Mo02(Cys-OEt), to form thiazolidines. 179 Reaction of aldehydes with Mo02(acac)z or M ~ O ~ ( E t ~ drequire t c ) ~ photochemical initiation and chlorinated solvents (CH2C12, CHC13) and are clearly not related to
Molybdenum
1392
enzymic activity. Photolysis of M o O , ( E t , d t ~ ) or ~ ~Mo02(dibenzoylmethanato)21sz ~~ leads to yet uncharacterized but interesting reduced Mo-containing products. MoOz(Et2dtc)2 is reported to catalyze the reduction of NO, to N20 by HC02H in DMF.lS3 This reaction involves the reduction of Mo02(Et2dtc), by HC02H, giving M ~ O ( E t ~ d t c )a~reaction ,'~ that may be similar to the one catalyzed by formate dehydrogenase. MoOz(Cys-OMe),, Mo02(S2CNEt,) and other complexes are reportedw to catalyze the air oxidation of benzoin to benzil. The complex M O O ~ ( C ~ S - O(R R )= ~ Me, Et, Pr' and CH2Ph) catalyzes the O2oxidation'28,18qof PPh3 in DMF b a mechanistically involved process in which excess H 2 0 prevents the catalytic redox cycle.184,IZ The electrochemistry of MOO:' core complexes has been extensively s t ~ d i e d . ~ ~ ' ~ ~ , ' ~ ~ * ~Virtually ~ ~ , ~ ~all~ compounds , ~ ~ ~ , ~ studied show irreversible electrochemistry. However, one complex, MoO2(SCHzCH2NMeCH2CH2NMeCH2CH2S), displays a reversible, one-electron reduction ~ a v e . ' ~ ~The , ' ~ one-electron reduction has been characterized by EPR spectroscopy as MoVO2H(SCH2CHZNMeCH2CH2NMeCH2CH2S)-. This MoV complex displays proton superhyperfine splitting reminiscent of that found in xanthine oxidase.'" The formation of MOO$+complexes and their substitution, ligand exchange and rearrangement (fluxionality) reactions have been reviewed.195 3~14991747186193
36.5.5
COMPLEXES WITH MOO"+ CORES
The Moo4+ core structure is found in Mo"' chemistry in a relatively small number of authenticated structures and in a few suggested examples shown in Table 3. The simplest set of complexes which potentially contains the Moo4+ core is MoO&(X=F,Cl,Br . While a square pyramidal structure seems probable for the chloro and bromo co~nplexes,Z~~ the fluoro complex is polymeric in the solid state and in solution as Table 3 Complexes Containing Moo4+ Cores Complex
n ( M 0 - 0 , ) (cn-')
MoOF", MoOCI, MoOBr, MoO&(Pr"dtc), MoOF,(E~dtc), MoOCI,(Et,dtc), MoOBr,(Et,dtc), Moo (C~HIONO)Z(O~C.~H~) MoOCl,(acda), MOO(abt)(acda), MOO(MeNHO),[MeN( O)C(S)S] MOO(PhCONNH)[ SCH,CH2NMeCH,CH,CH,NMeCH,S] [MoO(Et,dtc),]BF, MoO(MeNHO),[HNC( S)NMeO
C~[M00(~,),(~C(0)C(0)0)1 a
1030 1015 998 917 945 947 960 920 955 940 928 890 935 926
Mo-4
(A)
1.65(11)"
1.701(4) 1.701(4) 1.68(6) 1.689(6) 1.677(8) 1.691(3) 1.660(8)
Ref. 232 232 232 240 246 246 246 247 136 136 244 174 239,238 87 87 248
Polymeric solid.
The reaction of Mo02(R,dtc), (R = Me, Et, Pr") with HF, HCl or HB?'& eliminates H20 and gives M 0 0 F ~ ( R ~ d t cMoOC12(R2dtc), )~, and MoOBr,(R2dtc),, respectively. The structure of MoOCl2(Pr;dtc),, shown in Figure 11, reveals a seven-coordinate complex. An approximate pentagonal bipyramid is formed with an apical oxo and a truns chloro ligand. The four sulfur donors of the R2dtc- ligands and the second chloride ligand form the equatorial plane. An alternative route to M00Cl~(R,dtc)~ involves oxidative addition of Clz to M 0 0 ( R ~ d t c ) , .The ~~~ ligand C5H&H2CS, = acda- 237 displays similar with BF;, PF;, Cloy, The cation MoO(Rzdtc); has been isolated in a number of MqO&- and Mo60?G as counterions. The structure of the M%O&- salt shows that the cation has a seven-coordinate pentagonal bipyramidal structureu8 with terminal oxo in an apical position. Proton NMR indicates this structure to be present in solutionu9 although variable temperature work indicates significant fluxionality. The complex cation can be
Molybdenum (VI)
1393
Figure 11 The structure of MoOCl,(Prgdtc)240
prepared from MoOZ(R2dtc), and a hydrohalic acid. These reactions, although obviously complex, can give >50% yield of product with the remainder of the Mo presumably ending up as Mo60;C. The characteristic wo-0 stretching band appears at -935cm-' in the IR spectrum for MoO(Et2dtc);. The blue complexes MoOSz(R2dtc)z have been prepared by several r o ~ t e s . ' ~ The '~~ structure shown in Figure 12 again reveals a pentagonal bipyramid with Si- in the equatorial plane, one RZdtc- ligand in the equatorial plane and the second Rzdtc- ligand spanning axial and equatorial positions.
Figure 12 The structure of MoOSz(Pr;dtc),240
--
Among the reactions which lead to the MoOS2(RZdtc)2complex are those of equations (18) to (21). MoO(R,dtc), + S, MOOSz(RzdtC)z (18)" MoOZ(R2dtc)z + H2S MoOS,(Rzdtc)z (19)2r(03241 S S
II
II
M002Si- + RzNC--S-S-CNRZ Mo,03(R2dtc)4+ Na,S,
+ MoOSz(R2dtc)z
M00S,(R,dtc)~
The Mo'" complex M 0 0 ( R ~ d t c )is~ a potential source of new Moo4+ complexes. Thus, reaction with S8 gives MoOSz(Rzdtc)z"o while reaction with Clz gives M 0 0 C l ~ ( R ~ d t c ) ~ . ~ ~ ~ Reaction with MeOZCCkCCOzMe yields the acetylene adduct MoO(R2dtc)Z(MeO2CCCCO2Me)which, upon protonation with CF3C02€€,gives the complex MoC)(R2dtc),(MeC0zC=CHC0zMe)(02CCF3) with coordinated t r ~ u o r ~ a c e t a according t e ~ ~ ~ to equation (22).
MOO(R~~~C)~(M~O~CCCCO,M~), + CF3COZH-MoO(R,dtc),(MeOzCC-CHCOzMe)(02CCF~) (22)
Complexes containing a single oxo and hydrazido(2-) ligand are discussed separately below. The reaction of Mo02(MeNHO)Z in the presence of the hydroxylamine ligand and CS2 proceeds according to Scheme 4.2 The resultant formally seven-coordinate complex has a distorted pentagonal bipyramidal structure with the lone oxo group in the apical position. The five equatorial positions are occupied by two formally bidentate 0,Nhydroxylamido ligands of
Molybdenum
1394
the starting material and the sulfur atom of the N-methyl-N-oxodithiocarbamate ligand. The angle of oxygen of this ligand binds trans to M w O t in the apical position with the O,-Mo-O 162.1'. As discussed below other heterocumulenes such as RCN, SCN- and OCS are also reactive toward hydroxylamine to form related ligands and complexes.M Me MoO,(MeNHO),
+
\
N-OH
i
H
+ CS2
-
-
N'
MOO
Scheme 4
"Mo NMR spectra of a representative sampling of Moo4+ complexes containing R2dtcligands reveal a relatively narrow distribution (55 to 183p.p.m.) of chemical shifts. The complexes MoO(R,dtc),+ fall in the range 55-80p.p.m. A combination of IR, 95M0 and "0 NMR spectra should prove powerful in characterizing Mo04+-containing complexes in the future. There is one reported exam le in MoV1chemistry of a MoS4+ and MoSe4+ core. These are presumably found in M ~ s F ~ ~ M~ oa Sn ~d F ~The ? ~band ~ at 564 cm-' in the former compound is assigned to the Mo-S 36.5.6
NITRIDO COMPLEXES
The nitrido ligand N3- is isoelectronic with the oxo ligand 0'-. Known nitrido complexes contain a single nitrido ligand and are often structurally related to analogous oxo complexes. However, unlike the situation for oxo complexes, there are as yet no examples of dinitrido or trinitrido complexes. The majority of known nitrido complexes are mononuclear and can be five-, six- or seven-coordinate. Such complexes possess square pyramidal, distorted octahedral or pentagonal bipyramidal structures, respectively. Structural results are given in Table 4 where the Mo-N distance can be seen to be extremely short, even shorter than Mo-0 distances. This is due to stronger multiple bonding between Mo and N compared to 0. For similar reasons ~(Mo-N) is usually above 1000 cm-', whereas v(M-0,) is usually found below lo00 cm-l. The ability of the N3- ligand to a- and n-bond and hence donate very strongly to Mo may be the reason for the absence of multinitrido or oxo-nitrido complexes of Mow. Table 4 Nitrido Mo"' Complexes Complex
M e N (A) 1.662(7) 1.642(2) 1.643(3) 1.630(6) [1,83(4)1 1.66(4) 1.637(4) 1.63(2) 1.641(9) 1.638(11) 1.659(5) [2.150(5)]
~ ( M o - N ) (cm-')
Ref.
1030
253 260 261 252,253 253 249,251 250 263 264 265 266 255 261 254 268 269
948 1010 1020
1040 [%91 1054
1060 1058 1019 1014 1042
1025 1.65(1) 1.66(1) 2.12(2) 2.14(2)
257 259
Molybdenum (V I )
1395
Structurally, the Mo nitride bond exerts a significant tram influence. In the extreme this leads to empty sites tram to Mo-N, such as in MoNCI, or M O N ( N ~ ) ~ In ( ~the ~ )octahedral .~~~ complex, M ~ N ( N ~ ) ~ ( b ithe p y )nitrogen of the bipy ligand trans to Mo-N, is 2.42 A while the N of the same ligand cis to Mo-N, is 2.24 A.251In the seven-coordinate pentagonal bipyramidal complex MoN(Me&c), the S trans to Mo-0, has M0-S = 2.85 8, while S in the same ligand cis to Mo-N (in the equatorial plane) has Mo-S = 2.51 A. Clearly, Mo-N, has a structural trans effect comparable to or greater than that of oxo in analogous complexes. The most common preparative route for MoV' nitrido complexes involves the reaction of azides such as CINJ or Me3SiN3 with lower valent Mo complexes.252 The resultant azido complexes are often unstable and eliminate N2 to form the nitrido complex as shown in equations (23) and (24) or equation (25). MoCI,
MoCl,(MeCN),
---
+ ClN,
MoClSN3
+ Me,SiN, + bipy
+ hC12 + Nz + C1, MoNC13(bipy) + N, + 2MeCN + Me3SiC1 MoCl,N,
MoNC13
(23)
(24) (25)
Once the nitrido corntiexes are formed thev are reactive toward ligand substitution as in equations $26) and (27): Many complexes ha;e been prepared e x p l o h g this substitutional reactivity,2 1-257 MoNCl, MoNBr,
+ bipy
+ H,tpp
-
+ C1[MoN(tpp)+]Br; + H2 MoNCl,(bipy)
The nitrido nitrogen is reactive nucleophilically leading in some cases to di- or tetra-nuclear complexes in which nitrido binds to a second molybdenum as a weak ligand. For example, the structure of [ M O N C ~ ~ ( O P Cis~ illustrated ~)]~ in Figure 13. The complex MoN(R2dtc), reacts with S8 or propylene sulfide to give the thionitrosyl complex M ~ N s ( R ~ d t c ) ~ . ~ ~ *
Reaction of MoN(Et,dtc), with NH20S03H (in an attempt to make a hydrazido complex) yields an unusual trinuclear species.259 The diamagnetic tricationic brown complex [ M ~ N ( E t ~ d t c ) ~ ] ~ M o ( E tformed259 ~ d t c ) ~ has been shown by X-ray studies to have two bridging nitrido ligands such that the central Mo is eight-coordinate (dodecahedral) while the terminal Mo atoms have pentagonal bipyramidal seven-coordinate structures. The Mo-N distances to terminal Mo atoms are comparable to those in MoN(Me2dtch at 1.65(1) and 1.66(1) but the
Molybdenum
1396
distances to the central Mo are significantly longer at 2.12 and 2.14A. The bridging is reminiscent of that found in [MoNC13J4and [MoNC13(OPC13)J4. 36.5.7
IMIDO AND ARYL- AND ALKYL-IMIDO COMPLEXES
Imido (NH2-) and arylimido (NR2-) complexes have been prepared for Mo"' as well as for lower oxidation states of Mo. The alternative formulation for the ligand is that of a nitrene wherein the ligand is neutral. This, of course, makes the formal oxidation state of the metal two units lower for each ligand compared to the corresponding arylimido formulation. We prefer the arylimido formulation as the resultant complexes all have analogs in the chemistry of oxo Mo"' species and do not have analogs in lower oxidation state Mo chemistry. The lone structurally characterized imido complex is MOO(NH)C~~(OPE~P~~)~.~~~ Here the oxo and imido ligands are cis to each other with Mo-0 = 1.66 and Mo-N = 1.70A, respectively. The N-H hydrogen has been located and the Mo-N-H angle is 157'. The orange-yellow complex was prepared by the reaction of MoOC1, with Me3SiN3followed by the addition of the tertiary phosphine oxide.270 The arylimido complexes are prepared by the reaction of an arylazide with an appropriate Mo starting material. For example, the reaction of M O ( C O ) ~ ( R ~with ~ ~ Cphenyl ) ~ azide2'l yields the complex Mo(NPh)2(RZdtc)zwhose structure is shown in Figure 14. The structure is clearly analogous to that of Mo02(R2dtc)2with cis phenylimido ligands in place of the cis oxo ligands. The two phenylimido ligands differ significantly in their M e N - C angles and M-N bond lengths. These correspond to 139.4" and 1.789A, respectively, for one ligand and 169.4' and 1.754 A, respectively, for the other. Interestingly, the phenylimido ligand does not appear to exert a significant trans influence in this or other complexes. The reaction of MOO Rzdtc), These with phenyl azide gives the mixed oxo-phenylimido complex M00(NPh)(R2dtc)~.~~ complexes contain two oxo-equivalent ligands (Le. one oxo and one arylimido) and approximate octahedral coordination geometries. In contrast to these complexes is the p-tolylimido complex M O ( N - ~ - C ~ & M ~ ) C ~ ( Twhich H F ) has ~ ~ ~a single oxo-analog ligand and an octahedral six-coordination.
1
,
Figure 14 The structure of Mo(NPh),(R,dtc),
(unlabelled atoms are carbon)"'
Other aryl imido complexes seem to resemble more closely the seven-coordinate pentagonal bipyramidal complexes with a single oxo-equivalent ligand (in this case an arylimido or alkylimido ligand). The complex cation Mo(NMe)(Rzdtc); n4 is closely related both to the nitrido complex MoN(R2dtc)s and to the oxo complex MoO(R2dtc);. As anticipated, the Mo-N bond distance is, at 1.73A, significantly longer in the arylimido compared to either the corresponding nitrido or oxo complex. The complex Mo(NPh)Cl2(R2dt~)2"~*~'~ has the pentagonal bipyramidal structure found for its oxo analog MoOC12(R2dt~)2.~~
Molybdenum (VI) 36.5.8
1397
HYDRAZID0(2-) COMPLEXES
Hydrazine is one of the suggested intermediates on the pathway of the reduction of d i n i t r ~ g e nHydrazine .~~~ can serve as a conventional ligand using the lone pairs on each N atom as donors to a transition meta1. However, hydrazine can also be deprotonated by one or two steps to produce monoanionic and dianionic ligands, respectively. The double deprotonation of N21& leads to the hydrazido(2-pn which, in its disubstituted manifestation R2N$-, is a ligand to a significant number of Mo complexes. A majority of these have exact analogs in the chemistry of simple oxo complexes and so hydrazido(2-) joins the ranks of ligands that are oxo-like in character. In a significant number of cases a hydrazido(2-) complex can be prepared directly from the corresponding oxo complex by simply refluxing the oxo complex and the corresponding hydrazine in an appropriate solvent. For example, reaction of Mo02(SCH2CH2NMeCH2CH2NMeCH2CH2S) with NH2NPh2 yields the h drazido(2-) complex MoO(N2Ph2)(SCH2CH2NMeCH2CH2NMeCH2CH2S) str~cture"~, 178 is shown in Figure 15. Note the placement of the hydrazido(2-) ligandwhose cis to angle of 105.9(1)". oxo, the relatively short Mo-N distance of 1.778(3), and the 0-Mo-N These features are reminiscent of the starting cis dioxo complex. The near linear Mo-N-N angle of 172.9(2)0 and the N-N distance of 1.309 reveals some multiple binding between the N atoms of the hydrazide(2-) ligand.277 The Mo-N distance at 2.464(3) trans to oxo and 2.359(3) trans to hydrazido(2-) reveal that the oxo group has a stronger trans bond lengthening influence than does the hydrazido(2-) ligand."7 In most cases the Mo-N-N angle is near linear. A compilation of key distances in hydrazido(2-) complexes is given in Table 5 .
Figure 15 The structure of MoO(N,fh,)(SCH~CH~NMeCH2CH2NMeCH,CH,S)(unlabelled atoms are carbon)zn
Table 5 Hydrazido(2-) Complexes of Mom Complex
{Mo(NNPhEt)[(CH,),dtc],)BPh4 Mo(NNMe,)(SCH,CH,PPhCH,CH,S),
[Mo(NNMePh)(NHNMePh)(Mqdtc),]BPh, Mo(NNPhz),(Me,dtc)z Mo(NNMePh),(Me,dtc), [MoCI(NNM~~)~(PP~~),]C~ [Mo("Mez)z(biPY
PhJ
)21 (B
MoO(NNMe,)(Me,dtc), MoO(NNMe2)(oxine),
Mo0(NNPh,)(SCW,C~,~eC~~~,NMeCH2C~,S), [PPhJ [MOO("Me,) (SPh)31 .Et,O
Mo-N(A) 1.715(16) 1.775(6) 1.752(10) 1.790(8) 1.790(9) 1.763(2) 1.752(5) 1.80(1) 1.79(1) 1.85(2) 1.800(9) 1.778(3)
1.821[9)
N-N(A) 1.37(2) 1.37 1.285(14) 1.31(1) 1.30( 1) 1.291(7) 1.276(8) 1.27(3) 1.28(2) 1.24(2) 1.28(1) 1.309(4) 1.292(14)
Mo-O(A)
Ref 280 281 279 282 282 282
282
1.671(9) 1.696(2) 1.705(8)
283 284 277 285
Molybdenum
1398
Reaction of the Mo" complex MoO(SPh); with H2NNMe2leads to M O O ( N ~ M ~ ~ ) ( S P ~ ) ~ This complex anion has a distorted square pyramidal structure in the [PPh4][MoO(N2Me2)(SPh),].Et,O salt with the oxo ligand in the apical position.285 The complex has an unusually small Mo-N-N angle of 152.5(10)" in contrast to the near linear = 169"-180" found in all other complexes in Table 5. linkage Mo-N-N The oxo hydrazido complex MoO(N2Me2)(oxine)2 reacts with Hx (X = C1, Br, SPh, 3cat, 4tdt) to give M~O(N~Me~)X~(oxine)~.~~~ This reactivity is analogous to that of the dioxo complexes reacting with HX to give the corresponding seven-coordinate oxo dihalogeno complexes.2M The bis[hydrazido(2-)] complex Mo(NNMePh)z(Mezdtc)Zreacts with one equivalent of HCI to produce the cationic complex Mo(NNMePh)(NHNMePh)(Me2dtc)z+which contains one near-linear hydrazido(2-) and one hydrazido(1-) ligand bound to Mo through both N atoms.279The side-on bound hydrazido(1-) ligand has Mo-N = 2.069(8) and 2.185(9) and N-N = 1.388(12) A. Other side-on bound hydrazido complexes are known.262
36.5.9 PEROXO COMPLEXES Early work on peroxo compounds of molybdenum has been reviewed.286The stoichiometrically simplest Mo peroxo complex is the red-brown Mo(Oz):- ion formed by the reaction of MOO$- with H202.Although the complex is not stable in solution and decomposes slowly with the evolution of 0 2 , the anion can be crystallized as the Zn(NH&* salt whose structure has been determined. In a sense, Mo(O& is an intermediate in the thermodynamically favored decomposition of hydrogen peroxide to water and oxygen. As discussed above, the peroxido ligand, Og-, is capable of substituting for the oxo ligand, 02-,such that there is no gross change in geometry of the resultant complex. There is no more striking example of this than Mo(Oz)i-. The structure of the anion in [Z~(NH,),][MO(O~)~] is shown in Figure 16. It consists of a DZddodecahedron of eight oxygen atoms about MonB7 However, if the centroid of each 0-0 bond is taken, the four points describe a geometric figure close to the tetrahedron, which is, of course, the geometry of MoOf-.
Figure 16 The structure of Mo(0,);-
in [Zn(NH,),][Mo(02),~'s7
A substantial number of Mow peroxo complexes has been structurally characterized. These complexes all have nearly symmetrically side-on-bound peroxide. Among them are mononuclear, dinuclear, tetranuclear and heptanuclear compounds. The tetranuclear and heptanuclear complexes are related to the is0 olymolybdates. Structural studies have identified &[M07022(02)2]*8HzO (M = 1.38 A),2si) K5[HM070~(0&020)6] (0-0 = 1.38 and I C q [ M 0 ~ 0 ~ ~ ( 0 ~ ) ~ ]= ( 01.48 - 0 A).289 The designation of coordination number in peroxo complexes is equivocal since a side-on-bound peroxide can be considered to occupy either one or two coordination sites. For the purpose of the following discussion we initially consider that 0:- occupies two coordination sites when bound side-on.
36.5.9.1 Mononuclear Peroxo Complexes
Mononuclear peroxo compounds of known crystal structure are listed in Table 6. Except for Mo(02)$- and M ~ ( O ~ ) ~ ( t tall p )structurally , characterized mononuclear peroxo complexes have one Mo-0, in addition to one or two peroxo ligands. The M o - 0 , bond lengths fall in the range 1.63 to 1.73A. The Mo-0 (peroxo) distances span from 1.83 to 1.96A. The peroxo ligand is always found cis to the oxo group and two peroxo ligands are always found cis to each I
Molybdenum( V I )
1399
other. In mixed oxo-peroxo complexes, the plane determined by the Mo-0-0 ring is nearly perpendicular to the Mo-0, direction. The overall coordination geometry is often describable as a pentagonal bipyramid with the oxo ligand in an apical position and peroxo groups in the equatorial plane. Table 6 Structural Details for Mononuclear Peroxo Complexes
Cornpiex
0-0
(A)
1.55(5)
Mo-Oberoxo)
(A) Mo-O(oxo) (A)
2.00(2) 1.93(3)
287
1.36(3) 1.44(3)
1.445(6) 1.47(2) 1.44(2) 1.464(1) 1.481(1) 1.46-1.48 1.482(12) 1.478(3) 1.4748 1.44(2) 1.459(6)
1.459(6) 1.212(5) 1.414(5)
Re$
301
1.93(2) 1.94(2) 1.900(5) 1.920(5) 1.95(2) 1.93(2) 1.947(1) 1.962(1) 1.943(1) 1.932(1) 1.92-1.% 1.926(9) 1.975(8) 1.929(5 ) 1.952(5) 1.952(5) 1.935 1.94( 1) 2.04(1) 1.948(4) 1.912(5) 1.908(4) 1.953(4) 1.935( 5 ) 1.830(3) 1.916(3)
1.64(3)
332
1.647(5)
290
1.68(1)
333,292
1.680(1)
238
1.673(1) 1.655(8)
wa
1.662(5)
308
1.66(1)
308
1.682(4)
344
1.671(5) 1.733(2) 1.663(3)
335
1.670(5)
337
293
300
336
1.927(3) 1.447(&)
1.907(5)
1.912(5) 1.399(6) a
294
1.958(4)
The ligand is citrate bound through a single carboxylate oxygen and alcohol. The chiral ligand is (S)-N,N-dimethyllactamide.
The simplest diperoxo structure is that of M o O ( O ~ ) ~ ( H ~ which O ) ~ cocrystallizes with 18-crown-6 and H20.290In this pentagonal bipyramidal structure shown in Fi ure 17 the H 2 0 ligands occupy equatorial and axial sites at M w O , = 2 . 3 2 5 and 2.084 , respectively. Interestingly, this product was formed from the reaction of Moo3, THF and oxygen. The source of the reducing equivalents necessary to form the peroxo complex was not specified but is likely to be the THF molecule.291Moo3 appears to serve as a free radical initiator causing THF and O2 to form a-hydroperoxytetrahydrofuran,which leads in acid to H202.291
1
Figure 17 The structure of MoO(O~)~(H,O),(0, is an oxygen atom from a water ligand)zw
1400
Molybdenum
The structure of MoO(O&(phen) is formally related to that of M o 0 (OZ )~(H~0by )2 substitution of two water ligands by phen. Again the oxo ligand occupies one of the apical positions of the pentagonal bipyramid and the peroxo ligands occupy four of the five positions in the equatorial plane. The phen ligand spans the axial-equatorial edge. The structure of the yellow M O O ( O ~ ) ~ ( C Z Ocomplex ~ ) ~ - was recently redeterminedzw and is shown in Figure 18. It is representative in having a distorted pentagonal bipyramid around Mo with an oxo ligand in the apical position and the peroxo ligands and one 0 donor of the bidentate oxalate bound in the equatorial plane. The Mo-0 stretch is identified at 970 cm-’ and -865 cm-’ approximates the 0-0 stretching vibrational frequency.
Figure 18 The structure of MOO(O~)~(C~O~)’in its Kt saltZn
Interestingly, M O O ( O ~ ) ~ ( C ~ can O ~ be ) ~ -prepared from an aqueous solution of Moo3 and H202 in which the only carbon source is malonate, -OZCCHzCO; or malate, -O2CCHOHCHZCO;, but not succinate -OzCCHZCHzCO;. The formation of the degraded two-carbon oxalate complex is proposedz9’ to require a five- or six-membered ring chelated intermediate for the oxidation of the organic ligand to oxalate to occur. However, the isolation and structure determination of the apparently stable Mo0(0z)z(citrate)2- complex293which has a five-membered chelate ring must be noted. The structures of Mo0(0z)z(Pro)(Hz0)U8 and Mo0(02)z(Gly)(Hz0)238also show the pentagonal bipyramidal geometry familiar to this class. The glycine is bound as a monodentate ligand through its carboxylate group in an equatorial position. The water ligand is found trans to the terminal oxo group. Although this is the first known amino acid peroxy molybdenum complex, its biological relevance is uncertain as Mo peroxy complexes are not known to play a biological role. The Mo-0, stretch is found at 970-978 while the 0-0 stretch appears in the 870-880 cm-’ region in the IR spectrum for these complexes. Despite the utility of the bidentate formulation of 0;- for descriptive purposes, structural systematics of peroxo complexes are nicely understood by considering the 0%-ligand as an isomorphous replacement for an oxo ligand. As discussed previously, if the four oxo groups in MOO$- are replaced by properly oriented peroxo ligands, the Mo(02)$- structure quite nearly obtains. In the monooxo, monoperoxo complexes the cis arrangement of the oxo and peroxo ligands is reminiscent of the cis dioxo structure. Moreover, the orientation of the 0-0 bond allows it to be a n donor analogously to an oxo ligand. The analogy weakens somewhat for the oxo-diperoxo complexes. Although the oxo and peroxo groups are mutually cis as in Moo3 complexes, there are only two additional coordination sites filled, making the structures analogous to five-coordinate trigonal bipyramidal rather than the more common six-coordinate octahedral Moo3 core compounds. The unusual complex Mo(OZ)Z(ttp)(ttp = tetra-p-tolylporphyrin) has been synthesized and As shown in Figure 19, the peroxo ligands are bound side-on on structurally ~haracterized.~’~ opposite sides of the planar heme group. The two 0-0 vectors eclipse a perpendicular pair of trans N-Mo-N linkages. The 0-0 distance of 1.40 8, is marginally shorter than that found in the other peroxo complexes. This is perhaps not surprising insofar as the porphyrin may participate significantly in delocalizin the charge placed on Mo by donation from 0%JT levels. This delocalization allows further (I! Mo donation + and concomitant slight lengthening of the 0 - 0 bonds. The diperoxo porphyrin complex is photochemically active,z9sejecting an 0 2 ligand upon irradiation with a tungsten lamp to produce the cis dioxo complex MoOz(ttp). Interestingly, Mo(O+(ttp) is thermally stable and reacts neither with cyclohexene or triphenylphosphine .29 This complex is , however , electrochemically reactive displaying one quasireversible one-electron oxidation and two quasireversible one-electron reduction waves .296
Molybdenum( VI)
1401
The oxidation may involve extensive porphyrin contribution as a dramatic optical spectroscopic change is observed, reminiscent of porphyrin cation radical formation. Further, the EPR spectrum of the oxidized complex, presumably Mo(O,),(ttp)+, generated in situ, is devoid of observable 95,97M~ hyperfine splitting. In contrast, the first one-electron reduction leads to an EPR active species with g = 1.980 and 95*97M~ splitting characteristic of M0v.296Interestingly, neither redox process involves the peroxo ligands, a point to be poadered in attempting to understand the lack of reactivity of the ligands in this complex compared to other peroxo Mo"' complexes. The complex MoO(02)ttp reveals once again the oxo/peroxo interchangeability in the Mo coordination sphere.171
d Figare 19 The structure of Mo(O,),(ttp) (unlabelled atoms are carbon)294
Structurally yet uncharacterized peroxo complexes include M ~ O ( O ~ ) ( d t b c (dtbc) ~ ~ ~ '= 3,5di-tert-butylcatecholato) prepared by reaction of M 0 ~ 0 ~ ( d t b with c ) ~ [NMet][O:] and isolated as the (Me4N)z[MoO(02)(dtbc)z]~3DMS0 salt. The reaction presumably involves disproportionation of 0; according to equation (28). The first step may involve 0; acting as a nucleophile followed by electron transfer from free 0 2 to bound 02.M ~ ~ O z ( d t bisc )known ~ to be reactive to donor organic solvents.297The v(Mo-0) at -950 cm-I is clearly distinguished from a band at -870 cm-l assigned to the ~ ( 0 - 0 )characteristic of side-on bound Og-. Mo2Q,(dtbc),
+ 40;
--+
2MoO(Oz)(dtbc);-
+ 202
(28)
Many complexes written as M 0 0 5 * L or Mo05L' (L' = bidentate ligand) undoubtedly contain the M o0( 0& unit. For example, L may be Me3N0, BuZPO, PrgAsO, Ph3As0, CsHsNO, etc.298 The analytical data and the observation of v(M-0) at 950-970 and ~ ( 0 - 0 )at -85Ocm-' leave little doubt about the formulation of these c ~ r n p l e x eThe ~.~~~ complexes stoichiometrically epoxidize alkenes or catalyze alkene epoxidation by t-butylhydropero~ide."~ M~O(O~)(oxinate)~ can be formed299from MoOz(oxinate)z by treatment with H202.The cis dioxo and cis oxo peroxo complexes are electrochemically distinct with the latter converting to the former irreversibly upon electrochemical reduction.299 When 0;- is bound to a transition metal its formal oxidation state 'and that of the metal are ambiguous. In the compounds discussed above the Mov'-peroxide formulation is used. However, in the absence of structural data an MoIv-dioxygen or Mov-superoxide formulation is possible. The hypothetical reaction of MoIVand O2 can be considered an oxidative addition, wherein the extent of charge transfer determines the proper formulation. In the complexes discussed here the 0 - 4distance lies in the range 1.44 to 1.55A. Comparison of these
1402
Molybdenum
distances with 1.44 A in H202and 1.21A in O2 indicates that the Mov' peroxide formulation is quite appropriate for these compounds. Only Mo0(02)[PhN(0)C(0 Ph]2 with 0-0 distance 1.212(5) 8, seems to be an exception.300The intermediate 1.36(3) distance reported301 for (NH4)3[M~0(02)F4] may be inaccurate. The electronic structure of Mo(02)i- has been studied using SCF-MS-Xar molecular orbital calculations.302 The Movl-peroxide formulation is appropriate as a configurational starting point for discussion of the bonding. However, there is significant lengthening (weakening) of the 0-0 bond in the complex compared to that in free 0;-, a feature that may be responsible for the reactivity of peroxo-molybdenum complexes in epoxidation reactions. Moreover, the significant electrophilic character of the bound 0;- (in the form of low-lying virtual mantibonding orbitals) accords well with the proclivity of the peroxo-MoV' complex to attack and epoxidize electron-rich alkenes. I
d
36.5.9.2 Dinuclear Peroxo Complexes Dinuclear complexes have been structurally characterized with a single bridging oxo or fluoride ligand or with a set of two bridging hydroperoxo ligands. The dinuclear ion (H20)(02)20MoOMo0(02)2(H20)2- in its pyH+ and Kf salts305 has a non-linear monooxo bridge (136 and 148", respectively). The bridging oxo is found to be cis to the M M t group on each Mo. The peroxo ligands on each Mo are cis to the Mo-0, bonds as in the mononuclear compounds. The structure as illustrated in Figure 20 has two-fold symmetry and contains two pentagonal bipyramids bridged through an oxygen atom shared by their equatorial planes. It is clearly related to the M O O ( O ~ ) ~ ( H structure ~ O ) ~ shown in Figure 17. 30393b4
Fignre 20 The structure of (H20)(0,),0M~0M~0(0,),(H3~~*in its K+ salt (0, is an oxygen atom from a water ligand)
The unusual complex p-F-[Mo0(02)(pydca)]~(see 5) is formed as the NEt; salt upon treatment of Mo(Oz)(pydca)(OH2) with F- 418 in the presence of NEtt. The complex, shown in Figure 21, is a centrosymmetric &nuclear ion with each Mo atom possessing pentagonal bipyramidal seven-coordination,307 The linear O,MoV'FMoV'O, unit is to date unique to this complex. The Mo-0, distance is 1.659(3)A and the Mo-F fram to it has the very long distance of 2.135(1) A. The o--O distance of 1.43 A is similar to that found in mononuclear peroxo compounds of Mov'.
The structure of (pyH)z[(Oz)zMoO(OzH)2MoO(02)2]304~308 shown in Figure 22 has two bridging hydroperoxo ligands. Each Mo coordination sphere is a heptacoordinate pentagonal bipyramid. Sharing of an axial-equatorial edge gives a centrosymmetric structure. The peroxo ligands occupy the remaining equatorial sites while the remaining axial position on each Mo contains a terminal oxo ligand. The Mo-Ob distance is 2.05 A. The bridging and non-bridging peroxo groups both have distances near 1.47 8, consistent with their peroxide formulation.
MoZybdenum(V1)
1403
P
Figure 21 The structure of p-F[MoO(O,)(pydca)]; in its NEt: salt (unlabelled atoms are carbon)41s
Figure 22 The structure of (0,)2MoO(0,H)2MoO(02)z- in its RH"
salt (terminal H atoms on bridging
hydroperoxide are not shown)
36.5.9.3
Spectral Indicators
Spectroscopically, in the IR region the -0 stretch can be identified between 845 and 895cm-l. 170NMR has proven useful insofar as the oxo and peroxo ligands are clearly distingui~hable.~~~ For example, in Mo0(02)(CN)? the peroxo signal (although broad) appears at 487 p.p.m., while the oxo signal appears at 705 p.p,m. downfield from HzO.The latter value is well within the range found for oxo-molybdenum s p e ~ i e s . l " *The ~ ~ ~peroxo I7O chemical shift in Mo0(02),(HMPA) at -450 p.p.m. with oxo at 850 ~ . p . r n . ~again l l illustrates that oxo and peroxo are readily distinguished. Further, in this latter example oxo and peroxo are shown not to exchange their 170labels. Thus, 170NMR could prove a powerful t m l in elucidation of reaction pathways in oxo-peroxo complexes, Although oxo-peroxo exchange is indeed slow on the NMR time scale, it can occur at a significant Indeed, Mow compounds are reported to catalyze the exchange of l80between H20 and H202.312 36.5.9.4
Reactivity Toward Organic Molecules
Peroxomolybdenum complexes have been used stoichiometrically to effect a variety of organic conversions and catalytically, usually with t-butylhydroperoxide or H202, to effect COC3-SS.
1404
Molybdenum
similar reactions.313The Mow complexes have been anchored on and used under phase-transfer ~ o n d i t i o n s . ~Selective ~ ~ ~ ~ ” and asymmetric reactions have been r e p ~ r t e d . ~ ” ~ ~ ~ We cannot even partially review this area which is part of a general thrust in reaction chemistry of early transition metal facilitated or catalyzed peroxidation/epoxidation/oxidation ~ h e r n i s t r y . ~Reactions ~ ~ , ~ ~ catalyzed ~ , ~ ~ ~ or carried out include alkenes to sulfides to sulfoxides,323secondary alcohols to ketone^:'^*^^ aldehydes to carboxylic acids,316 azo benzenes to azoxy benzenes,325N-heterocyclics to N-oxides,326amides to hydr~xamates’~~ and more complex reactions.55*327-330 Theoretical studies offer potential mechanistic insights into reactivity of the Mo-02 group.331 There is no evidence that dioxygen or peroxides interact with Mo in enzymes in physiologically relevant processes. The evidence is strong that in Mo oxidases the dioxygen reagent interacts with the Fe-S, heme or flavin parts of the protein and not with the Mo ~ i t e . ~ The . ~ ’ ~reduced Mo states of enzymes, however, are susceptible to oxidation by O2 ita vitro.
36.5.10
HYDROXYLAMIDO(1-) AND HYDROXYLAMIDO(2-) COMPLEXES
Deprotonated substituted hydroxylamines have provided potent ligands to Mo”’. The N,N-dialkyl hydroxylamine, RzNOH, in its singly deprotonated form, R2NO-, N , N dialkylhydroxamido(1-), serves as an N-0 bound ligand with some similarity to peroxide. N-Alkylhydroxylamine, RNHOH, in its doubly deprotonated form, [RN02-, Nalkylhydroxamido(2-)] forms complexes quite analogous to those formed by peroxo or disulfido ligands. For example, the complex M 0 0 (R ~d t cb z(ONR )~~~ is analogous to MoO(R2dtc)&. The complex MO(O)(~~~C~)(ONR)(HMPA)~~* 39 whose structure is shown in Figure 23 has an exact analog in peroxo chemistry, Mo(O)O,Qydca)(HMPA). The hydroxyla,339 A systematic study of the mido complexes can be thought of as metall~oxaziridiiies.~~ one-electron reduction of M00(Et~dtc)~(oNR) complexes as a function of R and in comparison to MoOz(Etzdtc)z leads to the conclusion that the oxaziridine group is a better donor than the oxo group.340
Figure 23 The structure of MoO(pydca)(ONR)(HMPA) (unlabelled atoms are carbon)”’
The dioxo complexes containing two 0,N-coordinated N,N-dialkylhydroxylamido(1-) ligands have been extensively studied,86~87~89~90~191~z44~341 Th ey are prepared by the reactions of MOOS- and N , N-dialkylhydroxylamine, usually in aqueous solution. Mo02(C5H1$IJ0 reacts in toluene with H2S to give MoOS(CSHl$r10)2 and MoSz(C5H10NO)2~4z With H2Se only MoOSe(C5Hl&O)z is formed. The structure of the dioxo, disulfido and oxo-sulfido complexes are k n o ~ n . As~ discussed ~ ~ ~ above, ~ ~ they ~ *can~ ~
Molybdenum(VI)
1405
be viewed as distorted tetrahedral structures with each 'bidentate' hydroxylamido(1-) ligand occupying a single coordination site. The hydroxylamido(1-) complexes can therefore be considered as derived from MOO$-. Alternatively, the structure shown in Figure 6 is describable in terms of non-octahedral six-coordination in which the N and 0 of the hydroxylamido(-1) ligand lie in a plane with 0, atoms equidisposed above and below the plane. It is related to the skew-trapezoidal six-coordination found in M O O ~ ( S C M ~ ~ C H ~ NNMR M ~ ~studies )~.~ of ~MoOz(RzNO), ,~~ and MoS,(R,NO)~ reveal configurational lability attributed to rapid dissociation and reformation of the M-N bond .341 Spectroscopic characterization of the cis-MoOS2+unit in MoOS(R,NOk was used to discuss the occurrence of an MOOS'+ center in oxidized xanthine ~ x i d a s e . ~ Comparison ~~,~ of the voltammetric behavior of MoXY(CSH1&J0)2(X= Y = S, 0 ; X = S , Y = 0) leads to the finding that the presence of the S atoms lowers the redox potential of the comple~.'~'The great difference observed between the MoOS(C5Hl&10)2 and M O O ~ ( C ~ H ~ ~complexes N O ) ~ (voltammetry differing in potential by 0.6 V)19' contrasts with the tiny difference found between the proposed (inactive) MOO$+and (active) MOOS'+ forms of xanthine oxidase. This unusual behavior in the pseudotetrahedral complexes may be due to their significantly different electronic structure compared to that of the MOO$+ and Moos2+ cores found in xanthine oxidase. The reaction of Mo02(R,NO), with catechol derivatives yields complexes containing one oxo, two dialkylhydrox lamido(1-) and one catecholate ligand. The structure of M o O ( C ~ H ~ N O ) ~ ( C & ~ Ois~reminiscent )'~ of that of the analogous peroxo complex. Here the hydroxylamido (piperidinolato) ligand binds side-on to the Mo in the equatorial plane of a seven-coordinate Mo structure. The catecholate ligand bridges axial and equatorial positions with Mo-0, at 1.69A typical of monooxo Moo4* core corn lexes. The N-0 distance of 1.38-1.39 8,is shorter than that in MoO2(C5H5NO),(1.43-1.47 ), perhaps indicating a greater x donation by the l-piperidinolato(1-) group in this complex where it competes with only one oxo ligand. One remarkable feature of this structure is the virtual identity of M o - 0 distances cis and trans to oxo in the same cathecholate ligand Mo-O(cis)=2.016(5) and MO(trans) = 2.012(5) A. This complex also had its structure determined by 2D NMR techniques (lH-13C chemical shift correlations and 'H-lH scalar coupling correlations).247 Reaction of N-methylhydroxylamine and various unsaturated triatomic reactants yield chelating ligands that form yellow MoOzL MoO,(MeHNO), reacts with the appropriate unsaturated molecule to yield complexes with five-rnembered chelate rings. For example, in equation (29) the resulting complex has the cis dioxo distorted octahedral structure.
8:
MoO,[MeHNO]
+ [Bu'INCO
-
,O\N/Me
Moo( L H L I N
1
(291
Scheme 5 shows the relationship of the starting hydroxylamido ligand to the final ligand.341 Complexes that contain remaining hydroxylamido as well as these bidentate chelates are also formed (Table 3). For example, MOO(M~NH~)~[HNC(S)N(M~)O] forms by reaction of MOO:-, MeNHOH and SCN- at 60 "C.Its structure, illustrating the two ligand types, is shown in Figure 24.87
36.4.11 COMPOUNDS WITH M
A BONDS
Molybdenum(V1) compounds containing M d bonds are a recent addition to the literature. These compounds include both oxo and non-oxo species. The latter have an alkylidyne (alternatively formulated as an alkylcarbido) group which is a bonding equivalent to the oxo group. Several unexpectedly stable series of M A bonded compounds344~72~345~346 h ave been reported. The compounds are of the form MoOzRBr(bipy) (purple), Mo02R2(bipy) (yellow) and Mo02R- (in solution, colorless). R can be Me, CH2Me3 or CHZPh, which, by lacking p-hydrogen atoms, are unable to undergo /3 elimination. The compounds are prepared by reacting Mo02Br2(bipy) with a Grignard reagent according to Scheme 6.
c
Molybdenum
1406
NH-
S
Me
\
H
NC/ I
r"" +
CS,
I
RNCO,
Scheme 5
n
-
-
e
95
Figure 24 The structure of MoO(MeNHO)@NC(S)NMeo]
MoO,Br,(bipy)
RMoO,Br(bipy)
MoO,R,(bipy)
iS7
OH-,
RMoO;
Scheme 6
The last complex, which has yet to be isolated as a salt, decomposes, albeit slowly (tin is 26.5 h at pH 11) in basic aqueous solutions at room temperature according to equation (30). M-MoO,
+ OH-
I_)
MOO:-
+ CH4
(30)
The dimethyl complex, Mo02(Me)z(bipy), shows the expected cis dioxo structure345with the methyl groups trans to each other and bent back from the oxo roups (C-M0-C = 149"). The M A distance is 2.19A in the R = M e complex and 2.20 in the R=CH2Me3 complex.
1
Molybdenum (VI)
1407
Mo02Me2(bipy) shows v(M-0,) at 934 and 905 cm-' in the IR and has UV bands at 303 nm (24 900); 293 nm (17 000); and 245 nm (19 600). The near UV band may be responsible for the complexes' photo~ensitivity.~~ In the NMR the Mo-Me protons resonate at 0.58 p.p.m. p y ) , ~structure ~ is downfield from TMS.345The neopentyl analog M 0 0 ~ ( C H ~ C M e ~ ) ~ ( b iwhose shown in Figure 25, has high thermal stability (to 182°C in air) attributed to the absence of @hydrogen atoms. Structurally, it is very similar to the Me derivative with deviations attributed to repulsions due to the bulky neopentyl groups. Thermolysis of the complex in the presence of abstractable hydrogen atoms from silicone oil leads to formation of neopentane , presumably from neopentyl radicals. Reaction with alkaline Na+BH;, Z n in HCl or 1-thioglycerol yields neopentane, quantitatively, through reductive cleavage of the Mo--C bond, In H2S04 a mixture of dineopentyl and neopentane is formed.
Y
Figure 25 The structure of MoOz(CR,CMe,),(bipy) (unlabelled atoms are carbon)%
Use has been made of Grignard reagents and organolithium reagents to prepare other alkyl Mo"' complexes. For example, reaction of Mo02C12.2THF with (mesity1)MgBr (mesityl = 2,4,6-trirnethylphenyl) gives the four-coordinate Complex M ~ O ~ ( m e s i t y l ) ~This . ' ~ ~complex reacts with the ylide BugPCHz to give BuSPO and (mesityl)Mo02[C(mesityl)PBu~] wherein a P-substituted carbene is bound to M o . ~ ~ ' Reaction of LiCH2CMe3 with MoC15 gives the remarkable alkylidyne complex (Me3CCH2)3Mo(CCMe3).348This complex can also be prepared from MoOZCl2 and (Me3CCH2)MgC1 in ether.a53 Reaction of Mo(CCMe3)(CH2CMe3)3 with HX in DME (1,2-dimethox ethane) gives Mo(CCMe3)X3DME (X = C1, Br).353 These complexes are all formally Mov?if the alkylidyne ligand is considered as RC3-. The Mo"' complex Moz(OCMe3)e reacts with phenylacetylene in pentane to give the alkylidyne complex MO(CC&)(OCM~~)~ in 60% yield.354In the presence of quinuclidine (quin) both Mo(CCJi5)(OCMe3)3( quin) and Mo(CHI(0CMe3)3(quin) have been identified. These reactions are remarkable as equation (31) formally involves the breaking of metal-metal and carbon-carbon triple bonds and the formation of a metal-organic triple bond.354 (Me,CO),Mo=Mo(OCMe,)
+ C6H5CCH
--+
(Me,CO)&oCC,H,
(31)
The complex M o ( C C M ~ ~ ) ( O Ris) ~formally Mo"' having a substituted molybdate structure. Here the alkylidyne ligand, formally R3CC3-, obviously displays the powerful 0- and n-donor ability required to stabilize Mo"'. The complexes MoOCI(CH&Me3)3 and MoO(CH2CMe& have been isolated355by reaction of MOOC14 with bis(neopenty1)rnagnesiumdioxane along with the non-oxo Mo(CCMe3)(CH2CMe3)3complex prepared previously.I3O The organic reactivity of the alkylidyne complexes has been extensively studied especially with regard to alkene and alkyne metathesis reaction^.^^^,^^^
Molybdenum
1408 36.5.12
DINUCLEAR COMPLEXES OF Mow
In addition to the multitude of mononuclear and heteropolynuclear complexes that contain Mow there are a significant number of dinuclear complexes. These include complexes in which the Mo atoms only coincidentally find themselves in the same ion such as ( M 0 0 ~ ) ~ e d t a ~ (where the two Mo atoms are more than 4 A apart) to ions such as M O ~ O ~ ( C ~ O ~ ) ~ ( H ~ O ) Z (where the two Mo atoms are bridged by a single oxygen atom). In no case are Mo-Mo bonds present in the 4d0, Mow, dinuclear complexes. The dinuclear complexes are diverse in structure. They can have a single, double or triple bridge. The single bridge is almost always a lone oxo group. The double bridge contains either a single oxo plus a non-oxo ligand or, more commonly, two non-oxo ligands. Besides this variety in the nature of the bridge, the remaining three, four or five ligands on Mo can also be a diverse mixture of oxo and non-oxo groups. As many as three oxo groups in Mo20;- 197 to one oxo group as in M0~0~(3,5-dtbcat),3~~ may be present in non-bridging positions. There are even examples of dinuclear, formally Mow complexes, containing no oxo groups such as Mo2(02C&&)6357(see below under non-oxo complexes). The simplest dinuclear ion is the dimolybdate ion. Discrete Mo20$- ions exist in KzMo207-KBr,350in molten K2MoZO7 above 520 0C233and in MgMo207,351,358 the latter having a dichromate-like structure with an M-0-Mo bridging angle of 160.7(3)". However, the Mo20;- stoichiometry does not guarantee the presence of discrete Mo20;- ions as, for example, c stalline K2M0207 and (NH&Mo207 consist of infinite chains of edge-shared octahedra. 3 2 The discrete Mo20;- anion has been structurally characterized as its NBuZ 361*352 and [(C5€&Me)Mo(C0)2(y-dppm)Pt(dppm>]+360 salts. In the latter salt, Mo20?- shows Mo-0, distances from 1.68-1.79 and Mo-O,, distances of 1.82 and 1.95A with the M o - S M o angle equal to 160'. In the (NBL&Mo~O~complex, the Mo-O-Mo angle is 153.6'. This complex is soluble and stable in organic solvents and serves as a starting material in various preparations. Its stability in non-aqueous media compared to higher polynuclear species is attributed to counterion effects. "0 NMR studies show peaks at 741 and 248 p.p.m, down field from H20 in the expected 6: 1 ratio assigned to the terminal and bridging 0 atoms, re~pectively.~~~~'~~ Other dinuclear complexes having a single oxo group as the sole bridge contain Mo20!+ or Mo20g+ cores. Of these, the latter is by far the most common. Complexes of known structure containing the Mo20$' core with a single bridging oxo ligand are listed in Table 7. The Mo-0-Mo angle varies from 136 to 180'. For example, the structure of Mo205L2 (where L = hydrotris(3,5-dimethyl-l-pyrazolylborate)contains an Mo20Z+ core with a singly bridging 0 atom and an Mo-0-Mo angle of 167.1'. The MOO;+ unit of each Mo atom in the dimer is, as shown in Table 7, quite similar to the same unit found in mononuclear complexes displayed in Table 1. Table 7 Structural Data on Oxo-bridged M%O:+ Complexes
Dimnces
Complex
Mo-0,
(A)
1.664(12), 1.662(10) 1.715(6), 1.715(6) 1.714(S),1.709(6) 1.685(8), 1.737(7) 1.685(5), 1.688(4) 1.691(6), 1.694(5) 1.701(2), 1.696(2) 1.68(3) 1.680(19), 1.7OO(20) 1.680(6), 1.710(6) 1.695(3), 1.695(3) 1.700(3), 1.683(3) 1.691(5), 1.686(5) 1.694(7) 1.696(7) a
' h e ligand is N',N'',N"-trimethyl-l,4,7-triazacyclononane.
Angles
Moo-0,
(A) Mo-0-Mo
1.933(10) 1.929(5) 1.917(5) 1.908(3) 1.865(4) 1.885(4) 1.889(1) 1.W3) 1.876(2) 1.880(1) 1.870(3) 1.881(3) 1.864(1) 1.898(1)
(")
Ref.
136.1(4) 143.8(2)
305 107
147.0(5) 162.7(2)
382 211
167.1(2) 171 180 180 167.0(2)
383 299 384 385 386
180 180
387 388
Molybdenum (VI)
1409
In the salt K ~ [ M o z O ~ ( G O ~ ) ~ ( H ~ the O ) ~ ]complex , anion is crystallographically As shown in Figure 26, the two terminal oxo ligands on each Mo are c e n t ro~ ymm etr ic .~~ mutually cis and cis to the bridging oxo. One oxalate and one water ligand complete the six-coordination sphere of each Mo atom. The M e , distance of 1.698, is similar to those found in mononuclear cis dioxo complexes. The Mo-Ob distance is 1.8SA. The long Mo-OHz distance (trans to Mo-0,) of 2.33 8, illustrates the large structural trans effect of the 0, ligand. The two Mo-0 distances to the oxalate ligand of 2.09A (trans to 0,) and 2.19A (trans to 0,) indicate that 0,has a larger trans bond-weakening effect than Ob.The largest 0-Mo-0 bond angles correspond to the shortest M A bond distances in agreement with the idea that interligand or interbond repulsions determine the details of such coordination spheres. A centrosymmetric structure such as that in M0205(&04)2(H20)$- with an Mo,O;+ core is found in a number of complexes. In other cases, a structure closer to C2 symmetry is found with the dioxo moieties on the two Mo atoms adopting a quasieclipsed conformation. In the complex Mo205(phen)z(NCS)2,the phenanthroline ligands are eclipsed,211 i.e. stacked in almost parallel planes as shown in Figure 27.
Figwe 26 The structure of Mo,O,(~O,),(H,O)~- in its K+ salt (0,is an oxygen atom from a water
P
Figure27 The structure of Mo205(p
are carbon)*"
Molybdenum
1410
Complexes containing a triple bridge and an Mo20:+ core are listed in Table 8. The prototypical complex (NH4)2[Mo205(02C61&)2].2H20, shown in Figure 28,362has a triple bridge consisting of non-linear oxo and two oxygen atoms of different catecholate ligands. The structure can be considered as two shared distorted octahedra with the three bridging oxygen atoms at average distances of 1.92,2.16 and 2.37 A from each Mo. The catechol oxygen atoms do not bridge symmetrically as each catechol is chelated to a single Mo and bridges by one 0 atom to the second Mo. The two six-coordination spheres are completed by two terminal oxo groups and one non-bridging catechol oxygen on each Mo. The M-Mo distance is 3.13 A as expected for this 4d0 system where no Mo-Mo bonding is possible. A similar triple bridge has been found in the complex MozOs(PhenSQ)z (where PhenSQ is the 9,10-phenanthrene uinone radical anion)363 and in complexes containing d i t h i ~ t h r e i t o l ~2-~ ~ m e r c a t ~ p h e n o l a t oand ~ ~4-xantho~terin.~’~ ~ The latter complexn4 shown in Figure 29 is the first example of Mo bound to a pterin ligand (as proposed for the molybdenum cofactor Moco). Interestingly, the pterin binds in a mode reminiscent of 8-hydroxyquinolinate ligands. However, the relevance of this binding mode to Moco is uncertain in view of the dinuclear nature of the complex, the proposed reduced nature of the pterin in Moco and the suggested sulfur binding by the Moco ~ t e r i n . ~ , ” ~ Table 8 Doubly and Triply Bridged Complexes Containing M%O:+ ~~
Cornplexar
(NH~)[MozO,(C~H,,O~I.H,O” Na[Moz0,(C6Hl
.2H,0a
Mo,O,(phenSQ)z [~~~lz[MozOs(dbcat)zI (NH,),Moz0,(cat)~~2Hz0 Mo,O,(n-pobb),b [PPh,lz[Mo205(sC6~~),1 (NEt4)z[MozOs(dtt)l
[Na(DMSo)zl~[Mozos(xantho~)~l a
Mo-0, (A)
Mo--O,-Mo
1.68(1), 1.72(1) 1.69(l), 1.71(1) 1.727(3), 1.693(7) 1.716(2), 1.703(2) 1.691(5), 1.678(5) 1.7OO(1), 1.707(10) 1.701(9), 1.674(10) 1.68(2), 1.69(2) 1.73(2), 1.71(2) 1.698(3), 1.691(3) 1.683(3), 1.699(3) 1.712(2) 1.699(2) 1.702(12), 1.716(12) 1.674(9), 1.657(12) 1.691(7), 1.707(5)
107.98(7)
364
107.98(7)
389
112.7(2) 109.4(4)
390 363
119.3
362 391 368
(“) Ref.
367 85.3(3)
365
146.2(3)
274
The organic ligand is the ionized sugar, mannitolate. n-pobb = 1-n-2a-hydroxybenzimidazole(deprotonated).
V
Figure 28 The structure of M o , O , ( O , C ~ H ~ )in ~ its NH: salt (unlabelled atoms are carbon)362
Molybdenum( VI)
1411
Figure 29 The structure of Mo,O,(xanthopterinate):- as found in the salt [Na(DMSO),],[ Mo~05(xanthopterinate)2]z74
In certain Mo20:+ complexes, the bridging ligand would appear to have a choice between an 0 and S bridge. In the case of the ligand C6H40S2- (monothiocatecholate) the oxygen atoms of the ligand are the (non-oxo) bridging atoms.367Similarly, in M 0 ~ 0 ~ ( d t t 365 ) ~ again 0 rather than S bridging is found. In both cases this allows the S Iigand to occupy a position trans to the bridging oxo rather than trans to the terminal oxo. This is the more favorable position for the strong n-donor S atom. This preference for not being irans to oxo may be the dominant factor in the adoption of the observed structure. One interesting complex has a deprotonated 1-n-2~-hydroxybenzylbenzimidazole ligand whose 0 atom forms an additional bridge to an 0 x 0 . ~The ~ complex is unusual in having a double (rather than more common single or triple) bridge. Further, one of the MoV' atoms is five-coordinate while the other has the more conventional six-coordinate near-octahedral structure. 368 A related tetranuclear complex having Mo20:+-like units is formed between Mow and malate, and has the formula (NH4)2[(Mo02)403(C4H305)2]-H20.234 Yellow complexes with the Mo20i+ core are formed with substituted glycol ligands such as pinacol ( ~ i n H )For . ~ example, ~ the structure of Mo203(pin)2(Hpin)has a 162.2(9)" Mo-0Mo bridging angle with intramolecular H bonding presumably helping to stabilize the complex. There are a few examples in which only non-oxo bridges occur. In these cases alcoholate or phenolate type oxygen atoms form a double bridge. Some of the complexes have MOO;+ cores bridged doubly by akoholate ligands. For example, as shown in Figure 30, {MoOz[OCMe2CMezOHj}2(-p-OMe)p has a double methoxide bridge,370while [Mo204(pOCH2CMe2CH20)(H,O)z] has bridging oxygen from a chelating 2,2-dimethylpropanediolato ligand.371 A related compound is Mo2120f;, which can be thought of as containing MOO:+ groups bridged by 10:- octahedra as represented in Figure 31.372 We have not deaIt explicitly with preparative procedures for dinuclear complexes. These preparations have, in most cases, been fortuitous and no general protocol exists. Most preparations leading to dinuclear complexes did not seek them, whereas some efforts actually sought' to avoid them! One class of complexes, that of substituted catecholate ligands, has yielded a variety of mono- and di-nuclear Mow complexes. Mo(CO)~reacts with the oxidized form of the 3,5-di-tert-butylcatechol (i. e. 3,5-di-tert-butyl-l,2-benzoquinone) to yield Mo(dbcat)3 which reacts with O2 to give the dinuclear complex I M o O ( d b ~ a t ) ~In ] ~ contrast, . ~ ~ ~ reaction of Mo(CO)~ with 3,4,5,6-tetrachloro-l,2-benzoquinoneyields the dinuclear non-oxo complex Mo2(OZC6C1)6.357 The complex Mo205(phenSQ) has already been discussed.363This complex
1412
Molybdenum
Figure 30 The structure of
{MOO~[OCM~,CM~,OH]}(~-OM~)~~~
Figure 31 The structure of MoZI20?; as found in its K+ salt3n
differs by two electrons, i.e. is formally oxidized (neglecting the ligand difference) with regard to MozOs(cat)2- or Mo,O,(dbcat)z-, each prepared from MOO:- and the respective catechol. M0~0~(phenSQ reacts ) ~ by ligand substitution with 3,5-dbcatH2to give M ~(d b cat )~. At least in this class of catecholate complexes, there seem to be some reactions that systematically interconnect the mononuclear and various dinuclear complexes. Infrared and/or Raman spectroscopy are useful in identifying the core structures involved in dinuclear complexes. The Mo-0, vibrations stand out as they do for mononuclear complexes and the MO-Ob-MO stretch can often be detected between 650 and 770 cm-'. For example, in M O ~ O ~ [ S C M ~ ~ C C H ~ N H C H ~v(MO-0,) C H ~ N M ~is ~found ] ~ ,at 860 and 900 cm-' while v(Mo-ob-MO) occurs at 665 cm-' . I7O NMR spectroscopy on enriched samples is particuH ~7O N Mre~~]~,~ larly informative. For example, in M O ~ O ~ [ S C M ~ ~ C C H ~ N H C H ~ Cthe sonance for Mo-0, ligands occur at 873 and 857 while the bridging Mo-Ob-Mo resonance occurs at 366 p.p.m. downfield from HzO. 95Mo NMR spectra of dinuclear complexes are also observable375and, as more data become available, could also become a structurally useful technique. In a number of studies spectroscopic techniques have led to the postulation of a dinuclear Mow structure for complexes wherein crystallographic information is not yet available. For example, a fluoro-bridged structure has been assigned to Mo202FP by IR377and '9FNMR53 techniques. IR spectroscopy has been used to assign an Mo20z+ structure with a single oxo bridge to CpzMo20267(v(MD--Ot) = 920, 898 v(Mo--O-Mo) = 770 cm-').Certain oxalate, citrate37s380 and a-amino acid381complexes studied in solution (or isolated) are also indicated to be dinuclear. 36.5.13
COMPLEXES LACKING OXO-TYPELIGANDS
Although the oxo group and its analogs dominate the chemistry of Mo"', there are several examples of compounds that lack the oxo ligand or any of its analogs. Perhaps the best known of these is molybdenum hexafluoride. MoF6, prepared by reaction of Mo metal and F2, has a
Molybdenum (VI)
1413
regular octahedral structure. The volatile liquid (m.p. = 17.5, b. 35 “C) is reactive toward addition, substitution and reduction. Lewis bases including F-,P. 92= C1- 393 and Me20392form adducts with MoFZ- whose structures have not yet been determined. For example, KzMoFs has been isolated393 but not crystallographically characterized. Fluoride in MoF6 can be substituted by FsTeO- or CF3CH20- to form the series of complexes M O F , ( O T ~ F ~ and )~-~~~~ MoF,(OCH~CF~)~-,(n = 1-6),394 respectively. As an oxidant, h;loF6 has a potential of +0.93 V us. Cu/CuF2(1.0 M NaF/HF) in HF3%and +1.60 V us. Ag+/Ag’ in MeCN3” for the MoF6/MoF; couple as determined by cyclic voltammetry. MoF6 is thus a strong oxidant and reacts with metals?9s I*?’’ NO3w and many other molecules. For example, the explosively reactive amber MoSF4 and MoSeF4 molecules have been reported to be prepared from MoF6 according to equation (32).2”5The vibrational spectrum of MoF6 has been extensively studiedm and exploited in laser isotope separation.“’ Although MoC16 has been briefly reportedm its existence has not been confirmed and seems unlikely. Mo(OMe)6 has been reported as a low-melting solid (m.p. = 67.6 0C).403The unusual complex M O C ~ ( N S N S Ncan ) ~ ~formally be considered to contain an MoV1core, which is consistent with a qualitative bonding scheme for the dithiatriazene ligand.3MoF6 + Sb2X3
----f
3MoXF4+ 2SbF3 (X = S , Se)
(32)
The remaining non-oxo complexes contain 0-, N- or S-donor polydentate ligands. The tris(dithio1ene) complexes are the oldest known members of this class. For example M 0 (t dt) 3,~~ ,~~ M 0 ’( b d t ) 3 ~ ~ ’and ~ ” M O ( S ~ G H &are ~ six-coordinate trigonal prismatic complexes whose structural, spectroscopic and redox properties have been thoroughly investigated and r e v i e ~ e d , ~ * ~The ~ ~amidothiolate ligand C6H4SNp-= abt2- forms an analogous nearly trigonal prismatic complex Mo(abt)3 with the cis arrangement of the ligands illustrated in Figure 32.121,410Mo02(acac)z reacts with thioaroylhydrazines, e.g. NH2NHC(S)Ph, to give Mo(HNNHCSPh)3 which has a somewhat distorted trigonal prismatic coordination4” related to ~ ’ * complex containing three chemically distinct N,S chelates has that of M ~ ( a b t ) ~ . ~ ~A” ,related been r e p ~ r t e d . ~Interestingly, ” these S , S and N,S donor ligands react with oxo molybdenum starting materials to give non-oxo products. Significantly, the N donors are always found in a deprotonated state such that they can serve as n as well as 0 donors to the Mow i ~ n . ~ ’ The ~,~” amidothiolate complexes are more difficult to reduce than are the corresponding dithiolene complexes. Complexes of catecholate ligands such as Moa(02C&14)63s7 and M ~ ( p h e n Q ) ~ ~ l ’ have dinuclear and distorted trigonal prismatic coordination, respectively. These complexes are prepared from Mo(C0)6 and the oxidized (0-quinone) form of the catecholate ligand.41M1s
Figure 32 The structure of Mo(abt), = Mo(NHSC&),
(wdabelled atoms are carbon)410
1414
Molybdenum
In addition to these complexes of bidentate ligands, several related tri- and tetra-dentate ligands are capable of removing all oxo groups from an MoV1 coordination sphere. The tridentate ligand hbma3- forms Mo(hbma)2 (6), a six-coordinate complex with a structure halfway between meridianal octahedral and trigonal prismatic geometries.lS9 The related potentially tetradentate ligand H2bpmp2- (7) forms the yello,w Mo02(H2bpmp) complex420on reaction of the ligand with Mo02(acac)2. However, the brown complex M~(Hbprnp)~.MeOH formed by a less reliable procedure420 has an octahedral structure with a meridianal arrangement of two of the ligands acting in a tridentate fashion. Each ligand has one uncoordinated phenolic OH roup.42oComplexes of en(abt); of the form M ~ ? [ e n ( a b t ) [see ~]~ (S)] have also been reported. $12
The non-oxo Mo"' complexes have in common the presence of good 0-and a- donor ligands. These serve the same purpose as the oxo group, namely the neutralization of high formal charge on Mo by strong donation.
Acknowledgement I thank Professors Sharon J . N. Burgmayer, Karl Dehnicke, John H. Enemark, C. David Gamer, Jean-Marie Manoli, Philip C. H. Mitchell, Achim Miiller, Jon A. McCleverty, John W. McDonald, William E. Newton, Narayanan Kutty Pariyadath, Franklin A. Schultz, Dieter Sellman, Jack T. Spence, A. Geoffrey Sykes, Joseph L. Templeton, Richard A. Walton, Anthony G. Wedd, Karl Wieghardt and Jon Zubieta for providing me with results prior to publication. I thank Mary Reilly and Pat Deuel for outstanding word processing and Jeannette Stiefel for excellent proofreading of the manuscript.
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36.6 Molybdenum: Special Topics C. DAVID GARNER University of Manchester, UK 36.6.1 MOLYBDENUM-SULFUR SPECIES AS LIGANDS 36.6.1.1 lntroduction 36.6.1.2 Synthesis and Properties of [MoO,S,-J*- (n = &2) Complexes 36.6.1.3 Other Mo-S Species as Ligands
1421 1421 1421 1424
36.6.2 IRON-MOLYBDENUM-SULFUR CLUSTERS
1425
36.6.3 MOLYBDENUM CHALCOGENIDES
1431
36.6.4 NITROSYL AND CYANIDE MOLYBDENUM-SULFUR CLUSTERS
1433
36.6.5 CYCLOPENTADIENYL MOLYBDENUM-SULFUR SYSTEMS
1434
36.6.6 DITHIOLENE AND RELATED COMPLEXES
1436
36.6.7 OXOMOLYBDOENZYMES
1437
36.6.8 HYDRIDE AND DIHYDROGEN COMPLEXES
1438
36.6.9 REFERENCES
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36.6.1 MOLYBDENUM-SULFUR SPECIES AS LIGANDS 36.6.1.1 Introduction The thiomolybdate(V1) anions [MoO,S ~-~]~( n = 0-3) have been prepared by a number of methods13 and, with the exception of [MoO3SI2-, act as versatile ligands to a wide variety of metal ions4 (see Table 1). In each of the compounds listed in this table, the [Mo0,S4-,]*- ion acts as a bidentate ligand to the metal and the resultant structural versatility has been clearly demonstrated for copper(1) (Table 2). In addition, thiomolybdate(V1) anions are of interest since they have special relevance to aspects of bioinorganic chemistry. Thus, [MoS,]'- has been widely used in the synthesis of Fe-Mo-S clusters, examples of which serve as spectroscopic analogues for the iron-molybdenum cofactor of the nitrogenases (see Section 36.6.2), and Cu-Mo-S aggregates, which are relevant to the copper deficiency induced by molybdenum in ruminant animals.
36.6.1.2
Synthesis and Properties of [MOO,S~-,]~-(n = 0-2) Complexes
A variety of divalent d transition metal cations (M = Fe? C0,21-23Ni,23,24348 Pd and Pt,25n26 or Zn23,"345) react with thiomolybdate(V1) anions to form the corresponding bis{thiomolybdate(VI)} complex, which has usually been isolated as its quaternary ammonium or tetraphenylphosphoniuium salt. Copper(I1) reacts with [NH4I2[MoS4]to form a polymeric copper(1) derivative [N&],[CuMoS4],, the anion of which is composed of chains of edge-sharing CuS4 and MoS4 tetrahedra.27 Such polymerization of Cu' and MoV1S4is inhibited by a variety of ligands which bind to the copper. Discrete aggregates with up to four Cur atoms bound to a thiomolybdate(VI) anion have been characterized (Table 2). [(PhSCu)MoS4I2(type I) and [ ( P ~ S C U ) & ~ O S(type ~ ] ~ -11) have been prepared28 by the action of KSPh on the corresponding cyanide complexes and X-ray crystallography has shown that coordination to the copper causes relatively little perturbation of the MoS4 unit. Reaction of CuCI and [MoS4IZ(3: 1) in MeCN produces [(CICU)~MOS~]~(type 111), which has a T-shaped structure.36 [MoS4I2- reacts with an excess (>4moles) of CuX(X=Cl,Br) in acetone to form (type IV). The bromo complex is a one-dimensional polymer, in which four [(XCU)~MOS~]'edges of the M o S ~tetrahedron are bridged by copper atoms; two trans copper atoms have trigonal planar stereochemistry with a terminal bromine atom, while the other two coppers are
1421
Molybdenum
1422
Table 1 [MOO,S,-,]~- (n = 0-2) Complexes of the d Transition Elements Complex
Ref.
5 6,7,8 7,9,10 8,11,12 13 14 15 16,17 18 19 20 20 21.22 22 23 24 23
23 25 2s
Complex
Ref. 27 28 29,30,31 29 28 29,31 32,33 34 35 36 34,37 32,38,39 32,39 40 34 39 41 42,43 44
41
26
41
25 26
24,45 23
25
each tetrahedrally coordinated and are bonded to two bromines which are attached to the copper of an adjacent Cu4MoS4unit.40 Compounds of the type [{CU&IOS~C~}(PP~~)~Y] (type V) (Y = 0,S), which contain a distorted Cu-S-Mo-Cl cubane-like cluster, have been prepared from the reaction between [MoYS31P-, CuCl2 and PPh3 in CH2C12.32,38.39 [(CICU)~MOOS~]~(type VIII) involves three copper atoms each bound across an S-S edge of the tetrahedral anion; the MoS3Cu3 core corresponds to a cubane-like framework with one position Extraction of this complex with a solution of CuCN and PPh3 in CH2C12gives [(Ph3P)3Cu2MoOS3](type VII),34 which can be considered to have a cubane-like framework with two vacancies. The analogous complex of [MoS4I2-, [{(Ph3P)2Cu}(Ph3PCu)MoS4] (type IX),32,33and its silver analogue41 ~ ] ~ - 11) but with one coinage metal in a possess a structure similar to [ ( P ~ S C U ) ~ M O S(type trigonal planar and one in a tetrahedral environment, rather than two with a trigonal planar coordination geometry. [{(Ph3P),A } (Ph3PAg)MoS4]was obtained by reacting [MoS4I2- with PPh3 and AgN03 in CH2C1$ b adjusting the stoichiometry of the reaction [(MO&A~~}(PP~~)~] may be formed.”, The central unit of this latter compound is a novel cage, arising from the fusion of two MoS3Ag2rings. Gold(1) thiomolybdate(V1) derivatives have been prepared; [(R3PAu)2MoYS3](Y=S , R=Et;44 Y = O , S, R=Ph41) have been obtained by reacting [MoYS3I2- and R3PAuCI (1:2). [(R3PAu)2MoOS3]in CH2Clz reacts with Cs2[MoOS3] to form [{ (Ph3P)zAu}(PhjPAu)MoOS3], which has a structure related to type VI1 of Table 1. [(Bipy)2RuMoS4] has been prepared by reacting [MoS4I2- with [ ( b i ~ y ) ~ R u C(1l ~:1) ] and this compound will bind a further ruthenium centre to form [{ (bipy)2Ru}2MoS4]2+.Both complexes have been investigated as catalysts for C2H2reduction and, in the presence of MeOH or other proton source, the electrochemically reduced species react to give GH,and C2H6a2’ Thiornolybdate(V1) complexes of d8 metal centres [M‘(MoYS~)~]’-(M = Ni, Pd, Pt) have been pre ared by the reaction of MClz with [MoYS312- (Y= 0,S) (1:2) in H20/MeCN s o l u t i ~ n ? ’ - ~ ,The ~ ~ ~‘mixed ~~ ligand’ com lexes [(Et2NC&)M‘(MoS4)]- (M‘ = Ni, Pd) have been prepared by the reaction of [M0S4f- and [M‘(S2CNEt2)z] (1: 1) in acet0ne.2~These systems involve a square planar geometry at the d8 metal. [ C O ( M O S ~ ~ ]formally ~-, a Co’ complex, has been prepared by reacting CoCl2, [Et,NH][SPh] and [MoS4] - in MeCN21and the magnetic properties of the complex are consistent with a tetrahedral environment about the cobalt. Heating [N&I2[MoS4 in DMF, alone or in the presence of benzenethiol, leads to the formation of [Mo3S9I2-. [PPh4]2[MoO(MoS4)2]has been prepared by heating [NH4]2[MoS4]in
r
41
Molybdenum : Special Topics
1423
Table 2 Structural Diversity Observed for Thiomolybdate(V1) Complexes of Copper(1) Solid stare structure
Complex
S [(PhSCu)MoS$ [(NCCU)MOSJ-
X-Cu/
S ‘Mo’
s\’
Ref.
Type
I
28 29
s‘
28
111
36
IV
40
V
38,39 32,39
VI
29
VI1
32,33,34
VI11
34,37
I
X
I
PPh,
I
/““‘s
\?\? cucu c u
/i\
x
X
1424
Molybdenum
Mew- EtO H in the presence of Na[B&k2' These anions involve a central MoIVY (Y= 0, S) moiety coordinated by two bidentate Mo IS4 groups and the central MoIVatom has a square pyramidal geometry. In the case of [Mo3S9I2-the central molybdenum is located 0.62 A above the plane of the four thiomolybdate(V1) sulfurs; the shortest M-S bond (2.086(4) A) is that of the MoTV-S group; within the Mow& groups, the terminal M A bonds (M-S,= 2.231(3)-2.251(3) A) are significantly shorter than the bridging bonds (Mo-Sb = 2.369(3)2.429(3) A; Sb-Mo-Sb (104.1(1) and 103.0(1)") are reduced from the value in [MOS,]~-;and the Mo - - - Mo separations are ca. 2.95 A.46Several of these features, viz. the lengthening of the Ma-Sb bonds, the reduction in Sb-MeSb below the tetrahedral value, and the close approach of the metal atoms, are general to the complexes formed by [MoS4I2- and related anions. For the 3d transition metals (M = Fe, Co, Ni and Cu) the Mo - * M separations are in the range 2.63-2.80 A. Thus, the Mo * * M separations in the thiomolybdate(V1) complexes are sufficiently close to be consistent with the formation of an Mo-M bond; this could arise by donation of a air of electrons from a filled orbital on M into an empty orbital on Mo. [M00,S4-,{(n = 0-2) and their complexes have been extensively investigated by IR and Raman spectroscopy, especially in respect of their v(M0-4) and v(M0-S) stretching vibrations, which occur in the ranges 800-1000 and 400-500 cm-', respectively. The Mo-S stretching modes of [MoS4I2- have been a s ~ i g n e dthe ; ~ totally symmetric ( a l ) stretching mode occurs at 458 cm-' and the triply degenerate, asymmetric (t2)stretching mode at 472 cm-l. The symmetry of the MoS4 group is lowered upon complex formation and, in general, the number of IR and Raman active vibrations observed is consistent with the symmetry of the Resonance Raman spectra have been recorded for thiomolybdate(V1) with excitation using laser lines of energy within the contour of the lowest energy sulfur-to-metal charge-transfer transition. The totally symmetric v(M0-S) stretches are easily identified as giving rise to the most intense bands and, especially for [MoS4I2- complexes of CUI, the pattern (i.e. the number of bands, their frequencies and relative intensities) observed in the v(Mo-S) region is distinctive for the number of Cu' atoms coordinated to [MoS4I2-.50 The UV/visible absorption spectra of [MoO,S4-,I2- (n = 0-2) complexes are dominated by the sulfur-to-metal charge transfer transitions of the anion and these bands often serve to characterize the particular system, even though these absorptions may be correlated with those of the parent anion.4 The reduction in symmetry due to complex formation often leads to a broadening or splitting of the bands and, in the majority of the d transition metal complexes, the lowest energy band is red-shifted by an amount which is dependent both upon the nature of the transition metal and the number of such metals bound per MowOnS4-, group. "Mo NMR s ectra have been reported for CUI,Ag' and Au' complexes of the [MOO,S~-,]~.( = 0-2) ions.g,29,32A1.49 e resonance position is characteristic of the complex and, as is seen clearly for the CUI systems, the chemical shift (from [Mo04]*-) decreases regularly (by ca. 500 p.p.m.) for each CuX unit bound to the thiomolybdate(VI) core. Cyclic voltammetric i n v e s t i g a t i ~ n of ~ ~[M(MoO,S~-,)~]~~ ~ * ~ ~ ~ ~ ~ (M= Ni, n = 0-2; M = Pd, Pt, n = 0) complexes has shown that each can be reduced reversibly in a one-electron process and a further one-electron reduction i s possible for some of the complexes. The first reduction is primarily centred on the d s metal but the second reduction is not so easily described, since the nature of it shows a sensitivity to both the nature of M and the thiomolybdate(V1) anion.26 The electrochemical behaviour of [CO(MOS~)~]~has also been investigated and two reversible processes, assigned to the 2-13- and 3-/4- couples, have been observed.21
-
36.6.1.3 Other Mo-S
-
Species as Ligands
The molecular fragment ~is-Mo(SBu')~ is capable of acting as a bidentate ligand; reaction of [Mo(SBU')~]or c~~-[Mo(SBU')~(CNBU')~] with [CuBr(CNBut)3] in acetone under dinitrogen produces [(BU~NC)~MO(~-SB~')~CUB~]. Two isomers, differing in the relative arrangements of the r-butyl substituents on the thiolato bridges, have been crystallized and structural1 characterized and their interconversion in solution demonstrated by 'H NMR spectroscopy.E Similarly, [Mo(SBu'),] reacts with [Fe2(C0)9] in toluene to give [MOF~~(SBU~)~(CO),] as the main product. Two carbonyls have been transferred to the molybdenum in the reaction and the resultant coordination about this metal corresponds to that of a distorted trigonal prism. The 'HNMR spectrum shows two Bu'S signals at room temperature which coalesce upon warming.55 c~~-[Mo(SBU')~(CNR),] (R = cyclohexyl, t-butyl) reacts with FeX2 (X = Cl, Br) to
Molybdenum : Special Topics
1425
and the related system [Cp2Mo(p-SBut),FeCI2] involves an give [(RNC)4Mo(p-S3u')2FeX2], Mo * Fe separation of 3.660(3) cis-[M~S~(pipNO)~] (pipNO = piperidine-N-oxide) reacts with CuCl to form the chloridebridged dimer [ { (pipNO)zMoS2CuC1}2](Figure I . Coordination to the copper atom lengthens the Mo-S from 2.145(2)& in the parent Mo' complex, to 2.179(4)A; concomitantly the Mo separations S-Mo-S interbond angle is reduced from 115.0(1) to 110.3(1)". The Cu are 2.639(1) A.57All of these structural features are typical of the coordination of [MoS,]*molybdenum-sulfur centres as bidentate ligands to CUI and other metals.
Figure 1 Structure of [{piperidine N-oxide(1- ))2MoS,CuC1}2]57
36.6.2 IRON-MOLYBDENUM-SULFUR CLUSTERS Interest in the chemistry of F e - M d clusters has been stimulated by the discovery that the iron-molybdenum cofactor (FeMoco) of the nitrogenase enzymes is an aggregate of these elements, organized in a manner yet unidentified. The nitrogenases catalyze the reduction of dinitrogen to ammonia in nitrogen-fixing organisms. The enzyme complex consists of two types of protein, the Fe protein and the FeMo protein, and activity requires both proteins, ATP, a divalent cation (preferably Mg2+) and a reductant. The Fe protein, which involves an Fe4S4-cubane-likecluster, acts as an electron transfer agent to the more complex FeMo protein which contains the catalytic site for substrate reduction.58 There are two molybdenum atoms per FeMo protein molecule and each is involved with one FeMoco; these entities may be extracted from the proetni@ ': ' have been shown to be common to molybdenum-containing nitrogenases of various organisms, and can activate extracts of the FeMo protein component of inactive mutant strains of different microorganisms .61In the organism Klebsiella pneumoniue, a cluster of 17 genes in seven transcriptional units has been associated with nitrogen fixation. One class of mutants, the nif V - mutants, has altered substrate specificity and the enzyme horn these mutants is relatively ineffective at dinitrogen reduction. This altered substrate specificity has been shown to be associated with FeMoco,62 directly implicating this entity as part of the catalytically active site of the nitrogenases. The elemental composition of FeMoco has been estimated by a variety of analytical and spectroscopic procedures, leading to ranges of values for iron and sulfur per molybdenum of M o F ~ ~ S ~ There ~ . is~no~ evidence , ~ , ~for~ the presence of any organic entity associated with FeMoco, other than solventsa such as N-methylformamide (NMF) used in its a b ~ t r a c t i o n . ~ ~ FeMoco, both as a constituent of the FeMo rotein and an isolated entity, has been the subject of detailed spectroscopic examination8 =Fe Mossbauer and EPR studies of the cofactor have been interpreted in terms of an S = 3 centre that contains one molybdenum and cu. six irons in a spin-coupled structure. The EPR signal serves as a valuable fingerprint of FeMoco; furthermore, release of FeMoco from the FeMo protein produces an EPR spectrum with broader features, but the same profile, thereby indicating that the core of this cluster is little changed by the extraction procedure. Treatment of FeMoco with cu. one equivalent of
Molybdenum
1426
benzenethiol sharpens the EPR spectrum so that it more closely resembles that of the MoFe p r ~ t e i n . In ~ .the ~ ~FeMo protein, FeMoco is capable of existing in at least three distinct levels of oxidation. The S = 2 level is oxidized by the removal of one electron per molybdenum to an EPR-silent species and, during steady-state turnover, the cofactor is converted into an EPR-silent, super-reduced state. The oxidized state can be reproduced by oxidation of FeMoco in NMF solution by methylene The most definitive structural information concerning the molybdenum site in FeMoco and the FeMo protein has been provided by X-ray absorption near edge structure (XANES) and extended X-ray absorption h e structure (EXAFS) s p e c t r o s ~ o p y The . ~ ~ ~profile ~ of the molybdenum K-absorption-edge indicates the absence of terminal oxo groups and the position of this edge suggests an oxidation state of ca. +IV. Interpretation of the EXAFS has shown that, in the MoFe protein, the molybdenum possesses a first coordination sphere of three to four sulfur atoms 2.36 away, with a shell of two to three iron atoms at a distance of 2.68 A; these structural features remain essentially unchanged for FeMoco in NMF, but there is evidence for an additional shell of two to three oxygen (or nitrogen) atoms at 2.10w. These latter atoms could originate from the solvent and IR spectroscopic studies of solutions of FeMoco in NMF have identified an intense band at 1600 cm-', which has been assigned to the W stretching of N-bonded, deprotonated solvent.@ The lack of precision in the structural definition of FeMoco has allowed much scope for the synthesis of possible chemical analogues and, thereby, stimulated the preparation and structural and spectroscopic characterization of Fe--M& ~ l u s t e r s .The ~ ~ 'majority ~~ of these synthetic approaches employ [MoS4I2- as a reagent and they can be classified in two main categories: (a) ligand exchange reactions between Fe(L), complexes and [MoS4I2- to give complexes containing tetrathiomolybdate(V1) anions acting as ligands7' and (b) 'spontaneous self-assembly' reactions between [MoS4I2- and FeC13, in the presence of a thiol, which yield Fe3MoS4 cubane-like clusters in dimeric species which , by subsequent manipulations, have been converted into monomeric species.72 Examples of complexes formed between iron and tetrathiomolybdate(V1) anions are given in Table 1,%19 and aspects of the vibrational and electronic spectra associated with their molybdenum centres are discussed in Section 36.6.1.2. Magnetic moments and 57FeMossbauer spectra of [S2MoS2FeSzMoS2]3-,73[X2FeS2MoS2I2-(X = Ph, Cl, SPh, OPh; X2 = S5)7,8.11314and [Cl2FeS2MoS2FeCl2I2have been recorded and interpreted in terms of Fe" centres which are spin coupled to the (MoS4I2- groups, with some transfer of electron density from the former to the latter. [NEt4]Z[(PhS)2FeS2FeS2MoS2] has a 57Fe Mossbauer s ectrum consistent with an FeII' (S = 4) centre antiferromagnetically coupled to a formally {Fe' S2MoV'S2}(S = 2) unit to give an S = $ ground state." The "Fe isomer shift in [S2MoS2FeS2MoS2]3-is smaller than that expected for an oxidation state of I or I1 for the iron, suggesting extensive Fe-tMo charge transfer; the EPR spectrum indicates an S = 2 ground state, similar to that of F e M o ~ o .Electrochemical ~,~~ studies on [X2FeS2MoS2I2-complexes (X = SPh,7 OPh14) show an irreversible reduction at -1.34 V or -1.51 V us. SCE for the thiophenolate and phenolate complexes, respectively. The [(PhS)2FeS2FeS2MoS2]3ion displays an irreversible oxidation at 0.05 V and an irreversible reduction at -1.1 V us. SCE.19 The irreversible electrochemical reduction of C12FeS2MoS2FeC1212-occurs at -0.56V vs. SCE, with loss of a chloride to form woS$e&l,] -, which may exist as a dimer in solution.'6 Mo K-edge EXAFS spectra have been recorded for the binuclear complexes [X2FeS2MoS2]*- (X = C1, SPh, OPh) and the trhuclear com lexes [(4-Me3C6H4S)2FeS2FeS2MoS2]3-, [S2MoS2FeSzMoS2l3- and [ C I Z F ~ S Z M O S ~ C ~The ~ ] ~Mo-S - . ~ ~ , ~and ~ Mo-Fe distances determined agree with the X-ray crystallographic data to better than 0.5%. Comparisons of the Fourier transforms of the EXAFS spectra with that of the MoFe protein of nitrogenase show them to be significantly different, and suggest a first coordination sphere around molybdenum in the enzyme that is not simply tetrahedral. Despite the undoubted chemical interest of the complexes involving [MoS4I2- ligated to iron, the second category of Fe-Mo-S clusters, involving Fe3MoS4cubane-like cores, appear to have more relevance to FeMoco. Reactions between FeC13, [MoS4I2- and an excess of a thiolate anion in methanol solution yield several products based on cubane-like clusters containing one molybdenum and three iron atoms in a tetrahedral arrangement, with each face bridged by a p3-sulfide ligand." By varying the stoichiometry, reaction times and cations present in solution, three different types of product have been isolated from this system: [Fe6Mo2Sa(SR)9l3-, [F~~Mo~S,(SR},(OM~)~]~and [Fe7Mo2S8(SR)~2l3-(Figure 2). Each of these anions contains two { (RSFe)3(p3-S)4Mo}clusters with each molybdenum atom bonded to 16717,74
P
1
Molybdenum: Special Topics
1427
three atoms of a bridging system. The anions [ F ~ & I o ~ S ~ ( S R )(R ~ ]=~ Et, - Ph, 4-ClG&, 4MeC6H4) and [Fe6MqSs(SPh)6(0Me)3]3- contain three bridging (SR)- or (0Me)- ligands, which have been shown by 'H NMR spectroscopy not to interchange with the terminal ligands in solution.",78 [Me3NCH2Ph F ~ , M O ~ S ~ ( S E ~and ) ~ , ] the , ~ related (reduced) anion of [ N B U ~ ] ~ [ F ~ ~ M O ~ S ~ ( ,]8[involve S C H ~ P ~a) ~( ~ -I s ) ~ F e-S)3 ( p bridging arrangement containing a central Fe"', or Fen, atom, respectively. RS
\
R
S-Fe
\
R RS
SR
(4
Me
RS
SR (b) -
RS
-t3
SR .SR
Figure 2 Structure of some dimeric assemblies of Fe MoS, cubane-like clusters;n (a) [Fe6M02S,(SR)9]3-; (b) IFe$vro2s,(sR)6(oMe)31~-; (c) F+Mo8R(SR)l,i3-
The bridging system in [Fe7M~2S8(SEt)12]3can be broken by reaction with catechol, catHz, which yields [Fe4MoSd(SEt)3(cat)3l3-(Figure 3). The crystal structure of its [NEt4]+salt shows this anion to consist of a single {(EtSFe)3M~S4} cluster, with the molybdebum coordinated via three M o - S F e bridges to an Fe(cat)3 group acting as a tridentate ligand.80 The reaction between [NEt4]3[Fe7M02S,(SEt)12] and 3,&dipropylcatechol, Pr2catH2, yields [NEt4]4[Fe6M02S8(SEt)@r2Caf)2].This anion contains two (Fe3MoS4}cubane-like clusters with each molybdenum chelated by a (Pr2cat)2- ligand; two of the iron atoms in each cube are coordinated to one terminal ethanethiolate, and the third iron atom is bridged to the molybdenum atom of the other cube by a bridging ethanethiolate group. Similar products have been obtained with 3,6-diallylcatecholate, (alzcat)2- (Figure 4) and tetrachlorocatecholate, ( c L ~ a t ) ~ligands.81,82 -,
/
EtS
'"
Fipre 3 Structure of [F~,MOS,(SE~),(C~~),]~-
Ligand exchange reactions between [Fe6M02Ss(SEt)913-or [Fe7Mo&(SEt)l2I3- anions and thiols, acetyl chloride, benzoyl chloride, benzoyl bromide or phenol have shown that the COC3-TT
1428
Molybdenum
'
Q/ ~
Figure 4 Structure of [Fe&lo,S,(SEt),(3,6-diallyl~atecholate)~]
0
n
(0 0 = 3,6-diallyl~atecholate)*~
terminal thiolate ligands are labile and easily replaced by other thiolate groups, halides, or phenolates; the bridging ligands are inert to substitution.83-85However, the reaction between arenethiols . (RSH) and [Fe6MozS8(SEt)6(R~cat)z]4-(see Figure 4) yields [Fe6M~S8(SR)6(R;cat)z]4- (R= Ph, 4-MeC6H4, 4-C1c&; R' = Pr, allyl) in which both bridging thiolates have been substituted as well as the terminal ligands. In coordinating solvents, including DMSO, DMF, T W and MeCN, [ F e f l ~ ~ S ~ ( S R ) ~ ( R k a tdouble ) ~ ] ~ - cubane aggregates are cleaved to give the single Fe3MoS4 cubane-like cluster solvates [Fe3MoS4(SR)3(R~cat)(solvent)]2-(see Fi ure 5 . These systems possess specific reactivities at both the molybdenam and the iron sites.' 8 2 8 6 8 Thus, the solvent molecule is readily replaced by a variety of ligands (RS- (Figure 5), N;, CN-, RO- and PR,) and catecholate exchange can be effected at the molybdenum site. Normal thiolate exchange at the iron sites occurs and using acetyl chloride results in chloride for thiolate substitution. Reaction of the product [Fe3MoS4C13(cat)(solvent)]2- with CN- produces [Fe3MoS4(CN)4(cat)]3-,in which this substrate of nitrogenase is bound at each iron and molybdenum atom.
E .2 1
L
n
Elgue 5 Structure of [Fe,MoS4(SC6H,Cl-4),(3,6-diallylcatecholate~3-(R = 4-ClC6H4; L = 4-ClC6H4S;(0 0 = 3,6diallylcatecholate)
A novel double Fe3MoS4 cubane-like complex bridged by two persulfide groups has been prepared via an intermediate carbonyl cluster. Displacement of the chlorides from [Cl2FeS2MoS2I2- by [(C0)$e2S2Jz- yields [(C0)6FezSzFeS2MoSz]2-;this reacts with (4CIC6H4S)2to give IFe6M02S8(S,),(SC6~cl-4)6]2-,in which the two persulfides form a bridge between the molybdenum atoms of two {Fe3MoS4}units. This type of cluster can also be prepared in a self-assembly reaction from [MoS4]'-, FeC13, a thiolate and NazS2in methanol.88 The electronic structure of compounds containing the single and double Fe3MoS4cubane-like clusters has been investigated by 57Fe Mossbauer, EPR and NMR spectroscopy, and by magnetic moment measurements."T8M2 These results have shown that each {Fe3MoS4I3+ framework is electronically delocalized and the coupling of electron spins produces an S = 2 ground state. The EPR spectra of these single cubane systems show little dependence on the nature of the ligands bound to molybdenum and, of particular significance, closely resemble the EPR spectrum characteristic of FeMoco with the same magnetic ground state. Furthermore, none of these spectra manifest molybdenum hy erfine coupling. In the triply bridged double cubanes [Fe6Mo2S8(SR),J3-, the two {Fe3MoS4} sub-clusters are only weakly coupled, but in the doubly bridged anions [Fe6Mo$8(sR)6(R;cat&]4- there is strong spin coupling between the cubanes to give an S=O ground state. The 'Fe Mossbauer isomer shifts of compounds
E
Molybdenum : Special Topics
1429
possessing one or two {Fe3MoS4}'+ clusters compare closely with those for the S = state of FeMoco. Electrochemical studies have shown that each {Fe3MoS4}3+cubane-like cluster exhibits a reversible one-electron reduction and oxidation process, giving rise, for exam le, in the dimeric species [Fe6M02S8(SR)9]3-to 1-/2-, 2-/3-, 3-/4- and 4-/5- couples.7~~~s6~8991,93,94 57Fe Mossbauer isomer shifts indicate that the redox changes occur primarily at the iron centres." Chemical reduction using sodium acenaphthylenide generates the reduced complexes in solution, and the doubly reduced complex [Fe6Mo&(SPh)9j5- has been crystallized as its [NEt4]+ salt. This anion retains the double cubane structure of its parent, but with an increased Mo-Mo separation.% The reduced clusters react with PhSH to produce dihydrogen, and can catalyze electrochemical reduction reactions, including the conversion of ethyne to ethene and reduction of dinitrogen to ammonia,95101 Crystal structure determinations of the single and double cubane complexes have shown that the internal dimensions of the (Fe3MoS4} clusters are independent of the distances of 2.72f0.01A and nature of the other ligands present and, with Mo-Fe Mo-S bond lengths of 2.35 f 0.01 A, closely resemble the environment suggested for molybdenum in FeMoco on the basis of molybdenum K-edge EXAFS measurements. The molybdenum K-edge EXAFS of [ N B u ~ ] ~ [ F ~ ~ M ~ ~ S [NEt4]3[Fe6M02S9(SEt)s] ~(SE~)~~, and [NEt3(CH2Ph)]3[Fe6M02S8(SEt)3(OPh)6] agree well with crystallographic results, showing back-scattering from both a shell of sulfur atoms and a further shell of iron atoms. The molybdenum XANES of nitrogenases are similar to those of Fe3MoS4 cubane-like clusters possessing an MoS303 coordinati~n.'~However, unlike FeMoco, the iron K-edge EXAFS spectra of Fe3MoS4clusters show no net back-scattering from the molybdenum and iron atoms due to destructive interference of the Fe-Fe and Fe-Mo waves.lm Therefore, the FesMoS4 cubane-like clusters are deficient in this respect as models of FeMoco. A more fundamental objection is that their composition is not encompassed by the range of Fe:Mo:S values suggested for FeMoco. clusters have been made, especially using Other approaches to the synthesis of Fe-Mo-S metal carbonyls as starting materials. The reaction between [RMo(CO)~]~ (R = Cp, MeCp) and [FeS(CO)3]2 yields two isomeric iron-molybdenum-sulfur clusters with different {Fe2M02S2} .1037104 The skeletons, but both products can be formulated as [R2M02Fe2(p3-s)2(p-co)2(co)6] major isomer (80%) has the expected butterfly arrangement, with the two molybdenum atoms in the hinge positions and a butterfly angle of 104"; each sulfide bridges an {MozFe} face and two carbonyls bridge different Mo-Fe bonds; the Mo-Mo distance in the Cp product is 2.846(5) A, and the average Mo--Fe distance is 2.817(56) A.lM The minor isomer (20%) is a centrosymmetric cluster (Figure 6) with a planar .{Fe2Mq} skeleton, with the two (p3-S) groups bridging different {M%Fe} triangles above and below the plane; in the MeCp complex the Mo-Mo distance is 2.282(1) A, indicating considerable metal-metal interaction; the average Mo-Fe distance is 2.791(11) A.104 [R2M02S2(SH)21reacts with [Fe(C0)5] in the presence of Me3N0, or with [Fe2(CO)9] to yield [R2M02Fe2(p3-S)Z(p-S)2(C0)6], the structure of which (Figure 7) is related to that in Figure 6, by replacing the two p-CO groups by p-S2- ligands.lo5 [MoOC~~(THF)~] reacts with [Fe2(C0)6S2]2- to give [MOOFe&(C0)12]2-, in which the central {MOO} unit is bonded to two {Fe(C0)3} groups by two p3-sulfides, forming an {MoFe2} triangle, and to a tetrahedral {FeS4} group via two p,-sulfides; the other sulfides of the {FeS4}'form p3-bridges to two {Fe(C0)3} groups.'M MeCp
MdCp FCgare 6 Structure of
[(M~C~),MO,F~(~,-S),(~-CO),(CO),]'~
The interaction of [(Cpr)2M02S4] (Cpr= &Me5) with [Fe(NO)(C0)3]+ yields [(Cpr)2M02Fe2S4(N0)2]which, on the basis of spectroscopic evidence, is considered to have a
Molybdenum
1430
MeCp Figure 7 Structure of [(MeCp)2M02Fe2(~L,-s)~(~-s),(co)6]’”
cubane-like {Mo2Fe2S4}core. This reaction demonstrates how attack of a metal-metal bond by an electrophilic metal species can be used to build up larger clusters.’@’ [(Cp’)2M~2S4] reacts with [Fe2(C0)9] in THF to form [ ( C ~ ‘ ) , M O S ~ F ~ ( C O[(Cp’)zMo2S2(S2CO)Fe(CO)2] )~], and [(Cp‘)2M02S4{Fe(CO)2}2], the structures of which have been confirmed by X-ray crystallography, the last compound having an Mo2Fe2S4 cubane-like core. [(MeC5H4)2M02Fe2(CO)6S4] has also been reported. lo’ The addition of {Mo(CO),} fragments to iron carbonyl anions has also been used to synthesize iron-molybdenum-sulr compounds. Thus, the reaction between [Mo(CO)~(M~CN),]and [FesS&13- (X= Cl, 4-OC,H4Me) yields [Fe,S&{Mo(CO),},~(n = 3, 4). A Crystal structure determination accomplished for [ N E ~ ] ~ [ F e , s , c ~ , { M O ( ~ o ) ~ } ~ ] (Figure 8) has shown that the molybdenum atoms cap the top and bottom of the {Fe6S6CL} ‘prismme’ cluster. Derivatives containing such {Fe,MoS6} and {Fe7MoS6} units have been suggested as models for the Fe-Mo-S centre in nitrogenase but have not yet been synthesized.lW
CI
Figure 8
Structure of [Feas,Cl6(Mo(Co),},]‘-
Several complexes have been prepared containing tetrahedral iron atoms coordinated to two molybdenums through two pairs of p2-sulfido ligands. [NH4I2[MoS4] reacts with Na2(SCH2CH2S) and FeCl,; this mixture, on addition of [NMe4]Br, yields [NMe4]3[FeS4{MoS(SCH2CH2S)}2], the anions of which involve molybdenum atoms with a distorted square pyramidal geometry, and axial Me+ ligands.’” The catalytic activit of this and related complexes in the hydrogenation reaction of ethyne has been investigated. 171
Molybdenum: Special Topics
1431
36.6.3 MOLYBDENUM CHALCOGENIDES The principal ore of molybdenum is molybdenite, MoS2, which occurs as fine crystals embedded in quartz. Molybdenum deposits exist in many parts of the world, but few are sufficiently rich to warrant the extensive processing required for commercial exploitation. The two most important molybdenum mines are at Climax and Henderson in Colorado, USA and the alternative source of molybdenum is as a by-product of copper mining. MoS2 exists as an S-MOS * C M O S layer lattice, in which each molybdenum atom is surrounded by a trigonal prism of six sulfur atoms. MoS2 is of considerable commercial importance as a lubricant, catalyst precursor and support , precursor of intercalation superconductors, and as an electrode for photoelectrolysis. Therefore, the physical properties of MoS2 and its derivatives have been extensively examined.l13 MoSez and the low temperature form of MOT% adopt the same structure as MoS2, but in the high temperature form of MoTez each molybdenum atom is located in an octahedral arrangement of tellurium atoms and is involved in two close Mw-Mo approaches. ‘14 The amorphous trichalcogenides, MoS3 and MoSe3, have interesting electrochemical and other physical properties;ll’ molybdenum and selenium EXAFS studies have led to the proposal that an important structural unit of these systems is a binuclear metal site (Mo-Mo ca. 2.75 A) with a triple bridge consisting of one Y2- and one (Y = S , se) ion.l16 Mo-Mo and Y-Y bonding and their interrelationshi s are a dominant feature of the The clearest demonstration of chemistry of discrete molybdenum chalcogenide this interrelationship is the oxidation of [MoS4I2- by organic disulfides to form [MC&]~-, thereby achieving reduction of molybdenum (from Mow to Mov), production of persulfide ions and the formation of an Mo-Mo bond.’” Many other examples of oligomeric molybdenum systems bound to persulfide ligands exist ;118J19 two important examples are [Mq(S2)6]2(Figure 9)”l and [MO~S(SZ)~]’(Figure both as interesting systems in their own right and as synthetic precursors. Another important aspect of Mo-S chemistry is the demonstration that [NEt4Iz[MoS4]reacts with either elemental sulfur or organic trisulfides (RSSSR) in MeCN to produce [NEt4j2[MoS9].This anion involves an {MoS}~+(Mo-S = 2.128(1) A) core coordinated by two tetrasulfide ligands (Mo-S = 2.36(3) A) to give a square pyramidal geometry at the metal. Heating an MeCN solution of [MoS9I2- in air produces the [MOO(S~)~]’-analogue. lWOxomolybdenum centres ligated by groups containing one or more S-S bonds have been identified in many other systems, including: [ M O O S ~ ( S ~ C N P ~ ~ ) ~ ] , ’ ~ ~ [Moz(S~O)Z(SZCNE~$~] ,l25 [MO~S&Z)(S,O)]’- 12‘ and ~MO~O,S,-,(SZ)Z-,(S,),]~-(n = 2, x = 0, 1; n = 1, x = 1).12 Amons of this last type, with n = 0 and x = 0, 1 , 2 have been shown to participate in a complicated series of equilibria which arise in reactions of [MoS4jZ- with elemental sulfur, organic trisulfides or [NH4I2Sx,and thiols; these equilibria involve monomeric and dimeric molybdenum sulfide complexes and the equilibrium position depends upon several factors, including the nature of the solvent and the counter ion^.^'^,^^^ One product of these reactions [(S4)Mo(S)(p-S)zMo(S)(S4)]2- (Figure 11) is an isomer of [ M O ~ ( S ~ ) (Figure ~ ] ~ - 9) with a remarkably different arrangement of sulfur atoms. Complexes involving Sf and S:- ions ligated to molybdenum have also been characterized.
-
z-
en ti tie^."'"^
S
n
Figure 9 Structure of [Moa(S&jZ-
”’ I
Molybdenum complexes of polysulfide ligands art reactive and, for example, readily make and break sulfur-carbon bonds, a property undoubtedly relevant to the involvement of M A
1432
Molybdenum
us
Figure IO Structure of [Mo,s(s,)~]’-
”’
S
figure 11 Structure of [(s,)Mo(s)(~-s),Mo(s)(s~)~~-
centres
in
hydrodesulfurization catalysis. Addition
of
CS2 to
[PPh4I2[MoS9] and
[PP~,]Z[(S~)M~(S)(~-S)~M~(S)(S,)J produces [PPh4]2[(CS&MoSj and [PPh4]2[(CS,)Mo(S)(~-
S)2Mo(S)(CS,)], respectively, both products containing the perthiocarbonate ligand bound as an S,S chelate. The former anion contains an {MoS}~+core coordinated by two nearly planar (CS4}2- li ands in a trans configuration; the latter anion consists of a syn-{M~~S,}~+ core with two {CS4Q- ligands coordinated to the molybdenum atoms in a cis config~ration.’~~ 2-Butyne dimethanoate, M e 0 2 C M C 0 2 M e , reacts with [MoS9I2- to form, as one of the products, [Mo{S2Q(C02Me)2}3]2-, a trigonal prismatic complex with three dithiolene ligands.131 Similarly, M e 0 2 C M C 0 2 M e reacts with [(S)4Mo(S)(p-S)~Mo(S)(S4)]2- or [(CS4)Mo(S)and with [ M o O ( S ~ ) ~ ]to ~ - form (p-S)2Mo(S)(CS4)]2- to form [MozS4{S2C2(C02Me)2}2]zReaction of with [Mo 0{ S2C2(C02Me),}21 [(S4)Mo(0)( p -S12Mo(0)(S2)I2Me02Ck=CC02Me results in the formation of the dithiolene complex [MO~O~S~(S~C~(CO~M~)~}~]~-, which is an isomer, in respect of the C-S framework, of the vinyl disulfide complex (Figure 12) obtained by reacting [(S~)MO(O)(~--S)~MOO(S~)J~with
’-.
Figure 12 Structure of the bis(viny1 disulfide) complex [ M o 2 0 ~ S , { ~ ~ ( C O z M e ) z } 2 ] 2 ~
Molybdenum : Special Topics
1433
Me02CC=Cq02Me. Further important reactivities of M0-S-C systems have been demonstrated by Rakowski DuBois et al., for dimeric M c F S complexes involving each metal ligated by a cyclopentadienyl ligand (Section 36.6.5). 36.6.4
NITROSYL AND CYANIDE MOLYBDENUM-SULFUR COMPLEXES
The structural chemistry of these systems is interesting and a considerable versatility exists, especially in respect of the nature of the Mo-S framework to which nitrosyl or cyanide ligands are a t t a ~ h e d . l l ' > ~ ~ ~ * ~ ~ ~ [ M O ~ ( N O ) ~ ( S ~ ) can ~ O ]be ~ -obtained by the reaction of [MoO4I2- with NH20H-HC1 with H2S in H20. This anion (Figure 13) consists of a central oxygen atom bound to four molybdenum atoms, arranged as a tetragonal bisphenoid over whose edges two handle-shaped and four roof-shaped Sf ligands are located; each molybdenum is bound to a terminal nitrosyl ligand.136The related anion [ M O ~ ( N O ) ~ S$Figure ~ ~ ] ~ -14) involves a central sulfur bound to four MONO}^+ moieties, five S$- and two S - ligands (rather than the six S$- ligands in Figure 13). [ M O ~ ( N O ) ~ Sis~ ~one ] ~ -product of the reaction of molybdate with ammonium polysulfide in aqueous solution in the presence of hydroxylamine .134,137 [MO~(NO),(S~)~(S~)(OH)]~(Figure 15) has been obtained and involves an unusual (~6-s~)~ligand.13' Other studies of these and related anions have been r e ~ 0 r t e d . l ~ ~
8.
vs"s
Figure U Structure of [Mo,(NO),(S,),O]~-
13'
Nitrosyl and cyanide ligands can coexist bound to a molybdenum-sulfur core, as in [Mo&(NO)2(CN)6]6- 13* and [ M O ~ S ~ ( N O ) ~ ( C N ) , ]The ~ - . 'latter ~ anion has been obtained by reacting [Mo4(NO),(S&0]*- with KCN and it possesses a distorted cubane-like MO& framework, composed of two Mo2S2 quadrilaterals (Mo-Mo = 2.99(3) A) linked with long (3.67(4)A) Mo * Mo separations; each molybdenum is bound to one nitrosyl and two cyanide ligands external to the M04S4 cluster.lm [ M O ~ S ~ ( C N ) also ~ ~ ] ~contains an Mo4S4 cubane-like cluster, but the molybdenum and sulfur atoms are arranged as two, almost regular, interpenetrating tetrahedra with all the Mo-Mo separations essentially the same (2.85 A); each molybdenum is octahedrally surrounded by three CN- and three S2- 1iga11ds.I~~ K5[Mo3S4(CN)9].3KCN.4Hz0 has been structurally characterized and shown to involve cyanide ligation of a normal (M0&}4+ core.135 Cyanide ligation to { M o ~ S } ~(see + Section 36.4.1.3), { M o ~ S ~ } and ~ + (Mo&}~+ cores has also been demonstrated; [Mo2S2(CN),I4- involves a significantly longer (2.758(7) 'us.2.64(5)A) Mc-Mo bond than in [ M O ~ S Z ( C N ) ~ ] " - . ~ ~ ~ ' ~ ~ ~ The redox chemistries of di-, tri and tetra-nuclear Mo-S cyanide complexes have been Passage of oxygen into an aqueous discussed in relation to their electronic solution of [Mo2S2(CN),I6- leads to the formation of a dark violet mixed-crystal compound of the composition IC4+x[Mo2(S02)(S2)(CN)8]x[Mo2(S02)(S2)(CN)8]1-,~4Hz0 (x = 0.3). In the crystal the two anions, whilst structurally similar, are located at crystallographically independent positions; each involves both molybdenum atoms surrounded by an approximately
1434
Molybdenum
Figure 14 Structure of [Mo4(NO)4S,3]2-137 S
s Fiure 15 Structure of [MO~(NO),(S~),(S,)(OH)]~-
pentagonal bipyramidal array of four terminal CN- and three sulfur atoms of the bridging So"- and S2- ligands. However, these anions involve the significantly different Mo-Mo separations of 2.790 and 2.684 A, respectively, and the results of spectroscopic studies have led to the suggestion that the former is a mixed MO"*MO'~dimer whilst the latter involves two Molv atoms.143 36.6.5
CYCLOPENTADIENYL MOLYBDENUM-SULFUR SYSTEMS
Cyclopentadienyl molybdenum-sulfur compounds are useful s nthons for the preparation of Mo-M-S (e.g. M = Fe {see Section 36.6.2), Co, Ni) clusters10z1m,144in reactions with metai carbonyls. However, the principal interest in molecules of this class has arisen because of the reactivity of the M e S system, primarily in respect of the making and breaking of S-H
Molybdenum: Special Topics
1435
and/or S-C bonds, leading to novel chemistry and an illumination of processes relevant to hydrodesulfurization catalysis. [ C P ~ M O ~ ( C Oreacts ) ~ ] with elemental sulfur to form insoluble prod~cts,~" but the introduction of methyl substituents on the cyclopentadienyl ring permits the isolation of the soluble products [(Me5C5)2M02S4], [(MeC5H4)2MozS4] and [(MesC5)zMoz(S2)s].The first two products possess an ~ n t i - { M o ~ S core ~ } ' ~(Section 36.4.3.5.3) and the last one involves two $-SZ ligands bound to each metal with each bonded to a single sulfur atom of a p2-S2 ligand.146Molecules of the composition [R;Mo2S4] (R' = Me,C5H5-,, R = 0, 1, 5 ) may exist in one of three isomeric forms (Figure 16) and photochemical isomerization between them has been demon~ t r a t e d . ' ~Dimers ' ~ ~ related to form I with oxo or imido ligands have been characterized,lm the electronic structure of these molecules has been investigated,151and their intramolecular steric effects have been modelled.1s2 ( R -Mo-R' 1
I1
/?\ m111
Figure 16 Isomers of molecules with the composition (R&To,S,] (R' = Me,C,H,-,,
n = 0, 1, 5)16149
The molecules [R;Mo&] and their relatives manifest interestin! reactivity towards HZI alkenes, alkynes and other compounds with unsaturated linkages. 46,153-156 [R;MoS4] cornpounds react with HZ under mild conditions to form a quadruply bridged structure with two hydrosulfido-ligands, [ R I M o ~ ( ~ - S ) ~ ( ~ - SThese H ) ~ ] .systems catalyze H-D exchange between H2 and D2, as well as between HZand D20, and catalyze the reduction of elemental sulfur and S02.153-154 Both alkene and arenethiols (RSH) react with [ R ~ M O ~ ( ~ - S ) ~ ( ~to- release S H ) ~ ]HzS and form the corresponding [R;Mo2(p-S),(p-SR),]] derivative; the structure of the n = 1, R = Me derivative has been determined and the molecules shown to be centrosymmetric with an Mo-Mo separation of 2.582(1) A. [ R ~ M O ~ ( ~ - S ) ~ ( ~ -reacts - S H )with ~ ] various unsaturated molecules, for example ethylene or benzyl isocyanide, releasing Hz and formin dimers with two bridging ethane-1 ,Zdithiolate or dithiocarbonimidate groups, re~pective1y.l~The kinetics of the reactions of benzyl isocyanide with [(MeCp)ZMoz(p-S)z(p-SH)Z]and [(MeCp)2Mo.&S)z(p-SCHzCHzS)], to form the same dithiocarbonimidate complex, [(MeCp)2M02(pS2CNCH2Ph)2],have been determined and the results suggested that the former reaction proceeds by an associative mechanism whilst the latter proceeds by a dissociative pathway. Deprotonation of [R;MC&S)~(~-SH)~] with NaOMe (1:2) in the presence of excess CHzBrz results in the formation of the dimer [RiMozS2(&CH2)] with a bridging methanedithioiate ligand;156this compound will catalyze the homogeneous hydrogenolysis of CSz.15' Addition of GJ& or GH2 to [R;Mo2S4] yields a quadruply brid ed dimer with, respectively, two p-ethane-1 ,Zdithiolate or p-ethylene-1,Zdithiolate ligands.' % Complexes related to the former, viz. [(Cp)2Mo(p-SC,H,S)2] (R = 2 and 3), have been prepared by the reaction of ethylene sulfide and propylene sulfide, respectively, with [CpMoH(C0)3] or [(Cp);lMoz(CO)6] and their redox properties have been studied. These neutral dimeric complexes undergo a unique reaction with alkenes and alkynes, in which the hydrocarbon portion of the bridging dithiolate ligand is exchanged.15sA series of these complexes, in which the molybdenum atoms are bridged by two inequivalent dithiolate ligands, have been synthesized and the relative tendencies of the ethanedithiolate ligands to eliminate ethylene investigated. Alkenes and cumulenes react with these complexes, displacing ethylene to form derivatives with dithiolene bridges.15' [Cp2Moz(C0)6] reacts with [Zn(S3CPh)z] to form [ C ~ M O ( C O ) ~ ( S ~ Cand P ~ ) ][CpzMo~(pS)2(S2CPh)2],and with 3,Cdimercaptotoluene to yield a purple dimer which, on irradiation, decomposes to produce the green compound [(Cp)zMo&-SzC~3Me2)2],which has a structure in which the core is similar to that of [(Cp)zMoz(p-SCHCHS)z].159 [CpzMoz(p-SMe),] may be obtained by reacting [Cp2Moz(C0)6]with MeSSMe and oxidation by Ag[PF6] yields the monocation; the neutral and cationic species possess the same geometry and oxidation leads to no essential change in dirnensions.l6O A wide variety of other cyclopentadienyl molybdenum-sulfur compounds are known including: monomeric [CpzMo(SR)2],161,162 [CpzMoSz] and [Cp2MoS4] dimeric
5
COC3-TT.
1436
Molybdenum
[(BuC~)~MO~(~-S and ~)C [C~MOCI~(~-X)(~-SM~)~C~MOC~] ~~] (X= OH, SMe),lM trimeric ) ~ ] the + , ~ tetrameric ~ [Cp3M03(p-S)3(p3-S)]n+(n = 0, " 116') and [ C P ~ M O ~ ( ~ ~ - S ) ( C Oand [(Pr'Cp),Mo4(p3-S),]"+ ( n = 0, 1, 2)16' and related systems (see Section 36.3.3.2).la
36.6.6 DITHIOLENE AND RELATED COMPLEXES Transition metal complexes of unsaturated 1,Zdithiolates (metal dithiolenes) have attracted much attention because of their interesting structural and redox properties. 169 Molybdenum dithiolene complexes have featured prominently170in these studies and have special significance following the s u g g e ~ t i o nthat ~ ~the ~ ~molybdenum-containing ~~~ cofactor of the oxomolybdoenzymes (Section 36.6.7) incorporates a molybdenum complex of an unsymmetrically substituted alkene-l,2-dithiolate. The best characterized complexes are the tris(dithio1enes) which are known for several ~ ~ ~ ~ ~ (R= H, Me, CF3, C02Me, Ph, ligands, includin : {S2G(CN)2}2- ( ~ n n t ) , '(2S2GR2}24-MeC&), 131~17'183 {S2C&}2-, { S2C&-4-Me} - (tdt), { S2CJ12-4,5-Me2}2-,{ S2GjF4}2and quino~aline-2,3-dithiolate.~~~ These complexes are intensely coloured, due to S +Mo charge-transfer transitions and they have been synthesized by a variety of procedures, this versatility being required by the instability of some of the free ene-1,Zdithiols. cis-1,2Dicyanoethane-lY2-dithiolate(maleonitriledithiolate, mnt), however, is a stable species and MoC15 reacts with Na2mnt in THF to produce A1so, H2tdt reacts with ] ; reacts with molybdate in acidic solution,184or with MoC15 in CC14,182to form [ M ~ ( t d t ) ~H2tdt [ M O O C ~ ~ ( M ~ P Pin~ ~MeCN )~] in&be prisence of Et3N to form [M 0 (t d t )3 ]~-. ~~ Bis(trifluoromethyl)dithieten, CF3C(S)(S)CCF3 is a convenient reagent and reacts with [Mo(C0)6] to form [ M o { S ~ G ( C F ~ ) ~ } reduction ~);'~ of this compound by hydrazine in ethanol affords the corresponding dianion.In Procedures for the in situ synthesis of dithiolene ligands have extended the range of metal-dithiolene chemistry. Thus, cyclic thiophosphate esters, prepared from benzoins or acyloins and P4S10, have yielded ene-1,Zdithiolate complexes when treated with transition metal salts under acidic condition^.'^^ Also, hydrolysis of 2-(N,Ndialkylamino)-l,3-dithiolium salts affords a general method for the synthesis of ene-1,2dithiolates and allows different substituents to be bound to the alkenic carbons.lW Molybdenum tris(dithio1ene) complexes undergo electrochemically reversible, one-electron, redox processes. [Mo(mnt)#- dis lays redox changes corresponding to the O/l-, 1-/2-, 2-13and 3-/4couples;" [Mo(S~GR~)~]'- (R= H, Me, Ph, CF3)17731827191and [M~(tdt),]"- 182 manifest O / l - , 1-/2- and 2-/3- couples. These observations have encouraged chemical oxidations or reductions of these species, and stimulated a lively debate as to whether the processes are primarily metal- or ligand-based. The ligands are 'non-innocent' and this renders the precise definition of oxidation state difficult in these systems, but it is often assumed that each coordinated dithiolene contributes +I1 to the count of the metal's oxidation state. Discussions of the electronic structure of these systems have generally incorporated the information available from EPR studies of the paramagnetic species and X-ray crystallographic data. 131,175,181,183,189 Metal trisdithiolenes were the first non-octahedral, six-coordinate transition metal complexes to be structurally characterized. Much of the interest shown in these com lexes has concentrated on attempts to explain the observed trigonal prismatic geometry" and it is generally considered that the unusual structures of these complexes are due to electronic, rather than steric, effects. Two MO schemes have been proposed1mg182which differ slightly in the ordering of the valence orbitals and that due to Gray et al.1s2gives better agreement with spectroscopic data. In this scheme, the stability of the trigonal prismatic geometry is attributed to two n-bonding interactions. Firstly, the nh interaction which results from overlap of the metal dZz orbital with the sulfur sp2 lone pair orbitals at 120" to the M-S and S-C bonds and, secondly, the nu bonding which arises from overlap of the metal dxy and d x ~ ~orbitals y2 with the butadienoid n orbitals of the ligand S2C, groups. The optimal electron configuration for trigonal prismatic geometry is that of the neutral trisdithiolene complexes of the Group VI transition metals, Le. the bonding xh and x, orbitals are filled and their antibonding counterparts unoccupied. The LUMO in these complexes is an antibonding nh orbitd; therefore, the occupation of this orbital in monoanionic and &anionic trisdithiolene complexes of the Group VI transition metals should destabilize the trigonal prismatic structure and allow distortion towards octahedral geometry. In support of this argument, the structure of [ M ~ ( m n t ) ~ ]is~severely distorted from trigonal prismatic geometry.17'
Molybdenum : Special Topiu
1437
[MOH(C&&-1 ,2)313- has been ~ynthesized”~ and the selenium analogue of a trisdithiolene complex [MO(S~&(CF&}~] shown to involve the six selenium atoms arranged in a trigonal prismatic manner about the molybdenum. lg4 A wide variety of other molybdenum dithiolene complexes have been prepared and characterized. Mononuclear complexes involving other ligands in the primary coordination sphere include: [NB~][MO(~~~)~(S~CNE~~)], which possesses an essentially trigonal prismatic Moss a r r a ~ ~ g e m e n t[CpM~(mnt)~]-, ,’~~ [Cp2Mo(mnt)] and [Cp2Mo(tdt)].161,196 Several dimeric molybdenum complexes with dithiolene ligands are known and, of these, [ M ~ ~ ( t d t )is# ~ the only system based on dithiolene ligands alone. One of the products from the reaction of [NH4]2[Moz07J with benzoin and P4S10 is the binuclear dithiolene complex [Mo2(S2)(S2C2Ph2)4]. An X-ray analysis of the structure has revealed that each molybdenum atom is coordinated by seven sulfur atoms, in an approximately monocapped trigonal prismatic arrangement; the two metal atoms are bridged by four sulfur atoms of a shared face of the trigonal prisms, two of the bridging sulfur atoms are supplied by a brid in Sz ligand and the other two are supplied by two bridging dithiolene !he structure of [Mo202S2{S2G(CN)2};]2- has been determined; each molybdenum atom possesses distorted square pyramidal coordination geometry with the sulfur atoms forming the basal plane and an oxygen atom in the axial position.lW Other dimeric molybdenum complexes with dithiolene ligands have been considered earlier in Sections 36.6.3 and 36.6.5.
ligand^.'^'
36.6.7
OXOMOLYBDOENZYMES
The existence of a biological role for molybdenum was first recognized in 1930.200Since that time, an increasing volume of evidence has accumulated to show that molybdenum is essential for a significant number of biological processes. For example, molybdenum is required both for nitrogen fixation (Section 36.6.2) and nitrate reduction, and its participation in some animal hydroxylase enzymes and several fungal and bacterial enzymes is now firmly established.201The molybdoenzymes are all reasonably large proteins, the functions of which involve the molybdenum centres undergoing redox reactions in concert with other redox active constituents. An important aspect of the composition of the molybdoenzymes has been the demonstration of the existence of a low molecular weight prosthetic group (or cofactor) containing molybdenum necessary for catalytic activity and which may be dissociated reversibly from the main bod of the protein. The original concept was of a cofactor common to all the molybdoenzymes.’ Subsequently, studies d e r n o n ~ t r a t e d ’ ~that , ~ ~there are at least two such cofactors; nitrogenases yield FeMoco (Section 36.6.2) which is distinct from the molybdenumcontaining cofactor, or Mom, of enzymes such as xanthine oxidase and dehydrogenase, sulfite oxidase and nitrate reductase. Molybdoenzymes other than the nitrogenases are usually termed oxomolybdoenzymes. This prefix relates to the nature of the catalysis effected, i.e. the net effect of the conversion (xanthine to uric acid, sulfite to sulfate, nitrate to nitrite, or aldehyde to carboxylate) corresponds to the transfer of one oxygen atom to or from the substrate. Furthermore, molybdenum X-edge EXAFS studies have established that this metal is coordinated to one or more terminal oxo groups in each enzyme studied by this technique.2w EPR spectroscopy has been extensively applied to the oxomolybdoenzymes.zo5These studies have established that all of these enzymes have similar molybdenum centres, at which the substrate undergoes the catalytic conversion. The redox states of molybdenum, involved in the normal catalytic cycle of these enzymes, are Mow, Mov and MoTVand the EPR characteristics of the MoVcentres, and hence the Mow and Mo’” states, are mononuclear in molybdenum and coordinated to s ~ l f u r EPR . ~ spectroscopic studies have established that, at least in the EPR-detectable form of each of the oxomolybdoenzymes at low pH, there is a minimum of one proton magnetically coupled to the unpaired electron of the MoV centre which is chemically exchangeable with the protons of the external aqueous medium. Furthermore, for xanthine oxidase, EPR spectroscopy has provided direct evidencem in support of the hypothesism that the mechanisms of all the molybdoenzymes involve coupled proton-electron transfer at the molybdenum centre. EPR spectra using ”0-enriched H20have established that oxygen atoms are bound to the Mov centres209and studies with 33S suggest that the difference between active and desulfo xanthine oxidase involves the replacement of an M d t group in the former b the M-0, group in the latter?1° The results of molybdenum K-edge EXAFS studies’ are complementary to those of EPR spectroscopy, not only since it is easier to probe the EPR
Molybdenum
1438
silent states by the former technique, but also because the results provide additional information, in respect of the nature, number and distance of the ligand donor atoms attached to molybdenum. For example, in oxidized sulfite oxidase molybdenum is bound to two oxygen atoms at 1.68 A and two to three sulfur atoms at 2.41 A; reduction to the Mo’” state results in the loss of one of the oxo groups. The molybdenum centre of ChloreZla vulgaris nitrate reductase resembles that in sulfite oxidase, but some important differences occur for molybdenum in Escherichia coli nitrate reductase.’ll Also, molybdenum K-edge EXAFS studies have strengthened the view that an Mo=S, group is present in active, but not desulfo, xanthine oxidase and dehydrogenase2I2and identified a close approach (cu. 3.0A) of Mo and As atoms in arsenite-inhibited xanthine oxidase.’13 A major development in the definition of the constitution of Moco was the recognition214 that an oxidized, inactive, form of this cofactor has the fluorescence properties characteristic of a pterin. Further studies indicated that the fluorescent material was derived from a novel, sulfur-containing reduced pterin substituted at the C-6 po~ition.”~ Subsequently, a metabolic relationship was established between Moco and urothione, since persons genetically deficient in Moco are unique in having no detectable urothione in their urine. This metabolic relationship, and the chemical reactions of the cofactor, formed the basis from which Johnson and Rajagopalan proposed a structure (Figure 17) for Moco.’”
Figure 17 Proposed structure for the molybdenum cofactor of the oxomolybdoenzyrnes”*
The biological importance of molybdenum, and the challenge to reproduce the spectroscopic properties and the reactions of the molybdenum centres of the oxomolybdoenzymes, have been considerable stimuli for the development of the coordination chemistry of this element (see Chapters 36.4 and 36.5).
36.6.8
HYDRIDE AND DIHYDROGEN COMPLEXES
The most extensive group of molybdenum hydride complexes are those which contain phosphine ligands, and mononuclear systems with one to six hydride ligands have been identified. The reaction of [M~(a ca c)~] and diphos in toluene with AlEt, under Ar affords [Mo(C&L,)(dipho~)~]and [MoH(acac)(dipho~)~]. The former complex reacts with Hacac to produce the latter and this hydrido complex initiates the polymerization of acrylonitrile.215 [MoCI,(THF)~]reacts with PMe3 in Et,O or THF to form [ M o C ~ ~ ( P Mwhich ~ ~ ) ~reacts ] with Na[B&] to yield [ M O H ( B H ~ ) ( P M ~ ~whose ) ~ ] , structure has been determined by X-ray crystallography; the molecular unit involves a distorted octahedral environment at the molybdenum, with the bidentate [BHJ group considered to occupy one vertex, and Mo-H = 1.63 A.216 The carbonyl hydrides of molybdenum include [pH( M O ( CO) ,PM~P ~~}~]-, which has been obtained by reacting [~-H{Mo(CO)~}Z]with a 10-fold excess of PMePh2; its structure has been determined for the [NEt$ salt, the Mo - Mo separation in the three-centre, two-electron hydride bridge is 3.443(1 A, significantly shorter than that of 3.7436(1) 8, in [p-H{Moz(CO)gPPh3}]- and only 0.0’2 longer than in [ ~ - H { M O ( C O ) ~ ) ~ ] -A. ~ combined ~’ structural and spectroscopic study has been performed to examine the stereochemical consequences resulting from the deprotonation of the bent Me-H-Mo bond in [(p-H)(p-PMe2){CpMo(CO)z)z]; the most significant structural change is the reduction of the Mo-Mo separation from 3.262(2)A in the hydride to 3.157(2) in [(~-PM~,){C~MO(CO)~}~]-.~~~ Reduction of [MoC14(THF),] by Mg under H2 in the presence of PMe3 gives [MoH2(PMe3),] which possesses a distorted pentagonal bipyrarnidal stereochemistry with Mo-H bonds of length 1.67A. Both the ‘H and 31P{1H}NMR spectra of this compound are consistent with a fluxional structure; the former manifests a binomial sextet at S = -5.23 ~ . p . m . ’ ~[ M ~ O H ~ ( P M ~ , P ~reacts ) ~ ] with Ag[BF4] (1 :2) in MeCN to give
--
1
Molybdenum: Specid Topics
1439
[MoHz(PMezPh)4(NCMe)2][BF4)2,the cation of which has been shown by spectroscopy and X-ray crystallography to possess a fluxional dodecahedral structure. The same salt is obtained by treating the tetrahydride with H[BF4].Etz0 in MeCN, but [MoH4(PMePh2),] reacts with .’ % As a further Ag[BF4] or H[BF4].Etz0 in MeCN to produce [MOH~(PM~P~~)~(NCM~)~]’+ variation on this theme, [ M O I & ( P M ~ P ~ ~reacts ) ~ ] with H[BF4] in THF to produce [{MoH2(PMePhz)3}z(p-F)3][€3F4], the structure of which has been determined by X-ray crystallography. Only one hydride per metal was revealed directly by the crystallographic analysis, but the presence of the second hydride was confirmed by IR and NMR studies; the ligands adopt a dodecahedral arrangement about each metal centre.”l [ M ~ H ~ ( d p p ereacts )~] with Ph3CC1 in CHZCl2to form [M~H~Cl~(dppe)~]~C€€~Cl~~~ and [MoH2(diphos)J has been prepared by reactin [Mo(acac)3] with AlB& in benzene in the presence of di hos under an atmosphere of HZ,’ and by reacting [ M ~ ( N ~ ) ~ ( d i p h owith s ) ~ ][FeH4(PEtPhz)3].‘4 [MoH4L4](L = tertiary phosphine; LZ = diphosphine) complexes are conveniently prepared by borohydride reduction of [MoC14L]in the presence of the phosphine,225or by treatment of [Mo(N&L4] with ‘H NMR spectroscopy has shown that [MoH4L4] molecules are non-rigid; the nature of this non-rigidity has been discussed with reference to the molecular structure of [ M O H ~ ( P M ~ P ~ which ~ ) ~ has ] , been shown by X-ray analysis to contain a trigonal dodecahedral MOIj[4P4 framework, with the hydrogen atoms at the vertices of the elongated tetrahedron and the phosphorus ones at those of the flattened tetrahedron.’” The dinuclear complex [(PMe3)3HMo(p-H)2MoH(PMe3)3] has been mentioned in Section 36.3.2.1.1. [MoH&] (L = PPrj, PPhP&, PCy3, PCyzPh; Cy = cyclohexyl) have been prepared by reacting [MoC~~(THF)~], L and Na[A1H2(OCHzCH20Me)2]and characterized by ‘H and 31P{1H}NMR spectroscopy; the hydridic resonance occurs at cu. -4 p.p.m. These complexes have been claimed as the first examples of nine-coordinate mononuclear molybdenum, but attempts to prepare [MoH9I3- were unsuccessfuLzzs An exciting recent development has been the isolation of transition metal Complexes containing a coordinated dihydrogen molecule, bound as an q2 ligand. Toluene solutions of the 16-electron compound [Mo(CO)~(PC~&] react readily and cleanly with H2 (1 atm) to form mer-trans-[M~(CO)~(PCy~)~(H~)]. The H2 is extremely labile and, for the corresponding PPr$ complex, the addition is reversible in solution. Neutron and X-ray diffraction performed for [W(C0)3(PP&)2(H2)]confirmed the q’ bonding of the H2 molecule and this is consistent with vibrational spectra obtained for the Hz, Dzand HD These complexes are important, not only because of their relevance to the participation of metal centres in catalytic hydrogenation, but also since they pose an interesting theoretical challenge. In respect of this latter point, at present it is not clear why [MO(CO)~(PR,)~(H~)] complexes involve an q2-H2 molecule whereas, for example [MoH2(PR&] involve two hydride ligands. Dimeric molybdenum halide complexes involving a y-hydride ligand are described in Section 36.5.2.2.6. The novel dithiolene hydride complex [ M o H ( C & I ~ S ~ - ~ , ~ has ) ~ ] ~ been ~haracterized;’~~ ~ [ M O H ( C N ) ~ ] - ~has H ~been O prepared by acidification of K,[Mo(CN),] and its reactivity investigated.=’ Hydrido-cyclopentadienyl,23’ -q6-benzeneZ3’and -carbonylB3rnolybdenum complexes have a rich chemistry which is outside the scope of this review. [ M O ~ ( O A ~ ) ~ ( C H ~ S(0 ~Ar M ~=~2,6-dimethylphenoxide) )~] in the presence of pyridine undergoes the loss of Me4Si to form [ M O ~ ( ~ - H ) ( ~ - C S ~ M ~ & . ( O A ~ ) ~ ( ~ ~ ) ~ ] . ’ ~ ~
36.6.9
REFERENCES
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1444
Molybdenum
208. E. I. Stiefel, Proc. Natl. Acad. Sei. USA, 1973, 70, 988. 209. R. C. Bray and S . Gutteridge, Biochemistry, 1982,2l, 5992. 210. S. Gutteridge, S . J. Tanner and R. C. Bray, Biochem. J., 1978, 175, 887; J. P. G. Malthouse and R. C. Bray, Bwchem. J., 1980,191,265; J. P. G. Malthouse, G. N. George, D. J. Lowe and R. C. Bray, Biochem. J . , 1981, 199, 629. 211. S. P. Cramer, L. P. Solomonson, M. W. W. Adams and L. E. Mortempn, J. Am. Chem. Soc., 1984,104,1467. 212. S. P. Cramer, R. Wahl and K. V. Rajagopalan, J. Am. Chem. Soc., 1981,103,7721; J. Bordas, R. C. Bray, C. D. Gamer, S. Gutteridge and S . S . Hasnain, Biochem. J., 1980,191,499. 213. S. P. Cramer and R. Hille, J. Am. Chem. SOC.,1985,107,8164. 214. J. L. Johnson, B. E. Hainline and K. V. Rajagopalan, J. Biol. Chem., 1980,255,1783. 215. T. Ito, T. Kokuba, Y. Yamamato and S. I. Ikeda, J. Chem. SOC., Chem. ibmm., 1974, 136; J . Chem. Soc., Dalton Trans., 1974, 1783. 216. J. L. Atwood, W,E. Hunter, E. Carmona-Guzrnanand G. Wilkinson, J. Chem. Soc., Dalton Trans., 1980, 467. 217. M. Y. Darensbourg, J. L. Atwood, W. E. Hunter and R. R. Burch, Jr., J. Am. C h m . SOC., 1980, 102, 3290. 218. J. L. Peterson and R. P. Stewart. 3r.. horn. Chem., 1980,19, 186. 219. M. B. Hursthouse, D. Lyons, M. Thomntotb'ett and G. Wilkinson, 1. Chem. Sac., Chem. Commun., 1983,476. 220 L. F. Rhodes, J. D. Zubowski, K. Folting, J. C. Huffman and K. G. Caulton, Inorg. Chem., 1982, 21, 4185. 221. R. H. Crabtree, G. G. Hlatky and E. M. Holt, J. Am. Chem. SOC., 1983,105,7302. 222. E. B. Lobkovskii, V. D. Makhaev, A. P. Borisov, K. N. Semenenko, Yu. A. Antipin and Yu. T. Struchkov, Koord. Khim., 1985, 11, 983. 223. A. Frigo, G. Puosi and A. Turoo, Gazzetta, 1971, 101, 637. 224. B. Bell, J. Chatt and G. J. Leigh, J. Chem. SOC., Dalton Trans., 1972, 2492. 225. F. Pennella, Chem. Commun., 1971, 158; Inorg. Synth., 1974, 15, 42; R. H. Crabtree and G. G. Hlatky, Znorg. Chem., 1982, 21, 1273. 226. L. J. Archer and T. A. George, 1. Organomet. Chem., 1973,54, ( 2 5 . 227, P. Meakin, L. J. Guggenberger, W. G. Peet, E. L. Muetterties and J. P. Jesson, J , Am. Chem. SOC., 1973,95, 1467. 228. R. H. Crabtree and G. G. Hlatky, Znorg. Chem., 1984, U,2388. 229. G. J. Kubas, J. Chem. SOC., Chem. Commun., 1980, 61; G. J. Kubas, R. R. Ryan, B. I. Swanson, P. J. Vergamini and H. J. Wasserman, J. Am. Ckem. SOC., 1984,106,451; G. J. Kubas and R. R. Ryan, Polyhedron, 1986, 5, 473. 230. M. J. Mockford, A. M. Soares and W. P. Griffith, Transition Met. Chem. (Weinheim, Ger.), 1984, 9, 40;M. J. Mockford and W. P. Griffith, J. Chem. SOC., Dalton Trans., 1985, 717. 231. E. 0. Fischer and Y.Hristridu, 2.Nahcrforsch., 1960,156, 135; M. L. H. Green, J. A. McCleverty, L. Pratt and G. Wilkinson, J. Chem. SOC.,1961,4854; N. J. Cooper, M. L. H. Green, C. Couldwell and K. Prout, J. Chem. SOC., Chem. Commun., 1977, 145; J. C . Smart and C. J. Curtis, Znorg. Chem., 1978,17,3250. 232. M. L. H. Green, L. C. Mitchard and W.E. Silverthom, J. Chem. Soc., Dalton Trans., 1974, 1361. 233. H. D. Kaesz and R. B. Saillant, Chem. Rev., 1972, 72, 231; R. Bau, R. G. Teller, S. W. Kirtley and T. F. Koetzle, Acc. Chem. Res., 1979,112, 176. 234. T . W. Coffindaffer, I. P. Rothwelt and J. C. H u h a n , J . Chem. Soc., Chem. Commun., 1983, 1249. I
Subject Index A 23187 calcium transport, 66 Acetic acid, 1,2-diaminocyclohexanetetraalkaline earth metal complexes, 32 Acetylacetone alkali metal complexes, 25,30 Acetylene reduction vanadium(I1) complexes, 471 Acid salts, 2 Actinide complexes, 1129-1220 coordination numbers, 1131 geometry, 1131 kinetic stability, 1130 steric effects, 1130 Actinide(II1) complexes, 1131-1 136 amides, 1133 ammines, 1131 antipyrine, 1134 aqua, 1133 aromatic hydroxy acids, 1134 carbonates, 1134 carboxylates, 1134 N,N-diethyldithiocarbamates,1135 N,N-dimethylformamide, 1134 ethers, 1134 halogens, 1135 hydrides, 1135 hydroxides, 1133 b-ketoenolates, 1134 mixed donor atom ligands, 1136 multidentate macrocyclic ligands, 1136 nitriles, 1133 nitrogen ligands, 1131 oxides, 1133 oxoanions, 1134 oxygen ligands, 1133 pyridines, 1133 pyrimidones, 1134 selenides, 1135 sulfato, 1134 sulfides, 1135 sulfur ligands, 1135 tellurides, 1135 tribrornides, 1135 trichlorides, 1135 trifluorides, 1135 triiodides, 1135 Actinide(IV) complexes, 11361178 alcohols, 1146 aldehydes, 1151 aliphatic amines, 1137 aliphatic hydroxy acids, 1159 alkoxides, 1147 amides, 1161 amine oxides, 1165 ammines, 1137 aqua, 1144 aromatic amines, 1137 aromatic hydroxy acids, 1159 arsine oxides, 1170
arsines, 1144 aryl oxides, 1147 2,2’-bipyridyl, 1139 carbamaies, 1159 carbonates, 11.55 carboxyIates chelating, 1155 carboxylic acid hydrazides, 1164 catecholates, 1150 complexones, 1175 crown ethers, 1178 cryptands, 1178 cupfernon, 1170 cyanides, 1136 2,6-diacetylpyridine bis@-methoxybenzoyl hydrazone), 1177 dialkylamides. 1141 diamines, 1137 diarylamides, 1141 dicarboxylates, 1157 dipyridylamines, 1140 dipyridylethanes, 1140 diselenwarbamates, 1173 dithiocarbamates, 1172 dithiolates, 1172 dithiophosphinates, 1173 esters, 1151 ethers, 1150 ethylenediamine, 1137 halides, 1173 halogeno, 1173 hexafluoroacetonylpyrazolides, 1177 hydrazines, 1137 hydrides, 1175 hydrogen, 1175 hydroxamates , 1170 8-hydroxyquinolines, 1177 hyroxides, 1146 imidazole, 1140 isonitriles, 1136 isoquinoline, 1139 P-ketoenolates, 1148 ketones, 1151 monocarboxylates. 1153 monothiocarbamates, 1172 nicotine, 1138 nitrates, 1151 nitriles, 1143 nitroalkanes, 1159 nitrogen ligands, 1137 oxalates, 1156 oxides, 1146 oximes, 1143 oxohalides, 1173 peroxides, 1146 1,lO-phenanthroline, 1140 phenols, 1146 phosphates, 1152 phosphines, 1144 phosphine selenides, 1173 phosphorus oxides, 1165
1445
1446 phthalocyanine, 1178 piperazine, 1141 piperidine, 1139 pokypyrazol-1-ylborates, 1142 pyrazine, 1140 pyridine, 1139 pyrrolidines, 1138 quinoline, 1139 quinones, 1150 Schiff bases, 1175 selenates, 1153 selenides, 1173 selenites, 1153 selenocyanates, 1142 silyl oxides, 1147 sulfates, 1152 sulfides, 1170 sulfur oxides, 1170 tellurates, 1153 tellurides, 1173 teilurites, 1153 terpyridyl, 1140 tetracarboxylates, 1159 tetrahalides, 1173 thiocarboxamides, 1172 thiocyanates, 1142 thioethers, 1172 thiohydroxamates, 1172 thiols, 1171 thioureas, 1172 triazenes, 1142 tropolonates, 1148 ureas, 1143,1164 Actinide(V) complexes, 1179-1187 alcohols, 1180 aliphatic amines, 1179 aliphatic hydroxyacids, 1183 alkoxides, 1181 amides, 1183 amines, 1174 aqua, 1179 aromatic hydroxy acids, 1183 arsenates, 1182 arsenic ligands, 1179 aryl oxides, 1181 2,2’-bipyridyl. 1179 carbonates, 1183 carboxylates chelating, 1183 complexones, 1187 dihydroazirine, 1179 diphenyl diselenide, 1185 diphenyl ditelluride, 1185 dithiocarbamates, 1184 esters, 1182 ethers, 1182 halogeno, 1186 hydroxides, 1180 8-hydroxyquinolines, 1187 iodates, 1182 isoquinotine, 1179 fi-ketoenolates, 1182 ketones, 1182 mixed donor atom ligands, 1187 monocarboxylates, 1182 1,8-naphthyridine, 1179 nitrates, 1182 nitriles, 1179 oxalates, 1183 oxides, 1180 oxohalides, 1185,1186 pentahalides, 1185 1,lO-phenanthroline, 1179
Subject Index phenazine, 1174 phosphates, 1182 phosphine selenides, 1185 phosphine sulfides, 1185 phosphorus oxides, 1184 phthalazine, 1179 polycarboxylates, 1183 pyrazine, 1179 pyrazole, 1179 pyridine, 1179 quinoline, 1179 selenates, 1182 silyl oxides, 1181 sulfates, 1182 sulfur oxides, 1184 thiocarboxylates, 1184 thiocyanates, 1179 thiols, 1184 thiourea, 1184 trichloroacryloyl chloride, 1183 tropolonates, 1182 Actinide(V1) complexes, 1187-1215 alcohols, 1193 aldehydes, 1196 aliphatic amines, 1187 aliphatic hydroxyacids, 1202 alkoxides, 1194 amides, 1204 amine oxides, 1206 amino acids, 1212 ammines, 1187 aqua, 1192 aromatic amines, 1187 aromatic hydroxy acids, 1203 arsenates, 1197 arsenic oxides, 1207 aryl oxides, 1194 2,2’-bipyridyl, 1190 4,4’-bipyridyl, 1190 carbamates, 1202 carbonates, 1201 carboxylates chelating. 1198 carboxylic acid hydrazides, 1205 catecholates, 1195 complexones, 1213 crown ethers, 1214 cryptands, 1214 cupferron, 1208 cyanates, 1191 cynides, 1187 diamines, 1189 dicarboxylates, 1201 dioxo seven-coordination, 1131 dipyridylalkanes, 1190 di-2-pyridylamine, 1190 dithiocarbamates, 1209 dithiolates, 1210 dithiophosphinates, 1210 esters, 1196 ethers, 1195 ethylenediamine, 1189 hexacarboxykates, 12M hexahalides, 1210 hydrazines, 1188 hydroxamates, 1208 hydroxides, 1143 8-hydroxyquinoline, 1213 imidazole, 1190 isonitriles, 1187 isoquinoline, 1190 P-ketoenolates, 1195
Subject Index ketones, 1195 5-methylpyrazole, 1191 monocarboxylates, 1198 monothiocarbamates, 1209 nitrates, 1196 nitriles, 1191 nitroalkanes, 1202 nitroarenes, 1202 oxalates, 1201 oxides, 1193 oximes, 1192 oxyhalides, 1211 peroxides, 1193 1,lo-phenanthroline, 11W phosphates, 1197 phosphites, 1197 phosphorus oxides, 1207 phthalocyanine, 1214 piperidine, 1190 pyrazine, 1191 pyridine, 1189 quinoline, 1190 quinones, 1195 Schiff bases, 1212 selenates, 1197 silyl oxides, 119s sulfates, 1197 sulfur oxides, 1207 superoxides, 1193 superphthalocyanine, 1214 tellurites, 1197 thiocyanates, 1191 thioureas, 1210 triazenes, 1191 tropolonates, 1195 uranates, heptafluoro-, 1211 urea, 1205 xanthates, 1210 Actinide(VI1) complexes, 1214-1215 Actinides oxidation states, 1130 Actins alkali metal complexes, 64 Adenosine-2’-monophosphate lanthanide complexes NMR, 1103 Adenosine-3’-monophosphate lanthanide complexes NMR, 1103 Adenosine-5’-monophosphate lanthanide compIexes NMR, 1104 Adenosine triphosphate alkali metal complexes, 34 vanadyl complexes, 568 Alane, 123 amine adducts, 107 phosphine adducts, 111 Alane, alkoxy-, 124 Alane, amino-, 109 Alane, imino-, 109 Alkali metal complexes, 1-70 acid anions, 30 acid salts, 30 bipyridyl, 13 crown ethers cavity size, 38 cryptates alkali metal transport, 55 diacetamide, 15 1,2-diaminoethane, 11 1,2-dimethoxyethane, 14 ethylene glycol, 14
hexamethylphosphorarnide,6 hydration energies, 3 ionic radii, 3 organometallic compounds, 10 phenacylkojate, 15 o-phenanthroline, 13 ,, phosphine oxides, 15 phosphoramides, 15 polyamines, 12 polyethvlene polyamines, 11 Schiff bases, 28 solvates, 5 thiourea, 6 tri-n-octylphosphine oxidc, 6 triphenylphosphine oxide, 6 Alkali metal fluoroborates, 101 Alkali metals coordination numbers, 3 ionic potentials, 3 ionic size, 3 Alkaline earth metal complexes, 5 1 0 amides, 9 anions, 10 cations diacetamide, 15 crown ethers cavity sizc, 38 diacetamide, 15 1,2-dimelhoxycthane, 14 hydration energies, 3 ionic radii. 3 organometallic, 10 phenacylkojate, 15 o-phenanthroline, 13 phosphine oxides, 15 phosphoramides, 15 Schiff bases, 28 Alkaline earth metals coordination numhers, 3 ionic potentials, 3 ionk size, 3 Alkenes lanthanide shift reagents, 1104 Alkyl halidcs reduction vanadium(I1) complexes, 469 Alkynes reactions niobium(lI1) complexes, 661 tantalum(II1) complexes, 661 Alumina, 112 Aluminates, fluoro-, 121 Aluminates, heptachlorodi-, 123 Aluminates, tetrahalo-, 122 Aluminohydrides, 124 Alurninoxanes, 113, 114 Aluminum cumulative toxicity, 118 Aluminum, tris(acety1acetonato)-, 114 Aluminum, tris(tropo1onato)-, 115 Aluminum borohydride phosphine adducts, 111 Aluminum bromide, 121 Aluminum chloride, 121 Aluminum complexes, 105-142 ATP, 116 alkoxides, 113 alkoxyhydridcs, 114 amido, 109 amines, 107 ammines, 107 aqua, 112 arsenates, 115
1447
1448 azides, 110 borates, 115 borohydrides, 125 bromates, 117 carbonates, 115 carboxylates, 117 chlorates, 117 crown ethers, 119,126 cyanates, 110 cyanides, 106 dialkyl sulfides, 120 dimethyl sulfoxide, 119 cdta, 126 ethers, 118 fluorides, 120 fluorophosphato, 116 germy], 107 hydrides, 123 hydroxides, 112 imido, 109 iodates, 117 fi-ketoenolates, 114 metalla-P-diketones, 115 mixed halidcs, 121 mixed oxides, 112 molybdates, 116 nitrates, 115 nitriles, 111 oxides, 112 oxy anions, 115 perchlorates, 117 periodates, 117 phosphates, 115 phosphines, 111 phosphorus oxychloride, 119 phthalocyanines, 126 porphyrins, 126 Schiff bases, 125 selenates, 116 selenides, 120 silicates, 1 IS silyl, 107 silyl phosphine, 112 sulfates. 116 sulfides, 120 tellurates, 116 tellurides, 120 thiocyanates, 110 tungstates, 116 Aluminum halides antimony halide complexes, 112 2,2’-bipyridyl, 108 bismuth halides complexes, 112 morpholine adducts, 108 2,2’:6’,2”-terpyridyl, 108 1,lo-phenanthroline adducts, 108 Aluminum hydroxide, 112 Aluminum iodide, 121 Aluminum isopropoxide, 114 complexes, 108 Aluminum methoxide, 114 Aluminum nitride, 109 Aluminum phosphide, 111 Aluminum trihalides pyridine adducts, 108 Amavadin, 548,568 Ammonium molybdate, 1257 Anderson structure, 1044 Aniline, p,p’-methylenedialkali metal complexes, 14 Antamanidc alkali metal complexes, 64 Antimonates, 265
Subject Index Antimonates, hexafluoro-, 276 Antimonic adds, 265 Antimony, 237-294 biology, 277 carcinogenicity, 278 coordination number, 256 electronic structure, 256 in medicine, 278 pharmacology, 278 teratogenicity, 278 toxicology, 271 Antimony, di-pL-d-tartarato(4)-bisdipotassium trihydrate, 278 Antimony, (O-ethylxanthato)bis(quinolin-8-olato)-, 267 Antimony, pentaphenylbond lengths, 270 Antimony, triphenylbiscatecholate hydrate, 270 Antimony, triphenylcatecholate-, 270 Antimony, tris(diethy1dithiocarbamate)-, 266 Antimony, tris( dipheny1dithiophosphinato)-,267 Antimony, tris(1-pyro1idinecarbodithioato)-, 266 Antimony chloride, 270,276 Antimony complexes, 259 catenation, 262 double bonds, 261 Group IV ligands, 259 Group V ligands, 259 halogen, 270 nitrogen ligands, 259 organooxy, 264 organosulfur ligands, 265,269 pentacoordinate, 269 sulfur ligands, 265 Antimony(II1) complexes cyclic, 262 oxo acids, 263 oxyfluoro, 264 Antimony(V) complexes organohalo, 257 organotrihalo, 258 Antimony fluoride, 270 Antimony halides, 270,272 mixcd chloride/fluorides, 274 Antimony oxides toxicology, 277 Antimony oxoacids, 265 Antimony oxoiodides, 265 Antimony oxychlorides, 264 Antimony oxyhalides, 264 Antimony pentachloride, 272 toxicology, 278 Antimony pentafluoride, 272 Antimony phosphite, 263 Antimony pseudohalides, 276 Antimony spots, 278 Aplasmomycin, 96 Arsazine, lO-chloro-5,10-dihydro-,256 Arsenic, 237-284 biology, 255 environmental, 255 in medicine, 255 oxidation states, 239 tolerance lower marine life, 256 toxicity, 255 trace element, 256 uses, 239 ylides, 239 Arsenic, chlorobis(trihoromethy1)(trimethylsily1)aminobis-,242 Arsenic, dichlorobis(trifluoromethy1){ bis(trimethylsi1yl)amino 1 -,242
Subject Index Arsenic acid esters, 245 oxaorganic compounds, 245 Arsenic chloride complex anions, 253 Arsenic complexes coordinated open-chain, 244 germanium ligands, 240 Group V ligands, 240 Group VI ligands, 245 halogen, 250 nitrogen ligands, 240 oxygen ligands, 245 phosphorus ligands, 243,244 selenium ligands, 249 silicon ligands, 239 structures, 239 Arsenic(II1) complexes, 240 diselenocarbamate, 250 Arsenic(V) complexes, 242 Arsenic(V1) complexes haloselenocarbamate, 250 selenium ligands, 250 selenocarbamate, 250 Arsenic halides, 250,252 mixed, 253 reactions, 252 Arsenic oxyfluorides, 254 Arsenic pentachloride, 253 Arsenic pentafluoride, 252 Arsenic pseudohalides, 252 Arsenic sulfates, 246,247 bond lenths and angles, 247 Arsenic triastatine, 251 Arsenic trichloride, 252 Arsenic tricyanide, 252 Arsenic triiodide, 251 Arsenic triisocyanate, 252 Arsenic triisothiocyanate, 252 Arsenotungstates, 1042 Arsine, alkylamino-, 240 Arsine, dialkylamino-, 241 Arsine, dialkylaminodifluoro-, 242 Arsine, diphenylchloro-, 256 Arsine, diphenyicyano-, 256 Arsine, silyl-, 239 Arsine, trimethylin prawns, 256 Arsine, trisilyl-, 239 Arsorane spirocyclic, 246 Arsorane, 2-acetyl-3,4,5triphenylcyclopentadiienetriphenyl-,239 Aryl halides reduction vanadium(I1) complexes, 469 Aspartic acid terbium complexes, 1108 Barium complexes cryptands, 53 pyridine oxide, 9 urea, 9 Beauveracin alkali metal complexes, 63 Benzo-5-crown-5 alkali metal complexes cavity shape and size, 51 Benzo-15-crown-5 alkali metal complexes, 40 Benzoic acid, o-iodoxy-, 318 Beryllates, fluorocomplexes, 10
1449
Beryflates, tetrachloroalkali metal complexes, 10 Beryllium acetate complexes, 8 Beryllium borohydride ammine complexes, 8 Beryllium carboxylates basic, 31 Beryllium complexes, 3 amines. 7 anionic, 10 hydrates, 6 phthalocyanine, 59 polycarboxylic acid, 33 Schiff bases, 28 Beryllium dichloride ether complexes, 8 Beryllium difluoride ammine complexes, 7 Beryllium dihalides sulfide complexes, 10 Bcryllium haiides amine complexes, 8 ammonia complexes, 7 carbonyl complexes, 9 Beryllium nitrate basic, 32 1,I’-Bibismulane,284 Bis(chlorophylls), 58 Bismuth, 237-294 biology, 292 pharmacology, 292 toxicology, 293,294 Bismuth, amminebis(trich1oro)-, 282 Bismuth, chlorotriphenyl(8-quinolinolato)-, 280 Bismuth, dichlorotriphenyl-, 280 Bismuth, isopropyixanthogenatodiphenyl-, 288 Bismuth, triamminetrichloro-, 282 Bismuth, trichlorotris(3-sulfanilamido-6meth0xypyridazine)-, 293 Bismuthates, diphenylbis(trifluoroacetato)tetraphenyl bismuthonium salts, 279 Bismuth-azide bonds, 283 Bismuth complexes carbon ligands, 279 germanium ligands, 280,281 Group IV ligands, 279 Group V ligands, 282 Group VI ligands, 284 halogen, 290 nitrogen ligands, 282 organosulfur ligands, 285 oxygen ligands, 284 silicon ligands, 280 sulfur ligands, 284 tin ligands, 281 Bismuth(I11) complexes, 282 Bismuth(V) complexes, 283 Bismuth halides, 290 crystal structure, 291 Bismuthine, alkylamino-, 282 Bisrnuthine, alkylxanthogenato-, 288 Bisrnuthine, alkylxanthogenatodimethyi-, 289 Bismuthine, bis(diethy1thiocarbamato)methyl-,288 Bismuthine , bis(diorganodithiocarbamato)organo-, 289 Bismuthine, bis(organothi0)-, 287 Bismuthine, dithio-, 285 Bismuthine, organodithiocarbamato-, 288 Bisrnuthine, thio-, 285 Bismuthine, tris(organothi0)-, 287 Bismuthine azides organo-, 283 Bismuthines, 281
1450
Subject Index
tin, 281 Bismuth line, 294 Bismuthotungstates, 1042 Bismuth pentafluoride, 292 Bismuth tribromide, 291 Bismuth trichloride, 290 Bismuth trifluoride, 290 Bismuth triiodide, 292 Bismuth trioxide, 284 Bond dissociation S k C r , 184 Bond energies Group IVB elements, 185 Bonds antimony-antimony, 263 antimony-halogen, 271 arsenic-Group IV, 239 arsenic-Group V, 240 arsenic-halogen, 251 angles, 251 lengths, 251 arsenic-nitrogen, 240 bismuth-bismuth, 284 bismuth-selenium, 290 bismuth-silicon, 280 bismuth-sulfur angies, 285 lengths, 285 bismuth-tellurium, 290 carbon-nitrogen tungsten complexes, 1013 chromium-chromium, 749 molybdenum-carbon, 1405 molybdenum complexes metal-metal, 1301-1322 molybdenum-molybdenum, 1301,1317 molybdenum-molybdenum double, 1316 length, 1317 molybdenum-molybdenum quadruple, 13U8 activation energy for rotation, 1308 electronic structure, 1308 IR spectroscopy, 1308 lcngth, 1308 NMR spectroscopy, 1309 UV spectroscopy, 1308 molybdenum-molybdenum triple, 1301,1310-1315 cleavage, 1315-1322 oxidative addition, 1315 reaction with alkynes, 1315 reaction with CO, 1315 reaction with cyanamide, 1315 reaction with diazomethane, 1315 Boramycin alkaline earth metal complexes, 68 Borane electrophilicity, 82 Borane, alkoxyLewis acidity, 82 Borane, alkylthiocomplexes, 88 Borane, aminoLewis acidity, 82 Borane, carboxyamine complexes, 84 Borane, cyanatocomplexes, 88 Borane, cyanoamine complexes, 84 Borane,fluoroalkoxycornpiexes, 88 Borane, halogenocomplexes, 84-88 nitrogen donors, 87
oxygen donors, 86 stoichiometry, 8.5 syntheses, 84 relative acidities, 82 Borane, 1-methylbenzylaminocyanohydropyrrolyl-,84 Borane, thiocyqnatohalogenohydro-, 88 Borane, trialkoxyamine complexes, 88 Borane, trifluorocomplexes Lewis acids 87 van der Waals compleues, 84 Boranc complexcs arninecarboxy-, S4 arninehalogeno-, 84 amines, 82, 101 E-N bond polarity, 82 preparation, 83 reactions, 83 bonds B-N, 88 B-O,88 B-S, 88 n bonds, 82 carhon monoxide. 83 chiral boron, x4 dimcthyl suifide, 54 enthalpy of dissociation, 82 halogenohydro, 84 hydro, 83 monocyano, 84 neutral, 81-90 stability, 81 NMR, 83 relative stabilities?82 Borates E-C bonds, 97 R-N honds, 97 B-0 bonds, 94 B-P bonds, 97 B-Si bonds, 97 oxo acid anion complexes, 96 Borates, alkoxo-, 94 Borates, amidotrihydro-, 92 Borates, aryloxo-, 94 Borates, carboxylato-, 96 Borates, catechol, 95 Borates, chlorosulfato-, 97 Borates, dicarboxylato-, 96 Borates, dipyrazol-I-yl-, 92 Borates, halogeno-, 92 Borates, halogenohydro-, 90 Borates, hydro-, 90 Borates, hydrohydroxo-, 90 Borates, hydropyrazol-1-yl-,92 Borates, hydroxo-, 94 Borates, hydroxycarboxylato-, % Borates, inositol, 95 Borates, monoalkyLl 92 Borates, monophosphido-, 92 Borates, peroxohydroxo-, 94 Borates, polyol, 95 Borates, pyrrol-I-yl-, 92 Borates, sulfato-, 97 Borates, tetrabromo-, 92 Borates, tetrachloro-, 92 Borates, tetrafluoro-, 92 Borates, tetrahalogenomixed, 93 IIB NMR, 92 Borates, tetraiodo-, 92 Borates, tetranitrato-, 96
Subject Index
..
.
Borates, tetraperchhato-, 97 Borates, tripyrazol-I-yl-, 92 Borax, 10.1 Boric acid, fluoro-, 101 Boromycin, 96 Boron, 81-101 as ligand, 99 Boron complexes anionic, 9C-97 antibiotics, 101 applications, 101 cationic, 97 chelates, 89 Bromates, dinitrata-, 319 Bromine anions BrF,-, 316 Bromine cations Br3+, 314 BrF,+, 315 BrF,+, 316 CIFz+, 314 Bromine(II1) compounds, 317 [Br(OSO,F)],+, 320 Bromine fluorosulfate, 320 Bromine hexafluoroantirnonate difluoro-, 313 Bromine perchlorate, bis(quino1ine)-, 319 Bromine tetrafluoroborate, his(quinuc1idine)-, 319 Bromine tritluoride, 313,314 Butane, roc-p,p'-diaminodiphcnylalkali metal complexes, 1-1 Calcium complexes amino acids, 33 arsine oxides, 9 bipyridyl, 13 crown ethers, 39 dimethylphthalate, 16 hydrates, 7 , ionophores, 66 peptides, 33 , phosphines, 9 pyridine oxide, 9 Schiff bases, 29 urea, 9 Calixarenes alkali metal complexes, 57 Carbohydrates alkaline earth metal complexes, 24 Carbon dioxide alkali metal complexes, 16 Carbon monoxide reduction vanadium(I1) complexes, 471 Cell membranes alkali metal transport, 51 Ceric ammonium nitrate, 1114 Ceric sulfate, 1114 Cerium complexes tetrakis(dibenzoylmethanate), 1114 tetrapositive oxidation state, 1113 tetrapositive oxidation state catechol, 1115 tetrapositive oxidation state hydrated ions, 1113 organic oxyanions, 1114 Cerium(1V) complexes fluorides, 1115 Cerium tetrakis(acetylacetonate), 1114 Cesium complexes crown ethers, 40 Chcvrel phases, 1321 Chlorella vulgarik nitrate reductase structure, 1438 Chlorine cations
1451
CIFz +,314
CIFbfl 316 Chlorine trifluoride, 314 Chloroph ylls alkaline earth metal complexes, 58 Choline antimony analogue, 279 Chromates, 928,941 anionic oxo halides, 944 biological effects, 947 carcinogenicity, 947 mutagenicity, 947 YMR, 943 spcctroscopy, 943 structures, 943 Chromates. heptafluoro-, 927 Chromates, hexacyano-, 703,773 production, 704 Chromates, hexafluoro-, 927 Chromates, hexahalo-, 889 Chromates, hexaiodo-, 766 Chromates, oxohalo-, 935 Chromates. pentafluoro-, 927 Chromates, pentahalo-, 766 Chromates, tetrabromosolvated, 758 synthesis, 763 Chromates, tetrachloroantiferromagnetic, 761 ferromagnetic magnetic properties, 759 optical properties, 759 structure, 759 solvated, 758 synthesis, 759 Chromates. tetrahalo-, 889 Chromates. trifluoroelectronic spectra, 757 magnetic properties, 757 structures, 757 synthcsis. 756 Chromates, trihalaelectronic spectra, 764 magnetic properties, 764 structure, 764 synthesis, 764 Chromium dissolution in acids, 716 oxidation states, 701 Chromium complexes, 699-948 crystal structures, 702 mixed oxidation state(II1 and VI) oxo, 945 Chromium(0) complexes, 702 alkyl isocyanides, 704-709 aryl isocyanides, 704-709 2 ,Z'-bipyridyl, 709 electrochemistry, 713 electronic spectra, 712 ESR spectra, 712 IR spectra. 712 magnetic properties, 710 synthesis, 709 cyanides: 703 fluorophosphine, 714 germanium ligands, 709 isocyanides crystaIlography, 708 spectroscopy, 708 synthesis, 707 1,IO-phenanthroline, 709 electrochemistry, 713 electronic spectra, 712 ESR spectra, 712 "
1452 IR spectra, 712 magnetic properties, 710 synthesis, 709 quinones, 716 tertiary phosphines dinitrogen, 713 tris(bipyridy1) NMR, 711 tris(1,lo-phenanthroline) NMR, 711 2,2':6',2"-terpyridyl, 709 electrochemistry, 713 electronic spectra, 712 ESR spectra, 712 TR spectra, 712 magnetic properties, 710 synthesis, 709 Chromium(1) complexes, 702 alkyl isocyanides, 704-709 aryl isocyanides, 704-709 2 2'-bipyridyl, 709 electrochemistry, 37! electronic spectra, 712 ESR spectra, 712 IR spectra, 712 magnetic properties, 710 synthesis, 709 cyanides, 703 fluorophosphine, 716 isocyanides crystallography, 708 electrochemistry, 709 spectroscopy, 708 synthesis, 707 1,lO-phenanthrolire, 709 electrochemistry, 713 electronic spectra, 712 ESR spectra, 712 IR spectra, 712 magnetic properties, 710 synthesis, 709 tertiary phosphines dinitrogen, 713 tris(bipyridy1) NMR, 711 2,2':6',2"-terpyridyl, 709 electrochemisty, 713 electronic spectra, 712 ESR spectra, 712 IR spectra, 712 magnetic properties, 710 synthesis, 709 Chromium(11) complexes, 716-927 acetate aqueous solutions, 752 adenine, 726 alcohols, 737 balkoxides. 737 alkyl isocyanides, 704-709 amines bidentate, 720 polydentate, 721 amino acids, 768 2-aminobenzenethiol, 769 2-aminomethylpyridine, 726 8-aminoquinoline, 726 ammines, 718 magnetic properties, 718 spectroscopy, 718 structures, 718 synthesis, 718 aqua, 735 arsenic ligands, 732
Subject Index .
arsine oxides, 753 aryl isocyanides. 704-709 aryl oxides, 737 2,2'-bipyridyl, 726 biurets, 739 bromides, 758-766 carboranes, 731 carboxylates, 740 magnetic properties, 752 mass spectra, 752 structures, 748 synthesis, 745 chloridcs. 758-766 chlorosulfatcs, 740 complexones, 768 coordination numbers, 701 corrins, 770 dehydration, 717 dialkylamines synthesis, 729 1,2-diamino-2-methylpropane. 720 1,2-diaminopropane 720 1 ,3-diaminopropane7720 diethylenetriamine, 721 N,N-dimethylethylenediamine,720 N,N'-dimethylethylenediamine, 720 dimethyl sulfoxide, 753 meso-5,12-dimethyl-1,4,8,1l-tetraazacyclotetradecane, 770 dinuclear photoelectron spectroscopy, 750 disilamides synthesis?729 dithiocarbamates. 754 double sulfates hexahydrates, 736 ethane-1,Zdithiol. 755 ethylenediamine, 720 fluorides, 756 glucose tolerance, 904 glutathione, 905 halides, 755-766 anhydrous, 755 ~neso-5,7,7,12,14.14-hcxamcthyl-1,4,8,11tetraazacyclotetradecane, 770 hydrates, 735 hydrazines, 729 hydrides, 766 hydrogen, 766 hydroxy acids, 753 imidazoles, 726 inner-sphere electron transfer, 716 iodides, 758-766 isocydnides crystallography. 708 electrochemistry, 709 spectroscopy, 708 synthesis, 707 Jahn-Teller distortion, 701 P-ketoamines, 766 P-ketoenolates, 738 low-spin, 703 monohydrogenphosphate, 740 nicotinic acid, 725,904 nitriles, 732 oxo anions, 740 1,lO-phenanthroline, 726 o-phcnylenebis(dimethylarsine),734 phosphinato, 740 phosphine oxides, 753 phosphite, 740 phosphorus ligands?732 phthalocyanines, 770 ~
Subject Index polyazamacrocycles, 770 polyprazolylhoratcs, 731 porphyrins, 770,911 pyrazoles, 722,726 pyridines, 722 Schiff bases, 766 polymeric, 767 saccharine, 725 stereochemistry, 772 sulfato, 740 sulfur ligands, 754 synthesis, 716,717 non-aqueous solutions, 717 by reduction of chromium(1II) solutions, 716 solutions, 716 1,4,8,l2-tetraazacyclopentadecane, 770 1,4,8,11-tetraazacyclotetradecane, 77U tetrahydrofuran, 739 1,4,8,11-tetramethyl-l,4,8,1~-tetraazacyclotetradecane,
770 tetranuclear electronic spectra, 751 thiocyanates, 729 thiourea, 755 tricyanomethides, 709 triethylenetetramine, 722 tripod ligands tetradentate, 734 tris(2-dimethylarninoethyl)amine,722 urea, 739 Chromium(II1) complexes. 701 acetates, 869 acetylacetonates, 861 phase chemistry, 863 physical studies, 862 alcoholates, 860 alkoxides, 860 alkyl, 779 alkyl isocyanides. 704-709 amides, 835,852 amines bidentate, 78!WW306 monodentate, 787 amino acids, 902-908 hydroxy-bridged, 903 solution studies, 903 ammines, 780-787 polynuclear, 783 anilines, 788 antimony ligands, 852 aqua, 856 arsenic ligands, 852,901 aryl, 779 aryl isocyanides, 704-709 aziges, 843 azo, 833 biguanides, 850 bipyridyls, 816 his(oxalato), 871 1,6-bis(2’-pyridyl)-2,5-diazahexaneI 812 borates, 869 camphorates, 862 carboxylates, 869 catecholates, 865,866 citrates, 874 complexones, 908 corrins, 911 cyano binuclear, 777 multinuclear, 777 tetranuclear, 777 cyanoamines, 775 cyanoammines, 775
1453
2,4-diacetylpyridine, 899 diamines aliphatic, 788 bidentate, 797 diammines, 781 cis-dicyanobis(ethylenediamine), 794 diethylenetriamine, 806 P-diketonates chemical reactivity. 864 NMR, 864 optical activity, 863 dimethylfuran, 875 dimethyl sulfoxide, 874 disulfido, 876 dithiocarbamates, 883 1,I-dithiolates, 883 1,2-dithiolates, 883 dithiophosphates, 883 ethanolamine, 897 ethylenediamine, 789-806 ammonolysis, 795 fluoro, 796 polynuclear, 799 thiolato, 796 e thylenediamine-N,N’-diacetato, 910 ethylenediaminetetraacetic acid, 908 halides, 889 dimeric, 890
solution chemistry, 891 hexaammines, 783 hexamines aliphatic, 787 hydrazines, 834 hydroxamates, 866 hydroxy polymeric 857 hydroxy acids, 873 akpharic. 873 aromatic, 874 hydroxylamines, 834 S-hydroxyquinoline, 899 imidazoles, 821 iminoacetic acid, 910 isocyanates, 842 isocyanides?779 crystallography, 708 electrochemistry, 709 spectroscopy, 708 synthesis, 707 isoselenocyanates, 842 isothiocyanates, 837 I
6-ketoamines, 892-897
6-ketoenolates, 861 malonaldehyde, 861 malonates, 873 2-mercaptoethylamine, 900 monothio (3-diketonates, 901 nitrates, 867 nitriles? 844 nitrilotriacetic acid, 911 nitrosyls, 823 alkoxido, 830 amido, 830 arsines 826 azido, 823 chalcogens, 826 cyano, 823 hydroxylamines, 823 isocyanides, 826 macrocycles, 831 nitriIes, 826 phosphines, 826 pyridinecarboxylates. ~
1454 pyridines, 823 N-thiocyanato, 823 organometallic, 897 oxalates, 870 oxides, 859 hydroxides, 859 N-oxides, 874,875 P-oxides, 874,876 oximes, 849 pentaammines, 783 pentamines aliphatic, 787 perchlorates, 869 peroxides, 859 phenanthrolines, 816 phosphates, 868 phosphinates, 868 phosphoric acid, 867 phosphorus acid, 867 phosphorus ligands, 852 picolinic acid, 899 polyamines open chain, 806-815 polypyrazolylborates, 845 porphyrins, 911,913 1,3-propylenediaminetetraaceticacid, 910 pyrazines. 822 pyraroles, 822 pyridines, 815 I -(2-pyridyl)ethylamine di-phydroxo-, 805 1-(2-pyridyl)ethylene, 803 (2-pyridyl)methamine, 803 di-p-hydroxo-, 805 quinones, 865 Schiff bases, 892-897 selenium ligands, 889 o-semiquinones, 865 squarates, 873 sulfato, 867 sulfides binary, 882 ternary, 882 sulfoxides, 874 tartrates, 873 tellurium ligands, 889 terpyridyls, 816 tetramines aliphatic, 788 tetraammines, 781 tetraethylenepentamine, 814 tetrahydrofuran, 865,867 tetramerhylguanide, 850 tetramines, 812 tetranitrosyls, 832 thiobis(ethylenenitri1o)tetraaceticacid, 911 thiocyanates bridged, 838 linkage isomers, 841 thioethers, 876 thioglycolic acid, 901 thiolates, 876 thionitrosyls, 832 thiourea, 888 triamines aliphatic, 788 2,2’,2”-triaminotriethylamine, 811 triammhes, 781 triazinides, 845 triethylenetetramine, 808 trifluoromethylsulfate, 867 tris(oxalato), 870 urea, 852,865,867
Subject Index Chromium(1V) complexes, 701, 927 alkoxides, 928 alkyls, 928 amides, 930 chloro, 927 cyano,927 fluoro, 927 halo, 927 peroxo, 945 porphyrins, 916 Chromium(V) complexes, 701,931-938 cyclarn, 918 5,4-dimethyl-1,4,8,1l-tetraazacyclotetradeca-4,6,11,13tetraenates, 924 halogens, 932 5,7,7,7,12,14,14-hexamethyl-l,4,8,11tetraazacyclotetradecane, 920 oxides, 936 peroxides, 936,945 phthalocyanines, 924 polyazamacrocycles, 918-924 porphyrins, 916 salen, 917 tertiary a-hydroxycarboxylates, 936 1,4,7,10-tetraazacyclodecane,92 1 tris(p-hydroxo), 924 1,4,6,-triazacyclononane,924 Chromium(V1) complexes, 701,933 chloro, 940 L-cysteine, 948 fluoro, 940 glutathione, 947 hexafluoro, 938 organoimido, 945 peroxo, 945 Chromium dioxide, 928 Chromium fluoride. 932 Chromium tetrafluoride. 927 Chromium trioxide. 941 Chrornyl bromide, 940 Chromyl chloride, 940 Chromyl halides, 933,935,938 Chromyl nitrate, 941, Chromyl perchlorate, 940 Citratotellurates, 303 Cobalt complexes boron ligands, 99 Cobalt(I1) complexes reduction vanadiurn(I1) complexes. 472 ‘Cobaltitungstates, 1042 Coronands alkali metal complexes, 3,3745 18-Crown-6, dicyclohexylalkali metal complexes cavity shape and size, 50 Crown ethers alkali metal complexes, 3,37-45,60 lanthanide complexes, 1092 Cryolite, 121 Cryptands alkali metal complexes, 3,4,35,45,54 cavity shape and size, 50 conformational rigidity, 53 flexibility, 53 heteropoly, 1049 [2.2.C8]-Cryptands alkali metal complexes donor atom arrangements, 5U [2.2.2]-Cryptands alkali metal complexes donor atom arrangements, 50 Cryptates ~
Subject Index alkali metal complexes stability, 52 Cuprotungstates, 1042 Cyclophane chlorophylls, 58 Decatungstates, 1034 Decatungstometalates, 1045,1046 Decavanadates, 1027 Dehyrodgenase structure, 1438 Dianemycin alkaline earth metal complexes, 66 Dibenzo-18-crown4 alkali metal complexes. 35 electronic influences, 54 Dibenzo-24-crown-8 alkali metal complexcs, 40 Dibenzo-30-crown-10 alkali metal complexes, 40 Dibismuthines, 284 Dibismuthines, tetrakis(trimethylsily1)-, 284 Diborane Lewis base complexes. 83 Dichromates, 941, 943 Dihydrogen oxidativc addition niobium(I1) complexes, 678 niobium(ll1) complexes, 660 tantalum(I1) complexes, 678 tantalum(II1) complexes, 660 1,3-Diketones alkali metal complexes, 2 alkaline earth metal complexes, 25-30 Dimetallenes, 217 Dimolybdate, 1408 Dinitrogen activation niobium(II1) complexes, 6-51 niobium(V) complexes, 617 tantalum(II1) complexes, 661 tantalum(V) complexes, 617 fixation vanadium(I1) complexes. 473 reduction hydrazine in, 1397 vanadium(I1) complexes, 469 tungsten complexes, 1011-1014 reduction, 1012 Diorganoarsinates metal complexes, 1054 Dioxygen activation niobium(II1) cornplcxes, 661 tantalum(II1) complexes, Mil 1,4-Disilabenzene, hexamethyl-, 190 Disilene, tetramesityl-. l!M Disilenes Si=Si n bond energy, 184 stability, 190 Disiloxane, tetramesityl-, 206 Distannene, 217 Distannoxane, 1,3-dichloro-, 207 Dodecamolybdometalates, 1045 Dodecatantalates, 1029 Electronegativity, 184 Group IVB elements, 183 Enniatins alkali metal complexes, 63 synthetic analogues, 65 Enzymes molybdenum, 1334 oxomolybdenum, 1336
Escherichia c d i nitrate reductase structure, 1438 Europium complexes P-diketones, 1081 dipositive oxidation state hydrated ions, 1109
Fluorine bridges trifluoroxenon hexabismuthate, 313 a-D-Fructose alkaline earth metal complexes, 25 WU-FUWS~ alkaline earth metal complexes, 25 a-D-Galactose alkalinc earth mctal complexes, 25 Gallium alkoxides. 133 Gallium bromide, 139 Gallium chloride, 139 Gallium complexes, 105-142 amido, 130 aqua, 133 azides, 231 2,2'-bipyridyl, 130 cyanates, 131 hydroxides, 3 33 imido, 130 nitriles, 131 oxides, 133 1 ,lO-phcnenthroline, 130
phosphines, 132 piperidine, 130 pyrazolyl. 130 pyridine, 130 selenocyanates, 131 terpyridyl, 130 thiocyanates, 131 Gatlium(T) complexes, 127 halides, 127 oxides, 127 selenides, 127 sulfides, 127 Gallium(I1) complexes, 127 halides, 128 selenides, 128 sulfides, 128 tellurides, 128 Gallium(II1) complexes, 129 amines, 129 amino acids, 141 arsenates, 135 borates, 134 borohydrides, 141 bromates, 135 carbonates, 134 carboxylates, 135 chlorates, 135 citrates, 136 cyanides, 129 dithiocarbamates, 138 ethers, 136 fluorides, 138 gerrnanates, 134 hydrides, 140 iodates, 135 b-ketoenolates, 134 lactates, I36 mixed halides, 139 molybdates, 135 nitrates, 135 peptides, 141 perchlorates, 135
1455
1456 phosphates, 135 phosphites, 135 phthalocyanines, 141 porphyrins, 142 proteins, 141 Schiff bases, 141 salicylaldehyde, 136 selenates, 135 selenides, 137 silicates, 134 silyl, 129 sulfates, 135 sulfides: 137 tartrates, 136 tellurates, 135 tellurides, 137 thiophosphates, 138 tungstates, 135 xanthates, 138 Gallium iodide, 139 Gallium trihalides, 126, 130 pyridine adducts, 108 Gerrnaalkcnes, 191 Gcrmaarcnes, 191 Germaimines, 191 Germ.anate, trichloro-, 195 Germanates, 203 Germanediol, di-&butyl-,206 Germanium Elemental structurc, 183 Germanium, cyclopentadienyl-, 218 Germanium complexes, 183-223 bivalent stereochemistry, 188 coordination geometry, 185 dialkyl or diaryl, 216 multiple bonding, 188 tetravalent, 205 Germanium fluoride, 193 Germanium halidcs, 197 bivalent
structure, 192 Lewis acidity, 199 Germanium iodide, 194 Germanium oxides structure, 192,202 Germanium selenide structure, 202 Germanium sulfide structure, 202 Germanocene nomenclature, 217 Germanocenium ions pentamethyl, 218 Germanones, 191 Germaphosphimines, 191 Germathiones, 191 Germatranes, 212 Germylenes, 192 structure, 193 Glucose tolerance chromium(II1) complexes, 904 Glycobiarsol, 256 Glycols alkaline earth metal complexes, 17 Glymes alkali metal complexes, 17 Group IIA mctal complexes, 1-70 Group IV elements bivalent compounds Group IV, 216 Group V, 216 Group VI, 216
Subject h d e x coordination geometry, 185 elemental radii, 184 ionization energies, 326 ionization potentials, 326 properties, 326 tetravalent compounds, 205 Group IVB elements coordination geometries, 186 Group V elements tetravalent compounds, 20.5 Group VI elements tetravalent compounds, 205 Hafnates, bromo-, 430 Hafnates, chloro-, 430 Hafnates, fluoro-, 423 alkali metal salts, 425 alkylammonium salts, 427 divalent metal salts, 427 guanidinium salts, 427 hydrazinium salts, 427 vibrational spectra, 429 Hafnates, iodo-, 430 Hafnium bases benzoyl hydrazones, 434 Hafnium complexes, 363-440 coordination geometries, 364 oxidation states, 364 tetrakis-chelate stereochemistry, 364 Hafnium(0) complexes, 364 2,2’-bipyridyl, 366 nitrogen ligands, 366 1,lO-phenanthroline, 366 Hafnium(II1) complexes, 366-370 arsenic ligands, 369 nitrogen ligands, 366 phosphorus ligands, 369 Halnium(1V) complexes, 370-400 2-acetylpyridine, 373 aldehydes, 403 alkoxides, 389 reactions, 390 alkyl thioglycolates, 439 amides, 414 amines, 371 amino acids, 410 aminopolycarboxylates, 436 ammines, 371 aqueous solutions polynuclear, 384 Raman spectra, 384 arsenic ligands, 383 arsines, 383 aryloxides, 389 N-arylsalicylaldimines, 437 azides, 379 benzoquinoline, 372 2,2’-bipyridyI, 372 bis(amido)silane, 375 bis(diketonates), 400 bis(trimethylsily1amido). 375 carbamates, 410 carbonates, 410 carboxylates, 409 catecholates, 403 citric acid, 412 cupferron, 417 cyanates, 379 cyanides, 370 3-cyanopyridine, 373 dialkylamido,375 photoelectron spectra, 375
Subject Index reactions, 375 dichlorophosphates, 407 diethylenetriaminepentaacetate,436 dithiocarbamates, 419 dithiolenes, 420 esters, 403 ethers, 403 ethylenediaminetetraacetate. 436 fluorosulfates, 407 glycolic acids, 412 halogenoids, 381 hydrazine, 374 hydrides, 432 hydrohorates, 432 hydroperoxo, 387 hydroxamates, 417 hydroxide, 384 hydroxy acids aliphatic, 412 aromatic, 412 imidazoles, 372 indazoles, 372 interhalogen-halogenoids,381 interhalogenoids. 381 isoproxides NMR,389 13-kctoenolates,392 ketones, 403 lactic acid, 412 malic acid, 412 mandelic acid, 413 o-mercaptophenolato. 421 N-methyl-p-thiotoiylhydroxamato. 439 molybdates, 408 mono(diketonates), 401 monothiocarbamates, 419 N heterocyclic ligands, 372 IR spectra, 372 nicotinamide, 373 nitrates, 406 nitriles, 382 IR spectra, 382 Raman sepctra, 382 nitrilotriacetate, 436 nitrogen ligands, 371 nitro ligands, 374 octaethylporphinato, 439 oxalates, 411 oxides, 384 N-oxides, 414 P-oxides, 414 oxoanions, 406 oxygen ligands, 384 perchlorates, 408 peroxo, 387 IR spectra, 388 Raman spectra, 388 1,lO-phenanthroIine, 372 phosphines, 383 phosphorus ligands, 383 phthalocyanines, 440 poly(pyrazolyl)borates, 381 porphyrins, 439 pyridine, 372 8-quinolinol, 437 quinones, 403 Schiff bases, 434 salicylic acid, 413 selenates, 407 selenium ligands, 421 selenocyanates, 380 selenoxides, 414 sulfates. 407
sulfoxides, 414 sulfur ligands, 418 tartaric acid. 412 tetrahalides, 422 tetrakis(diketonates), 397 bromination, 398 '' ligand exchange, 398 mass spectra, 399 N M R ,398 volatility, 399 tetraphenylporphyrin, 440 thiocyanates, 380 thioethers. 418 thinlates,418 thiourea, 421 tin-containing ligands, 370 trialkysilyl oxides, 389 triazenes, 378 trihydroxyglutaric acid, 412 tris(diketonates), 399 tropolonates, 402 urea, 382 water, 384 Halogen cations, 3152 hexafluoro, 316 tetrafluoro, 315 Halogen compounds hqpewalent, 316 Halogen(1) compounds, 313,317,319 Halogen(II1) compounds, 313,317,320 Halogen(V) compounds, 315,318,320 hexafluoro anions, 315 Halogen(VI1) compounds, 316 Halogenium gases oxidation states, 312 Halogeniurn halides, 317 Halogenium species, 311-321 carhnn ligands, 316 coordination compounds, 312 coordination geometry, 312 halogen Iigands, 31S316 nitrogen ligands, 319 oxygen ligands, 319 selenium ligands, 321 selenocyanates, 321 sulfur ligands, 321 thiocyanates, 321 Halogen pentafluorides, 315 Halogens reduction vanadium(r1) complexes, 471 Hemispherands alkali metal complexes, 43 Heptafluorotellurates, 303 Heptamolybdates, '1030, 1376 Heptatungstates, 1033 Heteropolyanions, 1023-1055 Anderson structures, 1043,1044 heteroatoms, 1035 Keggin structures, 1037 ligands, 1046 multisubstituted, 1048 phosphorus(V), 1039 trivalent antimony, 1042 trivalent arsenic, 1042 Heteropoly blues, 1049,1052 Heteropolyrnolybdates, 1256 Anderson structure, 1045 Heteropolyvanadates, 1041 Anderson structure, 1045 Hexachloroselenates, 303 Hexahalogenotellurates, 302,303 Hexaiodoselenates. 303
1457
1458 Hexaniobares, 1028 Hexatantalates, 1029 Hexatungstates, 1034 Hexavanadates, 1027 Holmium complexes phenanthroline, 1069 Hornopolymolybdates, 1377 Hydrates, 4 amides, 6 ammines, 4 solvates, 4 Hydrodesuifurization molybdenum catalysts, 1435 Hydrogen peroxide reduction vanadium(T1) complexes, 471 Indium complexes, 153-175 Indium(1) complexes, 154 cyanates, 155 halogens, 155 0-ketoenolates, 155 macrocyclic ligands, 155 nitrogen ligands, 154 thiocyanates, 155 Indium(I1) complexes, 156 halogens, 157 nitrogen ligands, 156 oxygen ligands. 156 phosphorus ligands, 156 sulfur ligands, 156 Indium(II1) complexes, 157 alkoxides, 161 amido, 159 amines, 158 ammines, 158 aqua. 160 arsenic ligands, 160 azides, 159 bipyridyl, 159 borohydrides, 166 bromates, 163 bromides, 16.5 carboxylates, I63 chlorates, 163 chlorides, 165 crown ethers, 167 cyanates, 159 cyanides, 157 cyclam, 167 ethers. 163 fluorides, 165 fluorosulfates, 162 germanium compounds, 158 hydrides, 166
hydrogen, 166 hydroxides, 160 8-hydroxyquinoline, 166 imido, 159 iodates, 163 iodides, 165 P-ketoenolates, 161 macrocyclic ligands, 167 mixed halides, 166 nitrates, 162 nitriles, 160 nitrites, 162 orthophosphates, 162 oxides, 160 perchlorates, 163 1 ,lO-phenanthroline, 159 phosphorus ligands, 160 phthalocyanines, 167
Subject Index porphyrins, 187 pyridines, 158 selenides, 164 selenocyanates, 159 silicon compounds, 158 sulfates, 162,. sulfur compounds, 164 bidentate, 164 thiocyanates, 159 tin compounds. 158 cis-Inositol alkalinc earth metal complexes, 24 epi-Inositol alkaline earth metal complexes, 25 myo-Inositol alkaline earth metal complexes, 25 Iodates, dicyano-,317 Iodates, dinitrato-, 319 Iodates, octafluoro-. 316 Iodine, chlorodiphenyl-, 317 Iodine, triphenyl-, 317 Iodine anions IFG-, 316 IFs-, 316 Iodine cations, 319 (bipy)(IN&, 319 bis(thiourea), 32i ICl,+, 314 ICL.'.,315 IF,+, 315 IF6+,316 PY(Iz)z3319 Iodine complexes {[(en)zCo(SCHzCH2NHz)]~I}5+, 321 Iodine(II1) compounds [I(OSOZF)J+, 320 I(OSOZF),I, 320 Iodine dichloride, phenyl-, 317 Iodine dioxide,p-chlorophenyl-; 318 Iodine dioxide, phenyl-, 318 Iodine fluorosulfatc, 320 Iodine halides, diphenyl-, 317 Iodine heptafluoride, 316 Iodine,pentafluoride, 315 Iodine perchlorate, 320 Iodine tetrafluoride, phenyl-, 318 Iodine tetrafluoride, trifluoromethyl-, 318 Iodine trifluoride, 314 Iodine trifluoride, diphenyl-, 318 Ionomycin alkaline earth metal complexes, 68 Ionophores, 60 carboxylic, 66 neutral, 62-66 Ion selective clectrodes, 22 Iron complexes boron ligands, 99 Iron-molybdenum cofactor, 1425 composition, 1425 EPR, 1425 extended X-ray absorption fine structure, 1426 57FeMossbauer spectra, 1425 level of oxidation, 1426 X-ray absorption near edge structure, 1426 Iron-molybdenum-sulfur clusters, 1425-1430 cubane-like crystal structure, 1429 elcctrochcmistry, 1424 electronic structure, 1428 57FeMossbauer spectra, 1428 magnetic moments. 1428 NMR, I428 synthesis, 1426
Subject Index double cubane-like bridged, 1428 EPR, 1428 NMR, 1427 reactions, 1427 structure, 1426 synthesis, 1426 Isopolyanions, 1023-1055 Anderson structures, 1043 stoichiometry, 1025 Isopolymolybdatcs. 1029-1032, 1256, 1259 aqueous solutions, 1029 nonaqueous solutions, 1031 Isopolyniobates, 1028 Isopolytantalates, 102Y Isopolytungstates, 1032, 1033 Isopolyvanadates, 1025-1028 aqueous solutions, 1025 Kala-Azar, 278 Keggin anions, 1034 electrochemistry, 1050 Keggin structures, 1035 heteropolyanions, 1037 Klebsiella pneumoniae nitrogenases, 1425 Kroll process, 325 Krypton compounds difluorides, 313 fluoride, 313 o-Lactose alkaline earth metal complexes, 25 Lanthanide complexes, 1059 acetylacetone, 1077 acetylhydrazine, 1090 amine oxides, 1081 X-ray crystallography, 1081 amines, 1072 arsine oxides, 1082 aza-crown ethers, 1095 azidcs, 1075 bipyridyl, 1069 bis(methy1imido)triphosphoric acid pentakis(dimcthylamide)l1085 carboxylates, 1085 circularly polarized emission, 1108 complex halides, 1099 crown ethers, 1091 cryptates, 1097 2,6-diacetylpyridine bis(benzoic acid hydrazone), 1090 2,6-diacetylpyridine bis(semicarbazone), 1090 dialkylamido, 1071 diethylphosphate, 1084 diisopropyl N,N-diethykarbamylmcthylcncphosphonatc, 1084 p-diketonates, 1077 adducts, 1078 X-ray structure, 1079 dipicolinates, 1090 dipivaloylmethanates, 1077 dipositive oxidation state. 1109 electronic spectra, 1112 nitrogen donors, 1111 oxygen donors, 1111 phosphorus donors, 1111 solvation, 1110 dithia-18-crown-6, 1093 dithiocarbamates, 1087 dithiophosphates, 1086 dithiophosphina t es, 1087 edta, 1087 electronic spectra, 1105 emission spectra, 1106
1459
excitation spectra, 1107 glycolates, 1085 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyloctane-3.5-dione, 1078 hexarnethylphosphorarnide, 1084 hydrated carboxylates, 1076 hydrated iodates, 1076 hydrated perrhenates, 1076 hydrated pertechnetates, 1076 iminodiacetate, 1088 sminodiaceticmonomethylenephosphonic acid, 1085 macrocyclic ligands, 1091 l&naphthyridinc, 1069 nitrato, IO86 nitrilotriacetatc, 1088 nitrogen and oxygen donors, 1087 nitrogen donor ligands, 1069 nitrogen donor macrocycles, 1094 nitrogen-oxygen macrocycles, 1097 NMR, 11OC-1105 nonamethylimidodiphosphoramide, 1084 oxalates, 1085 o-phenanthroline, 1069 phosphine oxides, 1082 phthalocyanines, 1095 picoline oxides, 1082 polyamines, 1072 polyethylene glycols, 1093 4-pyridinecarboxylic acid hydrazide, 1090 pyridine oxides, 1081 rulfato, 1086 sulfur donor ligands, 1086 tetraphenylimidodiphosphate,1084 tetrapositive oxidation state, 1113 tetrapositive oxidation state catechol oxygen donors, 1115 tetrapositive oxidation state hydrated ions inorganic oxyanions, 1114 nitrogen donor ligands, 1114 oxygen donor ligands, 1114 tetravalent chlorides, 1116 tetravalent fluorides, 1115 thiocyanates, 1075 thioethcrs, 1087 thiosemicarbazidediacetic acid, 1090 triphenylphosphine oxide, 1083 2,4,6-tri-a-pyridyl-1,3,5-triazine, 1071 2,2':6',2"-terpyridyl, 1070 Lanthanides coordination numbers, 1068 ionic radii, 1068 magnetic interaction with coordinated ligands, 1101 separation, 1068 Lanthanide shift reagents applications, 1103 complexation, 1105 Lasolocid alkaline earth metal complexes, 67 calcium transport, 66 Lead elemental structure, 184 Lead, cyclopentadienyl-, 218 Lead bromide, 194 Lead bromide hydrate, 195 Lead carboxylates, 222 Lead cornplexcs, 183-223 q6-arene, 220 bivalent stereochemistry, 188 coordination geometry, 185 dialkyl or diaryl, 216 tetravalent, 205
I460 Lead fluoride, 193
Lead halides, 197 bivalent structure, 193 Lead iodide, 194 Lead oxide structure, 202 Lead phosphates, 222 Lead selenide structure, 202 Lead sulfate, 222 Lead sulfide structure, 202 h a d tclluride structurc, 2U2 Lewisite, 256 Lipophilicity alkali metal complexes, 53 Lithium aluminum hydride, 124 amine adducts, 124 Lithium complexes crown ethers, 40 urea, 6 Livingstonite, 265 M- 139603 alkaline earth metal complexes, 68 Macrocyclic effect, 48 Macrocyclic ligands alkaii metal complexes, 35-59 alkaline earth metal complexes, 35 Macrocyclic polyethers alkali metal complexes, 2 Magnesium complexes amines, 8 ammines, 8 arsine oxides, 9 6-diketonates, 13 dimethylphthalate, 16 hydrates, 7 naturally occurring, S8 octarnethylpyrophosphorarnide , 15 phosphines, 9 pyridine oxide, 9 Schiff bases, 24 , urea hydrates, 9 Magnesium dibromide complexes, 8 Magnesium halides complexes, 10 a-D-Mannofuranose alkaline earth metal complexes, 25 Metal complexes alkali metal cations, 16 alkaline earth metal cations, 16 ammines, 2 reduction vanadium(I1) complexes, 472 Metalloarsenates non- Keggin, 1042 Metallophosphates non-Keggin, 1042 Metathioarsenites, 248 Metatungstates, 1034 fluoro, 1034 isomers, 1035 Metavanadates, 1027 polymeric, 1027 Molybdates, 1376 as ligand, 1376 bonding, 1378 charge transfer spectra, 1378 IR spectra, 1379
Subject Index molecular orbital calculations, 1378 occurence, 1229 Raman spectra. 1379 reactions, 1379 reduction, 1238 salts, 1376 seleno, 1377 spectra, 1378 structure, 1376 thio, 1377 UV spectra, 1378 Molybdates, aquatetraoxodioxidation, 1255 Molybdates, heptacyano-, 1235 Molybdates, hexaaqua-, 1235 oxidation, 1236, 1237 substitution reactions, 1235 Molybdates, hexachloro-, 1234 Molybdates, hexaisocyanato-, 1235 Molybdates, hexaoxoreduction. 1242 Molybdates, octachlorodiaqueous solution chemistry, 1232 Molybdates, pentachlorooxoIsO-exchange, 1254 Molybdates, tetraoxo180-exchange, 1259 Molybdates, tetrathioiron complexes, 1426 Molybdenite, 1229,1431 Molybdenum. 1229 aqueous solution chemistry, 1229 biological relevance, 1229 occurrence, 1229 uses, 1229 Molybdenum, dicyciopentadienyltetracarbonyldi-,1316 Molybdenum, hexacarbonyl-, 1230 Molybdenum complexes alkoxides, 1307 alkyl, 1307 allyl, 1306 aqua, 1230 aryl, 1307 bis(triRuoromethyl)dithietene, 1436 bromide, 1306 carboxylates, 1232 decarboxylations, 1304 reactions, 1302 stereochemistry, 1304 structure, 1302 synthesis, 1302 chalcogenides, 1431-1436 Chevrel phases, 1321 tetrameric clusters, 1321 chlorides pentameric, 1321 tetrameric clusters, 1319 chromium derivatives, 1307 clusters, 1317-1322 bonding, 1318,1329 hexameric, 1321 95MoNMR, 1318 structure, 1317,1319 trimeric, 1317, 1319 cyanide sulfur, 1433 redox reactions, 1433 cyclooctatetraene, 1306 cyclopentadienyl dithiolene, I435 cyclopentadienyl p.-ethane-l,2-dithiolate, 1435 cyclopentadienyl sulfur, 1435 L-cysteine, 1360 ciS-l,2-dicyanoethane-1,2-dithiolate, 1436 dihydrogen, 1438
Subject Index dimeric, 1230,1355 carboxylates, 1362 chloride bridges, 1362 cyclic voltammetry, 1361 cysteinato, 1361 dioxo, 1358 dithiocarbamato, 1360, 1361 dithiocarbonate, 1356 dithiocarboxylate, 1356 1,l-dithiolate, 1356 dithiophosphate, 1356 electrochemistry, 1361, 136.5 clectronic absorption spectra, 1357 glycinate, 1361 heterocyclic ligands, 1362 L-histidine, 1361 2-hydroxyethanethiol, 1366 8-hydroxyquinoline, 1357,1366 IR spectra, 1357, 1361 isothiocyanate, 1357,1362 nitrogen bridges, 1362 NMR, 1357 one bridging ligand, 1355 oxalate, 1358 persulfide bridges, 1362 porphyrins, 1357 pyridazine, 1362 pyridine, 1362 rcalgar, 1363 thiolate bridges, 1365 thiol bridges, 1365 thiophenolates, 1361, 1365 thiourea, 1362 three bridging ligands, 1365 4-toly1, 1357 1,5,9-triazacycl~dodecane~ 1358 1,4,7-triazanonane,1361 trithiocarbonates, 1356 two bridging ligands, 1358 xanthate, 1356 dimeric dioxo CD spectra, 1361. dimeric disulfido CD spectra, 1361 dimeric oxo 1361 9 5 MNMR, ~ structure, 1355 dimeric sulfido gsMO NMR, 1361 dimetallic, 1302-1310 dinitrosyl, 1272 dithiocarbamates, 1305 dithiocarboxylates, 1305 dithiolenes, 1432, 1436 dithiophosphinates, 1305 double bonds, 1.316 extended arrays, 1321 halides, 1306 dimetallic, 1316 tetrameric, 1321 heteronuclear quadruple bonded. 1307 hexaaqua dimers substitution reactions, 1236 hydrides, 1438 hydrido-bridged, 1307 hydridotetraoxo reaction rates, 1260 2-hydroxypyridine, 1305 iodides tetrameric clusters, 1319 isothiocyanates, 1306 metal-metal bonds, 1301-1322 molybdenum-molybdenum bonds, 1301-1322
1461
monomeric, 1230 nitrosyl sulfur, 1433 oligomeric, 1355, 1363 hydroxide bridging, 1363 methoxide bridging, 1363 oxalate, 1363 sulfide bridges, 1364 trifluoroacetate, 1364 penulfides, 1431 perthiocarbonates, 1432 phosphines, 1306 polysulfides, 1431 porphyrins, 1305 quadruple honds. 1308 cleavage. 1310 ligand substitution, 1309 metal-metal, 1302-1310 oxidation, 1309 reactions, 1309 redox reactions, 1309 single bonds, 1317 sulfur ligands. 1421-1425 N,N,N'.N'-tetrakis(2-aceticacid), 1362 N,NIN,N'-tetrakis(2-acetic acid) 1,2-diamin~propane~ 1362 thiocarboxylates, 1305 thioxanthates, 1305 1,4,7-triazacyclononane,1358 N.N',N'-trimethyl-l,4,6-triazarronane, 1357 triple bonds, 1315 cleavage, 1315 molybdenum-molybde~um, properties, 1314 oxidative addition, 1315 reaction with alkynes, 1315 reaction with CO, 1315 reaction with cyanamide, 1315 reaction with diazomethane, 1315 triple metal-metal bonds, 1310-1315 alkoxy, 1310,1311 diethylamino, 1310 dimethylamino, 1310, 1311 dimethylcarbamoyl, 1311 ethyirnethylamino, 13'10 properties, 13'14 reactions, 1311-1314 reaction with alcohols, 1311 reaction with bidentate ligands, 1313 reaction with bridging ligands, 1313 reaction with C02, 1313 reaction with P-diketonates, 1313 reaction with isocyanates, 1313 reaction with isocyanides, 1313 reaction with N donor ligands, 1313 reaction with Pdonor ligands, 1313 reaction with PF3, 1314 reaction with phcnols, 1313 reaction with phosphines, 1313 silyl, 1310, 1311 thiolato. 1314 tris(thio1ene) electrochemistry, 1436 preparation, 1437 structure, 1436 tungsten derivatives, 1307 vinyl sulfide, 1432 xanthates, 1305 Molybdenum(0) complexes, 1265,1266 carhon dioxide, 1277 carbon monoxide, 1266 diphenylphosphinoethane,1266 9sMo NMR, 1266 carbonyl sulfide, 1277 dinitrogen, 1267, 1268
1462 binding, 1267 15NNMR, 1271 oxidation, 1271 preparation, 1267 reactions, 1271 dinitrosyl, 1271 dithiolene, 1277 isocyanides, 1266 oxidationlreduction, 1266 mononitrosyl, 1275,1276 nitrogenase, 1267 nitrogen donor ligands, 1276 nitrogen rnonoxidc, 1271 phosphine ligands, 1277 sulfur dioxide, 1.277 Molybdenum(1) complexes, 1265,1278 carbon monoxide, 1278 dinitrogen, 1278 isocyanide, 1278 nitrogen donor ligands, 1280 nitrogen monoxide, 1279 nitrosyl, 1279 phosphorus donor ligands, 1280 Molybdenum(I1) complexes, 1230,1265,1280 arsines, 1283 azido, 1296 bimetallic reduction, 1293 carbonato carbonyl, 1296 carbon monoxide, 1280 alkoxy, 1282 coordination number, 1281 dithiocarbamato, 1282 hydride, 1282 loss of carbon monoxide, 1282 monocarbonyl, 1283 preparation, 1281 structure, 1282 cyanide, 1283 diazenido, 1292,1294,1295 dicarbonyl, 1295 reactions, 1295 synthesis, 1293 dicarbonyl, 1281 diethyl dithiocarbamato reactions, 1296 disulfido, 1296 halides, 1321 hydrazide, 1292 isocyanide, 1283 alkyne, 1283 oxidation, 1283 protonation, 1283 ' monometallic reduction, 1293 q2-nitrosodurcne, 1296 nitrosyl, 1285,1286 nitrosyl dithiocarbamato, 1290 nitrosyl pyrazolylborates, 1287 pentachloronitrosyl, 1290 pentacyanonitrosyl, 1290 phosphines, 1283 phosphites, 1285 sulfido, 1296 tertiary phosphines, 1285 thionitrosyl, 1290 trichloronitrosyl, 1285 Molybdenum(II1) complexes, 1234, 132S1332 acetylacetone, 1331 cyanides, 1332 diamines, 1331 diazadienes, 1331 dimeric, 1332
Subject Index oxidation, 1251 preparation, 1239,1241 substitution reactions, 1239 dithiocarbamates, 1331,1333 dithiophosphates, 1331 electronic absorption spectra, 1332 electronic properties, 1332 ESR, 1332 ethylenediaminetetracetate, 1333 1,4,7,10,13,16-hexathiacyclooctadecane,1331 magnetic properties, 1332 nitrogen ligands, 1331 ' oxo-bridged dimers, 1333 oxygen ligands, 1331 phosphorescence, 1332 polymeric, 1332 sulfide bridges, 1333 sulfur ligands, 1331 thiols, 1330 1,4,7-triazacyclononane,1333 trichlorotris(tetrahydrofuran), 1331 1,4,7-trimethyl-1,4,7-triazacyclononane, 1331 Molybdenum(1V) complexes, 1243,1332-1347 alkoxy, 1343 azides, 1340 2,6-bis(2,2-diphenyl-2-mercaptoethyl)pyridine,1337 bromides, 1334, 1342 t-butylthio, 1344 carbonyldioxo, 1339 chloride bridges, 1347 chlorides, 1334,1342 cyanides, 1338,1345 cyanothio, 1347 cyclopentadienyl, 1346 dialkylamides, 1344 diazenides, 1340 dichloroseleno, 1334 dichlorothio, 1334 difluorodithiophosphate, 1337 dimeric, 1346 dioxide, 1334 dithiocarbamates, 1336,1343 dithiocarboxylates, 1343 dithiolenes, 1344 electrochemistry, 1340 electronic properties, 1345 electronic spectroscopy, 1339 fluorides, 1334.1342 halides, 1333,1342 hydrazides, 1340 imides, 1340 isocyanides, 133R, 1345 is0t hiocyanat o reduction, 1247 magnetic properties, 1345 molybdenum-nitrogen bonds, 1340 NMR, 1339 nitride, 1340 nitrogen ligands, 1338 oxo, 1336 bonding, 1335 crystal structure, 1336 monomeric, 1335 mononuclear, 1334 oxo-sulfido trimers synthesis, 1246 oxychloride, 1334 oxygen ligands, 1337 perthiocarhonates, 1344 1,lO-phenanthroline, 1339 phosphines, 1337 photolysis. 1345 phthalocyanate, 1338
Subject Index polymeric, 1346 8-quinolinolate, 1343 redox reactions, 1337 Schiff bases, 1338,1342 spectroscopy, 1244,1339 sulfur ligands, 1337 tetracyanoethylene, 1336 tetraphenylporphyrin, 1338 1,5,9,13-tetrathiocyclohexadecane. 1337 tetratolylporphyrin, 1338 thiocyanates, 1344 thio-1,3-diketones, 1343 thiolates, 1343 2,4,6-triisopropylbcnrenethiolatc, 1344 trimers oxidation, 1246 UV spectroscopy, 1345 vibrational spectroscopy, 1339,1345 Molybdenum(V) complexes, 1249,1347-1366 bidentate ligands, 1349 N,N’-bis(2-mercapto-2-methylpropyl-1.2ethylenediamine, 1351 N,N‘-bis(2’-mercaptophenyl-l,2-ethylenediamine, 1351 carbodithioates, 1350 chlorides, 1348 chloro alkoxides, 1354 cyanides, 1248,1354 cyclohcxane-l,2-dione dioximc, 1351 cyclopentane-1,2-dione dioxime, 1351 dichlorotrifluoro, 1348 dimers pox0 bridged, 1253 reduction, 1256 N,N’-dimethyl-N,N’-bis(2’-mercaptoethyl-l,2ethylenediamine, 1351 di-p-sulfido preparation, 1252 di-psulfidocysteinatodimers, 1247 dithiocarbamates, 1354 dithiolenes, 1354 electronic spcctra, 1353 ESR, 1352 fluorides, 1347 halide oxo monomeric, 1348 crystal structures, 1348 halides, 1347,1353 imido, 1353 8-mercaptoquinoline, 1351 monomeric oxo, 1250 mononuclear, 1248 nitrate reduction, 1350 nitrido, 13553 nitrite reduction, 1350 nitrogen bonds multiple, 1353 oxo monomeric, 1348 p-oxo-p-sulfido, 1252 polydentate ligands, 1349 polypyrazolylborat es, 1253 quinolines, 1349 Schiff bases, 1349,1351 selenium ligands, 1350 selenocyanates, 1351 spectroscopy, 1250,1352,1355 sulfur ligands, 1350,1354 2,3,17,18-tetrarnethyI-7,8,12,13-tetraethylcorrole, 1351 tetraphenylporphyrin, 1351 thiocyanates, 1351 thioxanthates, 1354 tribromooxo, 1348
1463
trichlorooxo. 1348 trichloroscleno, 1348 trichlorothio, 1348 Molybdenum(VI) complexes, 1256,1375-1414 alkyl, 1407 alkylidyne, 1407 alkylimido, 1396 arylimido, 1396 bipyridyldibromodioxo, 1388 bis(acety1acetonate) dioxo, 1388 catecholate dioxo, 1389 dimethyl, 1406 dinuclcar, 1408 IR spectroscopy, 1412 0 or S bridge, 1411 preparation, 1411 Raman spectroscopy, 1412 single oxo bridge, 1408 structure, 1408 triple bridge, 1410 dioxo as reactants toward enzyme substrates, 1391 catalysis of air oxidation of benzoin to benzil, 1392 electrochemistry, 1392 oxo removal, 1590 reactions with phosphines, 1390 reactions with cyclohcxyl isocyanate, 1391 reactions with heterocumulenes, 1390 rcactivity, 1390 redox reactions, 1390 dioxo Schiff bases, 1389 1,l-dithiolate dioxo, 1388 ferrocenecarbodithioatedioxo, 1388 hydrazido, 1397 hydroxylamido, 1404 reactions, 1405 structure. 1404 imido, 1396 monomer-dimer equilibria, 1259 M002’+,1380 cis structure, 1381 distorted trigonal prismatic, 1384 EXAFS, 1388 IR spectra, 1386 gsMO NMR, 1386 octahedral, 1381-1386 l7ONMR, 1386 preparation, 1388 Raman spectra, 1386 skew trapezoidal bipyramidal, 1385 spectra, 1386 tetrahedral, 1383 trigonal bipyramid, 1384 Moo4+,1392
NMR,1394
9 5 ~ 0
nitrido, 1394 preparation, 1395 reactions, 1395 structure, 1395 peroxo, 1398 coordination numbers, 1398 dinuclear, 1402 mononuclear, 1398 mononuclear, structure, 1399 reactivity toward organic molecules, 1403 spectral indicators, 1403 tetrahedral sulfido, 1260 tetrahedral thiolate, 1260 tetranuclear, 1411 1,4,7-trirnethyI-l,4,7-triazanonanehalodioxo, 1388 tris(dithiolene), 1413 without oxo ligands, 1412
1464 Molybdenum hexafluoride, 1412 Molybdenum telluridc, 1431 Molybdenum tribromide, 1330 Molybdenum trichloride, 1330 Molybdenum trifluoride, 1330 Molybdenum trihalides, 1330 bond lengths, 1330 magnetic moments, 1330 preparation, 1330 properties, 1330 structure, 1330 Molybdenum triiodide, 1330 Molybdenum trioxide complcxes, 1379 Molybdenum triselenide, 1431 Molybdenum trisulfide, 1431 Molybdoarsenatcs, 1041 non-Keggin, 1042 Molybdoborates, 1042 Molybdoenzymes, 1352 molybdenum site, 1391 Molybdogermanates, 1038 Molybdophosphates, 1041 non-Keggin, 1042 reduced, 1050 Molybdosilicates, 1038 Molybdovanadates non-Keggin, 1043 Monensin alkaline earth metal complexes, 66 Neodymium hypophosphate hydrated, 1076 Neptunium complexes, 1131-1215 Nigericin alkaline earth metal complexes, 66 Niobates, halo-, 592,640 Niobates, pentabromooxo-, 627 Niobates, pentachlorooxo-, 627 Niobium, 585-686 atomic radius, 588 coordination numbers, 588 heterocycles, 639 oxidation states, 587 zinc compounds, 639 Niobium alanate, 685 Niobium complexes clusters, 672,673,675 hexamethylbenzene ligands, 669 pdinitrogen, 418 fluor0 transfer reactions, 593 mixed valence, 685 non-integral oxidation states, 667-677 octahedral bonding, 676 octahedral clusters, 670 electronic spectra, 676 hydrates, 671 iodides, 677 ionic halides, 671 magnetism, 676 neutral binary halides, 671 oxidized, 573 standard potentials, 674 oxidation states, 587 pentahalo, 589 reactions with alkynes three-membered metallacycles, 661 tetranuclear clusters, 670 triangular clusters, 667 solvolysis, 668 Niobium( -111) complexes, 684
Subject Index carbonyl, 684 Niobium(-I) comple.xc.s,684 carbonyl, 684 Niobium(0) complexes, 683 Niobium(1) complexes. 679-683 alkenes, 682 ., carbonyl, 680 carbonylcyclopentadienyl. 682 dienes, 682 phosphines, 679-683 Niobium(I1) complexes, 677-679 dihalo, 667 Niobium(1H) complexes, 655-667 activation reactions, 661) alkoxo, 664 carboxylatotrihalo, hh4 dialkylphosphido, A65 fi-diketonatotrihah, 6@i dithiocarbamato, 661 halo anionic, 655 nitrenes, 665 organometallic, 665 oxalato, 666 pseudohalol 666 sulfato, 666 thiolato, 664 trihalo, 655 neutral adducts, 656 solvolysis, 663 Niobium(1V) complexes, 639-655 activation of srndl moleculesl 654 alkoxo, 649 alkylamido, 652,653 carboxylato, 650 chalcogeno, 654 clusters, 667 dialkyldithiocarbamato, 651 dialkylphosphido, 652 fi-diketonato, 650 ESR,645 halooxo, 646 haloseleno, 646,647 halotelluro, 446 halothio, 646,647 hydrazido, 653 iminoethyl, 653 nitrido, 654 phthalocyanines, 653 poly( 1-pyrazolyl)borato, 653 porphyrins, 653 pseudohalo; 645 silylamido, 653 tctrahalo, 639 neutral adducts, 640 solvolysis, 649 thiolato, 651 thiophosphato, 652 trihalo neutral adducts, 657 Niobium(V) complexes N-acetamido, 622 alkoxo, 601 oxygen ligands, 604 properties, 602 reactions with p-diketones, 603 reactions with nitrogen ligands, 603 reactions with Schiff bases, 605 structure, 601 alkoxohalo, 603,607 adducts, 606 alkoxooxo, 632 N-alkyl-N-nitroso hydroxylaminato, 623
Subject Index amido, 608 structure, 608 synthesis, 608 amidooxo, 634 amidopentahalo, 595 arsenato, 638 azido, 619 bromo anionic, 593 bromooxo, 625 carboxylatohalo, 606 carboxylatooxo, 633 chloro anionic, 593 chlorooxo, 625 chloroperoxo, 636 cyano, 623 N,N’-dialkylacetarnidinato, 622 dialkylamido, 609 dialkylcarbamato, 611 N,N-dialkyldithiocarbamato. 606 dialkyldithiophosphato, 639 difluorophosphinato, 639 $-diketonatooxo, 632 dithiocarbamatohalo, 607 fluoro anionic, 591 fluorooxo, 625 anionic, 626 halo arsenic donors, 600 phosphorus donors, 600 halooxo, 625-632 neutral adducts, 628 nitrogen donor adducts, 630 haloperoxo, 636,637 halophosphonato, 607 halo-8-quinolino1, 606 haloseleno, 626 halothio, 626 halothiolato, 607 hexafluoroperuxo, 636 iodo anionic, 593 iodooxo, 625 isocyano, 623 isoselenocyanato ,624 isothiocyanato, 624 isothiocyanatooxo, 634,635 metaphosphato, 639 mixed alkoxo, 602 mixed halo, 594 mixed pentahalo, 591 nitrato, 638 nitrenes, 613 reactivity, 616 structure, 613 synthesis, 613 nitrido, 620,621 nitrogen-containing insertion produc:ts. 623 orthophosphato, 639 oxo, 635-639 oxygen abstraction reactions. 631 pentabromo, 590 pentachloro, 590 pentadialkylamido cyclometallation, 611 reactivity, 610 pentafluoro, 589 pentahalo, 58S591 amines, 596 applications, 591 arsenic donors, 599
1465
arsine oxides, 595 dialkyl esters, 595 ethers, 59s imidazoles. 596 ketones, 595 neutral adducts, 594 nitrogen complexes, 597 nitrogen donors, 596 phosphines, 595 phosphorus donors, 599 Schiff bases, 5% selenium ligands, 596 solvolysis, 600 sulfur ligands, SY6 tclluriurn ligands, 596 pentahalonitrido, 596 pcntahalothicicyanatci, 596 pentaiodo. 590 peroxo, 635,636 peroxo acids, 635 phosphato, 638 phosphinato, 639 phosphineirninato, 619 phosphonato, 639 porphyrins, 620 pyrazolylboraro, 622 seleno, 638 selenocyanato, 624 sulfato, 638 tetrachloro with phosphorus donors, 642 tetrahalo amines, 641 oxygen ligands, 643 sulfur ligands, 643 with arsenic donors, 642 thio, 635-639,638 thiocarbatmato, 634 thimyanato. 624 N-thioacetamido, 622 trihalooxo solvolysis, 632 Niobium selenide, 685 Niobyl phosphoniobate, 685 Nitrate reductase, 1334 Chlorella vulgaris structure, 1438 Escherichuc coli structure, 1438 Nitrates reduction oxomolybdoenzymes, 1437 Nitrogenase dinitrogen tungsten complexes, 1011 iron-molybdenum cofactor, 1425 molybdenum bridging to iron, 1379 vanadium(I1) complexes, 473 Nitrogen fixation Klebsiella pneumoniae, 1425 oxornolybdoenzymes, 1437 Noble gas compounds oxo acids, 320 Noble gas(I1) compounds, 313,319 Noble gas(IV) compounds, 313,320 Noble gas(V1) compounds, 315,320 heptafluoroxenate, 315 Noble gas(VII1) compounds, 316 Noble gases, 311-321 carbon ligands, 316 coordination compounds, 312 coordination geometry, 312 halogen ligands, 313-316 nitrogen ligands, 329 I
1466 oxidation states, 312 oxygen ligands, 319 selenium ligands, 321 sulfur ligands, 321 Nonactin alkali metal complexes,.64 Nuclear relaxation rates lanthanide shift reagents, 1103 applications, 1103 Octamolybdates, 1054 isomerization, 1032 Organoantimony compounds bond lengths and angles, 258 Organuarsenic compounds, 239 Organoarsonates metal complexes, 1053 Organolead compounds, 210 structure, 212 Organophosphonates metal complexes, 1053 Organotin compounds, 210 structure, 212 Orthovanadates, 1025 1-Oxa-4,6-dithia-5-bismocane, 5-phenyl-, 285 1-0xa-4,6-dithia-5-stibocanc, 5-chloro-, 265 Oxomolybdoenzymes, 1436,1437 EPR spectra, 1437 Oxovanadium(V) complexes aqueous solution, 1026 Oxygen reduction vanadium(I1) complexes, 471 Oxygen abstraction reactions niobium and tantalum complexes, 631 Paramolybdates, 1030 Paratungstate A , 1033 Paratungstate B, 1033 Paratungstates, 1033 Pentabromotellurates, 301 Pcntachlorotellurates, 301 Pentafluoroselenates, 301, 302 PentaRuorotellurates, 301 Pentaiodopolonates, 302 Pentaiodotellurates, 302 Pentamolybdates, 1032 Pentamolybdobis(organophosphonates),1053 Pentatungstobis(organophosphonates), 1053 Peptides models protein denaturation, 6 Phaliusia mammilata
vanadium(II1) complexes, 486 Phenol, o-nitroalkali mctal complexes, 30 Phosphates protonation, 1026 Phosphonic acids alkaline earth metal complexes, 32-35 Phthalocyanines alkaline earth metal complexes, 58 Platinum(1V) complexes reduction vanadium(I1) complexes, 473 Plumbacarboranes, 219 Plumbates, 204 Plumbocene functionally substituted, 218 nomenclature, 217 Plumbonic acid, 206 Plutonium complexes, 1131-1215 Podands
Subject Index alkali metal complexes, 4, 17-23 NMR, 20 Polonium, 299-307 bonding, 299 coordination compounds, 300 geometry, 299 toxicity, 307 valence, 299 Polonium complexes diethyldithiocarbamate. 307 thiourea, 307 Polonium(1V) complexes solubility in carboxylic acids, 304 Polonium hydroxide solubility in acids, 304 Polonium perchlorate complexes with tributyl phosphate, 304 Polonium tetrachloride complexes with tributyl phosphate, 304 Polyanions complete ligands, 1047 lacunary ligands, 1W7 organic, 1052 Type I, 1050 Type 11,1050 Polycarboxylic acids alkaline earth metal complexes, 32-35 Polychromates, W1 structures, 943 Polyethers alkali metal complexes, 48 alkaline earth metal complexes, 48 Polymers boron-nitrogen, 101 Polymolybdates cyclic voltammetry, 1051 electrochemistry, 1051 electronic structure, 1051 spectroscopy, 1051 Polypodands alkali metal complexes, 23 Polytungstates electrochemistry, 1050 Polyvanadates, 1028 mixed valence, 1028 Porphyrins alkaline earth metal complexes, 58 niobium(V) complexes, 620 nomenclature, 350 tantalum(\.') complexes, 620 Potassium complexes o ,o'-catecholdiacetic acid, I6 crown ethers, 39 Potassium hexachioromolybdate, 1230 Praseodymium complexes edta, 1089 tetrapositive oxidation state hydrated ions, 1113 tetravalent fluorides, 1115 L-Proline lanthanide complexes NMR, 1103 Protactinium, 1131-1215 Proteins denaturation alkali metal salts, 6 Pseudometatungstates, 1034 Quat eren es alkali metal complexes, 57
Subject Index Rubidium complexes podands, 19 Ruthenium(I1) complexes reduction vanadium(I1) complexes, 472 Salicylaldehyde alkali metal complexes, 30 Salvosan, 255 Samarium complexes dipositive oxidation state hydrated ions, 1109 Sampsonite, 265 Scandium complexes, 1059-1 116 alkoxides, 1065 amides, 1062,1066 amine oxides, 1065 amines, 1060 amino acids, 1066 aromatic amines, 1061 arsenic compounds, 1065 2,2'-bipyridyl, 1061 1,2-bis(pyridine-a-carbaldimino)ethane,11061 bromides, 1067 Carboxylates, 1063 carboxylato ammines, 1060 chlorides, 1067 cyclic crown polyethers, 1065 6-diketonates, 1062 dimethylacetamide, 1066 dimethylformamide, 1066 ethylenediamine, 1060 fluorides, 1067 formates, 1063 hydrated carboxylates, 1063 hydrated ions, 1073 properties, 1074 structures, 1074 thermodynamic parameters, 1073 hydrates, 1064 hydrazines, 1060 hydroxo, 1064 hydroxyacetates, 1063 8-hydroxyquinolinate, 1066 malonates, 1063 nitrate hydrates, 1076 nitrato, 1064 nitrogen donor Ligands, 1060 octaethylporphyrin, 1061 oxalates, 1063 oxalato ammines, 1060 oxyanions, 1064 oxygen donor ligands, 1062,1073 1,lO-phenanthroline, 1061 phosphine oxides, 1065 phosphines, 1073 phosphorus donor Ligands, 1073 phosphoryl, 1065 phthalocyanine, 1062 porphyrins, 1061 pyridine, 1061 salt hydrates, 1073, 1075 sulfate hyrates, 1076 sulfato, 1064 sulfoxides, 1065 tetrahydrofuran, 1064 tetraphenylporphyrin, 1061 tripyridyltriazine, 1061 tris(silylamide), 1062 urea, 1065 2,2':6',2"-terpyridyl, 1061 Scheelite, 974 Sea squirts
vanadium(II1) complexes, 486 Selenates, pentafluoroelectronegativity, 319 Selenium, 2%307 bonding, 299 coordination compounds, 300 geometry, 299 toxicity, 307 valence, 299 Selsnium(I1)complexes cis-dibromobis(thiourea),305 cis-dichIorobis(thiourea),305 four-coordinate, 305 triselenocyanate, 305 dimeric, 305 triselenourea, 305 trithiocyanate dimeric, 305 Selenium monobromide complexes with amines, 304 Selenium oxychloride complexes, 302 Selenium tetrachloride complexes with amines, 304 with pyridine, 304 Selenium trioxide complexes, 304 Selenoarsenous acid, 249 esters, 249 Selenotungstates, 1042 Sesquisiloxanes,206 Siderophores reduction vanadium(I1) complexes, 473 Silabenzene, 190 Siladiimides, 191 Silaethylene, 1,l-dimethylSi=C n bond energy, 184 Sildethylenes preparation, 188 Silaimines, 191 Silanediol, di-t-butyl-, 206 Silanones, 191 Silathiones, 191 1-Silatoluene, 190 Silatranes, 212 Silica structure, 192,201 Silicates,202 structure, 203 Silicon elemental structure, 183 Silicon complexes, 18S223 coordination geometry, 185 multiple hnding, 188 tetravalent, 205 Silicones, 188 Silicon halides, 197 Lewis acidity, 199 Silicon monoxide, 191 Silylenes, 192 structure, 193 Sodiuim hexakis(formato)molybdate, 1235 Sodium complexes crown ethers, 37 Sodium molybdate, 1230 Sodium peroxoborate, 101 SodiumlpotassiumATPase vanadate inhibition, 567 Sodium pyroantimonate, 265 Sodium tetrahydroborate, 101
1467
1448
Subject Zndex
Solvolysis niobium and tantalum oxo trihalides, 632 niobium pentahalides, 600 tantalum pentahalides, 600 Spherands alkali metal complexes, 43 Spiroarsoranes, 245,269 2,2'-Spirobi( 1,3,2)5-dioxastibolan, 2-phenyl-4,4,4',4',5,5,5',5'-octamethyl-, 269 Stannacarborates, 219 Stannate, trichloro-, 195 Stannates, 204 Stannatranes, 212 Stannocene, 217 functionally substituted, 218 nomenclature, 217 Stannocenium salts, pentamethyl-, 218 Stannonic acid, 206 Stannylenes, 192 structure, 193 Stibine, amino-, 260,282 Stibine, aminochloro-, 259 Stibine, dihalo-, 258 Stibine, germyl-, 259 Stibine, stannyl-, 259 Stibine, tris(dimethy1amino)-, 259 Stibine, trisilyl-, 259 Stibine, tris(arganop1umbyl)-, 259 Stibines alkyl primary, 262 halogenation, 257 Stibonous acid thioesters, 269 Stibotungstates, 1042 Strontium complexes pyridine oxide, 9 Sucrose alkaline earth metal complexes, 25 Sugar phosphates alkaline earth metal complexes, 34 Sulfite oxidascs, 1334 structure, 1438 Sulfur, 299-307 bonding, 299 coordination compounds, 300 geometry, 299 valence, 299 Sulfur tetrafluoride complexes, 301 with pyridine, 304 Sulfur trioxide complexes with pyridine, 304 Tanning chromium(lI1) complexes, 907 Tantalates, halo-, 592,640 Tantalates, pentachlorooxo-, 627 Tantalum, 585-686 atomic radius, 588 coordination numbers, 588 oxidation states, 587 zinc compounds, 639 ' Tantalum complexes clusters, 612,673,675 p-dinitrogen, 418 Ruoro transfer reactions, 593 hexamethylbenzene ligands, 669 mixed valence, 685 non-integral oxidation states. 667-677 octahedral oxidized, 673
octahedral clusters, 670 bonding, 676 electronic spectra, 676 hydrates, 671 ionic halides, 671 magnetism, 676 neutral binary halides, 671 standard potentials, 674 oxidation states, 587 tetranuclear clusters, 670 triangular clusters, 667 solvolysis, 668 Tantalum(-111) complexes carbonyl, 684 Tantalum(-I) complexes, 684 carbonyl, 684 Tantalum(0) complexes, 683 Tantalum(1) complexes, 679-683 alkenes, 682 carbonyl, 680 carbonylcyclopentadienyl, 682 dienes, 482 phosphines, 679-683 Tantalum(I1) complexes, 677-679 dihalo, 667 Tantalum(II1) complexes, 655667 activation reactions, 6-N alkoxo, 664 carboxylatotrihalo, 664 dialkylphosphido, 665 P-diketonatotrihaio, 664 dithiocarbamato, 664 halo anionic, 655 nitrenes, 665 organometallic, 665 oxalato, 666 pseudohalo, 666 reactions with alkynes three-membered metallacycles, 661 sulfato, 666 thiolato, 664 trihalo, 655 neutral adducts, 656 solvolysis, 663 Tantalum(1V) complexes, 639-655 activation of small molecules, 654 alkoxo, 649 alkylamido, 652 carboxylato, 650 chalcogeno, 654 clusters, 667 dialkyldithiocarbamato, 651 dialkylphosphido, 652 0-diketonato, 650 ESR, 645 halooxo, 646 haloseleno, 646,647 halotelluro, 646 halothio, 646,647 nitrido, 654 phthalocyanines, 653 porphyrins, 653 pseudohalo, 645 tetrahalo, 639 neutral adducts, 640 solvolysis, 649 thiolato, 651 trihalo neutral adducts, 657 Tantalum(V) complexes N-acetamido, 622 alkoxo, 600,601
Subject Index oxygen ligands, 604 properties, 602 reactions with f3-diketones, 603 reactions with nitrogen ligands, 603 reactions with Schiff bases, 605 structure, 601 alkoxohalo, 603,607 adducts, 606 alkoxooxo, 632 N-alkyl-N-nitrosohydroxylaminato, 623 amido, 608 structure, 608 synthesis, 608 arnidooxn, 634 arnidopentahalu, 59.5 arsenato. 638 azametallacycles, 611 azido, 619 bromo anionic, 593 bromooxo, 625 carboxylatohalo, 606 carboxylatooxo, 633 chloro anionic, 593 chlorooxo, 625 chloroperoxo, 636 cyano, 623 N,N'-dialkylacetamidinato, 622 dialkylamido, 609 dialkylcarbamato, 611 N,N-dialkyldithiocarbamatol 606 dialkyldithiophosphato, 639 difluorophosphinato, 639 p-diketonatooxo, 632 dithiocarbamatohalo, 607 fluoro anionic, 591 fluorooxo, 625 anions, 626 halo arsenic donors, 600 phosphorus donors, 600 halooxo, 625-632 nitrogen donor adducts, 630 haloperoxo, 636,637 halophosphonato, 607 halo-8-quinolinol, 606 haloseleno, 626 halothio, 626 halothiolato, 607 hexafluoroperoxo, 636 iodo anionic, 593 iodooxo, 625 isocyano, 623 isosclenocyanato, 624 isothiocyanato, 624 isothiocyanatooxo, 634,635 metaphosphato, 639 mixed alkoxo, 602 mixed halo, 594 mixed pentahalo, 591 monothiocarbamato, 610 nitrato, 638 nitrenes, 613 reactivity, 616 structure, 613 synthesis, 613 nitrido, 620,621 nitrogen-containing insertion products?623 orthophosphato, 639 OXO,635-639
1469
oxygen abstraction reactions, 631 pentabromo, 590 pentachloro, 590 pentadialkylamido, 611 reactivity, 610 pentafluoro, 589 pentahalo, 589491,589 amines, 596 applications, 591 arsenic donors, 599 arsine oxides, 59.5 dialkyl esters, 595 ethers. 595 ketones, 595 neutral adducts, 594 nitrogen complexes, 597 nitrogen donors, 596 phosphine oxides, 595 phosphorus donors, 599 Schiff bases, 596 selenium ligands, 596 solvolysis, 600 sulfur ligands, 596 tellurium ligands, 596 pentahalonitrido, 596 pcntahalothiocyanato, 596 pentaiodo, 590 pcroxu, 635,636 peroxo acids, 635 phosphato, 638 phosphinato, 639 phosphineiminato, 619 phosphonato, 639 porphyrins, 620 pyrazolylborato, 622 seleno, 638 selenocyanatn, 624 silylamido, 610 sulfato, 638 tetrachloro with phosphorus donors, 642 tetrahalo amines, M1 oxygen ligands, 643 sulfur ligands, 643 with arsenic donors, 642 thio, 635-639 N-thioacetamido 622 thiocarbamato, 634 thiocyanato, 624 trihalooxo solvolysis, 632 Tellurates, pentafluoroelectronegativity, 319 Tcllurates, tartrato-, 303 Tellurium, 299-307 bonding, 299 coordination compounds, 300 geometry, 299 toxicity, 307 valence, 299 Tellurium(1I) complexes four-coordinate, 305 bidentate ligands, 305 tris(ethyfxanthato), 305 Tellurium(1V) complexes eight-coordinate, 307 seven-coordinate, 307 six-coordinate, 307 tram-tetrachlorobis(tetrarnethylthiourea), 3137 tetrakis(4morphoIinecarbodithioato),307 Tellurium dichlorobromide complexes, 302 ~
1470 Tellurium dioxide complexes with thiourea, 305 Tellurium dithiocyanate complexes, 303 Tellurium hexafluoride complexes, 303 with amines, 304 Tellurium sulfate complexes, 303 Tellurium tetrabromide complexes, 302 Tellurium tetrachloride complexes with acetamide, 304 with amines, 304 with pyridine N-oxide, 304 Tellurium tetrafluoride complexes with amines, 304 with dioxane, 304 Tellurotungstates, 1042 Temperature-jump studies molybdenum(V1) complexes, 1259 Terbium complexes fi-diketones, 1081 tetrapositive oxidation state hydrated ions:, 1113 Tetranactin alkali metal complexes, 64 Tetraselenomolybdates fR spectra, 1379 Raman spectra, 1379 Tetrathiomolybdates, 1377 IR spectra, 1379 Raman spectra, 1379 Thallium complexes, 153-175 Thallium(1) complexes, 167 alkoxides, 168 bromides, 170 carboxyiates, 169 chlorides, 170 crown ethers, 170 cyanides, 167 P-diketonates, 169 dithiocarbamates, 169 fluorides, 170 hydrides, 170 hydrogen, 170 hydroxides, 168 iodides, 170 macrocyclic ligands, 170 nitrogen ligands, 168 oxides, 168 oxyanions, 169 peroxides, 168 selenides, 169 sulfides, 169 tellurides, 169 thiourea, 170 Thallium(II) complexes, 171 Thallium(II1) complexes, 171 amines, 172 ammines, 172 aqua, 172 bipyridyl, 172 bromates, 173 bromides, 174 carboxylates, 173 chlorates, 173 chlorides, 174 cyanides, 171 dichlorophosphates, 173 ethers, 173
Subject Index fluorides, 174 hydrides, 175 hydrogen, 175 hydroxides, 172 iodates, 173 iodides, 174 , macrocyclicligands, 175 mixed halides, 174 nitrates, 173 orthophosphates, 173 oxides, 172 perchlorates, 173 lJ0-phcnanthroline, 172 porphyrins, 175 pseudohalides, 172 pyridines, 172 selenates, 173 sulfides, 174 tellurates, 173 terpyridyl, 172 Thallium cyanate, 168 Thallium nitride, 168 Thallium selenocyanate, 168 Thallium thiocyanate, 168 Thioantimonates, 265 Thioarsenates, 248 Thioarsenious acid, 248 esters, 248 Thioarsenites, 248 Thiomolybdate anions, 1421 copper derivatives, 1421 cyclic voltammetry, 1424 gold derivatives, 1422 IR spectoscopy, 1424 nickel derivatives, 1422 NMR, 1424 palladium derivatives, 1422 platinum derivatives, 1422 properties, 1421 Raman spectroscopy, 1424 resonance Raman spectroscopy, 1424 ruthenium derivatives, 1422 silvcr derivatives, '1422 structure, 1421 synthesis, 1421 UV spectroscopy, 1424 Thiostibinous acid esters, 269 Thorium complexes, 1131-1215 Tin elemental structure, 184 Tin, cyclopentadienyl-,218 Tin, methoxytrimethyl-,208 Tin, tri-n-butylmethoxy-,208 Tin bromide, 194 Tin bromide hydrate, 195 Tin carboxylates, 222 mixed valence, 222 Tin chloride, 194 Tin chloride dihydrate, 195 Tin complexes, 183-223 q6-arene, 220 bivalent stereochemistry, 188 coordination geometry, 185 dialkyl or diaryl, 216 multiple bonding, 188 tetravalent, 205 Tin fluoride, 193 Tin halides, 197 bivalent structure, 193 Lewis acidity, 199
Subject Index
’
Tin iodide, 194 Tin oxides structure, 192,202 Tin phosphates, 222 Tin selenide structure, 202 Tin sulfate, 222 Tin sulfide structure, 202 Tin telluride structure, 202 Titanium, 32S358 coordination numbers, 327 discovery, 324 isotopes, 325,326 oxidation states, 327 preparation, 324 properties, 325 Titanium, tetrakis(trimethysily1)oxy-,334 Titanium complexes alloy hydrides, 353 amino acids, 342 antimony, 345 arsenic, 345 bromides, 357 chlorides, 355,356 fluorides, 354 Group IV derivatives, 352 halides, 354 electron spectra, 358 hexamethylphosphoramide, 335 iodides, 357 macrocyclic ligands, 349 mixed donor ligands, 340 sulfur, 343 nitrogen ligands low valent, 327 oxygen-nitrogen ligands, 345 phosphorus, 345 phthalocyanine, 352 porphyrins, 349 Schiff bases, 340 stereochemistry, 327 sulfur ligands anionic ligands, 338 neutral donors, 338 trifluoromethanesulfonic acid, 335 tritium isotope effects, 353 Titanium(I1) complexes antimony, 346 arsenic, 346 phosphorus, 346 Titanium(II1) complexes antimony, 347 arsenic, 347 charge-transfer spectra, 357 nitrogen ligands, 328 magnetic properties, 328 oxygen ligands bidentate, 330 electronic spectra, 330 monodentate, 330 phosphorus, 347 porphyrins, 349 six-coordinate electronic spectra, 331 Titanium(1V) complexes alkoxides, 333 antimony, 347 arsenic, 347 charge-transfer spectra, 357 f3-diketonates, 336 nitrogen ligands
bidentate, 329 monodentate, 329 polydentate, 329 oxygen ligands bidentate, 335 monodentate, 333 peroxo, 350 crystallography, 351 phosphorus, 347 Titanium dihalide porphyrin complexes, 352 Titanium salts catalysts, 348 Titanyl complexes, 350 Transition metals arsenic compounds coordinated open-chain, 244 basicity borane complexes, 101 boron complexes, 99 Transplutonium complexes, 1215-1220 Transplutonium(I1) complexes, 1215 TranspIutonium(lI1) complexes, 1215 aliphatic hydroxy acids, 1217 aqua, 1215 aromatic hydroxy acids, 1217 carbonates, 1217 halogeno, 1218 hydrides, 1218 hydroxides, 1216 p-ketoenolates, 1216 monocarboxylates, 1216 oxalates, 1217 oxides, 1216 phosphates. 1216 phosphorus oxides, 1217 phthalocyanine, 1218 selenium ligands, 1217 sulfates, 1216 sulfur ligands, 1217 tellurium ligands, 1217 trihalides, 1218 Transplutoniurn(1V) complexes, 1219 halogeno, 1219 P-ketoenolates, 1219 oxides, 1219 tetrahalides, 1219 Transplutonium(V) complexes, 1219 aqua, 1219 carbonates, 1220 halogens, 1220 monocarboxylates, 1220 oxalates, 1220 oxides, 1219 Transplutoniurn(V1) complexes aqua, 1220 carbonates, 1220 carboxylates chelating, 1220 halogens, 1220 monocarboxylates, 1220 nitrato, 1220 oxides, 1220 oxoanions, 1220 Triethanolamine alkali metal complexes, 23 Triethylamine, 2,2’,2”-trimethoxyalkali metal complexes, 24 Trimolybdates, 1032 Trithioarsenates, 249 1,3,6,2-Trithioarsocane, 2-chloro-, 249 Trithiomolybdates, 1378 Trivanadates, 1027
1471
1472 Tungstates, decachlorooxydi-, 990 Tungstates, nonahalodi-, 999 Tungstates, thio-, 982 Tungstate X,1034 Tungsten, hexahalodi-, 1000 Tungsten complexes, 973-1015 dinitrogen, 1011-1014 dioxo, 978 hydrido, 1014-1015 nitrosyl, 1014 quadruple bonds, 1010 triple bonds, 1002 Tungsten(I1) complexes, 1005 hexanuclear clusters, 1011 monomeric, 1005 quadruple bonds, 1008 seven-coordinate, 1007 Tungsten(II1) complexes, 998-1005 dinuclear, 998 mononuclear, 998 Tungsten(1V) complexes, 988-998 carbon multiple bonds, 997 cyanides, 997 eight-coordinate complexes, 996 metal-metal bonds, 990 metal-metal double bonds, 991 nitrogen multiple bonds, 997 sulfur multiple bonds, 997 trinuclear clusters, 993 Tungsten(V) complexes, 984-988 metal-metal bonds, 986 oxo, 985 selenium, 986 sulfur, 986 thiocyanato, 988 Tungsten(V1) complexes, 974-983 alkylidene, 981 rtlkylidyne, 981 aimbenzene, 977 carbon multiple bonds, 979 dioxo, 977 dithiolene, 983 nitrido, 980 niuogen multiple bonds, 979 oxo,976 peroxy, 977 seknium multiple bonds, 979 structure, 980 sulfur multiple bonds, 979 trioxo, 978 Tungsten halides, 974,984, 988 synthesis. 974 Tungsten oxyhalides, 985,990 synthesis, 976 Tungsten steel, 974 Tungstoarsenates, 1040 12-Tmgstoborates, 1041 Tungstomolybdates, 1036 Tungstophosphates, 1038 cryptands, 1049 non-Keggin, 1042 reduced. 1050 Tungstwilicates, 1036 Tunicates-see Sea squirts Tunichrorne B-l,486
Subject Index Uranium complexes, 1131-1215 Urothione oxomolybdoenzymes and, 1438 Valence-shell electron-layer repulsion theory halogenium species, 312 noble gas compounds, 312 Valinomycin alkali metal complexes, 3 , 6 2 potassium compkxes, 60 synthetic analogues. 65 Vanadatcs biochemistry, 456 protonation, 1026 Vanadates, hexafluoro-, 482,531 Vanadates, oxoperoxo-. 501 Vanadates, pentacarbonyl-, 457 Vanadium oxidizing/reducing properties, 454 Vanadium complexes, 45S-569 biochemistry, 455 1,2-bis(dimethylphosphino)ethane, 460 low oxidation states, 457 hipyridyl, 457 cyanides, 457 dinitrogen, 459 diselenolene, 460 dithiolene ,46U hydrido, 462 nitrosyl, 458 o-phenanthroline, 457 phosphorus ligands, 460 terpyridyl, 457 mixed I V N valence, 510 oxidation state, 454 triiluorophosphine, 460 Vanadium(I1) complexes. 462 acetonitrile, 464 alcohols, 466 amines, 463 ammonia, 463 benzimidazole, 463 bipyridyl, 464 bromide, 467 carbonates, 467 chloride, 467 cyanides, 462 P-diketones, 466 dimethylimidazole, 463 ethers, 466 fluoride, 467 halides, 467 imidazole, 463 iodide, 467 isocyanides, 462 isoquinoline ,463 isothiocyanates, 464 macrocyclic ligands, 469 1-methylimidazole, 463 2-methylimidazole, 463 oxalates, 467 oxygen ligands, 465 phenanthroline, 464 phosphines, 465 P-picoline, 464 y-picoline, 464 polypyrazolylborate, 463 polypyrazolyl ligands, 463 poly(1-pyrazo1yl)rnethaneligands, 463 porphyrins, 469 pyrazole, 463 pyridine, 463 reducing agents, 469
Subject Index terpyridyl, 464 tris(1-pyrazolylethyl)amines,463 water, 465 Vanadium(II1) complexes, 473 adenine, 475 alcohols, 478 amides, 474,480 amines, 474 amino acids, 484 ammonia, 474 aqua, 477 arsines, 476 azide, 475 bipyridyl, 475 bromides, 483 carboxylates, 479 catecholates, 478 chlorides, 482 complexones, 485 cyanides, 474,476 dimethyl sulfoxide, 480 dioxygen, 478 dithiocarbamates, 481 dithiolates, 481 dithiophosphinates, 481 ethers, 478 ethylenediaminetetraacetic acid, 485 guanine, 475 halides, 482 hydroxy acids, 480 isocyanides, 474 fi-ketoenolates, 478 mixed donor atom ligands, 483 nitnlotriacetic acid, 485 oxoanions, 479 oxygen ligands, 477 phenanthroline, 475 phosphates, 479 phosphinates, 479 phosphines, 476 phosphonates, 479 purine, 475 pyridine, 475 Schiff bases, 483 sea squirts, 486 seleninates, 479 selenocyanates, 475 silazanes, 476 sulfates, 479 sulfinates, 479 sulfur ligands, 481 thiocyanates, 475 thioethers, 481 Vanadium(1V) complexes, 487 acetates, 513 acetylacetonates, 504 electrochemistry, 505 ESR spectra, 505 adenine, 568 adipates, 516 alcohols, 502 amines, 489 amino acids, 544 equilibria, 544 ammonia, 489 arsenates, 513 arsenic ligands, 496 arsines, 498 ascorbic acid, 502 benzilato, 522 biguanide, 496 binucleating ligands, 561 bipyridyl, 492,494
bromides, 530 butanediaminetetraacetate, 548 carbazole, 492 carbonato, 515 carboxylates, 513 carnosine, 547 catecholates, 502 chlorides, 530.531 chlorophyll, 557 citrates, 522 cornpartmental ligands, 561 complexones, 547 cyanides, 489,496 cysteine, 546 deoxophylloerythroetioporphyrin, 557
N,N-didkyl-2-arninoethanethiol, 554 dicydohexylphosphide,498 diethylenetriaminepentaacetic acid, 548 4,6-dihydroxycoumaran-3-one, 510 P-diketonates, 504 solvent effects, 505 3,7-dimethyl-7-hydroxyoctan-l-a1,510 2,6-dimethylpyrazine,492 dithioarsinate, 527 dithiocarbamates, 524,526 spectra, 524 structure, 524 dithiocarboxylates, 524 dithiolates, 528 dithiophosphates, 527 dithiophosphinates, 527 ENDOR, 489 ESR spectroscopy, 488 ethylenediaminetetraacetic acid, 548 fluorides, 529 formates, 513 fulvic acid, 568 glutarates, 516 glycine, 546 Group IV ligands, 489 guanine, 568 halogen ligands, 529 halogenoacetates, 513 heteropolynuclear, 565 histidine, 494,547 humic acid, 568 hydrazine, 494 hydrolysis, 489 hydroxyaldehydato, 510 hydroxycarboxylates,517 1-hydroxycyclohexanecarboxylate,522 hydroxyketonato, 510 2-hydroxy-6-rnethlylpyridineI 523 P-hydroxynaphthaldehyde, 510 hydroxyoximes, 552 3-hydroxyquinazoline-4(3H)-thiones,554 &hydroxyquinoline,551 imidazoIe, 494,547 iodides, 531 lactates, 520 lutidinato, 551 malates, 522 maleates, 516 malonates, 516 mandelates, 522 8-mercaptoquinoiine, 553 methanol, 502 2-methyl-8-quinolinol,552 mixed donor atom ligands, 531,553,555 N heterocycles, 492 stoichiometry, 492 N macrocycles, 557 naturally occurring ligands, 567
1473
1474 nitrates, 513 nitriilotriaceticacid, 548 nitrogen ligands, 489 nucleotides, 568 oxalates, 516 N-oxides, 522 P-oxides, 522 oxo aqueous solutions, 498 bonds, 488 oxoanions, 511 oxygen ligands, 498 perchlorates, 513 perfluoropicanolate, 523 phenanthroline, 494 l-phenyl-l,3-butanedionato, 504 phosphates, 512 phosphines, 496 [@hosphononomethyl)imino]diace tic atid, 5 51 phosphorus ligands, 496 phosphotungstates, 513 phthalocyanines, 559 potycarboxylates, 515 polynuclear, 561 porphyrins,557 structure, 557 proteins, 567 ENDOR, 567 ESR spectra, 567 purine, 568 pyrazine, 492 pyridine, 492,494 pyridinecarboxylicacid, 551 [1-(2-pyridyl)ethyl]iminodiaceticacid, 548 [1-(2-pyridyI)rnethyl]iminodiacetic acid. 548 pyrrole, 492 Schiff bases, 531 bidentate, 536 6-diketone diamines, 535 polydentate, 544 salicylaldehydediamines, 531 tridentate, 538 salicylaldehydato, 510 salicylates, 520 selenates, 513 selenides, 529 selenites, 513 selenium ligands, 529 selenoporphyrins, 559 spectra, 488 structure, 487 succinates, 516 sulfates, 511 sulfides, 529 sulfites, 513 5-sulfosalicylates, 520 sulfoxides, 522 suIfur ligands, 523 tartrates, 517 tetraaza[l4]annulenes, 559 N,N,N',N'-tetramethylurea, 523 tetrasulfur tetranitrides, 554 thio, 523 electrochemistry, 524 IR spectra, 523 stability, 523 structure, 523 thiocarbohydrazones, 553 thiocyanates, 495 thio-p-diketones, 554 thiophene, 529 triethylenetetraminehexaacetic acid, 547 tropolone, 510
Subject Index xanthates, 527 Vanadyl ions hydrolysis, 499 oxidation, 501 rate of exchange, 498 redox behavior, 501 Vanadyl porphyrins, 558 Vanadyl uroporphyrin I,558 Water reduction vanadium(I1) complexes, 471 Xanthine oxidase, 1334 molybdenum complex center, 1405 structure, 1438 Xenates, octafluoro-. 315 Xenon anions XeF,-, 316 XeFS2-, 315,316 Xenon cations fluoride, 313 XeF,+, 314 XeF,+, 315 Xe2Fl, ', 315 Xenon complexes FXeN(SO,F),, 319 nitrogen ligands. 319 oxygen ligands, 319 Xenon(I1) complexes difluorides, 313 FXeOS02F, 320 [(FeX0),SOFji, 320 nitrogen ligands, 312 Xe(OTeF&, 320 Xenon(1V) complexes Xe(OTeF& 320 Xenon(V1) complexes Xe(OTeF,),, 320 Xenon hexafluoride, 315 monomer, 315 Xenon hexafluorobismuthmate, trifluorofluorine bridging, 313 Xenon pentafluorotehrate, 320 Xenon tetrafluoride, 315 Ytterbium complexes dipositive oxidation state hydrated ions, 1109 Yttrium complexes, 1059 Zeolites, 113 Ziegler-Natta catalysis, 358 Zirconates, bromo-, 430 Zirconates, chloro-, 430 Zirconates, fluoro-, 423 alkali metal salts, 425 alkylammonium salrs, 427 divalent metal salts, 427 guanidinium salts, 427 hydrazinium salts, 427 vibrational spectra, 429 Zirconates, iodo-, 430 Zirconate salts tris(tetraphenyldisiloxanedia1ato)structure, 418 Zirconium complexes, 362440 coordination geometries, 364 oxidation states, 364 tetrakis chelate stereochemistry, 364 Zirconium(0) complexes, 364 2,2'-bipyridyl, 366 cyanides, 364
Subject Index Group IV ligands, 364 nitrogen ligands, 366 tns(l,lO-phenanthroIine), 366 Zirconium(I11) complexes, 366-370 acetonitrile, 366 ammines, 366 aqua, 370 arsenic ligands, 369 2,2'-bipyridyl, 366 l,2-bis(dimethylarsino)-3,3,4,4-tetrafluorocyclohutene 369 halogen ligands, 370 2,4-lutidine, 369 3,5-lutidine, 369 nitrogen ligands, 366 oxygen ligands, 370 1,lo-phenanthroline, 366 phosphines, 369 dissociation, 369 structure, 369 phosphorus ligands, 369 pyridine, 366 X-ray structure, 364 Zirconium(1V) complexes, 3 7 M O 2-acetylpyridine, 373 acylamido, 375 4-acyl-3-methyl-1-phenyl-2-pyrazol-5-one, 401 aldehydes, 403 aldirnines, 434 alizarin red S , 403 alkoxides, 389 bimetallic, 392 degree of association, 389 reactions, 390 reactions with isocyanates, 391 S-alkyldithiocarbazates Schiff bases, 434 alkyl thioglycolates, 439 amides, 414 amines, 37 I amino acids, 410 aminopolycdrboxylates, 436 ammines, 371 aqueous solution NMR, 384 polynuclear, 384 Raman spectra, 384 arsenic ligands, 383 arsines, 383 N-arylnaphthylaldimines,438 aryloxides, 389 N-arylsalicylaldimines, 437.438 azides, 379 azines, 434 benzoquinolines, 372 2,2'-bipyridyl, 372,374 bis(amido)silane, 375 bis(benzeneselenina)oxo, 418 bis(diketonates), 400 bis(trimethylsilylamido),375 NMR, 377 spectroscopy, 377 X-ray structure, 377 carbamates, 410 carbonates, 410 carboxylates, 409 catecholates, 403 chlorooxo, 386 citric acid, 412 cupferron, 417 cyanates, 379 cyanides, 370 cyanogen iodide, 381
cyanogen selenide, 381 cyanogen sulfide, 381 3-cyanopyridine, 373 diacyl hydrazines, 436 dialkylamido, 375 photoelectron spectra, 375 reactions, 375 diamines, 372 dichlorooxo anhydrous IR spectra, 386 dichlorophnsphates, 407 diethanolamine, 437 diethyldithiophosphate, 421 dicthylenetriaminepentaacetate,436 N,N-diethyl hydroxylamido, 438 2,h-dipicolinoyl &hydrazine, 440 dithiocarbamates, 419 dithiolenes, 420 esters, 403 ethanolamine, 437 ethers, 403 ethyl acetoacetato, 402 ethylenediaminetetraacetate,436 N-ethylsalicyialdiminato, 438 fluorosulfates, 407 glycolic acids, 412 glycoxides, 391 Group IV Iigands, 370 halogenoids, 38 1 hydrazine, 374 hydrides, 432 hydroborates, 432 hydroperoxo, 387 hydroxamates, 417 hydroxide, 384 hydroxy acids aliphatic, 412 aromatic, 412 hydroxyalkylamines, 434 imidazoles, 372,377 indazoles, 372 interhalogen-halogcnoids, 381 interhalogenoids, 381 isopropoxides Lewis base adducts, 391 NMR. 389 ketamines. 434 P-ketoenolates, 392 ketones, 403 lactic acid, 412 2,4-lutidine, 372 2,6-lutidine, 372 3,5-lutidine, 372 malic acid, 412 mandefic acid, 413 mercaptoalkylamines, 434 a-mercaptophenolato, 421 3-methoxysalicylaldehydato, 402 N-methylaminoalkoxides, 391 methyl isothiocyanate, 439 molybdates, 408 mono(diketonates), 401 monothiocarbamates, 419 N heterocyclic ligands, 372 IR spectra, 372 oxo,374 nicotinamide, 373 nitrates, 406 nitriles, 352 IR spectra, 382 Raman spectra, 382 niuilotriacetate, 436 nitrogen ligands, 371
1475
1476 nitro ligands, 374 octaethylporphyrin ,439 oxalates, 411 oxides, 384 N-oxides, 414 P-oxides, 414 oxoanions, 406 oxycyanogen, 381 oxygen ligands, 384 perchlorates, 408 peroxo, 387 IR spectra, 388 Raman spectra, 388 1,lo-phenanthroline, 372 Ll0-phenanthroline mono-N-oxide, 439 phenyl isocyanate, 439 phosphine, 383 phosphorus ligands, 383 phthalocyanines, 440 phthalonitrile IR spectra, 382 poly(pyrazolyl)borates, 381 porphyrins, 439 primary amines, 372 pyrazine, 373 pyridine, 372 N-(2-pyridyl)salicylaldimine, 438 pyrrolyi, 377 &quinolinol,437 quinones, 403 Schiff bases, 434 salicylaldehydato,402 salicylic acid, 413 seIenates, 407 selenium ligands, 421
Subject Index selenocyanates, 380 selenoxides, 414 semicarbazones, 434 sulfates, 407 sulfoxides, 414 sulfur ligands, 418 tartaric acid, 412 tertiary amines, 371 tetrahalides, 422 tetrakis(diketonates), 397 bromination, 398 intramolecular rearrangement, 398 ligand exchange, 398 mass spectra, 399 NMR, 398 volatility, 399 X-ray crystal structures, 397 tetrakis(perfluoroalkanesulfonates),418 tetraphenylporphyrin, 440 thioacetylacetonato, 439 thiocyanates, 380 thioethers, 418 thiolates, 418 thiosernicarbazones,434 thiourea, 421 tin-containingligands, 370 trialkylsilyloxides, 389,391 triazenes, 378 triethylamine, 371 trihydroxyglutaricacid, 412 tris(diketonates), 399 tropolonates, 402 urea, 382 water, 384
Formula Index AlCI,Br, 123 AIBr,CI AlClBr,, 123 A IBr, N, AIBr,(N,), 110 AlBr3 AlBr,, 121 AICCISO A1C13(COClJ, 119 AlCH3C13 [AIMeC13]-, 123 AlCH,Cl,NO, AIC1,(MeNO2), 118 NC*HN2 AlH(CN)z, 106 AICZHIICI~N AlCI,(MeCN), 111 AI&H3CL0 A1C13(AcC1), 118 AlCJi6Br3N2o4 AlBr3(MeN02)2,118 AlCzH6Cl AlClMe2, 122 AlCzH6C130 AIC13(OMez),119 AIC~H6CI3S AlCl3(SMe2),120 AIC&F AlMe,F, 121 AGWz [AlMeJ2]-, 123 AlCzH6N3 AlMe2(N3),110 AlC&,N, [A1Mez(N3W,111 AICZHBN AM2(NMez),109 AIC3H3O6 Al(O,CH),, 117 AIC3H,CI [AlMe,Cl]-, 123 AIC3&C13N AIC13(NMe3),107
AIC~H~I [AlMe31]-, 123 A1C3H9N03 [AlMe,(NO,)]-, 116 tUC3H9N3 [AIMe,(N,)]-, 111 AlC3H903 AI(OMe)3, 114 AlGHio [AlHMe3]-, 123
AGHizN AIH3(NMe3),107,123,124 AGHi3N2 AlHB (H2N(CH2)3NH2}, 108 AIC3NS Al{CN),, 106 A1CaN3SJ AI(Ncq3, 111
1477
Formula Index
1478
', 109
. 114
109
,124
1479
Formula Index AICISN20, AlC13(NOC1)2,119 AIC150zo [A1(C104)5]z-,117 AlCkOP Ak13(POCI3), 119 AlCkOa [Al(clod)6]3-, 117 AIClz1O,P, AlC!, (POCl, )6,119 AICrC4HI2O4 Cr{A1(OMe)4),860 AlCrCzsH41NOsPSi2
_
Cr(CO),(p-PPh2)A1(CHzSiMe3)2(NMe3), 112 I AIF, AlF,, 120,123 AlF, TAIFn)-, 121
Al(bipy),, 108 A~C~ZHI~FN~ AlF(phthalocyanine), 126 AlC36fhsN40 Al(OH)(octaethylporphyrin),126 AlC36HnN4 Al(N=CBuf2)4, 109 A1C36Hio8Ni806P6 [A1(HMPA)6I3+,119 AG5H3iN40 Al(OMe)(tetraphenylporphyrin),126 AG6&N4 AlEt(tetraphenylporphyrin), 126 AlC54H4503Si3 A1(OSiPh3)3,114 AlCl AlCl, 121 AlClS AlSCl, 120 AlC12H305 Al(OH)(OzCH)z, 117 AIC120.S [AlClz(SO4)]-, 117 AlClzS [AlSCI,]-, 120 A1Cl3 AlC13, 119,121,122 AlCIjFsP AICIs(PF,), 112 AIC1302S AlC13(SOZ), 119 AlC13012 Al(C104)3, 117 AICI~ [AlCI4J-, 111, 112,118,119,1~,123 AlC14N0 AlCl?(NOCl),119
[Al(S03Cl)4]-, 117 AIc14016 [Al(C104)4]-,117
AIF60$3 AI(POzFz),, 116 AlGeC&Izl AIMe(GeMe&, 107 A1GeC18Hle [AlH,(GeP h3)]-, 107 AlGe,CqHz, M(GeMe,),, 107 AlHClz AlHC\Z,l23,125 A IH CI209 A1(OH)(CI04),, 117 AIHC13 [AIHCI3]-, 125 AMFO4P AlF(HP04), 116 mo2
AlO(OH), 112
AlHiCl AlHlCl, 125 AlH2C12 IAIHzClJ, 125 AlHzNS ALSNHZ, 120 AIH20 ALH(OH), 113
A&, 123 AlH,Br,N AIBr,(NH,), 107 AIH3CI3N AlCI3(NHa), 107 AM3C@ AlC13(PH3),111 AlH303 AI(OH),, 112,133 AlH3OaP [AI(H3P04)]3+,116 AIH,O 12P3 [A1(HP0,)3l3-, 116 A W 0I& AI(HSO&, 117 AIHa [A&]-, 114,123-125 AlH4O4 [AI(OH)~]-,113 AHf,F,O3
Formula Index
1480
A109P3
A1(P03)3r116 AIPu03 PuAlO,, 1133 AlSbBr6 A1Br3(SbBr3),112 A1Si3&HZ7O3 A1(OSiMes)3, 113,114 A1TaC18H4804P4
Ta{H2AI(OCH2CHzOMe),}(dmpe)z, 679 117
116,117,118,133
.
AI14 [AlIJ, AlInS?
122,123
[ioz]-,112 A104 [~10,15-,
AlOaP
112
AlTlH4
Formula Index
1481
A12C17 [AlzCl7]-,112,122,123 A1,CLOS
110
1482 AISc16H56N8 { AlMe(NMe)}6{AlMe,(NHMe)} 2r 110 &3&7040
IA1i304(OH)zs(OHz)iiI6+, 113 A113bO40 [~~304(0H)z4(0Hz)iz17+~ 1036 AmCOS [Am02(C03)]-, 1220 hG06
[Am02(Cz04)]-, 1220 AmGO, [Am0,(C03)2]3-, 1220 AmC3H306 Am(OZCH),, 1216 Amc3011 [Amo2(co3)3]4-, 1220 ~ C ~ H S O ~ Am(citrate), 1217 Amca9os fArnO,(A~0)~]-, 1220 [Am02(OAc)3]2-, 1220 bc%5H2l06 A m ( a c a ~ ) 1216 ~, AmC21H170~~ Am(2-HOC6H4C02)3(H20), 1215 hC33H5706
A~(Bu'COCHCOBU')~, 1216 A~C~~H~~FISOI~PZ Am(hfacac)3{(Bu0)3PO}2,1217 AmC64H32N16
Am(phthalocyanine)2, 1218 bCI2 AmC12, 1215 hC!, AmC13, 1133,1218 AmC402 [Am02C14]2-,1220 [Am0,Cl,]3-, 1220 A&D, AmO,F,, 1220 AmF3 h F 3 , 1218 AmF, IAmF,]-, 1218 A d 3 AmH,,1218 Ad303 A ~ I ( O H ) 1216 ~, AmH12C1206 [Amclz(OH2)6]+, 1215 Am13
A d a , 1218 AmLiOp LiA1110z,1216 AmLi304 Li,Am04, 1219 ArnN3011
[Am02(N03),]-, 1220 Am0 AmO, 1215 Am02 AmOz, 1219 Am06P [Am02(P04)]-, 1220 Am08S2 [ ~ ( S O & ] - , 1216 AJ33014S3
[Am02(S04)314"",1220 AmS
AmS, 1215 Am2C207 Am2O(C03)2,1216 hZC309
Formula Index Am2(C03)3, 1216 AmzCs0iz Amz(C204)3, 1217 AmzOzS Am,02S, 1217 Am203 Am203,1216 A~zOIZS~ Am2(S04)2, 1216 AmzO28S7 [Am2(S04)7]8-,1216 Am2S3 Am,S3, 1217 ASAICI6 AlC13(ASC13), 112 AsBC,H&l, 4 BC13.AsMe3,82 AsBC~H~F~ BF3.AsMe,, 87 AsBC3H12 BH3.AsMe,, 82 AsBH3Br3 BBr,.AsH,, 86 AsB~AIC~H~~ AI(BH4)3(AsMe3), 1 AsBiCsH9NO6
AsO(OH)(OBiO)(C&NHCOCH20H-4), 256 AsBr, AsBr3, 251 AsCH~F~ AsMeF2, 251 AsCH,F,NOS AsFS(SOF2NMe),252 AsCH7Si AsH,(SiH,Me) ,239 AsCzHzC13 AsCl,(CH=CHCI), 256 AsC~H~CI~N AsC12(NMe,), 240 AsC3F9Se3 A S ( S ~ C F ~249 )~, AsC~HQ AsMe,, 256 AsC3HgBr [AsBrMe3]+,251 AsC~H~CI~ AsClzMe3,251 AsCSHgC13N AsCI3.NMe3,252 A~C3H904 A S O ( O M ~ )245 ~, AsC~N~ A~(CN)3,252 AsC3N30, As(NCO),, 252 AsC~N~S~ As(NCS)3,252 AsC~H&IS~ AsCl{(SCH,CH2)2S), 249 A sC~H ~O S A s ( O H ) ( O C H ~ C H ~246 ~)~, AsC4H12ClN2 AsCl(NMe&, 240 AsC~H~ZC~~P AsMeC12(PMe3),244 AsCSH1sClP AsMezC1(PMe3),244 AsC6F18N3 {AsN(CF3)213,243 AGHl8IZP2 As12(PMe&, 244 AsCsH18NP2 [AsMe2(PMezN=PMe2)]+,244
Formula Index AGHisN3 As(NMe,),, 240,241 AsC~HZIIZP~ A S M ~ I , ( P M ~244 ~)~, AsC8H18C12F6NSiZ A S ( C F ~ ) ~ C ~ ~ { N ( S ~242 M~~),}, AsC8Hi8F6NSiZ As(CF3),{N(SiMe3),}, 242 AsC9H14C12P AsPhCl2(PMe,), 244 AsC,HiaN306 As(02CNMe2),, 241 ASC9H18N3S6
AS(S~CNM~,)~, 241 AsCgHz7Si3 A s ( S ~ M ~239 ~)~, AsCl2H9C1N ASCI(C,H4NHC,H,), 256 AsClzH1oCl AsClPhz, 256 AsC12H2704 AsO(OBu)g, 245 AsC12H27S3 AS(SBU')~,248 AsCnHioN As(CN)Ph,, 256 AsC13H2704 A s M ~ ( O C M ~ , C M ~, O) 246~ , ASCISH~ON~S, As(S,CNEt,),, 249 AsCi5HwN&6 As(Se,CNEt,j,, 250
AsPh(OCMe2CMe20)2,246 AsC H. N Se 250 A:;S2JC&8H,),}3, ASC~OH~~PSI~ As{C(SiMe,),} {PC(SiMe3)3),243 ASCz7Hd3Se6 As(SezCNBu'z)3,250 A~CzsH4104 AS(OH)(O2C6H2BUf2)z,216 A~Cz9H4304 246 ASM~(O~C~H~B U'~)~, AsC43H330 AsPh,(&Ph=CPhCPh=CCOMc). ASC~~H~ZN~S~~ As { Se2CN(CH,Ph) z} 3, 250 AsClF, AsClF,, 253 AsCl, AsCl, ,251,252 ASCI, [AsC14]+,251,253 ASCI, ASCI,, 253 AsCrC,,H,,O, CrO(Oz),(OAsPh3), 946 AsCrC19H1403 CrO(2-PhzAsC,H4C02),901 AsCrF, CrF2.AsF,, 756 AsCr3HO13 [(Cr030)3AsOH]2~',1024 AsF3 AsF,, 251 AsF,
AsF, .25 1,252 AsF, [ASF,]-, 251.253 AsGa04 GaAsO,, 135 AsH5Si AsH,(SiH,). 239 AsH7Si2 AsH2(SiH2SiH3),239 Ad, Ad,, 250 AsI, Ad,, 25 1 ASL~S~,C,~H,.,O~ LiAs(SiMe3),.2TWF,240 ASMO~C,H,O~;
[MO~O~~(M~~A~O~)(OH)]~-, 1054 AsMoACH~,027 [ M O ~ ~ , , ( M ~ A ~ O ~ ) ( H 1054 ,~),]~-, AsMo~H,O,~ [A s M o ~ O ~ ~ ( H ~ O1042 )~]~-, ASNbCisHijC15 NbC15( AsPh,), 600 ASNbClaH ISClSN [NbCI,(NRsPh,)]-, 620 As0,S2 [AsO&J-. 2 d Y As0,S [AsO,SI3-, 249
As04
239
[As0+I3-,255 ASS, [AsSz]'-, 248 Ass3 [Ass3),-, 248 AsSe, [AsSeJ-. 249 AsTaC, ,H,,CI, TaCl,(AsPh,), 600 AsTiCl8HI5Cl4 TiC14(AsPb3),347 hUH06 UO2HAsO4,11% AsWC,H,,CI,OP WOC13(Me2PCH2CHzAsMeZ), 985 ASWg033 [AsW9033I9-, 1042 AsWiaHz060 [ A ~ ( H z ) W I ~ O ~ 1042 O]~-, A s ~ . ~ C ~ C ~ ~5 H ~ ~ C I N ~ CrC13(2-Me2A~C6H4NMeZ)1 j, 854 A s ~ . ~ C ~ C ~ ~ H ~ ~ C ~ ~ P ~ CrC13(2-Me2PC6H4AsMe2)l.s, 854 AS~BC~HZO [BH2(AsMe3),]+,98 As2CloHlBC12F12N2Siz {As(CF,),CI(NSiMe,)} ,, 242 As~CISS [AszSCk,]-, 251,253 ASZCI, [As,Cl,]-, 251 AszCleS [A S,SC~]~-, 253 As~CI~ [ A S ~ C I ~ ]253 ~-, A S ~ C ~ C ~ H ~ ~ C ~ ~ [C~C~,(M~,AS(CH~)~A~M~~}]-, 853 As2CrCloHl,C1, [CrCIJdiars)]-, 853 A&CrC,~H,,N;O6SZ [ C ~ ( P ~ - I A S O ~ H ) ~ ( H ~ N C S 888 NH~)~]+, AszC~CZ~H~~S~ [Cr(NCS),(diars)]-, 838
1483
1484
Formula Zndex UCI,(Et,AsO),, 1170 AszUCMH~ONL~O UOZ(N03)z(Ph3As0)2, 1207 ASzUC40H3608 UOz(OAc)z(Ph3AsO)z,1207 AszUZC~~H~ZOI~ { UOz( OAc)z(Ph,AsO)}z, 1207 AszVC4HizOS4 VO(SZAsMe,),, 527 AszVCi4Hz4C1, VC14{1,2,-~Et2A~)1C6H4), 498 AszWCIOHI&~~ WCl,(diars). 984 AS~WCI~HI~B~ZO~ W(CO)3(diars)Br2,1005 AS~WCI~HIJO~ [W(CO),(diars)I]+. 1007 ASzWCi6HzzC14Pz WCl4(AsMeZPh),,989 AszWCzsHzzIz03 W(CO),(dpam)I,, 1007 ASzW6CizHi10zs [W601s(PhAS03)z(OH)]5-, 1053 ASzW6CizHiz025 [ W ~ O ~ ~ ( P ~ A S O ~ ) ~ ( 1053 OH~}]~-, ASzWi8062 [AszW1@62]~-,1040 [ASzW,s06~]'-, 1052 ASzWidk7 [AS~Wis067]'~-,1042 AszWzoO68 [ASzWzoO68]lo-, 1042 ASZWZIHZO~O [AszWz1069(HzO)]~-,1042 AszZrC8Hl C13F4 ZrCl3{Fz~C(AsMe2)==€(AsMeZ)~Fz}, 366,370
CrC13{MezAs(CHz)3AsMe,}, S, 854 As3CrC11H2,C13
C~CI,{A~M~(CHZCH~CH~ASM~~)~}, 854 CrC13{MeC(CH2AsMez)3}, 854 As3CrCl,H,4C13 CrC13(diars)l,5,8.54 As3NaSb6W21085 [NaSb,As,W210,6]~88-, 1042 A s ~ P ~ C ~ ~ H ~ ~ ~ ~ ~ PaH302(PhA~03)3r 1182 As3TiCllHZ7Ch TiC14{MeC(CH,AsMez)3),344 As~T~C~~H~~CL TiC14{MeC(CHzAsPhz)3}, 338 As3ZrCllHZ7Br4 ZrBr4{M ~ A S ( C H ~ C H ~ C H ~ A S M 383~ , ) ~ } , As4Co~W4oH.Wi.i~ [As4W40(NH4)0,40{Co(OH2))2123-9 1049
[CrCI, {As(CHzCHzCH2AsMez),},855 A S,C~C~,H ~~CIS CrC13{As(CHzCHzCBzAsMez)3}, 854 A s ~ C ~ C ~ ~ H ~ ~ B ~ ~ CrBr2(diars),, 734 As~C~C~~H~~C~NO Cr(NO)Cl(diars)z, 830 [Cr(NO)Cl(diar~)~]+, 830 AS~C~C~~H~~CI~ CrClz(diars)2,734 [CrCl,(diars),]', 854 As~C~CZOWJZ~ Cr12(diars)2,734 As,CrC,,H,,CI, CrCI,(Ph2AsCH2CHzAsPhz)2, 854 AS.,C~C~~H~B~~O~ CrBr2(OAsPh3),, 754
Formula Index AS~C~C~ZH~OC~Z~IZ Cr (ClO,),( 0AsPh,) 4, 754 A~~E~C~Z&.ON,OIO [ E U ( N O ~ ) ~ ( O A S P ~1082 ~)~]+, A S ~ F ~Z C I O H I Z O ~ {Fe(CO)3}~(AsMe)4, 244 A~qHfCzoH32C14 HfCl,(diar~)~, 383 As4InCzoH3zClz [InCl,(diars),]+, 160 AS~I~C~ZH~O [ I r ~ ( A s P h ~ ) ~160 l~+, As~KW~OOI~O [KAs4W400140]Z7~, 1042 As4MoCz,H,,ClNzO, [MoCl(NO),(diars)J', 1272 As~MoCZOH~~CIZ MoC1z(diars)z,1284 As~MoCZOH~ZC~~ [MoC14(d i a r ~ ) ~,]1354 As4MoCzoH3zNzOz [M~(NO)~(diars),]~+, 1273 As~MoCZ~H~ZCINOZ MoC1(NO)(CO)(diars)2,1276 As~MoCZZH~ZC~OZ [MoCl(CO)z(diars),] +,1281 +
MoBrZ(CO)z(dparn)Z,1281 AS~MOC~ZH~~C~ZOZ MoCl,(CO),(dpam),, 1281 As4MoC5ZH4SN4
Mo(Nz)z(PhZAsCHzCH2AsPhz)~,1269 A S ~ M O G I H ~ I N Z P~ Mo(Nz)(triphos) (diars) , 1268 AS4MOzH4016 [ M O ~ ( H A S O ~ )1303,1309 ~]~-, As~MozOZSI~ [ M O ~ O ~ S ~ ( A S ~1363 S~~)]~-, A~4M0404Si4 [(M~zOz~z)z(A~z~s)zl"-, 1363 AS4M012CZ4HZ8046
1053 hMo12H4050 [G S ~ M O ~ ~ O & I ,1043 ]~-, As4NbCzoH3zBr4 NbBr4(diars)z,642 A~qNbCzOH32C14 NbC14(diars)z,644 [NbCL,(diar~)~]+, 599 AS~N~ZCZOH~ZC~~ NbZCl6(diars),,656 AS& AS.&, 239 A s ~ T ~ C ~ ~ H ~ ~ B ~ ~ [TaBr,(diars),]+, 594,599 As4TaC,,H,,C14 TaCl,(diars), 644 [TaCl,(diars),]*, 599 TiCL(diar~)~, 348,356,357 A s ~ T ~ C ~ ~ H ~ ~ I ~ Ti14(diars)z,349 A s ~ T ~ C ~ ~ H ~ ~ C ~ ~ TiC14{1,8-(MezAs)zCl,H,) 2 r 349 As~VCZOH~ZC~~ VCl,(diars),, 498 AS4VC72H6005
[VO(OAsPh3)4l2', 522 As4WCzoH3zIz W(diars)&, 1006 As4WCZ2H,,BrO2 [W(CO)z(diars)zBr]+,1005 As4WC53H44Br203
BCCI,NO [BCl,(NCO)]-, 93 BCFd [BF~cF,] - ,93 BCF603S [BF303ScF3]-, 93 BC&F30 BF3*OCH2,86 BCH,Br,S BBr,(SMe), 82 BCH3F30 [BF30Me]-, 87 BCH3N [BH,(SCN)]-, 90,91 BCH,O BH3*C0,84,91 BCHJOz Isw3(co,)l'-, 91 BCHSNzS BHz(NCS).NH3,85 BCH6NOZ
1485
1486
Formula Index [BH,P(OMe),]-, 91 BC-HwP BH;rPMe3, 82 BC4HsF30 BF,.THF, 87 BC H 0
[ii-$&am)z]-, 95
BCAHtnFqO . __ BF3.0Et,, 86,87 BC4H10F3S
BF-*SEti,87
BFBr,.NMe,, 86 BCjHgBr3N BBr3.NMe3,83 BC3H9CIFzN BFzCI.NMe3,86 BGH9C13N BC13.NMe3, 82,83,87 BC3HqC13P BCI3.PMe3,82,87 BC3HQF3N BF3-NMe3,83,87 3CfHJJP BF3.PMe3,87 BC3H913N B13.NMe3,83 BC,H,N,O, B(N03)3.NMe3,89 BC3HloClZN BHCl,.NMe,, 84
BC~H:~N [BH3(&N=CMeCHdMe)]-, 91 BC.H,,F,NP BHiP(CF3),.NMe3, 85 BCSH14C12N BC12Pr.HNMe2,86 BC5HisCizN2 BC1,( NMe2).NMe3,86 BCSHI~N, [B(NMe3)(NHMe2)]+,98 BCsHsC12Na BCIZCN.py,86 BC6H7CI2N [BClz(4-Mepy)]+,98 BC~H TCI~N BC13’HZNPh, 86 BC~H~F~NSB B(SCF3)3.NMe3,88 BCsHio04 [B(OH)(OJC&)I-, 95 BC6HioOs [BH(OAc),]-, 91 BC6H12N03
B(OCH,CHz),N,89 BC6HiSBrC12P BBrC12.PEt3,86 BC~HISCIN~
Formula Lndex
BClFi (BF,CI]-, 93 BCIFtO, [BF3C104]-, 93 BCIqN, [BCf3N3]-,93 B CI4 [BCIJ, 92 BC1,O.i
BCrCY3Hl2N8O3
[Cr(C0,){B(fiN=CHCH=~H)4)]-, 846 BCrCl7Hi7N8O2
Cr(CO)z{B(fiN=CHCH=cH)4}(n-C3H5)r 846 BFI5O3Te3 B(OTeF&, 88
GaMe,(BH,), 141 BGaC3HlZF3N GaH3(NMe3)(BF3),130
1487
1488 BGaCI, GaC13.BC13,139 BGa04 [GaBO4I2-, 134 BGeC16 [BCl3GeC1,]-, 93 BGeH, [BH3GeH3]-,91 BHC13S {BC13SH]-,93 BHF, [BHF,]-, 91 BHF3O [BFaOH] ,93 BWaS [BF,SH]-, 93 BHzUz [BH2CI2]-,90 BHzF3N [BF3NH2]-,93 BHZF3NO [BF,ONHJ, 93 BH2F30 BFs.HZO, 85,86 BHS3 [ B H A - , 91 BH3CIsP BC13*PH3, 86 BH,F [BH3F]-, 90,91 BH3F03 [BF(OH),I-, 93 BH3F3N BF,.NH3,81,86 BH3N3 [BH3N31-r 91 Brt iBH,]-, 90,101,125 BH4F302
BF3.2H20,85,86 BH40 [BH,OH]-, 90 BH402 [BH2(OH)zI-, 90
BtF403 IBH(OH),I-, 90 BH404 [B(OH),l-, 93,94 B%016S4
[B(HSO)d,I-, 96 B W IBH,SH]-, 90,91 BH5P [BH,PHJ, 91 BHfC12H40CINZSi4 HfCSI(BH,){N(SiMe,),},, 377,433 BI4 [ B W , 92 BLiC2&602
Formula Index BMoC~ZH~~CI N MoCl, HN H -) { HB ( N y = b H ) , ) , 1331 BMoCI~H~IP~ MoH(BH4)(PMe3).,, 1284,1438 B M o C ~ ~ HNI 0 MO{B(N=H)~}(NO)(CO),, 1276 BMOC15H15CIN8 Mo(N2Ph) { HB (&N=CHCH=c H),}CI, 1295 BMOC15H15CINgO MO(NO){HB(~~N=CHCEJ&H)~}C~(N~P~), 1288, 1294 BMoC15H,,Cl,N 0 MoOCI,{HB(dN=CMeCH=tMe),), 1352 BMOC~~H~~CI~N~P MoCl2(NO)(HB(~N=CMeCH=CMe),},1287 BMoCisH,,CI$N6 MoCl,{HB(&N=CMeCH=~Me),}, 1342 [MOC~,(HB(~~N=CM~CH=~M~)~}]~, 1331 BMoC~~HZZI~N~O MOI,(NO)(HB($JN=CM~CH=~M~)~}, 1287,1292 BMoCISH24IN80 Mo(NO)(HB(&N===CMcCH=~Me),}I(NH2), 1287 BMoCI~HZ~N~O~ Mo(NO)(HB(fiN=CMeCH=cMe),} (OM)2, 1287 BMOC~SH~~IN~O MO(HB(M~~C,NH~)~}I(NO), 1279 BMoC,,H,~IN~O
Mo(NO)(HB(&N=CMeCH=~Me),)I(NHNH,), 1288 B M o C ~ ~ HO~ ~ N ~ Mo{B(N-H),)(NO)( CO)(EtNC),1276 BMOC16H26ClN80 Mo(NO) { HB (& N =CMe CH d Me),} Cl(NHMe) , 1287 BMOC16H27INgO Mo(N0) { HB(&N=CMeCH=tMe),} I(NHNHMe), 1288 BMOC16H27N802
Mo(N0) { HB(NN=CMeCH=CMe),) (OMe)(NH2), 1287 BMoCI~HI~N~O~ Mo(N,Ph) {HB@N=CHCH=cH),} (CO)z,1294 BMoCl7HZ2N7O3 MO{HB(~~N=CM~CH~M~)~}(NO)(CO)~, 1276 BMOC~~HZ~CIN~O~ Mo(N0) { HB($JN=CMeCH=cMe),} C1(OEt), 1287 BMoCI~HZ~IN~OS Mo(N0) { HB(&N=CMeCH=tMe),)I( SEt), 1288 BMoCI~HJN~O Mo(NO){HB(dN=CMeCH=CMe),)I(OEt), 1292 BMoC~~H~~CINBO 1287 Mo(N0) {HB(aN=CMeCH&Me),}Cl(NHEt), MO(NO)(HB(~~N=CM~CH=~M~)~}CI(NM~~), 1287 BMoC~~H~~IN~O
Mo(NO){HB(fiN=CMeCH&Me),)I(NHNMe2),
1288 BMoC~~H~~C~N~OZ Mo(NO){HB(~N=CM~CH&M~)~}CI(OP~'), 1287 Li{2,4,6-Me3C6H3)2BHZ}(MeOCH2CH20Me), 14 BMGs&iNs0z BLiMoCI3Hz0T2N7O3 Mo(NO){HB(RN=CMeCH=eMe),} (OEt)NHMe), Mo{HB(&N=CMeCH=CMe),} (NO)I,Li(OEt),, 1287 1280 B M o C ~ ~ H0 ~~N BMOGHl&12N6O [Mo{ HB(-MeI3} (NO)(MeCN)2]f, MOOC~,{ HB(&N=CHCH=~ H ) ~ }1253 , 1279 BMOC~H~OCL,N~O BMOC~~H~~CIN~OS MoC12(NO)(HB(&N=CHCH=CH),}, 1287 Mo(NO){HB(hN=CMeCH=CMe),}Cl(SBu), 1288 BMoC9A,,CI,N, BMoCwHJW3 MOCI,{HB(I~N=CHCH=~H),),1342 Mo(NO){HB(f4N=CMeCH=~Me),}(OEt),,1287 BMoC,iHI,N,O, BMoCigHds02 Mo(NO){HB(&N=CMeCH=CMe),} (OEt)(NHEt), Mo{ HB($IN=CHCH=CH),} (NO)(CO), ,1276 BMOC~~HISBIN~O 1287 Mo(NO){HB(N-H),}Br(OEt), 1287 BMoC~~H~~N~O
Formula Index
1489
MO(NO){HB(NN=CM~CH=~M~)~}(NHE~)~, 1288 BSbCsHgF3 BF,-SbMe,, 87 BMoCzoHi7NioO Mo(N2Ph){B(N-H),}(CO),, 1294 BSbC3H12 BH3-SbMe,, 82 BMGoH34N703 BSi4ZrC12H40C1N2 Mo(N0) { HB(fiN=CMeCH=CMe)3) (OEt)( OPr'), 1287 ZrC1(BH4){N(SiMe3)z}2,377,433 BMoCz1H27CIN7OS BSnC3Hl Mo(NO)(HB(flN=CMeCHdMe)s)C1(SPh), rBH,SnMe,l-, 91 ,-_ , 1288 BMOC,,H&IN,OS BSIIC,IH,,N~Mo(N0) { HB( flN=CMeCH=tMe),}I( SPh), 1288 SnMe,{HB(fiN=CHCH=CH)3},185,211 BMoCziH27IN70 BSnCI, Mo(NO){HB(dN=CMeCH=CMe)3}1(OPh),1287 BMOC~~H,;CIN~O Mo(NONHB(fiN=CMeCH=cMel,)CI(NHPh), ,-, , , . 1288 BMoC,,H;,IN~O Mo(N0) { HB(fiN=CMeCH=CMe),}I(NHPh), 1288 BMoCZ~H~~IN~OS ~
Mo(NO){HB(fiN=CMeCH=CMe),}I(NHC,H,SH), 1288 1288.1293 1288.1293 , BMoC,~H,JN~OS Mo(N0) (HB(N;JN=CMeCH=C Me),} I(SC6Hll), 1288 BMoC22HZ1IN90 ~-~~
~
MO(NO){HB(~~N=CM~CH--CM~)~)I(NHNM~P~), 1288 BMOC~,H~~CIN~O Mo(N0) {HB(fiN=CMeCHdMe)3} CI(NHC6H4Me-4), 1288 BMoCzzHgoINsOz Mo(N0) { HB(fiN=CMeCH=CMe),)I(NHC,H,OMe-4), 1288 BMoC~ZH~ON~O~
1-1
BZrC,;H*2Ci,N6 ZrC1, { HB(fiN=CMeCH=cMe),} BZrC19H31C12N60
,381
Z~C~,(OBU'){HB(~~N=CM~CH=~M~)~), 381 B ZrC23H39C12N60
ZrC12(OBu'){BuB(&N=CMeCH=~Me),),381 BzAICRll
MO(NO){HB(~~N=CM~CH=~M~)~)(OP~)(OM~),
1287 BMoC~~H~~N~OS
Mo(NO){HB(fiN=CMeCH=cMe)3}(NHMe)(SPh), 1288 BMoC,~H,~N~O~S Mo(N0) { HB( NN=CMeCH=cMe),} (OEt)( SPh), 1288 BMOCZ~H~ON~OSZ Mo(N0) {HB(fiN=CMeCH=cMe)3} (SBu), ,1288 BMoCZ~H~~N~OS Mo(NO){HB(&N=CMeCH=CMe)3}(SPh),, 1288 BMoCz7H~;Ng0 Mo(NO){HB($JN=CMeCH=CMe),}(NHPh),, 1288 BMoC28HJNgO Mo(N0) (HB(fiN=CMeCH=C Me),) I(NHC6H4CH2CsH*NH>),1292 BMoC~,H,ZN~OP MO(N,C,H,M~){ HB (PSN=CHCH=CH)~} (CO)(PPh,), 1294
Be (BH4j;( PPh,) , I o BVCFAN
,-
'Cr(BH&(f'HF)z, 766 B2CrC12H16N8 Cr~H,B(I;IN=CHCH=CH),),,
732,772
1490
Formula Index
Cr {B(&N=CHCH=CH),} 2r 732 kCr{B(fiN=CHCH=CH),},]+, 84.5 B,CrF60n
A1(BHi)3(AsMe3), 111 B,CrF,O,
B~M&c~-;H~~N~o~
Mo~(OAC)~{E~~B(~~N=CHCH=~H)~)~, 1303 B2M02C30H4412N1403 {Mo(NO){HB(flN=CMeCH&Me),)I),O, 1288 BZM~zC30H4~N1205 M O ~ O ~ { H B ( ~ N = C M ~ C H = ~1408 M~)~}~, BZMO~CZZH~J'JI~~~~ [Mo204{HB(fiN=CHCH=CH),) (MeOH)) 2(OMe),, ~~
Hf(BH4)4,384,432,433 B~MOZC~OH~~N~~
Mo2{Et2B(fiN==CHCH=~H)2}4, 1305
Formula index W(BH4)sl-, 432 B7A1H20 A ~ ( B H ~ ) ( B ~125 H~z, B9SnC4H15 SnMe,C,B,H,, 219 B9SnCl4HZ3N2 SnMezCzB9H9(bipy),220 Bl0CrC4HZ6BrN4 Cr(BloH1o)Br(en)z,787 BloCrC4H3,,BrN4 Cr(B,oH,o)Br(McNH,),, 787 B20CrC4H24
[Cr(CZBlOHlzjal~-,732 BaC2H6N20,S2 Ba(NCS)2(H20)3,7 BaCloH1404 Ba(acac),, 27 BaC12H48C1ZN12 BaCl,(en),, 11 BaC14H3312N305 Barz{ HOCH,CHZN{CH2CH2N(CH,CH,OH),},} , 23 BaCi6H36Nz010 Ba(OAc)2{N(CH2CH2OH)&, 23 BaC18W36NZ06 [Ba([2.2.2]cryptand)]-. 46 BaC19H38N307S [Ba([2.2.2]cryptand)(NCS)(H20)]-,46 B~CZOH~OB~ZOIO Ba(15-~rown-5)~13r~, 39 BaC2,Hl~N,Sz
IBa(piienj2(H20)412+,13 BaC24H26N8,
’
_
~
[Ba{HN{CH,CH2N=CMe(2,6-py)C(Me)= NCH,CH,},NH)12’,
44
Ba(dibenz0-24-crown-8)(ClO,)~~ 40 BaCz4H34N604
1491
BeBZC18H23P Be(B%)2(PPh3), 10 BeC2H403 Be(OH)(OAc), 31 BeC,H,CI,N, BeCl,(en), 11 BeC204 [Be(C2O4)I2-,33 BeC3HloClN BeHCl(NMe,), 7 BcC4H,Cl,N, BeCI,(HCN),, 8 BeC,H,CI,N, ReCI,(MeCN),, 8 BeC,H,O, Be(OEt)(OAc), 31 BeC,H,,CI,N, BeC12(H2NMe)4,7 BeC,N,S, [Be(NCS),I2-, 10 BeCSH3N4S3 [Be(NCS),(MeCN)lz-, 10 BeC6H6N4S2 Be(NCS)2(MeCN)2,8 BcC8HzOBr2O2 BeBr2(Ef20)2,8 BeC&oC1202 BeCl,(Et,Oj,, 8 BeCIUH1404 Be(acac),, 25,26 BeC14H12N204 Be(2-OC6H4CH=NOH),, 29 BeC14H24N204 Be{MeCOCH,COCH,CH(NH)Me),, 29 BeC16H14N202 Be(salen), 29 BeCi6Hi6N203 Be(salen)(H,O), 29 BeC*oH*,N,O, Be(2-OC6H4CH=NPr’},, 28 BeC30H2204 Be(PhCOCHCOPh),, 26 BeC12HlbN8 Be(phthalocyanine), S9 BeC32H30N405
Be(salen),(H,O), 29 BeC14 [BeCI4l2-, 10 BeCI4Ol6 44 [Be(C104)JZ-,8 BeF, ._ _. Ba(dibenzo-24-crown-8){2.4.6-(O2N)3C6H~O}2(Hzo),, [BeF4l2-, 10 40 BeH4Br,Sz BeBr,(H,S),, 10 BeH41zS2 BeIdW2S)2,1fl BeH,C12N2 BeCldNH,),, 7 BeH,N, IBe(NH2),1-, 10 BeH804 [Be(Hz0)412+,6 , 8 BeHI2Cl2N4 BeC12(NH3)4,7 BeH16C!2N8 8 BeC12(H2NNH2)4, BeH,,Cl,N, BeCI2(NH3j,, 7 BeH36C12N12 BeC12(NH3)1~, 7 BeMWIIH2040 [BeW,,Mn(OH,)0,,]7-, BeN4OI2
[Ba{HOCH{CH,N==CMe(2,6-py)C(Me)=NCHz}-
zCHOH}(HzO),] ’+44 BaC32H32N804 [Ba{fi=CHC+CHCH+OCH=NCH,CH,N= CH+CHCH=COCH=NCH&H,),1Zt, I
~
coc3-w
1048
.
Formula Index
1492
Bi(NMeZ),, 282 BiC7H7Sz BiMe(l,2-S,CaH.),287 BiC7H15NZS4 BiMe(S2CNMeZ),, 288 .~ BiCBH9S2 BIP~(SCH ~CH ~S), 287 BiCsHllSe BiMe,(SePh), 289 BiCsHlSOS2 BiMe,(S,COBu), 289 BiCoHlis2BiMe{l,2-(SCH,)*C,H,). 287 BiC9H2,N2Si2 BiMe{NMe(SiMc,)),, 282 BiC10H130S2 BiPh{(SCH,CH,),O} + 285 BiCI0Hl5SZ BiPh(SEt),, 287 BiClJb3N2S4 BiMe(S2CNEt2)2,288 BiClzHloN3 BiPh,(N,), 283
BiClzHi7S3 Bi(SBu'),, 287 BiCJhPh Bi(NEt,),, 282 BiC13H13S~ BiMe(SPh)z, 287 BiC13H13Se2 BiMe(SePh),, 289 BiC13HZ3N& BiMe{S2CNTH2)5)2,288 BiC14HljOS2 BiPh,&COMe), 289
BiBr, [BiBr4j--.291 BiBrs [BiB@-, 291 BiBr6 [BiBr6I3-, 291 BiC2H6N3 BiMe,(N,); 283 BiC3H9 BiMes, 294 BiC4H90S2 BiMe,{S2COMe), 289 BiC4N4S, [Si(NCS)4]2', 285 BiC,H,,OS, BiMe2(S2COEt),289 BiC,H,& BiMe(SEt),, 287 BiC6H, (BiPh),, 284 BiC6H9N2S BiMe2(S&=NCH=CHCH=N), BiC6H1iOS2 BiMe,(S&OPr'), 289
287
BiC15H150S2 BiPh,(S2COEt), 289 BiC1,H;"F6O4 [BiPh2(0zCCF3)21-,280 BiC16H170S2 BiPh2(S2COPri),288,289 BiC16H25N~S4 BiPh(SzCNEt2),, 288 BiCl7Hl90S2 BiPhz(SzCOBu),289 BiCl&LS BiPh3, 294 BiClBHISCl2 BiPh3Clz,280 BiCisHi5N6 BiPh3(N3)2,283 BiClskS2 BiPh(SPh)2,2S7 BiClsHi& Bi(SPh),, 287 BiC1sHlsN3S3 Bi(2-SC6H4NHz) ,, 283 BiCdLN2S4 BiPh{SzCNTH2)S)2, 288 BiC H N S Bt~SZ&$$H2)5}3, 288 BiC1&d3 Bi(NPr,),, 282 BiCISH45SiS Bi(SiEt,),, 280 BiCl8HS4N3Si6 Bi{N(SiMe,),}3, 282
Formula Index Bi(SnPh,),, 281 Bi,Br, [BizBr8JZ-,291 BiCzlHzlS3 BizBr9 Bi(SCH,Ph),, 287 [BizB~gj-,291 BiCZ4H~o [BizBr9]3-,291 [BiPh4]+,280 BiZBrIO BiC2,HZ1CINO [Bi2Br,,14-. 291 BiPh3(8-quinolinolate)C1,280 Bi2Cl,H,,Si, B~CBBH~~C~NIZO,S~ BiCI3{~=NC(NHS02C,H4NH,)=CHCH=~OMe}3, Bi2(SiMe,),, 284 BizCls 293 BiCS4HlsSi3 [BizClalL-,291 Bi(SiPh,),, 281 Bi2Ge3C333o BiClO 5i2{Ge(C6F5)2}3, 281 BiOCI, 294 BiJq [Bi,I,I3-. 291 BiCI3 BiCI,, 290,291 Bi2MoC8H1204P2 BiC13N0 (BiMe),Mo(CO),(PMe),, 283 BiCl,(NOCI), 290 Biz03 BiCI,(NO), 290 Biz03,284 BiC13Ni04 Bi201,S3 BiCI3(N0&, 290 BiZ(S04)3r 284 BiCll B~SM [BiC14]-, 291 [Bi,S,,]"-, 284 BiC15 Bi3CTH2&19N14S7 [BiCl,]z-, 291 (BiC13)3(HZNCSNHZ)7, 290 BiC& Ri,H,O, [BiC1613-, 291 [Bi604(OH)4]6 ' ,284 BIF3 BkCisHsFixOb BiF3,290,291 Bk(hfacacI3, 1216 BiF4 BkC3zHioFizO, [BiF4]-, 291 Bk(CF,COCHCOBu'),, 1219 BiF5 BkCl3 BiFs, 291,292 BkCl,, 1218 BiF6 [BiF,1-, 292,314 Bib&;* Bi(SiF3)3,280 B k , , 1218 BiGe2C14H3S BkzQ BiEt(C;eEt3)2,281 Bkz03,1216 B~Gc~C~~H~F~O BiEt { Gc(C6F5)3}2,281 CaCHloN4010 BiGe3C,,H4, Ca(N03),(HzNCONH2)(HzO)3,9 Bi(GeEt,),, 280 BiGe3C54H45 Bi(GePh,),, 281 BiHO, BiO(OH), 284 BiH3C16N (BiCI3),(NH3),282 BiH303 Bi(OH)3, 284 BiH9C13N3 BiC13(NH3)3,282,ZW Bi13 Bi13, 291,292 BiI, [Bi14]-, 292 Bi16 [BiI6I3-,291,292 BiN,06 Bi(N03),, 284 BiN30 BiO(N3), 283 BiN309 Bi(N03)3r294 Bi04P BiP04, 284 BiSnC9HZ7 Bi(SnMe&, 281 BiSn,C18H4, Na(PhCOCHCOPh), 26 Bi(SnEt3)3,281 CaC15H21N303 BiSn3C54H45
BiC20H40C13N1 OI2S5
Bi(C104)3(S8NHCH2CH2CH2NH)5, 290
'
1493
~
1494
Formula Index
[Ca{fi=cH(2,6-py)CH=N(CHzC H,O)~CHZCI-I,}]'+. 44 CaC16H3608
[Ca{MeO(CH2CH20)3Me}z]z+,17 CaCl7H1,N4O8 ca{ (2-HOC6H4CH~NCH,),CH2) (NO,),, 29 C~CI~HZ~NZ~~SZ Ca(be nzo-15-crown-5)(NCS),( MeOH 1, 39
CaCzzHisN60Sz Ca(NCS),(bipy),(HzO), 13 aC30 22 4 Ca(PhCOCHCOPh),, 26
Ce(NO,),(OPPh,),, 11IS CeC60H4408
Ce(PhCOCHCOPh)4, 1114 CeCI, CeCI,, 1113 CeCI, [CeC16]2-,1116 CeCr,O, Ce(CrO,),, 943 CeF, [CeF,]-, 1115 CeF, [CeF,]' , 1115 CeF, [CeF7]'-, 1115 CcF, [CeFSl4-, 1115 CeKCz~H4406 K{O(CH2CH,0Me)2},Ce(cot)2, 17 CeM012042 [C~MO~~O~ 1045 ,]~-, CeN5OIs [Ce (NO,),] ~, 1086 CeN6018 [Ce(N0,),]2-, 1114 Ce016S4
[Ce(S04)4]4, 1114 CeWl"O36 [CeW,o036]s-, 1046 CeW340122P4 [Ce(P2wl7o6,)2]'6-. 1048 CeA4 [Ce2Fl4I6-, 1115 Ce2Hl8OZlS3 CeZ(S04)3(H20)9r 1086 Ce.831 [Ce6F3,I7-, 1115 CfBr, CfBr,, 1215 CfCI, CfCl,. -, 121s
crci,
CfCI3, 1218 cfF3 CfF3, 1218
cfF4 CfF,, 1219 CfOz [Cf02]+,1219 Cf2OZS Cf,O,S, 1217 CfzOs CfiO,, 1216
cfzo6s
[Ce(S,CNEt2)4]-, 1087 CeC~zHdW7 Ce(NO,),(H,O) { &=CMe(2,6-py)C(Me)= NCH2CHzN=CMe(2,6-py)C(Me)= m H , ) , 1096
Cf202(SO,), 1216 Cf2Ol'S, Cf,(S04),, 1216 CmC13 CmC13, 1218 CmF, CmF,, 1218 Cm,CO, Cmz02(C0,), 1217 CmzCzO7 Cm20(C0,),, 1217 CmX309 CmZ(C03)3,1216 Cm,O,S CmZ02S,1217 Cm,O, Crn203, 1216 CmZO6S Cm,0,(S04), 1216
Formula Index Cm1011S3
Cm,(SO,),, 1216 C0BWiiHz04o [ B W ~ ~ C O ( O H Z ) O ~1048 ~]~-, COBZC44HSZP4 Co(BPhz)z(diphos)Z.99 C°CSH20N6 [CO(PY)(NH~)SI~', 1110 C0CgHsN208 [Co(en)(Cz04)zJ-,472
Co(salen)Na(THF), 16 CoW,Ss [Co(WS,),j*-, 982 ICo(WS4),13-, 982
C0C6H19N7
[Co(NH,),(4-CNpy)l3 ' ,846 CoCdh& [Co(en),]3+, 1110 COC6012 [CO(C~O,),]~-, 472, 1236 CoC,H,1N,OzS [Co(en)z{SCH,CH(NH,)CO,) J +,904 COC&I~NSO~ Co(HDMG),(NH,)(N,), 472 CoCsHzoNs04 [Co(NH,),(HDMG),]', 472 COCgH17N604S Co(HDMG),(NH,)(NCS), 472 COCi6N40z [C~(NHZ)~(H 846~ ~ I ~ I ~ ~ ~ C0ClsHz4Nh04 ICO(HDM~)Z(PY)Zl+~ 472 COC2oHmN604 [Co(HDMG)z(PhNH&]+, 472 COC36Hz4N6 [Co(phen),13+,712 COC~C~H~ON~OS Cr(HzO)5( pNC) Co(CN), 777 COC~C~H~~N~OO Cr(NH,),(H,O)(y-NC)Co(CN),. 779 COCTC6H1~N11 CO(NH,)&-NC)C~(CN)~.779 COC~C~H~SN~IS~ Co(NH,),( pSCN)Cr (NCS)F, 842 CoCrC6N6 Cr(CN),Co, 779 Cr(NC),Co, 779 CoCrCloHI6FN [CrF(en)z(p-NC)Co(CN)5]-, 778 CoCrClzHzoFNlo [CrF(pn),( p-NC)Co(CN),]-, 778 CoCrzS4 CoCrzS4,883 CoFeWllHZO,o [CoWllFe(OHz)0,,]7-, 1048 COHI~NTO [CO(NH,),(H,O)(N,)I", 472 CoH1,CINs [COCI(NH,),]~+,472 CoHI,NSO [Co(Nb3)5(H20)]3+,1259 CoHqsN, [c;;(Nk3)6]3+, 1110 COKCZ~H~~NZOS Co(Pr-salen)K(CO,)(THF),16 CoLiCi6Hl4N2O2 Co(salen)Li, 16 COMOH~~N~O~ [CO(NH,)~MOO~]+, 1377,1379 COMOH~SNSO~ [ C O ( N H ~ ) ~ M O O~ 1377,1379 ]+, [CO(NH,)~OMOO~]+, 1379 CoMo,S, [Co(MoS4)#-, 1422,1424 COM06HbOz4
[COM06H&4]3-, 1044
((Co(salen)),N~(THF),] ', 16 CozWiiH2040 [ C O W ~ ~ C O ( O H ~ ) 1037 O~~]~-, Co3NaC,,Hq6N409 {Co(~alen)}~NaCo( CO),(THF), 16 C03W&040Si [W9Si{Co(OH2)}3037]10-,1049 CO4Hd3024 [COI,Co,O,,(OHz),]'-, 1044 [Co4I3024H12I3-, 1024 Co4WisHsOJ'z [ ( W ~ P O , ~ ) ~ C W ~ ( H ~1049 O)~]~O-~ CrAgCH15N6S [Cr(N~I,),(p-NCS)Ag13+,841 CrAgC6HI6N6 [Ag(CN),Cr(en),lZ+, 776 CrAgO, AgCrO,. 943 CrAgS, AgCrSz, 882 CrAgSe, AgCrSe,. 889 CrAs204 Ag,CrO,, 943 CrAlC4Hl2O4 Cr{AI(OMe),}, 860 CrAISi2C2sH41NOSP Cr(CO)5(pPPh,)AI(CH2SiMe,),(NMe3), 112 CrA~2C1z€Iz806 CrfOAl(0P&l2, 738 . CrAl;Cl, (AlC13)2(CrClz),122 CrAl2C1,, 755 CrA13C&4012 CT{AI(OP~')~},, 860 CrAsC18H1506 CrO(Oz)z(OAsPh3),946 CrAsC, sH,,03 Cr0(2-Ph,AsC,H4C0,), 901 CrAsF, CKF,.ASF,,756 CTAS~.~CI&~~C~~NI.~ C~CI,(~-M~,ASC,H~NM~~)~., ,854 C~ASI.SC&Z~C~J'~.S CrC13(2-MezPC6H4AsMez)l.s, 854 CrAs2C7HlSCl4 [CrC14{Me2As(CHz)3AsMez}]-,853 CI' A SZCIO H ~~C~~ [CrCl,(diars)]-, 853 CrAsZC14HZON406S2 [CI(P~ASOSH)~(H~NCSNH~)~]+, 888 [Cr(NCS),(diars)J-, 838 C ~ A S ~ C ~ ~ H ~ ~ C ~ ~ [CrCI,(Ph,AsCH=CHAsPh,)l-, 853 CrAsZC3,H3,Cl2O2 CrClz(OAsPha)z, 754
1495
Formula Index
1496
~. .~
Cr(2-Ph,AsC6H4C02)2(~H)2, 901 C ~ A S ~ C ~ ~ . ~ H ~ ~ C ~ ~ CrC13{Me,As(CH2)3AsMez}l.5, 854 CrAs3Cl1HZ7Cl3 CrCI2{A SM ~ ( C H , C H~ C H~ AS M~854 ,)~}, CrC13(MeC(CH2AsMe2)3}, 854 CrAs3C,,H,,Cl3 CrCl,(diars)l.s, 854 CrAs,HO,, ~(C~O,O),ASOH]~-, 1024 CrAs4CISH3,C12 CrC12(As(CH2CF~2CH2AsMe2)3~, 855 CrAs,ClsH3,Cl, CrCla{As(CH2CH2CH2AsMe2)3}, 854 CrAs4C20H32Brz CrBr2(diars),, 734 CrAs4CzoH32C1N0 Cr(NO)Cl(dlars),, 830 [Cr(NO)Cl(diars),]+, 830 C~AS,C~,,H~~CI~ CrCl,(diars),, 743 [CrCl,(diars),]+, 854 C ~ A S ~ C ~ ~ H ~ ~ L CrI,(diars),, 734 C~AS,C~~H~~CI, C~CI~(P~~ASCHZCH~A 854 SP~~)Z. C ~ A S ~ C ~ ~ H ~ ~ B ~ ~ ~ ~ CrBr2(OAsPh3),, 754 C~AS~C~Z&OC~~~IZ Cr(C104)z(OAsPh3)4,754 C~A~~CZ~HS~IZ [Cr12{MeC(CHzAsMe2)3)2]+, 856 CrBCl3Hl2N8O3 [Cr(CO,) {B(RN=CHCH=CH),)]-, 846 CrBC17H~7N80~ Cr (CO), { B(fiN=CWCI-I=c A),} (;c-C,H,). 846 CrB,CaH7,O7 - - _. Cr(BH,),(%F),, 766
CrB2C20H32N8
Cr{Et2B(fiN=CHCH=CH)2}z, 732,772 CrB2C2JW%6 Cr{B(fiN=CHCH=eH),},, 732 [Cr{ B(fiN=CHCH=cH),} ,845 CrB,F,O, [C;d4(BF,)2]Z -,944 CrBzHR Cr(BH,),, 92Y CrB4C~~H300~2 Cr (B,O(OAc),} z, 752 Cr31,C4H30BrN4 Cr(BloHlo)Br(MeNHz)4,787 CrBrO3 [CrO,Br]-, 944 CrBrz02 CrOaBr2,940 CrBr, [CrBr,]-, ~. 764,765 CrBr, [CrBr,]Z-, 753,763 CrBrr [C;Br5l3-, 766 CrBr,O [CrOBr5la-, 935 CrBr, +
[CrBr614-,763 CrCH3C1,0 {Cr(OMe)Cl,),, 860 CrCH,N205 ICr(NO)(CN)(H,O),l+, 823,825 CrCH;NO4S2 [Cr(NCS)(SH)(H,O),I+ 877 CrCHlnNO,Sz [Cr(NCS)(H,O),( H2S)]Z+q 877 CrCH,,NO, [Cr(CN)(HzO),I", 774 [Cr(NC)(H20),]2-, 774,842 CrCH,,NO,S [Cr(NCS)(H20),12' ,842 [Cr(SCN)(H20),J2+, 841, 842 CrCH1,NsO [Cr(CN)(NH3),(OH )I' 776 CrCHl,NsO [Cr(CN)(NH,),(Hz0)}", 776 crCHl~N6 [Cr(CN)(NH3)J2', 776 CrCHIsN6O [Cr(NCO)(NH,),]*+ 783, 844 C~CH~SN~S [Cr(NCS)(NII3j5l2+,844 CrCH,,N,Sc [Cr(NCSe)(NH,),]?+, M 4 CrCH18N70 [Cr(NW3)5(NHCONH2)]2 ' , 852 [Cr(NH,),(OC(NH)NH,}]z L, 852 CrCH19N70 852 [Cr(NH3)5(H2NCONH2)]3+. CrC2ClN0, Cr(NO)CI(CO),. 826,827 CrCzC1212N202 Cr0zC12(ICN)2,941 CrC2F3N07 Cr02(N03)(0,CCF3),940 CrC2F6O& Cr02(0qSCF3)2,940.941 CrC,H,CI50, [C~CI,(ACOH)]~-, 848,936 CrC2H4N02 [Cr(Gly-0)]2 ' ,769 CrC2H,CI,N ICrCI,(MeCN)12-, . _ 848 CrC2H6N304 Cr(NO)(CN),(H,O),, 825 CrC2H602 C;(OMe)z, 737 CrCzH8C1zN40z CrCI,(H2NCONH2),, 739 CrC2H&&"S2 CrCI,(HZNCSNH2),,754 CrC,HaFsN, p5F;(en)l-, 796,798 CrCZH8NZ04S2 [Cr (NCS)(SCN)(H,O),] + ,F42 [Cr(NCS),(H,O),]+, 838,862 ~~
~
[Cr(CN)2(NW3),]', 716,777,782,919 CrC2H13N05 [Cr(MeCN)(H20),]3', a 6
Formula lndex CrC,H17N05S [Cr(SCH2CH2NH3)(H20)51"+. 879 CrC,HI8C1N4OS [CrCI(DMSO)(NH3)4 ] ,782 CrCzHz3N70 [Cr(NH3),(MeNHCONHMe)I3+, 852 CrC204 Cr(Cz04),752 CrC3F9OgS3 Cr(03SCF3),, 941 CrC3H2N4OZ [Cr(NO)(CN),(H,O)]-, 825 CrC3H6N303 C ~ ( C N ) ~ ( H Z O775 )~, CrC3H.&O& Cr(NCS),(H20),, 838 CrC3H9F1AV" Cr{(PF,)2NMe}3, 714 CrC3HgN& Cr(S2CNHNH2)3,885 CrC3H,03 Cr(OMe),, 860 CrC3H906Se3 Cr(0zSeMe)3, 869 CrC3H90gS3 Cr(03SMe),, 815 CrC3H9S3 Cr(SMe),, 881 CrC,HloC12N2 CrC12{H2N(CH2)3NH2}, 721 CTC3H&l&& 888 CrC13(H2NCSNH2)3, CrC3H12N~0~ C ~ ( P ~ ) ( H Z O ) ( O946 Z)~, CrC3H1309 [Cr(H,0)5(0,CCHzCOzH)]Z+.873 CTC~H~~NZO~S [Cr(H20),(S~NHCH,CH,fiH)]3' ,888 CrC,HlsN,OS [Cr(CN)(DMSO)(NH,),IZ+, 776,782 CrC3H2,N60 [Cr(NH3)5(DMF)]3+,853 CrC3N304 [Cr(CN)3(02)213.,947 CrC& [Cr(CS,),]'-, 885 CrC4F606 Cr02(02CCF3)2,939-941 CrCaHOs Cr(C404)(0H), 873 CrC4H2N502 [Cr(NO)(CN)4(H,0)]2-I 825 crC4w406 Cr(02CCHOHCHOHC02),753 CrC&010 [Cr(C204)2(H,0),]-, 87&872 C~C~H~CIZNZ CrC12(MeCN),,717,732,899 CrC4H6Cl& [CrC14(MeCN)2]-,848 CrC4H606 Cr(C2H303)2r753 CrC4HiBr404' [C~B~,(ACOH),]~-, 758,762 CrC4HsC120 C~CI&THF),739 CrC4HsC1404 [CrC14(AcOH)2]-,848.936 CrC4HSN2O4 Cr(Gly-0)2,768,769 CrC4Hs05 Cr(OAc),(H,O), 740 CrC&S4 ~
1497
lICr(S2C,H4)d,l"-, 886 Cr(SCH,CH,S),, 772 (Cr(SCHzCHzS)2]Z-,755 CrC4H10C12N604 CrC1z(HzNCONHCONH2)2,739 CGH,oO2 Cr(OEt),, 737 C~C~HIOO~ CrO(Oz),(OEt,), 945 CrC4H1,Ci,N, CrCI,(H,NCH,CMezNH&, 721 CrC,H, ,Cl,O,S, CrC1,(DMSO)z, 753 CrC4HI2Cl2P2 {CrCI,(MePPMe,)),, 733 CrC4Hl,CI,N3 CrCl,(dien), 722 CrC4Hl,CI,N3 CrC13(dien),807 C&4Hi?N3 [Cr(dien)12+,721 CrC4H13N3o4 Cr(dien)(O,),, 807, 946 CrC4HI4NZOS [CI(N=CHCH=NCH=~ H)( HZ0)5]31,822 CrC,H,,CbN,O [CrCl,(dien) (HzO)]+, 807 CrC4H,6BrFN4 [CrBrF(en),] ' 795 CrC4HI6BrN5O2 [CrBr(NO,)(en),]+, 795 [CrBr(ONO)(en),]+, 789 CrC4Hl6Br2N4 CrBr,(en),, 721 [CrBr,(en),l+, 795 CrC,H,~Br,&$, CrBr24(H2NCSNH,), 755 CrC4H,,C1FN4 [CrCl$(en),] I , 796,798 CrC,H,,C12N, [CrCl,(en),]+, 7Y5.796,7Y8,809,816 CrC4HlhClZN804 CrC1,(H,NCONHz)4, 739 CrC,H,,Cl,04 [Cr(MeOH),Cl,]' , 860 CrC4HldZN4 [CrFz(en),]-, 796,798 CrC4Hi6N303 Cr(OH),(dien) ,807 C~C~HI~N.,O~S Cr(SO,)(en),, 721 C~C~HI~NX~ Cr(en)2(N03)3,867 C~CIHISNIO ICdNdden)zl+3844 CrC4Hl7ClN3O2 [CrCl(dien)(H20)Z]2', 807 CrC,H,,BrN40 [CrBr(H20)(en),]z', 794,796 CrC4HIBCIN40 [CrCl(H20)(en)2]2+, 795 CrC4H18FN40 [CrF(H,O)(en) ,I+, 796 [CrF(HzO)(en)2]Z+,796-798 CrC4H18N404 C T ( O A C ) ~ ( N H746 ~ ) ~ ,. CrC4HlaN70 ICr(N,)(en)2(H~O)]2'1844 CTC4H18N1002 [Cr(H,NC(=NH)NHC(-NH)NH2}z(H20)2]7+. 851 CrCdIz qCIN j [CrC1(NH3)(en)2]2',795 CrC4H19m5 -,
~
Formula Index
1498
Crk6H8clN302 CrCl{O2CCH(NH,)CH2~=CHNHCH=fi}, 768 CrC,HsCl2N4 CrCI,(HfiN=CHCH=~H),, 727
, ._
.
[Cr(NO)(CN)4]2-,825 CrC,HN,O ICr(CN),(OH)]3 ,774,775 CTC~H~CIN 2 02 CrCp(NO),Cl, 833 CIC~HSCI,NO, CrO,Cl2(PY), 935 CrCSH5NO3 Cr03py, 941 CrCSHSNOS C a O z M P y ) , 945 CssHISN05 [ C ~ ( PY ) ( H Z O ) S815 I~+~ CI-C&~FN~S [CrF(NCS)(cn)$, 796 [CrF(SCN)(en),]+,796 CrC5H,,N,0 [Cr(CN) (H,O) (en)z]2 + , 795 CrC&oN06 {C~(NO)(M~OH)S]~+, 824 CrC5HzSBrN5 [Cr3r(MeNHz)s1z+, 787 -. ~Ck&;CINs [CrCI(MeNH,)s]Zf,787,788 CGHwN,O [Cr(MeNH,)s(HzO)]3+,787,788 crC~HzaN6 [Cr(MeNH,),(NH3)]", 787 CrCSNSS ICr(CN),S]*-, 774
[Cr{SCH,CH(NH,)CO,})~~904 CrC6Hl,C1,N,S C K ~ , ( H N ~ 754 H ~ ~ ~ , CrC6Hi2NzS4 Cr(S2CNMe2),,754 CrCbHizN306 Cr(Gly-O),, 902 CrC6HizN40& Cr(N0),(S2CNMe2)*,826 CrC6HizN404 Cr(hexamine)(O,),, 947 CrCc,Hi,NzO, [Cr(HZO),(4-NCpy)]", 848 CTC6HdzO6
[Cr(OzCCHzNHCH~CH~NHCH~C0~)(H~O)~]+, 910 CrC6H~4N~004 [Cr{H,NC(=NH)NHC(=NH)NH,j,(C,O,)lt, 851 CrChH14N~2S2 [Cr{H,NC(=NH)NHC(=NH)NH,}Z(NCS),]" ,851 CrCbHlsClzP { CrCI2(PEt3)},,733 CrC6HisC13P {CrCI3(PEt3)},,853 CrC6HisS3 Cr(SEt),, 881 c~c,H,,N,o, [Cr(en)z(Cz04)l+,871 CrC6Hi6N6 [Cr(CN),(en),]+, 776,777,795,797 CrCLH,&N,S [&(CNRNCS)(en)Z]+,838 C~C~HI~N~SZ [Cr(NCS),(en),]+,838 CrC6H17NOsS [Cr(4-SC6H4NH,)(H,0),I3 877 CrC6H18BrCIN4 [CrClBr{(H2NCHzCHZ)3N}]C, 811 CrChHlaBr,03S3 CrBrz(DMS0)3,754 CrC6H&l2N4 [CrCl,{ (HzNCH,CH,NHCHz)z}]', 808,809,810 [CrCl, { (H2NCH2CH2)3N)]+, 811 CrC6Hl&IjN2 CrC13(Me3N)Z, 788 CrC,H&13N4 CrCl,{ (H,NCH,CH,),N}, 788,812 I .
,~
Formula Index
Cr(NMeJ3, 835 CrC6Hi~N303 Cr(OCHzCHzNH2)3, 897 CrCsHisN4 [Cr{(H2NCHzCHzNHCH2)z}]z+, 722 CrC6HlaN40zS [Cr(SCH2COz)(en),]+,796,879 CrC6Hldlo [Cr(N3)2{(H2NCH2CHzNHCHz)2]]+,811,844 [C~(N~)Z{(HZNCH~CHZ)~N)~+, 811 CrC6H18N15 Cr{H,NC(=NH)NC(=NH)NH,),, 850 CrC6H1804SiZ CrOZ(OSiMe3),,941 C~C~HI~N~OZ [ C ~ ( ~ - P Y C O ~ ) ( N H899 ~)~I~+, crC6HzoBrm4 [CrFBr{HzN(CH2)3NH2)z]f, 797 CrC6Hz0C1FN4 4+,798 [CrCIF{H2N(CH2)3NHz) CrC6H20ClN40 [CrCI{(HzNCNzCW2NHCH~]2}(H20)]z+, 808 [CrC1{(HzNCH2CH2)3N}(Ha0)]2+, 811 CrC6H2,,C12N4 CrCI,{H”H2)2~2}2,721 [CrCIz{HzN(CHz)~NH2~zIf, 798 CrC6Hz,,FN40 [CrF{(HzNCHzCHz)2N} (HzO)]zf,811 C~C~HZOF~N~ [CrFz(pn)zl+,798 CrCsHzoN4Sz [Cr(SCHzCHzNH,)2(en)]+.879,900 CrC6H21CIN40 [CrCI(HzO)(trenH)]3f,811 CrC6HZICi3N3 CrC13(EtNH2)3,788 C~C~HZIN~O~ [Cr(l,4,7-tria~acyclononane)(H,0)~]~~, 924 CrC6HzlN40, 809,810 [Cr(OH){ (H,NCHzCHzNHCH,),}(HzO)]z+, CrC6HziN403 Cr(OH)3{(H2NCHzCH2NHCH2)2}, 810 CrC6HziNis [Cr{H,NC(=NH)NHC(=NH)NH,)3]3+,850 CrC6HZ2FN40 [C~F(OHZ)(P~)ZI”, 798 [CrF(HzO)(HzN(CH~),NH,)z12’, 797 CrC,H,,N,S [Cr(SCH2CHzNHz)(en)2]2+, 879,880 CrC6Hz3ClN,O2 [CrCl(EI,0),(trenH)]3+,81 1 CTC6H&N5 [CrCl(en)(dien)12+,807 CrC6HZ3m5
[G(OC(NH&}6l3’ 9 867 C~CQHZSN~OZ 810 [Cr(t~ienH)(H~O)~ ]~+, CTC~HZ~N&O~S [Cr(OS02CF3)(MeNHz)s]Z+, 788 CrCdHz6N6 [ C I ( M ~ N H ~ ) , ( ~ ~ )787 ,]~+, CrC6Hd703 [ Cr(N3)(H20)3(trienHz)]4+, 811 CrC6H2aN404 [Cr(trienH2)(H20)4]5+, 810,811 CrC6H25”704
[Cr(N3){H20)4(trienH3)]5+, 811 CrC6&oN6 [Cr(MeNH,)6]”, 787 CrC6H31N405
[Cr(trienH3)(HzO),]6+,810 CIC6N6 [Cr(CN)6]3-, 703,708,774,775,797.823,838 [Cr(CN),14-, 703,708,774,775 [Cr(CN),I6-, 704,774 CrC6N606 [Cr<XCO),]3-, 843 CrC6N6S [Cr(CN),(NCS)I3-, 841 crc$IJ6s7. [Cr(CN)4(NCS),]3-, 841 CrC,N,S, [Cr{CN)3(NCS)3]3-,841 CrC,N6S4 [Cr(CN)2(NCS)4]3-,841 CrC6N6S5 [Cr(CN)(NCS),I3-, 841 CrC6N& [Cr(NCS)J3-, 837,838,842,843 [Cr(NCS)6]4-, 730 CrC6NeSe6 [Cr(NCSe),l3-, 842 CrC606 Cr(C404)3,1,873 0cSo6s6
[Cr(C&O2)3l3-, 885 CrC6O9S3 [Cr(C2SO3)J3-,901 CrC6012 [Cr(Cz04),]3-,799-871 Cc7H403 Cr(2-OC&,COz), 753 CGHSN02S CrCp(CO),(NS),832 CrC,H7N2O7
C~(NO)I~,~-PY(CO~)~~(H~O)~, 826,835 CrC,EI,Cl,N,
CrCl,{H,C(~N=CHCH=~H)2}, 728 C~C7H9N4O7
~(NO)(~,~-~~(COZ)Z}(NHZ~H)Z, 826,835 CGHioCW, CrCI2(2-pyCH2CHzNHZ), 727 CrC7H1~Nd07
[CrF(NH3){H2N(CH*)3N~~}2l2-, 797,798 CrC6Hz3N05S [Cr(4-SC6H4NMe3)( H20),l3’, 877 CrC6Hz3N50 [Cr(en)(dien)(HZO)l3+,807 C ~ C ~ H Z ~ N ~ ~ Z [Cr(N3)(trienH)(H,0),1’+, 811,844 CrC6H24IzNizS6 Cr12.6(H2NCSNHz),755
Cr{2,6-py[COz)i)(NH~0)(NH20H)2, 835 CrC7HllN1407 Cr(NHZO){2,6-py(COZ)Z)(NH~OH)Z, 826 CrC7H18CW5S [CrCl(NCS){(HzNCHzCH2)3N)]+,811 CrC7HI8ClNSSe [CrCI(NCSe){(HzNCH2CHz)3N}]+, 811 CrC7H&l2N4
CrC6H24N6
CW-bAN,
[C~(en)~]*, 838 [ C r ( e r ~ ) ~ ]721 ~+, [C~(en)~]j+, 797 CrC&di206
CrC7HmN4OzS [Cr(SCHzCH2C02)(en)2]+, 879 CGH,,Cl2N3OS
coc3-W’
1499
[CrC12((H2NCHzCH2NHCH2CHz)2CHz)]+, 812 [CrFz{(H2NCH,CH2NHCHzCHz)zCH2)]+, 812
1500
Formula Index
[CrCI,{ (HZNCH2),CMe}(DMSO)]+,808 CrC,H&lNs [crCi( H,N( CH,),NH,} (dien)]z+, 807 [CrCl@n)(dien)],+,807 CK7H25N50
[Cr(pn) (dien)(Hz0)l3+,807
[Cr{H,N(CH2)3NHz}(dien)(H,0)]3+, 807 .CrC7HZSN5S
. [Cr(MeSCHzCH,NH,)(en)z]3+, 880 CrCSFP2010 [Cr02(02CCF,)4]2-,944 Cr C,H9N403 [Cr(NO)(CO)Z(MeCN)3].C, 827 CrCsH1,CIN,O [Cr(NO)Cl(MeCN)41*, . _ 827 Ck8€&,C1,N~ CrCl,(Me$lCH=NCH=cHh. ,-. 727 CrC8H,N4 [Cr (MeCN),I2+, 732 CrC8H14N204S~ Cr{ (02CCH(NH2)CHZCH2S}z}r 768 CrC8H16C1202 CrC12(THF)2,717,739 CrCsHd204Sz Cr{O,CCH(NH,)CH,SMe},, 768 CrCSHl8C1,S, Cr{CMe(CH,SMe),}Cl,, 886 C68H18C13S4
'
CF((M~SCH~CH~SCH~)~}C~~, 886 CrC$IisN404 [Cr(C204){ (H,NCH,CH,NHCH,),)]', 809 [Cr(Cz04){(H2NCH2CH2)3N)I+,811 CrCsH1804 CIQZ(OBU')~,928 CGHzoCIzN4 [CrCl,{ 1,4,7,10-tetraazacyclododecane)]-.922 CGHZONZ Cr(NEt&, 931 CC&z&Sz ICr(NW2(~n)21+1 838 [Cr(NcS),{H2N(CH2),NH2)~l' ,798,838 CGHZ~NQOS [Cr(C,0,)(H,0)(trienH)]2809, +, 811 CGHzQzN [CrCI2{H,N(CHz)3NHCHzCH2NH(CHz)3N€iz}]+, 812 CrC$IZ2N4Oz [Cr(OH),(1,4,7,10-tetraazacyclododecane)]+, 922 [Cr{(H2NCH,CH2N=CMe),}(H,0)21f, 895 CrC8HzzN402S [Cr(SCMe,CO,)(en)z]+, 879 CrC8H2,CIN5 [CrQ{HN(CH2CHzNHCH2CH2~Hz)2}~z+, 814 C&&LJ'LOz [Cr(OH)(l ,4,7,10-tetraazacyclododecane)(H20)]z~, 922 CGH24Br2N4 CrBr2(H2NCH,CH,NMe2),, 721 CrCBH&I2N4 CrCl2(H2NCH2CMe2NH2),,721 CrClz(MeNHCHzCHzNHMe)z,721 CrCsHZ4Cl2N8O4 CrC12(MeNHCONH2)4,739 CGfIz4ClJ'b CrC13{H2N(CH2)4NH2},,798 C~BHWI204S4
CrI,(DMSO),, 754 CrCBeZ4N4O2
[Cr[1,4,7,10-tetraazacyclododecane)(H20) 922 CrCsHzsNsO
[Cr{HN(CH2CH2NHCH2CH2NH&} (HzO)]~', 814 CGHwN6 [Cr(dien),12+,721 [Cr(dien),I3*, 806
Formula Index
1501
[CrC12(1,4,8,12,-tetraazacyclopentadecane)]+,921 CrC11H2BCIN40 [CrCl(1,4,8,12-tetraazacyclopcntadecane)(HzO)]2+, 923 CrC11H30tI402 [Cr(1,4,8,12-tetraaza~yclopentadecane)(H,0)~]~+,921
Cr(NCS)2{HflC(Me )=NCH& H},, 727,773 CrCloH14Br2N20z CrBr2(HzO)&& 725 C~GOHI~NZOS [Cr { (0,CCHz)2NMe}J, 911 CrC10H14N209 [Cr(edta)(HzO)]-, 1259 CrC10H1404 Cr(acac),, 717,738,772,912 CrCi OH,sC120~ CrClz(acacl(acacH),861 CrC,oH~sN,O~ Cr{(0zCCHz)zNCH2CH~~(CH2C02H)CHzCOz}Hz0, 908 C~CIOHISN~O [Cr(NO)(MeCN)s]2-C,826,829,848 [Cr(NO)(MeNC)s]+,826 [Cr (NO)(MeNC)s]2C,824 C~CIOHISN~S [Cr(NS)(MeCN),I2+,832,848 CrCioHi6C14Pz [CrCL1{2'(MeZP)ZCdH4}J-, 853 C~CIOH~~O, [CrO(OICOCMeEt),]-, 937 r~
..
CrC10H18C12N40Z
Cr(NO),C12(CNBu')z ,827 C ~ C I O H I S N ~ PZ S ~ [Cr(NCS),(Me,P),]-, 838 CrCioHis06 [ C r ( a ~ a c ) ~ ( H ~ O )841 ~]+, CrCloHZoN204Sz Cr{02CCH(NH2)CH2CH2SMe)2, 768 CrCl~HzoNZS4 Cr(SZCNEt2),,754,884 CrCl~HzONCS~' [Cr(NCS),( 1,4,7,10-tetraazacyclododecane)]+, 922 Cl'CloHzzNaO~ ~
[Cr(C~04){H,N(CH,)3N€ICH,NH(CH,),NH(CHz)3NH2}]+, 812 CrClOHZ4ClZN4 [CrCI2(1,4,7,1l-tetraazacycloterradecane)] ,922 [CrC1,(1,4,8,11-tetraazacyclotetradecane)]~, 919 CrCioHz4N604 [Cr(N02),(1,4,8,11-tetraazacyclotetradecane)]+,919 C~CIOH~~N~~ [Cr(N,),(1,4,8,1 1-tetraazacyclotetradecane)]+, 919 CrCl&6ClN40 [CrC1(1,4,7,11-tetraazacyclotetradecane)(H,0)12+,922 [CrCI(l,4,8,11-tetraazacyclotetrade~ane)(H~O)]~+, 919 CrCioHz6N40z [Cr{(H2NCH2CH,CHzN=CMe)2}(HzO)z]+, 895 CrCioH26Ns04 [Cr(l ,4,8,11-tetraazacyclotetradecane)(N03)(HzO)]z+, 919 CrCloH2,N03Siz CrO(NBu')(OSiMe,),, 945 ., . CrCloH27N402 [Cr(OH)(1,4,8,11-tetraazacyclotetradecane)(H20)1"+, 919 CrC10H28N402 [Cr(l,4,7,11-tetraazacyclotetradecane)(HzO)z]3+, 922 [Cr(l ,4,8,11-tetraazacyclotetrade~ane)(H~O)~]~+, 919 CrC10H3sC1Ns [CrC1(EtNH,),]2+, 788 +
CrCllH17N404S
Cr { 02CCH(NH,) CH,&=CHNHCH=N } {SCMe2CH(NH2)CO2},904
CrCllH21N704
[Cr {2,6-py{ C(Me)=NHNHCONH 2 } 2} (Wz0)2I3+,899 [Cr{2,6-py{C(Me)=NHNHCONH2}Z}(H@)#+, 896 CrC11H26ClZN4
CrC,,H,,Ci N, CrCl2(H&CH=NCH=c H)4, 727 CrC1, (H$iN=CHCH=c H)4,727
[Cr{OlCCH(NHz)CH2c=CHNHCH=$4 } 904 CrCIPHIBBrZN CIB~,(M~N-€I)~, CrC,,H,.CIN,Ot
727
CiCl,(bipy)(NCMe), 936 CrC1~Hl8N2O2 Cr( IMeCOCHC[Me)=NCH,),), . , - _- _ 772 crc1;H1~2o10 [Cr{(02CCH2)2NCH2CH20H}2]-, 911 CrC12H1sN,O -CHCH=CMeNCH,CH,NC(Me)= c r ( N o b H .* l *., , 832 Ce12HlSN6 [Cr(MeCN)6]2+,718,732 CG2H18P3SS Cr { S,PMe(C=CMe)),, 885
Formula Index
1502
C I C ~ ~ H ~ ~. F ~ O ~ CrF,(THFL. 855
CrC12H241303
Cr13(THF)3,856 Cr'&Hz& [Cr:r(CN),(1,4,8,1l-tetraazacyclotetradecane)l+ 777, 919 CGzHwN6Sz [Cr(NCS)z(1,4,8,11-tetraazacyclotetradecane)]+, 919 CrCizHz4Ss [Cr(MeSCH=CHSMe)3]3+,886 CrCi2Hz7Cl3P (Cfl13(PB~3)},,853 CrC1&L7N04 C~(NO)(OBU')~, 831 CrCI2Hz7O3 Cr(OBU')~,928 CrGJ%@J' Cr0(02)2{OP(OBu)3},946 C~CI~H&LN~ CrCl,(Me,[ 1