The noble metals

16

The noble metals

Peter Thornton Consulting Chemist, 124 Malford Grove, South Woodford, London, UK E18 2DG

1

Introduction

As last year, it has been necessary to exclude some good work to meet space restrictions. In particular, some solution studies and theoretical work has been omitted and organometallic chemistry has been included only to the extent that it casts light on the behaviour of the metal atoms rather than the ligands. Within each section compounds are normally described in order of decreasing oxidation state, and, within each sub-section, in the order halogen > chalcogen > pnictide > Group 14 > metal. A major three-volume work on metal clusters includes chapters on noble metals.1 General reviews including noble metal chemistry cover metallomesogens,2 and hemilabile ligands, many of which are chelating phosphines in which the other donor is so weak it can easily be substituted without losing the phosphine co-ordination.3 A general review on metallodendrimers describes the shape of the topic systematically.4 These metals also feature in a review which discusses metal boryl compounds and M^B links.5 Among organometallic topics, reviews cover cluster chemistry,6 indenyls,7 and oxazoline complexes.8

2

Ruthenium

Simple and co-ordination compounds The most interesting new ternary oxide is Sr2Ru3O10, obtained as a minor product from the hydrothermal reaction of hydrated Sr(OH)2 and RuO2 under oxidising conditions. The structure contains rutile-like RuO2 units linked by RuO6 octahedra; the Sr2‡ ions are nine-co-ordinate.9 A new synthesis of AgRuF6 is reported, using the reaction of AgF with a high oxidation state £uoride or £uoro-complex in anhydrous HF.10 Studies of [GeF3]ÿ complexes prepared in aqueous HF include [RuCl6ÿn(GeF3)n]4ÿ (n ˆ 1 to 3); some complexes of Os, Rh and Ir were also reported.11 DOI: 10.1039/b003155k

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1-Nitrosonaphthol (HL) forms [RuL3]ÿ with RuII, showing N,O-chelation; the three unco-ordinated oxygens can then bind to Ni2‡, giving [Ni{RuL3}2].12 The bridging formates in [Ru2(O2CH)2(CO)4(PPh3)2] can oxidise primary alcohols RCH2OH to give [Ru2(m-O2CR)2(CO)4(PPh3)2] (R ˆ Pr or Bui) but other ruthenium(I) dinuclear carboxylate-bridged complexes of this type reacted with R0 OH to give [Ru2(m-OR0 )2(CO)4(PPh3)2] (R ˆ Ph, Bun or Me2CHCH2CH2).13 Treating [Ru2(O2CBut)4(H2O)2]‡ with the nitroxide L1 gives the polymeric chain complex [{Ru2(O2CBut)4(L1)2}] [{Ru2(O2CBut)4(H2O)2}]2‡n, which appears to show antiferromagnetism between nitroxides and ferromagnetism between these and the metal together with a high zero ¢eld splitting.14a The ¢rst room temperature mesomorphism in a mixed valence compound was found in the palmitoleate [Ru2{O2C(CH2)7CHˆCH(CH2)5Me}5], made from [Ru2Cl(OAc)5] by removal of chloride with Ag‡ followed by carboxylate exchange.14b

In chemistry involving co-ordination by other chalcogen ligands there have been advances in polysul¢de anion complexes. The curious reactivity of these in [Ru2(m-Sn)(m-Me2dtc)(Me2dtc)(CO)2(PPh3) 2] is shown in its reaction with hydrazine to give partial desulfurisation to [{Ru(Me2dtc)(CO)(PPh3)2}2(m-S4)(m-N2H4)].15 Further studies include the reaction of the known ruthenium( II,III) complex [{RuCl[P(OMe)3]2}2(m-S2)(m-Cl)(m-N2H4)] with dioxygen to form ¢rst a cation with a superoxide linked to S2 followed by decomposition to sulfate species such as [{RuCl[P(OMe)3]2}2(m-S2)(m-Cl)(m-N2H4)][HSO4].16 The versatility of the ruthenium(III) dinuclear complex [RuCl{P(OMe)3}2(m-S2)(m-Cl)2] in producing new clusters during redox processes is further demonstrated by its reduction with Na/Hg to the tetranuclear ruthenium(II) complex [{RuCl[P(OMe)3]2}2(m-S2)(m-Cl){m-P(OMe)-P,O}Ru{P(OMe)3}]2 with two parallel Ru2 cores.17 The ¢rst ruthenium(IV) complex with all sulfur donors was obtained by the iodine oxidation of [Ru(mnt)3]3ÿ to give the dianion. The NEt4‡ salts of each have been shown by crystal structure analysis to have exceptionally regular octahedra for mnt complexes.18 Reaction of PhMeSO (L) with RuCl3
xH2O gives the unsymmetrical linear trinuclear complex [{RuClL}(m-Cl)3Ru(m-Cl)(m-L)2{RuClL}], in which L shows each of the bonding con¢gurations S, O and (m-S,O).19 The tridentate ligands MeC(CH2EMe)3 (L; E ˆ Se or Te) react with [Ru(dmf)6]3‡ to give [RuL2]2‡, the ¢rst homoleptic seleno- and telluro-ether complexes.20 Among new ditelluroether complexes are trans-[RuCl2{PhTe(CH2)3TePh}2] and other ruthenium(II) complexes. Unlike sulfur and selenium analogues, these resist oxidation by halogen to RuIII; platinum(II) complexes were also prepared.21 280

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Turning to nitrogen donors, the use of 3- or 4-ferrocenylpyridine (L) to form [Ru(NH3)5L]2‡ allowed an unusually thorough study of intervalence transitions, including their energy, solvent dependence, kinetics and relationship to electrochemistry.22 The reaction between Na3[Fe(CN)5]
3H2O and [Ru(NH3)5L]3‡ (L ˆ 4-dipyridylamine) gives [{Fe(CN)5}L{Ru(NH3)5}]ÿ, which can be oxidised to the neutral molecule and the monocation.23 The role of glutathione (GSH) in the reduction of ruthenium(III) anticancer compounds was investigated through studies of its reaction with [RuCl(NH3)5]2‡, from which GSSG and [Ru(OH)(NH3)5]2‡ were identi¢ed, but with the probable involvement of some ruthenium(II) species.24 Although studied in solution for many years, salts of trans-[Ru(NH3)4(NO)(H2O)]3‡ have only now been isolated in the solid state, this being achieved by the reduction of trans-[Ru(NH3)4(NO)(SO3)] Cl with Zn/Hg followed by a cation exchange process.25 Developments in the use of [RuCl4(C7H6N2)2]ÿ (C7H6N2 ˆ indazole) as an antitumor agent are enhanced by its use as a very soluble sodium salt and by the crystal structure determination of the [PPh4]‡ salt con¢rming its trans con¢guration. Despite the compound's paramagnetism a COSY spectrum was obtained.26 NMR (1H) spectral studies show that [RuII(edta)(H2O)]2ÿ is co-ordinated by GMP at the imidazole N(7) (30%) or by the pyrimidine N(3) (70%), but the ruthenium(III) monoanion gives only the former, as in platinum anticancer drugs.27 The redox series [Ru(hedta)(NO)] (Z ˆ 1, 0, or ÿ) has been prepared and studied by NMR spectroscopy and electrochemistry. The conversion of NO‡ into NO2ÿ was also studied. The paper presents a new analysis of the factors governing the behaviour of NO as NO‡ or as singlet or triplet NOÿ.28 cis-[Ru(2,20 -bipy)2(NCMe)(NO)]3‡, whose crystal structure as the triperchlorate was determined, reacts with nitrite to give [Ru(2,20 -bipy)2(NCMe)(NO2)]‡ and the acetamidate complex [Ru(2,20 -bipy)2(NHCOMe)(NO)]2‡.29 The reaction of HBr and [RuCl5(NO)]2ÿ gave [RuBrnCl5-n(NO)]2ÿ (n ˆ 0 to 5), with a crystal structure being obtained for the monochloride and IR and Raman spectra for all.30 The neglected co-ordination chemistry of 2-HOCH2C5H4N (2-pyridinemethanol, HL) has been extended by a study of its reaction with [RuCl3(NO)], from which [RuCl3L(NO)]ÿ, [RuCl2L(HL)(NO)] and two isomers of [RuCl2L2(NO)] were isolated.31 Ruthenium nitrosyl porphyrins react with nitrido complexes of Os and Re to give nitride-bridged complexes, as when [Ru(oep)(O3SCF3)(NO)] and [OsO3N]ÿ give [{Ru(oep)(NO)}(m-N)(OsO3)].32 A kinetic study of the reaction of NO with [Ru(por)(CO)] [por ˆ oep or meso-tetra-4-tolylporphyrinate(2ÿ)], giving [Ru(por) (NO2)(NO)] and nitrous oxide, shows this may go through the formation of [Ru(por)(NO)2], formed after loss of CO from [Ru(por)(CO)(NO)].33 The synthesis of [Ru(por)L2] (L ˆ O, or NTs) and their roles in enantioselective alkene epoxidation and aziridation have been reviewed.34 The vacuum pyrolysis of mixed [Mo(oep)(PhCCPh)] or [W(oep)(PEt3)2] with [Ru(por)(py)2] (por ˆ oep or tpp) gives heterodinuclear [MRu(oep)(por)] (M ˆ Mo or W) with two unpaired electrons. [MoRu(tpp)(oep)] can be oxidised to its monocation. This probably has its p* orbital below the d*, unlike other metalloporphyrin dimers. All these metal^metal bonds are slightly stronger than in homodinuclear porphyrins. There are also some osmium examples.35 Annu. Rep. Prog. Chem., Sect. A, 2000, 96, 279^315

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The environmentally important dechlorination of aryl chlorides can be achieved quickly and completely in alcohols using NaOH with catalytic [RuXH(H2)2(PCy3)2] (X ˆ Cl or H).36 A new convenient one-pot synthesis of the above complex (X ˆ H) has been developed from [Ru(cod)(Z3-C4H7)2], hydrogen and PCy3.37 The bis(dipyridylphosphine) ligand L2 often behaves as a simple diphosphine, but it shows P,P,N co-ordination in [RuX2L(PPh3)] (X ˆ Cl, Br or I) with both merand fac-P3 con¢gurations found.38

Polypyridyl complexes This ever-growing ¢eld continues to £ourish. New supramolecular dendrimer complexes having a central [Ru(2,20 -bipy)3]2‡ core and containing 12 or 24 naphthyl units seem to have a very ef¢cient antenna effect in transmitting UV absorption to the centre while protecting the luminescent core from quenching by dioxygen.39 For these complexes one oxidation and three one-electron reduction steps can be identi¢ed in the central core and other redox processes can be found in the outer dendrimer components.40 Bonding [Ru(2,20 -bipy)3]2‡ to myoglobin allows photochemistry resulting in the generation of oxidised myoglobin in the iron(IV) state. An intermediate porphyrin radical cation is formed and the mechanism was deduced, including an explanation of the pH dependence.41 The binding of [Ru(phen)3]2‡ and complexes of various methyl-substituted analogues of phen with DNA depends on many factors, including the concentration of the complex. At low concentrations binding occurs in the minor groove, but the slotted mode is more favourable at higher concentrations. However for the L form the interaction with the minor groove is favoured at all concentrations.42 A major area of study has been to use variants of 2,20 -bipy and phen. Thus 2-(2-pyridyl)quinoxaline L3 forms [Ru(L3)3]2‡, with a mer con¢guration, and various mixed-ligand complexes containing 2,20 -bipy.43 The Br atom in [Ru(2,20 -bipy)2(3-Brphen)]2‡ acts as a starting point for substitution reactions that incorporate ruthenium(II) tris-diimine units into peptides.44 The use of [Ru(2,20 -bipy)2{5,6-(NH2)2-phen}]2‡ in Schiff base forming reactions with salicylaldehyde followed by reaction with Cu2‡ or Ni2‡ gave dinuclear complexes with much reduced emission properties.45 Phenyl-substituted phen derivatives such as L4 form [Ru(2,20 -bipy)2(L4)]2‡ whose photochemistry and structures suggest the phenyl ring forms intramolecular p^p interactions with an adjacent 2,20 -bipy, resulting in reluctance to undergo photochemical substitutions.46 282

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A study of polynuclear ruthenium(II) complexes of 2,3-bis(2-pyridyl)pyrazine (L5) includes the ¢nding that [{Ru(2,20 -bipy)2(m-L5)}4{Ru2(m-L5)}]12‡ shows 26 reversible reduction processes, detectable over an unprecedentedly wide range, and that the size of the cation of the supporting electrolyte is relevant, more steps being detected with [NEt4][BF4] than with the [NBu4]‡ salt.47 The observation by 2-dimensional NMR of all the rotational isomers of [Ru(2,20 -bipy)2(C8H8N2)2]2‡ (C8H8N2 ˆ 1-methylbenzimidazole) reveals the rotational properties of the monodentate ligands, with implications for understanding the co-ordination of guanine to anticancer drugs.48 This next section covers derivatives of 2,20 -bipy. Among many studies on ruthenium complexes which may be suitable for use as photosensitizers, work on [RuX2L2] {L ˆ 4,40 -(EtO2C)2bipy; X ˆ Cl, I, CN or NCS} gives many insights. The ruthenium(III) dichloride derivative is very stable. UV and EPR spectroscopy show oxidation has occurred at the metal, but similar oxidations for the diiodide complex led to loss of ligand and, in CH2Cl2, formation of ¢ve-co-ordinate [RuIL2]‡. For the dicyanide derivative, polynuclear compounds and cyanogen are formed, while the dithiocyanate gives elimination of sulfur with formation of the dicyanide complex.49 The ¢rst synthesis of trans-[Ru(NCS)2{4,40 -(HO2C)2bipy}2] from the dichloride counterpart and NH4NCS in dmf allowed comparison with the cisisomer, which has a higher MLCT absorption and is a very ef¢cient photosensitizer.50 The subject of con¢gurational preference in ruthenium(II) complexes has neatly been developed by the discovery that irradiation of dmso and cis-[RuCl2(2,20 - bipy)2] gave trans-[RuCl(2,20 -bipy)2 (dmso)]‡, whereas the cis-form is formed in the dark. However, with bulkier 4,40 -Me2bipy the cis isomer was formed under both conditions.51 In another report, extended to tetramethylene sulfoxide complexes, optical resolutions were achieved.52 It is also suggested that the conversion of the cis-2,20 -bipy^dmso complex into cis-[Ru (2,20 -bipy)2(4,40 -Me2bipy)]2‡ is achieved with 97% retention of chirality and that this procedure might be valuable in future synthetic work.53 A crystal structure of cis-[Ru(O2CH)(CO)(2,20 -bipy)2]‡ reveals monodentate formate and allows an updating review of IR spectra of carboxylate complexes.54 An important result in work on photosensitizers arises from the use of [{Ru[4,40 -(HO2C)2bipy](4,40 -Me2bipy)}(m-dpp){Ru(terpy)(H2O)}]4‡, in Annu. Rep. Prog. Chem., Sect. A, 2000, 96, 279^315

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which the carboxylate can bind to nanocrystalline TiO2 on Sn-doped InO. When this is irradiated in 2-propanol oxidation to acetone occurs, probably by reaction with an [RuO]2‡ moiety. The dehydrogenation of 2-propanol is not a spontaneous reaction.55 The ¢rst ruthenium(III) carbonyl ^ diimine complexes are claimed as products from the reaction of RuCl3
xH2O and 2,20 -bipy or phen in formic acid to give [RuCl3(CO)L] (L ˆ 2,20 -bipy or phen).56 Despite their involvement as effective catalysts in the water gas shift reaction, the chemistry of these mono(2,2 0 -bipy) complexes has been neglected. However, this has been recti¢ed by ¢nding that the reaction of 2,20 -bipy with [RuCl(CO)3]2 in ethylene glycol gave [RuCl(O2CC2H4OH)(CO)2(2,20 -bipy)], [RuClH(CO)2(2,20 -bipy)] or [RuCl(CO)2(2,20 -bipy)]2.57a An extensive survey of 99Ru NMR spectra ¢nds a correlation of chemical shift with absorption energy in various diimine (L) complexes [RuXY(CO)2L] (X,Y ˆ monodentate anions), con¢rming the assignment of electronic transitions and allowing the detection of isomers in complexes of mixed 2,20 -bipy derivatives.57b There has also been a great deal of activity concerning ruthenium complexes of terpy and its derivatives. Unlike [Ru(2,20 -bipy)3]2‡, [Ru(terpy)2]2‡ salts do not show luminescence, but this property does appear in a new trinuclear complex in which two 2,5-thiophenediyl spacers link three cations in a linear array.58 The ¢rst reported crystal structure of a [Ru(terpy)2]2‡ salt, that with [PF6]ÿ and crystallised from MeCN, allowed an assessment of the effects of substitution, particularly at the 40 position, though these were found not to be great. 59 The incorporation of an [Ru(terpy)2]2‡ unit into a catenate by linking at the 6 position to a similar terpy pair which co-ordinate various d10 cations produced complexes showing intermetallic luminescence processes.60 The electroreduction of [Ru(terpy)2]2‡ gave crystals of the uncharged [Ru(terpy) 2]. The structure of this compound, reported with that of the bis(hexa£uorophosphate), shows the Ru^N distances are similar but some C^C and C^N distances are shorter in the uncharged molecule, which is also more distorted from regular octahedral.61 A new convenient synthesis of [Ru(terpy)(H2O)3]2‡ comes from ¢rst converting [RuCl3(terpy)] into [Ru2(m-O)(mO4PH)2(terpy)2] and reducing this with zinc. The paper also describes the ¢rst crystal structure of [RuCl3(terpy)].62 Studies have continued on complexes of more elaborate ligands related to polypyridyls. There has been an unusually thorough study of the spectra, redox properties and photochemistry of many isomers of dendritic ruthenium(II) tetramers bridged by L6 and capped by phen.63a [RuCl3(terpy)] reacts with qtpy and ligands (L) to give [RuL(terpy)]2‡ with L behaving as if it were terpy and generating interesting conformational effects; [{Ru(terpy)}(m-L){RuCl(terpy)}]3‡ was also prepared, with the bridging qtpy acting as a bidentate ligand to one Ru and a tridentate to the other.63b The availability of 4-Ph2Pterpy allowed the preparation of [Ru(Ph2Pterpy)2]2‡, which uses its P atoms to co-ordinate to Mo or W (M) to form [Ru{(m-Ph2Pterpy)M(CO)5}2]2‡; a triosmium complex was also described.64 The terpy analogue L7 generates interesting ruthenium(II) chemistry through the synthesis of [RuCl(L7)L]‡ (L ˆ 2,20 -bipy, Me2bipy or phen), the substitution of chloride by water or NCMe and the study of the one-electron oxidation of many of these by spectroelectrochemistry, in which an {[RuIVˆO]} complex reported earlier could 284

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not be detected.65 A spectacular group of rigid dendritic complexes is produced when the complexes of 4,40 -(IC2C6H4)2bipy (L), [ML3]2‡ (M ˆ Ni, Cu or Ru), couple with [Ru(terpy)2]2‡ complexes containing 40 -alkyne substituents; the largest product gives a cation with 19 metal ions and a charge of 38‡.66

Mononuclear organoruthenium compounds Hydride ^ carbonyl ^ phosphine complexes are dominant in this area. The reaction of [RuClH(CO)(PPh3)3] with CH2ˆCMeCH2OH is unusually varied depending on the conditions; one phosphine is lost, but the resulting complex may be one with Z4-CH2 ˆ CMeCO, a dimethylacetate or a chelating complex of HOCH2CMeCH2ÿ.67 Developments in the chemistry of 16-electron square pyramidal ruthenium(II) complexes include (a) the crystal structures of [RuC1X(CO)(PPri3)2] (X ˆ Cl or H) and of [RuClPh(CO)(PBut2Me)2], (b) the multistep removal of chloride from the last of these to give [RuPh(CO)(PBut2Me)2], whose IR spectrum shows that agostic interactions persist in solution, (c) the cleavage of the Ru^Ph bond by catechol^borane or Me3SiCCH and (d) the progressive substitution of chloride by methyl in [RuCl2(CO)L2] (L ˆ PPri3 or PBut2Me).68 The crystal structure of [Ru(H)2(CO)(PPh3)3]
CH2Cl2 shows a very short H


H contact of 204 pm between co-ordinated hydride and a hydrogen at the 2 position of a phenyl. Studying the published data showed 24 other ruthenium compounds which showed such distances shorter than 220 pm.69 A search for new simple carbonyl and carbene hydridoruthenium complexes was rewarded by ¢nding that RuCl3
xH2O reacts with PPhPri2 in methanol and triethylamine to give [RuClH(CO)(PPhPri2)3], which reacts with CO, CH2N2 and PhCHN2 (L) to give [RuClH(CO)L(PPhPri2)2] and [RuCl2H(CO)(PPhPri2)2]ÿ, the latter having cis-chlorides and trans-phosphines.70 A rare organometallic cyanate complex [RuH(NCO)(CO)(PPh2Me)3] is formed from [RuCl3(PPh2Me)3] and NaNCO in ethanol, but using the PPh3 analogue led to solvent involvement in the reaction to form [Ru(NCO)(OAc)(CO)(PPh3)2].71 The attractive ¢eld of aquaorganometallic complexes has been boosted by the reaction of [Ru(O3SCF3)2(CO)2(dppe)] with atmospheric water to yield [Ru(O3SCF3)2(CO)(H2O)(dppe)] and with water in chloroform to give [Ru(CO)(H2O)(dppe)]2‡.72 The use of the bulky diphosphine But2PC2H4PBut2 (L) to make [RuCl2(CO)L], with subsequent conversion into trans-[RuClH(CO)2L], allowed formation of ¢ve-co-ordinate [RuH(CO)2L]‡ by substitution of chloride Annu. Rep. Prog. Chem., Sect. A, 2000, 96, 279^315

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by an unco-ordinating anion.73 The use of para-enriched hydrogen allowed the discovery of isomers of [Ru(H)2(CO)2(PPh3)L] (L ˆ CO or PPh3) which have inequivalent hydrogens. One of the products of the reaction of H2 with [Ru3(CO)9(PPh3)3] is [Ru3H(m-H)(CO)8(PPh3)3], not [Ru3H(m-H)(CO)11] as previously supposed.74 [Ru(H)2(CO)(PHPh2)(PPri3)2] reacts with [Ir(m-OMe)(cod)]2 to give the ¢rst heterometallic complex with mixed hydride ^ alkoxide bridging, [{RuH(CO)(PPri3)2}(m-H)(m-OMe){Ir(cod)}].75 Among complexes with unsaturated hydrocarbons, the ¢rst allenylidene complexes of Ru and Os (M) have been prepared from [MCl2(PPh3)3] and HCCCPh2OH, giving [MCl2(ˆCˆCˆCPh2)(PPh3)2]. The ruthenium complex is particularly versatile and can be used to make a wide range of other complexes.76 The ¢rst report of the molecular activation of C^H bonds in silyl complexes shows [RuH(SiMe3)(PMe3)3] in equilibrium with [Ru(H)2(Z2-H2C ˆ SiMe2)(PMe3)3]. 77a The ¢elds of polythienyl and polyyne chemistry are combined in the preparation from [RuCl2(dppm)2] and Bu3SnCCR of [Ru(CCR)2(dppm)2]n‡ (R ˆ 2-thienyl, -dithienyl, or -trithienyl; n ˆ 0 or 1), the electronic spectrum of the monocation indicating electron delocalisation from Ru to acetylide.77b Turning to cyclopentadienyl complexes, [Ru(CO)2(Cp)]ÿ reacts with B2Br2(NMe2)2 to give a new example of the uncommon boryl complexes, [Ru{B(NMe2)B(Br)NMe2}(CO)2(Cp)], the new ligand co-ordinating through B.78 The organometallic macrocyclic complex [Ru(L8)(CCC6H4X-4)2] (X ˆ H, F, Cl, Me or OMe) has been prepared from [Ru(L8)Cl2]‡, the alkyne, NaOMe and Zn/Hg. Oxygen converts one acetylide into CO for X ˆ H. The complexes could be oxidised to RuIII with Ce4‡, leading to a study of their LMCT bands, and their oxidation to RuIII or RuIV electrochemically.79 The range of half-sandwich compounds which may be useful synthons has been extended by the synthesis of [Ru(NCMe)2(PPh3)(Cp)]‡ (R ˆ Me, Ph or Cy) by reaction of PR3 with [Ru(NCMe)3(Cp)]‡. Thus [Ru(NCMe)2(PPh3)(Cp)]‡ reacts with allyl bromide to give [RuBr(Z3-C3H5)(PPh3)(Cp)]‡ and treatment of this with NEt3 gave [RuBr(Z2-CH2 ˆ CHCH2NEt3)(PPh3)(Cp)]‡.80 The useful synthetic reagent [Ru(NCMe)3(Cp*)]‡ can be made conveniently by reducing [RuCl2(Cp*)]n with zinc in MeCN. Further reaction with PMe3 or diphosphines results in replacement of two MeCN ligands.81 Reactivity studies of the P^H bonds in [RuCl(PHPh2)(Cp*)] include hydrolysis to [Ru(H)2(OPPh2){P(OH)Ph2}(Cp*)] and chlorination to [RuCl(PClPh2)(Cp*)].82 The reaction of [RuCl(Cp*)]4 with 3,5-R2pzH (R ˆ Me, But or Ph) in thf gave [Ru(Cp*)(R2pz)], the ¢rst Z5-pyrazolate complexes.83

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There are also useful reports on Z6 complexes. [RuCl2(C6H6)]2 reacts with [C(CHPh)3]ÿ to give two isomers of [Ru{C(CHPh)3}(C6H6)], with the tribenzylidene ligand adopting different Z4 con¢gurations.84 The synthesis and structures of [Ru(Cp)(Z6-L)]‡ (L ˆ C6H6 or trans-stilbene) are reported, starting from [RuCl2(C6H6)2] and going through [Ru(NCMe)3(Cp)]‡.85 The new bis(oxazolinylmethyl)phenylphosphine (L9) forms facial N,P,N complexes such as [RuCl2(PPh3)(L9)], but is a P,N-bidentate in [RuCl(L9)(Z6-C6H6)]‡.86 Studies on potentially bridging diphosphinites (4-Ph2POC6H4)2X (L; X ˆ CMe2, O or S) included the synthesis of [{RuCl2(Z6-p-PriC6H4Me)}2(m-L)], whose structures and reactivity were investigated.87

Polynuclear organoruthenium compounds A binuclear complex was obtained from the alkene metathesis catalyst [RuCl2(ˆCHPh)(PCy3)2] by reaction with AgO2CCF3 to give [{Ru(O2CCF3)(ˆCHPh)(PCy3)}2(m-O2CCF3)(m-H2O)], which is also active.88 Four binuclear complexes arose from renewed investigation of the reaction of [Ru3(CO)12] with ethyne to give [Ru2(C2H2)m(CO)n] (m ˆ 2, n ˆ 7; m ˆ 4, n ˆ 4, 5 or 7); in the (m ˆ 4, n ˆ 4) complex the [Ru(CO)2]2 unit is bridged by an Z1,Z4:Z1,Z4-C8H8 chain.89 KOH induces the reductive elimination of HCl from [RuCl2(CO)3(thf)], ultimately yielding [Ru3(CO)12], but using Et4NOH surprisingly gave polymeric [Ru2Cl3(CO)5]n and, in the presence of trace NaHCO3, (Et4N)Na2[{Ru(CO)2}8(m-Cl)4(m-CO3)4].90 The reaction of [Ru2S4(Cp*)2] with [W(CO)3(NCMe)3] gave the isomers [{Ru(CO)Cp*}2(m-S)2(m3-S)(WS)] and [{Ru(CO)(Cp*)(m-S)2}2W] and [Ru2W2S4(CO)6(Cp*)2], which show isomerism involving bridging sul¢de.91 [Ru3(CO)12] may be prepared quickly and conveniently by treating [RuCl2(CO)3(thf)] with KOH and CO. The probable intermediate [RuCl2H(CO)2] was trapped, for example, by conversion into [Ru(CPhˆCHPh)Cl2(CO)2]ÿ.92 Another report in similar vein covered not only [M3(CO)12] (M ˆ Ru or Os), but also [M4H4(CO)12], [M4H3(CO)12]ÿ and [Ru6C(CO)16]ÿ; these were made, under carefully controlled conditions, by the carbonylation of MCl3 or [MCl2(CO)3]2 in high boiling alcohols in the presence of Na2CO3 or K2CO3.93 Yet another newly reported synthesis of [Ru3(CO)12] uses 20 atmospheres pressure of CO reacting with RuO2
xH2O in toluene at 160  C.94 The crystal structure determination of [PMePh3]2[Ru3(CO)11] con¢rmed the IR and 13C NMR spectral indications that the anion contained two {Ru(CO)3} units bridged by CO and by {Ru(CO)4}.95 Annu. Rep. Prog. Chem., Sect. A, 2000, 96, 279^315

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Reactivity studies of the methoxynitrene complex [Ru3(CO)9(m3-CO)(m3-NOMe)] have continued with the reaction with phosphines to give [Ru3(CO)7(m3-CO)(m3-NOMe)(dppm)] and [Ru3(CO)9ÿn(m3-CO)(m3-NOMe)(PPh3)n] (n ˆ 1 or 2). Heating the (n ˆ 1) complex gave [Ru3(CO)8(m-NCO)2(CO)8(PPh3)2].96 [Ru3H(CO)11]ÿ reacts with an excess of PCy3 to give the ¢rst 44-electron triruthenium complex [Ru3H2(CO)6(PCy3)3].97 Structural studies allowed a thorough reinvestigation of the reaction of [Ru3(CO)10(NCMe)2] with ethyne; the products characterised include [Ru3(CO)10(m3-C2H2)], [Ru3(m-H)(CO)9(m3-C2H)], the pseudooctahedral (Ru4C2) complex [Ru4(CO)12(m4-C2H2)], the edge-bridged tetrahedron [Ru5(CO)15(m4-CCH2)] and [Ru6(CO)16(m4-CCH2)2], with an RuˆRu bond linking two butter£y structures.98 Earlier work on the reactions of metal carbonyls with dppmSe2 to prepare clusters containing Se and dppm has been developed by the synthesis of such new compounds as the cubane [Ru4Se4(CO)10(dppm)].99 Reactions of the boron-centred cluster [Ru4Rh2B(CO)16]ÿ with gold complexes [{AuCl}2(m-L)] [L ˆ 1,4-bis(diphenylphosphino) butane] gave dinuclear linked clusters [{Ru4Rh2B(CO)16}(m-Au2L)] with Au added across an Ru2Rh face. Cluster degradation also occurred to give [Ru4RhB(CO)14{Au(dppm)Au}], which has a square pyramidal Ru4Rh core with the [Au(dppm)Au] unit added to the opposite (Ru and Rh) corners of the Ru3Rh square base.100 [Ru(CO)5] reacts with the hexatriyne C2(CCSiMe3)2 (L) to give [Ru3L(CO)10] and [Ru4L(CO)10] with m3 and m4 bridging central alkynes respectively.101 Os3 and Os3Ru analogues were also made.102 In a systematic study of the reactivity of [Ru3IrH2(CO)12]ÿ it was found that various ER3 (PPh3, PMe3, P(OPh)3, AsPh3 or SbPh3) ligands could replace one CO and that subsequent protonation gave neutral [{M(CO)3}3(m-H)3{M0 (CO)2(ER)3}] (M or M0 ˆ Ru or Ir), which for PMe3 only gave two identi¢able isomers arising from the phosphine's coordination to Ru or Ir. 103 The thermolysis of the ethyne complex [Ru3(C2H2)(CO)10], already known to give [Ru3H(C2H)(CO)9], has now also been found to give, inter alia, [Ru5(m4-CHCHCCH2)(CO)15] (Ru4C3 pentagonal bipyramid) and [Ru6H(m4-C)(m3-CCMe)(CO)17] (based on an Ru4 butter£y), the formation of each product requiring alkyne coupling and hydrogen migration.104

3

Osmium

High oxidation states [Os2(m-O)2(m-SO4)2L4] (L ˆ monodentate ligand) is formed by displacement of four terminal sulfates from [Os2(m-O)2(m-SO4)2(SO4)4]8ÿ, but reaction of this anion with [OsBr6]4ÿ gives [Os5(m3-O)2(m-O)2(m-SO4)8(SO4)4], in which an Os atom links two dinuclear units through bridging oxygens.105 The crystal structure showed that the black product from the reaction of [OsO2(en)2]2‡ with Fe2‡ and sulfate was the [OsO2(en)2]2‡ salt of the polymeric anion [{Fe(SO4)2(H2O)}2{m-OsO2(en)2}]2ÿ, containing Fe^O^Os bridges and polymerising through Fe^SO4 chains.106 A new synthetic method for osmium oxo complexes of diols (H2L) from OsO4 involves the use of PPh3 as a reductant, forming [OsOL2].107 The synthesis, IR and Raman 288

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spectra and crystal structures of a number of new simple osmium complexes have been reported, using [Os2O2(OH)4]2ÿ as starting material; products include [OsO2X(CN)2]2ÿ (X ˆ oxalate, malonate or oxamidate),108 trans-[OsO2(CN)4]2ÿ and trans-[OsO2(CN)2(m-OR)]2 (R ˆ H or Me).109 Similar work on both isomers of [OsCl4F2]ÿ used oxidation of the dianion with KBrF4 or PbO2 in the synthesis.110 Os4‡ in 0.2% dilution in Cs2ZrBr6 shows upconversion luminescence, in which excitation in the NIR region generates luminescence in the visible. The process is known for f-block elements, but is more unusual elsewhere.111a Reaction of [OsH2(OAc)2(PPri3)2] with HBF4
OEt2 gave binuclear [{OsH2(OAc)(PPri3)2}2(m-OAc)]‡, but using HBF4
H2O gave [OsH2(OAc)(H2O)(PPri3)2]‡. The latter shows differing reactions with alkynes, HCCBut and HCCSiMe3 giving the carbyne complexes [OsH(OAc)(PPri3)2(CCH2R)]‡ (R ˆ H or But), whereas HCCPh gave the metallacyclopropane [OsH(OAc)(PPri3)2(CH2CPh)]‡.111b Linear trimers with OsˆN bonds were found in the mixed valence [Os3N2Cl8L6] (L ˆ 3-Mepy or 4-Etpy) formed in the pyrolysis of [OsNCl3L2] in methanol.111c Osmium( III) [OsX4(py)2]ÿ (X ˆ Br or Cl) have been made by the reduction of [OsX6]2ÿ with py and BH4ÿ and characterised by IR and Raman spectra and crystal structure determination.112 The crystal structure of [OsCl2F(py)3] shows the py are in the mer con¢guration with trans-£uorines.113a Other osmium(III) complexes similarly characterised by the Kiel group are mer-[OsX3(py)3] (X ˆ Cl, Br, I or NCS).113b,c Osmium( II) [OsF(py)5]‡, obtained as a minor product in the reduction of [OsF6]2ÿ with glycerine and pyridine, shows an Os^N bond length trans to F shorter by 2 pm that the other four.114 The oxidation of [OsCl(dcpe)2]‡ has further been studied to show that the known product [OsCl(Z2-O2)(dcpe)2]‡, with a very short O^O bond of 131.5 pm, is converted to the ¢rst stable osmium oxo complex [OsOCl(dcpe)2]‡, which has two unpaired electrons, by reaction with HCl and an oxygen abstractor such as PPh3.115 Reinvestigation of the complex formulated as [OsClH(PPh3)3], which would be a valuable analogue of the familiar rhodium catalyst, shows that the product from [OsCl2(PPh3)3], hydrogen and NEt3 is really [OsClH(H2)(PPh3)3], but this can exist in equilibrium with free H2 and the previously proposed complex.116 A spectroelectrochemical study using IR, UV/VIS and EPR spectroscopy showed the reversible reduction of [Os(CN)5(NO)]2ÿ occurred by the addition of an electron to an NO orbital.117 Polypyridyl complexes Thionitrosyl co-ordination chemistry has been advanced by the synthesis of [OsCl2(tpa)(NS)]‡ and [OsCl2(Tp)(NS)] from the corresponding nitrides, CS2 and azide. The former is especially versatile and can be converted into an NH3 complex Annu. Rep. Prog. Chem., Sect. A, 2000, 96, 279^315

289

by reduction, an NSO complex by reaction with Me3NO and an NPPh3 complex with PPh3.118 The crystal structure of [Os(2,20 -bipy)3]2[Cr(CN)6]Cl
8H2O and its ruthenium analogue have been reported. The complex anion partly quenches the luminescence of the cation, probably by involving the 2T2g state of the anion.119 The electronic spectra of the new mixed valence pyrazine (L) complexes [{Os(2,20 -bipy)(terpy)}2(m-L)]5‡ indicate that these are unusual in sometimes being described by delocalised and sometimes by localised charge formulations.120 The interesting new ligand L10, described as exoditopic, forms diastereomeric complexes containing two {Os(2,20 -bipy)2}2‡ units.121 The crystal structures of a series of osmium hydrazido complexes, including [OsCl2{N2(C2H4)2O}(terpy)]2‡, [OsCl2(N2Et2)(tpm)]‡ and [Os{N2(C2H4)2O}(terpy)(NCMe)2]2‡, neatly show that the Os^N bond lengthens and the Os^N^N angle deviates from linearity more as the oxidation state decreases.122 The formation of osmium(V) hydrazide complexes [Os(NR)Cl2(terpy)]‡ (RH ˆ morpholine or piperidine) produced from [OsNCl2(terpy)]‡ and HR, and their subsequent study by electrochemistry, showed many resemblances to related RuIV ˆ O complexes.123 The crystal structure of trans,trans-[{OsCl2(terpy)}2(m-N2)][BF4] shows the osmium(II) ion is structurally distinct from the osmium(III) ion, in line with the conclusion from ¢nding v(N^N) at 2007 cmÿ1. It was concluded that similar Os25‡ (m-N2) complexes had similarly distinct osmium(II) and osmium(III) centres; the electronic spectra were assigned to various IT and d^d bands.124 Reactivity studies on [OsCl2{N2(C2H4)2O}(terpy)]2‡ showed it reacts with bases such as morpholine (RH) to give substitution of R at the 40 position of terpy.125

Organo-osmium compounds A new general synthesis of cis,cis,trans-[OsCl2(CO)2L2] (L ˆ tertiary phosphine) has been achieved by starting with [Os2Cl4(CO)6]; the work is supplemented by a crystal structure determination of the PEt3 complex.126 [OsClH(CO)(PPh3)3] reacts with bis-[2-(2-pyridyl)phenyl]mercury (HgX2) to give chelating [OsCl(CO)(PPh3)2X]. The work led to many reactivity studies, including the bromination of the phenyl ring, which occurs at the position opposite Os. Crystal structures and some ruthenium analogues are also reported.127 The chemistry of the hydroxide complexes [OsH(OH)(CO)L2] (L ˆ PBut2Me or PPri3) includes their reaction with hydrogen to give [OsH(CO)(H2)L2].128 [Os(SnI3)(CO)(PPh3)2(Me2dtc)] reacts with N(C2H4OH)3 to give ¢rst a stannatrane complex [Os{Sn(OC2H4)3N}290

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CO(PPh3)2(Me2 dtc)]; the ligand includes an Sn^N bond distance of 242.2 pm.129 The methylimide group in [Os(CH2SiMe3)2(NMe)(Cp)] reacts with PPh3 to give Ph3P ˆ NMe and the osmium(III) hydride [OsH{CH(SiMe3)C6H4PPh2}] as well as the methyleneamide complex [Os(CH2SiMe3)(N ˆ CH2)(Cp)].130 A new route to methylidene complexes is provided by treating [OsH(dppm)(Cp*)] with MeO3SCF3 to give [Os(CH2)(dppm)(Cp*)]‡.131 In polynuclear organometallics the value of photosubstitution over reactions with Me3NO for inducing reactivity in [M3(CO)12] (M ˆ Os or Ru) is shown in new convenient syntheses of [M3(CO)n(PR3)12ÿn] and [M3(CO)n(NCMe)12ÿn]. A difference was found between Os and Ru in the photochemical reaction with ethene, which gives [Os(C2H4)(CO)4] but no ruthenium complex.132 The ¢rst application of time-resolved microwave conductivity in organometallics was in the photolysis of [Os3(CO)10(Me2CHN ˆ C(Me)C5H4N-2)] to give heterolytic ¢ssion of the Os^Os bond to form a zwitterion with the positive charge on the Os carrying the diimine.133 The reaction of [Os3(CO)12] with Me3NO in thf gives [{Os(CO)3}2(m-H){m-Os(CO)4}(m-MeNHCO){Os(CO)2(NMe3)}], with the methylcarboxamide bridging through C and O.134 Many products arise from the activation of 2-methylsulfanylthiophene by [Os3(CO)10(NCMe)2], involving cyclometallation and co-ordination by alkene or either sulfur, cleavage of the SMe group and its migration to the 3 position. One example is [{Os(CO)3}2(m-H){m-Os(CO)4}(m-Z2-MeSC4H2S)].135 The structurally characterised ethylene glycolate complex [{Os(CO)3}2(m-H){m-Os(CO)4}(m-OC2H4OH)] is prepared more conveniently from [Os3(m-H)(m-OH)(CO)10] by substitution of the hydroxide with glycol.136 [Os3(CO)10(PPh3)2] has been found to have two isomers, a cis,trans con¢guration in addition to the normal trans,trans structure, to which the former reverts on recrystallisation.137 On the basis of NMR spectral evidence an unusual hydrogen bond is claimed to occur between the terminal H on Os and the imine H in [Os3H(m-H)(HNˆCHMe)(CO)10], formed from acetaldehyde and [Os3(H)2(CO)10(NH3)].138 The reaction of [Os3(CO)12] with [Ir(CO)4]ÿ gave [Os3Ir(CO)13]ÿ, containing one Os^Os bridging CO. Protonation gives [Os3IrH(CO)13], with H bridging 2 Os and 3 CO bridging Ir and Os. Hydrogenation gave [Os3IrH2(CO)12]ÿ, with two Os^Os bridging hydrogens and no bridging CO.139 Using 2-sulfanyl-substituted heterocyclic bases (L) in PhHgL gave a variety of products with [Os3(CO)10(NCMe)2], including [Os3(CO)10(m-Z2-L)(m-Z2-HgL)] (HL ˆ 2-sulfanylbenzothiazole) and [{Os5(m-Z2-L)(CO)14}2(m4-Hg)] {HL ˆ HSC(NˆNPh)(ˆNNHPh)}. Some ruthenium examples are also described.140 Pyridopyrazine (L11) and 2,20 -bipy give rise to the ¢rst N-donor complexes of Os4 clusters in [Os4(m-H)4(CO)11(L11)] and [Os4(m-H)4(CO)10(2,20 -bipy)], formed from the ligands, [(Os4(m-H)4(CO)12] and Me3NO in MeCN.141

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291

4

Rhodium

High oxidation states Only one report of a mixed oxide deserves mention, the synthesis of the ¢rst rhodium(IV) perovskite compounds La2MRhO6 (M ˆ Mg or Zn) by heating Rh2O3 with metal nitrates. These have the expected low magnetic moments, attributed to both spin^orbit coupling and antiferromagnetism.142 Rhodium( III) A new form of SrRh2O4 was obtained by heating SrCO3 and Rh in air at temperatures of up to 1150  C; it contains RhO6 octahedra and trigonal prismatic Sr2‡.143 The normal spinel CuRh2O4 has been prepared from CuO and Rh2O3 at 1150  C; it shows an antiferromagnetic transition at 21.9 K.144 Treating carboxylic acids or amino acids with the superoxide complex [Rh2(O2)(OH)2(H2O)n]3‡ gave [Rh2(O2)(OH)(O2CR)4], with the amino acids conferring stability on RhIII2^superoxide units.145 A mono-Schiff base derivative of 2,6-diformyl-4-methylphenol reacts with [RhCl(PPh3)3] and dilute HCl to give a chelating acylphenoxide (Lÿ) complex [RhCl2L(PPh3)2]; similar compounds can be made by replacing HCl with HNO3 or RCO2H.146 The limited range of chalcogenophenoxide complexes of RhIII is extended by the reaction of [Rh(CO)(PMe3)Cp] with Ph2E2 (E ˆ S, Se or Te) to give [Rh(EPh)2(PMe3)Cp], allowing multinuclear NMR spectroscopic studies.147 The reaction of RhCl3 with meso-(tetramesityl)octaphenylporphyrin in PhCN gives a metalloporphyrin polymer with links through a nitrile N and the deprotonated 3 position on the phenyl ring of PhCN.148 A thorough study of rhodium(III) Tp complexes includes the syntheses and structures of various complexes [RhCl2(Tp)L] and [RhCl3Tp]ÿ.149 A new convenient synthesis of [RhCl2Ph(PPh3)2] uses the reaction of RhCl3
3H2O with SbPh3, to give [RhCl2Ph(SbPh3)3], followed by treatment with PPh3.150 Some binuclear bis(carbene) complexes show remarkable reactivity. [Rh2Cl2(m-CPh2)2(m-SbPri3)] undergoes stepwise replacement of Cl by acac on reaction with Tl(acac), but without change in the bridging system. The reaction of this product with CO results in substitution of CO for the stibine, but reaction with PPri3 gives [{Rh(PPri3)(m-CPh2)2{Rh(acac)2}] with migration of acac and generation of a mixed valence complex, carbonylation of which gave [{Rh(CO)3}(m-CPh2){Rh(acac)2}].151 Rhodium( II) The synthesis of [Rh2(OAc)4(H2O)2] from RhCl3
3H2O, HOAc and NaOAc ¢rst gives [Rh2Cl2(OAc)4]2ÿ, which has now been crystallised and found to have the expected paddle-wheel structure; the cations of the hydrated sodium salt are co-ordinated to one Cl and one OAc oxygen on adjacent dimers and to two water molecules.152 [Rh2(O2CCF3)4(thf)] exists as two isomers, one being polymeric with 292

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bridging O atoms from thf. The other is a dimer of dimers, with thf bonded to only one Rh and interdimer links through a carboxylate O binding to a neighbouring Rh. Both isomers are formed during sublimation from an equimolar mixture of the bis(thf) adduct and the uncomplexed carboxylate.153 The solid state reaction of C6Me6 with [Rh2(O2CCF3)4] gives the ¢rst arene complex of a dinuclear rhodium(II) carboxylate. The polymeric structure has opposite C 6 ring edges closer to Rh, but the aromatic system is regular.154 Unlike the paddle-wheel dimeric rhodium(II) carboxylates given by reducing RhIII, the oxidation of [RhCl(CO)2]2 by copper(II) acetate gives [{Rh(CO2Me)(CO)L}2(m-O2CMe)2] (L ˆ MeOH or PPh3) with both O,O0 -bridging and C-monodentate co-ordination.155 Further carboxylate reactivity studies showed that the reaction of [Rh2(O2CCX3)4] (X ˆ H or F) with Et3OBF4 or Me3SiO3SCF3 in RCN (L; R ˆ Me or Et) gave [Rh2L10]4‡. Oxidation with NOBF4 gave [RhL6]3‡ and electrolytic reduction gave polymeric [Rh2(BF4)3L8]n. Attempts to form a new rhodium(I) compound gave [Rh(CO)2L2]‡.156 More work on C2S42ÿ complexes has appeared. Binuclear [{RhCl(Cp*)}2(m-S4C2)], made from C2S42ÿ and [RhCl(Cp*)]2, can be reduced to [{RhCp*}2(m-S4C2)], the diamagnetism of which indicates strong coupling of the formally d7 rhodium(II) centres, but the contracted C^S bonds imply the presence of C2S44ÿ; an iridium analogue was also made.157 Low oxidation states A wide ranging survey of the 103Rh NMR spectra of diphosphine (L) complexes [Rh(acac)L] concludes that chemical shifts are much more affected by geometrical effects than electronic ones.158 The solvent dmf acts as a source of CO when RhCl3
3H2O is re£uxed in it to give [RhCl2(CO)2]ÿ. Similar behaviour is found for Ir, but RuCl3
3H2O gives [RuCl3(CO)2(dmf)]ÿ.159 The reaction of [RhR(C2H4)(triphos)] (R ˆ Me, Et or Ph) with white phosphorus in thf gave [Rh(Z1:Z2-P4R)(triphos)], the ¢rst example of converting P4 into a P^C bond using a transition metal complex.160 The Tp derivative HB(2,4-Cl2-3,5-Me2pz)3 (L) forms [RhL(CO)(PMePh2)2], with only one N atom bonded to Rh, but it also has an agostic B^H^Rh bond. 161 The ¢rst oxidative additions of [RhR(pnp)] (R ˆ Me or Ph) have been reported to give [RhRI2(pnp)] with I2 or [RhMeRI(pnp)] with MeI; for the latter pair, that with R ˆ Me underwent reductive elimination of C2H6 with TlBF4 in acetone, but that for R ˆ Ph did not.162 Mixed metal cubane complexes derived from sul¢de-bridged [M2S2(m-S)2(Et2dtc)2] (M ˆ Mo or W) include [{Rh(cod)}2{MCl(Et2dtc)}2(m3-S)4], made from a reaction with [RhCl(cod)]2; an iridium analogue was also prepared as well as compounds derived from [M0 Cl(PPh3)3] (M0 ˆ Rh or Ir).163 The ¢rst linkage isomers of ketene, Ph2CˆCˆO (L), in complexes have been found in the rhodium(I) indenyls [Rh(C9H7)(PPri3)L], one isomer using co-ordination through C ˆ C and the other through C and O.164 Use of the diphosphine ligand MeN(PF2)2 (L) allowed a photochemical investigation of the reactions of the rhodium(II) dimer [Rh2Br4L] with other phosphines (L0 ). These ¢rst gave [Rh2Br2L3L0 ] which contains Rh0 and RhII not RhI. Further reaction with L0 gave [Rh2L3L0 2].165 Annu. Rep. Prog. Chem., Sect. A, 2000, 96, 279^315

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5

Iridium

Two useful reviews describe iridium chemistry. One discusses tetra£uorobenzobarrelene complexes166 and the other oligomeric complexes with N^C^X ligands (X ˆ N, O or S), such as 2-pyridonate.167 Both include examples from rhodium chemistry. Iridium( V) A new synthesis of [IrF6]ÿ salts in anhydrous HF uses the reaction of AgF, or the Ag‡ salt of a complex £uoroanion, with a high oxidation state £uoride or a £uoro-complex.10 The iridium(V) organometallics [IrMe3X(Cp*)] (X ˆ Cl or O3SCF3) have been made from HX and [IrMe4Cp*]; treatment with phosphines gave such complexes as [IrMe3(PMe3)Cp*]‡, while an excess of phosphine gave loss of MeCp* and formation of, for example, [IrMe2(dppm)2]‡.168 Iridium( III) New CuIr2Te4, made from the elements at 950  C, is believed to have a defect NiAs structure; its magnetism shows a maximum susceptibility and electrical resistivity around 190 K, thereby differing from other CuIr2X4 compounds.169 The study of some hydride complexes was assisted by the use of para-hydrogen to assist assignments and mechanistic studies. The molecules studied included [IrCl(H)2(PPh3)2], now shown to have inequivalent hydrides, and the reaction products of hydrogen with [IrCl(AsPh3)3].170 The few known anionic iridium hydride complexes have been augmented by [IrX3H(PPri3)2]ÿ formed in the reaction of [IrX(H)2(PPri3)2] (X ˆ Cl or I) with Xÿ.171 The chemistry of tellurium(IV) ligands has been extended by the reaction of Vaska's compound with TeCl4 to give [IrCl2(TeCl3)(CO)(PPh3)2] and the synthesis of ruthenium complex.172a The lability of the two hydride ligands in the diphosphinoalkyl complex [Ir(H)2{CH(C2H4PBut2)2}] allows reaction with CO2 to give an isolable bidentate bicarbonate complex; a formate intermediate was found by NMR spectroscopy.172b A new improved synthesis of [Ir(terpy)2]3‡ and its analogues uses the reaction of IrCl3 with the ligand in glycol at 160  C, followed by heating under re£ux with more ligand, all this in the dark.173 The reaction of [IrCl(cod)]2 with Ph2PC5H4N, CdI2 and CO gave a donor ^ acceptor Ir!Cd complex with bridging Ph2PC5H4N, but reaction of [IrCl(CO)(Ph2PC5H4N)2] with HgCl2 resulted in oxidative addition to [{IrCl2(CO)(m-Ph2PC5H4N)2HgCl].174 [Ir2Cl2(L12)4] is the ¢rst paddle wheel/

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Chinese lantern structure for IrIII.175 Protonation of [Ir(CN)3Cp*]ÿ in aqueous acetone gives [H3O][Ir(CN)3Cp*]. Me2CO, in which 14 Ð diameter channels contain exchangeable solvent molecules.176 The reactive hydroxide organoiridium complex [IrPh(OH)(PMe3)Cp*], already known to form hydroxyalkyls with alkenes, forms carboxamide complexes [IrPh(NHCOR)(PMe3)Cp*] (R ˆ Me or aryl) with RCN in the presence of catalytic [IrPh(O3SCF3)(PMe3)Cp*]; other products prepared include [IrPh(CH2CN)(PMe3)Cp*] and [IrPh(PMe3){C5Me4CH2C(C6H4CF3)NH}]‡.177 The value of NOE effects in elucidating metalloborane NMR spectra was shown clearly in a study of [Ir(Cp*)(Ph2C2B9H9)].178 New iridium(III) complexes containing two cyclometallated 2-phenylpyridines and a bidentate terpy represent a new class of luminescent complex whose photophysical properties can be tuned by substituents in the 40 position of terpy, which may also be used as the basis for more synthetic chemistry.179 Iridium( II) Chloride abstraction from [IrCl(H)2(PBut2Ph)2] gave [Ir(H)2(PBut2Ph)2] with two agostic interactions from But groups, but there are no such interactions in the PPhPri2 analogue.180 [Ir2Cl2(CO)2(m-OAc)2] has been prepared from H2IrCl6 and LiOAc under oxygen; it forms complexes with two additional axial monodentate ligands NCMe, py, 4-Pripy or dmso.181 A dazzling series of reactions begins with methyl C^H bond cleavage in the reaction of [Ir2(CO)3(dppm)2] with CF3SO3Me to give [Ir2H(m-CH2)(CO)3(dppm)2]‡. One CO can be removed from this with Me3NO, giving [Ir2Me(CO)(m-CO)(dppm)2]‡, and various bases L give more C^H cleavage to form [Ir2H(m-CH2)(CO)2(dppm)2]‡.182 Low oxidation states Use of water-soluble phosphines (L) allowed the synthesis of many cationic iridium(I) complexes including [Ir(CO)L3]‡, [Ir(CO)2L3]‡ and trans-[Ir(CO)(H2O)L2]‡.183 Complexes made from the new triphosphine MeC{CH2P(C6H4CF3-3)2}3 include [IrLL0 2]‡ [L0 2 ˆ (CO)2 or cod]; IrI is more stable in these than in triphos complexes.184 A wide range of Vaska compounds [IrX(CO)(ER3)2] (E ˆ P, As or Sb) has been obtained from [IrX2(CO)2]ÿ (X ˆ Cl or Br) and ER3 in CO; this is considered a better method than starting with [IrCl(cod)]2.185 Iridium was found to follow Rh in forming an Ir0: IrII mixed valence complex [Ir2Cl2L3] {L ˆ NMe[P(OCH2CF3)2]} on reaction of [Ir2Cl2(cod)2] with the newly synthesized ligand. The structure contains two bridging L and one chelating, giving trigonal bipyramidal Ir0 and square pyramidal IrII. Preliminary chemical studies suggest this undergoes oxidative additions to IrI, IrIII complexes.186 The homoleptic complexes [Ir(dppf)2]n‡ (n ˆ ^ 1 or 0) have been obtained from the monocation by reduction with sodium dihydronaphthalide, giving the monoanion, comproportionation of which with the monocation gave the uncharged molecule. Heating a CH2Cl2 solution of the monocation gave [IrIIIH(dppf-H)(dppf)]‡.187 An improved synthesis of [Ir9(CO)20]3ÿ uses the reaction of [Ir4(CO)12] with K2CO3 Annu. Rep. Prog. Chem., Sect. A, 2000, 96, 279^315

295

in methanol. The product can be converted through [Ir10(CO)21]2ÿ into [Ir11(CO)23]3ÿ. This new cluster has three octahedra sharing a common edge and two faces with neighbours, the structure following the pattern of Cs11O3.188

6

Palladium

High oxidation states The silver salt of [PdF6]ÿ has been formed from AgF or the silver salt of a £uoroanion and a high oxidation sate £uoride or £uoro complex.10 The ¢rst alkaline earth metal palladates(IV), M4PdO6 (M ˆ Ca or Sr), have chains of face-shared PdO6 octahedra.189 Palladium( II) Reviews on palladium(II) chemistry include one on the formation of its polyamine complexes in aqueous solution.190 A volume of reviews on organopalladium chemistry puts special emphasis on the catalysis of C^C bond forming processes known as the Heck reaction.191 [PdCl4]2ÿ regioselectively cleaves peptide amide bonds involving histidine carbonyls, but [Pd(H2O)4]2‡ cleaves this and the amino group of histidine units.192 The crystal structure determination of CsPdCl 3 shows this contains [Pd2Cl6]2ÿ, the ¢rst example to contain a metallic cation.193 A tetranuclear palladium(II) anion, [Pd4Cl12]4ÿ, previously prepared by hydrothermal synthesis from PdCl2 and TlCl as Tl4Pd3Cl10, is shown crystallographically to comprise two [PdCl4]2ÿ ions cis-bridged by two PdCl2 units.194 The belief that square planar complexes cannot be chiral is challenged by the crystal structure determination of trans-[PdCl2(2-Mepy)2]. In discussion the authors comment that many earlier structure determinations were in fact of chiral complexes, though this feature was not recognised by the scientists involved.195 A spin-labelled phosphine such as L13 forms normal complexes [PdCl2L2] or [Pd(C3H5)ClL], but the electron spin allows EPR spectroscopy to be applied to this area of chemistry; [RuCl2L(p-PriC6H4Me)] was also studied.196 Treating [PdCl2(NCPh)2] with PPh2H in toluene gave the elusive [PdCl2(PPh2H)2], but suspension of this in ethanol gave the known [{PdCl(PPh2H)}2 (m-PPh2)2] and further treatment with the phosphine gave [Pd(PPh2H)4].197 A reinvestigation of the effects of added halide ions on the electrochemistry of [PdX2(PPh3)2] (X ˆ Cl, Br or I) shows these result in the formation of [PdX3(PPh3)]ÿ.198 Tetrahedral PdII is found in [PdCl2(dppfO2)], ¢rst isolated as a product from COS and [Pd2(dppf)2(m-S)2], but better prepared from [PdCl2(NCMe)2] and dppfO2; its room temperature magnetic moment of 2.48 mB is curious.199 Various alkynes RCCH insert into the Pd^P bond of [PdCl2(dppm)] to give complexes of alkenyl phosphorus ylides such as L14.200 The crystal structure determination con¢rmed the cross-bridged macrocycle L15 must be folded to give the trigonal bipyramidal complex [PdClL15]Cl.201 296

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Turning to compounds of PdII with oxoanions, the reaction of metallic Pd with iodine in dmf is reported to give [Pd2I6(dmf)]2ÿ, but no structure was proposed.202 Having been neglected since its ¢rst report in 1975, the reactivity of [{Pd(Me4en)}2(m-OH)2]2‡ has been studied by exploiting the deprotonation of weak acids HX to give a large number of monunuclear [PdX(Me4en)]‡ and some binuclear complexes such as [{Pd(Me4en)}2(m-HNPh)(m-OH)]2‡.203 An improved preparation of [Pd3(OAc)6] involves treating the precipitate from the reaction between Na2PdCl4 and NaOH with glacial HOAc.204 During studies of the catalytic activity of this compound, IR and Raman spectra con¢rmed its trinuclear character in acetic acid, but suggested that addition of KOAc gave some binuclear species. 205 The crystal structure of [Pd(NO3)2(dppm)], prepared as a useful source of [Pd(dppm)]2‡ by the reaction of the dichloride complex with AgNO3, shows the nitrates are monodentate O-donors.206 Among studies of sulfur donors, SP(SSiMe3)3 was found to be a good reagent for making thiophosphate complexes, as shown by its reaction with PdCl2 phosphine complexes under various conditions to give products such as [{PdCl(PEt3)}2(m-S4P)] or the cyclic [Pd6(S4P)4(PPh3)6].207 Treating [PdCl2(NCPh)2] with [Nb2(m-S2)2(Et2dtc)4] gave ligand transfer, forming new [Pd3Cl2(Et2dtc)4].208 The synthesis of [Pd(SC6F5)2(dppm)] from the dichloride complex and Pb(SC6F5)2 is described as well as its subsequent reaction with more [PdCl2(dppm)] to give [{Pd(dppm)}2(m-SC6F5)2]2‡. The study includes other EAr (E ˆ S, Se or Te), other diphosphines and some platinum examples.209 The bis complex of L16 with Pd2‡ has the cis con¢guration and p^p stacking, both thought to account for very high single harmonic generation (SHG) effects.210 Many new dithiolene complexes have been reported; these have intense 1010 nm absorptions, leading to speculation on their suitability as switching for an Nd/YAG laser. 211 The solvatothermal reaction of Pd2‡ with Se2ÿ, Se and Rb‡ gives Rb2[Pd(Se4) 2]
Se8, which is unlike any other polyselenopalladate salt in having the Se8 ring round the Rb‡, but with long Rb^Se distances of 400 pm.212 The ¢rst well de¢ned palladium(II) azide is formed from Pd2‡ and NaN3 in water. Despite its detonation at 117  C, its conversion into trans-[Pd(N3)2(py)2] and [Pd(NH3)4][Pd(N3)4] was studied.213 A striking structure was found when three Annu. Rep. Prog. Chem., Sect. A, 2000, 96, 279^315

297

trans-[Pt(NH3)2] units were linked by three [Pd(en)(L)] (L ˆ 2,20 -bipyrazine) units to give triangular arrays with perchlorates inside and ¢tting together in leaning towers.214 The use of L17 allowed the synthesis of the cage structure [Pd18(NO3)36(L17)8] comprising a trigonal bipyramidal structure with two Pd along each edge.215 Studies of palladium(II) complexes of NH2C2H4SEt (L), [PdCl2L] and [PdL2]2‡, together with some platinum examples, indicate that L forms bidentate complexes, this ¢nding possibly being relevant to the role of methionine in the metabolism of platinum anticancer drugs.216 Re£uxing palladium(II) acetate with 1-(2-pyridyl)indole and related compounds (HL) in MeCN unexpectedly gave nitro cyclopalladate complexes [Pd(NO2)L(NCMe)].217

The tridentate binding of Ph2PC2H4PPhC2H4PPh2 (L) in [PdClL]‡ can be transformed into bidentate binding in [PdL2]2‡, while H2O2 oxidation gave [Pd(L^O)2]2‡ with the pendant P oxidised.218 Using isocyanides RNC with co-ordinating atoms included in the R group allowed the formation of co-ordination polymers, such as one involving trans-[PdI2(CNC5H4N-3)2] and [Cu(acac)2], while other R groups gave square Pd8 or Pd4Pt4 complexes.219 Low oxidation states Pyridine reacts with the phosphido-bridged palladium(I) complex [Pd2(PBut2)(PPh3)3]‡ to give [Pd2(m-PBut2)(py)(PPh3)3]‡ and [Pd2(m-PBut2)(py)2(PPh3)2]‡. The lability of pyridines allows the formation of the doubly unsaturated complex [{Pd(PPh3)}2(m-PBut2)].220 In studies of Pd0, the product of the reaction of palladium(II) acetate with L18 is an uncharacterised palladium(0) species, but adding PEt3 gave cyclic [{Pd(PEt3)}3(m-L18)3].221 Reaction of SO2 and P(CH2Ph)3 (L) with [Pd2(dba)3] gave [(PdL)2298

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(m-SO2)(m-dba)]. The surprisingly retained, but normally labile, dba allows further reaction with COS, giving [(PdL)4(m3-S)(m-CO)(m-SO2)],222 while its substitution by dppm gave complexes with shorter Pd^Pd bonds, implying formation of PdI and [SO2]2ÿ. Reaction with more L gave new [Pd(SO2)L2] with pyramidal SO2 in the ¢rst crystal structure of a Pd^SO2 complex. Reaction with CO gave [Pd3(m-CO)2(m-SO2)L3], with short Pd^Pd bonds averaging 270 pm and said to be in accord with a 42-electron count.223 In related work the rather confused picture of the nature of the products resulting from the reaction of [Pd2(dba)3] with phosphines (L) is partly clari¢ed by the synthesis and characterisation of many complexes [Pd(dba)L2].224

The ¢rst palladium(0) dicarbonyl complexes [Pd(CO)2L] (L ˆ diphosphine) have been prepared from [Pd(C2H4)L] and CO. These tetrahedral compounds lose CO in solution, forming [(PdL)2(m-CO)].225 The substitution of NMe for CH2 in dppm gives a change in redox properties on the formation of [CoPd2(CO)2(Ph2PNMePPh2)], with any charged species becoming unstable.226 The synthesis of new M2PdH4 (M ˆ Ba or Sr) zerovalent palladium hydrides is achieved using MH2 and Pd under hydrogen. Their structures can be considered either as interstitial or as salts of tetrahedral [PdH4]2ÿ.227 Encapsulation of nanoparticles of 40 or 60 Pd or Pt atoms into polyamidoamine dendrimers gives good hydrogenation catalysts, the best results being obtained for N-isopropylacrylamide on Pd40 in a fourth generation dendrimer.228

7

Platinum

Platinum( V) and (IV) The silver salt of [PtF6]ÿ has been formed from AgF or the silver salt of a £uoroanion and a high oxidation sate £uoride or £uoro complex, while LiPtF6 has been prepared from Li2PtF6 and £uorine in HF.10 [Pt(OAc)Me3(dppe)], prepared from [PtIMe3(dppe)] and AgOAc, loses MeOAc on thermolysis, the ¢rst high yield C^O reductive elimination from PtIV.229 Crystal structure determinations for [PtL2(NH2CH2CMe2CH2NH2)] (H2L ˆ CH2ˆCHCH2CR(CO2H)2; R ˆ H or CH2ˆCH) show that for R ˆ H, L co-ordinates through two carboxylates, but for R ˆ CH2ˆCH, L co-ordinates through one carboxylate and one alkene. The co-ordination mode in solution depends on solvent and temperature.230 It is surprising that no platinum complexes of MeC(NH2)3 (L) have been reported, but octahedral [PtL2]4‡ can be made from L and K2PtCl6 with catalytic K2PtCl4. Annu. Rep. Prog. Chem., Sect. A, 2000, 96, 279^315

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Electrochemical reduction gave the dication, which, in the protonated complex [Pt(HL)2][ZnCl4]2, has two pendant nitrogens.231a The use of [Pt(OH)4L] (L ˆ diamine) as a synthon gave a better yield of [Pt(OAc)4L] than an earlier route using [Pt(OAc)2L]. The 1,2-diaminocyclohexane complex has slightly longer equatorial Pt^O bonds than the axial ones.231b Contrary to a 1998 report, [(PtMe3)4F4] can be made from the iodide and AgF; controlled hydrolysis gave [(PtMe3)4Fn(OH)4ÿn] (n ˆ 0 to 3).231c Intermediate oxidation states A mixed valence 1,2-diaminobenzene (L) complex of formula PtI3L was prepared from [PtCl4]2ÿ, iodide and L. Initially [PtI2L] is formed but oxidation by iodine gave the ¢nal product with a structure containing [PtIII2L] and [PtIVI4L] units.232 Oxidising [{Pt(C6F5)2}2(m-PPh2)2]2ÿ with Ag‡ gave the neutral platinum(III) complex, with the Pt^Pt separation decreased from 362.1 to 272.5 pm, indicating a single Pt^Pt bond.233 [PtI(S2CMe)4] has a polymeric structure with linear Pt^Pt^I chains and additional S^S contacts. It is metallic above 300 K with charge delocalisation; below this temperature it is a semiconductor with alternate platinum(II) and platinum(III) centres and at 80 K it becomes insulating with ordering of the type PtIII, PtIII, PtII, PtII. The paper gives a good review of the general area.234 Platinum( II) The complex photochemistry of [PtCl(terpy)]‡ is extended by the synthesis of salts of [PtCl(40 -Phterpy)]‡, whose solid state emission shows transitions from a 3MMLCT and mixed 3MLCT/3IL states.235 The familiar ligand PMePh2 has been modi¢ed to PPh2CH2C6H3(OMe)2-2,5. This forms usual bis complexes with platinum(II) halides, but removal of the protecting methyls with BBr3 and treatment with base produced a phosphinohydroquinonate complex, with interesting electrochemical properties.236 SO can be formed during the oxidation of various dialkyl sulfoxides with H2O2 and can be trapped as the complex [{PtX}2(m-dppm)2(m-SO)] (X ˆ Cl or I), but a better synthesis is the H2O2 oxidation of [{PtX}2(m-dppm)2(m-S)].237 A kinetic study of the exchange of ethene with [PtCl3(C2H4)]ÿ showed that free C2H4 attacked the site trans to the co-ordinated ethene, and that this site might be occupied by chloride or a solvent molecule.238 Studies of halide complexes which shed light on biological or medicinal chemistry will be described later. Platinum(II) acetate and [S2P(OEt)2]ÿ (Lÿ) give [Pt4(OAc)4L4] as two isomers; both have Pt4 squares with bridging acetate, but one isomer has chelating L and the other has bridging L, but with similar Pt^Pt distances.239 Ascorbate forms platinum(II) complexes as a bidentate ligand, co-ordinating through two oxygens when the co-ligand is a diphosphine, but through C and O with diamines.240 The use of a ¢lled dz2 orbital allows square planar PtII to act as a ligand to Ag‡ as shown by the synthesis of the two-dimensional polymer [Pt(acac)2][Ag2(O3SCF3)4][Ag(H2O)3]2, in which the aqua-complex bridges the paddle-wheel 300

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tri£ate and the O-bonded platinum complex. Another example is provided by three-dimensional Ag2[Pt(ox)2]
2H2O, prepared from aqueous K2[Pt(ox)2] and [Ag(py)2][O3SCF3], which contains bridging oxalates and Pt^Ag bonds.241 A new reaction of the platinum(II) salicylaldoxime complex [PtCl2(HOC6H4CHˆNOH)(dmso)] with chlorine gave inter alia chlorination of the benzene ring to yield [PtCl3{OC6H2Cl2CH ˆ NOH}(dmso)] with Cl o and p to the phenolate O.242 A new synthesis of phosphonate PO(OMe)2ÿ complexes was achieved by the reaction of [PtCl2L] (L ˆ dppm, dppe or dpp) with P(OMe)3, giving [Pt{PO(OMe)2}L]. Addition of TlPF6 suppresses demethylation of the phosphite to give such complexes as [Pt{PO(OMe)2}(dppe){P(OMe)3}]‡.243 Synthesis of the sodium salt of tellurocarboxylic acids RCOTeH (R ˆ Et or Ph) and of Me3SiOC(Te)R stimulated the synthesis of the ¢rst tellurocarboxylate complexes by allowing either class of compounds to react with [MCl2(PR0 3)2] (M ˆ Pd or Pt; R0 ˆ Et or Ph).244 Co-ordination of various potentially ambidentate (S,O) ligands to {Pt(dien)}2‡ units showed these were all bonded through S, suggesting further the role of biological sulfur species in the metabolism of cisplatin.245 Sulfur inserts an S atom in the Pt^H bonds of some [PtH(R)(dcpe)] (R ˆ alkyl or aryl), giving [PtR(SH)(dcpe)]; SO2 inserts into [PtMe2(dcpe)2] to give the S-bonded sul¢nate [PtMe(SO2Me)(dcpe)].246 [PtCl2(NCMe)2] reacts with Ph2SˆNH to give [PtCl(Ph2SNH)(Ph2SNCMeNH)]‡, wherein the sul¢mide has added to MeCN and gives the ¢rst S-co-ordinating sul¢mide; this reaction does not occur for Pd.247 A review describes the structures and reactions of compounds containing the {Pt2S2} core, along with some palladium and nickel analogues.248 Turning to nitrogen donors, a thorough survey reviews the structures and 1H, 13C and 195Pt NMR spectra of [PtX2(Mepy)2] with special reference to isomerism, the trans in£uence and other structural effects.249 The limited co-ordination chemistry of naphthyridines has been extended to include substitution at the positions adjacent to nitrogen, thus allowing considerable variation in structure, and giving complexes containing up to three Pt atoms with bridging by the ligand N atoms; some palladium examples were also made.250 Salts of the new platinum blue [Pt(NHCOMe)(2,20 -bipy)]‡ were prepared from [PtCl2(2,20 -bipy)], Ag‡ and MeCONH2; the tri£ate shows a dimer of dimer cations with a Pt^Pt separation of 291 pm within the dimer and 321 pm between them.251 An unusual ligand centred reaction occurred when AgNO2 reacted with [PtL2] (HL ˆ PhNˆC6H4ˆNH). A deprotonated N in one L adds an H and the MPh of the other adds an NO.252 Unlike [Pt(CN)2(2,20 -bipy)], the emission from the stacked structure of [Pt(CN)2L] (L ˆ 3,30 -biisoquinoline) shows a shift to lower wavelength at low temperature, attributed to the emitting state being 3MLCT for the former complex and a 3p^p* for the latter.253 The reaction of [NBu4]2[Pt(CN)4] with Cu(NO3)2
52H2O and 2,20 -bipy gave [{Cu(bipy)(H2O)Pt(CN)4}2]
2H2O, which crystallised from NH4OH on allowing NH3 to evaporate; the structure has Cu2Pt2 squares bridged by CN with the particular con¢guration stabilised by hydrogen bonds from water molecules.254 Studies of the electronic spectra of cyanometalate-bridged complexes lead to the conclusion that such complexes as [{Fe(dppe)Cp}2{m-Pt(CN)4}] show metal^metal interactions across trans-Pt con¢gured bridges, but not cis ones.255 Attempts to Annu. Rep. Prog. Chem., Sect. A, 2000, 96, 279^315

301

make a metallalactone complex using the reaction of Ag2O with CNCH2CO2H and cis-[PtCl2(PPh3)2] gave instead a rare example of a cyanomethyl, cis-[Pt(CH2CN)2(PPh3)2].256 The unusual CP (phosphalkyne) ligand was further investigated in the improved synthesis of [{PtCl(PEt3)2}(m-CP){Pt(PEt3)2}], in which the chlorinated Pt is co-ordinated by C and the other by the phosphaalkyne bond. This complex is made from [PtCl(PEt3){CClˆPN(SiMe3)2}] and sodium^benzophenone, and can be used to make other complexes through co-ordination of the phosphorus lone pair to [PtCl2(PEt3)] or [W(CO)5] units, while treatment with MeI gives [{PtI(PEt3)2(m-CˆPMe){PtCl(PEt3)}].257 The ¢rst structural characterisation is reported of a cyclic bis(silyl) platinum complex in the reaction of [Pt(C2H4)(PPh3)2]. with 1,2-B10C2H10(SiHMe2)2 to give [Pt{SiMe2)2B10C2H10}(PPh3)2]. Reactivity studies with unsaturated organic molecules afforded new heterocycles.258 It is unusual to ¢nd a hydridometal alkyl which is stable to air and water but one such is [PtH2Me(Tp)], formed from water and [Pt(Me)Tp(CO)].259 Studies of various complexes [MHLn]‡ (M ˆ Pt or Ni, L ˆ phosphine or diphosphine) including crystal structures and hydride reactivity, concluded that [PtH(dmpe)2]‡ is the best hydride donor of those studied, being a 5d-metal complex of a small-bite chelating ligand.260

Biological and medicinal platinum chemistry An important new book on cisplatin has been published,261 the editor of which has also written a review on platinum complexes which emphasises those with nucleobases.262 Seven new platinum(IV) dipeptide complexes are claimed to be the ¢rst of this type and are readily characterised by 195Pt NMR spectroscopy.263 The functionalisation of cyclopeptides using metal atoms was achieved by the synthesis of cyclo-[Gly-L-CysPt(terpy)]nCln (n ˆ 3 or 4), leading to possible interesting metal and organic chemistry.264 Protonation of the 1-methyluracilate (L19) complex trans-[Pt(L19)2(NH3)2] to form trans-[Pt(HL19)2(NH3)2]2‡ gives greatly enhanced water solubility. This property is employed in reactions with metal salts to give trans[{Pt(L19)2(NH3)2}Ag3]3‡ and trans-[{Pt(L19)2(NH3)2}2AgNa(H2 O)4]2‡; the latter is claimed to be the ¢rst nucleobase complex of three different metal ions.265 The tetranuclear [Pt(L)(en)]4 (L ˆ uracilate) complex can adopt a cone conformation in which exocyclic oxygen atoms can co-ordinate other metal ions at the cone apex. Anions such as [Me3SiC3H6SO3]ÿ are held in the calixarene-type cone for the complexes of Be and Zn.266

302

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There have been many studies involving GMP. [PtI2(OAc)2(en)] reacts with GMP stereospeci¢cally under visible light illumination with replacement of the two iodides by nucleotides and reduction by removal of the acetates, to give [Pt(GMP) 2(en)].267 The reaction of various cytotoxic platinum(IV) compounds with GMP led to the conclusion that the ¢rst step is the very slow formation of a complex containing oxidised GMP, after which further reaction with more GMP gives a platinum(II) complex. The acceleration of the process by a small amount of cis-[PtCl2(NH3)2] is attributed to its involvement in the slow ¢rst step.268 A new study on substitutions of the anticancer compound trans-[PtCl2(NH3)(C9H7N)] (C9H7N ˆ quinoline) included the ¢rst replacement of one chloride by nitrate, giving greater reactivity. This includes co-ordination of methionine S to yield a compound which reacted rapidly with GMP, hinting at the formation of cross links in DNA binding.269 cis-[PtCl2(NH3)(NH2C6H11)], the probable metabolite of the antitumor drug [PtCl2(OAc)2(NH3)(NH2C6H11)], was reacted with GMP in a kinetic study using 15 N NMR spectroscopy, which showed that aquation occurred before substitution of chloride and that the reaction was faster at the chloride trans to the amine.270 9-Ethylguanine (L) forms a platinum(II) complex through the N(7) atom, as expected, to form trans-[Pt(NH2Me)2L(7,9-Me2guanine)]; this complex forms hydrogen bonds to 1-methylcytosine in solution in the manner of DNA base pairing.271 The ¢rst study of the reactions of platinum(0) complexes with nucleobases showed that [Pt(PPh3)4] reacted with 1-methylthymine (HL) to give [PtL(OH)(PPh3)2], with N(3) co-ordination, but in the presence of KCl, chloride replaced hydroxide.272 Studies continue of the intercalation of DNA with various pyridyl platinum(II) complexes. UV and CD spectroscopy, used with melting point and viscosity methods, showed the furthest extending aromatic planes of (quaterpyridine)platinum(II) formed the strongest complexes and that the electronic effect of methyl substituents assisted the interaction provided the methyls did not sterically clash with the DNA structure.273 A combination of crystallographic studies on cis- and trans- [PtCl2(NH3)L] (L ˆ planar cyclic N-donor), including experiments on the unwinding of DNA by these complexes, led to an assessment of the varied roles of speci¢c co-ordination, ring^ring interactions and electrostatic effects on their cytotoxicity.274 Extensive cytotoxicity studies on platinum(II) terpy complexes showed that many were active, typical examples being [Pt(terpy)(4-Me2Npy)]2‡ and [Pt{(40 -BrC6H4)terpy}(4-Mepy)]2‡;275a many of these compounds were more effective than carboplatin for ovarian cancer cytotoxicity.275b Using the tetradentate ligand [C2H4N(C3H6NH2)2]2 to bind four trans[PtCl(NH3)2]‡ moieties gave a complex with enhanced cytotoxicity attributed to its potential in forming many interstrand cross links.276 Studying the reactions of [PtI2(en)] and [PtI2(OH)2(en)] with human albumin showed very different behaviour from the analogous chlorides, indicating that the biological chemistry of platinum complexes is much more varied than sometimes supposed.277

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303

Low oxidation states Organoplatinum chemistry of Pt0 includes many studies of polynuclear and heterometallic compounds. Reaction of [Fe2(CO)6(m-SeCHˆC(CH2OH)Se)] with [Pt(PPh3)4] gave a low yield of [{Pt(PPh3)(CO)}2{m-Fe(CO)3}(m-Se)], the ¢rst FePt2Se cluster.278 A PtRu2 complex has been obtained by a new easier method using [Pt(dppm)2]2‡ and [RuH(CO)4]ÿ to give [PtRu2(CO)5(m-CO)(m-dppm)2] via [PtRuH(CO)3(m-dppm)2]; chloride substitution for CO gave [PtRuH(Cl)CO(m-CO)(m-dppm)2] and [PtRuCl2(CO)2(m-dppm)2].279 The condensation of [Pt19(CO)22]4ÿ into [Pt38(CO)44]2ÿ using NOBF4 is found to involve NO complexes, and the structure of [Pt19(CO)21(NO)]3ÿ has been determined.280 Crystallographic studies of [PtCu2(CCPh)4] show this compound can adopt at least three polymorphs. The most singular of these comprises a near linear trimer of trans-Pt2Cu4 octahedra with inter-octahedral Pt^Pt distances of 300 pm. The emission spectra are different for each polymorph and are concentration dependent in solution.281 EXAFS shows that [Pt309O30L36] (L ˆ sulfonated phen) has cubic close packed Pt with a 1% contraction in the cluster compared to the pure metal.282 The reduction of [Pt(Me)2(cod)] by hydrogen in supercritical CO2 at 80  C is said to offer a promising new route to platinum ¢lms at lower temperatures than CVD processes, particularly for the metallisation of porous solids.283 The reductive carbonylation of H2PtCl6 in the molecular sieve FSM-16 leads to [Pt5(CO)15]2ÿ and, for the ¢rst time in mesoporous media, [Pt6(CO)18]2ÿ, according to IR and UV spectra. Heating above 200  C gives platinum particles, but exposing the H2PtCl6 to an PriOH/H2O atmosphere and UV light gives nanowires with very high catalytic activity for the water gas shift reaction. 284 The derivatisation of metal nanoparticles, already known as a means of organising gold thiol structures, has successfully been applied to Pt by using dodecyl isocyanide to give nanoparticles of about 150 Pt atoms.285

8

Silver

High oxidation states Further to the syntheses of silver salts of high oxidation state metal ions mentioned earlier,10 treating Ag‡ with AgF3 in anhydrous HF gave red-brown AgIAgIIIF4. This diamagnetic compound then decomposes to paramagnetic AgF2. Some [AgF]‡ chemistry was also studied, ¢nding that [AgF][AsF6] disproportionated to Ag[AsF6] and [AgIIF]2[AgIIIF4][AsF6], which could also be prepared by another route.286 A kinetic study of amino acid oxidations by silver(III) periodate complexes showed [Ag(O6IH2)(H2O)2] was the best oxidant.287 Silver(I) A simple preparation of uniform nanoparticulate Ag2Se was achieved by the reaction of silver nitrate, Se and KBH4 in pyridine, the product being free from the B-containing impurities often generated in tetrahydroborate reductions.288 The ¢rst 304

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mixed AgCu oxide, Ag2Cu2O3, containing planar CuO4 and linear AgO2 units with tunnels which might facilitate diffusion of oxygen species, was prepared by adding NaOH to aqueous copper and silver nitrates followed by heating at 90  C.289 Silver(I) co-ordination chemistry produces a stream of unusual structures, and only a selection can be presented here. An improved synthesis of [Ag3(O3SCF3)3(PR3)3] involves the addition of PR3 to a diethyl ether solution of silver tri£ate; the crystal structure of the PPh3 complex shows the anion adopting three different co-ordination modes, m-Z1, m-Z2 and m3-Z3.290 The double salts AgSCN
2AgNO3 and AgSCN
AgClO4 show a new co-ordination mode for thiocyanate, the S being bound to three silvers and the N to two.291 Tridentate di-4-pyridyl sul¢de (L) gives [Ag(O3N)3L2]
2H2O which, on reaction with methanolic AgNO3, gives a structure with a wide range of co-ordination types and intermetallic and interligand interactions in a double stranded polymeric structure.292 Sebaconitrile, NCC10H20CN (L), forms particularly attractive polymeric complexes, [AgL2]‡, which are two- or three-dimensional with interweaving chains for the PF6ÿ and SbF6ÿ salts; [AgL]‡ is a chain polymer.293 Hexamethylenetetramine forms complexes with silver sulfate and acetate which have square and hexagonal cavities in their structures, while earlier reports on other complexes of silver salts with this ligand gave different networks.294 In studies of aerosolassisted chemical vapour deposition of silver ¢lms using diketonate derivatives the best result was obtained from [Ag{OC(CF3)CHC(CF3)ˆNC6H13}(PPh3)], the ligand being the ketoimine derived from hexa£uoropentanedione and n-hexylamine.295 New Ag4 clusters have been prepared using 6,60 -(Ph2P)bipy (L). These include [Ag4X2L2L0 2] (X ˆ Br or I; L0 ˆ NCMe or dmso) and [Ag4L2]4‡ which have a diamond-shaped core with luminescent properties sensitive to CO and NO.296 More revelations about silver(I) halide phosphine complexes come from a crystal structure which shows that [Ag4Cl4(PMe3)3] is a one-dimensional polymer of Ag4Cl4 cubes, and from IR and 31P solid state NMR spectra which indicate that [AgX(PMe3)2] (X ˆ Cl or Br) are halide-bridged dimers.297 The synthesis of KAgC2 has been achieved without using explosive Ag2C2 by the reaction of KC2H with AgI to give KAg(C2H)2, which gives KAgC2 on heating to 120^130  C; other MAgC2 were prepared, all these having AgC2 chain structures.298 1,2-Bis(4-pyridyl)ethyne (L) forms [Ag2L5]2‡ with an interpenetrating array of polymer cations described as ``entangled ladders''.299 5,50 -Dicyano2,20 -bipyridine forms a wide range of silver(I) complexes involving co-ordination by pyridyl N and cyanide C to give a helical polymer for the nitrate and a hexagonal planar polymer with [PF6]ÿ.300

9

Gold

High oxidation states New reviews include one on AuII which makes careful distinction between genuine examples and compounds containing both AuI and AuIII.301 Another review deals with compounds AuIII and AuI derived from acac.302 Annu. Rep. Prog. Chem., Sect. A, 2000, 96, 279^315

305

Included among the high oxidation state £uoride complexes prepared in anhydrous HF, K[AuF6] was obtained from gold metal, KF and £uorine and M[AuF6] (M ˆ Li or Cs) from MF, AuF3 and £uorine.10 Heating ThF4 and AuF3 gave Th2AuF11, containing [AuF4]ÿ.303 ThF4 or CeF4 with AuF3 gave Au3F8, whose structural similarity to PdAu2F8 suggested it contained AuII as well as AuIII, but no magnetic data were provided.304 The crystal structure of K[AuBr4] shows the expected planar anion structure.305 A more complex structure was found when Au or AuI was treated with various R3SIx to give chain polymeric complexes in which {AuIIII4} units are linked by an I3ÿ ion or two I2 molecules.306 Gold vapour interacts with a beam of C5D5N to give complexes [Au(NC5D5)n]2‡ on ionisation in a mass spectrometer, showing that gold(II) complexes can be stabilised against disproportionation under appropriate conditions.307 Studies of the principles of sonochemistry were enhanced by investigating the reduction of [AuCl4]ÿ to gold metal in the presence of various surfactants and noting variation in the sonoluminescence.308

Gold(I) The compound formed from aqueous NaOH and [Au(HL)2]‡ (HL ˆ 2-imidazolidinethione), formerly believed to be [(AuHL)2O], has now been shown crystallographically to be tetranuclear [AuL]4.309 The steric hindrance in the thiols 4-Me2NC6H4SH (HL) and (4-Me3NC6H4SH)‡ (HL0 ‡) prevents the usual Au^Au interactions occurring in the linear complexes [AuL(PPh3)] and [AuL0 (PPh3)]‡.310 The crystal structure of the pyrimidinethiol (HL) complex [Au2L2] shows this has the expected head to tail con¢guration with a short Au-Au intramolecular distance of 273.7 pm, but the predominant intermolecular attraction is through the overlap of the pyrimidine rings rather than an aurophilic interaction.311 Intermolecular interactions in gold chemistry are very subtle, as shown by complexes of polyphosphines and quinoline-2-thiol (HL). Among products of this type are [(AuL)2(m-Ph2PC4H8PPh2)]2, with two dinuclear units associated by Au^S links, [(AuL)4{m-C2H4PPh2)3}], with two intramolecular aurophilic attractions, and [(AuI)3{m-PhP(C2H4PPh2)2}], which has both Au^Au and Au^I intermolecular contacts.312 Following examples involving O or S, the ¢rst m4-Se gold cluster, [(AuPPh3)4Se]2‡ has been made from [(AuPPh3)2Se] and [Au(SO3CF3)(PPh3)].313 The product from fusing K2Te, SnTe, Sb, Au and Te under Ar reacts with Bu4NBr in en to give [NBu4]3[AuSnTe6], which can be considered as a gold complex of [Te2Sn(m-Te)2SnTe2]4ÿ which acts as a bidentate ligand through one terminal Te on each Sn and has a severely bent Te^Au^Te angle of 166.6 . The cluster is described as the smallest ternary Zintl telluride anion yet made. 314 The relatively neglected area of gold(I) N-donor complexes should be stimulated by the discovery that the complex [AuCl(2-NH2C2H4N)], formed from the amine (L) and [AuCl(SMe2)], contains equimolar amounts of [AuClL], [AuL2]‡ and [AuCl2]ÿ, all linear and with co-ordination by the pyridine N atom. 315 The ¢rst complex of acetonine (L20), a useful but elusive synthetic intermediate, was formed by reaction of [AuCl(tht)] with NH3 and acetone, giving [Au(L20)2]‡.316 Aziridine 306

Annu. Rep. Prog. Chem., Sect. A, 2000, 96, 279^315

(C2H4NH) reacts with [(AuPPh3)3O]‡ to give a ligand reorganisation forming [(AuPPh3)3{m-NCH2CH2(NC2H4)} ]‡.317 Crystal structure, IR and 31P (solid state) NMR spectral studies show compounds of stoichiometry Au(PCy3)2X (X ˆ Cl, Br or I), formed from [AuX2]ÿ and PCy3, have diverse structures. The chloride is [Au(PCy3)2]Cl, but the iodide gave molecular compounds, one having co-ordinated I and the other semico-ordinated I (Au^I 289.5 or 301 pm respectively), while the bromide shows all three forms with minor variations.318 Gold(I) forms a very strained tetrahedral complex [AuClL] [L ˆ PhP(C6H4PPh2-2)2]. With AgBF4 this complex is converted into [Au2L2]2‡ which contains one three- and one ¢ve-co-ordinate Au, although both silver atoms are tetrahedral in the silver analogue.319 Treatment of the known [Au(PMePh2)2][AuCl(SiPh3)] with PPh3 gave [Au(SiPh3)(PPh3)], but this very quickly converted into [Au(PPh3)2][Au(SiPh3)2]. However, using PMe3 gave stable [Au(SiPh3)(PMe3)] (crystal structure).320 Low oxidation states Rare gold(0) complexes of some macrocyclic phosphines such as L21 have been made starting with AuGaCl4.321 A patient wait of six weeks allowed completion of the reaction of [Au55Cl6(PPh3)12] with Na2B12H11SH, to form Na24[Au55(B12H11SH)12Cl6]. The exchange of sodium for tetraoctylammonium confers useful organic solubility on the salt.322

Ligand/reagent abbreviations used in this chapter carboplatin cisplatin

diammine(1,10 -cyclobutanedicarboxylato)platinum(II) diammine dichloroplatinum(II) Annu. Rep. Prog. Chem., Sect. A, 2000, 96, 279^315

307

dba dcpe dmpe dpp Et2dtcÿ GMP2ÿ GSH GSSG H3hedta Me2dtcÿ mnt2ÿ pnp qtpy tht tpa tpm triphos Ts

dibenzylideneacetone 1,2-bis(dicyclohexylphosphino)ethane 1,2-bis(dimethylphosphino)ethane 1,3-bis(diphenylphosphino)propane N,N-diethyldithiocarbamate guanosine 50 -monophosphate(2ÿ) glutathione glutathionedisul¢de N-carboxymethyl-N-(2-hydroxyethyl)ethylenediminodiacetic acid N,N-dimethyldithiocarbamate maleonitriledithiolate 2,6-bis(diphenylphosphinomethyl)pyridine 2,20 :60 ,200 :600 ,200 -quinquepyridine tetrahydrothiophene tris(2-pyridyl)amine tris(1-pyrazolyl)methane 1,1,1-tris(diphenylphosphinomethyl)ethane toluene-p-sulfonyl

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