Platinum Metals Rev., 1967, 11, (4), 148
Carbonyl Halide Complexes of the Platinum Metals
The many advances in the chemistry of the platinum metals made in recent years have been particularly evident in the field of carbonyl group-containing complexes. Improved methods of preparation have been developed and the uses of such complexes in homogeneous catalysis have been and are being actively studied.
The preparation of these carbonyl halide complexes by a variety of techniques has been reported. The production of ruthenium complexes by the decarbonylation of formic acid by ruthenium (II) species has been studied (1) and there are reports of the reaction of [IrCl6]3− or [IrCl6]2− with formic acid to yield carbonyl halide species of uncertain formulae (2, 3). Ruthenium complexes have been prepared by the long passage of carbon monoxide through solutions of ruthenium (III) halogen compounds (4) and iridium complexes by the reaction of halogen compounds with carbon monoxide under pressure (5). Similar osmium compounds have not, so far, been reported.
Recent investigations into the reactions of platinum metal compounds with formic acid have led to the discovery of a new and convenient method of preparing carbonyl halide complexes of osmium, ruthenium and iridium by refluxing the metal halide or halo-complex with 90 per cent formic acid (6). The compounds formed were initially recognised as carbonyl rather than formate complexes by infra-red spectroscopy in the 2000 cm−1 region.
The complexes that have so far been prepared and the reaction conditions are set out in the table opposite; all the compounds have been characterised by elemental analysis and their full infra-red spectra have been determined and recorded.
To isolate salts of the anionic species it was found necessary to use caesium as the cation since lighter atomic weight alkali metals yielded salts of very high solubility.
The reaction between sodium hexachlorosmate (IV) (used because of the low solubility of other [OsCl6]2− salts) and formic acid is of particular interest since it takes place much more slowly than those between ruthenium and iridium compounds and formic acid. This has made it possible to identify by infrared spectroscopy intermediate formato-halo species containing one and two monodentate formate groups. These species probably contain formally divalent osmium, that is [Os11(COOH)Cl5]4− and [Os11(COOH)2Cl4]4−, and they further react to give the mono- and dicarbonyl species respectively. The reaction proceeds quickly to the diformate stage and the monocarbonyl complex is formed in small quantities only.
Triphenylphosphine derivatives are easily prepared by warming the complexes with triphenylphosphine in formic acid solution or, more conveniently, by treating the solutions prior to isolation of a carbonyl halide salt with triphenylphosphine. The complexes [Os(CO)2(PPh3)2X2] and [Os(CO)3(PPh3)X2] have been prepared from Cs2[Os(CO)2X4] and Cs[Os(CO)3X3] respectively, while [Ru(CO)2(PPh3)2X2] and [Ru(CO)(PPh3)3Cl2] have been prepared from Cs2[Ru(CO)2X4) and Cs2[Ru(CO)(H2O)Cl4] respectively (X = Cl, Br).
The author’s thanks are due to Dr W. P. Griffith of Imperial College of Science and Technology for help and guidance in this work.
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- 6M. J. Cleare and W. P. Griffith, Chem. and Ind., in the press