Platinum Metals Rev., 1970, 14, (2), 62
Platinum and Palladium Complexes of Unsaturated Hydrocarbons
A Comprehensive Review of the Literature
To generations of chemists trained before about 1950 the possibility of a marriage between organic and inorganic chemistry was rarely considered. These two sub-divisions of chemistry remained in separate water-tight compartments, and only the unusually inquiring or perspicacious research man dared to explore the boundaries. Within the space of twenty years a complete transformation has occurred: beginning with the discovery of ferrocene, a very large number of compounds are known in which organic molecules or derivatives thereof are combined with inorganic salts to form complexes of great interest and usefulness. The study of these complexes constitutes organometallic chemistry, and with it has come the realisation that the previous sub-divisions were artificial and restrictive, and that the interaction of one on the other is both stimulating and necessary. Our concept of chemical bonding has been greatly extended through the study of organometallic complexes, and the end of our increasing understanding of the fundamentals of chemical forces is not yet in sight.
Of the many types of organometallic complexes, those formed between unsaturated hydrocarbons and palladium and platinum in their d 8 (i.e., PdII and PtII) and d 10 (i.e., Pd0 and Pt0) states are among the most fascinating and useful. A comprehensive and scholarly review of them by F. R. Hartley of the Commonwealth Scientific and Industrial Research Organisation of Australia, has recently appeared (1), reviewing the literature up to mid-1968 and containing no less than 605 references. It is impossible to do full justice to such a valuable survey in a short review; one can do little more than indicate the ground which is covered.
The bond formed between ethylene and, say, a palladous ion consists of two parts: in one part, referred to as the σ -bond, π -electrons from the olefin are donated to a vacant orbital of the ion, while in the other part, referred to as the π -bond, electrons from an orbital of the ion are back-donated into vacant anti-bonding orbitals of the ethylene. As a result, changes occur in the structure of the ethylene which are detectable in X-ray structure analysis, by infra-red and nuclear magnetic resonance spectroscopy. In complexes of Pd0 and Pt0, however, the π -character of the bond is almost entirely absent, as is also the case with acetylene complexes. In the tetracyanoethylene complex with Pt0, and with acetylene complexes, the bonding is almost pure π in character because of the weak donor ability of the hydrocarbon. The strength of the olefin to metal bond is measured as a “stability constant”, and numerous values are quoted in this review: complexes with PtII are however notably stabler than those with PdII. Much stronger complexes are formed by non-conjugated diolefins such as 1,5-cyclooctadiene, but the increased stability is predominantly due to the fact that two olefin-metal bonds are formed.
Particular interest attaches to olefin-metal complexes by reason of their being intermediates in a number of important homogeneously catalysed processes. The action of coordination renders the olefin more susceptible to nucleophilic attack, for example, by hydroxyl and acetate ions, than is the free olefin, and this enables the well-known Wacker reaction (the oxidation of ethylene to acetaldehyde) to occur, and also the oxidation of ethylene to vinyl acetate. Significantly, only PdII and not PtII complexes are sufficiently reactive: this results from the lower stability of the former, referred to above. Other potentially important homogeneous processes involving metal-olefin complexes as intermediates are carbonylation, hydrogenation, isomcrisation, and polymerisation.
The concept of donor-acceptor bonding in olefin-metal complexes has also been applied to understanding the mechanisms of heterogeneous catalysis (2), as well as in other areas of organometallic chemistry. It forms a basis for rationalising the properties and reactivities of olefin-metal complexes and possibly for predicting more reactive complexes for homogeneous catalysis.
It is unfortunate, but perhaps inevitable, that Dr Hartley’s review was not extended to cover the remaining platinum group metals, for there are interesting trends in the stability of olefin-metal complexes in passing through ruthenium, rhodium and palladium, and also systematic differences between the second and third row metals.
- 1F. R. Hartley, Chem. Rev., 1969, 69, 799
- 2G. C. Bond, Platinum Metals Rev., 1966, 10, 87