Crystal structures relating to the interactions of small molecules with transition metals.

The results of crystal structure investigations on several rhodium and platinum organometallic complexes are given. Some amplification of the X-ray methods used, and of some general features of the complexes studied, is initially made. A review of previous structural data on olefin complexes is included. The structures illustrate either the co-ordination of some small unsaturated molecule in the complex, or the reaction of such a molecule at a transition metal. The former complexes generally involve co-ordinated tetrafluoro-ethylene and the results are discussed in terms of the Chatt-Dewar bonding scheme. The latter group are examples of the use of X-ray methods to determine the nature of a reaction product. Comparison of the molecular geometries (in particular the Rh-C distances) of the complexes Rh(acac)(C2F4)(C2H4) and Rh(acac)(C2H4)2 indicates that C2F4 is more closely bound to rhodium than ethylene, a consequence of increased ?-bonding. The complexes Pt(bipy)(C2F4) and Pt(PMe2Ph)2(C2F4) have greatly different J(Pt-F). A difference in the structures is also apparent, the degree of bending of the four fluorine atoms away from the C=C bond is greater in the bipy complex. This is understood to indicate greater ?-bonding and sp3 character of the olefin carbon atoms in the bipy complex. The structure determination of [Rh(MeCN) (C2F4)(acac)]2 serves to confirm the formulation given this compound in the literature and also to provide additional data on the non-planarity of co-ordinated tetrafluoroethylene. Rh(C9H7F6O2)(C6(CF3)6) is formed by the reaction of four moles of C4F6 with one of Rh(cod)(acac). The structure shows that while 3 molecules of C4F6 per rhodium cyclize to a C6(CF3)6 ring, the remaining molecule adds 1-4 to the rhodium-acac ring. A study of the product [RhC12(C8H15O2)(4-Mepy)2] shows that RhCl3.3H2O reacts with 2-methyl allyl alcohol to form a Rh-saturated furan ring system. The geometry clearly indicates the trans effect of a o-carbon atom. The structure of [Pt(S2CF)(PPh3)2][HF2] shows that carbon disulphide inserts into a Pt-F bond on reaction with [PtF(PPh3)3][HF2]. The molecular geometry shows some asymmetry which is discussed.