trans to cis Isomerisations of platinum(II) olefin phosphine complexes, [PtX2(ol)(PR3)](X = Cl or Br; ol = ethene, propene, or 1-heptene; PR3= PBu3 or PMe2Ph)

Abstract

Isomerisation reactions of trans-[PtX₂(ol)(PR₃)](X = Cl or Br; ol = ethene, propene, or 1-heptene; PR₃= PBu₃ or PMe₂Ph), formed by treating [Pt₂X₄(PR₃)₂] with olefin, have been examined by ³¹P n.m.r. spectroscopy in CDCl₃ solution with both excess and deficiency of olefin. The trans isomers readily lose olefin and equilibrate with the halide-bridged dimers. The positions of these equilibria are dependent upon incident light. The more stable cis isomers steadily form in these solutions, but whereas the rate of trans to cis isomerisation is markedly retarded by excess olefin when X = Cl, it is slightly increased when X = Br. Exchange of free and co-ordinated olefins is rapid with all the complexes compared to the isomerisation rate. The mechanisms of these reactions are discussed and it is concluded that, in addition to photochemical processes, more than one isomerisation pathway operates. A dissociative route predominates for the chloride compounds but an associative process is more important for the bromide complexes. Related studies have been performed on the dimethyl sulphoxide analogues, cis- and trans-[ PtX₂(dmso)(PR₃)], and broadly similar results were obtained, emphasising the similarity between the ligand properties of olefin and dmso at platinum.