Dissociative Pathways in Platinum(ll) Chemistry

Steric and electronic factors favor the addition of a fifth ligand to a square planar d8 complex to form five coordinate species either as discrete compounds or reaction intermediates. The search for factors promoting the conversion of the normal associative mode of reaction into a dissociative process has attracted much attention. The attempt to achieve this goal by using bulky ancillary ligands has been unsuccessful. More convincing evidence in favor of a dissociative mechanism involving a Pt(II) 3-coordinate intermediate comes not from classical Werner compounds but from reactions involving organometallic substrates. It is now reasonably clear that the formation of such 3-coordinate 14-electron compounds offers a favorable reaction route to a number of fundamental processes as an alternative to the intermediacy of 4- and 5-coordinate species. Among them, the uncatalyzed cis to trans isomerization of complexes of the type cis-[Pt(PE3)2(R)X] (R = alkyl or aryl groups; X = halide ion), β-hydride elimination from symmetrical and unsymmetrical dialkyldiphosphino platinum(II) complexes and other processes that will be briefly discussed in this paper. Very recently, kinetics of solvent exchange and displacement by bidentate ligands on cis-[Pt(R)2(L)2] (R = CH3 or C6H5; L = sulfoxide or thioether) complexes have provided a clear-cut example of ligand dissociation as the dominant step in substitution reactions of square-planar complexes. While in most of the cases extensive electron density transfer associated with the presence of a Pt-C σ bond seems to be a prerequisite to produce a sharp changeover of reaction pathways, we are far from a complete understanding of the factors that promote the formation and stabilization of coordinatively unsaturated species, as well as of the efficiency with which they can be intercepted in solution by ligands or by the solvent itself.