Ligand-to-ligand charge transfer states and photochemical bond homolysis in metal-carbon bonded platinum complexes

This paper reviews the photochemical Pt-C bond homolysis reactions of metal-carbon bonded platinum compounds. Three types of complexes are considered: [Pt(R)(2)(COD)], [Pt(R)(2)(alpha-diimine)] and [Pt(Me-4)(alpha-diimine)]. This photoreactivity originates in the involvement of the sigma-bonded co-ligands in low-lying excited states. This involvement was proved by time-dependent density-functional theory (TD DFT) calculations and resonance Raman (rR) spectroscopy. In the case of the platinum(II) complexes, the contribution of the co-ligand to the high-lying filled orbitals increases with increased sigma-donor strength of this ligand. Concurrently, the photoreactivity increases dramatically in the case of the [Pt(R)(2)(COD)] complexes. The corresponding alpha-diimine analogues are much less reactive due to the smaller co-ligand participation in high-lying filled orbitals. The platinum(IV) complexes [Pt(Me-4)(alpha-diimine)] are very photoreactive, due to the fact that the HOMO has almost exclusively sigma(C-ax-Pt-C-ax) character. (C) 2002 Elsevier Science B.V. All rights reserved.