Mechanisms of ultrafast metal-ligand bond splitting upon MLCT excitation of carbonyl-diimine complexes

Using CO dissociation from [Cr(CO)(4)(bpy)], alkyl homolysis from [Re(R)(CO)(3)(dmb)] and dissociative isomerisation of [Mn(Br)(CO)(3)(Pr-i-DAB)] as characteristic examples, it is shown how excitation into charge transfer excited states can lead to ultrafast metal-ligand bond splitting. The overall course of organometallic photochemical reactions is clearly determined by excited state dynamics, which occur at the earliest times after excitation. Branching of the evolution of the optically prepared Franck-Condon excited state between reactive and relaxation pathways seems to be a general mechanism that limits photochemical quantum yields. CO dissociation and alkyl homolysis can be described as an adiabatic evolution on potential energy surfaces of metal-to-ligand charge transfer (MLCT) or sigma-bond-to-ligand charge transfer (SBLCT) excited states, respectively. These states acquire a dissociative character from upper states with which they interact along the reaction coordinate. Coupling of an excited state with the dissociative continuum of the electronic ground state provides an alternative reaction mechanism. This was demonstrated for the dissociative isomerisation of fac-[Mn(Br)(CO)(3)(Pr-i-DAB)], which occurs from ligand-to-ligand charge transfer (LLCT) excited state. In general, it follows that an understanding of organometallic photochemistry requires a knowledge of the energies and characters of the relevant electronic states as a function of possible reaction coordinates. (C) 2000 Elsevier Science S.A. All rights reserved.