Photoreactivity of metal-to-ligand charge transfer excited states

Generally, transition metal complexes in metal-to-ligand charge transfer (MLCT) excited states are considered to be less reactive than in other states (e.g. ligand field, ligand-to-metal charge transfer) because the orbitals which participate in MLCT transitions are frequently of the pi type and, thus, less involved in strong bonding interactions. However, contrary to these expectations, numerous complexes, in particular the organometallic compounds, are characterized by intrinsically reactive MLCT states. The photoreactivity may be correlated with the electron distribution in the excited state which roughly corresponds to an oxidized metal and reduced ligand when compared with the ground states. An attempt is made to classify reactive MLCT states according to the reactive center. The reactivity of a MLCT state may be based predominantly on the oxidation of the metal or reduction of the ligand. Finally, charge-transfer-to-solvent (CTTS) transitions which are closely related to MLCT transitions are included in this discussion. (C) 1998 Elsevier Science S.A. All rights reserved.