Mechanistic roles of metal-to-ligand charge-transfer excited states in organometallic photochemistry

Metal-to-ligand charge-transfer (MLCT) excitation of organometallic and coordination complexes has diverse chemical and physical consequences, depending on the molecular structure and the medium. It can promptly start an ultrafast ligand dissociation, populate a long-lived, charge-separated, 3MLCT state that shows typical reactivity of the oxidised metal atom or reduced ligand (electron transfer, associative substitution, oxidative addition), or lead to a long-lived 3MLCT state from which another, more reactive, excited state is populated thermally. Alternatively, a MLCT state may be unreactive with respect to the breaking or formation of metal-ligand bonds but engaged in electron- or energy-transfer reactions or just decaying to the ground state. The MLCT photochemistry is discussed in terms of the bonding properties and dynamics of the MLCT excited states which are, to some extent, dependent on interactions with other excited states of different orbital origins (LF, sigma pi*, IL). An examination of actual mechanistic roles of MLCT states in representative classes of photochemical reactions allows to outline more general relations between the molecular structure and photoreactivity of MLCT-active organometallic compounds. (C) 1998 Elsevier Science S.A. All rights reserved.