Spectral intensities of transition metal complexes

Electronic dipole 'd-d' transitions in transition metal complexes, and 'f-f' in lanthanoid species, are formally disallowed. This article describes how they are forced. A broadly chronological review of the literature on 'f-f' electric dipole transitions focuses upon mechanisms which have been proposed to effect the required parity mixing within functions involved in those transitions. It begins with the ''static coupling'' (SC) model and, following recognition of hypersensitive transitions, develops into an exploration of the ''dynamic coupling'' (DC) model and of their combination (SC + DC) upon which most intensity analyses before the late 1980s have rested. The central importance of effective electric dipole transition operators is emphasized as is the neglect of the effects of covalency in the SC + DC approach. Against this background, the cellular ligand-field (CLF) model of 'd-d' intensities is presented detail and the description includes a critique of its relationship with the SC + DC model and of its incorporation, in principle, of all contributions to open-shell intensities, including those deriving from covalency and overlap. The CLF scheme is introduced in two parts, relating first to acentric chromophores in which parity mixing arises within the static environment and then to centric species in which it arises vibronically. An early section of review briefly summarizes some fundamental concepts relating to normal coordinates and vibronic coupling. The final section provides a survey of the quality and parametrizations of all CLF analyses completed to date. (C) 1997 Elsevier Science S.A.