Excited States of Transition Metal Complexes

The assignment of the various bands which appear in the absorption spectra of transition metal complexes is a very difficult problem because the absorption spectra reflect, of course, the complexity of the electronic structure of these molecules. From a photochemical point of view2), it is convenient to make reference to 

Transitions between MO s predominantly localized on the central metal these are usually called metal centered (MC), ligand field or d-d transitions. 

Another important type of electronic transition is the charge transfer to solvent (CTTS) transition. Recently, it has also been found5 that electronic transitions may occur between MO s which are localized on different ligands of the same complex. 

These transitions have been called inter-ligand transmetallic charge transfer transitions. Other types of transitions are present in polynuclear transition metal complexes6 7- but we will not deal with such compounds. 

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Spectroscopic investigations of excited states of transition metal complexes. G. A. Crosby, Acc. Chem. Res., 1975, 8,231-238 (53). 

The interconversion between different spin states is closely related to the intersystem crossing process in excited states of transition-metal complexes. Hence, much of the interest in the rates of spin-state transitions arises from their relevance to a better understanding of intersystem crossing phenomena. The spin-state change can alternatively be described as an intramolecular electron transfer reaction [34], Therefore, rates of spin-state transitions may be employed to assess the effect of spin multiplicity changes on electron transfer rates. These aspects have been covered in some detail elsewhere [30]. 

Balzani, V., Bolletta, F., Gandolfi, M. T., and Maestri, M. Bimolecular Electron Transfer Reactions of the Excited States of Transition Metal Complexes. 75, 1-64 (1978). 

Bimolecular Electron Transfer Reactions of the Excited States of Transition Metal Complexes... 

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