Inorganic chromophores: Optical charge transfer in coordination compounds

Charge transfer (CT) transitions of coordination compounds of transition, main group, and f-block metals are classified according to the redox sites at the metals and ligands: ligand-to-metal (LMCT), metal-to-ligand (MLCT), metal-to-metal (MMCT), ligand-to-ligand (LLCT) and intraligand (ILCT). The energy of these CT transitions can be correlated with the reducing and oxidizing strength of the donor and acceptor, respectively. In many cases the colors of metal complexes are determined by CT absorption bands. A variety of examples demonstrate the importance of optical CT in coordination compounds.