Optical Ligand to Ligand Charge Transfer of Metal Complexes Including Ligand-Based Mixed-Valence Systems

The electronic spectra of metal complexes which contain a reducing and an oxidizing ligand (Lred–M–Lox) are characterized by ligand to ligand charge transfer (LLCT) absorptions. A specific form of Lred–M–Lox complexes are ligand-based mixed-valence (LBMV) compounds which contain the same ligand in two different redox states. In this case the ligand-ligand interaction may lead to a partial or even complete electronic delocalization between the reduced and oxidized form of the ligands. As a consequence, the “LLCT” transition loses its CT character since it takes place between delocalized orbitals.