Valence tautomeric transition metal complexes

Valence d-orbital energies of the first row transition metals are close to the frontier pi-orbital energies of o-benzoquinones. Complexes prepared with quinone ligands most commonly have the quinone coordinated with the metal in the form of a semiquinonate (SQ) radical-anion or as a catecholate (Cat) dianion. In a few unique complexes it has been possible to observe intramolecular electron transfer between localized metal and quinone electronic levels. Electron transfer is accompanied by changes in magnetism and spectral properties that have made it possible to observe metal-ligand electron transfer under equilibrium conditions in solution and in the solid state. This effect has been considered as an example of valence tautomerism (VT). In this review we present the results of studies on the physical properties of complexes that undergo VT, with a view of the scope of VT for complexes containing a variety of quinone ligands and with different metal ions.