Studies on charge distribution and valence tautomerism in transition metal complexes of catecholate and semiquinonate ligands

Transition metal complexes containing first row metal ions chelated by catecholate (Cat) and semiquinonate (SQ) ligands have been found to have localized electronic structures with quinone ligands bonded in both SQ and Cat electronic forms. In exceptional cases the balance of metal and quinone orbital energies is sufficiently close as to permit magnetic and spectroscopic observations on isomers differing in charge distribution together under equilibrium conditions. Equilibria occurring between M-n(SQ(.-)) and Mn+1(Cat(2-)) redox isomers have been observed in solution and in the solid state for complexes of Co, Mn, and Cu. The temperature range over which equilibria may be observed. is defined by enthalpy and entropy changes associated. mainly, with changes in charge and spin-state of the metal ion. This effect has been considered as an example of valence tautomerism (VT). In this review we present an overview of recent research on complexes of quinone ligands that exhibit reversible shifts in charge distribution under equilibrium conditions. (C) 2001 Elsevier Science B.V. All rights reserved.