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

Transition metal complexes containing first row metal ions chelated by cate cholate (Cat) and semiquinonate (SQ) ligands have been found to have locali zed electronic structures with quinone ligands bonded in both SQ and Cat el ectronic forms. In exceptional cases the balance of metal and quinone orbit al energies is sufficiently close as to permit magnetic and spectroscopic o bservations on isomers differing in charge distribution together under equi librium 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 m ay be observed. is defined by enthalpy and entropy changes associated. main ly, 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 t hat exhibit reversible shifts in charge distribution under equilibrium cond itions. (C) 2001 Elsevier Science B.V. All rights reserved.