Valence tautomerism in metal complexes: Stimulated and reversible intramolecular electron transfer between metal centers and organic ligands

The defining characteristic of a valence tautomeric (VT) transition in a metal complex with a redox-active organic ligand is a stimulated intramolecular electron transfer between metal and ligand. Reversible VT behavior is well established for octahedral cobalt complexes with o-dioxolene ligands, with other combinations of metal centers and organic ligands also capable of exhibiting the phenomenon. Although the thermodynamic basis of VT transitions in cobalt-dioxolene systems has been understood for some time, it has not necessarily been elucidated for the less common systems. In general a number of factors additional to the redox-active metal-ligand unit influence the manifestation of the VT transition. These include the ancillary ligands, counterions, solvent molecules and intermolecular interactions, with cooperativity between VT molecules important for hysteresis and bistability. Recent advances in the field include the development of new metal-ligand combinations that can display VT transitions and the recognition that stimuli in addition to heat, light and pressure can induce the transitions. Considerable effort is also currently being directed toward assembling multiple VT moieties into polynuclear and polymeric complexes and combining VT behavior with other physical and chemical properties to generate bifunctional molecule-based materials. (C) 2014 Elsevier B.V. All rights reserved.