Kinetics of the electron-transfer reactions of tetranuclear isomeric species of cobalt(III)-ammine complexes with hexaaquochromium(II) and hexaaquovanadium(II) ions

Abstract

The electron-transfer reaction kinetics of the Cr(OH 2) 6 2+ and V(OH 2) 6 2+ complexes with tetranuclear isomeric species of cobalt(III)-ammine complexes containing pyridine 2,6−, 3,5−, 2,4− and 2,5-dicarboxylate bridging ligands ( I– IV) have been studied in solution with various hydrogen ion concentrations. Inner-sphere mechanism with attack of Cr(II) at a deprotonated remote pyridine nitrogen atom and subsequent electron transfer through the organic ligands are assigned for complexes II, III and IV. Due to steric hindrance by carboxylate groups, an outer-sphere mechanism is observed for complex I. The electron-transfer rates for complexes III and IV are 20–50 times faster than that in complex II, and are discussed in terms of the lowering of the energy barrier caused by a significant π orbital interaction of oxidant and reductant through a heterocyclic bridge ligand. Outer-sphere mechanism for reduction of I is assigned on the basis of a k Cr/ k V ratio of 0.026. Reduction of complex II by V(II) is independent of [H +] and proceeds by the outer sphere.