THE OXIDATION-REDUCTION POTENTIALS OF COPPER COMPLEX IONS

Abstract

The effects of complex-forming species on the observed potential of the copper (II)-copper (I) couple in aqueous solution can be discussed conveniently in terms of the differences in the stability constants of the copper (I) and copper (II) complexes. Some of the physical properties involved can also be seen by considering suitable thermodynamic cycles. Few reliable values for the oxidation-reduction potentials of copper complexes have so far been reported, probably because of the ease with which hydrated cuprous ion disproportionates into cupric ion and free copper. Methods for calculating such potentials will be described, together with results obtained using a range of ligands. A major factor in determining the relative stabilities of copper(I) and copper(II) complexes is their stereochemistry. Depending on the nature of the ligand, copper(I) can form linear or tetrahedral complexes, whereas copper (II) shows a preference for square planar or distorted octahedral complexes, with coordination numbers of four or six. In 1:2 monodentate complexes, copper (I) is more stable, especially if the ligands are highly basic, whereas if the ligands are bi- or higher- dentate, copper (II) is usually favoured for steric reasons. These considerations enable some predictions to be made about the effects of complex-forming species on the copper (II)—copper (I) couple.