Entropy contribution to the relative solution stability of copper(III) and nickel(III) tetraazamacrocyclic complexes in aqueous perchloric acid

Solutions containing equimolecular amounts of [NiII(cyclam)][ClO4]2 and [CuII(cyclam)][ClO4]2(cyclam = 1,4,8,11-tetraazacyclotetradecane) of varying concentration of HClO4(4–11.56 mol dm–3) were investigated through voltammetric techniques and the potentials associated with the NiII–NiIII and CuII–CuIII redox changes determined. The potential difference, ECu–ENi, which is related to the metathetic equilibrium [CuIIIL]3++[NiIIL]2+[CuIIL]2++[NiIIIL]3+, becomes less positive with increasing HClO4 concentration, indicating a stabilizing effect on the copper(III) complex, compared to NiIII. This effects is related to the competition by the ions of the electrolyte for the water molecules co-ordinated to the electroactive species. The entropy change ΔS associated with the metathetic equilibrium was determined through voltammetric investigations performed at varying temperatures (–15 to +40°C). On the basis of ΔS values, it is suggested that, below an HClO4 concentration of 8 mol dm–3, water molecules occupy the available co-ordination sites in the electroactive metal complexes; at a higher concentration the solvent molecules are replaced by ClO4– anions.