Thermodynamics of the chelate effect in the formation of copper(II) ethylenediamine complexes in water-organic solvents

The changes in the Gibbs free energy of chelation in aqueous ethanol, dimethyl sulfoxide, and acetone solutions are calculated using the formation constants of the copper(II) ammine and ethylenediamino complexes. The results are compared with the data for analogous nickel(II) complexes. It is found that, irrespective of the nature of the central ion, the chelate effect is enhanced-in aqueous ethanol solutions and shows little dependence on the composition of water-dimethyl sulfoxide and water-acetone solvents. In going from water to water-organic solvents, the chelation equilibrium is shifted because of a change in the solvation components of the enthalpy and entropy contributions.