Potentiometric studies in mixed solvents, II Complexes of Nickel(II), Copper(II) and zinc(II) with pyridine, ethylenediamine and glycine

Abstract

Formation constants for the complexes of pyridine, ethylenediamine and glycinate with protons, nickel(II), copper(II) and zinc(II) were determined potentiometrically in a 0.5 M (Li)ClO4 medium in water, 54.3% w/w methanol and in 48.1% w/w dioxan at 25°C. The stepwise protonation constants for pyridine decrease in the order water>methanol>dioxan, while both stepwise protonation constants for ethylenediamine follow the order water>dioxan>methanol and both constants for the glycinate ion follow the order dioxan>water, methanol. The first hydrolysis constants for the three ligands, and the second hydrolysis constants for ethylenediamine, all decrease in the order water>methanol>dioxan. For the metal chelate complexes, stability increases in the order water⪡ methanol<dioxan, the increases being larger for the glycinate complexes than for those of ethylenediamine. The stability constants of pyridine complexes are but little influenced by change in the solvent. For complexes of a given metal cation with ethylenediamine and glycinate, the ratios of successive stability constants are almost independent of the ligand or of the composition of the solvent. The observed trends in the different equilibrium constants are discussed in terms of the number of water molecules released during the reaction and of the estimated free energies of transfer of the various species between an aqueouss and a partly organic medium.