Potentiometric and Calorimetric Studies on Formation of Glycinato Complexes of Nickel(II) in Water and in an Aqueous Dioxane Solution

Abstract

Abstract Protonation equilibria of glycinate (gly−) ion and complex formation equilibria between nickel(II) and glycinate ions have been studied by potentiometry and calorimetry in water and in 0.2 mole fraction (55.0%w/w) dioxane–water mixture, each containing 3 mol dm−3 LiClO4 as a constant ionic medium at 25 °C. A series of mononuclear complexes, [NiLn](2−n)+ (L−=gly− and n=1–3) were found in both solutions and their formation constants, enthalpies and entropies were determined. It elucidated that the stepwise Gibbs energies of formation of the metal complexes varied in the sequence, ΔG1°<ΔG2°<ΔG3°, and the stepwise enthalpies, ΔH1°>ΔH2°>ΔH3°, and thus, the stepwise entropies markedly decreased in the order, ΔS1°>ΔS2°>ΔS3°. The result was discussed in relation to changes in the metal–water and metal–ligand bond lengths within [NiLn(H2O)6−2n](2−n)+ (n=1–3) with n. As to the solvent effects, the formation constants of each metal complex increased with the addition of dioxane to water, the increase was caused by both enthalpies and entropies of formation of the complexes, the former mainly contributing to the formation of [NiL2] and the latter to [NiL]+ and [NiL3]−. The Gibbs energies of transfer of single species from water to the dioxane–water mixture were also evaluated. The enthalpy and entropy of transfer of any ionic species mostly compensated each other, so that the Gibbs energies of transfer of ions were rather small.