Kinetics of the substitution of 2,2'-bipyridine into aquanickel(II) ions in water + acetonitrile mixtures

Abstract

As deviations from the Eigen–Wilkins mechanism can occur in pure liquids other than water, the effect of changing solvent structure on the kinetics is better investigated in water-rich mixtures of water + co-solvent. The co-solvent used here is acetonitrile as rates of substitution in pure acetonitrile appear to be abnormally fast. Extrema in the enthalpy and entropy of activation for the substitution of 2,2'-bipyridine into Ni²⁺_aq correlate well with the extrema in the physical properties of the mixture which are related to sharp changes in the solvent structure: this is contrasted with the variation in methanol + acetonitrile where no sharp changes in solvent structure occur. Application of a free-energy cycle to this I_d reaction Ni²⁺⋯H₂O with a bond stretching in the transition state long enough to approximate to that for a D reaction shows that the dominance of the effect of change in solvent structure on the Ni²⁺ in the initial or in the transition state is variable according to the composition of the mixture. This latter behaviour is compared and contrasted with similar observations for the substitution of a range of ligands into Ni²⁺_aq in mixtures of water and various co-solvents.