The kinetics of the substitution of 2,2′-bipyridine into aquanickel(II) ions in water + dimethylsulphoxide mixtures

Abstract

Rates of substitution of 2,2′-bipyridine (bipy) into Ni²⁺_aq in water-rich H₂O + DMSO mixtures can be discussed strictly on the basis of the Eigen–Wilkins mechanism with the first-order rate constant k for the substitution of bipy from the outer sphere to the inner sphere equal to k_ex for the rate of exchange of water between Ni²⁺_aq and the bulk solvent. The effect of changes of solvent structure induced by the addition of DMSO to water can be investigated for this specific reaction without interference from complexes containing DMSO in the coordination sphere. ΔH* and ΔS* for the reaction decrease sharply in the composition region mole fraction of DMSO x₂≈ 0.1–0.2 corresponding to the minimum in the apparent molar volume of DMSO; thereafter, ΔH* and ΔS* decrease more slowly towards x₂≈ 0.35 where extrema in other physical properties suggest that maximum formation occurs in the mixture. Changes in the solvent structure have a greater effect on the initial state than on the transition state. The effects on this reaction of adding DMSO to water are compared with the effects produced when other co-solvents are added to water.