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 kex 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.