Complex-formation reactions of aquacobalamin revisited: effect of chloride on the rate and activation parameters

Abstract

Complex-formation reactions of aquacobalamin were reinvestigated to clarify the role of high chloride concentrations (up to 1 mol dm^–3). It was found that high chloride concentrations retard the reactions due to the formation of the less substitution-labile chloro complex, for which the formation constant was found to be 0.8 dm³ mol^–1 at 25 °C and ionic strength 1.0 mol dm^–3. The pH dependence of the complex formation with hydrazoic acid was also re-examined. Volumes of activation were determined for the reactions with HN₃ and N₃^– in NaClO₄ medium and are discussed in comparison with those measured in KCl medium. Direct kinetic evidence for the occurrence of a reverse, acid-catalysed aquation reaction was found and this step is characterised by an activation volume of +8.3 ± 1.6 cm³ mol^–1. The results are all in agreement with the concept of a dissociative interchange substitution mechanism.