The appearance of a stable intermediate in some substitution reactions of (1,5-diamino-3-azapentane)pyridinepalladium(II) perchlorate

Abstract

The kinetics and equilibria of the reversible displacement of pyridine (py) from [Pd(dien)(py)]²⁺(dien = 1,5-diamino-3-azapentane) in aqueous solution have been studied. In acidic solution the reaction is reversible and the solvolytic rate constant =(2.5 ± 0.1)× 10^–3 s^–1 at 25 °C is independent of [H⁺]. In the presence of excess of hydroxide the reaction goes to completion and the [OH^–] dependence takes the form 10³k_obs.=(2.33 ± 0.1)+(6.3 ± 0.8)[OH^–]+(8.05 ± 0.9)[OH^–]² s^–1 at 25 °C, I= 1.0 mol dm^–3. Chloride ions interact readily with the complex but the change in absorbance, while qualitatively consistent with the displacement of pyridine by Cl^–, can only be accounted for if an intermediate species containing both chloride and pyridine is formed in significant quantity. The kinetics and equilibria of the formation of this species have been studied in neutral aqueous solution at 25 °C, I= 1.0 mol dm^–3. The nature of this intermediate is discussed.