The displacement of the heterocyclic base R-py from [Pd(dien)(R-py)]2+(dien = 1,5-diamino-3-azapentane, R-py = 4-chloropyridine or 4-aminopyridine) by water in acid and basic solution and by chloride. The involvement of a stable five-co-ordinate species

Abstract

The kinetics of displacement of R-py from the [Pd(dien)(R-py)]²⁺ cation in acid and basic solution in the absence and presence of chloride ions are reported. In acid solution the rate of displacement of 4-chloropyridine is independent of [H⁺] but that for 4-aminopyridine is acid catalysed, the pyridylammonium ligand being a much better leaving group. In basic solution, the rate of displacement of 4-chloropyridine obeys the rate equation l0³k_obs.= 8.7 + 11.5[OH^–]+ 122[OH^–]²s^–1, while that for the 4-aminopyridine complex is independent of [OH^–]. The reaction between [Pd(dien)(4Cl-py)]²⁺ and Cl^– leads to the rapid formation of a stable intermediate species which is in equilibrium with [Pd(dien)Cl]⁺ and 4Cl-py. No such intermediate is observed in the reaction of the 4-aminopyridine complex which loses the ligand by the normal path for substitution in square-planar complexes. In the presence of acid the entry of Cl^– is irreversible and both substrates revert to the classical mechanism for ligand substitution.