KINETICS OF THE CIS-DICHLORODIAMMINEPLATINUM(II)-OXALATE REACTION IN AQUEOUS SOLUTION

Abstract

The substitution of the chloride ligands in cis-Pt(NH₃)₂Cl₂ by oxalate has been studied in neutral and slightly acidic aqueous solutions. At 50°, the reaction is first-order with respect to complex concentration. The estimated first-order rate constant (3.9 × 10⁻⁴sec⁻¹ at 50°) and the activation energy (17 kcals/mole) are similar to the corresponding values reported for the aquation of the complex ion. The reaction rate is only slightly pH dependent in the pH range 3–7 indicating that C₂O₄ ⁻² and HC₂O₄ ⁻ have similar reactivities. The rate of the reaction is inhibited by the presence of excess free chloride. These data have been interpreted in terms of a mechanism involving the rate-determining aquation of the complex. By comparison with previous studies in which amine molecules were employed as substituting groups oxalate is a very weak nucleophile. It is suggested that the known biological activity of dichloro complexes of platinum(II) involves reaction at nitrogen-containing rather than oxygen-containing sites in the biologically active molecules.