Kinetics of anation in platinum(II) and palladium(II) complexes of N-alkylated diethylenetriamine

Abstract

The kinetics of replacement of H2O in the pseudo-octahedral complexes [Pt(L–L–L)(H2O)][ClO4]2(L–L–L = Me2N[CH2]2NMe[CH2]2NMe2) and [Pd(L–L–L)(H2O)][ClO4]2(L–L–L = R2N[CH2]2NMe[CH2]2NR2, R = Me or Et] by the nucleophiles Cl–, Br–, I–, (NH2)2CS, SCN–, and S2O32– have been studied in water at 25 °C. All the reactions exhibit a first-order dependence on both the substrate and the nucleophile concentration. The kinetic behaviour of the complexes depends markedly on the size of the substituents at the nitrogen atoms in the ligand L–L–L. When L–L–L = Me2N[CH2]2NMe[CH2]2NMe2 the second-order rate constants follow the usual nucleophilicity scale and in the case of the platinum complex the kinetic behaviour can be related to that of the unhindered complex [Pt(dien)(H2O)]2+(dien = H2N[CH2]2NH[CH2]2NH2). In the case of the palladium substrate where the ligand L–L–L bears four bulky ethyl groups the reactivity is largely reduced with respect to that where L–L–L = Me2N[CH2]2NMe[CH2]2NMe2 and the usual scale of nucleophilicity is no longer valid. The experimental findings are interpreted in terms of the usual bimolecular attack at the metal on the basis of the electrophilicity and the steric hindrance at the reaction centre caused by the substituents of the tridentate ligand.