The Kinetics and Mechanism of Substitution of Square-Planar Complexes of Nickel(II), Palladium(II) and Platinum(II) Containing Sulfur Ligands

Abstract

The kinetics of substitution of nickel(II) ethylxanthate, dialkyldithiophosphate, diphenyldithiocarbamate and monothioacetylacetonate complexes, and palladium and platinum(II) ethylxanthate and dithiophosphate complexes by dithiocarbamate ion have been studied in acetone. In general, second order kinetics were observed; in some systems the mixed-ligand complex was observed as an intermediate while in others no intermediates were detected. An intermediate observed in the platinum ethylxanthate and diethyldithiocarbamate reaction is square-planar with two monodentate dithiolate ligands. Rates of substitution of the dithiophosphates for nickel, palladium and platinum are in the ratio 9 × 10⁵ : 1900 : 1 and the sequence is shown to be an effect of increasing activation enthalpy. Contrary to earlier reports, variations of rates of substitution of nickel(II) complexes do not reflect in any simple way the ability of the nickel atom to expand its coordination shell, nor the thermodynamic stability of the complexes, and it is proposed that there is a significant contribution from bond-breaking in these reactions.