Equilibria, kinetics and mechanism for complex formation between hydrogen sulfate/sulfate and palladium(II). Hydrolysis of tetraaquapalladium(II)

Abstract

Spectrophotometric equilibrium measurements indicate formation of the complexes [Pd(H2O)3HSO4]+ and [Pd(H2O)3SO4] in the reaction between [Pd(H2O)4]2+ and hydrogen sulfate/sulfate in the region: 0.10≤[H+]≤0.80 M. The stability constants are 0.7±0.2 and 19±6 M-1, respectively, at 25 °C and 1.00 M ionic strength. The protolysis constant for coordinated hydrogen sulfate, i.e. the equilibrium constant for the reaction [Pd(H2O)3HSO4]+ + H2O⇌[Pd(H2O)3SO4]+H 3O+, is 2.5±1.0 M. The stability constant for [Pd(H2O)3HSO4]+ and the protolysis constant for coordinated HSO4- are also derived from kinetic measurements as 0.6±0.2 M-1 and 2.3±1.3 M, respectively. The kinetics for the reversible complex formation reaction, studied by use of stopped-flow spectrophotometry, is first order with respect to palladium complex and total concentration of sulfate, [S(VI)], with an observed pseudo-first-order rate constant kobsd=kf[S(VI)]+kr for excess sulfate. Here kf and kr denote observed forward second-order and reverse first-order rate constants, respectively. The kinetic data are interpreted in terms of a reaction mechanism which involves parallel and reversible reactions between [Pd(H2O)4]2+ and HSO4- and SO42-, respectively, and between [Pd(H2O)3OH]+ and HSO4-. Forward and reverse rate constants for complex formation between [Pd(H2O)4]2+ and HSO4- are 119±6M-1s-1 and 210±60 s-1 at 25°C, indicating that HSO4- has a similar nucleophilicity as other oxygen-donor ligands. The rate constants for the reactions of [Pd(H2O)4]2+ with SO42- and of [Pd(H2O)3OH]+ with HSO4- cannot be resolved because of a proton ambiguity. The mononuclear protolysis constant of [Pd(H2O)4]2+ is pKh=3.0±0.1 at 25°C and 1.00 M ionic strength as determined from rapid spectrophotometric equilibrium measurements. © Acta Chemica Scandinavica 1998.