Short Communication: Kinetic Study of the Interactions of Palladium(II) Complexes with Thiols in Acidic Aqueous Solution

Abstract

The kinetics of complex formation between the monofunctional palladium(II) complexes [Pd(dien)Cl]⁺ and [Pd(gly–met-S,N,N´)Cl], where (dien) is diethylentriamine and (gly–met) is glycyl–L-methionine, and L-cysteine, DL-penicillamine and glutathione were studied in an aqueous 0.10 mol dm^–3 perchloric acid medium by using variable-temperature stopped-flow spectrophotometry. Second-order rate constants, k₂²⁹⁸, varied between 17.02 and 1.4410³ M^-1 s^-1. Higher reactivity was observed for the [Pd(dien)Cl]⁺ complex, whereas glutathione was found to be the strongest nucleophile. The reactivity of the thiol nucleophile follows the sequence DL-penicillamine < L-cysteine < glutathione. However, the variation in size, bulk and solvation of the entering thiols reflects their properties as nucleophiles. Negative entropies of activation support a strong contribution from bond making in the transition state of the process.

Manuscript received: 4 April 2001.

Final version: 2 August 2001.