Kinetic study of the interactions of palladium(II) complexes with thiols in acidic aqueous solution

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 dieth ylentriamine and (gly-met) is glycyl-L-methionine, and L-cysteine, DL-penic illamine and glutathione were studied in an aqueous 0.10 mol dm(-3) perchlo ric acid medium by using variable-temperature stopped-flow spectrophotometr y. Second-order rate constants, k(2)(298), varied between 17.02 and 1.44x10 (3) M-1 s(-1). Higher reactivity was observed for the [Pd( dien) Cl](+) com plex, whereas glutathione was found to be the strongest nucleophile. The re activity 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.