Kinetics and mechanism of ligand interchange in the [RuIII(edta)L] complexes;: L = cysteine and related thiolates

Complexes of the [Ru-III(edta)SR](n) series, with SR- = deprotonated cysteine, N-acetylcysteine, 2-mercaptoethanol, glutathione and penicilamine, were prepared from [Ru(edta)H2O](-) and the corresponding RSH thiols, at pH = 5.5. The complexes exhibit intense visible absorption bands at ca. 520 nm (epsilon congruent to 3500 M-1 cm(-1)), associated with LMCT from the sulfur ligands bound to Ru-III. The kinetics of the formation reactions were first order in [Ru-III(edta)H2O](-) and thiol reactants, with k(1) values ca. 1-5 x 10(2) M-1 s(-1) (25 degrees C) for all the sulfur ligands except penicilamine, which reacted slower by a factor of 10. Activation parameters suggest an associative mechanism, as for the coordination of other S- and N-bound ligands to [Ru-III(edta)H2O](-). A reactivity decrease is apparent at low and high pH's (ranges 1-3 and 8-10, respectively), associated with acid-base equilibria involving the less reactive [Ru-III(Hedta)H2O] and [Ru-III(edta)OH](2-) species. A significant rate increase was found for cysteine and penicilamine at ca. pH = 8.0, because the thiol reactants deprotonate. The equilibrium constants for all the ligands showed that robust complexes were formed, with K = ca. 1 x 10(5) M-1 (25 degrees C). The dissociation rate constants, k(-1), were in the 10(-3)-10(-4) s(-1) range. The influence of nucleophilic and steric effects increasing and decreasing the formation rates, respectively, is discussed for the thiolate ligands, with adequate comparisons with other L species bound to[Ru (edta)H2O](-).