KINETICS AND MECHANISM OF LIGAND INTERCHANGE IN THE RUIII(EDTA)L COMPLEXES; L = CYSTEINE AND RELATED THIOLATES

Abstract

Complexes of the [RuIII(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. 520nm (e≅3500M−1 cm−1), associated with LMCT from the sulfur ligands bound to RuIII. The kinetics of the formation reactions were first order in [RuIII(edta)H2O]− and thiol reactants, with k1 values ca. 1–5×102 M−1s−1 (25°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 [RuIII(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 [RuIII(Hedta)H2O] and [RuIII(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. 1×105 M−1 (25°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 [RuIII(edta)H2O]−.