Kinetics and mechanism of substitution of aquoethylenediaminetetraacetatoruthanate(III) with cysteine in aqueous solution

Reaction of the edta (ethylenediaminetetraacetate) complex of rurhenium(III) with cysteine has been studied spectrophotometrically as a function of [cysteine], pH (1.3-9.5) and temperature (20-45 degrees) in aqueous solution. Stopped-flow kinetic studies revealed that the substitution is first-order in both reactants. A maximum observed rate constant (k(obs)) was obtained in the pH region 7.8-8.0 which is in contrast to the characteristic bell-shaped k(obs)-pH profile for the substitution of Ru(III)-edta reported so far the (where maximum k(obs) is usually attained in the pH range 4.5-6.0). Activation parameters determined from the temperature dependence of observed rate constants at pH = 5.0 are comparable to those for other thio-ligand substitutions and consistent with the proposed mechanism.