Relative reactivity of N- and S-donor ligands in substitution reactions of aquaethylenedinitrilotetraacetatoruthenium(III) in aqueous solution*

Abstract

4-Sulfanylpyridine, a potential bridging ligand, reacted with [Ru(edta)(H₂O)]^– following a facile aqua substitution path through S-co-ordination of the pendant thione group or through N-co-ordination. The substitution reaction was studied as a function of pH (0.4–5.0), temperature (25–45 °C), pressure (0.1 to 100 MPa) and concentration. Second-order rate constants for the N- and S-donor complex-formation reactions are 4950 ± 60 and 1560 ± 50 M^–1 s^–1 at 25 °C respectively. The N-co-ordinated ruthenium(III) complex reacts to give the S-co-ordinated product with a rate constant of 0.07 ± 0.02 s^–1 at 25 °C. Detailed kinetic studies, including the determination of all activation parameters (ΔH ^‡, ΔS ^‡ and ΔV ^‡), revealed unambiguously that formation of the S-substituted product is favored thermodynamically, whereas that of the N-substituted product is favored kinetically. The N-co-ordinated complex reacts to give the S-co-ordinated complex via a dissociative process. Depending on the reaction conditions, mono- and weakly interacting asymmetric bi-nuclear complexes have been synthesized and characterized.