Kinetics and Mechanism of the Reaction of Platinum(II) Complexes with Thioglycolic Acid

Abstract

Complex formation of [Pt(H2O)4]2+ and [PtCl4]2− with thioglycolic acid has been studied as a function of temperature (288.2–308.2 K) using spectrophotometry in 1.00 M aqueous perchloric acid; the statistically corrected corresponding second-order rate constants are k1298=5.25×10−2 M−1ċs−1 and 0.624 M−1ċs−1, respectively. The temperature dependence of the rate constants gives the following activation parameters: ΔH1‡=34±2 kJċmol−1, ΔS1‡=−142±8 Jċ K−1ċmol−1 for [Pt(H2O)4]2+, ΔH1‡=31±2 kJċmol−1, ΔS1‡=−133± 6 JċK−1ċmol−1 for [PtCl4]2−. The negative values for the entropies of activation are consistent with an associative mode of activation. A LFER diagram observed for the complex formation of [Pt(H2O)4]2+ with sulfur containing ligands support an A mechanism. The reactivity of sulfur-bonding ligands towards [Pt(H2O]4]2+ follows the order H2O <Me2SO <SHCH2COOH <SCN− <S(CH2)4O<Et2S< S(CH2)4S<Me2S<SC(NH2)2 (1:3:7.4×103:4.8×104:5.2×104:6.8×104:8.8×104:1.3×105:5×105). A trigonal bipyramidal transition state stabilized by hydrogen bonds between the entering thioglycolic acid and the leaving aqua or chloro ligand is suggested..