A Kinetic and Mechanistic Study on Substitution of Aqua Ligands from cis-diaqua-bis-(bipyridyl)-ruthenium(II) Complex by Thiosemicarbazide in Aqueous Medium

Abstract

The kinetics of the interaction of thiosemicarbazide with cis-[Ru(bipy)2(H2O)2]2+ (bipy = α α′-bipyridyl) have been studied spectrophotometrically as a function of [Ru(bipy)2(H2O) 2 2+ ], [bipyridyl] and temperature, at a particular pH (4.8), where the substrate complex exists predominantly as the diaqua species and thiosemicarbazide as the neutral ligand. The reaction proceeds via an outer sphere association complex formation, followed by two slow consecutive steps. The first is the conversion of the aforementioned complex into the inner sphere complex, and the second step involves the entrance of another thiosemicarbazide molecule in the coordination zone of Ru(II) whereby, in each step, an aqua ligand is replaced. The association equilibrium constant (KE) for the outer sphere complex formation has been evaluated together with rate constants for the two subsequent steps. Activation parameters have been calculated for both steps using the Eyring equation (ΔH 1 # = 25.37±1.6 kJ mol−1, ΔS 1 # = −215.48 ± 4.5 J K−1 mol−1, ΔH 2 # = 24.24 ± 1.1 kJ mol−1, ΔS 2 # = −207.14 ± 3.0 J K−1 mol−1). The low enthalpy of activation and large negative value of entropy of activation indicate an associative mode of activation for both aqua ligand substitution processes. From the temperature dependence of KE, the thermodynamic parameters calculated are: ΔH0 = 10.75±0.54 kJ mol−1 and ΔS0 = 84.67 ± 1.75 J K−1 mol−1, which give a negative ΔG0 value at all temperatures studied, supporting the spontaneous formation of an outersphere association complex prior to the first step.