Kinetics and mechanism of the interaction of thiourea with cis-diaquaethylene-diamineplatinum(II) perchlorate in aqueous medium

The kinetics of the interaction of thiourea with [Pt(en)(H2O)(2)](2+) have been studied spectrophotometrically as a function of [Pt(en)(H2O)(2)](2+), [thiourea] and temperature at a particular pH(4.0), where the substrate com plex exists predominantly as the diaqua species and the thiourea ligand as a neutral molecule. The reaction proceeds via a rapid outer sphere associat ion followed by two slow consecutive steps, the second step exhibiting firs t order dependence on the aqua ion and thiourea concentrations. The activat ion parameters for both the steps have been evaluated: (Delta H(1)double da gger = 54.8 +/- 1.2 kJ mol(-1), Delta S(1)double dagger = -96 +/- 4 J K-1 m ol(-1), Delta H(2)double dagger = 27.9 +/- 0.8 kJ mol(-1) and, Delta S(2)do uble dagger = -183 +/- 2.6 J K-1 mol(-1)). The low enthalpy of activation a nd large negative values of entropy of activation indicate an associative m ode of activation for both consecutive steps.