Kinetics and mechanism of aqua ligand substitution from cis-diaqua(cis-1,2-diaminocyclohexane)platinum(II)perchlorate by diethyldithiocarbamate anion in aqueous medium

Abstract

The kinetics of the interaction of diethyldithiocarbamate (Et2DTC) with [Pt(dach)(H2O)2]2+ (dach = cis-1,2-diaminocyclohexane) have been studied spectrophotometrically as a function of [Pt(dach)(H2O)22+], [Et2DTC] and temperature at a particular pH (4.0). The reaction proceeds via rapid outer sphere association complex formation followed by two slow consecutive steps. The first step involves the transformation of the outer sphere complex into an inner sphere complex containing a Pt–S bond and one aqua ligand, while the second step involves chelation when the second aqua ligand is replaced. The association equilibrium constant KE and two rate constants k1 and k2 have been evaluated. Activation parameters for both the steps have been calculated (∆H1# = 66.8 ± 3.7 kJ mol−1, ∆S1# = −81 ± 12 JK−1 mol−1 and ∆H2# = 95.1 ± 2.8 kJ mol−1, ∆S2# = −34.4 ± 9.1 JK−1 mol−1). The low enthalpy of activation and negative entropy of activation indicate an associative mode of activation for both the steps.