Kinetics and mechanism of the interaction of pyridine-2-thiol with cis-diaqua(ethylenediamine) platinum(II) perchlorate in aqueous medium

The kinetics of the interaction of pyridine-2- thiol with [Pt(en)(H2O)(2)]( 2+) have been studied spectrophotometrically as a function of [Pt(en) (H2O) (2)(2+)],[pyridine -2-thiol] and temperature at a particular pH (4.0) where the substrate complex exists predominantly as the diaqua species and pyrid ine -2-thiol as the neutral molecule. The reaction proceeds via a rapid out ersphere association followed by two slow consecutive steps, having first o rder dependence on the aqua ion and pyridine-2-thiol in the second stage. A ctivation parameters have been calculated using Eyring equation. The low De ltaH(1)(++) ( 36.0 +/- 3.0 kJ mol(-1)) and large negative value of DeltaS(1 )(++) (- (1.66 +/- 0.08) x 10(2) J K-1 mol(-1)) as well as DeltaH(2)(++) (2 6.0+/- 1.0 kJ mol(-1)) and DeltaS(2)(++) (-(2.18+/- 0.04) x 10(2) J K-1 mol (-1)) indicate associative mode of activation for both the aqua ligand subs titution process in the two consecutive steps.