Kinetics and mechanism of the interaction of DL-methionine with cis-diaquaethylenediamine platinum(lI) perchlorate in aqueous medium

The kinetics of the interaction of DL-methionine with [Pt(en)(H2O)(2)](2+) have been studied spectrophotometrically as a function of [Pt(en)(H2O)(2)(2+)], [DL-methionine], PH and temperature. The reaction proceeds via a rapid outer sphere association followed by two slow consecutive steps having first order dependence on the aqua ion and methionine concentrations. Activation para meters have been evaluated using the Eyring equation. The low Delta H-1(double dagger) (15.6 kJ mol(-1)) and large negative value of Delta S-1(double dagger) (-230 J K-1 mol(-1)) as well as Delta H-2(double dagger) (19.5 kJ mol(-1)) and Delta S-2(double dagger) (-226 J K-1 mol(-1)) indicate an associative mode of activation for both the aqua ligand substitution processes in the two consecutive steps.