KINETICS OF THE REACTION OF DICHLORO-TRANS-RAC-1,2-DIAMINOCYCLOHEXANEPLATINUM(II) AND DICHLORO-CIS-MESO-1,2-DIAMINOCYCLOHEXANEPLATINUM(II) WITH PYRIDINE

The kinetics of the reaction [Pt(dach)Cl2] + py- [Pt(dach)(py)CI]Cl (dach = either trans-rac- or cis-meso-1,2-diaminocyclohexane, py = pyridine) has been studied in dimethylformamide (dmf) solution, under pseudo-first-order conditions (excess of pyridine), in the range 50-80-degrees-C, by measuring the increase in conductance of the solution. In dmf the products behave like weak electrolytes, with dissociation constants of about 1.4 x 10(-3) mol dm-3 . The reactions were found to be reversible under the experimental conditions. Owing to the uncertainty in evaluating the equilibrium concentrations, the kinetic constants k(obs) have been calculated from the initial slopes (up to 1 5% conversion) of first-order plots. Their values obey the equation k(obs) = k1 + k2[py], k1 being about two orders of magnitude lower than k2. The values of k(obs) for the cis derivative are always slightly but significantly higher than those of the trans. The activation parameters have been evaluated only for k2, since the k1 values are affected by too high uncertainties: DELTAH(double dagger) trans 76(1), cis 79(1) kJ mol-1; DELTAS(double dagger) trans -101(3), cis -92(3) J K-1 mol-1. The higher rate for the cis isomer is interpreted in terms of higher flexibility of the diamine chelate ring.