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

The kinetics of the reaction [Pt(dach)Cl2] + py- [Pt(dach)(py)CI]Cl (d ach = either trans-rac- or cis-meso-1,2-diaminocyclohexane, py = pyrid ine) has been studied in dimethylformamide (dmf) solution, under pseud o-first-order conditions (excess of pyridine), in the range 50-80-degr ees-C, by measuring the increase in conductance of the solution. In dm f the products behave like weak electrolytes, with dissociation consta nts of about 1.4 x 10(-3) mol dm-3 . The reactions were found to be re versible under the experimental conditions. Owing to the uncertainty i n evaluating the equilibrium concentrations, the kinetic constants k(o bs) have been calculated from the initial slopes (up to 1 5% conversio n) 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 valu es of k(obs) for the cis derivative are always slightly but significan tly higher than those of the trans. The activation parameters have bee n 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 highe r rate for the cis isomer is interpreted in terms of higher flexibilit y of the diamine chelate ring.