Kinetics of the reaction of dichloro-trans-rac- and -cis-meso-1,2-diaminocyclohexaneplatinum(II) with pyridine

Abstract

The kinetics of the reaction [Pt(dach)Cl₂]+ py →[Pt(dach)(py)Cl]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 °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 × 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 15% 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: ΔH^‡trans 76(1), cis 79(1) kJ mol^–1; ΔS^‡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.