Substitution kinetics of Cr(CO)5(η2-Z-cyclooctene) with tetracyanoethylene

The weakly bound Z-cyclooctene (zco) ligand in Cr(CO)5(η2-zco) is replaced by tetracyanoethylene (tcne) at an observable rate in the temperature range between −5 and +10 °C yielding the complex Cr(CO)5(η2-tcne) as the final product. The kinetics of this substitution reaction was studied in toluene solution containing 5% by volume zco by quantitative FTIR spectroscopy. The substitution reaction obeys a pseudo-first order kinetics with respect to the concentration of the starting complex. The observed rate constant, kobs, was determined at four different temperatures and five different concentrations of the entering ligand tcne in the range of 0.033–0.33 M. From the evaluation of kinetic data a possible reaction mechanism was proposed in which the rate-determining step is the cleavage of metal–olefin bond in the complex Cr(CO)5(η2-zco). A rate law was derived from the proposed reaction mechanism. From the dependence of kobs on the entering ligand concentration, the rate constant k1 for the rate determining step was estimated at all temperatures. The activation enthalpy (ΔH°=100±3 kJ·mol−1) and the activation entropy (ΔS°=59±3 J·K−1·mol−1) were determined for this rate-determining step from the evaluation of k1 values at different temperatures. The large positive value of the activation entropy is consistent with the dissociative nature of reaction. The large value of the activation enthalpy, close to the chromium–olefin bond dissociation energy, also supports this dissociative rate-determining step of the substitution reaction.