Substitution kinetics of W(CO)5(η2-bis(trimethylsilyl)ethyne) with triphenylbismuthine

The labile complex W(CO)(5)(eta(2)-btmse) undergoes replacement of bis(trimethylsilyl)ethyne, btmse, by triphenylbismuthine in cyclohexane solution at an observable rate in the temperature range of 35-50 degrees C yielding almost solely W(CO)(5)(BiPh3) as the final product. The kinetics of this substitution reaction was studied in cyclohexane solution by quantitative FT-IR spectroscopy. The substitution reaction obeys a pseudo-first-order kinetics with respect to the concentration of the starting complex. The observed rate constant, k(obs), was determined at four different temperatures and three different concentrations of the entering ligand BiPh3 in the range 16.8-65.4 mM. From the evaluation of kinetic data a possible reaction mechanism was proposed in which the rate determining step is the cleavage of metal-alkyne bond in the complex W(CO)(5)(eta(2)-btmse). A rate law was derived from the proposed mechanism. From the dependence of kobs on the entering ligand concentration, the rate constant k(1) for the rate determining step was estimated at all temperatures. The activation enthalpy (106 +/- 2 kJ mol(-1)) and the activation entropy (111 +/- 6 J K-1 mol(-1)) were determined for this rate determining step from the evaluation of k(1) 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 calculated tungsten-alkyne bond dissociation energy, also supports this dissociative rate-determining step of the substitution reaction. (c) 2006 Elsevier B.V. All rights reserved.