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Substitution kinetics of the pentacarbonylbis(trimethylsilyl)ethynetungsten(0) with triphenylbismuhine

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2004
Bayram, Ercan
The reaction between pentacarbonylbis(trimethylsilyl)ethynetungsten(0), W(CO)5(?2-btmse), and triphenylbismuthine, BiØ3, yields pentacarbonyltriphenylbismuthinetungsten(0), W(CO)5(BiØ3), as the sole product. The kinetics of the substitution of btmse by BiØ3 was studied by means of quantitative FT-IR Spectroscopy. The starting complex was prepared photochemically from hexacarbonyltungsten, W(CO)6, in the presence of excess btmse in n-hexane and identified by FT-IR and NMR Spectroscopies. The substitution reactions were performed in cyclohexane solutions at different concentrations of both leaving and entering ligand to observe the dependence of observed rate constant, kobs, on the concentration of entering and leaving ligands and also, at different temperatures to evaluate the activation enthalpy (?H?) and the activation entropy (?S?). The IR extinction coefficients for CO stretching were determined for both the starting complex W(CO)5(?2-btmse), and the product W(CO)5(BiØ3). Quantitative IR Spectroscopy does not show any significant reduction in the total amount of substance (material balance). Formation of W(CO)6 (in small amount) was attributed to the decomposition of the product, W(CO)5BiØ3. From the evaluation of kinetic data, a mechanism was proposed in which the rate determining step is the cleavage of btmse ligand from the starting complex, W(CO)5(?2-btmse) and the formation of solvated complex, W(CO)5(solvent). Thus, the reaction is essentially dissociative. The large positive value of activation entropy (?S?) and large value of activation enthalpy (?H?) are indicative of a dissociative mechanism.