Reactions of the Transient Species W(CO)5(Cyclohexane) with Thiophene and Tetrahydrothiophene Studied by Time-Resolved Infrared Absorption Spectroscopy

The transient species W(CO)5(CyH) (CyH = cyclohexane) is prepared by laser photolysis of a CyH solution of W(CO)6. Time-resolved infrared spectroscopy is used to probe the ligand substitution reactions of W(CO)5(CyH) with thiophene and tetrahydrothiophene (THT). From the temperature dependence of the second-order rate constants derived from the kinetic study, we obtain for reaction with thiophene ΔH ⧧ = 5.7 ± 0.1 kcal mol-1 and ΔS ⧧ = −11.2 ± 1.1 eu, while for reaction with THT, ΔH ⧧ = 2.6 ± 0.1 kcal mol-1 and ΔS ⧧ = −15.5 ± 0.8 eu. As expected, THT, the stronger electron donor, is the more reactive ligand of the two. THT is much more reactive than would be expected based on its strength as an electron donor, however. We show that the relative reactivities of O-, N-, and S-containing five-membered heterocycles toward ligand substitution at W(CO)5(CyH) can be accounted for by a model that takes into account the attacking ligand's polarizability in addition to its electron-donating ability.