Electronic and steric effects in ligand substitution at a transient organometallic species: The reaction of W(CO)₅(Cyclohexane) with (CH₃)_nTHF and (CH₃)_nFuran (n = 1, 2)

Time-resolved infrared absorption spectroscopy is used to study the reactions of the solvated transient intermediate W(CO)₅(cyclohexane) with the molecules (L) 2-methyltetrahydrofuran (MeTHF), 2,5-dimethyltetrahydrofuran (Me₂THF), 2-methylfuran (MeFur), and 2,5-dimethylfuran (Me₂Fur). In all four cases, the only reaction observed on the microsecond to millisecond time scale is substitution of the cyclohexane molecule to form W(CO)₅(L). From the temperature dependence of the reaction rate constant, measured over the range 15-60 °C, we determine the following activation parameters: for L = MeTHF, ΔH‡ = 4.0 ± 0.3 kcal mol^-1, ΔS‡ = -14.1 ± 1.0 eu; for L = Me₂THF, ΔH‡ = 4.3 ± 0.3 mol^-1 ΔS‡ = -14.8 ± 1.2 eu; for L = MeFur, ΔH‡ = 6.0 ± 0.2 kcal mol^-1, ΔS‡ = -10.2 ± 1.2 eu; and for L = Me₂Fur, ΔH‡ = 6.3 ± 0.4 kcal mol^-1, ΔS‡ = -9.8 ± 1.0 eu. We find that the relative rates of reaction of MeTHF and Me₂THF can be explained solely in terms of a steric effect, while for MeFur and Me₂Fur, the inductive effect of the electron-donating substituents leads to an electronic effect that competes with the steric effect.