Reaction of the transient species W(CO)₅(cyclohexane) with cyclo-C₄H_nO (n = 4, 6, 8) studied by time-resolved infrared absorption spectroscopy

Time- and temperature-resolved infrared absorption spectroscopy is used to monitor the ligand substitution reaction of W(CO)₅(cyclohexane) with a series of ligands L of the form cyclo-C₄H_nO (n = 4, 6, 8). Second-order rate constants for the formation of W(CO)₅(L) are obtained over the temperature range 20-60 °C, and from these temperature-dependent rate constants, activation parameters for the ligand substitution reaction are obtained. We find that for L = furan, ΔH‡ = 7.1 ± 0.7 kcal mol^-1 and ΔS‡ = -7.1 ± 5.1 eu; for L = 2,3-dihydrofuran, ΔH‡ = 5.9 ± 0.5 kcal mol^-1 and ΔS‡ = -8.9 ± 7.3 eu; and for L = 2,5-dihydrofuran, ΔH‡ = 3.8 ± 0.2 kcal mol^-1 and ΔS‡ = -14.0 ± 3.5 e.u. We find that as L becomes less strongly electron-donating, as determined by the trans C-O stretching force constant of W(CO)₅L, ΔH‡rises and ΔS‡ becomes less negative. The correlation of the activation enthalpy with the properties of the products and the low activation enthalpies relative to the (CO)₅W-cyclohexane bond dissociation energy are consistent with an associative interchange (I_a) mechanism for the ligand exchange.