Reaction of the transient species W(CO)₅(cyclohexane) with pyrrolidine and with pyrrole

Time-resolved infrared spectroscopy is used to examine the reactions of the transient intermediate W(CO)₅(CyH) (CyH = cyclohexane), formed by photolysis of a cyclohexane solution of W(CO)₆, with L = pyrrole and pyrrolidine. Time- and temperature-dependent rate constants for the ligand substitution reaction to form W(CO)₅(L) are determined. These reactions appear to go through an associative interchange mechanism. Eyring activation parameters for the ligand exchange are derived. For L = pyrrolidine, ΔH^‡ = 1.8 ± 0.1 kcal mol^-1 and ΔS^‡ = -18.9 ± 0.4 eu; for L = pyrrole, ΔH^‡ = 5.1 ± 0.2 kcal mol^-1 and ΔS^‡ = -11.6 ± 1.2 eu. Most unusually, the observed pseudo-first-order rate constant for reaction with pyrrole is not a linear function of pyrrole concentration. This deviation from simple first-order behavior is explained in terms of association of pyrrole in solution to form dimers and higher clusters that lower the ligand's effective concentration. The enthalpy of dimerization of pyrrole is estimated from the kinetic data to be approximately 4 kcal mol^-1. The results of reaction of W(CO)₅(CyH) with pyrrole and pyrrolidine are discussed in terms of the behavior of this intermediate with other ligands.