Ligand substitution reactions of the CpMn(CO)2(cyclohexane) intermediate studied by time-resolved infrared absorption spectroscopy

Abstract

Time-resolved infrared absorption spectroscopy is used to observe the ligand substitution reactions of the photolytically generated solvated intermediate CpMn(CO)₂(CyH) (CyH = cyclohexane) with L = cyclopentene, THF, furan, and pyrrolidine to form CpMn(CO)₂(L). Second-order rate constants and reaction activation parameters are derived. Although the pseudo-first order reaction rate constant is a linear function of ligand concentration over the entire concentration range studied, the observed kinetics for these reactions are most consistent with a dissociative interchange mechanism. A lower limit of 34 ± 3 kJ mol⁻¹ for the [Cp(CO)₂Mn]–CyH bond dissociation enthalpy is derived. Reactivity trends among the ligands are discussed, and comparisons are made between the ligand substitution behavior of CpMn(CO)₂(CyH) and that of other alkane-solvated transition metal intermediates. From these comparisons, it appears that for reaction of a coordinatively unsaturated transition-metal in a given solvent, the relationship between ΔH^‡ and ΔS^‡ is determined by the solvent independent of the particular metal complex reacting.