Estimating the (CO)5Cr–(η2-benzene) bond dissociation enthalpy: reaction of the (CO)5Cr(η2-benzene) complex with a series of (CH3)nTHF (n = 0, 1, 2, and 4) ligands

Abstract

The displacement of the benzene ligand from the photolytically generated (CO)₅Cr(η²-benzene) complex by THF and a series of methyl substituted (CH₃)_nTHF (n = 1, 2, and 4) ligands was studied using the technique of laser flash photolysis. The data suggest that two independent pathways (D and I_d) contribute to the overall displacement of the benzene solvent from the Cr center. The contribution of the I_d pathway increases as the steric bulk of the Me_nTHF ligand is reduced from Me₄THF to MeTHF. The activation enthalpy of 11.4 ± 1.1 kcal mol⁻¹ obtained when the entering ligand is Me₄THF is considered to be a lower estimate of the (CO)₅Cr–(η²-benzene) bond dissociation enthalpy. A detailed energetic profile of the substitution reaction is presented.