An unusual solvent isotope effect in the reaction of W(CO)₅(solv) (solv = cyclohexane or Cyclohexane-d₁₂) with THF

Time- and temperature-resolved infrared absorption spectroscopy is used to probe the reaction of THF with W(CO)₅(solv) (solv = cyclohexane, cyclohexane-d₁₂) to form W(CO)₅(THF). In cyclohexane-do, δH‡ for the reaction is determined to be 3.6 ± 0.2 kcal mol^-1 (15.2 ± 0.9 kj mol^-1) and δS‡ = -13.7 ± 2.5 eu (-57.3 ± 10.5 J mol^-1 K^-1). In cyclohexane-d₁₂, δH‡ = 2.4 ± 0.6 kcal mol^-1 (10.2 ± 2.5 kJ mol^-1) and δS‡ = -18.3 ± 3.5 eu (-76.6 ± 14.6 J mol^-1 K^-1). The low activation enthalpy in comparison to the (CO)₅W-cyclohexane bond dissociation energy, the negative entropy of activation, and comparison with other spectroscopic experiments involving reactions of W(CO)₅(cyclohexane) indicate that the reaction proceeds by an associative interchange reaction mechanism. An unusual isotope effect, in which reaction proceeds more slowly in cyclohexane-d₁₂ even though δH‡ is lower, is explained in terms of relative populations of low-frequency vibrations and the relative C-H and C-D bond strengths in the solvent molecule released upon uptake of THF. These experimental results are supported by the results of an ab initio DFT calculation of the structure and vibrational spectrum of W(CO)₅(cyclohexane).