Synthesis and thermochemistry of HMo(CO)₃C₅Me₅; Comparison of cyclopentadienyl and pentamethylcyclopentadienyl ligands

Toluenemolybdenum tricarbonyl reacts quantitatively with pentamethylcyclopentadiene in THF at room temperature yielding HMo(CO)₃C₅Me₅. The heat given off in this reaction has been measured by solution calorimetry and indicates there is little difference in the MoC₅H₅ and MoC₅Me₅ bond strengths. Thermochemical measurements of the reaction of HMo(CO)₃C₅R₅ (R = H, CH₃) with pyridine yielding (py)₃Mo(CO)₃ confirm this result. In THF solution, HMo(CO)₃C₅Me₅ is a weaker acid than HMo(CO)₃C₅H₅, ΔpK_a ≥ 3. The heat of hydrogenation of (Mo(CO)₃C₅R₅)₂ to yield two mol of HMo(CO)₃C₅R₅ is more favorable by about 2 kcal/mol for R = CH₃ compared to R = H, probably due to steric repulsion in (Mo(CO)₃C₅Me₅)₂.