Heats of Reaction of RMo(CO)₃C₅H₅ (R = H, CH₃, C₂H₅) with Phosphines and Phosphites: Thermodynamic Study of the CO-Insertion Reaction

The heats of reaction Of HMo(CO)₃C₅H₅ with a series of phosphines and phosphites, producing HMo(CO)₂(PR₃)C₅H₅, have been measured by solution calorimetry. The order of relative Mo-PR₃ bond strengths, PMe₃ ≈ P-n-Bu₃ > P(OMe)₃ > PMe₂Ph > PMePh₂, closely resembles that determined earlier for the complexes fac-(PR₃)₃Mo(CO)₃. The heats of reaction of RMo(CO)₃C₅H₅ (R = CH₃, C₂H₅), producing the phosphine-substituted acyl complexes RC(O)Mo(CO)₂(PR₃)C₅H₅, were also studied thermochemically. This reaction can be viewed as involving both phosphine substitution and carbonyl insertion. With use of the enthalpy of phosphine substitution of HMo(CO)₃C₅H₅ as a model, the enthalpy of carbonyl insertion is calculated. For both R = CH₃ and R = C₂H₅ the carbonyl-insertion reaction is favored for more basic phosphines with a difference of about 2 kcal/mol between PMe₃ (most favored) and P(OMe)₃ (least favored). Carbonyl insertion into the Mo-Et bond is about 3 kcal/mol more favorable than into the Mo-CH₃ bond. Attempts to prepare the unsubstituted acyls RC(O)Mo(CO)₃C₅H₅ by carbonylation of the alkyls in THF led to facile reductive elimination of RC(O)C₅H₅ (35°C, 1 atm of CO), producing Mo(CO)₆. Migration of the acyl group to the coordinated cyclopentadienyl ring in the presence of CO pressure and thermodynamic instability in the absence of CO pressure explain the lack of success in preparing RC(O)Mo(CO)₃C₅H₅ for simple alkyl groups.