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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry