Bond Energies, Reaction Volumes, and Kinetics for σ- and π-Complexes of Mo(CO)5L

The photosubstitution reactions of molybdenum hexacarbonyl with σ and π donor ligands were investigated using photoacoustic calorimetry and computational methods in a series of linear alkane solvents (pentane, hexane, heptane, octane, decane, and dodecane). The results show that reaction volumes make a significant contribution to the photoacoustic signal and must be considered during thermodynamic calculations based on photoacoustic measurements. The enthalpies of CO substitution by an alkane solvent and subsequent substitution by each Lewis base were determined. Corresponding Mo–L bond energies (kcal mol–1) were calculated: L = linear alkanes (13), triethylsilane (26), 1-hexyne (27), 1-hexene (27), and benzene (17). The relative energies are in agreement with computational results. The experimental reaction volume for CO substitution by alkane was positive (15 mL mol–1) and negative or close to zero for alkane substitution by a Lewis base (for example, −11 mL mol–1 for triethylsilane and 3.6 mL mol–1 for benzene). The errors in the experimental and computational reaction volumes are large and often comparable to the reaction volumes. An improved calibration of the methods as well as a better understanding of the underlying physics involved is needed. For the Lewis bases reported in this study, the second-order rate constants for the displacement of a coordinated alkane are less than diffusion control (5 × 106–4 × 107 M–1 s–1) and decrease monotonically with the alkane chain length. The rate constants correlate better with steric effects than with bond energies. An interchange mechanism is consistent with the results.