Time-resolved photoacoustic calorimetry of organometallic ligand substitution in alkane solvents: Molybdenum-alkane σ bond controls fast ligand addition to molybdenum pentacarbonyl


The kinetics and energetics of molybdenum hexacarbonyl ligand substitution with lone pair donor ligands in alkane solvents have been investigated by time-resolved photoacoustic calorimetry (PAC). Two mechanistic steps can be resolved following irradiation: (1) substitution of CO by solvent to form an alkane sigma complex and (2) the displacement of the alkane by a donor ligand. The enthalpy of the first step was 29 kcal/mol for all ligands, while the second step varies from −4 (benzene) to −24 (tetrahydrothiophene) kcal/mol for ten σ π and lone pair donors. Stereospecific solution bond energies are reported for the alkane solvent and ten ligands. In heptane, second-order rate constants vary nearly 600-fold (benzene to triethylphosphite), correlate poorly with bond energies but do correlate well with the steric environment of the donor electrons: σ and π < lone pair on trivalent atoms < lone pair on divalent atoms < lone pair on monovalent atoms.

Publication Title

Chemical Physics