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

Abstract

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.