Photoacoustic Calorimetry and Quantum Yields of Mo(CO)₆ Ligand Exchange in Linear Alkanes:  Determination of Volume of Reaction, Energetics, and Kinetics of Nucleophile Displacement of Alkane from Mo(CO)₅(Alkane)

Abstract

Photoacoustic calorimetry (PAC) and actinometry studies were used to determine the enthalpies and volumes of reaction for the production of a transient intermediate, Mo(CO)(5)-alkane, and for its subsequent reaction with tetrahydrofuran (THF). Both the enthalpy and the volume of reaction contribute to the photoacoustic signal and have been resolved by changing the solvent thermal expansion properties with a series of linear alkanes. The enthalpies for substitution of CO on Mo(CO)(6) by an alkane and of coordinated alkane on Mo(CO)(5)(alkane) by THF are 30 and -14 kcal/mol, respectively. Likewise, the volumes of reaction are 18 and -1 mL/mol. From available data for the Mo--CO bond energy, these results allow the calculation of the Mo-alkane and Mo-THF bond energies (11 and 25 kcal/mol, respectively). The Mo-alkane result is 7 kcal/mol less than that from our previous PAC study, which ignored the volume of reaction, and is in better agreement with the results of kinetic studies. The large absolute difference in the reaction volumes for each step is partially attributed to a void volume created during the formation of the Mo--THF bond. In general, the volume of reaction cannot be neglected in the calculation of enthalpies of ligand substitution from PAC studies. The quantum yields for photosubstitution of Mo(CO)(6), in contrast to Cr(CO)(6), were found to be insensitive to the chain length of the alkane solvent.