Photoacoustic calorimetry at high pressure: A new approach to determination of bond strengths. Estimation of the M-L bond dissociation energy of M(CO)₅L (M = Cr, Mo; L = H₂, N₂) in n-heptane solution

The enthalpy changes of the reactions M(CO)₆ → M(CO)₅(L) (L = n-heptane, H₂, and N₂) are studied in a new way by photoacoustic calorimetry (PAC) using high pressures (130 bar of H₂ or N₂). This high pressure increases the concentration of H₂ or N₂ dissolved in the solvent, thereby accelerating the reactions. Previously, these reactions could not be studied in alkane solvents because at atmospheric pressure they proceed too slowly to be studied by PAC. The enthalpy change associated with the reaction M(CO)₆ → M(CO)₅(heptane) under 130 bar of Ar is measured by high-pressure PAC to be 105 kJ mol^-1, consistent with measurements made by other workers using PAC at low pressure. This demonstrates the reliability of the high-pressure technique. The kinetics of the reaction M(CO)₆ → M(CO)₅(L) (L = H₂ and N₂) are studied by time-resolved infrared spectroscopy, and it is shown that the reactions at high pressure are sufficiently fast so as to be studied by PAC. The enthalpy change of the reaction Cr(CO)₆ → Cr(CO)₅(H₂) is measured to be 84 kJ mol^-1, and the Cr-H₂ bond energy derived from this measurement is 78 kJ mol^-1, again consistent with measurements made by other workers at low pressure. The Mo-H₂ bond energy is estimated to be 81 kJ mol^-1, and the Cr-N₂ and Mo-N₂ bond energies are estimated to be 81 kJ and 82 kJ mol^-1, respectively.