Direct measurement of the enthalpy of decomposition of HCr(CO)3C5H5 to [Cr(CO)3C5H5]2 and H2 was made by differential scanning calorimetry. The heat of hydrogenation of 1,3-cyclohexadiene by HM(CO)3C5H5 for M = Cr, Mo, and W was measured by solution calorimetry. The enthalpies of iodination of [M(CO)3C5H5]2 and HM(CO)3C5H5 were measured for M = Mo and W. These data have been used to calculate the heats of hydrogenation for each of the metal-metal bonded dimers, [M(CO)3C5H5]2 (M = Cr, Mo, and W). C5H5(CO)3M-M(CO)3C5H5(s) + H2(g) → 2HM(CO)3C5H5(s). Addition of hydrogen has been found to be exothermic for M = Cr, W (-3.3 kcal/mol and -1.5 kcal/mole, respectively) but endothermic for M = Mo (+6.3 kcal/mol). These results are consistent with the trend of increasing MH bond strengths upon descending Group VI. Addition of H2 to [Cr(CO)3C5H5]2 is favored by the unusually weak chromium-chromium bond.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry
- Materials Chemistry