Direct solution calorimetric measurements of enthalpies of proton and electron transfer reactions for transition metal complexes. Thermochemical study of metal-hydride and metal-metal bond energies

Enthalpies of reaction with sodium amalgam, sodium benzophenone ketyl, and metal carbonyl anions of organometallic complexes in THF solution have been measured. Relative to reduction with Na metal, the following enthalpies of reduction (kcal mol^-1) are reported: benzophenone = -38.5, Cr(CO)₂(PEt₃)C₅H₅ = -54.3, Cr(CO)₂(PPh₃)C₅H₅ = -59.8, 1/2[Mo(CO)₃C₅Me₅]₂ = -59.9, 1/2[W(CO)₃C₅H₅]₂ = -61.7, 1/2[Mo(CO)₃C₅H₅]₂ = -63.5, Cr(CO)₂(P(OMe)₃)C₅H₅ = -64.4, 1/2[C₅H₅(CO)₃Mo-Cr(CO)₃C₅H₅] = -66.1, 1/2[Cr(CO)₃C₅H₅]₂ = -68.2, Cr(CO)₃C₅Me₅ = -72.3, Cr(CO)₃C₅H₅ = -75.6, 1/2Co₂(CO)₈ = -77.9. The value for the enthalpy of reduction of benzophenone is 14.5 kcal mol^-1 more exothermic than recent literature data. The enthalpies of electron transfer and proton transfer between NaCr(CO)₂C₅H₅ and other chromium radicals and hydrides have been measured and span about 20 kcal mol^-1. These measurements are used in thermochemical cycles to calculate enthalpies of H atom transfer. The role of substituents in determining enthalpies of reaction relevant to electrochemical and pK_a measurement enthalpies of reaction is discussed. The heat of hydrogenation of Mn₂(CO)₁₀ has been determined using calorimetrically determined enthalpies of H atom transfer. The heat of hydrogenation, +4 kcal mol^-1 in THF, allows estimation of the MnH bond strength = 68 kcal mol^-1.