Comprehensive Thermochemistry of W-H Bonding in the Metal Hydrides CpW(CO)2(IMes)H, [CpW(CO)2(IMes)H]•+, and [CpW(CO)2(IMes)(H)2]+. Influence of an N-Heterocyclic Carbene Ligand on Metal Hydride Bond Energies

The free energies interconnecting nine tungsten complexes have been determined from chemical equilibria and electrochemical data in MeCN solution (T = 22 degrees C). Homolytic W-H bond dissociation free energies are 59.3(3) kcal mol(-1) for CpW(CO)(2)(IMes)H and 59(1) kcal mol(-1) for the dihydride [CpW(CO)(2)(IMes)(H)(2)](+) (where IMes = 1,3-bis(2,4,6-trimethylphenyOhnidazol-2-ylidene), indicating that the bonds are the same within experimental uncertainty for the neutral hydride and the cationic dihydride. For the radical cation, [CpW(CO)(2)(IMes)H](center dot+), W-H bond homolysis to generate the 16-electron cation [CpW(CO)(2)(IMes)](+) is followed by MeCN uptake, with free energies for these steps being 51(1) and -16.9(5) kcal mol(-1), respectively. Based on these two steps, the free energy change for the net conversion of [CpW(CO)(2)(IMes)H](center dot+) to [CpW(CO)(2)(IMes)(MeCN)](+) in MeCN is 34(1) kcal mol(-1), indicating a much lower bond strength for the 17-electron radical cation of the metal hydride compared to the 18-electron hydride or dihydride. The pK(a) of CpW(CO)(2)-(IMes)H in MeCN was determined to be 31.9(1), significantly higher than the 26.6 reported for the related phosphine complex, CpW(CO)(2)(PMe3)H. This difference is attributed to the electron donor strength of IMes greatly exceeding that of PMe3. The pK(a) values for [CpW(CO)(2)(IMes)H](center dot+) and [CpW(CO)(2)(IMes)(H)(2)](+) were determined to be 6.3(5) and 6.3(8), much closer to the pK(a) values reported for the PMe3 analogues. The free energy of hydride abstraction from CpW(CO)(2)(IMes)H is 74(1) kcal mol(-1), and the resultant [CpW(CO)(2)(IMes)]+ cation is significantly stabilized by binding MeCN to form [CpW(CO)(2)(IMes)(MeCN)](+), giving an effective hydride donor ability of 57(1) kcal mol(-1) in MeCN. Electrochemical oxidation of [CpW(CO)(2)(IMes)](-) is fully reversible at all observed scan rates in cyclic voltammetry experiments (E degrees = -1.65 V vs Cp2Fe+/0 in MeCN), whereas CpW(CO)(2)(IMes)H is reversibly oxidized (E degrees = -0.13(3) V) only at high scan rates (800 Vs(-1) For [CpW(CO)(2)(IMes)(MeCN)](+), high-pressure NMR experiments provide an estimate of Delta G degrees = 10.3(4) kcal mol(-1) for the displacement of MeCN by H-2 to give [CpW(CO)(2)-(IMes)(H)(2)](+).