Thermodynamic Hydricity of Transition Metal Hydrides

Transition metal hydrides play a critical role in stoichiometric and catalytic transformations. Knowledge of free energies for cleaving metal hydride bonds enables the prediction of chemical reactivity, such as for the bond-forming and bond-breaking events that occur in a catalytic reaction. Thermodynamic hydricity is the free energy required to cleave an M–H bond to generate a hydride ion (H<sup>–</sup>). Three primary methods have been developed for hydricity determination: the <i>hydride transfer</i> method establishes hydride transfer equilibrium with a hydride donor/acceptor pair of known hydricity, the <i>H<sub>2</sub> heterolysis</i> method involves measuring the equilibrium of heterolytic cleavage of H<sub>2</sub> in the presence of a base, and the <i>potential–p<i>K</i><sub>a</sub></i> method considers stepwise transfer of a proton and two electrons to give a net hydride transfer. Using these methods, over 100 thermodynamic hydricity values for transition metal hydrides have been determined in acetonitrile or water. In acetonitrile, the hydricity of metal hydrides spans a range of more than 50 kcal/mol. Methods for using hydricity values to predict chemical reactivity are also discussed, including organic transformations, the reduction of CO<sub>2</sub>, and the production and oxidation of hydrogen.