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–). Three primary methods have been developed for hydricity determination:
the hydride transfer method establishes hydride transfer equilibrium
with a hydride donor/acceptor pair of known hydricity, the H2 heterolysis method involves measuring the equilibrium of heterolytic
cleavage of H2 in the presence of a base, and the potential–pKa 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 CO2, and the production and oxidation of hydrogen.