Hydricity of Transition-Metal Hydrides: Thermodynamic Considerations for CO2 Reduction

The hydricity ΔG°H– of a metal hydride is an important parameter for describing the reactivity of such complexes. Here, we compile a comprehensive data set consisting of 51 transition-metal hydride complexes [M-H](n−1)+ with known ΔG°H– values in acetonitrile for which the one-electron reduction of the parent complex [M] n+ is reversible. Plotting the hydricity as a function of respective E 1/2(M n+/(n–1)+) yields a robust linear correlation. While this correlation has been previously noted for limited data sets, our analysis demonstrates that this trend extends over a wide range of metal identities, ligand architectures, structural geometries, and overall charges of the metal hydride. This correlation is modeled using established thermochemical cycles relating the hydricity and homolytic bond free energy of the metal–hydride bond. The linear trend of the model enables the estimation of hydricity simply on the basis of the reduction potential of the parent complex and thus provides a guide for the rational design and tuning of metal hydride catalysts for small-molecule reduction, such as CO2 to formic acid.