Thermodynamic Hydricity of Transition Metal Hydrides

• Published in: Chemical reviews Vol. 116; no. 15; pp. 8655 - 8692
• Main Authors: Wiedner, Eric S, Chambers, Matthew B, Pitman, Catherine L, Bullock, R. Morris, Miller, Alexander J. M, Appel, Aaron M
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 10-08-2016
• Subjects: carbon dioxide; catalysis; Chemical reactions; Electrons; hydricity; hydride; hydrogen; Hydrogen bonds; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Oxidation; Protons; Thermodynamics
• ISSN: 0009-2665; 1520-6890
• DOI: 10.1021/acs.chemrev.6b00168

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–pK a 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.