Aqueous hydride transfer is a fundamental step in emerging alternative energy transformations such as H2 evolution and CO2 reduction. “Hydricity,” the hydride donor ability of a species, is a key metric for understanding transition metal hydride reactivity, but comprehensive studies of aqueous hydricity are scarce. An extensive and self-consistent aqueous hydricity scale is constructed for a family of Ru and Ir hydrides that are key intermediates in aqueous catalysis. A reference hydricity is determined using redox potentiometry and spectrophotometric titration for a particularly water-soluble species. Then, relative hydricity values for a range of species are measured using hydride transfer equilibria, taking advantage of expedient new synthetic procedures for Ru and Ir hydrides. This large collection of hydricity values provides the most comprehensive picture so far of how ligands impact hydricity in water. Strikingly, we also find that hydricity can be viewed as a continuum in water: the free energy of hydride transfer changes with pH, buffer composition, and salts present in solution.

This review of thermodynamic and kinetic hydricity provides conceptual overviews, tutorials on how to determine hydricity both experimentally and computationally, and salient case studies.

A series of rhenium diimine carbonyl complexes was prepared and characterized in order to examine the influence of axial ligands on electronic structure. Systematic substitution of the axial carbonyl and acetonitrile ligands of [Re(deeb)(CO)(NCCH)] (deeb = 4,4'-diethylester-2,2'-bipyridine) with trimethylphosphine and chloride, respectively, gives rise to red-shifted absorbance features. These bathochromic shifts result from destabilization of the occupied d-orbitals involved in metal-to-ligand charge-transfer transitions. Time-Dependent Density Functional Theory identified the orbitals involved in each transition and provided support for the changes in orbital energies induced by ligand substitution.