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.

The dicationic complex [CpCo(azpy)(CHCN)](ClO) 1 (azpy = phenylazopyridine) exhibits a reversible two-electron reduction at a very mild potential (-0.16 V versus Fc) in acetonitrile. This behavior is not observed with the analogous bipyridine and pyrazolylpyridine complexes (3 and 4), which display an electrochemical signature typical of Co systems: two sequential one-electron reductions to Co at -0.4 V and Co at -1.0 to -1.3 V versus Fc. The doubly reduced, neutral complex [CpCo(azpy)] 2 is isolated as an air-stable, diamagnetic solid via chemical reduction with cobaltocene. Crystallographic and spectroscopic characterization together with experimentally calibrated density functional theory calculations illuminate the key structural and electronic changes that occur upon reduction of 1 to 2. The electrochemical potential inversion observed with 1 is attributed to effective overlap between the metal d and the low-energy azo π* orbitals in the intermediary redox state and additional stabilization of 2 from structural reorganization, leading to a two-electron reduction. This result serves as a key milestone in the quest for two-electron transformations with mononuclear first-row transition metal complexes at mild potentials.

Reactions of indigo with anilines provide a simple route to indigo N,N'-diaryldiimines ("Nindigo"), a new binucleating ligand with two beta-diketiminate-type metal binding sites. Bis-palladium complexes have interesting ligand-centred properties such as redox activity and intense near infrared absorption.

Hydride transfer between transition metal hydride complexes and carbon dioxide is a known reaction, where the thermodynamically favored direction of hydride transfer determines whether CO 2 reduction or formate oxidation occurs. Analysis of a growing database of thermodynamic parameters for transition metal hydride complexes now provides clear demarcation between metal hydrides which will function as oxidases and as reductases. The turning point is set at the hydricity of formate (44 kcal/mol in acetonitrile). Here, we utilize hydricity as a framework to reevaluate the catalytic activity and proposed mechanisms for formate oxidation and CO 2 reduction with several Ni and Rh P 2 N 2 (P 2 N 2 = 1,5-diaza-3,7-diphosphacyclooctane) complexes, respectively. The series of Ni P 2 N 2 complexes have hydricities between 55−64 kcal/mol and are active catalysts for the electrochemical oxidation of formate. A surprising correlation of increased rate of electrochemical oxidation with decreased overpotential, η, is observed. The Rh P 2 N 2 complexes have hydricities between 28−34 kcal/mol and function as hydrogenation catalysts for the reduction of CO 2 to formate. Learning from the reactivity of these catalysts, design principles for future metal hydride complexes are presented that focus on the ultimate goal of catalyst optimization for improved energy efficiency (overpotential) with high selectivity (Faradaic efficiency) for both formate oxidation and CO 2 reduction to formate.

The ruthenium hydride [RuH(CNN)(dppb)] (1; CNN = 2-aminomethyl-6-tolylpyridine, dppb = 1,4-bis(diphenylphosphino)butane) reacts rapidly and irreversibly with CO2 under ambient conditions to yield the corresponding Ru formate complex 2. In contrast, the Ru hydride 1 reacts with acetone reversibly to generate the Ru isopropoxide, with the reaction free energy ΔG°(298 K) = -3.1 kcal/mol measured by (1)H NMR in tetrahydrofuran-d8. Density functional theory (DFT), calibrated to the experimentally measured free energies of ketone insertion, was used to evaluate and compare the mechanism and energetics of insertion of acetone and CO2 into the Ru-hydride bond of 1. The calculated reaction coordinate for acetone insertion involves a stepwise outer-sphere dihydrogen transfer to acetone via hydride transfer from the metal and proton transfer from the N-H group on the CNN ligand. In contrast, the lowest energy pathway calculated for CO2 insertion proceeds by an initial Ru-H hydride transfer to CO2 followed by rotation of the resulting N-H-stabilized formate to a Ru-O-bound formate. DFT calculations were used to evaluate the influence of the ancillary ligands on the thermodynamics of CO2 insertion, revealing that increasing the π acidity of the ligand cis to the hydride ligand and increasing the σ basicity of the ligand trans to it decreases the free energy of CO2 insertion, providing a strategy for the design of metal hydride systems capable of reversible, ergoneutral interconversion of CO2 and formate.