Cationic Pd(II) complexes catalyzed the dehydrogenation of ammonia borane in the most efficient manner with the release of 2.0 equiv of H2 in less than 60 s at 25 °C. Most of the hydrogen atoms were obtained from the boron atom of the ammonia borane. The first step of the dehydrogenation reaction was elaborated using density functional theory calculations.

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.