Circumventing\nKinetic Barriers to Metal Hydride Formation\nwith Metal–Ligand Cooperativity

We report the two-electron, one-proton\nmechanism of cobalt hydride\nformation for the conversion of [Co<sup>III</sup>Cp(P<sup>Ph</sup><sub>2</sub>N<sup>Bn</sup><sub>2</sub>)(CH<sub>3</sub>CN)]<sup>2+</sup> to [HCo<sup>III</sup>Cp(P<sup>Ph</sup><sub>2</sub>N<sup>Bn</sup><sub>2</sub>)]<sup>+</sup>. This complex catalytically converts CO<sub>2</sub> to formate under CO<sub>2</sub> reduction conditions, with\nhydride formation as a key elementary step. Through a combination\nof electrochemical measurements, digital simulations, theoretical\ncalculations, and additional mechanistic and thermochemical studies,\nwe outline the explicit role of the P<sup>Ph</sup><sub>2</sub>N<sup>Bn</sup><sub>2</sub> ligand in the proton-coupled electron transfer\n(PCET) reactivity that leads to hydride formation. We reveal three\nunique PCET mechanisms, and we show that the amine on the P<sup>Ph</sup><sub>2</sub>N<sup>Bn</sup><sub>2</sub> ligand serves as a kinetically\naccessible protonation site en route to the thermodynamically favored\ncobalt hydride. Cyclic voltammograms recorded with proton sources\nthat span a wide range of p<i>K</i><sub>a</sub> values show\nfour distinct regimes where the mechanism changes as a function of\nacid strength, acid concentration, and timescale between electrochemical\nsteps. Peak shift analysis was used to determine proton transfer rate\nconstants where applicable. This work highlights the astute choices\nthat must be made when designing catalytic systems, including the\nbasicity and kinetic accessibility of protonation sites, acid strength,\nacid concentration, and timescale between electron transfer steps,\nto maximize catalyst stability and efficiency.