The reactivity of cobalt pincer complexes supported by the bulky tetramethylated PNP ligands Me 4 PNP R (R = i Pr, t Bu) has been investigated. In these ligands, the undesired H atom loss reactivity observed earlier in some classical CH 2 -arm PNP cobalt complexes is blocked, allowing them to be utilized for promoting two-electron catalytic transformations at the cobalt center. Accordingly, reaction of the formally Co I Me complex 3 with H 2 under ambient pressure and temperature afforded the Co III trihydride 4-H, in a reaction cascade reasoned to proceed by two-electron oxidative addition and reductive eliminations. This mechanistic proposal, alongside the observance of alkene insertion and ethane production upon sequential exposure of 3 to ethylene and H 2 , prompted an exploration into 3 as a catalyst for hydrogenation. Complex 4-H, formed in situ from 3 under H 2 , was found to be active in the catalytic hydrogenation of alkenes and alkynes. The proposed two-electron mechanism is reminiscent of the platinum group metals and demonstrates the utility of the bulky redox-innocent Me 4 PNP R ligand in the avoidance of one-electron reactivity, a concept that may show broad applicability in expanding the scope of earth-abundant first-row transition-metal catalysis.

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