Trialkylborane-Assisted CO₂ Reduction by Late Transition Metal Hydrides

Trialkylborane additives promote reduction of CO₂ to formate by bis(diphosphine) Ni(II) and Rh(III) hydride complexes. The late transition metal hydrides, which can be formed from dihydrogen, transfer hydride to CO₂ to give a formate–borane adduct. The borane must be of appropriate Lewis acidity: weaker acids do not show significant hydride transfer enhancement, while stronger acids abstract hydride without CO₂ reduction. The mechanism likely involves a pre-equilibrium hydride transfer followed by formation of a stabilizing formate–borane adduct.