A tunable bimetallic effect on product selectivity in catalytic CO2 reduction was identified using N-heteroc-yclic carbene-ligated Cu complexes. While the monometallic Cu-only system catalyzes hydroboration of CO2 with pinacolborane to produce formate exclusively, introducing a bimetallic effect with analogous Cu-Fe, Cu-W, and Cu-Mo catalysts produces mixtures of formate and CO. Within a series of isosteric catalysts, the selectivity of CO versus formate was controlled by tuning the electronic nature of the Cu/M pairing, with high selectivity for CO being achieved using a Cu-Mo catalyst.