The metal-mediated redox transformation of CO2 in mild conditions is an area of great current interest. The role of cooperativity between a reduced metal center and a Lewis acid center in small-molecule activation is increasingly recognized, but has not so far been investigated for f-elements. Here we show that the presence of potassium at a U, K site supported by sterically demanding tris(tert-butoxy)siloxide ligands induces a large cooperative effect in the reduction of CO2. Specifically, the ion pair complex [K(18c6)][U(OSi(OtBu)3)4], 1, promotes the selective reductive disproportionation of CO2 to yield CO and the mononuclear uranium(IV) carbonate complex [U(OSi(O tBu)3)4(u-K2:K1-CO 3)K2(18c6)], 4. In contrast, the heterobimetallic complex [U(OSi(OtBu)3)4K], 2, promotes the potassium-assisted two-electron reductive cleavage of CO2, yielding CO and the U(V) terminal oxo complex [UO(OSi(OtBu)3) 4K], 3, thus providing a remarkable example of two-electron transfer in U(III) chemistry. DFT studies support the presence of a cooperative effect of the two metal centers in the transformation of CO2. © 2014 American Chemical Society.