Multimetallic cooperative binding of heteroallenes provides an attractive route to their activation, but the reduction of CS2 at heterobimetallic sites, associating an electron-rich metal with a main group Lewis acid has not been explored. Here we show that the presence of a heterometallic U, K site plays an important role in the CS2 reduction by uranium(III) complexes of the electron-rich and the sterically demanding tris(tert-butoxy)siloxide ligand. Specifically, the ion-pair complex [K(18c6)][U(OSi((OBu)-Bu-t)(3))(4)], 1, leads preferentially to the reductive disproportionation of CS2 to K2CS3 and CS. The crystal structure of the thiocarbonate intermediate complex [U(OSi((OBu)-Bu-t)(3))(4) (mu(3)_kappa(2):kappa(2):kappa 2-CS3)K-2(18c6)(2)], 3, isolated from the toluene reaction mixture has been determined. In contrast, the heterobimetallic complex [U(OSi((OBu)-Bu-t)(3))(4)K], 2, promotes preferentially the reductive dimerization of CS2 to K2C2S4 and K2C3S5. The K2C2S4(DMSO)(3), 5, and U(OSi((OBu)-Bu-t)(3))(4)K-2(C3S5), 6, polymeric compounds were isolated from this reaction and structurally characterized.