We have shown here that four-or six-electron reduction of CO can be accomplished by the use of anionic aluminium(I) ('aluminyl') compounds of the type Mn[(NON)Al]n to give topologically linear or branched C4/C6 chains, depending on reaction conditions. The mechanism for homologation to the [C4O4] 4-system proceeds via rate-limiting formation of the central C=C double bond from two carbene fragments, and rationalizes the synthesis of this system from both monomeric (M = Li) and dimeric (M = K) precursors. Initial adduct formation with CO relies on the highly electron-rich nature of the aluminyl reagent, which drives an unusual (primarily Z-type) mode of interaction of the CO molecule with the metal centre. The onward formation of [C6O6] 4-from [C4O4] 4-demonstrates a homogeneous process which brings about chain branching via complete C-O bond cleavage. Comparison of the linear [C4O4] 4-system with the [C4O4] 6-congener formed under more reducing conditions, reflects the net conversion of C-O bonds to C-C bonds in the presence of additional reductant.