The possibility for assembling [CnOn] 4-chains with n > 4 was probed via the reaction of [2-K(C7H8)]2 with CO at higher temperatures/pressures. Use of 2 atm pressure in THF-d8 at ca. 65 o C over 4 d leads to quantitative conversion by H NMR to a single product featuring two inequivalent NON ligands, and which can be shown by X-ray crystallography to be K[K(THF)2][{(NON)Al}2(C6O6)], (3; Figure ). 3 features a [C6O6] 4-fragment formed by the assimilation of two further equivalents of CO, and which presents distinct (O,O and C,O) coordination modes at the two aluminium centres. The geometry of the [C6O6] 4-group is unique, featuring a six membered pyran core, with the monobranched nature of the six-carbon chain implying that C-C bond formation is also accompanied by complete cleavage of one of the C-O bonds. While the homologation of CO by highly reducing molecular species to give linear or cyclic products has literature precedent, the formation of branched carbon chains in this fashion, with accompanying cleavage of the exceptionally strong C-O bond is much less common. Examples of branched-chain carbon skeletons have been reported previously -via subsequent derivatization reactions of linear CO-derived chains with electrophiles such as CO2 or MeI.