Overall charge balance implies that the bridging unit is [C4O4] 4-, a finding consistent with the presence in the (dimeric) aluminyl starting material of two Al(I) reductants, i.e. with four-electron reduction of four molecules of CO. To a first approximation, C-C distances within the [C4O4] Four-carbon homologation has some literature precedent. In terms of a metal-bound ligand system, the C/O skeleton bears some resemblance connectivity-wise to the enedione diolate [R2C4O4] 2-system reported by Marks and co-workers, derived from insertion of two molecules of CO into a thorium(IV) alkyl bond (IV), and to the mixed d-block/Al systems reported by Crimmin et al. (VI). Structurally, however it is most closely related to Stephan's B/N functionalized system Cy2N{C4O4(BCy2)2}NCy2 derived from the trapping of CO by a combination of LiNCy2 and ClBCy2, and which also features a C(=O)-C(-O)=C(-O)-C(=O) skeleton (V). In addition, a similar C4 system was reported very recently by Coles and co-workers (VII). The isolated yield of [2-K(C7H8)]2 obtained from [1-K]2 and CO is low (ca. 10 %), and in situ 1 H NMR monitoring is consistent with the formation of other (NON)Al-containing species under these reaction conditions. Hypothesizing that this is due to the formation of other homologation products of the type [CnOn] 4-, we sought (i) to probe the reactivity of isolated samples of [2-K(C7H8)]2 with CO under more forcing conditions, to see if homologation could be driven towards longer C-C-bonded chains; and (ii) to investigate if greater control of the delivery of the carbon monoxide (and with it the product distribution) could be achieved by the use of alternative CO-containing reagents. . Targeting the second of these objectives, we employed Fe(CO)5 as a labile (and readily soluble) source of the CO molecule in reactions with [1-K]2. Reaction with four equivalents of Fe(CO)5 in benzene-d6 in the presence of the crown ether 12-crown-4 (to aid crystallization), leads to the formation of [2-K(12-c-4)]2, the solid-state structure of which is very closely related to that of [2-K(C7H8)]2 (see Supporting Information). While the potential involvement of the iron centre in Fe(CO)5 in this chemistry cannot be ruled out, it is noteworthy that conversion to the same [C4O4] 4- fragment via this approach is markedly higher (20-30 %) than that obtained using gaseous CO.