The availability of structural data for 5, [2-K(C7H8)]2 and [4-K2(BEt3)]2 provides a unique opportunity to probe the effects on the carbon/oxygen aggregate of stepwise (formal) reduction of the carbocyclic fragment (Figure ). The two p-electron squarate dianion has been described as possessing a 'moderate' aromaticity, a factor which underpins the prevalence of this planar carbocycle in four-carbon systems derived from two-electron reduction processes of CO. In the case of 5, DFT calculations imply that an alternative (linear) isomer featuring a chain of four -C(=O)units (akin to a doubly oxidized form of [2-K(C7H8)]2) is not an energetic minimum, rearranging to give a system featuring a bridging ethynediolate fragment ([OCCO] 2-), and two aluminium-bound CO ligands (which even then lies ca. 10.8 kcal mol -1 above the squarate form; see Supporting Information). The topologically linear butenedione diolate structure associated with the more heavily reduced [C4O4] 4-tetra-anion avoids the formation of a Huckel 4p anti-aromatic carbocycle derived from the addition of an additional pair of electrons to the LUMO of the squarate system. An alternative carbocyclic isomer featuring a more pronounced rectangular 'squarate' core (akin to the Jahn-Teller distorted structure of the related 4p electron system cyclobutadiene) is found to lie significantly higher in energy (by > 48 kcal mol -1 ). The HOMO-1 of [2-K(C7H8)]2 (Figure (lower)), corresponds to a p-bonding orbital across the central CC linkage of the [C4O4] 4-ligand. The formal addition of a further pair of electrons would then generate the [C4O4] 6-hexa-anion which adopts a butadienetetraolate structure, via reduction of the enedione component to the corresponding bis(enolate). The structural changes associated with the CC and CO linkages on transitioning from [C4O4] 4-to [C4O4] 6-are consistent with the form of the LUMO of [2-K(C7H8)]2 (and the HOMO of [4-K2(BEt3)]2), which features out-of phase p interactions in the terminal CO and central CC units (Figure ).