[1-K]2 has been shown by DOSY NMR measurements to retain a dinuclear structure in solution in arene solvents. With this in mind, we targeted the reactivity of CO with related aluminyl compounds known to possess a mononuclear structure. The reaction of the (monomeric) lithium aluminyl complex (Et2O)2LiAl(NON) ([1-Li(OEt2)2]) with CO was therefore probed under similar conditions. Intriguingly, this chemistry leads to the formation of a very similar dinuclear product, [2-Li(OEt2)]2, featuring an analogous [C4O4] 4-fragment, and two encapsulated Li + counter-ions each ligated by two of the oxygen atoms of the [C4O4] 4-unit and an additional molecule of diethyl ether. The structural metrics relating to the Al2(C4O4) unit (Table and Supporting Information) are very similar to those measured for [2-K(C7H8)]2, being consistent with a centrosymmetric four-carbon chain incorporating two C-C and one C=C bonds. Moreover, the formation of [2-Li(OEt2)]2 from [1-Li(OEt2)2] in this manner (in greater conversion to [2-K(C7H8)]2, 40 vs. 10 %) suggests that the state of aggregation of the aluminyl reagent is not critical in determining the stoichiometry of this carboncontaining product. This observation is consistent with the mechanism proposed by Marks and coworkers for the formation of [Cp*2Th{OC(CH2 t Bu)C(O)}]2, in which the central C=C bond is postulated as being constructed in the final mechanistic step through the dimerization of two mono-metallic carbene units (themselves accessed by bending of a coordinated ketene ligand). Coles has also proposed that the dimerization of a dioxocarbene moiety is the final step in the formation of an ethenetraolate ligand, [C2O4] 4-. In the case of the (NON)Al-derived systems presented here, this hypothesis is consistent with the results of mechanistic calculations carried out using Density Functional Theory -for a model system in which the backbone t Bu groups are replaced by Me, and the cation is omitted for computational efficiency (Figure ). The rate-determining step for the lowest energy pathway is found to involve dimerization of two carbene fragments, with these each being derived from the coupling of two molecules of CO. Kinetically, the transition state energy for the C=C forming step is located ca. 34 kcal mol -1 above the starting materials (and ca. 37 kcal mol -1 above the preceding singlet carbene intermediate). This barrier height reflects, at least in part, the sterically encumbered nature of the two NON-supported fragments being brought together, and presumably underpins the relatively low yields associated with systems such as [2-K(C7H8)]2 (and the possibility for forming longer Cbased chains via carbene/CO coupling). 33 In terms of the initial interaction of the CO molecule(s) with the (model) [(NON')Al] -unit, we find that end-on approach of CO perpendicular to the AlN2 plane in a manner analogous to that determined for CO-adducts of isoelectronic silylene systems, X2Si•CO, does not correspond to a local minimum on the potential energy surface. Rather, reflecting the nucleophilic nature of the aluminyl reagent, the approach of the carbon monoxide unit gives rise to a bent Al-C-O unit, and the predominant orbital interaction involves electron donation from Al to the p* orbital of CO. For the model C-bound bis(carbonyl) adduct [(NON')Al(CO)2] -, the HOMO (Figure ) defines a threecentre interaction involving the aluminium-centred lone pair and the p* orbitals of the two carbonyl ligands. Moreover, ETS-NOCV analysis determines that this three-centre 'back-bonding' interaction accounts for the majority of the orbital interaction energy (DEorb = 261 kcal mol -1 ). By contrast, the complementary 3-centre interaction, involving donation from the CO lone pairs to the Al-centred pp-orbital accounts for only 25 kcal mol . By means of comparison, the corresponding ETS-NOCV-determined data for a mono-carbonyl adduct of a cationic borole (in which the CO ligand behaves primarily as a s-donor) are reported to be 26 kcal mol -1 (back-bonding to CO) and 106 kcal mol -1 (sigma donation from CO). As such, the primary role of the CO donors in [(NON')Al(CO)2] -is strongly suggested to be as Z-type ligands. The barrier to this isomerization within the coordination sphere of aluminium (ca. 40.2 kcal mol -1 ) is found to be significantly higher than that for simple dissociation/re-association. Moreover, while I2 is ca. 17.5 kcal mol -1 higher in energy than I2', it is capable of undergoing a very facile C-C coupling to generate carbene intermediate I3 which is lower in energy than either of the carbonyl adducts.