Carbon monoxide can be a useful C₁ building block. Understanding how C–C bonds can form between CO units and how the resulting chain-growth reactions work can be important, e.g., for industrial processes. Metal complexes have been used as soluble model systems in this context. For example, complexes of CO-chain-based ligands such as [C₄O₄]^4– have been synthesized.

Mathew D. Anker, Martyn P. Coles, Victoria University of Wellington, New Zealand, and colleagues have performed a reduction and homologation of CO to give C₄ and C₅ chains using an aluminyl anion. The team exposed a solution of the complex (NON^Dipp)Al–K(TMEDA)₂ (NON^Dipp = [O(SiMe₂NDipp)₂]^2–, Dipp = 2,6-ⁱPr₂C₆H₃, TMEDA = tetramethylethylenediamine) to one atmosphere of CO. They obtained a complex with a [C₄O₄]^4– ligand, i.e., K₂[{Al(NON^Dipp)}₂(C₄O₄)].

When the researchers reacted the dimeric potassium aluminyl [K{Al(NON^Dipp)}]₂ with CO, they obtained a complex containing a [C₅O₅]^5– ligand, which had been unknown so far (partial structure pictured). This complex crystallized as a mono-TMEDA adduct, i.e., K₅[K(TMEDA)][{Al(NON^Dipp)}₄(C₅O₅)₂]_. According to the team, this [C₅O₅]^5– ligand represents the first non-cyclic C₅-oligomer of CO.

• Extending chain growth beyond C₁ → C₄ in CO homologation: aluminyl promoted formation of the [C₅O₅]⁵⁻ ligand,
  Matthew J. Evans, Michael G. Gardiner, Mathew D. Anker, Martyn P. Coles,
  Chem. Commun. 2022, 58, 5833–5836.
  https://doi.org/10.1039/d2cc01554d