When the reaction was performed using one molar equivalent of CO per 1, the CO dimerization product 2 was isolated in 53% crystalline yield. The molecular structure of 2 in the solid state (Figure ) was unambiguously confirmed by XRD studies and revealed the formation of the dinuclear complex [Tm(Cp ttt )2]2(µ-k(O):k(Oʹ)-C2O2) featuring a bridging ethynediolate ligand between the two oxidized Tm(III) metal centers. The C-C bond distance in the C2O2 2-moiety of 1.226 (10) Å is within the typical range for related ethynediolate bridged dimers of f-block elements, 16e,17f-j,17l and is consistent with a C≡C triple bond.