The 1 H NMR spectra of 6 in the 20-80 °C temperature range display a complex pattern of paramagnetically shifted signals (Figure ), as a result of the non-symmetric nature of the complex. In the IR spectrum of 6, the intense absorption band at 1583 cm -1 corresponds to the asymmetric stretching band of the coordinated carboxylate groups, similarly to the absorption band observed in 5. In addition, a weak band is observed at 1938 cm -1 and assigned to the asymmetric C=C=C stretching vibration. Although the mechanism for the formation of 6 is elusive to date and involves several rearrangement steps, the observed reactivity finds its origin in the reactive Tm-C bond in 3. It should be stressed that the stoichiometry of the reaction is strictly respected as one equivalent of both 6 and [Tm(Cp ttt )2I] are formed from two equivalents of 3, in other words, one tris-anionic {MeC3(CO2)3} framework and one iodide ligand are the result of the reaction of two di-anionic {C3O3} 2-fragments with one molecule of MeI. Overall, this reaction corresponds to an unprecedented oligomerization and functionalization of CO through formation of an allene-tricarboxylate complex in which the {C6O3} core exclusively arises from six CO molecules.