Since the central carbon atom (C2, Figure ) in 3 presents no C-O connectivity, complete cleavage of one CO triple bond has occurred. Although CO cleavage and further homologation is thought to occur in heterogeneous Fischer-Tropsch systems, such reactivity is not common in homogeneous systems. The observed CO reductive homologation induced by the Tm(II) complex 1, leading to the selective formation of either the ethynediolate complex 2 or the ketenecarboxylate complex 3, is the result of a fine balance between steric protection and coordinative unsaturation. A similar observation was already reported in the case of U(III) mixed-sandwich complexes reported by Cloke and co-workers, which allowed possible reductive CO di-, tri-or tetramerization depending on the steric demand of the supporting ligands and amount of CO. 17j Since selective formation of the CO dimerized product 2 is observed in the presence of one molar equivalent of CO per Tm(II) center, we further examined whether 2 may be an intermediate in the formation of 3. In a control experiment, the reaction of isolated 2 in toluene solution with excess CO was monitored by paramagnetic 1 H NMR spectroscopy (see Figure ), revealing clean and quantitative transformation into 3. This reaction is relatively slow at room temperature, requiring ca. 1-2 days to reach completion, which suggests a relatively high activation barrier. The gain further insights in the elementary steps leading to the formation of 2 and 3, i.e. the overall mechanism of this unusual reaction, DFT calculations were performed and confirmed that 2 appears indeed as an intermediate in the formation of 3 (see below). To the best of our knowledge, this result is unprecedented since all reported ethynediolate complexes have been either synthesized under excess of or the isolated products were found inert towards external CO. 17f To date, systems allowing controlled carbon chain growth by sequential insertions of CO on isolated intermediates are exceedingly rare. Such examples include CO addition on the boron-boron triple bond of a diboryne compound described by the group of Braunschweig, 15a and recent work by Crimmin and co-workers on systems based on transition metal carbonyls together with aluminum(I) reductants. Finally, to investigate whether other reaction conditions have an influence on the selectivity, temperature and solvent effects were examined. Performing the addition of CO at low temperature (-78 °C) or in different solvents (pentane, benzene, Et2O and THF) led to similar results signifying that the stoichiometry in CO is the only parameter controlling the selectivity in this reaction. It should be however noted that the reaction proceeds much more slowly in coordinating solvents such as THF, requiring several days until complete consumption of 1, which can be traced back to competitive coordination of THF and CO to the metal center. An inhibitory role of THF in the reduction of organic substrates by Sm(II) complexes has previously been observed in the literature. Interestingly, in the U(III) systems reported by Arnold and co-workers, THF coordination was found to completely stop the reactivity towards CO. 17g,17h Such a different behavior may originate in the steric demand of the Cp ttt ligands that induces weak and reversible coordination of the usually strongly bound THF donor. 21a