The formation of complex 3 has been investigated at the DFT level (B3PW91) and the computed enthalpy profile at room temperature is depicted in Figure . The reaction begins with the side-on coordination of CO to 1. This coordination induces the reduction of CO and the oxidation of Tm(II) to Tm(III), the radical on CO being stabilized by η 2 coordination to the metal center. This step is computed to be endothermic by 16.3 kcal.mol -1 . Unlike the calculated pathway for CO reductive coupling at U(III) mixed-sandwich complexes, 18b the CO moiety is not further reduced by the coordination of a second molecule of 1 but rather a radical coupling between two Tm(III)(CO °-) fragments is observed. This step also contrasts with that calculated for CO reductive oligomerization on Mg(I) dimers where the addition of CO occurs sequentially. It can be noted that a radical pathway for CO oligomerizations has been recently suggested on the basis of fragmentation reactions by mass spectrometry and computational quantum calculations on squarate species, i.e. CO tetramerization products. The associated transition state (TS1) has been located on the Potential Energy Surface (PES). The radical coupling barrier is 33.4 kcal.mol -1 from 1 (but only 17.1 kcal.mol -1 from the Tm(III) intermediate complex) in line with a kinetically accessible reaction. The radical nature of the TS is highlighted by the computed unpaired spin densities (see Figure ) where 0.6 unpaired electron is located on the two carbons that couple. Following the intrinsic reaction coordinate, the ethynediolate complex iso2 is obtained, which formation is favorable (-9.0 kcal.mol -1 with respect to 1). This ethynediolate complex displays a "zig-zag" geometry which is the key intermediate in this reactivity (see Figure ) and is similar to the "zig-zag" intermediate suggested in previous studies. This intermediate can either yield the more stable linear structure corresponding to 2, or bind another CO molecule. In our case, these two possibilities are thermodynamically competitive (the CO coordination is exothermic by 29.4 kcal.mol -1 from iso2 while the linear ethynediolate complex 2 is 35.2 kcal.mol -1 more stable than the "zig-zag" one). The formation of the stable CO adduct is easily explained by the nature of the HOMO of iso2, that can easily overlap with the p* orbital of CO (see Figure ). From the CO adduct, a C-C coupling TS has been located and corresponds to a nucleophilic attack of the ethynediolate to the p system of CO (see the HOMO at the TS in Figure ). The associated barrier is low (17.0 kcal.mol -1 ), yielding a stable ketene-type intermediate (-43.8 kcal/mol -1 ). This ketene intermediate can isomerize with a barrier of 37.7 kcal.mol -1 (TS3) to form complex 3 (see details in the Supporting Information), which is the most stable complex of the entire profile (-111.3 kcal.mol -1 ). The relatively high activation barrier calculated for TS3 is consistent with a relatively slow (1-2 days) transformation of 2 to 3 in the presence of excess CO at room temperature.