Depending on the reaction conditions and steric bulk of the supporting ligands, selective formation of oxocarbon dianions of the type ethynediolate (A), deltate (B) or squarate (C) was reported (Figure ). DFT calculations pointed to a "zig-zag" intermediate complex featuring a bridging trans-bent [C2O2] 2-dianion as a key intermediate in the formation of the different species. A similar intermediate was also recently suggested by Jones and co-workers in the reductive coupling of CO using activated b-diketiminate Mg(I) complexes (Figure ). 15e,15g However, in these studies, the reactivity of the CO oligomerization products was not further explored. Very recently, Crimmin and co-workers described the sequential formation of C1-C4 chain compounds starting from CO and CO2 feedstocks in the presence of transition metal carbonyl complexes and using b-diketiminate Al(I) reductants. Other systems allowing sequential chain growth by reductive homologation of CO from isolable intermediates are exceedingly rare, and, to the best of our knowledge, have not been used in further functionalization reactions. We have been interested in the use of highly reducing Tm(II) and other divalent lanthanide complexes for homogeneous transformations and small molecule activation. Herein, we report the reactivity of the Tm(II) complex [Tm(Cp ttt )2] 21 (Cp ttt = 1,2,4-tris(tert-butyl)cyclopentadienyl) towards CO, leading to selective CO reductive di-and trimerization products. The reactivity of the corresponding complexes, which feature central dianionic {CnOn} 2-(n = 2,3) oxocarbon moieties, was systematically investigated towards electrophiles (CO2, silylating/alkylating agents). Novel multicarbon oxygenated frameworks were obtained through unusual reactivities, opening new avenues for the functionalization of CO and CO2.