The conversion of carbon monoxide (CO) to hydrocarbons and oxygenates on industrial solid catalysts (the Fischer-Tropsch reaction) largely relies on the cooperation of heteromultimetallic active sites composed of main group (such as alkali) and transition metals, but the mechanistic details have not been fully understood at the molecular level. Here we report the cooperative trimerization of CO by molecular lithium and samarium boryl complexes. We have found that, in the coexistence of a samarium boryl complex and a lithium boryl complex, the trimerization of CO selectively occurred to give a diborylallenetriolate skeleton "BC(O)C(O)C(O)B", in sharp contrast with the reaction of CO with either the lithium or the samarium boryl compound alone. The C-13-labeled experiments and computational studies have revealed that the CO trimerization reaction took place exclusively by coupling of a samarium boryl oxycarbene species, which was generated by insertion of one molecule of CO into the samarium-boryl bond, with a lithium ketenolate species formed by insertion of two molecules of CO into the lithium-boryl bond. These results offer unprecedented insight into CO oligomerization promoted by heteromultimetallic components and may help better understand the industrial F-T process and guide designing new catalysts.