In turn, we envisioned that the in-situ combination of [Co2(CO)8] with a Lewis base (LB) could lead to its disproportionation, and thus to the generation of an active species for the carbonylation of PL, with the formula: Lewis acidic (di)cation + [Co(CO)4] -(Scheme 2c). The effect of Lewis basic additives on cobalt carbonyl-catalyzed carbonylations have been studied since the 1950s, with varied results according to their coordination force, their lability, their Brønsted basicity, or their concentration. Although their exact role in the catalysis was not always clear, the authors suggested that these Lewis bases could promote the formation of the active species from the precursor [Co2(CO)8], i.e., [HCo(CO)4] or [Co(CO)4] -, and/or stabilize some reaction intermediates. The speciation of the complexes resulting from the coordination of the LB on [Co2(CO)8] has been established for some LB: 19 for instance, phosphines may yield [Co(CO)3(LB)2] + +[Co(CO)4] -(along with the substitution complex [Co(CO)3(LB)]2), 32 whereas pyridine, N,N-dimethylformamide (DMF) or acetonitrile lead to [Co(LB)6] 2+ +2[Co(CO)4] -. It is noteworthy that this does not necessarily inform the nature of the exact active species during the carbonylation, because of the many possible equilibria between cobalt complexes. In the past few years, efforts have been made to identify them thanks to in-situ IR spectroscopy. In 2021, the group of Dong showed that the active species for the carbonylation of thietane arises from the disproportionation of [Co2(CO)8] by the substrate itself, and comprises an unidentified Co 2+ dication alongside [Co(CO)4] -. In 2024, the groups of Gusevskaya and Beller described the catalytic performances of [Co2(CO)8]/OPCy3 for the hydroformylation of epoxides: their IR studies confirmed the promoting effect of OPCy3 on the formation of the active species [HCo(CO)4], but they could only hypothesize on the structure of the disproportionation product, [Co(LB)6][Co(CO)4]2. Another example was provided by the group of Alexanian, who reported a photochemical version of the hydroaminocarbonylation of alkenes: they proposed that [Co2(CO)8] would be disproportionated by the amine substrate, yielding the ionic pair [cation] + [Co(CO)4] -, followed by its light-promoted decarbonylation to the active species [Co(CO)3] -. These recent works show also a regain interest in studies of the potential of generating active species from the disproportionation of [Co2(CO)8].