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 this year 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]. In 1962, Iwanaga and coworkers published two studies in which they were able to titrate [Co(CO)4] -from mixtures of [Co2(CO)8] in methanol, ethanol, ethyl acetate, acetone, THF, dioxane, or acetonitrile. This suggests that all of these solvents are able to disproportionate [Co2(CO)8] to some extent. We reasoned that a Lewis basic enough solvent would be able to generate [Co(CO)4] -as well as a cobalt cationic species exhibiting Lewis acidity (Scheme 3). We screened multiple solvents, including some classified as 'recommended' as regards their environmental impact, 38 in order to finetune the Lewis acidity of the in-situ formed cobalt (di)cation. The carbonylation of β-lactones was selected as a benchmark carbonylation reaction performed by our system (Scheme 2). A screening of ten solvents in the presence of catalytic amounts of the commercial precursor [Co2(CO)8] was thus run for the carbonylation of β-propiolactone (PL) to succinic anhydride (SA). Our study includes acrylic acid (AA), a substrate more challenging than PL despite these two substrates being isomers, and whose cyclizing carbonylation has been recently achieved by our group. The scope was also extended to substituted derivatives of PL and AA: β-butyrolactone (BL), crotonic acid (CA), and methacrylic acid (MA).