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. We screened multiple solvents, including some classified as 'recommended' as regards their environmental impact, 38 in order to tune the Lewis acidity of the in-situ formed cobalt (di)cation and applied our catalytic system to the carbonylation of β-lactones (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 catalytic 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).