To understand the lower activity and selectivity for the carbonylation of AA compared to PL, further analyses were run to check for side-products. After carbonylation runs, propionic acid could be detected in the liquid phase, but in too small amounts (<5%) to make up for the low mass balance recovered when taking only AA and SA into account (see S.I., section 4.1). After carbonylations in toluene, the formation of a purple slurry was also observed, which, after washing and drying under vacuum, rendered a lilac solid, [Co-AA] (see S.I., section 2.2). The IR bands of [Co-AA] suggest that it is a cobalt (II) species, with a structure of the type [Co II (acrylate)2]. The formation of [Co-AA] could thus consume at least 2 equivalents of AA per Co atom, which would explain the lower selectivity in favour of the carbonylation product (with 5 mol% of [Co2(CO)8], around 20% of AA may be lost). In addition, a lilac solid with similar IR bands was synthesized in less than 30 min by reacting an excess of AA with [Co2(CO)8] under argon (see S.I., section 2.3): the formation of [Co-AA] is thus quite fast comparatively to the duration of the carbonylation runs, and would be particularly favoured at low CO pressure. a This is consistent with the poor performances at 5 bar of the catalytic systems reported in Table . It is noteworthy that [Co-AA] is not an active complex for the carbonylation of AA. In other words, the substrate itself is prone to deactivating the precursor [Co2(CO)8] (or cobalt carbonyl species formed in-situ) by coordinating on the metal centre as an acrylate. This gives hints on the difficulty of the carbonylation of AA to SA.