Lowering the CO partial pressure from 45 to 20 bar, while maintaining the H2 partial pressure constant at 5 bar, was also detrimental to the selectivity toward SA (Table , entry 4). A minimal CO pressure is thus needed, probably either for the carbonylation step to be favoured, or to stabilize the active species. The positive influence of the presence of H2 may be explained by the formation and stabilisation of cobalt hydride intermediates, which can be proposed as active species. 46 a Conditions: AA (0.5 mol‧L -1 ) and mesitylene (internal standard, 10 mol%) with [Co2(CO)8] (5 mol%) in the solvent (2 mL), heated for 6 h under pressure of desired gas mix. AA conversions and SA yields measured by GC-MS analysis; selectivity calculated as the ratio between yield and conversion.