AcQvaQon barriers for this step are again universally low (DG ‡ 373K = 3.5 -9.6 kcal mol -1 ). TS-2 connects to the monomeric palladium acyl complex Int-6. Consistent with the low barrier, it has previously been shown that reacQon of three-coordinate palladium aryl complexes with CO occurs rapidly at 25 °C. An analogue of Int-6 bearing P t Bu3 in place of PAd2 n Bu has been isolated and crystallographically characterised. While this species is monomeric in the solid-state, analogous complexes bearing the PAd2 n Bu are dimeric in the solid-state. The overall reacQon is only modestly exergonic (DG°373K = -2.5 to -8.5 kcal mol -1 ) and as such is likely to be reversible under catalyQc condiQons. The migratory inserQon step is calculated to be more exergonic and occur with lower acQvaQon barriers for more electron-rich aryl subsQtuents (e.g. more negaQve sp). A contribuQon of the migratory inserQon step to the turnover-limiQng sequence is consistent with the Hammee analysis and the influence of CO pressure on reacQon rate and selecQvity.