The Hammee analysis shows that for electron donaQng subsQtuents (sp > 0) there is no net influence of charge stabilisaQon on the rate of the reacQon, while for electron-deficient subsQtuents (sp < 0) rate increases with increasing ability of the substrate to stabilise posiQve charge with r = -1.4. These data also effecQvely rule out oxidaQve addiQon (and its microscopic reverse reducQve eliminaQon) as being turnover limiQng. AddiQon of aryl halides to palladium(0) complexes has been studied under a variety of condiQons and typically occurs with r = +2 to +5 due to the ability of electron-withdrawing groups to stabilise the buildup of negaQve charge on the ipso-carbon in either oxidaQve addiQon or SNAr transiQon states. NegaQve r values observed for electron-deficient subsQtuents are instead consistent with the substrate playing a role of a nucleophile in the turnover-limiQng sequence and suggest that the migratory inserQon step, while known to be fast and reversible, may contribute to the overall turnover-limiQng sequence. A hypothesis that is also consistent with the observaQon of reduced rates of reacQon, and increased side-product formaQon under lower parQal CO pressures.