Kine;c Isotope Effects: KineQc isotope effects (KIEs) were measured by comparing absolute rates of reacQons carried out with either CO/D2 or CO/H2 (1:1, 5 bar). Using the power of automaQon, KIEs were readily collected on a range of electron-rich and electron-poor substrates. These experiments revealed small KIE values ranging from 1.1 -1.2 (Fig. ). KIEs for dihydrogen acQvaQon can involve a contribuQon from both an inverse KIE for dihydrogen coordinaQon and a normal KIE for the breaking of the H-H bond. DFT calculaQons were used to model KIEs for the proposed dihydrogen acQvaQon steps. Dihydrogen binding to Int-6 to form Int-7 was calculated to occur with a small inverse KIE, while deprotonaQon of the dihydrogen complex Int-7 through TS-3 was predicted to occur with a small normal KIE (Fig. and). Hence, DFT calculaQons predict that a base-assisted mechanism for hydrogen splifng, involving deprotonaQon of an intermediate Pd dihydrogen complex should occur with a small normal KIE, in good agreement with the experiment data. inserQon can be considered to be a fast and reversible pre-equilibrium step that occurs prior to dihydrogen acQvaQon (Fig. ). This equilibrium is expected to be sensiQve to the concentraQon of CO in soluQon. Dihydrogen acQvaQon has the highest energy transiQon state on the potenQal energy surface, but migratory inserQon is expected to contribute to rate as it will determine the effecQve concentraQon of the key Pd acyl intermediate involved in this step (e.g. Int-6).