At this stage, density functional theory (DFT) calculations were used to discriminate between the possible reaction mechanisms. An initial question relates to whether oxidative addition occurs by way of the neutral phosphine-modified iodidebridged Pd(I) dimer [Pd(I)(P t Bu3)]2 (Pd-A) or the dianionic Pd(I) dimer [Pd2I4][NBu4]2 (Pd-B). The oxidative addition of 2-bromo-6-methylpyridine 1a and 2-fluoro-5-iodotoluene 2c to Pd-A were determined to be endergonic with activation barriers of 28.3 and 20.2 kcal/mol (TS1a and TS1b, respectively, Figure ). This result is consistent with previous DFT calculations by Schoenebeck on the reverse process (the C-Br reductive elimination of a Br/I-bridged Pd(II) dimer to form PhBr and Pd-A), which is exergonic by 11.9 kcal/mol. In contrast, oxidative addition of aryl halides 1a and 2c to Pd-B were found to be exergonic and require lower kinetic barriers (21.2 and 15.7 kcal/mol, respectively) (Figure ). These results indicate that the electron-rich dianionic complex Pd-B is more reactive in oxidative addition than the phosphinemodified complex Pd-A. It should be noted that the equilibrium between Pd-A and Pd-B may affect the concentrations of these dimeric Pd(I) species, and thus the relative rates of the two oxidative addition pathways. Although the conversion of Pd-A to Pd-B is computed to be endergonic (ΔG = +16.0 kcal/mol) (Figure ), the concentration of exogenous iodide is high compared to free P t Bu3 and increases over the course of the reaction. Hence, we posit that oxidative addition occurs from the phosphine-free dianionic complex Pd-B to form the dimeric aryl Pd(II) complexes 5a and 5b. Ligand exchange to replace iodide with P t Bu3 in 5a and 5b was found to be highly disfavored (Figure ). The stronger binding of iodide to the oxidative addition complexes 5a and 5b compared to that of phosphine is an important factor that promotes the oxidative addition with the phosphine-free dianionic Pd(I) dimer Pd-B.