Our DFT calculations indicate dissociation of the dimeric oxidative addition complexes 5a and 5b to form monomeric anionic T-shaped Pd(II) complexes is highly endergonic (Figure ), although such species could be present in small concentrations. An equilibrium between 5a and 5b with several aryl Pd(II) iodide dimers (6a-1, 6a-2, 6a-3) via exchange and isomerization is more facile (Figure ). The most stable isomer 6a-1 involves chelation of the pyridine nitrogen to the adjacent Pd center (2.10 Å) (Figure ), accounting for the unusual facility of couplings adjacent to heteroaromatic ring nitrogen atoms. The transmetalation process is thermodynamically driven by the exergonicity of reduction of [Pd2I6] 2- (Pd-C) by formate to regenerate Pd(I) dimer [Pd2I4] 2-(Pd-B). As for 5a and 5b, dissociation of 6a-1 to monomeric Pd(II) complexes is highly endergonic (Figure ). Therefore, Pd-to-Pd transmetalation is expected to occur from the dimeric Pd(II) complexes (Figure ). From 6a-3, a higher energy isomer of 6a-1, the transmetalation of the 4-fluoro-3methylphenyl group between Pd(II) centers within the dimeric complex was found to be kinetically facile (TS3a), with a low barrier of only 2.7 kcal/mol relative to 6a-3 and 13.9 kcal/mol with respect to the more stable isomer of the Pd(II) dimer 6a-1. The Pd-to-Pd transmetalation transition state TS3a features a four-membered cyclic structure involving the migrating ipso carbon, the bridged iodide, and the two Pd centers. The low barrier to transmetalation is likely due to π-complexation of the migrating aryl group with the adjacent Pd center (2.43 Å) (Figure ) and the bridging iodide in 6a-3, which causes minimal structural distortion in the developing transition state. The alternative transmetalation pathway involving migration of the 6-methyl-2-pyridyl group from 6a-1 was also considered and was found to be less favorable (see Supporting Information). The transmetalation leads to the diaryl Pd(II) complex 7a, which is 7.2 kcal/mol less stable than 6a-1. The C(sp 2 )-C(sp 2 ) reductive elimination (TS4a) then takes place from the Pd(II) dimer to form the cross-coupling product 3f and regenerate the Pd(I) dimer Pd-B.