Next, we investigated the origin of selectivity for crosscoupling versus homo-coupling (Figure ). Based on the computed reaction energy profiles, the Pd-to-Pd transmetalation is reversible, and the C(sp 2 )-C(sp 2 ) reductive elimination is the selectivity-determining step. In the competing pathways leading to the homo-coupling products, the aryl Pd(II) iodide dimers 6b and 6c were found to be 9.2 and 6.6 kcal/mol less stable than 6a-1. After the reversible Pd-to-Pd transmetalation (TS3b and TS3c), the reductive elimination transition states from 7b and 7c leading to the two homocoupling products (TS4b and TS4c) require 28.0 and 30.6 kcal/mol, respectively, which are both substantially higher than the reductive elimination transition state leading to the cross-coupling product (TS4a). Compared to the homocoupling reductive elimination transition states, TS4a is stabilized by two factors. First, reductive elimination en route to hetero-coupled product is faster than reductive elimination en route to homo-coupled products. Second, TS4a is stabilized by coordination of Pd to the pyridyl nitrogen (d(N⋯Pd) = 2.12 Å) in the dimeric complex, once again accounting for the unusual facility of couplings adjacent to heteroaromatic ring nitrogens. Based on the collective experimental and computational data, a simplified catalytic cycle for formate-mediated reductive crossed-coupling is as follows (Figure ). The Pd(I) precatalyst [Pd(I)(P t Bu3)]2 (Pd-A) is converted to the dianionic Pd(I) dimer [Pd2I4][NBu4]2 (Pd-B), which is more reactive toward oxidative addition. Upon oxidative addition, the resulting mono-arylpalladium dimers participate in rapid, reversible Pd-to-Pd transmetalation to form iodide-bridged diarylpalladium dimers in a process driven by the highly exergonic reduction of [Pd2I6][NBu4]2 (Pd-C) (a known, catalytically competent dimer) by formate to regenerate the Pd(I) dimer [Pd2I4][NBu4]2 (Pd-B). The hetero-diarylpalladium dimers are more stable than the homodimers and have lower barriers to reductive elimination, which results in high crossselectivity. Notably, monometallic reactive intermediates are not believed to intervene due to the highly endergonic dissociation of the iodide bridged dimers.