In summary, we report the first efficient reductive crosscouplings of aryl halides mediated by an abundant feedstock reductant, sodium formate. These processes, which are uniquely enabled by air-stable Pd(I) iodide dimers, occur through a novel mechanism and display selectivities that complement classical cross-coupling protocols. For example, due to chelation of σ-(2-pyridyl)palladium dimers, normally challenging coupling adjacent to heteroaromatic ring nitrogen atoms occur with great facility. Additionally, as pinacol boronates and anilines are tolerated, the present protocol is orthogonal to Suzuki and Buchwald-Hartwig couplings. Finally, unactivated aryl chlorides and bromides do not suffer formate-mediated transfer hydrogenolysis. Our combined experimental and computational studies are consistent with conversion of the neutral Pd(I) precatalyst [Pd(I)(P t Bu3)]2 (Pd-A) to the dianionic Pd(I) dimer [Pd2I4][NBu4]2 (Pd-B), which is more reactive toward oxidative addition. Following rapid, reversible Pd-to-Pd transmetalation, iodide-bridged diarylpalladium dimers are formed. The hetero-diarylpalladium dimers are both more stable than the homodimers and have lower barriers to reductive elimination, resulting in high cross-selectivity. These processes and other work from our laboratory demonstrate the broad applicability of hydrogenative protocols as alternatives to reactions that traditionally require premetalated reagents or metallic reductants.