The species presumably involved in the associative ligand substitution mechanism for the Stille cross coupling of vinyl bromide and trimethylvinyl stannane with Pd(PMe3)(2)/PMe3 as catalysts in DMF (as ligand and solvent) have been structurally and energetically characterized. The cyclic four-coordinate transition state for the rate-determining transmetalation step explains the retention of configuration in the Stille coupling of chiral nonracemic alkyl stannanes.