Unsaturated N-heterocyclic phosphenium cations (uNHP) stabilize the [Pd 0 (PR3)2X] -anion proposed over the past decade to be the crucial but elusive intermediate in palladium-catalyzed cross-coupling reactions (X = halide). Insertion of metal into the PBr bond of the precursor mesityl-substituted bromophosphine gives the structurally characterized Pd(0)-phosphenium complex (uNHPMes)Pd(PPh3)2Br, which features a long Pd-Br bond (2.7240(9)Å) and the shortest known Pd-P bond (2.1166(17)Å). The reaction is proposed to proceed by an associative pathway involving a Pdbromophosphine complex that undergoes P-to-Pd bromide transfer.