Analysis by ESI(-)-MS of filtrate B (t = 15 min) from the reaction of iodide 1a' also indicated the formation of tetrameric (4a') and trimeric (5a') iodide adducts, but monomer 7a' was found to be the major species (Figure ). Analysis of filtrate B after 120 min revealed that tetramer 4a' and trimer 5a' had been consumed, and a series of palladium iodide salts with formulas [Pd2I4]n -(n = 1 or 2) and [Pd2In] -(n = 3 or 5) were detected (Figure ). These palladium iodide salts have been previously detected and isolated in other studies, and due to their high stability they were considered off-cycle species. Their formation may explain the slower kinetics in the reaction of 1b, as the concentration of active palladium species leading to product is diminished (Figure ). This is also in agreement with the findings by Ananikov and coworkers, who determined that in dynamic systems there are factors other than ease of oxidative addition (iodides > bromides) that can affect the efficiency of a process, e.g., the stabilization of Pd(II) and Pd(0) intermediates by halides. Although aryl iodides undergo faster oxidative addition than aryl bromides, iodide anions exert a greater stabilization on Pd(II) than bromide anions, thereby "trapping" these palladium species as kinetically stable salts.