Palladium(II) iodide NHC complexes catalytic actvity wise ....Is it iodine counter ion poisioning the reaction? compared with chloro or bromo around palladium..
The link provided by Shaban answers the question quite well. Iodine can be problem. It depends on the choice of solvent and ligand, because you can form nonreactive palladium iodide complexes that will shut down catalysis.