N-heterocyclic carbenes (NHCs) have become one of the most important ligands in transition-metal catalysis. The success of normal NHCs is greatly attributed to their superior σ-donating capabilities as compared to phosphines, which is even greater in abnormal NHC counterparts. Employed as ligands in palladium complexes, NHCs contributed greatly to the stabilization and activation of precatalysts and have therefore found application in many areas of organometallic homogeneous catalysis, including Sonogashira couplings.
Interesting examples of abnormal NHCs are based on the mesoionic 1,2,3-triazol-5-ylidene structure. 
An efficient, cationic palladium catalyst of PEPPSI type, i.e., iPEPPSI (internal pyridine-enhanced precatalyst preparation stabilization and initiation) was demonstrated to efficiently catalyse the copper-free Sonogashira reaction in water as the only solvent, under aerobic conditions, in the absence of copper, amines, phosphines and other additives.