When is used as a catalyst, {HEAD ~ a neutral Lewis acid} or a cationic Lewis or Bronsted acid whose counterion is {TAIL ~ a weakly coordinating anion} is also present as part of the catalyst system  
is
When is used as a catalyst , { HEAD ~ a neutral Lewis acid } or a cationic Lewis or Bronsted acid whose counterion is { TAIL ~ a weakly coordinating anion } is also present as part of the catalyst system
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH