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