By {HEAD ~ a neutral Lewis acid} is meant a compound which is a Lewis acid capable for abstracting X from to form {TAIL ~ a weakly coordinating anion} 
By { HEAD ~ a neutral Lewis acid } is meant a compound which is a Lewis acid capable for abstracting X from to form { TAIL ~ a weakly coordinating anion }
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 TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL