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