It is widely accepted that trigonal bipyramidal (TBP) hydroxyphosphoranes (such as'3) or their conjugate bases, phosphoranoxide anions (such as 2), are intermediates in nucleophilic displacements at tetracoordinate phosphorus in cyclic P= O bond containing compounds.'Thermally* and hydrolytically3 stable cyclic hydroxyphosphoranes and phosphoranoxide anions have recently been prepared. Spectroscopic evidence for ...