Berry pseudorotation phosphorus compounds

Phosphorus v) Compounds. According to CNDO/2 calculations, the only way in which the axial and equatorial fluorines in PF, can interchange is by the Berry pseudorotation mechanism,and an estimate of the barrier to such an interchange, (1371 cm or 3.92 kcal moP ) has been obtained from a detailed Raman study of the Vj, equatorial bend, of PF.-,. A new normal-co-ordinate analysis for the PF5 molecule has been published. 

It has been suggested that pseudorotation, i.e. intramolecular ligand rearrangement by the Berry mechanism, should be faster at arsenic(v) incorporated into a five-membered spiroarsorane ring than at phosphorus(v) in an analogous spirophosphorane. However, variable-temperature n.m.r. studies of compounds (22), with R = Me, CHaPh, Ph, p-QHa OMe, or p-CeHi NOa, and (23), with R = Me or Ph, show that barriers are comparable, at around 20—23 kcal mol, for such arsenic and phosphorus compounds. Pseudorotation in AsFs has been discussed theoretically. ... 

The study of pseudorotation in five-co-ordinate phosphorus(v) compounds is undertaken not only out of intrinsic interest but also for its relevance to rearrangements in transition states for nucleophilic substitution at tetrahedral phosphorus(v) compounds (c/. pp. 118—120). Pseudorotation in the stable compound MeaN PF4 takes place via a square-pyramidal transition state. The variation of line widths with temperature for various lines in the n.m.r. spectrum permits a distinction to be drawn between possible processes involving simultaneous interchange of two pairs of fluorine atoms, as required for the Berry mechanism for pseudorotation, or of only one pair. Inversion or pseudorotation in... 

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