Berry Pseudorotation Mechanism BPR

BPR was introduced53 to explain the observed equivalence of the five fluorines of PF5 (7) in the n.m.r. above —100°. The apical and the equatorial positions of the TBP are not symmetry-equivalent and, as was later shown,26 the apical P—F bonds are indeed longer than the equatorial ones in (7). Berry suggested53 that the magnetic equivalence was due to arelativelyrapidpositionalexchangeofligands, (7) (7 ), whichresulted 

Motions of ligands in Berry Pseudorotation. F =Pivot. F =Apical. F = Equatorial. 

The transition state for BPR was given tetragonal-pyramidal symmetry, C4v in some recent calculations mentioned in footnotes 34 and 35 of Ref. 49. 

As the pivot, the dimethylamino-group remains in the equatorial position. 

The Whitesides-Mitchell experiment80 can be interpreted by BPR using the nitrogen as pivot. The experiment rules out, for this particular molecule, mechanisms discussed by Muetterties78 as alternatives 

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