Structural non-rigidity and Berry pseudorotation

Pentaphenyl bismuth, BiPhs, is also approximately square pyramidal in the crystalline phase, the largest ZCBiC angles being 150° and 164°. The apical Bi-C bond distance, 221 pm, is considerably shorter than the mean basal bond distance of 232 pm. 

Problem 15.1 Calculate the mean values of the angles ZCapECbj in crystalline SbPhs and BiPhs respectively. 

In 1959 R.S. Berry postulated a general mechanism for the exchange of axial and equatorial ligands in trigonal bipyramidal molecules [5]. See Fig. 15.8. 

When the process is complete, the ligands marked a occupy equatorial positions, while the ligands marked e occupy axial position. The process does not involve any rotation, but when it is complete, it looks as if the entire molecule has been rotated 90° about the p-E bond. The process is therefore referred to as a pseudorotation. A series of such pseudorotations with different pivotal ligands, will lead to complete scrambling of all ligands. 

At some stage of the process the two angles ZeEe and ZaEa will be equal, say about 150°. At this point the coordination geometry of the central atom E is square pyramidal, and 

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