Berry-Pseudorotation

Fig. 1. Berry pseudorotation about pentacoordinate ( ) phosphorus, where (Q) represent fluorine atoms, (a) Original trigonal bipyramid (b) square...

The F nmr spectrum of this compound gives only one signal over a wide range of temperatures, a result attributed to Berry pseudorotation (144). No alkyl- or aryltetrabromoarsorane has been reported. There is, however, an early report on the preparation of tetraiodomethylarsorane from methylarsonic acid and hydriodic acid (145). 

Phosphonium hexafluorophosphate, benzotriazolyl-N-hydroxytris(dimethylamino)-in peptide synthesis, 5, 728 Phosphonium salts chromene synthesis from, 3, 753 reactions, 1, 531 Phosphonium salts, vinyl-in pyrrole synthesis, 4, 343 Phosphonium ylides in heterocyclic synthesis, 5, 165 Phosphoramide, triethylene-as pharmaceutical, 1, 157 Phosphoramide, triethylenethio-as pharmaceutical, 1, 157 Phosphorane, pentaphenyl-synthesis, 1, 532 Phosphoranes, 1, 527-537 Berry pseudorotation, 1, 529 bonding, 1, 528... 

Figure 12.13 Interchange of axial and equatorial positions by Berry pseudorotation (BPR).

The pentacoordinate molecules of trigonal bipyramidal form, like PF5, are a very nice example for the study of the formal properties of stereoisomerizations. They are characterized by an appreciable nonrigidity and they permit the description of kinetics among a reasonable number of isomers, at least in particular cases (see below). Therefore the physical and chemical properties of these molecules have been thoroughly investigated in relation to stereoisomerization. Recent reviews may be found in the literature on some aspects of this problem. Mislow has described the role of Berry pseudorotation on nucleophilic addition-elimination reactions and Muetterties has reviewed the stereochemical consequences of non-rigidity, especially for five- and six-atom families as far as their nmr spectra are concerned. 

C. Pseudorotation.—An alternative intramolecular exchange process to Berry pseudorotation has been suggested, which also occurs with conservation of angular momentum. It has been called a turnstile-rotation process because it involves the rotation of an apical-radial pair of ligands... 

Fig. 27. Energy profile for the Berry pseudorotation in PH5 (schematic). The reaction progress abscissa is made of an appropriate combination of two HPH angles. The energy barrier is in Kcal/mole.

These conclusions can be drawn by simple inspection of the molecular orbitals of PHS. The next question is the energetics of the interconversions of the various conformations. The result sketched in Fig. 27 refers to the Berry pseudorotation (J5) process of PHS. The square pyramid is a transition state for isomerization of the trigonal bipyramid, but the barrier is extremely small, in agreement with available experimental evidence. The more complex process ... 

A potential energy surface was computed for Berry pseudorotation in IrCl4N02- and Mn(CO)4NO the results confirm the qualitative conformational trends obtained from orbital interaction arguments. 

FIGURE 14.8 The Berry pseudorotation that leads to interchange of axial and equatorial groups. The letters a, e, A, etc., indicate only positions because all five groups are identical. 

R. S. Berry. Known as the Berry pseudorotation, the mechanism involves the trigonal bipyramid (D)fl) passing through a square based pyramid (C4v) as shown in Figure 14.8. This behavior is somewhat similar to the inversion of the ammonia molecule (C3 ) as it passes through a planar (D)fl) structure. 

CH3CN, 33-11, 33 H20, 34, 35-jCH3CN, and 37jCH3CN represent nearly ideal or (more or less) distorted square pyramids, with the carbon atom Cl in the apical position. In most cases, the geometries of the Si-coordination polyhedra in the crystal are located on the Berry-pseudorotation coordinate (pivot atom Cl). 

Berry pseudorotation process, 34 239 Bertrand diagram, 36 3 Bertrand-Gayda Model, 38 176-178 Beryllium... 

C. Comparison of the Turnstile Rotation and Berry Pseudorotation Processes... 

The phosphorane 70 can also undergo stereomutation without bond cleavage by the Berry process (Berry pseudorotation with the PH bond as pivot) or by the Ramirez process... 

Percentage displacement along the Berry pseudorotation coordinate. 

The stability of five-coordinate intermediates also makes possible the ready racemization of optically active silanes by catalytic amounts of base. The base can add readily to form a five-coordinate intermediate. The latter can undergo Berry pseudorotation with complete scrambling of substituents followed by loss of the base to yield the racemized silane. 

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