Pseudorotation square pyramid/trigonal bipyramid

A map that describes associative substitution of chiral tetrahedra to give isomerizable square-pyramids has been described 4). A diamond lattice map with an internal network has also been developed 4) that traces the Berry pseudorotations of chiral trigonal-bipyramids. This earlier paper provided the same generalizations as did the more detailed map of Fig. 7. 

Figure 6. The HOMO-LUMO crossing occurring during the dsd rearrangement (Berry pseudorotation) of a trigonal bipyramid through a square pyramid intermediate.

Concept of " pseudorotation introduced by R. S. Berry to interpret the stereochemical non-rigidity of trigonal bipyramidal PF5 (and SF4, ClFi) the 5 F atoms are equivalent (1953) due to interconversion via a square pyramidal intermediate. 

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 ... 

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. 

Pseudorotation.—A number of spirocyclic phosphoranes possess square-pyramidal structures rather than the trigonal-bipyramidal structures previously assumed, and this could have important consequences on the interpretation of their variable-temperature spectra. There is, as yet, no evidence that acyclic or monocyclic phosphoranes favour the square-pyramidal geometry, and the variable-temperature XH... 

Pseudorotation is well established in 5-coordinated species involving the main group elements and is best described by the Berry mechanism which interconverts two trigonal bipyramids via a square pyramid. Its operation here is difficult to reconcile with the highly stereospecific nature of substitution in Pt(II). Nevertheless, the mechanism has had substantial support. It may very well be that (a) is favored by polar solvents and that (b) is prevalent in nonpolar media. The associated reaction profiles are shown in Fig. 7.11. 

Arsenic pentafluoride (b.p. -52.8 °C) is the only stable well-characterized arsenic(V) halide. Electron diffraction49 shows it to be a trigonal bipyramidal molecule, with equatorial d As—F) 1.656 A and axial d(As—F) 1.711 A 19F NMR shows that the five fluorines are equivalent. The apparent inconsistency displayed by the two techniques is due to the differing time scales of the measured effects. On the slower NMR time scale the axial-equatorial bonds rotate via a square pyramidal intermediate resulting in the observed equivalence (pseudorotation). 

Fig. 4 Berry pseudorotation for the transition from C4v to Dih symmetry transforming a square pyramidal geometry into a trigonal bipyramidal geometry...

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