Pseudorotation square pyramidal

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 ... 

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... 

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). 

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...

FIGURE 11 Steps in the conversion of an asymmetric phosphorane (top left) into its enantiomer (top right) by a Berry pseudorotation mechanism. In each step, labeled Kx), one of the three equatorial ligands (the pivot ligand x) remains equatorial and is at the apex of the square pyramid in the transition structure. 

Crystal structures are now available for 2,2-diethoxyvinylidenetriphenyl-phosphorane, Ph3P—C=C(OEt)2,246 and the salt, [Ph3P=CH—PPh3]+Br .247 In the latter the P—C (bridge) distances are 1.710(4) and 1.695(4) A, and the PCP angle is 128.2°. Temperature-dependent H n.m.r. data for substituted aryl-bis(4,4 -dimethyl-2,2 -biphenylylene)phosphoranes are interpreted as showing that pseudorotation proceeds here by way of a square-pyramidal intermediate rather than by a simple Berry process.248... 

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