Berry mechanism

In pentacoordinate chemistry, the Berry mechanism may be depicted by a graph which was first used in chemistry by Balaban. Equivalent illustrations of the topology of Berry steps have been provided... 

To conclude, we think that valuable information can ce obtained from such relaxation experiments. They could provide a direct, kinetic proof of the conjecture that the Berry mechanism is the most probable one, as is indicated by some recent experimental and theoretical work. The applicability of this model is however restricted to situations where the energy of the molecule does not depend on the distribution of the ligands on the skeleton and where, as a consequence, there is one rate constant for each process. If this is not true, the present description could be the first-order approximation of a perturbation calculation. Such a work will be undertaken soon. 

It is interesting to note that many of the techniques developed in phosphorus chemistry are npw being routinely applied to hypervalent molecules of other elements. For instance, Martin et al. have studied the pseudorotational (Berry) mechanism for the inversion of 10-Si-5-siliconates (1) by 19F n.m.r. and demonstrated a linear correlation between AG for inversion at silicon and the a values of the variable ligand, Y The energy barriers for... 

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. 

Fig. 30 A—C. The Berry mechanism for electron-pair-coordination-number 5. (A) Trigonal bipyramidal coordination. Ligands bonded through shaded electron-domains are in axial positions. (B) Tetragonal pyramidal coordination. A slight distortion of structure A. (C) Trigonal bipyramidal coordination. A slight distortion of structure B. Ligands bonded through shaded electron-domains are now in equatorial positions...

J The Berry mechanism in the case of PF5 gives the impression that the whole molecule has been rotated, but since in Berry s definition there is no rotational motion, the process was named pseudorotation. The term pseudorotation was originally applied to the rapid up-and-down motion of the carbon atoms in cyclopentane, and later extended to the puckering of rings in general see Ref. 54. Objections can be raised to this duplication in terminology, and in this review we denote the Berry mechanism simply as BPR. 

Reaction of Os P(OMe)3 2Cl4 and P(OMe)3 with sodium amalgam in THF yields the white Os P(OMe)3 5. Study of the 31P[ H] NMR spectra showed simultaneous slow exchange of axial and equatorial ligands consistent with a Berry mechanism.429... 

Figure 1-7 The tbp-sp-tbp interconversion, the so-called Berry mechanism or pseudorotation for 5-coordinate molecules.

Because the rearrangement of a trigonal bipyramid is so widely important, it is useful to have a graphical representation of how the various permutations of vertex atoms are interrelated by the relevant pathway for rearrangement, which is assumed to be the Berry mechanism or pseudorotation just discussed. Necessary but not sufficient criteria for defining such a graph are ... 

A useful article has appeared on the published crystal structures of metallated phosphoranes. 2 Of the fifteen structures reviewed the majority when compared to the non-metallated phosphoranes exhibit a marked preference for tbp geometry and the review suggests that this may be due to the 7C-donating abilities of the metal substituents. In a few cases, however, the metallated phosphoranes exhibit distortion along the turnstile coordinate in contrast to the Berry mechanism followed by the non-metallated phosphoranes. It is suggested that the considerable steric demands of the metal substituents in the equatorial plane of the phosphoranes may be responsible for the exceptional behaviour. 

The study of pseudorotation in five-co-ordinate phosphorus(v) compounds is undertaken not only out of intrinsic interest but also for its relevance to rearrangements in transition states for nucleophilic substitution at tetrahedral phosphorus(v) compounds (c/. pp. 118—120). Pseudorotation in the stable compound MeaN PF4 takes place via a square-pyramidal transition state. The variation of line widths with temperature for various lines in the n.m.r. spectrum permits a distinction to be drawn between possible processes involving simultaneous interchange of two pairs of fluorine atoms, as required for the Berry mechanism for pseudorotation, or of only one pair. Inversion or pseudorotation in... 

The ED data were best fitted by distorted trigonal bipyramidal models of Civ symmetry with large-amplitude motion. A simple dynamical model for the ED data based on the ab initio results allowed the refinement of three stmctural parameters to yield good agreement with the ED data. The results of the two methods agree if a relatively flat potential for the distortion towards the C4V form is assumed. ED data as well as ab initio calculations exclude a possible pseudorotation by the Berry mechanism over a C4V barrier. 

Square-pyramidal metal carbonyls [M(CO)s] (M = Cr, Mo, or W) are generated on u.v. photolysis of metal hexacarbonyls and related compounds and appear as intermediates in photochemical replacement reactions of the six-co-ordinate species. The reaction step relevant in this context is an internal rearrangement of the square-pyramidal [M(CO)6] via a trigonal-bipyramidal intermediate, and back to the square-pyramidal ground state. This spy - tbp spy process represents an inverse Berry mechanism and also accounts for many other aspects of the photochemistry of these compounds. ... 

Intramolecular Processes. A -Ray work has shown the existence of a square-pyramidal structure (12). The n.m.r. data on spirocyclic oxyphosphoranes have been discussed, and were concluded to be consistent with the AT-ray data. Mechanisms for intramolecular exchange processes are proposed. The rate of pseudorotation in XPF4 increases in the sequence MejN < SR, H < Cl < Me, F. It was concluded that even if a square pyramid were an intermediate in pseudorotation, it would not be possible to detect it directly with present-day equipment. However, in a study of the H n.m.r. spectrum of 2-isopropylphenylbis-(4,4 -dimethyl-2,2 -biphenylene) phos-phorane, it is suggested that pseudorotation does not occur by a simple Berry mechanism, but involves a square-pyramidal intermediate. An n.m.r. study of (13)... 

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