Phosphorus pyramidal geometry

This is an important parallel to the analogous process first discovered for phosphorus ylides in the late 1980s,166 and extended to simple hydrocarbons in the mid-1990s.15 Note that carbon is again found to become hypercoordinate in a square-pyramidal geometry (Scheme 37). 

One of the few phosphorus compounds that exhibit square pyramidal geometry is shown in Fig. 6 31. Rationalize the preferred geometry of SP over TBP in terms of the presence of Ihe four- and five-membered rings. (Holmes. R.R./. Am. Chem. Soc. 1975.97.3379.) 6.17 Consider the cyclic compounds I and II. In I the rapid exchange of the fluorine atoms is inhibited just as it is in (CH,hpb - However, exchange in II is very rapid. Suggest a reason. 

In the absence of X-ray data for an arsenic ylide, it was, therefore, predicted that the carbanion in arsenic ylides should possess pyramidal geometry as opposed to the planar geometry in the phosphorus ylides. 

Donor/acceptor-substituted phosphole 22 exhibits classical properties, namely the phosphorus atom has a pyramidal geometry and the aromatic character of the heterole is similar to that of cyclopentadiene <2000JOC2631>. Due to the push-pull substitution pattern, significant delocalization of the endocyclic 7t-electron density over the entire system... 

Phospholes are less stable than azoles because phosphorus prefers a pyramidal geometry rather than a planar one. However, P-mesityl derivatives are forced into planarity. 1,3-Di- and 1,2,4-triphospholes with a P-mesityl group are nearly or fully planar, and so are also 1,2,4-azadiphospholes, 1,2,4-thiadiphospholes and 1,2,4-diazaphosp-holes. Most interestingly, unlike the explosive pentazoles, pentaphosp-holes are thermally stable but dimerize by a [4- -2]-cycloaddition. Pentaphosphole P5H is planar, in contrast to phospholes (96JPC13447). 

No Au-Au interactions occur in the monocation [RP(AuPPh3)3]+ (R = o-tolyl group) in which the central P atom has an ideal tetrahedral geometry. However, by treatment of this compound with [Ph3Au]+(BF4) (Scheme (3)), the dication [RP(AuPPh3)4] +, which has square-pyramidal geometry with the phosphorus atom at the apex, is formed. 

Interest in the controversial issue of the aromaticity of phosphole continues. The photoelectron spectroscopic (p.e.s.) data of Schafer et al (noted in the previous Report), which were interpreted as indicating that the phospholes (164) are aromatic systems which do not involve appreciable delocalization of the lone pair on phosphorus with the diene jr-system, have now been re-evaluated, using a different theoretical approach. It has been concluded that there is a significant interaction between the lone pair on phosphorus and the diene n -system, leading to a traditional aromatic stabilization of the phosphole system in both planar and pyramidal geometries, consistent with earlier assessments of the aromaticity of phosphole that were based on studies of n.m.r. spectra, structure, and reactivity. The non-planarity... 

The very fast Berry and tumstile pseudorotation processes can lead to the presence of a number of stereoisomers. The Berry process occurs through a transition state possessing the C4V square-pyramidal geometry with an energy barrier of ca. 2-3 kcal/mol in the case of phosphorus derivatives.48,49 (Scheme 2.5)... 

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