Pseudorotation and the Trigonal Bipyramid

Some important features of phosphoranes can be summarised as follows  

Stable phosphoranes with 5-coordinated P almost always have a trigonal bipyramidal configuration. 

The trigonal bipyramidal configuration is believed also to represent an intermediate state in many reactions of tetrahedral phosphorus compounds. 

Trigonal bipyramidal phosphoranes may in some cases exist in eqniUbrium with pyramidal and octahedral as well as tetrahedral molecules. 

The stability of a trigonal bipyramidal compound generally increases with the total electronegativity of the substituent groups. 

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