Equilibria of Trigonal Bipyramidal Molecules

Many trigonal bipyramidal phosphorus molecules exist in equilibrium with tetrahedral phospho-nium cations and octahedral anions. In some instances all forms can be isolated and characterised. Phosphorus pentachloride, for example, exists as PCI5 in the vapour state, but as an assembly of PCI4 PCI in the solid state. Dimerisation also occurs to a small extent in some solvents (3.41). 

In some cases trigonal bipyramidal and pyramidal P molecules can exist in equilibrium. Thus in (3.42) the trigonal bipyramidal form exists in methylene chloride solution, but in dimethyl furan the phosphite form predominates. 

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