Structural distortions of cyclic phosphoranes and the Berry exchange coordinate. A quantitative description

Structural distortions of cyclic phosphoranes are shown to form a continuous series between the idealized trigonal bipyramidal (D 3h) and square pyramidal (C 4v) representations. The particular form of the distortions is along the Berry intramolecular exchange coordinate. Despite the lack of symmetry in the makeup of the cyclic substituents, a local C 2v constraint is closely followed. The origin of the mode of distortion is discussed in relation to the two sets of bond properties peculiar to five-coordinate geometries and the closeness in energy of the two idealized structures, enhanced by the presence of ring constraints. These findings reinforce the operation of successive Berry rearrangements postulated to account for NMR exchange data on a wide variety of phosphorane derivatives.