Turnstile Rearrangement and Pseudorotation in the Permutational Isomerization of Pentavalent Phosphorus Compounds

Abstract

This chapter deals with turnstile rearrangement and peudorotation in the permutational isomerization of pentavalent phosphorous compounds. The phosphoranes are derivatives of the pentahydride of phosphorus, in which the five ligands are covalently bonded to phosphorus. The stereochemistry of pentavalent phosphorus relates to the trigonal bipyramid, just as that of tetravalent carbon relates to the tetrahedron. X-Ray crystallographic analysis provides details of the static molecular structure of cyclic oxyphosphoranes. Group theory and topology provide a formal analysis of the way in which trigonal bipyramidal pentacoordinated phosphorus can undergo permutational isomerizations. Theoretical calculations of the energies associated with model situations in the positional exchange of ligands in phosphorus pentafluoride support two mechanisms for this exchange: Berry pseudorotation (BPR) and Turnstile rotation (TR). Cyclic pentacoordinated phosphorus compounds with trigonal bipyramidal symmetry cannot undergo positional exchange of ligands unless a TR mechanism is involved. The TR process may involve single or multiple TR, or TR with “switches.” The TR mechanism could also explain the positional exchange of ligands in other pentacoordinated, trigonal bipyramidal molecules where the central atom is an element other than phosphorus. Regular processes are not the only way by which trigonal bipyramidal phosphorus can undergo permutational isomerization. Irregular processes, involving bond ruptures and reformations, must always be considered as alternatives and some experimental justification or reasoning-by-analogy must be provided in discussions of permutational isomerization itineraries. Irregular permutational isomerizations, which involve a decrease in the pentacoordination of the phosphorus are known, and they are prone to acid catalysis.