Pentacoordinated molecules .27. A model for calculating conformational energies in pentacoordinate phosphorus compounds (Reprinted from J Am Chem Soc, vol 100, pg 433, 1978)

A model is developed which allows the estimation of the relative stability of all trigonal bipyramidal (TP) and square pyramidal (SP) isomers of a given pentacoordinate phosphorus compound. It is based on the widely used pseudorotational hypothesis. The model makes use of activational energies governing fluxional behavior for simple acyclic derivatives of known structure and results of theoretical and spectroscopic studies defining TP-SP energy differences. Based on these criteria, apicophilicity scales are constructed for the TP and SP with the additional constraints that they have the same range and correlate linearly with each other as a function of ligand electronegativity, The resultant model which incorporates ring strain and steric terms has general applicability and should prove of considerable predictive value in quantifying pentacoordinate intermediates and transition states postulated in phosphorus reaction mechanisms. The model gives relative isomer energies in excellent agreement with those obtained from ab initio calculations and reproduces measured Delta G double dagger for intramolecular ligand exchange on cyclic and acyclic derivatives to within +/- 1.5 kcal/mol.