The synthesis of a series of cyclic pentaoxyphosphoranes containing a sulfonyl group was carried out by the reaction of either P(OCH2CF3)(3) or P(OPh)(3) with the appropriate diol in an oxidative addition reaction: O2S[(t-Bu)MeC6H2O](2)P(OCH2CF3)(3) (1), O2S[(t-BU)MeC6H2O](2)P(OPh)(3) (2), O2S[(t-Bu)(2)C6H2O](2)P(OCP2CF3)(3) (4), and O2S[(t-Bu)(2)C6H2O](2)P(OPh)(3) (5). Reaction of 2 with catechol yielded O2S[(t-Bu)MeC6H2O](2)P(OPh)(C6H4O2) (3). X-ray studies provided the structures of 1-5, although 1 was badly disordered. The geometries of 2, 3, and 5 are octahedral due to P-O coordination provided by the sulfonyl group, whereas 1 and 4 are trigonal bipyramidal, Solution P-31, H-1, and F-19 NMR spectra demonstrate that 1 and 4 exist in isomeric modifications, These are formulated as a pentacoordinate structure, as in the solid state, and a hexacoordinate structure, This represents the first study establishing intramolecular intraconversion between penta-and hexacoordinate isomers of phosphorus. An activation free energy of 17 kcal/mol for the exchange process for 1 and I was obtained from variable temperature H-1 NMR spectra.