Members of a new class of bicyclic oxyphosphorane compounds containing a six-membered oxaphosphorinane ring, Me(2)C(CH2O)(2)P(OC12H8)(OXyl) (1), CH2(MeCHO)(2)P(OC12H8)(OXyl) (2), and C6H4(CH2O)(2)P(OC12H8)(OXyl) (3), were synthesized in 85-90% yield by the oxidative addition reaction of a diol to the cyclic phosphine precursors. X-ray analysis revealed trigonal bipy;ramidal (TBP) geometries with the rings spanning axial-equatorial positions. P-31 NMR spectral data showed that pentacoordination was retained in solution. Variable-temperature H-1 NMR spectra recorded over the range of-90 to 85 degrees C supported intramolecular ligand exchange processes (pseudorotation) where each of the rings was required to be located in diequatorial positions in exchange intermediates. Application of a well-parametrized model treatment, including element effect and ring strain terms, resulted in excellent agreement between calculated and experimental activation energies for ligand exchange in 1-3. Strain energies for diequatorial placement of rings in 1-3 follow the order dibenzo-fused oxaphosphorinane > dioxaphosphorinane > benzo-fused dioxaphosphepane. The results of this study have application to nucleophilic displacement reactions of cyclic phosphorus compounds involving oxyphosphorane intermediates, e.g., cAMP. 1 crystallizes in the triclinic space group P (1) over bar with 8.607(1) Angstrom, b = 16.817(2) Angstrom, c = 17.153(2) Angstrom, alpha = 115.15(1)degrees, beta = 90.78(1)degrees, gamma = 91.46(1)degrees, and Z = 4. 2 crystallizes in the monoclinic space group P2(1)/n with a = 9.315(2) Angstrom, b = 17.422(3) Angstrom, c = 14.096(2) Angstrom, P = 102.04(1)degrees, and Z = 4; 3 crystallizes in the monoclinic space group P2(1)/c with a = 9.734(1) Angstrom, b = 27.769(4) Angstrom, c = 9.263(1) Angstrom, beta = 113.17(1)degrees, and Z = 4. The final conventional unweighted residuals are 0.037 (1), 0.040 (2), and 0.035 (3).