New bicyclic tetraoxyphosphoranes all containing a six-membered oxaphosphorinane ring, C6H8(CH2O)(2)P(OC12H8)(OXyl) (1), (C6H4O)(2)P(OC12H8)(OXyl) (2), CH2[(t-Bu)(2)C6H2O](2)P(OC12H8)(OXyl) (3), O2S[(t-Bu)MeC6H2O](2)P(OC12H8)(OXyl) (4), and S[(t-Bu)MeC6H2O](2)P(OC12H8)(OXyl) (5), were synthesized by the oxidative addition reaction of the cyclic phosphine P(OC12H8)(OXyl) (6) with an appropriate diol in the presence of N-chlorodiisopropylamine. X-ray analysis revealed trigonal bipyramidal (TBP) geometries for 1-4 where the dioxa ring varied in size from six-to eight-membered, With a sulfur donor atom as part of an eight-membered ring in place of a potential oxygen donor atom of a sulfone group as in 4, the X-ray study of 5 showed the formation of a hexacoordinated structure via a P-S interaction. Ring constraints are evaluated to give an order of conformational flexibility associated with the (TBP) tetraoxyphosphoranes 4 > 3 similar to 1 > 2 which parallels the degree of shielding from P-31 NMR chemical shifts : 4 > 3 > 1 > 2. The six- and seven-membered dioxa rings in 1 and 2, respectively, are positioned at axial-equatorial sites, whereas the eight-membered dioxa ring in 3 and 4 occupies diequatorial sites of a TBP. V-(TH)-H-1 NMR data give barriers to xylyl group rotation about the C-OXyl bond. The geometry of 5 is located along a coordinate from square pyramidal toward octahedral to the extent of 60.7%. Achieving hexacoordination in bicyclic tetraoxyphosphoranes of reduced electrophilicity relative to bicyclic pentaoxyphosphoranes appears to be dependent on the presence of a sufficiently strong donor atom.