New bicyclic oxyphosphoranes, S[(t-Bu)MeC(6)H(2)O](2)P(OC(6)H(5))(O(2)C(6)H(3)F) (1) and S[(t-Bu)MeC(6)H(2)O](2)P(OC(6)H(5))(O(2)C(6)H(4)) (3), were synthesized by displacement reactions of a monocyclic pentaoxyphosphorane by a diol, and S[(t-Bu)(2)C(6)H(2)O](2)P(OCH(2)CF(3))(O(2)C(6)Cl(4)) (2) and S[(t-Bu)(2)C(6)H(2)O](2)P(C(6)H(5))(O(2)C(6)Cl(4)) (4), by oxidative addition reactions of a phosphite or phosphine with tetrachlorobenzoquinone. X-ray studies revealed hexacoordinated structures formed by the presence of a sulfur donor atom incorporated in a flexible eight-membered ring. The structures were displaced along a coordinate from a square pyramid toward an octahedron. (31)P and (1)H NMR data are also reported. Comparisons are made between bicyclic tetraoxyphosphoranes and monocyclic and bicyclic pentaoxyphosphoranes which show the importance of ligand electronegativity in increasing the degree of hexacoordination. Of the various series now studied, the extent of sulfur donor atom coordination increases in the following order: phosphates < phosphites < oxyphosphoranes. It is concluded that, in general, sulfur donor atom coordination will take place with phosphorus in any of the common coordination geometries in the presence of sufficiently electronegative ligands.