Oxidative addition reactions of the eight-membered cyclic phosphite CH2[(t-Bu)(2)C6H2O]2POCH2CF3 (8) with 1,2-dicarbonyl compounds yielded the new bicyclic pentaoxyphosphoranes CH2[(t-BU)(2)C6H2O]P-2(OCH2CF3) (O2C6-Cl-4) (1), CH2[(t-BU)(2)C6H2O]P-2(OCH2CF3)(O(2)C(2)Ph(2)) (2), and CH2[(t-BU)(2)C6H2O]P-2(OCH2CF3) [O2C6H2(t-Bu)(2)] (3). They were characterized by H-1 and P-31 NMR. (31)p chemical shifts of 1-3 correlate with previous data showing characteristic downfield shifts for bicyclic pentaoxyphosphoranes relative to monocyclic members. An X-ray study of 2 revealed the eight-membered ring in diequatorial positions of a trigonal bipyramid (TBP) with the five-membered ring situated at axial-equatorial sites. This represents the first crystallographic evidence showing rings occupying the two different sets of positions of a TBP for a pentaoxyphosphorane. 2 crystallizes in the orthorhombic space group Pna2(1) with a = 20.211(4) Angstrom, b = 10.479(1) Angstrom, c = 19.784(3) Angstrom, and Z = 4. The final conventional unweighted residual is 0.061.