The conversion of the hexacoordinate zwitterionic phosphorus compound (p-tert-butylcalix[4]arene)P(H)NHMe2 (1a) to the tricoordinate hydroxyphosphite (OH) (p-tert-butylcalix[4]arene)P(4a) proceeds through a pentacoordinate intermediate 5a which can be spectroscopically characterized. This intermediate contains three calixarene P-O bonds; in addition, a proton and dimethylamine remain attached to the phosphorus. No evidence is found for a pentacoordinate species in which all four calixarene oxygens are bound to the phosphorus. Treatment of 1a with methyl trifluoromethanesulfonate leads to 4a and trimethylammonium trifluoromethanesulfonate. Treatment of 1a with butyllithium followed by addition of methyl trifluoromethanesulfonate leads to 6a, the trimethylamine analog of 1a. Oxidation of the phosphorus in 4a with tert-butyl hydroperoxide gives the phosphoryl derivative 7a. If the tert- butyl groups in the starting calixarene are replaced with hydrogens, the analogous unsubstituted compounds 1b, 4b, and 7b can all be synthesized and isolated under similar reaction conditions. Both 1a and 1b can be converted to 4a and 4b, respectively, by heating in the solid state or in solution; however, significantly higher temperatures are necessary for conversion of the tert-butyl analog. The species 4 can be deprotonated with potassium tert-butoxide to give the potassium salt 9; spectra suggest that the overall geometry of 9 is similar to 4 except for the absence of the hydroxyl proton. However, when 4 is treated with butyllithium, a mixture of two products is formed. One, 10, is the lithium analog of 9. The other, 11, appears to be a hexacoordinate derivative in which the bottom of the basket has reclosed via attack of the butyl anion on phosphorus and proton transfer from oxygen back to phosphorus. The X-ray crystal structures of 1b and 7b were obtained. The structure of 1b shows the geometry about phosphorus to be distorted octahedral, with the calixarene in the cone conformation and the phosphorus lying 0.14 angstrom above (inside the basket) the plane of the four calixarene oxygens. The P-H bond is inside the basket, and the P-N bond is trans to it. In 7b, the calixarene backbone lies somewhere between a partial cone and 1,2-alternate conformation. The phosphorus is bound to three calixarene oxygens. X-ray data : 1b, P1BAR, a = 11.677(3) angstrom, b = 14.055(4) angstrom, c = 18.824(7) angstrom, a = 90.47(3)degrees, beta = 101.85(3)degrees, gamma = 112.19(2)degrees, Z = 2; 7b, P2(1)/c, a = 15.064(5) angstrom, b = 15.424(4) angstrom, c = 13.073(3) angstrom, beta = 108.52(2)degrees, Z = 4.