Molecular cleft selectivity and exo- vs endo-phosphorus site regioselectivity of cyclotetraphosphazanes [C6H4N2-(Phe)(2)](2) (1) and [C6H4N2(PhP)(PhPS)](2) (2) have been examined. Oxidation of 1 with O-3 or Sg occurs regioselectively to form exo-phosphorus substituted [C6H4N2(PhP)(PhPO)](2) (4) and 2, respectively; Ag+ ion coordination and CH3I quaternization occurs at exo sites with no apparent involvement of the cleft (endo-) phosphorus atoms. Insertion of O atoms into the deft of 4 yields endo-substituted (C6H4N2)(2)(PhP)(PhPO)(3) (5) and [C6H4N2(PhPO)(2)](2) (6); insertion of S, AgNO3, or Ni(CO)(2) into the cleft of 2 yields (C6H4N2)(2)(PhP)(PhPS)(3) (10), [C6K4N2(PhPS)(2)](2) (11), {[C6H4N2(PhP)(PhPS)](2) . Ag}NO3 (12), and [C6H4N2(PhP)(PhPS)]Ni-2(CO)(2) (15). Selenium oxidizing reagents (elemental Se and KCNSe), elemental Te, the larger metal moieties Mo(CO)(3) and Mo(CO)(4), and BH3 and CH3I are excluded from reactivity in the cleft of 2. Complex 12 undergoes further reaction to form the mono- and dioxides (C6H4N2)(2)(PhP)(PhPO)(PhPS)(2) (13) and [C6H4N2(PhPO)(PhPS)](2) (14), via a process whereby oxidation by novel Ag+ activation occurs. New compounds are characterized by spectral data; structures of 4, 10, 11, and 15 were established by single crystal X-ray analysis : 4, monoclinic, C2/c, a = 19.154(8) Angstrom, b = 9.081(4) Angstrom, c = 18.832(9) Angstrom, beta = 103.09(4)degrees, Z = 4, V = 3190(2) Angstrom(3), d(obs) = 1.400 g/cm(3), R = 0.041, R(w) = 0.050; 10, triclinic, P (1) over bar, a = 11.209(2) Angstrom, b = 13.449(2) Angstrom, c = 13.872(3) Angstrom, alpha = 92.373(14)degrees, beta = 96.703(14)degrees, gamma = 105.718(14)degrees, Z = 2, V = 1993.4(6) Angstrom(3), d(obs) = 1.357 g/cm(3), R = 0.047, R(w) = 0.060; 11, monoclinic, C2/c, a = 26.674(5) Angstrom, b = 9.976(2) Angstrom, c = 19.732(5) Angstrom, beta = 120.47(1)degrees, Z = 4, V = 4525.5(17), d(obs) = 1.358 g/cm(3), R = 0.045, R(w) = 0.053; 15, monoclinic, C2/c, a = 28.583(13) Angstrom, b = 15.524(9) Angstrom, c = 18.409(8) Angstrom, beta = 114.75(5)degrees, Z = 8, V = 7418(6) Angstrom(3), d(obs) = 1.467 g/cm(3), R = 0.075, R(w) = 0.073. The basic cyclotetraphosphazane structure characterized by 1 and 2 is maintained upon insertion into the molecular cleft of one S atom (10)or a Ni(CO)(2) moiety (15); however, two S atoms in the cleft (11) causes severe distortion of the ring system.