The phosphadiazonium cation [Mes*NP](+) reacts quantitatively with the fluorenylide anion, Mes*NH2, and Mes*OH (Mes* = 2, 4, 6-tri-tert-butylphenyl), resulting in formal insertion of the N-P moiety into the H-Y (Y = C, N, O) bonds. Specifically, reaction of Mes*NPCl with fluorenyllithium gives the aminofluorenylidenephosphine [crystal data : C31H38NP, monoclinic, P2(1)/c, a = 9.568(8) Angstrom, b = 24.25(2) Angstrom, c = 11.77(1) Angstrom, beta = 101.38(8)degrees, Z = 4]. Similarly, reaction of [Mes*NP][GaCl4]s*NH2 gives the diaminophosphenium salt [Mes*N(H)PN(H)Mes*][GaCl4] [crystal data : C36H60Cl4GaN2P, monoclinic, C2/c, a = 24.921(2) Angstrom, b = 10.198(4) Angstrom, c = 16.445(2) Angstrom, beta = 93.32(1)degrees, Z = 4], and reaction with Mes*OH gives the first example of an aminooxyphosphenium salt [Mes*N(H)POMes*][GaCl4]. It is proposed that the reactions involve nucleophilic attack at phosphorus followed by a 1, 3-hydrogen migration from Y to N. Experimental evidence for the formation of sigma-complex intermediates is provided by the isolation of [Mes*NP-PPh(3)][SO3CF3] [crystal data : C37H44F3- NO3P2S, triclinic, P1, a = 10.663(1) Angstrom, b = 19.439(1) Angstrom, c = 10.502(1) Angstrom, alpha = 103.100(7)degrees, beta = 113.311(7)degrees, gamma = 93.401(7)degrees, Z = 2]. As part of the unequivocal characterization of the aminooxyphosphenium salt, detailed solid-state P-31 NMR studies and CIAO calculations on the phosphenium cations have been performed. Contrary to popular belief, the phosphorus shielding in dicoordinate cations is not caused by the positive charge but results from efficient mixing between the phosphorus lone pair and pi* orbitals.