A series of phosphine-stabilized phosphoranimine cations [R3P center dot PR2'=NSiMe3](+), which can be regarded as derivatives of the proposed transient reactive intermediate [PR2'=NSiMe3](+) in the thermal condensation polymerization of phosphoranimines (R '' O)PR2'=NSiMe3 to form poly(alkyl/arylphosphazenes) [PR2'=N](n) at 180-200 degrees C, have been prepared. The bromide salts [R3P center dot PR2'=NSiMe3]Br [R' = Me ([6](+)), OCH2CF3 ([8](+)); R3P = Me3P (a), Et3P (b), (Bu3P)-Bu-n (c), dmpm (d, dmpm = dimethylphosphinomethane), dmpe (e, dmpe = dimethylphosphinoethane)] were prepared from the direct reactions between BrMe2P=NSiMe3 (5) and Br(CF3CH2O)(2)P=NSiMe3 (7) and the corresponding tertiary phosphines R3P or the diphosphines Me2P(CH2)(n)PMe2 (n = 1, 2). Cations of the type [6](+) and [8](+), with electron-donating and -withdrawing groups at the phosphoranimine phosphorus center, respectively, undergo facile phosphine ligand substitution with the strong N-donor 4-dimethylaminopyridine (DMAP) to yield the corresponding DMAP-stabilized salts [DMAP center dot PR2=NSiMe3]Br [R = Me ([9](+)), OCH2CF3 ([10](+))]. Cations [6](+) with Br- anions are particularly labile: for example, [6a]Br slowly releases PMe3, BrSiMe3, and forms cyclic phosphazenes such as [Me2P=N](4). Anion exchange reactions between the salts [6b]Br or [8c]Br and AgOTf (OTf = CF3SO3) quantitatively afforded the corresponding and more stable triflate salts [6b]OTf and [8c]OTf. Phosphine ligand abstraction reactions with B(C6F5)(3) were observed for the bromide salts [6b]Br and [8c]Br, which regenerated the phosphoranimines 5 and 7, respectively, and formed the adduct R3P center dot B(C6F5)(3). In contrast, the triflate salts [6b]OTf and [8c]OTf were unreactive under the same conditions. X-ray structural analysis of the P-donor stabilized cations revealed longer P-P and P-N bond lengths and smaller P-N-Si bond angles for cations [6](+) compared to analogs [8](+). These structural differences were rationalized using the negative hyperconjugation bonding model. In addition, the (1)J(PP) coupling constants for the cations [6](+) observed by both solution and solid-state P-31 NMR are remarkably small (13-25 Hz), whereas those for [8](+) are substantially larger and positive (276-324 Hz) and are as expected for P(IV)(+)-P(V) systems. DFT studies suggest that the significant difference in (1)J(PP) couplings observed for [6](+) and [8](+) appears to be related to the electronegativity, of the R' substituents at the phosphoranimine phosphorus center rather than the strength of the donor-acceptor P-P bond, which is slightly weaker in [6](+) relative to that in [8](+), as indicated by the X-ray data and reactivity studies.