A molybdenum complex containing diamino-substituted phosphite, fac-[(bpy)(CO) 3Mo{PNN(OMe)}] (PNN stands for PN(Me)CH 2CH 2N Me) reacts with a Lewis acid such as BF 3·OEt 2 to give a cationic phosphenium complex fac-[(bpy)(CO) 3Mo{PNN}] +, where an OMe on a phosphorus is abstracted as an anion. The facial isomer spontaneously isomerizes into its meridional form. Group 6 transition metal complexes, fac-[(bpy)(CO) 3M{PXY(OMe)}] (M=Cr, Mo, W; XY=(NEt 2) 2, N(Me)CH 2CH 2O, (NEt 2)(OMe) and OCMe 2CMe 2O, (OMe) 2) have been subjected to reaction with BF 3·OEt 2. These reactions reveal that the stability of cationic phosphenium complexes increases with: (i) going to a heavier congener; (ii) increasing the number of amino substituents on the phosphenium phosphorus; and (iii) adding an ethylene bridge between X and Y when at least one of X and Y is an amino substituent. The cationic phosphenium complex reacts with a nucleophile (Nu=OEt −, Me −) at the phosphenium phosphorus to give fac-[(bpy)(CO) 3M{PXY(Nu)}], and also reacts with a tertiary phosphorus compound (L) to give [(bpy)(CO) 2LM{PXY}] +. The reaction of a cationic monoaminomonoalkoxy phosphenium complex of Mo, mer-[(bpy)(CO) 3Mo{PNO}] + (PNO stands for PN(Me)CH 2CH 2O ) with a diamino-substituted phosphorus compound, PNN(Y) (Y=OMe, OEt, SEt, N(CH 2) 3CH 2) proceeds with substitution for CO and then with the Y group migration to the coordinating phosphenium phosphorus to give [(bpy)(CO) 2{PNO(Y)}Mo{PNN}] +. The reaction is irreversible. Reactions of iron complexes containing a Group 14 element ligand (ER 3; E=C, Si, Ge, Sn) and diamino-substituted phosphite, Cp(CO)(ER 3)Fe{PNN(OMe)} with a Lewis acid have been examined. The reaction product depends on E. In any case, an OMe anion abstraction by a Lewis acid uniformly takes place at the first stage of the reaction to give a cationic phosphenium iron complex containing an ER 3 ligand. The subsequent reaction is strongly dependent on E. When E is C, migratory insertion of the phosphenium ligand into the FeC bond or more simply an alkyl migration from Fe to phosphenium phosphorus occurs. When E is Si or Ge, the cationic phosphenium complex is stable and FeSi and FeGe bonds remain intact. In contrast, when E is Sn, one alkyl group on the Sn, and not SnR 3, migrates to the phosphenium phosphorus to give a stannylene complex. The corresponding Ru complexes show similar reactions. X-ray structures of cationic phosphenium complexes of Mo and Fe have been employed and reveal that there is considerable double bond character between a transition metal and the phosphenium phosphorus. 31P- and 95Mo-NMR spectroscopic data also support the double bond character. Activation parameters concerning phosphenium ligand rotation along the PFe axis could be estimated from VT-NMR studies.