Cationic imidazoliumyl(phosphonio)-phosphanides [L_C-P-PR₃]⁺ (1a-e⁺, L_C = 4,5-dimethyl-1,3-diisopropylimidazolium-2-yl; R = alkyl, aryl) are obtained via the nucleophilic fragmentation of tetracationic tetraphosphetane [(L_C-P)₄][OTf]₄ (2[OTf]₄) with tertiary phosphanes. They act as [L_C-P]⁺ transfer reagents in phospha-Wittig-type reactions, when converted with various thiocarbonyls, giving unprecedented cationic phosphaalkenes [L_C-P═CR₂]⁺ (5a-f[OTf]) or phosphanides [L_C-P-CR(NR₂^′)]⁺ (6a-d[OTf]). Theoretical calculations suggest that three-membered cyclic thiophosphiranes are crucial intermediates of this reaction. To test this hypothesis, treatment of [L_C-P-PPh₃]⁺ with phosphaalkenes, that are isolobal to thioketones, permits the isolation of diphosphirane salts 11a,b[OTf]. Furthermore, preliminary studies suggest that the cationic phosphaalkene [L_C-P═CPh₂]⁺ may be employed to access rare examples of η²-P═C π-complexes with Pd⁰ and Pt⁰ when treated with [Pd(PPh₃)₄] and [Pt(PPh₃)₃] for which analogous complexes of neutral phosphaalkenes are scarce. The versatility of [L_C-P]⁺ as a valuable P₁ building block was showcased in substitution reactions of the transferred L_C-substituent using nucleophiles. This is demonstrated through the reactions of 5a[OTf] and 6c[OTf] with Grignard reagents and KNPh₂, providing a convenient, high-yielding access to MesP═CPh₂ (16) and otherwise difficult-to-synthesize 1,3-diphosphetane 17 and P-aminophosphaalkenes.