Two equivalents of boron trifluoride-diethyl ether complex or 1 equiv of trifluoromethanesulfonic acid reacts with C-[bis(diisopropylamino)phosphino]-C,P-bis(diisopropylamino) phosphaalkene 1, cleaving one of the diisopropylamino substituents at the sigma(3)-phosphorus atom. This affords the corresponding 1 sigma(2),3 sigma(2)-diphosphaallylic cation 4, which is isolated as the four-membered-heterocycle 3. Addition of a catalytic amount of base to 3 leads to a 1 sigma(1),3 sigma(3)-diphosphaallylic cation 5, which is isolated as the diphosphirenium salt 2. Mesityllithium and the lithium salts of diisopropylamine or dicyclohexylamine react at the sigma(2)-phosphorus atom of the cationic heterocycles 2 and 3, affording the corresponding phosphaalkenes 8, 1, and 6, respectively. All the results demonstrate the high electrophilicity of low-coordinated diphosphaallylic cations, which are only isolable as intramolecular donor-acceptor complexes.