The first N-donor-stabilized phosphasilene LSi(SiMe3)=PSiMe3 (L = PhC(NtBu)(2)) has been synthesized in 87% yield through 1,2-silyl migration of the (Me3Si)(2)P-substituted, N-heterocyclic silylene [LSi-P-(SiMe3)(2)]. Remarkably, the latter reacts with dichlorotri-phenylphosphorane Ph3PCl2 to give the unprecedented 4 pi-electron Si2P2-cycloheterobutadiene [(LSi)(2)P-2] with two-coordinate phosphorus atoms. The striking molecular structures as well as the Si-29 and P-31 NMR spectroscopic features of both products indicate the presence of zwitterionic Si=P bonds which is also in accordance with results by DFT calculations.