An Ylide-like Phosphasilene and Striking Formation of a 4π-Electron, Resonance-Stabilized 2,4-Disila-1,3-diphosphacyclobutadiene

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)2P-substituted, N-heterocyclic silylene [LSi−P(SiMe3)2]. Remarkably, the latter reacts with dichlorotriphenylphosphorane Ph3PCl2 to give the unprecedented 4π-electron Si2P2-cycloheterobutadiene [(LSi)2P2] with two-coordinate phosphorus atoms. The striking molecular structures as well as the 29Si and 31P NMR spectroscopic features of both products indicate the presence of zwitterionic Si═P bonds which is also in accordance with results by DFT calculations.