in the R2Si=PM system. Phosphasilene with amino-substituents at Si Due to the n(N)–π*(Si=P) orbital interaction, there exists strong delocalization of electron density from nitrogen to phosphorus in phosphasilene with amino-substituents at silicon. Among the resonance structures shown in the figure on the left, the structure in the middle with zwitterionic structure was calculated to have a significant contribution. Therefore, compared to the parent phosphasilene, the double-bond character of the Si–P is significantly reduced in phosphasilene with amino-substituents at Si, giving rise to longer Si–P bond distance. Reactivity Reaction with Lewis acids and bases Due to the higher electronegativity of phosphorus and the polarized nature of the Si=P moiety, phosphasilene tend to react with Lewis acids and bases at the phosphorus atom and silicon atom, respectively. The reaction of phosphasilene with Lewis acids, which usually occurs at the Lewis-basic two-coordinate phosphorus atom, is also dependent upon the nature of the reactants. Lewis bases have been shown to react with both three-coordinate and four-coordinate silicon atoms. The coordination of Lewis bases to the three-coordinate silicon atom of phosphasilene can lead to effective stabilization of the Si=P moiety. For example, it has been demonstrated that unstable phosphasilene can react with DMAP and small N-heterocyclic carbene (NHC) to afford the corresponding stable complexes. In phosphasilenes stabilized by NHC, the Si=P bonds are elongated and the negative charge gets localized on the phosphorus atom. Among the resonance structures shown in the figure below, Natural Resonance Theory (NRT) indicates that the third structure with a Si–P single bond is predominant, carrying a resonance weight of 76.3%. Therefore, these stabilized phosphasilene can also be interpreted as a silyene-phosphinidene adduct. For donor-stabilized phosphasilene with four coordinate silicon, the reaction with Lewis bases may lead to ligand exchange at the silicon atom. Metalation Metalation of phosphasilenes gives rise to either complexes featuring the coordination of the phosphorus lone pair to a metal center or P-metalated phosphasilenes. In the former case, the binding of the phosphasilenes to transition metals via the phosphorus lone pair reduces the double-bond character of the Si–P bond. Some examples of this type of phosphasilene transition metal complexes are shown below. Driess and coworkers first observed the formation of P-metalated phosphasilenes of the latter case: P-ferrio-substituted phosphasilene R2Si=P[Fe(CO)2(η5-C5H5)] (R = 2,4,6-iPr3C6H2). They further demonstrated that P-metalated phosphasilenes R2Si=PM can be obtained by metalating "half"-parent phosphasilenes, which substitutes the R2Si=PH hydrogen atom with transition metal-containing fragments. Phosphinidene