DFT calculations at the B3LYP/6-31G** level. The HOMO was calculated to represent mostly the 3pπ(Si–P), while the LUMO featured significant contribution from the 3pπ*(Si–P)–2pπ*(phenyl) conjugation. The HOMO-1 orbital involves the 3n–2pπ conjugation, which originate from the presence of lone pair on the phosphorus atom and the π-orbital on the Eind benzene ring. "Push-pull" phosphasilene By installing electron-donating substituents on silicon and electron-withdrawing substituents on phosphorus, the Si=P bond polarization can be decreased and even reversed through the "push-pull" interaction of the substituents with opposing electronic effects. Applying this design strategy, Escudié et al. prepared stable "push-pull" phosphasilene (tBu2MeSi)2Si=PMes* (Mes* = 2,4,6-tri-tertbutylphenyl) with electron-donating silyl groups on Si and an electron-withdrawing aryl group on P. Computations on the model compound (Me3Si)2Si=PMes (Mes = 2,4,6-trimethylphenyl) demonstrate that n(P) and π*(Si=P) correspond to the HOMO and LUMO, respectively. The relatively small energy gap between the interacting occupied (n(P)) and vacant (π*(Si=P)) molecular orbitals gives rise to a large paramagnetic contribution, which explains the extreme deshielding of the doubly bonded Si and P atoms, as well as the red shift in the UV spectrum that are observed in (tBu2MeSi)2Si=PMes*. In the "push-pull" phosphasilene prepared by Escudié and co-workers, the Si=P bond length was reported to be 2.1114(7) Å, which is longer than what was observed in most other reported phosphasilenes (2.062(1)–2.094(5) Å). Metallophosphasilenes R2Si=PM Driess et al. demonstrated that stable metallophosphasilenes of the type R2Si=PM can be prepared from metalation reaction of "half"-parent phosphasilenes R2Si=PH (R2Si = (tBu3Si)(iPr3C6H2)Si). NMR spectroscopic studies demonstrated that metalation led to the deshielding of the 31P nucleus, while the low coordinate 29Si atom in the metallophosphasilene became more shielded. Driess and coworkers explained this observation by proposing that the stabilization of the non-bonding orbital at phosphorus through n(P) → σ*(Si-Si) hyperconjugation is more effective after metalation. This is due to the higher negative partial charge at the phosphorus atom in the metallophosphasilene. As shown in the scheme below, this shielding effects is analogous to what has been observed for related alkali-metal-substituted disilenylides of the type [M(R3Si)Si=Si(SiR3)2]. The Si=P distance in the metallophosphasilene that Driess et al. synthesized was reported to be 2.064(1) Å, which is significantly shorter than that of the "half"-parent compound (R2Si=PH) from which R2Si=PM was derived. This contraction of the Si=P bond, together with a slight elongation of the Si–Si bond and a shrinkage of the P-Si-Si angle, has been rationalized by the increased hyperconjugative interactions