page_title: Phosphasilene
section_title: The parent phosphasilene H2Si=PH
hierarchical_section_title: Phosphasilene / Structure and bonding / The parent phosphasilene H2Si=PH
caption_attribution_description: English: Laplacian map for the parent phosphasilene by AIM analysis with critical points
context_page_description: Phosphasilenes or silylidenephosphanes are a class of compounds with silicon-phosphorus double bonds. Since the electronegativity of phosphorus is higher than that of silicon, the "Si=P" moiety of phosphasilene is polarized. The degree of polarization can be tuned by altering the coordination numbers of the Si and P centers, or by modifying the electronic properties of the substituents. The phosphasilene Si=P double bond is highly reactive, yet with the choice of proper substituents, it can be stabilized via donor-acceptor interaction or by steric congestion.
  The landmark discovery of the first phosphasilene by NMR spectroscopy was made in 1984 by Bickelhaupt et al. The first phosphasilene came with bulky aryl substituents at the phosphorus and silicon atoms. Almost a decade after this spectroscopic observation, the first structural characterization of phosphasilene was achieved in 1993 by Niecke et al. The successful isolation of phosphasilenes with silicon-phosphorus double bonds represents one of the discoveries that challenged and disproved the "double-bond rule".
context_section_description: The unstable parent phosphasilene H₂Si=PH has been generated in the gas phase by the reacting atomic silicon with phosphine PH₃, and identified via matrix isolation spectroscopy methods. Density functional theory (DFT) calculations suggests that in the ground state, H₂Si=PH exists in a singlet spin state, with Cₛ-symmetric planar geometry. At the B3LYP/6-311+G** level of theory, the Si=P bond length and the Si-P-H bond angle are calculated to be 2.084 Å and 90.7ᵒ. The Si=P bond dissociation energy is 75.0 kcal mol⁻¹ at the B97-D/6-31G(d) level of theory; while the π-bond energy, Dπ(Si=P) is 36.6/35.9 kcal mol⁻¹. The frontier orbitals of the parent phosphasilene consists of the π bonding and π anti-bonding orbitals: π(Si=P) and π*(Si=P) correspond to the HOMO and LUMO, respectively. HOMO-1 was calculated to be the lone pair on phosphorus n(P).
caption: Laplacian map of the parent phosphasilene H2Si=PH in the plane of the π bond. Selected critical points and bond paths are shown. The structure of H2Si=PH was optimized at the B97-D/6-311+G** level of theory.[13] Multiwfn was used to carry out the AIM analysis.
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