π orbitals are calculated to be 87.53% and 12.47%, respectively. Structure and bonding The parent phosphasilene H2Si=PH 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). "Half-Parent" Phosphasilene R2Si=PH Driess and coworkers prepared thermally stable "half"-parent phosphasilene R2Si=PH (R2Si = (tBu3Si)(iPr3C6H2)Si), which is the first example of phosphasilene with a terminal PH group. This species was obtained as a mixture of E/Z isomers, thus its 31P NMR spectrum featured two doublets with 29Si satellites (δ=123.1, 1J (P, H)=123 Hz, 1J (P, Si)=157 Hz and δ=134.2 ppm, 1J (P, H)=131 Hz, 1J (P, Si)=130 Hz). These 1J (P, H) coupling constants are much smaller compared to those of secondary phosphanes (R2PH) and phosphaalkenes with a PH group, which indicates that the phosphasilene phosphorus atom possesses more 3p character. X-ray crystallography of this "half"-parent phosphasilene species shows that the silicon atom occupies a trigonal-planar coordination environment. The Si=P bond distance was reported to be 2.094(5) Å, which is about 7% shorter than a typical silicon-phosphorus single bond, but only slightly longer than that of P-silyl-substituted phosphasilenes, which suggests that the potential of Si=P bond on a potential energy surface is relatively shallow. π-conjugated phosphasilenes Tamao et al. reported a series of π-conjugated phosphasilenes stabilized by Eind groups (Eind=1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl). These systems feature Si=P units that are highly coplanar with the aromatic ring, allowing strong π → π* absorptions. The coplanarity is made possible by the rigidity of the two Eind groups that are oriented trans and perpendicular with respect to the Si=P bond. The Si=P bond length observed by X-ray crystallography are ca. 2.09-2.10 Å, which are typical for phosphasilenes. The bonding of π-conjugated phosphasilenes has been probed by