Ab initio mo study of P-Si π systems: Structures and energies

The origin of the relative stability of isomers of 1,2-diphosphinodisilene and related compounds is discussed. The geometries of 18 isomers of (H₂X)HE=EH(XH₂), (H₂X)HE=EH(XH₂)²⁺, and HX=EH-HE=XH (E = C, Si; X = P, N) have been optimized at the MP2/6-311++G(d, p) level of theory. Since nitrogen and phosphorus have lone-pair electrons, six electrons can delocalize in four orbitals of (H₂X)HE=EH(XH₂) (6e/4o), while four electrons can delocalize in four orbitals of (H₂X)HE=EH(XH₂)²⁺ and HX=EH-HE=XH (4e/4o). To examine the interaction that governs the structure and relative stability of their cis and trans (or cisoid and transoid) isomers, the π-σ* delocalization energies and isodesmic reaction energies of the compounds were calculated. It is concluded that the π-σ* delocalization affects the bending structure of disilene derivatives, and phosphorus substituents prefer 2e/4o and 4e/4o systems.