AB Initio MO Study of P--Si π Systems: Structures and Energies

Abstract

The origin of the relative stability of isomers of 1,2-diphosphinodisilene and related compounds is discussed. The geometries of 18 isomers of (H 2 X)HE=EH(XH 2 ), (H 2 X)HE=EH(XH 2 ) 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 (H 2 X)HE=EH(XH 2 ) (6e/4o), while four electrons can delocalize in four orbitals of (H 2 X)HE=EH(XH 2 ) 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 ~ - * delocalization energies and isodesmic reaction energies of the compounds were calculated. It is concluded that the ~ - * delocalization affects the bending structure of disilene derivatives, and phosphorus substituents prefer 2e/4o and 4e/4o systems.