Importance of π–σ* Orbital Mixing in Bonding and Structure of Bicyclo[1.1.0]tetrasilane and Related Compounds

Abstract

The structural diversity of bicyclo[1.1.0]­tetrasilane has been analyzed using perturbation MO theory and DFT calculations. Among its four planar-<i>cis</i> (<i>C</i>_2<i>v</i>), planar-<i>trans</i> (<i>C</i>_2<i>h</i>), long-bond (<i>C</i>_2<i>v</i>), and short-bond isomers (<i>C</i>_2<i>v</i>) found theoretically, the first two are derived as the results of second-order Jahn–Teller distortion of the <i>D</i>_2<i>h</i> isomer associated with the π–σ* orbital mixing. The last two are formed through orbital mixing in an isomer with a folded ring. The orbital mixing of the two σ*­(Si–H) orbitals at the 1- and 3-silicon atoms into a π-type orbital between the atoms plays a crucial role in stabilizing the system by causing pyramidalization at the silicon atoms and by introducing the inverted σ-bond nature between the atoms. The π–σ* orbital mixing model is applied to predicting the structure of related compounds