THE ELECTRONIC-STRUCTURE OF BORABENZENE - COMBINATION OF AN AROMATIC PI-SEXTET AND A REACTIVE SIGMA-FRAMEWORK

The electronic structure of borabenzene (C5H5B, known also as borinane , borinine, borine) is studied using modern valence bond theory in its spin-coupled (SC) form. Three different types of SC wave functions-wi th six active pi orbitals and with four and eight active sigma orbital s-are used to describe the pi system of the molecule and the sigma-bon d framework around the boron atom. It is demonstrated that the SC pict ure of the pi space in borabenzene is very similar to that in benzene: The spins of six distorted nonorthogonal 2p(pi) orbitals are combined in a spin-coupling pattern involving two dominating Kekule-type and t hree less important Dewar-type Rumer spin functions. This indicates th at it is appropriate to consider the pi-electron sextet in borabenzene as aromatic and that the reason for the reactivity of this molecule s hould lie with its sigma framework. The two SC models of the sigma bon ding around B show that the boron-carbon sigma bonds in borabenzene in volve orbitals are ''bent'' to the outer side of the six-membered ring . This creates an orbital ''hole'' at the boron, which should represen t the preferred attachment site for Lewis acids. (C) 1997 John Wiley & Sons, Inc.