Three-dimensional aromaticity in deltahedral borane anions: Comparison of topological and computational approaches

Abstract

The molecular orbital energy parameters obtained from ab initio SCF MO Gaussian 82 computations on the series of deltahedral borane anions B nH n 2− ( n=5, 6, 7, 8, 9, 10, 12) correspond more closely to D n deltahedral core bonding topology than to K n complete core bonding topology. However, core-surface mixing, as noted previously for icosahedral B 12H 12 2−, raises the energies of all of the core molecular orbitals, except the lowest energy fully symmetric core orbital, to antibonding levels. For this reason the graph-theory derived model using K n complete core bonding topology gives the correct number of bonding and antibonding orbitals and hence the correct number of skeletal electrons for the deltahedral boranes. Extension of this topological analysis to self-consistent molecular orbital computations on the perchloroborane anions B n Cl n 2− ( n=6, 12) suggests that substitution of chlorine for hydrogen in the deltahedral boranes leads to more nearly deltahedral core bonding as well as a weakening of core bonding relative to surface bonding in accord with the expected electron withdrawing tendencies of chlorine relative to hydrogen. In addition, deltahedral borane anion computations using Gaussian orbitals rather than Slater orbitals lead to core bonding topology which is more nearly deltahedral rather than complete and weaker core bonding relative to surface bonding in accord with the more rapid decrease in the electron density of Gaussian orbitals at increasing distances relative to Slater orbitals.