Calculation of the electronic structure of boranes by the self-consistent molecular orbital method. Part II. Highly symmetrical cage anions

Abstract

The electronic structures of the cage borane anions B₆H₆ ^2–, B₆Cl₆ ^2–, B₁₀H₁₀ ^2–, B₁₂H₁₂ ^2–, and B₁₂Cl₁₂ ^2– and the species B₂₀H₂₀ are studied by the self-consistent molecular orbital method. All valence electrons are included, as also are the terminal hydrogen atoms attached to the borons. The closed-shell ground state of the first five systems is confirmed but the symmetries of the highest bonding levels are not as previously calculated: the discrepancy stems from our inclusion of the terminal hydrogen atoms. Some of the eigenvalues are found to be displaced upwards in energy by the crystal field exerted by the anionic charge. In B₆H₆ ^2– and B₁₂H₁₂ ^2– the excess of charge is distributed between boron and hydrogen with a preponderance towards the boron atom: this situation is changed by substitution of chlorine for hydrogen and in B₆Cl₆ ^2– and B₁₂Cl₁₂ ^2– the chlorine has the greater formal negative charge. Electron-density contours, plotted across planes of symmetry in the hydrogen-substituted ions, show that, for B₆H₆ ^2–, the charge is concentrated both at the centre of the cage and in the centres of the trigonal faces. For each of the two larger boron cages the electron density forms a symmetrical spherical shell inside the cage, the density decreasing towards the centre. The electronic structure of the B₂₀H₂₀ species predicts that the closed-shell configuration is that of a tetranegative ion. More extensive work, however, suggests that the closed-shell structure does not have the lowest energy and that there are several highly spin-degenerate excited states which are more stable.