Bonding in B6H 2–6 and other closo-boranes

Abstract

The HOMO of B₆H^2–₆ has a positive orbital eigenvalue in restricted SCF calculations which persists as the basis is enlarged to 6–31G^*. This is interpreted as indicating instability of the free anion to electron loss, confirmed by calculations on B₆H^2–₆, B₆H^–₆ and B₆H₆, which find B₆H^–₆ more stable than B₆H^2–₆ by 0.01 E_h at the B₆H^2–₆ experimental geometry.

Ab initio calculations, including a point-charge model of the electrostatic environment of B₆H^2–₆ in [(CH₃)₄N]₂(B₆H^2–₆), stabilize B₆H^2–₆ and give a bonding pattern consistent with the conventional picture of a closo cluster. In the simulated crystal the HOMO of B₆H^2–₆ is bonding with respect to the B 2p level by 199 kJ mol^–1.

In free B₁₂H^2–₁₂ the RHF 6–31G eigenvalues are all negative, but 15 electron pairs occupy orbitals that are less bound than the B 2p orbital. Crystal-field effects on these energies are discussed.