Magic electron counts and bonding in tubular boranes

Abstract

Ring stacking in some closo-borane dianions and the hypothetical capped borane nanotubes, predicted to be stable earlier, is analyzed in a perturbation theoretic way. A "staggered" building up of rings to form nanotubes is explored for four- and five-membered B_nH_n rings. Arguments are given for the stacking of B₅H₅ rings being energetically more favorable than the stacking of B₄H₄ rings. Elongated B-B distances in the central rings are predicted for some nanotubes, and the necessity to optimize ring-cap bonding is found to be responsible for this elongation. This effect reaches a maximum in B₁₇H₁₇^2-; the insertion of additional rings will reduce this elongation. These closo-borane nanotubes obey Wade's n + 1 rule, but the traditional explanation based on a partitioning into radial/tangential molecular orbitals is wanting.