The Occurrence of π Molecular Orbitals in Planar Boron Clusters

The energy of the lowest pi molecular orbital in a planar boron cluster can be estimated from the connectivity of constituent boron atoms, or the mean valency of the boron atoms. Low-energy pi molecular orbitals were then predicted to occur in all realistic planar polycyclic boron clusters. In fact, all such boron clusters studied have one or more pi molecular orbitals with two or more pi electrons. They are aromatic with positive topological resonance energies. pi Conjugation and aromaticity must be totally or partially responsible for the planarity of low-energy boron clusters.