Valence bonds in elongated boron clusters

A well-defined class of planar or quasi-planar elongated boron clusters, of type B7+3nq-, serves as a basis to identify the valence bond picture of delocalized boron networks. The origin of the series is the B7- cluster, which exhibits sigma-aromaticity. The cluster generating step is the repetitive expansion by three boron atoms in the direction of elongation. Specific electron counting rules are obtained for -bonding, peripheral sigma-bonding and multicenter inner sigma-bonding. A valence bond structure is introduced which explains the remarkable regularity in the bonding pattern. The analysis supports 4c-2e bonds as an alternative to the common 3c-2e bonds. The results are validated by symmetry induction and ab initio calculations.