Valence bonds in elongated boron clusters

A well‐defined class of planar or quasi‐planar elongated boron clusters, of type , serves as a basis to identify the valence bond picture of delocalized boron networks. The origin of the series is the cluster, which exhibits σ‐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 π‐bonding, peripheral σ‐bonding and multicenter inner σ‐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.