Hepta- and octacoordinate boron in molecular wheels of eight- and nine-atom boron clusters: Observation and confirmation

Two types of aromaticity: Experimental and theoretical evidence shows that eight- and nine-atom boron clusters are perfectly planar molecular wheels, with a hepta- or octacoordinated central boron atom, respectively (see picture, B green). The radii of the miniature molecular wheels are found to be 1.8 and 2.0 Å. Analyses of their chemical bonding reveal that they possess double (σ and π) aromaticity, which is responsible for the wheel structures and the extreme coordination environments.