Planar Networks of Boron Triangles: Analogies to Benzene and Other Planar Aromatic Hydrocarbons

The lowest energy structures for B species (n = 6 to 23 except for 20) observed experimentally in the gas phase with a mass spectrometer are planar networks of boron triangles. Such networks are considered to consist of trigonal planar sp²-hybridized boron atoms having perpendicular p orbitals similar to the carbon atoms in benzene and other planar aromatic hydrocarbons. Electron bookkeeping for reasonable chemical bonding topologies of wheel-like structures such as B@B (n = 6-9) leads to two π-electrons for B₆ and B₇⁺ similar to the cyclopropenyl cation and six π-electrons for B₈^2- and B₉^- similar to benzene. Related chemical bonding topology analyses for low-energy oval B₁₀ and B₁₁^- structures as well as for larger planar boron triangle networks with 12, 13, and 14 boron atoms suggest six π-electrons in such structures. Planar networks of boron triangles having 16-19 boron atoms are shown to be systems with 10 π-electrons similar to naphthalene. Similarly, low-energy planar B₂₂ and B₂₃^- structures are shown to be 4 π-electron systems 1analogous to linear anthracene and angular phenanthrene, respectively. Intermediate B₁₅^- and B₂₁^- systems are shown to be systems with 4k rather than 4k + 2 π-electrons with 8 and 12 π-electrons, respectively. Structures based on planar networks of boron triangles are strongly energetically disfavored for B₂₀ relative to a nonplanar decagonal antiprism structure with ideal D₁₀ symmetry.