Boron compounds have many diverse 2D and 3D structures. Anionic boron clusters have been shown to be mostly planar up to 27 boron atoms. Various transition metals can be bound by these boron rings, forming wheel-like structures. A record coordination number for planar compounds of 10 has been achieved in the complexes NbB₁₀^– and TaB₁₀⁻.

Alexander I. Boldyrev, Utah State University, Logan, UT, USA, Lai-Sheng Wang, Brown University, Providence, RI, USA, and colleagues have observed CoB₁₆^–, a drum-like structure with the highest coordination number observed so far for a metal atom. The researchers synthesized the complex by laser vaporization of a target composed of cobalt and boron. They characterized the compound using photoelectron spectra and quantummechanical calculations.

The spectra point to a high-symmetry structure, with two B₈ rings sandwiching the cobalt atom between them. According to computations, two structures of almost equal energy with D_8d and C_4v symmetry, respectively, should be the most favorable isomers. The researchers posit that the transition metal causes the boron structure to change from a 2D wheel to a 3D structure in this complex. The team hopes this discovery opens up new possibilities for designing boron-based nanomaterials.

• Cobalt-centred boron molecular drums with the highest coordination number in the CoB16− cluster,
  Ivan A. Popov, Tian Jian, Gary V. Lopez, Alexander I. Boldyrev, Lai-Sheng Wang,
  Nature Commun. 2015, 6, 8654.
  DOI: 10.1038/ncomms9654