The central atom in an umbrella-shaped boron cluster can bind like a transition metal in two different ways, new research shows. It increases boron ‘s tendency to form unusual bonds, including other recent studies showing that it can combine with sodium and neon.

An image with molecular orbitals

The partially filled d-shell of transition metal atoms means that they can bind to CO ligands, which is probably one of their most formative binding behaviors. Due to its partially filled p orbitals, boron has similar metallomimetic properties: A monovalent boron in a borylene dicarbonyl complex can coordinate two CO ligands via donor-acceptor bonds. Another important behavior of transition metals, however, is the formation of sandwich complexes with aromatic arene ligands. A collaboration between researchers from Shanxi University, China, and Brown University, USA, has now discovered that the central boron atom in a boron cluster boronyl complex can exhibit these two transition metal bonding behaviors.

The B9ÖClusters were first synthesized by laser evaporation of a bismuth-based target with residual impurities as an oxygen source. The team used photoelectron spectroscopy to study the boron cluster and found two isomers that contributed to the spectra. Fascinated by these results, they examined the global minimum and discovered that the most stable isomer is an unusual umbrella-like structure. In this structure, a central boron atom forms a σ-bond with the boronyl ligand, but also shows an unexpected bond to an aromatic h7th-B7th Ligand. To further substantiate the results, the team simulated spectra for the isomers that showed good agreement with their experimentally observed results.

While planar aromatic boron clusters can form half-sandwich complexes with transition metals, the bond to a boron atom is unprecedented. Here the central boron atom forms a doubly aromatic B7th3- by sharing its valence electrons with the aromatic ligand, making it unexpectedly stable.

Julia Stauber, an inorganic chemist at the University of California, Los Angeles, USA, comments that “the unusual structural and electronic properties of the” [(h7-B7)-B-BO] Anions are sure to provide insight into new and exciting classes of metallomimetic boron clusters that have the potential to show reactivity and coordination properties that may only have been realized in the context of transition metal chemistry. ‘

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