Bare and Ligand-Stabilized Planar Hexacoordinate Boron\n(phB) in ABeC<sub>2</sub>B<sub>4</sub> (A = P, As, Sb, and Bi) Clusters

Planar\nhexacoordination is an extremely uncommon phenomenon for\nthe atoms that belong to the main group. Within this article, we have\nanalyzed the potential energy surfaces (PES) of ABeC<sub>2</sub>B<sub>4</sub> (A = N, P, As, Sb, and Bi) clusters in neutral, monocationic,\nmonoanionic, dicationic, and dianionic states using density functional\ntheory (DFT). Among which PBeC<sub>2</sub>B<sub>4</sub>, PBeC<sub>2</sub>B<sub>4</sub><sup>–</sup>, AsBeC<sub>2</sub>B<sub>4</sub><sup>–</sup>, AsBeC<sub>2</sub>B<sub>4</sub><sup>2–</sup>, SbBeC<sub>2</sub>B<sub>4</sub><sup>–</sup>, and BiBeC<sub>2</sub>B<sub>4</sub><sup>–</sup> clusters contain a planar\nhexacoordinate boron (phB) atom in the global minimum energy structures\nwith <i>C</i><sub><i>s</i></sub> symmetry. The\nglobal minima of the remaining clusters do not correspond to a phB\natom. According to the results of the natural charge computations,\na significant amount of negative charge is accumulated on the phB\natom for each global minimum. Based on the values of the nucleus independent\nchemical shift (NICS), the phB structures are predicted to possess\nσ/π-dual aromaticity. The most intriguing aspect is that\nthe planarity of the phB core is preserved in the complexes that are\ncoupled to the N-heterocyclic carbene (NHC) ligand. The stability\nof these complexes has been depicted here for the first time. As a\nresult, it is our hope that both bare and ligand-stabilized clusters\nare viable options for gas-phase observation and large-scale synthesis,\nrespectively.