Realization of heptagonal pyramidal octacoordination in highly robust [OB–M©B7O7]− (M = Co, Rh, Ir) complexes via boron–oxygen synergy

Abstract

Non-spherical heptagonal pyramidal octacoordination has remained elusive due to extreme repulsion among equatorial ligands. Herein, we computationally demonstrate this unprecedented configuration in monoanionic [OB–M©B7O7]− (M = Co, Rh, Ir) complexes through synergistic integration of an equatorial heptadentate B7O7 ligand and an axial boronyl (–BO) group. Advanced quantum chemical analyses confirm these species as global minima with substantial thermodynamic stability (6.7–7.5 kcal mol−1 below isomers), dynamic integrity up to 1000 K, and exceptional electronic robustness (HOMO–LUMO gaps: 5.12–5.28 eV; VDEs: 3.68–3.79 eV). Critical stabilization arises from (i) B–O σ/π bonding networks compensating equatorial strain, (ii) σ+π+δ triple aromaticity (10σ + 6π + 2δ electrons satisfying Hückel’s 4n + 2 rule), and (iii) electrostatic-dominated ligand–metal interactions (56.4%–58.0% attraction per EDA–NOCV). The electron-compensation strategy—using oxygen atoms to electronically compensate electron-deficient boron centers while providing steric protection—enables condensed-phase viability. This work establishes boron–oxygen clusters as versatile platforms for non-classical coordination geometries.