Planar Heptacoordinate Halogens

Planar hypercoordinate halogens remain challenging because of the high electronegativity of halogens. Through a systematic exploration of 200 X©M7E7– combinations (X = F–At; M = Be–Ba, Zn–Hg; E = O–Po), we identify four global minima featuring planar heptacoordinate halogens: Br©Zn7O7–, I©Zn7O7–, At©Zn7O7–, and At©Cd7O7–. These systems have large HOMO–LUMO gaps, considerable singlet–triplet separations, and stability up to 1000 K in molecular dynamics simulations. Bonding analyses (WBI, AdNDP, EDA–NOCV, and IQA) indicate that the central halogen behaves as an anion confined within a covalently supported M7O7 ring, with interactions dominated by electrostatics and minor covalent contributions. Magnetic-response and EDDB analyses do not support aromatic stabilization. Stability instead arises from the combination of a rigid metal–oxide framework and electrostatic confinement. A size-dependent stabilization pattern emerges. Zn-based rings stabilize Br, I, and At, whereas the larger and softer Cd-based framework stabilize only At. Rather than introducing a distinct bonding mechanism, these systems extend the electrostatic confinement mechanism previously established at lower coordination numbers, revealing both its persistence and limitations at n = 7. These results show that planar halogen hypercoordination can be sustained up to n = 7 only when the cavity size and ring rigidity simultaneously satisfy a narrow electrostatic confinement window, thereby defining the stability limits of the oxide-ring families examined here.