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.