Planar Tetracoordinate Hydrogen Stabilized Within Alkali Metal-Halogen Frameworks

A D_4h screening of the 25 H©M₄X₄ ^- combinations (M = Li-Cs; X = F-At), followed by unbiased global-minimum searches on the nine vibrationally stable candidates, reveals eight previously unreported planar tetracoordinate hydrogen (ptH) global minima adopting D_4h symmetry, bringing the total number of known D_4h ptH clusters in the alkali-metal-halogen family to nine. Born-Oppenheimer molecular dynamics confirms dynamic stability at 300 K across the entire series, with thermal isomerization observed at 600 K in three of the nine clusters. In all cases, the central hydrogen carries a substantial negative charge and is stabilized predominantly by electrostatic H^-···M⁺ interactions, while the peripheral M-X framework compensates the intrinsic metal-metal repulsion and preserves the planar arrangement. Although a small non-Coulombic contribution exists within the M-X framework, multicenter bonding plays no significant role in the stabilization of the central hydrogen. The magnetic response is dominated by localized diatropic vortices at the atomic cores, with no evidence of a global ring current, confirming the nonaromatic character of these assemblies. These results reframe ptH stabilization within a predominantly electrostatic framework and provide a predictive framework for further expansion of this emerging family.