Planar Tetracoordinate Hydrogen Stabilized Within Alkali Metal-Halogen Frameworks.

A D4h 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 D4h symmetry, bringing the total number of known D4h 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.