Superatomic Shell Closure Defines the Stability of Octacoordinate M[NH3]8 Complexes Across Phases

Octacoordinate ammine complexes of electropositive metals (M[NH ₃ ] ₈ , M = Ca, Sr, Ba, Eu, Yb) maintain full ligand saturation and robust structural integrity across gas, liquid, and solid phases, challenging conventional coordination theory. Using a multiscale quantum chemical framework, we show that their stability originates from superatomic shell closure. In the gas phase, an 18-electron configuration (1S ² 1P ⁶ 1D ⁸ 2S ² ) formed by metal-ligand hybridized orbitals ensures closed-shell stability independent of d -orbital participation. Upon solvation, the system adapts to a 16-electron configuration (1S ² 1P ⁶ 1D ⁸ ), preserving the superatomic hierarchy through orbital reorganization. In the crystalline state, these orbitals propagate into delocalized bands, enabling a seamless transition from molecular to bulk coordination behavior. These findings define a phase-independent coordination paradigm based on electronic shell closure and expand the applicability of superatomic models to weak-field and non-classical complexes.