Water Encapsulated [(Fe4Se4)Se4]6– Clusters in [Na6(H2O)18][Fe4Se8]

[Na₆(H₂O)₁₈][Fe₄Se₈] was synthesized using hydrothermal methods and characterized by single-crystal X-ray diffraction, ⁵⁷Fe Mössbauer spectroscopy, magnetization, and muon spin resonance (μSR) measurements. The cubic crystal structure (space group I23, a = 11.785 Å, Z = 2) contains heterocubane-type ${\left[{Fe}_4{Se}_4\right]}^{2+}$ clusters with T_d symmetry. Four additional selenium atoms complete the tetrahedral coordination around the iron atoms, forming ${\left[\left({Fe}_4{Se}_4\right){Se}_4\right]}^{6-}$ units. Notably, the selenide ligands do not interact with the sodium cations, as the Fe₄Se₈^6- cluster in [Na₆(H₂O)₁₈][Fe₄Se₈] is exclusively surrounded by water molecules via weak Se···H–O hydrogen bonds. This is particularly remarkable given that [Fe₄Q₄] clusters are typically unstable in water. Quantum chemical calculations (DFT) confirm the crystal structure and improve the positions of the hydrogen atoms. ⁵⁷Fe Mössbauer spectroscopy reveals that the spin ordering of the mixed-valence [Fe₄^2.5+Se₄]²⁺ cluster core mirrors the antiparallel spin arrangement found in analogous iron–sulfur clusters, such as ${\left[{Fe}_4{S}_4\right]}^{2+}$, in protein-bound systems. An increase of the quadrupole splitting at low temperatures indicates changes of the orbital occupations. μSR Experiments show a reduction of the spin fluctuation frequency until the system becomes static at low temperatures, but no evidence of a S = 0 state above 2 K.