Water Encapsulated\n[(Fe<sub>4</sub>Se<sub>4</sub>)Se<sub>4</sub>]<sup>6–</sup> Clusters in [Na<sub>6</sub>(H<sub>2</sub>O)<sub>18</sub>][Fe<sub>4</sub>Se

[Na<sub>6</sub>(H<sub>2</sub>O)<sub>18</sub>][Fe<sub>4</sub>Se<sub>8</sub>] was synthesized using hydrothermal methods\nand characterized\nby single-crystal X-ray diffraction, <sup>57</sup>Fe Mössbauer\nspectroscopy, magnetization, and muon spin resonance (μSR) measurements.\nThe cubic crystal structure (space group <i>I</i>23, <i>a</i> = 11.785 Å, <i>Z</i> = 2) contains heterocubane-type [Fe4Se4]2+ clusters with <i>T</i><sub>d</sub> symmetry. Four additional selenium atoms complete the tetrahedral\ncoordination around the iron atoms, forming [(Fe4Se4)Se4]6− units. Notably, the selenide ligands do\nnot interact with the sodium cations, as the Fe<sub>4</sub>Se<sub>8</sub><sup>6‑</sup> cluster\nin [Na<sub>6</sub>(H<sub>2</sub>O)<sub>18</sub>][Fe<sub>4</sub>Se<sub>8</sub>] is exclusively surrounded by water molecules via weak Se···H–O\nhydrogen bonds. This is particularly remarkable given that [Fe<sub>4</sub>Q<sub>4</sub>] clusters are typically unstable in water. Quantum\nchemical calculations (DFT) confirm the crystal structure and improve\nthe positions of the hydrogen atoms. <sup>57</sup>Fe Mössbauer\nspectroscopy reveals that the spin ordering of the mixed-valence [Fe<sub>4</sub><sup>2.5+</sup>Se<sub>4</sub>]<sup>2+</sup> cluster core mirrors the antiparallel spin arrangement\nfound in analogous iron–sulfur clusters, such as [Fe4S4]2+, in protein-bound systems. An increase\nof the quadrupole splitting at low temperatures indicates changes\nof the orbital occupations. μSR Experiments show a reduction\nof the spin fluctuation frequency until the system becomes static\nat low temperatures, but no evidence of a <i>S</i> = 0 state\nabove 2 K.