Structure and Mössbauer Study of the First Mixed‐Valence Iron Diphosphonate: $^{2}_{\infty}[\rm \{Fe^{II/III}(H_{2}O)_{2}\}\{Fe^{III/II}Cl\}_{2}(\mu_{4}-O_{3}P-C(OH)\{(CH_{2})_{4}NH_{3}\}-PO_{3})_{2}]\cdot4H_{2}O$

The title compound bis(μ 4 ‐5‐ammonio‐1‐hydroxypentyl‐idene‐1,1‐bisphosphonato‐κ 7 O , O ′, O ″, O m , O ′ v , O v , O v ′, O v ″)‐(diaquairon)‐di(chloroiron)(II/III)‐tetrahydrate was prepared by hydrothermal synthesis and consists of nets of corner sharing {FeO 6 } or {FeClO 5 } octahedra linked by {CPO 3 } tetrahedra. Neighboring layers interdigitate with their ammoniopentylidene tails and connect to a three‐dimensional network by N‐H···O and O(hydrate)‐H···O hydrogen bonds. The ligand [PO 3 ‐C(OH){(CH 2 ) 4 + NH 3 }‐PO 3 ] 3− is a zwitter‐ion of overall charge –3. Different from other known iron diphosphonates, which contain only either Fe II or Fe III , the iron bisphosphonate here is found from Mössbauer‐Spectroscopy to be a trapped mixed‐valence system with Fe II and Fe III disordered over both crystallographic sites Fe1 and Fe2 in what could be described as a coherent superposition of different states. At 4.2 K the Fe III is completely magnetically ordered.