The Hexamethylenetetramine Adduct [Mg 3 (H 2 O) 14 ][Fe(CN) 6 ] 2 (C 6 H 12 N 4 ) 4 (H 2 O) 4.18 , Its Potential as Testing Agent for Mg 2+ Ions, and the Formation of Ferromagnetic Fe 3 C

Isotypic compounds [Mg 3 (H 2 O) 14 ][M(CN) 6 ] 2 ·4HMT· x H 2 O (HMT = hexamethylenetetramine or urotropine) with M = Fe, Cr, and Co are crystallized from aqueous solutions. The iron compound (M = Fe) is most remarkable because it crystallizes with distinctive thin, deep‐yellow crystal plates. The crystal structure of [Mg 3 (H 2 O) 14 ][Fe(CN) 6 ] 2 (C 6 H 12 N 4 ) 4 (H 2 O) 4.18 is solved and refined by X‐Ray single‐crystal diffraction, revealing the tetragonal space group P 4 c 2, with a  = 14.8156(5) Å, and c  = 32.890(3) Å (Z = 4). The structure contains finite cyanido‐bridged linear sequences of the type [(H 2 O) 5 Mg–(Fe(CN) 6 )–(Mg(H 2 O) 4 )–(Fe(CN) 6 )–Mg(H 2 O) 5 ] being arranged parallel to the tetragonal axis with HMT and crystal water molecules in‐between these patterns. The straightforward synthesis and the characteristic appearance of crystals of this compound can make this reaction a useful test to proof the presence of Mg 2+ ions in aqueous solution. Thermoanalytical studies, designed to investigate the loss of water molecules and the decomposition pathway, reveal this compound as a useful precursor for the preparation of ferromagnetic cementite (Fe 3 C).