The hexachloroferrate(III) anion stabilized in hydrogen bonded packing arrangements. A comparison of the X-ray crystal structures and low temperature magnetism of tetrakis(methylammonium) hexachloroferrate(III) chloride (I) and tetrakis(hexamethylenediammonium) hexachloroferrate(III) tetrachloroferrate(III) tetrachloride (II)

Single crystal X-ray diffraction studies of the title compounds, both having a nominal FeCl7 stoichiometry, indicate that I, with molecular formula C4H24Cl7FeN4, contains [FeCl6](3-) units and is isomorphous with the In(III) compound of similar formula, while II, molecular formula C24H72Cl14Fe2N8, has an unusual symmetric structure containing the three anions [FeCl6](3-), [FeCl4](-) and Cl-. I crystallizes in the space group P2/n, Z = 4 with a = 15.998(3), b = 7.292(1), c = 16.056(3) Angstrom and beta = 103.57(2)degrees; for II: I4(1)/a, Z = 4 with a = 19.216(4) and c = 14.458(4) Angstrom. A combination of Fe-57 Mossbauer and a.c. susceptometry confirm long range magnetic order with 1.39 K <T-critical <1.60 K in I leading to a 3-D antiferromagnetic ground state, which is not magnetically saturated at 1.33 K, while for II the results suggest likely 3-D ordering only in the range 1.2 K > T-critical > 1.0 K. The low ordering temperatures are consistent with the relatively long Fe ... Fe distances in these compounds.