THE HEXACHLOROFERRATE(III) ANION STABILIZED IN HYDROGEN-BONDED PACKING ARRANGEMENTS - A COMPARISON OF THE X-RAY CRYSTAL-STRUCTURES AND LOW-TEMPERATURE MAGNETISM OF S(METHYLAMMONIUM)HEXACHLOROFERRATE(III)CHLORIDE(I) AND RRATE(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 wi th the In(III) compound of similar formula, while II, molecular formul a C24H72Cl14Fe2N8, has an unusual symmetric structure containing the t hree anions [FeCl6](3-), [FeCl4](-) and Cl-. I crystallizes in the spa ce group P2/n, Z = 4 with a = 15.998(3), b = 7.292(1), c = 16.056(3) A ngstrom and beta = 103.57(2)degrees; for II: I4(1)/a, Z = 4 with a = 1 9.216(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 st ate, which is not magnetically saturated at 1.33 K, while for II the r esults suggest likely 3-D ordering only in the range 1.2 K > T-critica l > 1.0 K. The low ordering temperatures are consistent with the relat ively long Fe ... Fe distances in these compounds.