Magnetochemistry of the tetrahaloferrate(III) ions. 2. Crystal structure and magnetic ordering in (4-Br(py)H) sub 3 Fe sub 2 Cl sub 1. 3 Br sub 7. 7 and (4-Cl(py)H) sub 3 Fe sub 2 Br sub 9. The superexchange paths in the A sub 3 Fe sub 2 X sub 9 salts

A series of neutron diffraction investigations at 25 K are reported on single crystals of stoichiometry (4-chloropyridinium){sub 3}Fe{sub 2}Br{sub 9} and on (4-bromopyridinium){sub 3}Fe{sub 2}Cl{sub 1.3}Br{sub 7.7}. The compound of stoichiometry (4-bromopyridinium){sub 3}Fe{sub 2}Cl{sub 1.3}Br{sub 7.7} is found to belong to the space group P2{sub 1}/n with four formula units in the unit cell, and the related compound (4-chloropyridinium){sub 3}Fe{sub 2}Bn{sub 9} was found to belong to the same space groups. Structural analysis shows that the iron is present as the (FeX{sub 4}){sup {minus}} ion (X = Cl, Br). The results of the determination of the crystal structure of the compounds are reported. Both materials are isomorphous with the previously reported bis(4-chloropyridinium tetrachloroferrate(III))-4-chloropyridinium chloride. Magnetic measurements on single crystals show that the S = 5/2 material (4-bromopyridinium){sub 3}Fe{sub 2}Cl{sub 1.3}Br{sub 7.7} orders as a canted antiferromagnet at 5.67 K, while (4-chloropyridinium){sub 3}Fe{sub 2}Br{sub 9} orders in a similar fashion at 7.96 K. Magnetic susceptibility data are compared with the theoretical predictions for the simple cubic (sc) Heisenberg high-temperature series expansion model, and superexchange pathways for the isostructural series of monoclinic A{sub 3}Fe{sub 2}X{sub 9} salts are examined. 19 refs., 6 figs., 6 tabs.