Low-frequency infrared spectra of iron(III) halide complexes

The infrared spectra of the complexes (pyH)3Fe2X9, (quH)FeX4, and Cs3Fe2Cl9(X = Cl or Br; pyH = pyridinium; quH = quinolinium) have been recorded in the 450–200 cm.–1 range. The frequencies of the iron–halogen stretching vibrations indicate that in the pyridinium and quinolinium salts the only iron-containing species are the tetrahedral anions FeX4–. With the caesium cation, two different species may be formed, namely an α-form containing the µ-trichloro-anion Fe2Cl93–, and a β-form containing essentially the tetrahedral anion FeCl4–. These conclusions are in agreement with the most recent magnetic studies on certain of the complexes. All attempts to prepare the reported complexes (pyH)5Fe2Cl11 and (quH)4Fe2Cl10 led only to the isolation of the complexes (pyH)3Fe2Cl9 and (quH)FeCl4.