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

Abstract

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 cæsium cation, two different species may be formed, namely an α-form containing the µ-trichloro-anion Fe₂Cl₉^3–, 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.