Abstract

Raman spectra of concentrated aqueous solutions of ferric chloride were recorded in the spectral range 50 to 500 cm⁻¹. In near-saturated solutions, spectra show a four-line pattern typical of the tetrahedral FeCl₄− complex: 390 cm⁻¹ (w); 335 cm⁻¹ (vs, p); 135 cm⁻¹ (w); 110 cm⁻¹ (s). At lower concentrations a new species dominates the spectrum, showing characteristic Raman lines at 318 cm⁻¹ (s, p) and 165 cm⁻¹ (vw). Conventional investigations of spectral changes with concentration of Fe(III) and chloride, coupled with group theoretical arguments, indicate that the stoichiometry of the complex must be FeCl₃ or lower. Of the several symmetries that a FeCl₃ complex might assume (C_2v, C_3v, D_2h (dimer), and D_3h), the D_3h trigonal bipyramid structure FeCl₃·2H₂O with three equatorial chloride ligands and two axial waters is most compatible with the spectral evidence.

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