Raman Spectral Investigation of the Complex Species of Ferric Chloride in Concentrated Aqueous Solution

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.