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

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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 (C2v, C3v, D2h (dimer), and D3h), the D3h trigonal bipyramid structure FeCl3·2H2O with three equatorial chloride ligands and two axial waters is most compatible with the spectral evidence.