Abstract
Raman spectra of concentrated aqueous solutions of ferric chloride were recorded in the spectral range 50 to 500 cm<sup>−1</sup>. In near-saturated solutions, spectra show a four-line pattern typical of the tetrahedral FeCl<sub>4</sub>− complex: 390 cm<sup>−1</sup> (w); 335 cm<sup>−1</sup> (vs, p); 135 cm<sup>−1</sup> (w); 110 cm<sup>−1</sup> (s). At lower concentrations a new species dominates the spectrum, showing characteristic Raman lines at 318 cm<sup>−1</sup> (s, p) and 165 cm<sup>−1</sup> (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 FeCl<sub>3</sub> or lower. Of the several symmetries that a FeCl<sub>3</sub> complex might assume (C<sub>2v</sub>, C<sub>3v</sub>, D<sub>2h</sub> (dimer), and D<sub>3h</sub>), the D<sub>3h</sub> trigonal bipyramid structure FeCl<sub>3</sub>·2H<sub>2</sub>O with three equatorial chloride ligands and two axial waters is most compatible with the spectral evidence.
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