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

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 FeCl 4 – 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 FeCl 3 or lower. Of the several symmetries that a FeCl 3 complex might assume (C 2v , C 3v , D 2h (dimer), and D 3h ), the D 3h trigonal bipyramid structure FeCl 3 ·2H 2 O with three equatorial chloride ligands and two axial waters is most compatible with the spectral evidence.