Question

Why is characterization of complexes trans-CrCl₂(OH₂)₄⁺, CrCl(OH₂)₅²⁺, and Cr(OH₂)₆³⁺ following cation exchange chromatography done by visible spectroscopy rather than IR spectroscopy?

Answer 1

Oxidation state of chromoim in all three complexes trans-[CrCl2(OH2)]4+, [CrCl(OH2)5]2+, and [Cr(OH2)6]3+  is +3 which d3 system. These complexes show colour due to d-d transitions in visible region (wavelengths 350 nm and 750 nm) of spectrum. The characteristic colors of chromium(III) solutions, e.g., the violet of the hexaaquo ion [Cr(OH2)6]3+, the light green of the chloropentaaquo ion [CrCl(OH2)5]2+ and the dark green of the dichlorotetraaquo ion trans-[CrCl2(OH2)]4+ .This why UV-visible spectroscopy is best suited for their analysis. While there are no characterstic distinct bonding features in these complexes to distinguish by IR specroscopy.