Single-crystal Raman spectroscopy of the α alums CsM(SO4)2· 12H2O (M = Co or Ir) between 275 and 1200 cm–1

Abstract

Low-temperature polarised single-crystal Raman spectra of CsM(SO₄)₂·12H₂O (M = Co or Ir) have been recorded between 275 and 1200 cm^–1. A full assignment of the spectra has been completed, including all the Raman-active internal modes of the tervalent hexaaqua cation [ν₁(MO₆) 548 (Co) and 554 (Ir), ν₂(MO₆) 509 (Co) and 540 (Ir) and ν₅(MO₆) 332 av. (Co) and 314 av. cm^–1(Ir)]. The relationship between the wavenumber of the ν₁(MO₆) mode and the metal–oxygen bond length is significantly different for the tervalent cations of the second- and third-row transition metals than for those of first-row transition metals and Group 13 metals. Solution Raman spectra of [Ir(OH₂)₆]³⁺ have enabled determination of ν₁(MO₆)_aq(536 cm^–1); the relationship between this value and that obtained at low temperature in the solid state closely follows that found previously for the chromium(III) and rhodium(III) aqua ions. The external modes of water co-ordinated to the tervalent cation are highly sensitive to the alum type, the metal–ligand bond length, and the stereochemistry of water co-ordination and provide additional insight into the metal–water interaction. This, in turn, casts light on the assignment of the external modes to particular molecular motions.