Low‐temperature polarized single‐crystal Raman spectra of the β‐alums CSM(SO4)2. 12H2O (M = Mo or Ru) between 275 and 1200 cm−1

Low‐temperature, polarized, single‐crystal Raman spectra of the β‐alums CsM(SO4)2. 12H2O (M = Mo or Ru) and their deuterated analogues were recorded between 275 and 1200 cm−1. The internal modes of sulphate and [M(OH2)6]3+ and the external modes of water coordinated to the mono‐ and tervalent cation were assigned. This allowed a comparison between the wavenumbers of the internal modes of second‐ and first‐row transition metal hexaaquacations. The relationship between v1(MO6) and the MIII—O bond length is similar for the first‐row transition and Group 13 metals, but is significantly different for the second‐row transition metals. This suggests a non‐ionic contribution to the metal‐water bonding in the latter case. The relative intensities of the v1(MO6) modes are highly dependent on the identity of the metal; however, the trend in this property is not simply related to either the bond strength, the polarizing power of the cation or the covalent interaction.