Infrared and Raman studies of the anion ordering transitions in paramagnetic organometallic radical cation salts [Cp2Mo(dmit)]X (XÂ =Â PF6, SbF6)

Temperature dependence of infrared and Raman spectra of the two isostructural salts [Cp2Mo(dmit)]PF6 and [Cp2Mo(dmit)]SbF6 is studied. At room temperature the physical properties of both compounds are very similar but at lower temperatures they undergo phase transitions associated with anion ordering, which are surprisingly different. The phase transitions in [Cp2Mo(dmit)]PF6 salt at T1 = 120 K and T2 = 89 K have no important influence on infrared and Raman spectra, while the phase transition in [Cp2Mo(dmit)]SbF6 salt at T1 = 175 K causes a splitting of Raman bands assigned to the CC stretching at about 1334 cm−1 and the in-plane Mo(dmit) ring deformation at about 353 cm−1, and also an infrared band at about 939 cm−1 related to the C-S stretching. The splitting of vibrational bands demonstrates a clear distortion of [Cp2Mo(dmit)]+ cations in the [Cp2Mo(dmit)]SbF6 salt. This molecular distortion is related to a lattice distortion providing thus a good argument for applicability of the compressible model of the anion ordering transition.