Vibrational spectra of cluster anions. 2. Vibrational spectra of compounds with the cluster anions [E4]4-: M4E4 (M = K, Rb, Cs; E = Ge, Sn) and β-Na4Sn4

Abstract

Vibrational spectra of the compounds M4E4 (M = K, Rb, Cs; E = Ge, Sn) and of β-Na4Sn4 with the cluster anions [E4]4− were analysed based on the point group equation image of isolated tetrahedranide units. The lower individual equation image symmetry of the anions in the real structure being more patterned and complex primarily affects the spectra of the tetrahedro-tetragermanides. ν3(F2) clearly splits both in Raman and IR and in the case of K4Sn4 only in IR. Rb4Sn4 and Cs4Sn4 exhibit very simple spectra with three bands in Raman and one band in IR. The breathing mode ν1(A1) for the quasi isolated [E4]4− cluster appears only in the Raman spectrum and is hardly influenced by the structural environment and by the nature of the alkali metal cations: ν1(A1) = 274 cm−1 ([Ge4]4−) and 183-187 cm−1 ([Sn4]4−), respectively. The calculated valence force constants fd(E–E) are: [Ge4]4−: fd = 0.89 Ncm−1 (K), 0.87 Ncm−1 (Rb), 0.86 Ncm−1 (Cs) and [Sn4]4−: 0.67 Ncm−1 (Na), 0.66 Ncm−1 (K), 0.67 Ncm−1 (Rb), 0.68 Ncm−1 (Cs). Both, the frequencies and the force constants fit well into the range previously reported.