Vibrational Spectra of Cluster Anions. 2. Vibrational Spectra of Compounds with the Cluster Anions [E 4 ] 4− : M 4 E 4 (M = K, Rb, Cs; E = Ge, Sn) and β‐Na 4 Sn 4

Vibrational spectra of the compounds M 4 E 4 (M = K, Rb, Cs; E = Ge, Sn) and of β‐Na 4 Sn 4 with the cluster anions [E 4 ] 4− were analysed based on the point group ${\bar 4}{\rm 3}m-T_{d}$ of isolated tetrahedranide units. The lower individual ${\bar 4}{\rm 2}m-D_{2d}$ symmetry of the anions in the real structure being more patterned and complex primarily affects the spectra of the tetrahedro ‐tetragermanides. ν 3 (F 2 ) clearly splits both in Raman and IR and in the case of K 4 Sn 4 only in IR. Rb 4 Sn 4 and Cs 4 Sn 4 exhibit very simple spectra with three bands in Raman and one band in IR. The breathing mode ν 1 (A 1 ) for the quasi isolated [E 4 ] 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: ν 1 (A 1 ) = 274 cm −1 ([Ge 4 ] 4− ) and 183‐187 cm −1 ([Sn 4 ] 4− ), respectively. The calculated valence force constants f d (E–E) are: [Ge 4 ] 4− : f d = 0.89 Ncm −1 ( K ), 0.87 Ncm −1 ( Rb ), 0.86 Ncm −1 ( Cs ) and [Sn 4 ] 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.