The vibrational spectra of crystals of K2TaF7

The [TaF7,]2−on exhibits no symmetry in the monoclinic K2TaF7, crystal but it nevertheless closely resembles the geometry of a monocapped trigonal prism of C2v− symmetry. The vibrational spectra of K2TaF7 at ambient temperatures are not in agreement with a low symmetry for the anions in the crystals and resemble the spectra of seven-coordinated species which are known to have a pentagonal bipyra = nridal structure of D5h-symmetry. It is shown that non-rigid behaviour of the [TaF7]2−-groups gives rise to this discrepancy. Below 1̃50 K a large number of additional bands appear in the spectra and it can be concluded that when the crystal becomes rigid its equilibrium structure is reflected in the vibrational spectra. Neglect of the effects of non-rigid behaviour can therefore lead to serious misconceptions as far as the interpretation of the vibrational spectra of hepta-coordinate salts is concerned.