Single-crystal Raman spectrum of bismuth pentafluoride and of antimony tetrachloride fluoride and the vibrational spectrum of vanadium, niobium, tantalum, and antimony pentafluorides
Single-crystal Raman polarisation data on α-BiF5 enable an unambiguous assignment to be made of the Raman-active modes of this polymer. The corresponding i.r. data define a Bi–F–Bi bridge stretching mode at 450 cm.–1, suggesting a force constant equal to one quarter of the terminal force-constant. Single-crystal Raman studies and normal-co-ordinate calculations (based on this bridge force-constant) are used to assign the vibrational spectrum of SbCl4F. The vibrational spectra of solid VF5, of NbF5 and TaF5 in the solid and liquid states, and of liquid SbF5 are examined to study the position of the ‘fluorine-bridging’ modes and to investigate the stereochemistry of the liquid states. An interpretation in terms of cis-fluorine-bridged polymers is satisfactory for NbF5 and SbF5. In the case of SbF5 the Raman spectrum of the melt is the same at 20 and 125°. Normal-co-ordinate calculations are used to assign the vibrational spectrum of solid NbF5.