Trans effect in halobismuthates and haloantimonates revisited. Molecular structures and vibrations from theoretical calculations.

Ab initio and density functional theory computations have been carried out to calculate the structures and vibrational spectra of halobismuthates and haloantimonates of formulas MX6(3-), M2X10(4-), and M2X9(3-) for M = Bi or Sb and X = I, Br, or Cl. The results have been compared to experimental crystal structures and the infrared and Raman spectra of these species as well as the (MX5(2-))n and (MX4(1-))n anions. Even though the calculations neglect the effect of which cation is present, they do a good job in verifying the observed trends in bond distances and bond stretching vibrational frequencies. External bonds across from bridging bonds are the shortest and have the highest stretching frequencies for all of the ions investigated. This supports the previously postulated "trans effect". Since the calculations were carried out for individual noninteracting anions, the computed results can be expected to best represent the idealized species unperturbed by the effect of the cations present. The trans effect results in shortening of the M-X bonds by 0.08-0.13 Å. It also leads to frequency increases of about 20% for the M-X stretching vibrations.