Low-Frequency Vibrational Spectra of Bromo- and Iodobismuthates and the Observation of a Trans Effect

The far-infrared and Raman spectra of 17 bismuth halide complexes of formulas BiX₆ ^3-, BiX₅ ^2-, Bi₂X₉ ^3-, and BiX₄ ^- have been recorded for X = Br or I. The spectra of these anions, which show bands only below 200 cm^-1, have been analyzed in terms of known structures. Several of the spectra have been used to confirm predicted structures for other molecules. Although the solid-state spectra are seen to vary somewhat with the cation present, they can be used to distinguish between the different types of complexes. These spectra also establish frequency ranges expected for Bi-Br and Bi-I stretches, for both the external and the bridging types. Approximate force constants were calculated for the various types of Bi-X stretches; from these it was determined that the nature of the bonding of the halide trans to a particular Bi-X bond significantly affects the strength of that bismuth-halogen bond. Moreover, the strength of the Bi-X bonds was found to be greatest in the anions in which the largest number of the other bonds were to bridged halogens.