A raman spectrophotometric study of the stability of aqueous coordinatively-saturated mixed bromo-iodo complexes of cadmium

The substitution equilibrium constantsfor the reactions CdI_3-nBr_n+1^2- + I^- ⇋CdBr_nI_4-n^2- + Br^- (n = 0, 1, 2 and 3) have been measured in aqueous media at a constant ionic strength (1·634 M) and varying bromide to iodide ratios. Quantitative Raman spectrophotometry was used, with dilute (0·167 M) cadmium solutions and an internal standard (LiClO₄). It is shown that the intensity of the Raman scattering of a species is determined by the peak height (rather than area), which is proportional to the concentration. The values of the mixing constants, for the reactions (( n 4)CdI₄^2- + (1 - n 4)CdBr₄^2- ⇋ CdI_nBr_4-n^2-, corrected for statistical effects, give the stabilization constants 0·4 for CdBr₃I^2-, 0·7 for CdBr₂I₂^2- and 1·6 for CdBrI₃^2-. Only the latter is thus stabilized beyond the statistically expected level, but the reasons for this are obscure.