The nature of pb(II)-bromide complexes in propylene carbonate

The greatly increased solubility of PbBr₂ in propylene carbonate (PC) caused by addition of LiBr indicates that PB(II) interacts strongly with bromide ion in propylene carbonate to form stable Pb(II)-bromide complexes. Solubility and spectrophotometric data for LiBr solutions saturated with PbBr₂ indicate the presence of two Pb(II)-bromide complexes whose bromide:lead ratios are respectively 3.00 and 2.75. The former, which predominates in dilute (<0.03 m) solutions of LiBr saturated with PbBr₂, was shown by potentiometry and solubility data to be PbBr₃^?, a species also formed when PbBr₂ dissolves in PC in the absence of LiBr. The latter, which is predominant in more concentrated LiBr solutions saturated wih PbBr₂, was shown by spectrophotometry and solubility data to be the polynuclear complex Pb₄Br₁₁^?3. Both complexes are sufficiently stable that the free bromide ion concentration in these solutions corresponds to only a small fraction of the total bromide concentration. In LiBr solutions unsaturated with respect to PbBr₂, the major Pb(II) species are PbBr₃^? and PbBr₄^?2.