Isotopic exchange of bromine in aqueous system: cis-dibromodiammineplatinum(II), tetrabromoplatinate(II), bromide

The aquation equilibria for cis-dibromodiammineplatinum(II) have been measured. For the first and second aquations at 25°C: K₁ = 1.13 mM. ΔH° = 3.9 ± .7 kcal/mol, K₂ = 0.042 mM, ΔH° = 10 ± 3 kcal/mol. Some isotopic exchange of ⁸²Br between c-Pt(NH₃)₂Br₂ and PtBr₄²⁻ occurs without the formation of a free bromide ion and with a rate expression: Rate Exchange = (k_ac′ + k_ac″/[Br⁻])[Pt(NH₃)₂Br₂] [PtBr₄²⁻]. It has been shown that the presence of c-Pt(NH₃)₂Br(H₂O)⁺ serves to catalyze the aquation of PtBr₄²⁻, but that the catalysis step does not account for all the bromide dependent exchange.