Bromide Anation Kinetics of Some Platinum(IV) Bromo Aqua Complexes

Bromide anations of PtBr5H2O− in the presence of PtBr42− or bromine, of trans-PtBr4(H2O)2 in the presence of PtBr42− and of cis-PtBr4(H2O)2 in the presence of bromine have been studied in 0.50 M perchloric acid medium at 25 °C.
The rate law for the bromide anation of PtBr5H2O−indicates two parallel reaction paths. The term k′Br−2 corresponds to a bromide assisted mechanism. The second term in the rate expression can be accounted for by assuming that a dimeric intermediate is formed from the platinum(IV) substrate and hydrated PtBr42−. The formation of the reaction product PtBr62− is preceded by a substitution of water by bromide in this dimer.
A bromide assisted mechanism can also describe the bromide anation kinetics of cis-PtBr4(H2O)2, the rate law determined. The bromide anation of trans-PtBr4(H2O)2 in the presence of PtBr42− occurs by a two-term rate law, indicating two parallel paths, k′ corresponding to formation of PtBr5H2O− and k″ to formation of PtBr62−.
Trans-PtBr4(H2O)2 reacts so slowly with bromide in the absence of PtBr42− that no reaction can be observed. The large difference in bromide anation rates for PtBr5H2O− and cis-PtBr4(H2O)2 compared to trans-PtBr4(H2O)2 might indicate that halide assisted anations are possible only for complexes having a halide (or some other effective bridging group) in trans-position to the aqua ligand to be replaced.