Bromide anation kinetics for some platinum(IV) bromo aqua complexes

Abstract

Bromide anations of PtBr 5H 2O − in the presence of PtBr 2− 4 or bromine, of trans-PtBr 4(H 2O) 2 in the presence of PtBr 2− 4 and of cis-PtBr 4(H 2O) 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 PtBr 5H 2O − ▪ 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 PtBr 2− 4. The formation of the reaction product PtBr 2− 6 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-PtBr 4(H 2O) 2 having the rate law ▪ The bromide anation of trans-PtBr 4(H 2O) 2in the presence of PtBr 2− 4 occurs by the two-term rate law ▪ indicating two parallel paths, k′ corresponding to formation of PtBr 5H 2O − and k″ to formation of PtBr 2− 6. Trans-PtBr 4(H 2O) 2 reacts so slowly with bromide in the absence of PtBr 2− 4 that no reaction can be observed. The large difference in bromide anation rates for PtBr 5H 2O − and cis-PtBr 4(H 2O) 2 compared to trans-PtBr 4(H 2O) 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.