Kinetics and mechanism of the oxidation of bromide ions by aquocobalt(III) ions in aqueous perchlorate media

The kinetics of the oxidation of bromide ions by aquocobalt(III) ions have been investigated in aqueous perchlorate media at ionic strength μ = 2.00 over a temperature range 5–40°C using the stopped-flow technique. At 25°C and below the rate α[H+]−1, but above 25°C the rate may vary according to a + b[H+]−1, a ⪡ b: this agrees with earlier observations at μ = 3.00 and 25°C, but is contrary to other observations at μ = 0.50 in the temperature range 2–15°C where a ∼ b. These results are compared with other results at μ = 8.00, where the rate α[H+], which are shown to require the formation of CoIII + Br− complexes in rapid pre-equilibria followed by slow oxidative steps. A general mechanism is developed which can be adapted to explain all these observations found under differing conditions and which can be applied to the oxidations of Br− by other aquocations where the rate increases with increasing [H+]. The transition state parameters are discussed in terms of slow re-arrangements of solvent molecules and rapid electron tunnelling from ligand to cation.