Kinetics and mechanism of the reaction between chromium(II) and 4,4′-bipyridine in acidic aqueous solution

Article abstract of DOI:10.1021/ic50207a010

The reaction between chromium(II) and the diprotonated 4,4′-bipyridine ion was studied at 25°C and ionic strength 1.0 M (LiClO₄-HClO₄). A highly absorbing, blue intermediate is produced, reaches a maximum concentration, and then fades to yield a solution containing chromium(III) and N,N′-dihydro-4,4′-bipyridine. The intermediate was identified as the 4,4′-bipyridine radical. The kinetics of the formation and disappearance of the radical were studied as a function of the concentrations of 4,4′-bipyridine, chromium(II), and hydrogen ions. Analysis of the observed kinetics indicates a sequence of two pseudo-first-order steps, the radical intermediate being present in transient equilibrium. The first step is the outer-sphere reduction of diprotonated 4,4′-bipyridine (bpyH₂²⁺) by chromium(II) according to the rate law k_d[Cr²⁺][bpyH₂²⁺], where k_d = (2.0 ± 0.1) × 10^-2 M^-1 s^-1 at 25°C and μ = 1.0 M. The second step involves the disappearance of the radical according to the rate law a + b[Cr²⁺][H⁺], with a = (4.79 ± 0.62) × 10^-2 s^-1 and b = 6.46 ± 0.39 M^-2 s^-1 at 25°C and μ = 1.0 M.