Studies in nickel(IV) chemistry. Kinetics of electron transfer from dimethyl sulfoxide to oxohydroxonickel(IV) in aqueous acid medium

Abstract

AbstractThe kinetics of decomposition of “oxohydroxonickel(IV)” [Ni(IV)] with concomitant intramolecular electron transfer to produce hexaaquanickel(II) and dioxygen in aqueous acid solutions show pseudo‐first‐order dissappearance of the Ni(IV). The pseudo‐first‐order rate constants for the acid decomposition (kad) satisfy where KMH and kd refer to the equilibrium protonation constant and the decomposition constant of the protonated species of the Ni(IV) respectively. The values of KMH and kd in aqueous medium at 45°C and μ = 2.0M are 25.5 ± 1M−1 and (1.7 ± 0.1) × 10−5 s−1, respectively.The kinetics of the intermolecular electron transfer from dimethyl sulfoxide (DMSO) to the Ni(IV), producing Ni(H2O)62+ and dimethyl sulfone as products, have been investigated by monitoring the formation of Ni(H2O)62+. The pseudo‐first‐order rate constants for the electron transfer kobs are linearly dependent on [DMSO]0 or [H+], attaining limiting values at higher relative [DMSO]0 or [H+], in accordance with where K1c and K2c represent the formation constants of the precursors involving DMSO and the unprotonated and one‐protonated Ni(IV) species, respectively, and k1x and k2x are the corresponding decomposition rate constants of the precursors. The values of K2c and k2x are (2.3 ± 0.1) × 104M−1 and 19 ± 1 s−1, respectively, at 45°C and μ = 1.0M. Results are interpreted in terms of probable mechanisms involving (1) a rate‐determining decomposition of the protonated Ni(IV) followed by rapid product formation steps, and (2) precursor complex formation between DMSO and the unprotonated or the protonated species of the Ni(IV) followed by rate‐determining decomposition with electron transfer.