Studies in nickel(IV) chemistry. Kinetics of the Cu(II) ion‐mediated acid decomposition of tris(dimethylglyoximato) nickelate(IV) in aqueous acidic medium

Kinetics of the Cu(II) ion‐mediated acid decomposition of tris (dimethylglyoximato)nickelate(IV), Ni(dmg) 3 2− (dmg 2− = dimethylglyoximate dianion), are reported in aqueous medium in the range of 3.6 ⩽ pH ⩽ 6.6 at 35°C and μ = 0.57 M . The pseudo‐first‐order rate constants of the disappearance of Ni(IV) k obs(M) satisfy the equation\documentclass{article}\pagestyle{empty}\begin{document}$$ k_{{\rm obs(M)}} = k_{ad} + k_{dec(M)} $$\end{document}where k ad refers to the pseudo‐first‐order rate constants for the proton‐assisted decomposition of the Ni(IV) complex determined independently and is a function of [H + ], and k dec(M) to that for the Cu(II) ion‐mediated route and is a function of [H + ] and [Cu 2+ ]. Both k obs(M) and k dec(M) are found to increase with increasing [Cu(II)] 0 , tending to attain limiting values at higher relative [Cu(II)] 0 . At low [Cu(II)] 0 the k dec(M) is found to register a decrease with increasing pH in the pH range of 3.6–4.4, then an increase in the range of 4.4–5.76, and again a decrease in the range of 5.76–6.6. Results on the Cu(II) ion‐mediated acid decomposition are interpreted in terms of a probable mechanism involving pH‐dependent adduct formation equilibria involving the one‐protonated and the two‐protonated species of Ni(IV) and the various species of Cu(II) ion in the media, followed by rate‐determining acid decomposition of the adduct(s) to give Ni(II) aq. and Cu(dmgH) 2 . While the two‐protonated Ni(IV) complex apparently reacts about five orders of magnitude faster than the one‐protonated species, the aquacopper(II) reacts about two orders of magnitude slower than the hydroxoaquacopper(II).