Studies in nickel(IV) chemistry. Part 1. Preparation, characterization, and kinetics of formation of tris(dimethylglyoximato)nickelate(IV) in aqueous alkaline media

Abstract

Potassium tris(dimethylglyoximato)nickelate(IV), K₂[Ni(dmg)₃], has been prepared in aqueous alkaline medium from nickel(II) and dimethylglyoximato (dmg^2–) in the presence of an excess of two-electron oxidant. Electronic, i.r., and ¹H n.m.r. spectra are in conformity with an octahedral (D₃) geometry for this compound, with dmg^2– units bonded through nitrogen donors. The lack of any significant e.s.r. signal establishes that the central nickel is in the oxidation state IV. The formation of [Ni(dmg)₃]^2–, from Ni^II and dmg^2– in the presence of hypoiodite generated in situ from tri-iodide and hydroxide, exhibits pseudo-first-order kinetics. The rate constants of formation, k_obs, show linear dependence on [oxidant]_O in the concentration range employed. At [OH^–]= 4.8 × 10^–2 mol dm^–3, the k_obs. values show a linear increase with increasing [dmg^2–]_o, tending to limiting values at higher relative [dmg^2–]_o. The k_obs, values remain almost constant with decreasing [OH^–] in the range 0.105 [OH^–] 1.1 × 10^–2 mol dm^–3, but exhibit a significant decrease with decrease in [OH^–] in the range 1.1 × 10^–2[OH^–] 6 × 10^–4 mol dm^–3. The kinetic results are interpreted in mechanistic terms.