Spectroscopic and Electrochemical Studies of Nickel(III) and Nickel(IV) Complexes derived From Oximato Ligands

Abstract

Nickel(II) complexes with tridentate oxime chelate HRR′Y system in alkaline media are oxidized by ammonium persulfate to produce red-violet crystals of Ni(HRR′Y)-(RR′Y)(ClO₄)₂ which (i) oxidize iron(II) to iron(III), (ii) have μ_eff≃2.0 B.M., (iii) exhibit characteristic polycrystalline epr spectra of S = 1/2 in the axial field (g_∥ = 2.04, g_⊥ = 2.16) and also are oxidized by concentrated nitric acid to yield dark-violet diamagnetic nickel(IV) species Ni(RR′Y)2(ClO₄)₂. Cyclic voltammetry (CV) of Ni(RR′Y)₂(ClO₄)₂ has been thoroughly done in well-buffered media over a wide range of pH values. Below pH 6.0, CV data show a single, reversible, two-electron, two-proton transfer process followed by an irreversible chemical decomposition of the nickel(II) species. In the pH range 7.00 – 9.00 two distinct reversible, one-electron processes are observed corresponding to Ni(IV) – Ni(III) and Ni(III) – Ni(II) couples. The first of these has no Proton involvement while the other is proton dependent. The E^0′ ₂₉₈ values for the various couples have been accurately determined from CV data.