Chemistry of tetravalent nickel and related species. 3.¹ Characterization and cyclic voltammetry of new [NiN₆] species based on tridentate ligands

Abstract

High-spin octahedral (NiN₆ core;Dg≈ 1250cm^-1) nickel(II) complexes of type Ni(HRRI)₂X₂ (where HRRT is a tridentate ligand of class N.β-aminoethylisonitroso ketimine and X = CI0₄. NO₃) are readily oxidized by concentrated nitric acid to yield the diamagnetic nicket(IV) species Ni(RRT).K.. Cyclic voltammcry (CV) of Ni(RRT)₂(CIO₄)₂ has been thoroughly done in well-buffered aqueous media over a wide range of pH. 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. The rate constant for this decomposition reaction has been determined to be 0.16s^-1 (283 K) from CV data. In the pH range 7.0-9.0. two distinct reversible onc-elcctron 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 involves one proton. The E°′₂₉₈ values for the various couples have been accurately determined front CV data. Representative results are as follows: Ni(Me₂T)₂²⁺-Ni(HMe₂T)₂²⁺,0.71 V; Ni(Me₂T)₂²⁺-Ni(Me₂T)2⁺,0.40 V; Ni(Me₂T)2⁺-Ni(HMe₂T)(Me₂T)⁺,0.66 V; Ni(Me₂T)2⁺-Ni(Me₂T)2,0.07 V (estimated). The nickel(II) species involved in each of the above couples is what is expected from acid dissociation (oxime protons) data of Ni(Hme₂T)₂²⁺(pK₁=7.80;pK₂=10.00).