Chemistry of tetravalent nickel and related species. 3. Characterization and cyclic voltammetry of new [NiN6] species based on tridentate ligands

Abstract

High-spin octahedral (NiN 6 core;Dg≈ 1250cm -1 ) nickel(II) complexes of type Ni(HRRI) 2 X 2 (where HRRT is a tridentate ligand of class N.β-aminoethylisonitroso ketimine and X = CI0 4 . NO 3 ) are readily oxidized by concentrated nitric acid to yield the diamagnetic nicket(IV) species Ni(RRT).K.. Cyclic voltammcry (CV) of Ni(RRT) 2 (CIO 4 ) 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(Me 2 T) 2 2+ -Ni(HMe 2 T) 2 2+ ,0.71 V; Ni(Me 2 T) 2 2+ -Ni(Me 2 T)2 + ,0.40 V; Ni(Me 2 T)2 + -Ni(HMe 2 T)(Me 2 T) + ,0.66 V; Ni(Me 2 T)2 + -Ni(Me 2 T)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(Hme 2 T) 2 2+ (pK 1 =7.80;pK 2 =10.00).