Chemistry of tetravalent nickel and related species. 4. The situation where nickel(III) is stabilized by amine-imine-oxime ligands and nickel(IV) is not formed

Abstract

New hexadentate ligands (HRR'Y) generated by reaction of isonitroso ketones with tetraethylenepentamine in 1:1 ratio yield pscudooctahedral nickel(II) complexes Ni(HRR'Y)(CIO₄)Dq≈1250 cm^-1 ;μ_eff≈3μ_B). These are oxidized by alkaline ammonium persulfate to produce red crystals of Ni(RR'Y)(CI0₄)₂ which (i) oxidize iron(II) to iron(III), the reaction stoichiometry being Fe:Ni=1:1, (ii) have μ_eff≈2.1 μB, (iii) exhibit polycrystalline EPR spectra characteristic of S=1/2 in axial field (gp≈2.04; g⊥≈2.14), and (iv) undergo a ono-electron electrochemical reduction but no oxidation. Thus Ni(RR'Y)2+ contains nickel in the oxidation state +3. Extensive cyclic voltammetric studies have led to the identification of the following quasi-reversible process near pH 6: Ni(RR'Y)²⁺ +e^-+ H⁺ Ni(HRR'Y)²⁺ with E°′₂₉₈≈0.68 V vs. SCE. No sign of the formation of nickel( IV) species could be detected up to +0.9 V. Comparison of the ocnnplexing behavior of the various amine-imine-oxime ligands described in this paper and elsewhere and the various oxidation states of nickel achievable with each suggests that for each unit increase (above +2) in the oxidation state of the metal, the presence of at least one oxime function is needed. The HRR'Y ligand system having only one oxime function stabilizes nickel(III) but not nickel(IV). The low-spin iron(II) complexes Fe(HRR'Y)(C10₄)₂ are also reported. The IR and electronic spectra of these and the nickel(II) and nickel(III) complexes are described and compared. The iron(II) and nickel(III) complexes exhibit MLCT and LMCT transitions, respectively, in the visible region (∼500 nm).