Trivalent and Tetravalent Nickel in Oxime-Imine-Amine Coordination

Model complexes of nickel(III) and nickel(IV) are furnished by hexadentate and tridentate oxime-imine-amine ligands derived by the condensation of biacetyl monoxime with amines. The synthesis, characterisation, structure, redox equilibria and reactivity of these complexes are critically reviewed. Correlations and generalisations pertaining to oxidation state stability, electroprotic equilibria and reaction mechanisms are examined. The possible significance of the present complexes in the context of the nickel-containing methanogenic enzymes is noted.