Chemistry of tetravalent nickel and related species. I.MN₆ coordination octahedra generated from hexadentate oxime ligands

Hexadentate ligands (H2RR'L) of type 1 readily yield high-spin nickel(II) complexes. Ni(H2RR'L)X2 (X = CIO4, NO3), in which the coordination polyhedron is octahedral NiN6 and Dq is∼1300m-1. These arc readily oxidized to red diamagnetic Ni(RR'L)X2 by chemical and electrochemical means. The oxidized complexes aintain tctravakru nickel held in an octahedral NiN6 core. They show two intense ligand → metal charge transfer transitions in the visible region. The charge transfer state also makes some contribution to the ground state fir data). Ni(RR'L)2+ oxidizes quantitatively 2 mol of Fe2+ to Fe3+. The potential of the half•cell reaction Ni(RR'L)2+H++2e=Ni(H2RR'L)2+ is determined using a direct potentiometric method. E°298 (vs. normal hydrogen electrode) is found to lie in the range 0.90-0.94 V for the systems studied. The possible role of the negative charge on the =N-0- moiety in minimizing the effective pasiEvc charge on the metal in Ni(RR'L)2+ is discussed. The spectroscopic properties of the low-spin complexes of H2Me2L with two other d6 ions, viz., iron(II) and cobalt(III) are compared with those of the nickel(IV) species