Valence tautomerism in octahedral and square-planar phenoxyl-nickel(II) complexes: Are imino nitrogen atoms good friends?

The two tetradentate ligands H2L and H2LMe afford the slightly distorted square-planar low-spin Ni-II complexes I and 2, which comprise two coordinated phenolate groups. Complex I has been electrochemically oxidized into 1(+), which contains a coordinated phenoxyl radical, with a contribution from the nickel orbital. In the presence of pyridine, 1(+) is converted into 1(Py)(+), an octahedral phenolate nickel(m) complex with two pyridines axially co-ordinated: An intramolecular electron transfer (valence tautomerism) is promoted by the geometrical changes, from square planar to octahedral, around the metal center. The tetradentate ligand H2LMe, in the presence of pyridine, and the hexadentate ligand H2LPy in CH2Cl2 afford, respectively, the octahedral high-spin Ni-II complexes 2(Py) and 3, which involve two equatorial phenolates and two axially coordinated pyridines. At 100 K, the one-electron-oxidized product 2(Py)(+) comprises a phenoxyl radical ferromagnetically coupled to the high-spin Ni-II ion, with large zero-field splitting parameters, while 3(+) involves a phenoxyl radical antiferromagnetically coupled to the high-spin Ni-II ion.