Valence Tautomerism in Octahedral and Square‐Planar Phenoxyl–Nickel( II ) Complexes: Are Imino Nitrogen Atoms Good Friends?

The two tetradentate ligands H 2 L and H 2 L Me afford the slightly distorted square‐planar low‐spin Ni II complexes 1 and 2 , which comprise two coordinated phenolate groups. Complex 1 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( III ) complex with two pyridines axially coordinated: An intramolecular electron transfer (valence tautomerism) is promoted by the geometrical changes, from square planar to octahedral, around the metal center. The tetradentate ligand H 2 L Me , in the presence of pyridine, and the hexadentate ligand H 2 L Py in CH 2 Cl 2 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.