A Square-Planar Dinickel(II) Complex with a Noninnocent Dinucleating Oxamate Ligand: Evidence for a Ligand Radical Species

The new bimetallic nickel(II) compound (PPh4)4[Ni2(2)]·6H2O (3), where H8[2] stands for N,N′,N′′,N′′′-1,2,4,5-benzene-tetrayltetrakis(oxamic acid), has been synthesized and its crystal structure determined by single-crystal X-ray diffraction. The structure of 3 consists of [Ni2(η4:η4-2)]4– anions, tetraphenylphosphonium cations, and water molecules. Facile one-electron oxidation of the square-planar diamagnetic dinickel(II) complex [Ni2(η4:η4-2)]4– generates the metallo-radical species [Ni2(η4:η4-2·+)]3– with characteristic intra-ligand π-cation radical transitions in the visible region (475–550 nm) as well as a typical quasi-isotropic EPR signal at g ≈ 2.0.