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)[Ni-2(2)]. 6H(2)O (3), wher e H-8[2] stands for N,N',N",N'''-1,2,4,5-benzenetetrayltetrakis(oxamic acid ), has been synthesized and its crystal structure determined by single-crys tal X-ray diffraction. The structure of 3 consists of [Ni-2(eta(4):eta(4)-2 )](4-) anions, tetraphenylphosphonium cations, and water molecules. Facile one-electron oxidation of the square-planar diamagnetic dinickel(II) comple x [Ni-2(eta(4):eta(4)-2)](4-) generates the metallo-radical species [Ni-2(e ta(4):eta(4)-2(.+))](3-) with characteristic intra-ligand pi-cation radical transitions in the visible region (475-550 nm) as well as a typical quasi- isotropic EPR signal at g approximate to 2.0.