Square Planar vs Tetrahedral Coordination in Diamagnetic Complexes of Nickel(II) Containing Two Bidentate π-Radical Monoanions

The reaction of three different 1-phenyl and 1,4-diphenyl substituted S-methylisothiosemicarbazides, H2[L1-6], with Ni(OAc)2·4H2O in ethanol in the presence of air yields six four-coordinate species [Ni(L1-6•)2] (1−6) where (L1-6•)1- represent the monoanionic π-radical forms. The crystal structures of the nickel complexes with 1-phenyl derivatives as in 1 reveal a square planar structure trans-[Ni(L1-3•)2], whereas the corresponding 1,4-diphenyl derivatives are distorted tetrahedral as is demonstrated by X-ray crystallography of [Ni(L5•)2] (5) and [Ni(L6•)2] (6). Both series of mononuclear complexes possess a diamagnetic ground state. The electronic structures of both series have been elucidated experimentally (electronic spectra magnetization data). The square planar complexes 1−3 consist of a diamagnetic central Ni(II) ion and two strongly antiferromagnetically coupled ligand π-radicals as has been deduced from correlated ab initio calculations; they are singlet diradicals. The tetrahedral complexes 4−6 consist of a paramagnetic high-spin NiII ion (SNi = 1), which is strongly antiferromagnetically coupled to two ligand π-radicals. This is clearly revealed by DFT and correlated ab initio calculations. Electrochemically, complexes 1−6 can be reduced to form stable, paramagnetic monoanions [1−6]- (S = 1/2). The anions [1−3]- are square planar Ni(II) (d,8 SNi = 0) species where the excess electron is delocalized over both ligands (class III, ligand mixed valency). In contrast, one-electron reduction of 4, 5, and 6 yields paramagnetic tetrahedral monoanions (S = 1/2). X-band EPR spectroscopy shows that there are two different isomers A and B of each monoanion present in solution. In these anions, the excess electron is localized on one ligand [NiII(L4-6•)(L4-6)]- where (L4-6)2- is the closed shell dianion of the ligands H2[L4-6] as was deduced from their electronic spectra and broken symmetry DFT calculations. Oxidation of 1 and 5 with excess iodine yields octahedral complexes [NiII(L1,ox)2I2] (7), [NiII(L1,ox)3](I3)2 (8), and trans-[NiII(L5,ox)2(I3)2] (9), which have been characterized by X-ray crystallography; (L1-6,ox) represent the neutral, two-electron oxidized forms of the corresponding dianions (L1-6)2-. The room-temperature structures of complexes 1, 5, and 7 have been described previously in refs 1−5.