Square planar vs tetrahedral coordination in diamagnetic complexes of nickel(II) containing two bidentate pi-radical monoanions

The reaction of three different 1-phenyl and 1,4-diphenyl substituted S-methylisothiosemicarbazides, H(2)L(1-6)], with Ni(OAc)(2).4H(2)O in ethanol in the presence of air yields six four-coordinate species Ni(L(1-6)(*))(2)] (1-6) where (L(1-6)(*))(1-) represent the monoanionic pi-radical forms. The crystal structures of the nickel complexes with 1-phenyl derivatives as in 1 reveal a square planar structure trans-Ni(L(1)(-3)(*))(2)], whereas the corresponding 1,4-diphenyl derivatives are distorted tetrahedral as is demonstrated by X-ray crystallography of Ni(L(5)(*))(2)] (5) and Ni(L(6)(*))(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 pi-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 Ni(II) ion (S(Ni) = 1), which is strongly antiferromagnetically coupled to two ligand pi-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) S(Ni) = 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 Ni(II)(L(4-6)(*))(L(4-6))](-) where (L(4-6))(2-) is the closed shell dianion of the ligands H(2)L(4-6)] as was deduced from their electronic spectra and broken symmetry DFT calculations. Oxidation of 1 and 5 with excess iodine yields octahedral complexes Ni(II)(L(1,ox))(2)I(2)] (7), Ni(II)(L(1,ox))(3)](I(3))(2) (8), and trans-Ni(II)(L(5,ox))(2)(I(3))(2)] (9), which have been characterized by X-ray crystallography; (L(1-)(6,ox)) represent the neutral, two-electron oxidized forms of the corresponding dianions (L(1-6))(2-). The room-temperature structures of complexes 1, 5, and 7 have been described previously in refs 1-5.