SYNTHESIS, MOLECULAR-STRUCTURE, AND MAGNETIC-PROPERTIES OF BIS(3,6-DI-TERT-BUTYL-O-BENZOSEMIQUINONE)NICKEL(II) AND BIS(3,6-DI-TERT-BUTYL-O-BENZOSEMIQUINONE)COPPER(II)

The title complexes of Ni and Cu with symmetrical 3,6-di-tert-butyl-o-benzosemiquinone are synthesized. Their EPR spectra and magnetic properties are investigated. The x-ray structure studies [Siemens R3/PC diffractometer, lambda Mo Kalpha, theta/2theta-scanning in the range 2 less-than-or-equal-to 20 less-than-or-equal-to 54-degrees, 2084 reflections with F > 4sigma(F), R = 0.034, R(w) = 0.039, monoclinic crystals, a = 9.982(2), b = 11.548(2), c = 12.145(2) angstrom, beta = 95.05(3)-degrees, Z = 2, d(calc) = 1.19 g/cm-3, space group P2(1)/c) demonstrated that the complex is monomeric with square-planar coordination for the Ni with the o-semiquinone ligands. The Cu complex is isostructural with the Ni (a = 9.88, b = 11.60, c = 12.15 angstrom, beta = 95-degrees]. The dependence of the magnetic moment of the Cu complex on temperature is consistent with the presence in it of two pathways for exchange interaction. These are antiferromagnetic ligand-ligand J12 = -179 cm-1 and ferromagnetic metal-ligand J13 = 100 cm-1 (mean-square deviation 2%). the Ni complex is diamagnetic over the whole studied temperature range despite the fact that it contains free-radical o-semiquinone ligands. Such an effect involving electrons belonging to the free-radical ligands is observed for the first time in the magnetochemistry of d8- and d9-transition-metal complexes. It is explained by incorporation of vacant p(z) and/or occupied d(xz) and d(yz)-orbitals on Ni in molecular orbitals containing the pi-MO of the semiquinone ligands.