PENTACOORDINATED BINUCLEAR COMPLEXES OF COBALT(II) AND NICKEL(II) WITH 3-IMIDAZOLINE NITROXIDES

The imidazoline nitroxide free radical 4-(2-oxoethen-1-yl)-2,2,5,5-tetramethyl-3-imidazolyl-1-oxy (LH) has been reacted with nickel(II) and cobalt(II) acetates to give centrosymmetric pentacoordinated binuclear complexes of formula M(2)(L(4)). The two complexes are isomorphous with metal coordination spheres intermediate between square pyramids and trigonal bipyramids. Around each metal ion, the nitroxide ligands are cis-coordinated. Assuming non-degenerate ground states for the metal ions, coupling constants are as follows (H=-2JS(i)S(j)): J(Ni-Ni)=-31.1(6) cm(-1), J(Ni-NO)=+1.7(7) cm(-1), J(Co-Co)=-9.8(2) cm(-1), J(Co-NO)=-2.1(1) cm(-1). The ferro- and antiferromagnetic natures of the metal-nitroxide interaction for the nickel and cobalt complex have been rationalized considering delocalization of metal spin density in ligand orbitals of different symmetry. Relevant structural parameters for the cobalt complex are as follows: space group P2(1)/c, a=12.169(8), b=11.300(7), c=15.944(9) Angstrom, beta=101.16(1)degrees, Z=2.