2,2'-BIPYRIMIDINE (BIPYM)-BRIDGED DINUCLEAR COMPLEXES .1. PREPARATION, CRYSTAL-STRUCTURE, AND MAGNETIC-PROPERTIES OF [NI2(H2O)8(BIPYM)][NO3]4 AND [NI2(H2O)8(BIPYM)][SO4]2.2H2O

Two new dinuclear nickel(II) complexes of formula [Ni2(H2O)8(bipym)][NO3]4 1 and [Ni2(H2O)8(bipym)][SO4]2.2H2O 2 (bipym = 2,2'-bipyrimidine) have been synthesised and their crystal structures determined by X-ray crystallographic methods. Crystals of 1 and 2 are monoclinic, space group P2(1)/n with a = 10.020(2), b = 10.521 (2), c = 11.01 9(2) angstrom, beta = 90.27(1)-degrees and Z = 2 for 1 and space group P2(1)/c with a = 8.094(2), b = 11.550(3), c = 11.719(2) angstrom, beta = 91.85(1)-degrees and Z = 2 for 2. Both structures consist of centrosymmetrical dinuclear cations [Ni2(H2O)8(bipym)]4+ with nitrate counter ions for 1 and water of crystallization and sulfate counter ions for 2. Each nickel atom is six-co-ordinated in a slightly distorted octahedral NiN2O4 chromophore: the average Ni(II)-N distance is 2.094 angstrom in 1 and 2.115(2) angstrom in 2, whereas the mean Ni(II)-O distance is 2.094(3) angstrom in 1 and 2.056(2) angstrom in 2. The bipyrimidine group joins two adjacent nickel atoms acting as a bis(bidentate) ligand. The C-C bond between the pyrimidine rings of bipym is perpendicular to the Ni ... Ni vector giving two five-membered chelate rings. The intramolecular Ni ... Ni separation is 5.569(1) angstrom for 1 and 5.660(1) angstrom for 2. Intramolecular antiferromagnetic spin-exchange coupling between the two nickel(II) ions is observed in both complexes with J = -14 and -12 cm-1 for 1 and 2, respectively (J is the intradimer exchange parameter in the isotropic Hamiltonian H = -JS(A).S(B). The efficiency of bipym to transmit electronic effects is compared to that of related bis(chelating) ligands such as oxalate and oxamidate in the parent oxalate- and oxamidate-bridged nickel(II) complexes.