Structural and Spectroscopic Studies of Transition Metal Nitrite Complexes. VI Crystal Structures and Spectra of Hexakis(imidazo1e)nickel(II) Dinitrate, trans-Tetrakis(2-methylimidazo1e)dinitrito-nickel(II) and cis-Tetrakis(2-methylimidazo1e)-(nitrito-O, O′)nickel(II) Nitrate-Methanol (2/1)

The crystal structures and spectroscopic properties of the complexes cis-tetrakis(2-methy1imidazole)-(nitrito-O, O′)nickel(II) nitrate-methanol (2/1) and trans-tetrakis(2-methylimidazole)dinitritonickel(II) are reported. The nitrite group in the former compound chelates, giving a distorted octahedral ligand coordination geometry. The ligand stereochemistry in the second complex is also a distorted octahedron, with both anions coordinating as trans nitrito groups though with significantly different Ni-O bond lengths and the nickel(II) ion being raised out of the plane of the four imidazole ligand nitrogen atoms towards the closer nitrite group. The Ni-O bond directions also deviate substantially from the fourfold symmetry axis in this latter complex. These distortions apparently occur in order to minimize ligand-ligand repulsions, particularly those associated with the amine methyl substituents. The electronic spectra of both complexes show significant band splittings due to the departure of the ligand field from regular octahedral symmetry and these are discussed in terms of the relative field strengths of the imidazole and nitrito ligands. The structure of hexakis(imidazole)nickel(II) nitrate (redetermination) is also reported.