Unidentate versus symmetrically and unsymmetrically bidentate nitrate co-ordination in pyrazole-containing chelates. The crystal and molecular structures of (nitrato-O)[tris(3,5-dimethylpyrazol-1-ylmethyl)amine]copper(II) nitrate, (nitrato-O,O′)[tris(3,5-dimethylpyrazol-1-ylmethyl)amine]nickel(II) nitrate, and (nitrato-O)(nitrato-O,O′)[tris(3,5-dimethylpyrazol-1-ylmethyl)amine]cadmium(II)

Abstract

The crystal structures of three compounds ML(NO₃)₂ are described, where M = Cu, Ni, or Cd and L is the tripodal quadridentate ligand tris(3,5-dimethylpyrazol-1-ylmethyl)amine. The structure of the copper compound can best be described as tetragonally distorted trigonal bipyramidal. As in the other compounds, the ligand L utilizes each of its four potential donor sites. One nitrate ion is unidentate, yielding a CuN₄O chromophore. In the nickel compound a nitrate ion is symmetrically bidentate, yielding a distorted octahedral cis-NiN₄O₂ chromophore. In the cadmium compound one of the nitrate ions is unsymmetrically bidentate, the other symmetrically bidentate, yielding a CdN₄O₃ chromophore. The structure is best described as distorted bicapped octahedral. The nitrate co-ordination modes were investigated using several spectroscopic techniques. Criteria to differentiate between the unidentate, unsymmetrically bidentate, and symmetrically bidentate modes are presented. In this type of compound discrimination between the co-ordination modes solely on the basis of spectroscopic behaviour appears not to be possible. Slight changes in the i.r. spectra of CuL(NO₃)₂, and of the analogous cobalt and zinc compounds, upon applying pressure can be understood in terms of changes in the nitrate co-ordination.