Structural and spectroscopic studies of transition metal nitrite complexes. V. Crystal structures and spectra of trans-Tetrakis(pyridine)dinitritonickel(II)-pyridine (1/2), trans-Tetrakis(4-methylpyridine)dinitritonickel(II) and trans-Tetrakis(pyrazole)-dinitritonickel(II)

Abstract

The crystal and molecular structures of the compounds [Ni(py)₄(ONO)₂],2py_, [Ni(γmpy)_,(ONO)₂] and [Ni(prz)₄(ONO)₂] are reported. All three are trans nitrito complexes, the pyridine (py) compound containing two pyridine molecules of solvation. The aromatic rings in the first two complexes adopt 'paddle wheel' conformations with pitch angles varying between 40 and 70º. The nitrite ions are positioned so as to minimize repulsive interactions with the amines, and it seems likely that these groups bond through oxygen rather than nitrogen because this allows a lesser degree of interligand steric interference. The amine rings in [Ni(prz)₄(ONO)₂] are orthogonal to the plane containing the nickel and coordinated pyrazole nitrogen atoms; the nitrito groups are disordered between two inequivalent positions, each of which involves hydrogen bonding with the pyrazole NH groups. The nitrite infrared frequencies are similar to those observed for other nickel(II) nitrito complexes except that the antisymmetric NO stretching mode of one of the groups in the pyrazole complex is much lower in energy than expected, being in the range normally associated with a nitrogen-bonded or chelated nitrite group. It is suggested that this deviation may be caused by the hydrogen bonding in the complex. The electronic spectra of the compounds yield 10Dq values of 9100 and 8500 cm^-1 for the nitrite ligands in [Ni(py)₄(ONO)₂] and Ni(prz)₄(ONO)₂], respectively, placing the nitrito group towards the weaker end of the spectro-chemical series.