Configuration of Octahedral Metal Compounds − Equilibrium, Crystal and Molecular Structure of Nickel(II) Complexes of Linear N2O4, N4S2 or N6 Donors Set Atoms Ligands

Two new nickel(II) complexes of symmetric hexadentate mixed-ligand N,O [1,12-bis(2-pyridyl)-5,8-dioxa-2,11-diazadodecane (pydado)] and N,S [1,12-bis(2-pyridyl)-5,8-dithia-2,11-diazadodecane (pydadt)] donor atoms have been synthesised as perchlorate salts and characterised by X-ray crystallography and ligand-field spectroscopy. In both complexes the Ni2+ ion is hexacoordinate. The cation [Ni(pydado)]2+ is pseudo-octahedral with the two pyridyl groups in trans position; all Ni−N and Ni−O bond lengths are practically equivalent. In [Ni(pydadt)]2+ complex, however, the size of the thioether sulfur atoms imposes a C2v symmetry; the pyridyl groups and the sulfur atoms are in trans positions and all Ni−N bond lengths are equivalent. The comparison of these structures with those of octahedral cobalt, copper and zinc complexes with the same ligands shows that their configurations depend not only on the nature of the two central donor atoms of the ligand, but also on the nature of the metallic ion. In aqueous solution, the stability constants of the NiII chelates with these two ligands, determined by potentiometry, show the formation of [Ni(LH)]3+ and [NiL]2+ species in all cases. The chelating power of the pydadt ligand is slightly greater than that of pydado. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)