Pyridine-2,6-dihydroxamic acid, a powerful dihydroxamate ligand for Ni2+ and Cu2+ ions

Pyridine-2,6-dihydroxamic acid was found to be the most effective ligand for Ni2+ and Cu2+ ions among the known dihydroxamates. The stability constants of the title complexes are many orders of magnitude higher than those obtained for the other dihydroxamates. Only equimolar species are formed with Cu2+ ions, while NiL and bis-complexes are formed in the case of Ni2+. The [NiL2]2− complex anion contains compressed octahedral hexanitrogen surroundings of two meridionally coordinated ligands. The equatorial Ni–N(Py) bonds are significantly shorter than the axial Ni–N(hydroxamate) distances although the latter are formed by the deprotonated groups.