Complexes of Nickel(II) with 2-Pyridinaldoxime

Abstract

THE complexing properties of 2-pyridinaldoxime (I) would be expected to follow from those of 2,2′-bipyridine (II) and dimethylglyoxime (III) since this new ligand incorporates in its structure the functional groups of both the latter substances. In the course of synthesizing and characterizing the nickel(II) complexes of pyridinaldoxime (HPAO) it has become apparent that some very unusual deviations from the expected behaviour occur. By analogy with the nickel(II) complexes of 2,2′-bipyridine¹, two complexes containing molecular pyridinaldoxime should form: bis(pyridinaldoxime)nickel(II) and tris-(pyridinaldoxime)nickel(II). In addition, a third complex, containing one molecule of pyridinaldoxime co-ordinated to each nickel(II) atom, might also be expected to exist. The first two complexes, [Ni(HPA0)₂]⁺⁺ and [Ni(HPA0)₃]⁺⁺, have been prepared as the chloride and iodide salts, respectively. The bis complex is prepared by mixing 1 mole of nickel chloride 6-hydrate with 2 moles of the ligand in hot absolute ethanol. The compound is precipitated as fine green crystals. The tris complex may be isolated as fine tan crystals by mixing 3 moles of ligand with 1 mole of nickel(II) chloride in aqueous solution, followed by the addition of solid potassium iodide. The colour changes occurring in aqueous solutions indicate that the bis-pyridinaldoxime complex is considerably less subject to disproportionate, and consequent formation of the tris complex, than is true of the corresponding 2,2′-bipyridine complex. Both the bis and tris complexes containing the neutral pyridinaldoxime molecule are paramagnetic, having corrected effective magnetic moments of 3.16 and 3.12 Bohr magnetons, respectively. It may be concluded that these compounds are normal representatives of the classes of nickel(II) complexes formed by the aromatic heterocyclic diamines and other related ligands.