The co-ordination chemistry of divalent cobalt, nickel, and copper. Part 10. Characterisation and stability constants of nickel(II) species isolated from a mixed-ligand solution: crystal structure of tris(1,2-diaminoethane)nickel(II) bis(pyridine-2,6-dicarboxylato)nickelate(II) tetrahydrate

Abstract

The complexes, Ni(pydca)(en)·2H2O (1), Ni(pydca)(en)3·2H2O (2), and Ni(pydca)(en)1.5·2H2O (3) were isolated from solutions of nickel(II), pydca, and en (pydca = pyridine-2,6-dicarboxylate, en = 1,2-diaminoethane). Spectral and magnetic data indicate that species (1) and (2) are octahedral and can be formulated as [Ni(pydca)(en)(H2O)]·H2O and [Ni(en)3][pydca]·2H2O, respectively. The structure of (3) was shown by Patterson and Fourier methods to be in accordance with the formula [Ni(en)3][Ni(pydca)2]·4H2O, with a= 9.395(5), b= 10.924(5), c= 15.662(8)Å, α= 70.95(2), β= 101.36(2), γ= 104.91(2)°, Z= 2, triclinic, and space group P. Heating (1) gave a compound with the empirical formula Ni(pydca)(en). Infrared and magnetic data indicate that this species is monomeric and five-co-ordinate. A compound with the same empirical formula, Ni(pydca)(en), was obtained by heating (3), but spectral data and a magnetic moment of 3.40 B.M. for this product suggest the formula [Ni(en)2][Ni(pydca)2], containing [Ni(en)2]2+ with Cs symmetry. A plot of complex-ion concentrations as a function of Ni : en ratio in solution was derived from measured stability constants and was used to explain the stepwise isolation of species (1), (3), and (2).