ABSTRACT
Nickel (Ni) is element 28 and, along with iron (Fe) and cobalt (Co), forms the first transition series group VIIIb of the periodic table. In aqueous solutions, nickel is most often divalent and exists primarily as the hexaquonickel [Ni(H2O)6]2+ ion; other valences include –1, 0, +1, +3, and +4 (1). In solution, Ni2+ is 4-or 6-coordinated and most commonly occurs in square planar configuration and less often in tetrahedral or octahedral configurations (2). Ni2+ exhibits both “hard” and “soft” acid properties (3) and thus combines with nitrogen, oxygen, and sulfur-containing ligands in addition to donors from rows IV, V, VI, and VII of the periodic table. Nickel also can combine with carbon monoxide at atmospheric pressure to form highly toxic nickel carbonyl (Ni(CO)4). The acetate, nitrate, sulfate, and halogen salts of nickel are all water soluble whereas the oxides, sulfides, carbonates, phosphate, and elemental forms of nickel are insoluble in water (4).
