Nickel, Palladium, and Platinum

Nickel, together with iron and cobalt, completes one group of the transition series in which 10 electrons have been added to the argon electronic core according to the electron distribution A4s²3d⁸. The relatively small energy of the electrons present in the s and d levels leads to nickel compounds with oxidation numbers ranging from 0 to +4. The chemistry of nickel in aqueous solutions is principally that of Ni²⁺ and its related compounds. NiO prepared at temperatures above 200°C corresponds to NaCl crystal structure. Only a few complexes of low stability are reported for the +3 state. Most of the standard potentials of electrode reactions involving different nickel species are computed from thermal data. The oxidation of nickelous hydroxide in alkaline solution appears to form solid solutions of Ni²⁺ and Ni³⁺ compounds with a variable water content.