Octahedral-Tetrahedral Coordination Equilibria of Nickel (II) and Copper (II) in Concentrated Aqueous Electrolyte Solutions

The coordination slates of Ni(II) and Cu(II) in aqueous magnesium chloride solutions have been studied spectrophotometrkally at temperatures up to 320° and concentrations up to 8 M in magnesium chloride. For Ni(II), increase in the MgCl₂ concentration results in the replacement of H₂O in the first coordination sphere of Ni(H₂O)₆²⁺ by Cl- to give NiCl₆⁴⁻ groups. With increasing temperature, transformation to NiCl₄²⁻ groups occurs through a two-species octahedral-tetrahedral equilibrium. Thermodynamic data are presented for this equilibrium. For Cu(II), at similar temperatures and MgCl₂ concentrations, the coordination state is the CuCl₄²⁻ configuration, but in this case the transition from Cu(H₂O)₆²⁺ cannot be described in terms of a two-species equilibrium but seems to occur by means of a continuous ligand exchang0e and distortion mechanism. In the MgCl₂-H₂O system, H₂O coordinates strongly with Mg(II). For compositions near H₂O/Mg = 6, the Mg(H₂O)₆²⁺ species appears to exist as an independent entity. It is pointed out that changes in the Ni(II) coordination states can serve as indicators for the determination of Mg(II) hydration, numbers as well as hydration numbers of other noble gas configuration cations in molten hydrate systems.