Octahedral-tetrahedral coordination equilibria of nickel (II) and copper (II) in concentrated aqueous electrolyte solutions

Charles Angell, D. M. Gruen

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The coordination states of Ni(II) and Cu(II) in aqueous magnesium chloride solutions have been studied spectrophotometrically 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)6 2+ by Cl- to give NiCl6 4- groups. With increasing temperature, transformation to NiCl4 2- 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 CuCl4 2- configuration, but in this case the transition from Cu(H2O)6 2+ cannot be described in terms of a two-species equilibrium but seems to occur by means of a continuous ligand exchange and distortion mechanism. In the MgCl2-H2O system, H2O coordinates strongly with Mg(II). For compositions near H2O/Mg = 6, the Mg(H2O)6 2+ 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.

Original languageEnglish (US)
Pages (from-to)5192-5198
Number of pages7
JournalJournal of the American Chemical Society
Issue number22
Publication statusPublished - 1966
Externally publishedYes


ASJC Scopus subject areas

  • Chemistry(all)

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