Alkaline-earth-metal(II) complexes with hydroxide and fluoride in molten NH4NO3· 1.5H2O at 50 °C

Abstract

The formation of alkaline-earth-metal(II) hydroxide and fluoride complexes in molten NH₄NO₃· 1.5H₂O has been investigated by potentiometric, NMR and Raman spectroscopic methods at 50 °C. Complexes with the formal composition MgOH⁺, Mg₂OH³⁺ and CaOH⁺ were detected in the hydroxide systems. The formal stability constants are: β₁₁[MgOH⁺]= 3.2 ± 0.1 kg mol^–1, β₂₁[Mg₂OH³⁺]= 1.6 ± 0.5 kg² mol^–2, β₁₁[CaOH⁺]= 0.8 ± 0.1 kg mol^–1 obtained from potentiometric measurements. In the fluoride systems the following complexes were detected: MgF⁺, MgF₂, Mg₂F³⁺, CaF⁺, CaF₂, SrF⁺, Sr₂F³⁺ and BaF⁺ with formal stability constants: β₁₁[MgF⁺]= 3.3 ± 0.5 × 10² kg mol^–1, β₁₂[MgF₂]= 1.2 ± 0.2 × 10⁶ kg² mol^–2, β₂₁[Mg₂F³⁺]= 2.5 ± 0.2 × 10² kg² mol^–2, β₁₁[CaF⁺]= 1.0 ± 0.2 × 10² kg mol^–1, β₁₂[CaF₂]= 2.9 ± 0.6 × 10⁵ kg² mol^–2, β₁₁[SrF⁺]= 1.5 ± 0.2 kg mol^–1, β₂₁[Sr₂F³⁺]= 0.9 ± 0.2 kg² mol^–2 and β₁₁[BaF⁺]= 0.7 ± 0.1 kg mol^–1 . ¹⁹F NMR experiments reveal only small changes in the ¹⁹F chemical shift upon addition of Mg²⁺ or Ca²⁺, a fact which indicates a predominantly ionic M—F bonding character in the complexes formed. ¹⁴N NMR and Raman spectroscopy experiments using M(NO₃)₂· 3H₂O–NH₄NO₃· 1.5H₂O melts (M = Mg, Ca, Sr, Ba) indicate that nitrate ions are weakly coordinated to the metal ions and suggest the following order of increasing metal ion–nitrate interaction: Ba²⁺ < Sr²⁺ < Ca²⁺ < Mg²⁺. Difference Raman spectra of melts with magnesium or calcium ions, with and without fluoride present, show that nitrate ions, rather than water molecules, are most likely released upon metal–fluoride complex formation according to: M(H₂O)_x(NO₃)_y^2–y+nF^–⇌ MF_n(H₂O)_x(NO₃)_y–z^{2 –n–y+z}+zNO₃^–.