Formation and structure of mono- and di-bismuth hydroxide and fluoride complexes in molten NH4NO3· 1.5H2O at 50 °C

Abstract

The formation of bismuth hydroxide and fluoride complex species in NH₄NO₃· 1.5H₂O has been investigated by potentiometric, NMR and Raman spectroscopy and large-angle X-ray scattering (LAXS) methods at 50 °C. Complexes with the formal composition BiOH²⁺, Bi₂OH⁵⁺, BiF²⁺ and Bi₂F⁵⁺ were detected. The stability constants, as derived from potentiometry, are; β^*₁₁[BiOH²⁺]=(5.6 ± 0.1)× 10^–3 mol kg^–1, β^*₂₁[Bi₂OH⁵⁺]=(3.37 ± 0.02)× 10^–2, β₁₁[BiF²⁺]=(1.54 ± 0.02)× 10⁴ kg mol^–1 and β₂₁[Bi₂F⁵⁺]=(7.3 ± 0.5)× 10³ kg² mol^–2. ¹⁹F NMR shift studies indicate that the predominant contribution to the Bi–F bonding character is most likely to be electrostatic. ¹⁴N NMR and Raman spectroscopic experiments on a number of compositions in the system Bi(NO₃)₃· 4.5H₂O–(H,NH₄)(F,NO₃)· 1.5H₂O indicate the presence of nitrate ions convalently bonded to bismuth. The Raman spectra show a loss of degeneracy of the internal ν₃ and ν₄ nitrate vibration modes in the 1300 and 700 cm^–1 regions, respectively, and a residual contribution to the ν₁ band of NO₃^– at 1048 cm^–1 due to direct bismuth–nitrate interactions. A weak polarized band at 240 cm^–1 is assigned to a Bi—O₂NO stretching vibration. Intensity correlations in the 1050 cm^–1 region yield a coordination number of four for the bismuth nitrate complex in an acidic melt. The quantitative analysis suggests that nitrate ions are released upon formation of bismuth hydroxide and fluoride complexes. Results of the structural investigation on the [Bi(NO₃)₄(OH₂)₂]^– complex, supported by ab initio calculations, reveal a slightly asymmetric bidentate coordination of NO₃^–., d_Bi–O(NO₃)= 2.52, 2.69 and 4.30 Å, d_Bi–N(NO₃)= 3.08 Å. Rather long Bi—OH₂ contacts (2.90 Å) were obtained. The Bi—Bi distances in Bi₂OH⁵⁺ Bi₂F⁵⁺ are 3.70 Å, implying a bent structure (C_2v symmetry). The large Bi—Bi separations and negligible Bi—Bi orbital overlap (as revealed by ab initio calculations) indicate that these complexes may be stabilized by bridging nitrate ions. Approximate MO calculations suggest that a monodentate bridging configuration is the most stable one.