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

Abstract

The formation of lead(II)–hydroxide and –fluoride complexes in NH₄NO₃· 1.5H₂O has been investigated by potentiometric, spectroscopic and X-ray scattering methods at 50 °C. Complexes with the formal composition PbOH⁺, Pb₂OH³⁺, PbF⁺ and Pb₂F³⁺ were detected. The formal stability constants are: β₁₁[PbOH⁺]= 18.8 ± 0.3 kg mol^–1, β₂₁[Pb₂OH³⁺]= 4.3 ± 0.5 kg² mol^–2, β₁₁[PbF⁺]= 7.92 ± 0.05 kg mol^–1 and β₂₁[Pb₂F³⁺]= 2.59 ± 0.06 kg² mol^–2 as derived from potentiometry. The formation of lead fluoride species was also monitored by ¹⁹F NMR experiments, indicating a significant covalent contribution to the Pb—F bonding character. ¹⁴N and ¹⁷O NMR data support the basic assumption that complexes of lead(II) and ammonia are not formed in significant amounts. ¹⁴N NMR and Raman spectroscopy experiments using Pb(NO₃)₂· 3H₂O–NH₄NO₃· 1.5H₂O melts indicate the formation of direct or solvent-separated lead–nitrate ion pairs. Although the lead–nitrate interaction is most likely to be of electrostatic character, the Raman spectra reveal an increased tendency towards splitting of the internal ν₄ nitrate vibrational mode at 718 cm^–1 and the ¹⁴N NMR spectra exhibit a small but significant shift of the nitrate signal and an increase in band halfwidth. A structural model for the Pb²⁺ coordination, including four nitrates and two water molecules in an irregular octahedral arrangement, fits the experimental data well. The nitrate ions adopt an asymmetric bidentate coordination as supported by a geometry optimization on the Hartree–Fock level. The distances obtained by a visual fit based on such a geometrical model to experimental large-angle X-ray scattering (LAXS) data are: Pb—O(NO₃), 2.70 (O_I), 3.10 (O_II) and 4.40 (O_III)Å; Pb—N(NO₃), 3.30 Å; and Pb—O(OH₂), 2.75 Å.