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 (OI), 3.10 (OII) and 4.40 (OIII)Å; Pb—N(NO3), 3.30 Å; and Pb—O(OH2), 2.75 Å.