Bertil Holmberg

The structure of room temperature molten polyiodides

The structure of molten polyiodides, Et3SI x (l) (Et = ethyl; x = 3, 4, 5 and 7), have been investigated by liquid x-ray scattering and Raman spectroscopy techniques at room temperature. The predominant iodine-containing species in Et3SI3(l) is a centrosymmetric I3 - ion with a closest I-I distance of 2·915(2) A. The Raman spectra indicate a large bond flexibility of the triiodide ion. The structural results of the iodine-rich melts Et3SI x (l), x > 3, are consistent with a three-dimensional network of interconnected I3 - ions and I2 molecules. The short-range order bears close similarities to that of pure liquid and solid iodine. The triiodides are on the average solvated by one, two and three iodine molecules for x = 4, 5 and 7, respectively. The coordination mode is flat-on with the I3 - and I2 units almost parallel but slightly tilted away from each other like the nearest-neighbour contacts in pure iodine. Previously published conductivity results are consistent with such a structure model.

Cationic lead(II) hydroxide complexes in molten alkali-metal nitrate

The complexation between lead(II) and hydroxide has been studied in molten (K,Na)NO3 at 280 °C by various techniques. An inert platinized platinum wire bathed with a gaseous mixture of oxygen and water served as a hydroxide electrode in potentiometric determinations of the hydroxide activity in the melts. Only PbOH+ and Pb2OH3+ can be detected at the low hydroxide concentrations used. Raman spectra and 1H NMR experiments gave strong indications that hydroxide retains the proton also when coordinated to lead(II). Oxide ions added to the melts readily transform into hydroxide even at very low water activities. Studies of the solubility of Pb(NO3)2 in melts with various concentrations of dissolved PbO show the formation of [PbOH+]q-species at fairly high hydroxide concentrations. Liquid X-ray scattering data yield q= 2 at these hydroxide concentrations, i.e. Pb2(OH)2+2 is the predominant complex ion. The Pb—O and Pb—Pb distances are 2.7 and 3.4 A, respectively. The short Pb—Pb distance is the same as that observed in the halide species Pb2X3+, and it is suggested that the complex is stabilized by a Pb—Pb interaction.

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

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–., dBi–O(NO3)= 2.52, 2.69 and 4.30 A, dBi–N(NO3)= 3.08 A. Rather long Bi—OH2 contacts (2.90 A) were obtained. The Bi—Bi distances in Bi2OH5+ Bi2F5+ are 3.70 A, implying a bent structure (C2v 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.

Cationic lead(II) halide complexes in molten alkali-metal nitrate. Part 3.—The structure of Pb2X3+ and the solvated PbII ion, determined by liquid X-ray scattering and raman spectroscopy

Liquid X-ray scattering and Raman spectroscopy measurements have been made at 280 °C on binary and ternary mixtures in the system PbX2–Pb(NO3)2–(K,Na)NO3, where X = F, Cl, or Br. The furnaces and experimental techniques are described. The nitrate coordination of Pb(II) is shown to be Pb(NO3)2–4 in halide-free melts of Pb(NO3)2–(K,Na)NO3. The nitrate ions are asymmetrically bound via two oxygen atoms at distances of 2.4 and 3.1 A and the lead(II) atoms is positioned slightly out of the nitrate plane. In melts containing halide ions X– complexes of formal composition Pb2X3+ are formed, with Pb—X—Pb angles of 84 and 81° for Pb2Cl3+ and Pb2Br3+, respectively. In Pb2F3+ only the Pb—Pb distance can be experimentally determined. The Pb—F—Pb angle ranges between 96 and 139°. A remarkable characteristic of these complexes is the occurrence of very shot Pb—Pb distance of 3.4–3.8 A, suggesting a significant metal–metal interaction.

The crystal structure of Ag2IF · H2O—A compound containing Ag2+2 pairs

Abstract Ag 2 IF · H 2 O is monoclinic, space group P 2 1 , with a = 4.7206(5), b = 7.8117(8), c = 6.3747(4) A˚, β = 93.345(9)°, Z = 2, D m = 5.35, and D x = 5.37Mg m −3 . The structure was determined from single-crystal X-ray and neutron diffractometer data. X-Ray data were refined anisotropically to an R value of 0.033. The Ag atoms are grouped together in Ag 2+ 2 pairs with very short (2.81A˚) Ag Ag distances. X-Ray photoelectron spectra do not reveal, however, any significant effect on the Ag electron-binding energy due to a possible metallic Ag Ag interaction. The I atom has four nearest Ag neighbors (two Ag 2 pairs) describing a distorted square pyramid with the I atom in the top. These square pyramids are linked up to chains via shared edges. Each Ag atom has a tetrahedral anionic surrounding of O, F, and I atoms. These tetrahedra share edges and corners, creating sheets which are linked by a system of comparably short hydrogen bonds between the O and F atoms.

Cationic lead(II) halide complexes in molten alkali-metal nitrate. Part 1.—A thermodynamic investigation of the fluoride system

The complexation of fluoride ions with lead (II) ions in molten equimolar (K, Na)NO3 has been investigated in the temperature range 240–280 °C. Complexes with formal compositions PbF2, PbF+ and Pb2F3+ were identified and the stability constants for these species were determined at five temperatures from fluoride activity measurement in the systems (K+, Na+, Pb2+)–(NO–3, F–) by means of a fluoride ion-selective electrode. ΔH°mn and ΔS°mn for the stepwise formation of PbmF2m-nn have been estimated, yielding : ΔH°12=– 7.0 kJ mol–1, ΔS°12= 38.3 J K–1 mol–1 for PbF++ F–→ PbF2; ΔH°11=– 15.3 kJ mol–1, ΔS°11= 30.0 J K–1mol–1 for Pb2++ F–→ PbF+; ΔH°21=– 5.9 kJ mol–1, ΔS°21= 1.9 J K–1 mol–1 for PbF++ Pb2+→ Pb2F3+. The thermodynamic results deviate from what is predicted by a quasi-lattice ideal configuration. These deviations are assumed to arise from coordination effects between lead (II) and nitrate and Pb–Pb interactions in Pb2F3+. The solubility of PbF2 in excess of Pb(NO3)2 in (K, Na)NO3(l) was measured at 280 °C. The complexation model derived from the e.m.f. measurements in dilute melts, expressing all activities in mole-fraction units, does accurately describe the solubility of PbF2 in the range CPbCF. In melts with an excess of fluoride over lead(II) the model fails though, probably because polynuclear, i.e. polyfluoric, species are formed.

Cationic lead(II) halide complexes in molten alkali-metal nitrate. Part 2.—A thermodynamic investigation of the chloride, bromide and iodide systems

The complex formation between lead (II) ions and chloride, bromide and iodide ions in molten equimolar (K, Na) NO3 has been studied. The systems were investigated at four temperatures between 240 and 300 °C. The iodide system was only studied at one temperature, since I– is oxidized to I2 in melts rich in Pb(NO3)2. Halide-ion activities were measured in the systems (K+, Na+, Pb2+)–(NO–3, X–), X–= Cl–, Br– and I–, by means of Ag/AgX electrodes. The formation of PbX2, PbX+ and Pb2X3+ was observed. Special interest was focused on the formation of cationic complexes with anions as coordination centres. The thermodynamic parameters ΔH°mn and ΔS°mn have been evaluated for the chloride and bromide systems. The results are : ΔH°11=– 7.9 kJ mol–1, ΔS°11= 26.3 J K–1mol–1 for Pb2++ Cl –→ PbCl+; ΔH°11=–4.6 kJ mol–1, ΔS°11= 35.2 J K–1 mol–1 for Pb2++ Br–→ PbBr+; ΔH°21= 1.5 kJ mol–1, ΔS°21= 16.1 J K–1 mol–1 for PbCl++ Pb2+→ Pb2Cl3+; ΔH°21=–7.1 kJ mol–1, ΔS°21= 1.9 J K–1 mol–1 for PbBr++ Pb2+→ Pb2Br3+. The parameters are compared with previous literature data and recent results obtained from the lead (II) fluoride system in (K, Na)NO3(l).

Speciation, structural characteristics and proton dynamics in the systems NH4NO3· 1.5H2O and NH4NO3· 1.5H2O–(HNO3, NH4F, NH3)–H2O at 50 °C

In order to obtain a good basis for exploring metal-ion complex formation in molten NH4NO3· 1.5H2O, some fundamental characteristics of the pure hydrous melt and a number of compositions in the NH4NO3· 1.5H2O–(HNO3, NH4F, NH3)–H2O system, have been investigated at 50 °C. Several aspects have been taken into consideration, e.g. thermodynamics of solvent autoprotolysis and HF formation, dynamics of proton exchange and structural properties. The acid dissociation constant of NH+4, Ka, and the equilibrium constant for formation of HF, KHF, were obtained from potentiometric measurements; Ka=(2.2 ± 0.2)× 10–9(mol kg–1)2 and KHF= 2160 ± 40 (mol kg–1)–1. Results from 19F NMR spectroscopy indicate that unprotonated fluoride, F–, probably exists as an H3NH+⋯F– ion pair in the solvent. The change in the 19F chemical shift with increasing HNO3 content in (NH4NO3–NH4F–HNO3)· 1.5H2O verifies the conclusion from potentiometric data that HF is the only proteonated fluoride species present. Raman spectroscopy and 14N NMR experiments give clear evidence for an increased tendency to NH+4⋯NO–3 ion-pair formation with decreasing water content in the systems NH4NO3–H2O. However, no loss of degeneracy of the internal ν3 and ν4 nitrate bands at 1380 and 718 cm–1, respectively, was observed. The D3h symmetry of NO–3 seems to be preserved in the NH4NO3· 1.5H2O melt. Results from Raman scattering, 1H NMR and 14N NMR experiments show significant changes in the spectra upon acidification with HNO3. These observations suggest an increase in hydrogen-bonding ability with increasing acidity. Results from large-angle X-ray scattering experiments on NH4NO3· 1.5H2O cannot be explained by a model comprising only interactions between water molecules and ions. A residual contribution to the overall radial electron density distribution at 1.8 A is tentatively assigned to remarkably short N(NH4)–O(NO3) distances. 1H NMR spectroscopy shows a strong retardation of the proton exchange between NH+4 and H2O in the acidic region. The rate constant, kH, for the proton-exchange step H3N ·HOH(OH2)s–1+ H2O → H3N ·(OH2)s+HOH, is estimated at (4.3 ± 1.5)× 107 s–1.

On the structure of Hg2I3+ in dimethylsulfoxide and aqueous solution. An X-ray scattering and raman spectroscopic study

Abstract Dimethylsulfoxide and aqueous solutions of mercury(II) in large excess over iodide have been investigated by X-ray scattering techniques supported by Raman spectroscopic measurements. The composition of the solutions has been selected to ensure that the cationic complex Hg 2 I 3+ is the predominant iodide species. The structure parameters of the solvated Hg 2 I 3+ ion have been refined by a least- squares procedure on the scattering data, using known structural parameters for the additional molecular entities present. The Hg 2 I 3+ entity is more or less identical in DMSO and water. The HgI bond distance is 2.613(12) and 2.632(5) A and the HgHg distance is 3.66(5) and 3.70(1) A in DMSO and water, respectively. This yields a HgIHg angle of 89° in both solvents. The mercury(II) atom in this complex is most probably solvated in a tetrahedral fashion by three DMSO or H 2 O molecules. The structure of Hg 2 I 3+ is discussed in the light of recent results for the Ag 4 I 3+ complex in solution and relevant crystal structures.

Thermodynamics of formation and the structure of polymetal alkaline-earth-metal(II) fluoride complexes in molten-nitrate media

The complexation reactions between alkaline-earth-metal ions, Mg2+, Ca2+, Sr2+ and Ba2+, and fluoride in molten equimolar (K,Na)NO3 have been studied by potentiometric titrations at temperatures between 240 and 300 °C, using a fluoride ion-selective electrode. A structural investigation at 280 °C was performed by detecting the liquid X-ray scattering from melts containing Sr2+ or Ba2+, with and without fluoride present.Equilibrium constants were determined at temperatures between 240 and 300 °C. No polymetal complexes were detected in the magnesium(II) and calcium(II) systems owing to the high stability of the polyfluoride species MgF–3, MgF2 and CaF2. The standard enthalpy and entropy changes for the stepwise formation of MmF2m– 1 in the strontium(II) and barium(II) systems have been evaluated from the temperature dependence of ΔG°m1, yielding: ΔH°11=–9.5 kJ mol–1, ΔS°11= 25 J K–1 mol–1 for Sr2++ F–→ SrF+; ΔH°21=–27 kJ mol–1, ΔS°21=–30 J K–1 mol–1 for SrF++ Sr2+→ Sr2F3+; ΔH°11=–4.7 kJ mol–1, ΔS°11= 26 J K–1 mol–1 for Ba2++ F–→ BaF+; ΔH°21=–20 kJ mol–1, ΔS°21=–21 J K–1 mol–1 for BaF++ Ba2+→ Ba2F3+. These results are compared with the predictions of the statistical quasi-lattice model for specific association in ionic melts. The structural investigation shows that new peaks arise in the radial distribution functions following the addition of fluoride to the Sr2+- and Ba2+-containing nitrate melts. The new peaks at 4.1 and 4.3 A in the Sr and Ba systems, respectively, are considered as metal–metal correlations due to the presence of the M2F3+ complexes in the melts.The thermodynamic and structural parameters indicate that the dimetal complexes M2F3+, M = Sr or Ba, might be stabilized by direct M–M interactions and/or bridging nitrates.