Metal Hexafluorides Research Articles

Structure and Excitation Spectra of Third-Row Transition Metal Hexafluorides Based on Multi-Reference Exact Two-Component Theory.

The structures and some vertical excitation energies of third-row transition metal hexafluorides (MF6, M = Re, Os, Ir, Pt, Au, Hg) were calculated using the generalized-active-space configuration interaction (GASCI) theory based on the exact two-component (X2C) Hamiltonian. The spin-orbit coupling (SOC) was included at the Hartree-Fock level, enabling us to analyze the SOC at the orbital level (spinor-representation). The excitation spectra were assigned based on the double group, a relativistic group theory applicable to states with the SOC. This study provides a fundamental understanding of the ligand field splitting, including the SOC, that is useful for the photochemistry and spin chemistry involving heavy elements.

Deposition products predicted from conceptual DFT: The hydrolysis reactions of MoF6, WF6, and UF6.

Metal hexafluorides hydrolyze at ambient temperature to deposit compounds having fluorine-to-oxygen ratios that depend upon the identity of the metal. Uranium-hexafluoride hydrolysis, for example, deposits uranyl fluoride (UO2F2), whereas molybdenum hexafluoride (MoF6) and tungsten hexafluoride deposit trioxides. Here, we pursue general strategies enabling the prediction of depositing compounds resulting from multi-step gas-phase reactions. To compare among the three metal-hexafluoride hydrolyses, we first investigate the mechanism of MoF6 hydrolysis using hybrid density functional theory (DFT). Intermediates are then validated by performing anharmonic vibrational simulations and comparing with infrared spectra [McNamara et al., Phys. Chem. Chem. Phys. 25, 2990 (2023)]. Conceptual DFT, which is leveraged here to quantitatively evaluate site-specific electrophilicity and nucleophilicity metrics, is found to reliably predict qualitative deposition propensities for each intermediate. In addition to the nucleophilic potential of the oxygen ligands, several other contributing characteristics are discussed, including amphoterism, polyvalency, fluxionality, steric hindrance, dipolar strength, and solubility. To investigate the structure and composition of pre-nucleation clusters, an automated workflow is presented for the simulation of particle growth. The workflow entails a conformer search at the density functional tight-binding level, structural refinement at the hybrid DFT level, and computation of a composite free-energy profile. Such profiles can be used to estimate particle nucleation kinetics. Droplet formation is also considered, which helps to rationalize the different UO2F2 particle morphologies observed under varying levels of humidity. Development of predictive methods for simulating physical and chemical deposition processes is important for the advancement of material manufacturing involving coatings and thin films.

Potential rules for stable transition metal hexafluorides with high oxidation states under high pressures.

High pressure is a powerful tool in material sciences which can lead to the discovery of novel inorganic species in high oxidation states. Based on the prediction of the stability of PdF6 with a high Pd oxidation state of +6, we propose three potential guiding rules for finding stable transition metal (TM) fluorides with high +6 oxidation states: (1) the existence of a large (>7 eV) valence orbitals energy differences of atoms between the TM d orbital and the F 2p orbital; (2) an appropriate number of valence electrons within the range of 6-11; and (3) suitable electronegativity values less than 2.3 on the Pauli scale. More importantly, by synergistically invoking all of these rules, we predict, by combining a particle swarm optimization algorithm with first-principles calculation on the phase stabilities of the various TM-F compounds, a collection of new TMF6 species with the space group Pnma that have a +6 oxidation state. Subsequently, we develop an understanding of the high +6 oxidation state for the TM elements. These findings are expected to play a crucial role in the predictive discoveries of new fluorides with high oxidation states of +6.

Investigating the hydrolysis of cryogenically layered molybdenum hexafluoride through a disordered hydrogen-bonded network.

Molybdenum hexafluoride (MoF6) is used as a non-radioactive substitute for uranium to study the hydrolysis of metal hexafluorides. Molybdenum hexafluoride gas and water vapor, from the air, were sequentially layered onto a diamond substrate kept at liquid nitrogen temperature using a custom designed cryogenic cell with a copper cold finger. Reaction progress was monitored by transmission Fourier Transform Infrared Spectroscopy (FTIR) through the layers and diamond substrate over several hours while allowing the substrate to warm. Changes in the modes in the 500-1000 cm-1 region are tracked as the reaction progresses in order to identify intermediate species. Strong absorption features are also observed in the 1000-3000 cm-1 range, suggesting the presence of ionic dissociation intermediates trapped in a disordered H-bonded network of cryogenic hydrofluoric acid. A possible reaction pathway is proposed and the final hydrolysis product is characterized by FTIR, UV-vis, and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS).

Hydraulic Resistance of Fine Packing in Uranium Hexafluoride Rectification

The hydraulic resistance of irregular fi ne packing is studied as a function of the process parameters for the rectification of metal hexafluorides. It is shown that the dependence of the hydraulic resistance of irregular fi ne packing on the rectification parameters coincides with the dependence presented in the literature but with different coefficients and exponential factors.

IrF8 Molecular Crystal under High Pressure.

An important goal in chemistry is to prepare F-rich transition metal fluorides due to the high oxidation states and potential applications such as oxidating and fluorinating agents. Thus far, the highest F stoichiometry in the neutral transition metal fluorides is 7. Here, we identify a hitherto unknown IrF8 compound through first-principles swarm-intelligence structure search calculations under high pressure. The three identified IrF8 phases exhibit typical molecular crystal characters, showing +8 oxidation state in Ir. The spatial symmetry of the basic building block in the three IrF8 phases gradually increases with pressure (e.g., dodecahedron [Formula: see text] square antiprism [Formula: see text] quasicube). The pressure-induced faster increase of Ir 5d orbital energy level with respect to F 2p provides a strong charge transfer driving force from Ir 5d to F 2p, facilitating the formation of F-rich compounds. More interestingly, the predicted electron affinities of the three predicted IrF8 phases are comparable/larger than that of PtF6, the strongest oxidation agent in the third row transition metal hexafluorides. The built high-pressure phase diagram of Ir-F binary compounds provides useful information for experimental synthesis.

Electron Affinities, Fluoride Affinities, and Heats of Formation of the Second Row Transition Metal Hexafluorides: MF6(M = Mo, Tc, Ru, Rh, Pd, Ag)

High-level electronic structure calculations were used to evaluate reliable, self-consistent thermochemical data sets for the second row transition metal hexafluorides. The electron affinities, heats of formation, first (MF(6) --> MF(5) + F) and average M-F bond dissociation energies, and fluoride affinities of MF(6) (MF(6) + F(-) --> MF(7)(-)) and MF(5) (MF(5) + F(-) --> MF(6)(-)) were calculated. The electron affinities are higher than those of the corresponding third row hexafluorides, making them stronger one-electron oxidizers. The calculated electron affinities, in good agreement with the available experimental values, are 4.23 eV for MoF(6), 5.89 eV for TcF(6), 7.01 eV for RuF(6), 6.80 eV for RhF(6), 7.95 eV for PdF(6), and 8.89 eV for AgF(6). The corresponding pentafluorides are also very strong Lewis acids, although their acidities on the pF(-) scale are about one unit lower than those of the third row pentafluorides. The performance of a wide range of DFT exchange-correlation functionals was benchmarked by comparing them to our more accurate CCSD(T) results.

ChemInform Abstract: Third Row Transition Metal Hexafluorides, Extraordinary Oxidizers, and Lewis Acids: Electron Affinities, Fluoride Affinities, and Heats of Formation of WF6, ReF6, OsF6, IrF6, PtF6, and AuF6.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Third Row Transition Metal Hexafluorides, Extraordinary Oxidizers, and Lewis Acids: Electron Affinities, Fluoride Affinities, and Heats of Formation of WF6, ReF6, OsF6, IrF6, PtF6, and AuF6

High level electronic structure calculations were used to evaluate reliable, self-consistent thermochemical data sets for the third row transition metal hexafluorides. The electron affinities, heats of formation, first (MF(6) --> MF(5) + F) and average M-F bond dissociation energies, and fluoride affinities of MF(6) (MF(6) + F(-) --> MF(7)(-)) and MF(5) (MF(5) + F(-) --> MF(6)(-)) were calculated. The electron affinities which are a direct measure for the oxidizer strength increase monotonically from WF(6) to AuF(6), with PtF(6) and AuF(6) being extremely powerful oxidizers. The inclusion of spin orbit corrections is necessary to obtain the correct qualitative order for the electron affinities. The calculated electron affinities increase with increasing atomic number, are in good agreement with the available experimental values, and are as follows: WF(6) (3.15 eV), ReF(6) (4.58 eV), OsF(6) (5.92 eV), IrF(6) (5.99 eV), PtF(6) (7.09 eV), and AuF(6) (8.20 eV). A wide range of density functional theory exchange-correlation functionals were also evaluated, and only three gave satisfactory results. The corresponding pentafluorides are extremely strong Lewis acids, with OsF(5), IrF(5), PtF(5), and AuF(5) significantly exceeding the acidity of SbF(5). The optimized geometries of the corresponding MF(7)(-) anions for W through Ir are classical MF(7)(-) anions with M-F bonds; however, for PtF(7)(-) and AuF(7)(-) non-classical anions were found with a very weak external F-F bond between an MF(6)(-) fragment and a fluorine atom. These two anions are text book examples for "superhalogens" and can serve as F atom sources under very mild conditions, explaining the ability of PtF(6) to convert NF(3) to NF(4)(+), ClF(5) to ClF(6)(+), and Xe to XeF(+) and why Bartlett failed to observe XePtF(6) as the reaction product of the PtF(6)/Xe reaction.

Solubility of fluorine in liquid uranium, tungsten, and molybdenum hexafluorides

Experimental data on the solubility of fluorine in liquid uranium, tungsten, and molybdenum hexafluorides in the temperature range 66‐90°C and fluorine partial pressure 226‐1066 kPa are presented. Computed values of Henry’s constant for these systems and constants in the equation expressing the linear approximation of the temperature dependence of Henry’s constant are given. Uranium hexafluoride exists as a liquid only at temperature and pressure above the triple point, i.e., 64.05°C and 0.155 MPa. Since it is obtained for commercial purposes by means of the interaction of uranium tetrafluoride with excess fluorine gas [1], liquefaction from commercial gas from fluoridation apparatus can be done only at high pressure and in the process fluorine becomes dissolved in the liquid uranium hexafluoride. For this reason, the solubility of fluorine in liquid uranium hexafluoride affects the choice of the technological arrangement of any facility that is used to liquefy uranium hexafluoride from commercial gases containing fluorine. The objective of the present work is to determine the solubility of gaseous molecular fluorine in liquid metal hexafluorides. The solubility of fluorine in liquid uranium, tungsten, and molybdenum hexafluorides was determined using the apparatus shown schematically in Fig. 1. The nickel vessel 13 with a copper cover 4 had a magnetic mixer 10, placed in plain Teflon bearings 6 and finished on top by a bar made of a ferromagnetic material in aluminum casing 7. The mixer was put into motion by a permanent magnet 3, placed outside and rotated by an electric motor 1 with frequency ranging from 3 to 15 rpm. Three thermocouple pockets 9, made of nickel tubing with 0.2 mm thick wall were uniformly arranged over the height of the vessel. Two couplings with small, nickel, packed gland valves were present on the vessel wall. The initial components were loaded through one of them 5. First, the setup was evacuated, checked for vacuum, and passivated with fluorine gas at 100°C. Metal hexafluorides were loaded by means of condensation into a nickel vessel at liquid-nitrogen temperature. A standard manometer 2 was used to monitor the progress of the loading. The amount of condensed metal hexafluoride permitted the existence of at least 1 cm 3 of vapor above the liquid at the temperature of the experiment. When condensation was completed, fluorine was introduced into the vessel. Next, the vessel 13 with a differential manometer 8 was placed into a water thermostat 14 ,w here the temperature was maintained to within ±0.2°. The total pressure in the vessel was measured with a differential manometer 8. Samples of the liquid metal hexafluoride were extracted every 20 min into the space between the valves 11, whence the liquid sample was completely evaporated into the vessel 12 and delivered in the gas phase to a chromatograph for analysis. Equilibrium was considered to be attained when the solubility with decreasing gas pressure was the same as with increasing gas pressure. Methods for purifying the initial substances and checking their purity as well as the technique for performing chromatographic analysis of extracted samples are described in [2]. The computed error in determining the fluorine concentration did not exceed 0.1%.

The transition metal hexafluorides

The nine transition metal hexafluorides present a unique series of closely related compounds. Deviations from octahedral structures are influenced by Jahn-Teller effects and spin orbit coupling. The most pronounced chemical behaviour is the increasing electron affinity in the direction WF(6)--> PtF(6) and MoF(6)--> RuF(6), so that with PtF(6) and RuF(6) values of 7 eV or higher are reached. All hexafluorides can be used as one electron oxidants, starting with mild (WF(6)) up to extreme (PtF(6), RuF(6)) oxidation power.

Investigation of Luminescence from Dy3 + in AlN

The preparation of powder samples of aluminum nitride activated by dysprosium, , was achieved for the first time using a low-temperature, solution-based approach. Aqueous solutions of aluminum and dysprosium nitrates was first converted to hydroxides and then to ammonium–metal hexafluorides (. Finally, this complex ammonium salt was converted to finely divided powders of via a pyrolysis reaction in a high-purity ammonia flow inside a tubular quartz furnace at . The phase purity and crystallinity of the material were determined using X-ray diffraction and energy dispersive spectroscopy. The intraconfigurational transitions of were investigated using photoluminescence (PL) and photoluminescence excitation (PLE) measurements. The excited states of in this large-gap nitride were identified from a careful analysis of PL and PLE results. It was also observed from the PLE measurements that the ions in this host are excited through energy transfer from both the excited state of the host lattice and defects. Measurements using site-selective spectroscopy suggest multiple sites for ions in AlN. A theoretical model describing the excitation process and saturation of excited states of at higher intensity of the exciting radiation is also proposed.

Solid State Molecular Structures of Transition Metal Hexafluorides.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Solid State Molecular Structures of Transition Metal Hexafluorides

Single-crystal structure determinations of all nine transition metal hexafluorides (Mo, Tc, Ru, Rh, W, Re, Os, Ir, and Pt) at -140 degrees C are presented. All compounds crystallize alike and have the same molecular structure. The bond length sequence r(w-F) congruent with r(Re-F) congruent with r(Os-F) < r(Ir-F) < r(Pt-F) is confirmed and paralleled by the sequence r(Mo-F) congruent with r(Tc-F) congruent with r(Ru-F) < r(Rh-F). Within the limits of precision, no systematic deviation from octahedral symmetry can be established. DFT and ab initio calculations predict octahedral structures for MoF6 and RhF6 and tetragonally distorted structures for ReF6 and RuF6. The energy barrier toward octahedral structures is only 2.5 kJ mol(-1) in the two latter cases. Calculated electron affinities are in the sequence MoF6 < TcF6 < RhF6 < RuF6 with a value of 6.98 eV for the latter. O2+RhF6- crystallized in an undisordered manner in P, isostructural to the low-temperature form of O2+AuF6-. RhF6- has a D4h compressed octahedral structure, while AuF6- is essentially octahedral. The absorption spectrum of TcF6 and the 19F and 195NMR spectra of PtF6 are presented.

The reactions of the third transition series metal hexafluorides, MF 6, M = Pt, Ir, Os, Re and W, with carbon monoxide in superacid media

The reactions of the third transition (5d) series metal hexafluorides PtF 6, IrF 6, OsF 6, ReF 6 and WF 6 with CO are studied in the Lewis superacid SbF 5, the Brønsted superacid HF and the conjugate Brønsted–Lewis superacid HF–SbF 5, in order to evaluate their potential as reactants in reductive carbonylations. These reactions are aimed at the generation of homoleptic metal carbonyl cations of the type [M(CO) n ] m+ , M = Pt, Ir, Os or Re, n = 4 or 6 and m = 1, 2 or 3. Under very mild conditions, PtF 6 is cleanly converted in liquid SbF 5 to give [Pt(CO) 4][Sb 2F 11] 2 in quantitative yield. In anhydrous hydrogen fluoride, PtF 6 is reduced and partly carbonylated to the mixed valency product [Pt(CO) 4][PtF 6], the first carbonyl fluoride of platinum. With IrF 6 in SbF 5, [Ir(CO) 6][Sb 2F 11] 3 is obtained, which contains with [Ir(CO) 6] 3+ the first and so far only known homoleptic tripositive metal carbonyl cation. The reaction of OsF 6 with CO in HF–SbF 5 produces single crystals of [Os(CO) 6][Sb 2F 11] 2, which are suitable for a molecular structure determination. ReF 6 reacts in SbF 5 very slowly with CO, and the previously known [Re(CO) 6] + is only obtained in low yield. Finally WF 6 does not react with CO in any of the superacid media under comparable conditions. As by-product in the reduction of MF 6, M = Pt, Ir and Os, only carbonyl fluoride, COF 2, forms, which allows very facile product isolation. The observed order of reactivity: PtF 6 > IrF 6 > OsF 6 > ReF 6 > > WF 6 follows a previously established order of oxidizing ability and electron affinity of the metal hexafluorides. The mixed valency compound [Pt(CO) 4][PtF 6] is studied by vibrational spectroscopy and is found to contain a square planar [Pt(CO) 4] 2+ cation and a [PtF 6] 2− anion with a trigonally distorted octahedral geometry of pointgroup D 3d. A vibrational assignment of [Ir(CO) 6] 3+ is presented, aided by 13C substitution and supported by ab initio calculations.

Time-of-flight neutron powder diffraction study on the third row transition metal hexafluorides WF6, OsF6, and PtF6

A neutron diffraction study on the third-row transition metal hexafluorides MF6 (M≡W, Os, Pt) has been performed using the high resolution neutron powder diffractometer (HRPD) at the spallation source ISIS, England. The previously unknown structures of the low-temperature phases of OsF6 and PtF6 are reported. WF6, OsF6, and PtF6, which exhibit a (5dt2g)0, (5dt2g)2, and (5dt2g)4 electronic configuration, respectively, are found to be isostructural and crystallize in the UF6 structure, space group Pmnb, (No. 62). The geometry of the MF6 molecules is to good approximation octahedral for each compound, the mean M–F bond length increasing only slightly from 182.5 (W) to 185.0 (Pt). For WF6 deviations from ideal octahedral geometry are only marginally significant [181.8(2) to 183.2(2) pm] and may be interpreted on the basis of packing effects. Deviations for the d2 complex OsF6 are somewhat larger [181.5(2) to 184.4(3) pm] and may be assumed to be caused by packing effects essentially the same as for WF6, in addition to a first-order Jahn–Teller effect arising from the (5dt2g)2 electronic configuration. While eliminating the effects of packing by a comparison of individual M–F bond lengths for WF6 and OsF6, the OsF6 molecule shows to have D4h symmetry with two apical M–F bonds about 1.8 pm longer than the four equatorial bonds as a result of the Jahn–Teller distortion. Only small deviations from ideal octahedral geometry [184.4(3) to 185.8(3) pm] are found for the d4 complex PtF6. Within the series W to Pt a substantial shortening of the F...F van der Waals contact distances is observed. This shortening more than compensates for the increase in the M–F bond lengths and leads to unit cell volumes and cell parameters decreasing continuously from W to Pt. The variation of F...F contact distances and M–F bond lengths may be rationalized in terms of polarization of the F-ligands in the field of the highly charged nuclei of the central atoms which are only incompletely shielded by the 5d electrons.

Intercalation of fluorometallate anions of Group VI metals and uranium in graphite

Systematic investigations have been made of the formation of graphite fluorometallates of the Group VI transition metals (molybdenum and tungsten) and uranium. The reactions were performed by interacting liquid or gaseous metal hexafluorides in the presence or absence of elemental fluorine at ambient temperature. The degree of intercalation of these metal fluorides depends on the formation enthalpy of the fluorometallate anion from the original metal hexafluoride, as has been found for other intercalation reactions of binary fluorides.

Infrared spectroscopy of fluoride molecules in noble gas solutions

Infrared spectra of the spherical top molecules XF6 (X = S, U, Mo, W) dissolved in liquefied argon, krypton and xenon have been studied. It is shown that the observed Lorentz shape of the bands results from two processes: vibrational dephasing and rotational diffusion. In contrast to fundamental transitions A1g - F1u, for the (v 2 + v 3) state it has proved necessary to apply two limiting cases: when the anharmonic level splitting ΔE anh is much greater than the Coriolis coupling, and when ΔE anh is small relative to the Coriolis coupling. Measured values of band halfwidths are compared with the theoretical values, obtained with the help of a rotational diffusion model. It is concluded that, in contrast to SF6, vibrational dephasing and intermolecular interactions contribute essentially to the formation of band contours of heavy metal hexafluorides. The second moment for the (v 2 + v 3) band has been estimated. The characteristic times of angular momentum correlation for SF6, UF6, MoF6 and WF6 molecules i...

Exafs studies on solutions of metal hexafluorides in anhydrous HF

The metal L(III)-edge EXAFS for the 5 d transition metal hexafluorides in anhydrous HF solution have been recorded for the first time. In addition to the first coordination sphere, additional features in the EXAFS have been satisfactorily modelled resulting from the first HF solvation spheres. These data indicate that significant order exists in HF solution and offer the first estimate of the occupation number of the first solvation sphere for this solvent.

Chromium Site Specific Chemistry and Crystal Structure of LiCaCrF 6 and Related Chromium Doped Laser Materials

The crystal structure of LiCaCrF 6, a fully Cr-substituted lithium metal hexafluoride representative of laser materials with the generic formula Li Me II Me III 1- x Cr x F 6 ( Me II = Ca, Sr; Me III = Al) was determined by X-ray single crystal diffraction and neutron powder profile refinement. LiCaCrF 6 was confirmed as a LiCaAlF 6 (Colquiirite)-type (space group P3̄1 c (No. 163), Z = 2, a = 5.1054(08) Å, c = 9.7786(10) Å, Li in 2 c, Ca in 2 b, Cr in 2 d, and F in 12 i with x, y, z of 0.3652(3), 0.0183(4), 0.1402(1)). Deviation from centrosymmetry ( P31 c; No. 159) and full occupation were examined and appear to be insignificant. We selected LiCaCrF 6 as a model compound for the determination of EXAFS phase shifts to provide Cr site specific information in 3% Cr-doped LiCaAlF 6 laser crystals where Cr occupies a slightly distorted CrF 6 octahedron with a Cr-F distance (1.89(8) Å) practically identical to Cr-F in LiCaCrF 6 (1.901 Å) and CrF 3 (1.899 Å). The pseudobinary laser material LiSrAl 1- x Cr x F 6 forms a true solid solution over the complete composition range 0 ≤ x ≤ 1.