Molybdenum hexafluoride

Molybdenum hexafluoride

Names
IUPAC names molybdenum(VI) fluoride
Other names molybdenum hexafluoride
Identifiers
CAS Number	7783-77-9
EC Number	232-026-5
Jmol interactive 3D	Image
PubChem	82219
InChI InChI=1S/6FH.Mo/h6*1H;/q;;;;;;+6/p-6
SMILES F[Mo](F)(F)(F)(F)F
Properties
Chemical formula	MoF6
Molar mass	209.93 g/mol
Appearance	white crystals[1] or colorless liquid hygroscopic
Density	3.50 g/cm^3[2]
Melting point	17.5 °C (63.5 °F; 290.6 K)[1]
Boiling point	34.0 °C (93.2 °F; 307.1 K)[1]
Solubility in water	hydrolyzes
Structure
Crystal structure	Orthorhombic, oP28
Space group	Pnma, No. 62
Coordination geometry	octahedral (Oh)
Dipole moment	0
Related compounds
Other cations	Tungsten hexafluoride Uranium hexafluoride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Molybdenum hexafluoride, also molybdenum(VI) fluoride is the inorganic compound with the formula MoF₆. It is the highest fluoride of molybdenum. A colourless solid, it melts just below room temperature. It is highly unstable toward hydrolysis.^[3] It is one of the seventeen known binary hexafluorides.

Synthesis

Molybdenum hexafluoride is made by direct reaction of molybdenum metal in an excess of elemental fluorine gas.^[2]

Mo + 3 F
2 → MoF
6

Typical impurities are MoO₂F₂ and MoOF₄.^[4]

Description

Molybdenum hexafluoride is a liquid at room temperature. It reacts violently with water with release of hydrofluoric acid and molybdenum oxides.^[1]

At −140 °C, it crystallizes in the orthorhombic space group Pnma. Lattice parameters are a = 9.394 Å, b = 8.543 Å, and c = 4.959 Å. There are four formula units (in this case, discrete molecules) per unit cell, giving a density of 3.50 g·cm⁻³.^[2] The fluorine atoms are arranged in the hexagonal close packing.^[5]

In liquid and gas phase, MoF₆ adopt octahedral molecular geometry with point group O_h. The Mo–F bond length is 1.817 Å.^[2]

Applications

Molybdenum hexafluoride has few uses. In the nuclear industry, MoF₆ occurs as an impurity in uranium hexafluoride since molybdenum is a fission product of uranium. It is also an impurity in tungsten hexafluoride, which is used in the semiconductor industry. MoF₆ can be removed by reduction of a WF₆-MoF₆ mixture with any of a number of elements including molybdenum at moderately elevated temperature.^[6][7]

References

1. CRC Handbook of Chemistry and Physics, 90th Edition, CRC Press, Boca Raton, Florida, 2009, ISBN 978-1-4200-9084-0, Section 4, Physical Constants of Inorganic Compounds, p. 4-85.
2. T. Drews, J. Supeł, A. Hagenbach, K. Seppelt: "Solid State Molecular Structures of Transition Metal Hexafluorides", in: Inorganic Chemistry, 2006, 45 (9), S. 3782–3788; doi:10.1021/ic052029f; PMID 16634614
3. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9.
4. W. Kwasnik "Molybdenum(VI) Fluoride" Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 259.
5. J. H. Levy, J. C Taylor, A. B. Waugh: "Neutron Powder Structural Studies of UF₆, MoF₆ and WF₆ at 77 K", in: Journal of Fluorine Chemistry, 1983, 23 (1), pp. 29–36; doi:10.1016/S0022-1139(00)81276-2.
6. US-Patent 5234679: Method of Refining Tungsten Hexafluoride Containing Molybdenum Hexafluoride as an Impurity, 10 August 1993.
7. US-Patent 6896866: Method for Purification of Tungsten Hexafluoride, 24 May 2005.