Molybdenum hexafluoride

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 and boils in 34 °C.[3] It is one of the seventeen known binary hexafluorides.

Molybdenum hexafluoride
Names
IUPAC names
molybdenum(VI) fluoride
Other names
molybdenum hexafluoride
Identifiers
3D model (JSmol)
ECHA InfoCard 100.029.114
EC Number
  • 232-026-5
UNII
Properties
MoF6
Molar mass 209.93 g/mol
Appearance white crystals[1] or colorless liquid
hygroscopic
Density 3.50 g/cm3[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]
hydrolyzes
−26.0·10−6 cm3/mol
Structure
Orthorhombic, oP28
Pnma, No. 62
octahedral (Oh)
0
Related compounds
Other cations
 Tungsten hexafluoride
Uranium hexafluoride
Molybdenum(VI) chloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

Synthesis

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

Mo + 3 F
2 → MoF
6

Typical impurities are MoO2F2 and MoOF4, reflecting the tendency of the hexafluoride to hydrolyze.[4]

Description

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 Oh. 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 978-0-08-037941-8.
  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 Archived 2011-06-12 at the Wayback Machine, 10 August 1993.
  7. US-Patent 6896866: Method for Purification of Tungsten Hexafluoride Archived 2011-06-12 at the Wayback Machine, 24 May 2005.
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