Molybdenum hexafluoride, also molybdenum(VI) fluoride, is the
inorganic compound with the formula MoF
6. It is the highest
fluoride of
molybdenum. It is a colourless solid and melts just below
room temperature and boils in 34 °C. 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:
: + 3 →
Typical impurities are MoO
2F
2 and MoOF
4, reflecting the tendency of the hexafluoride to hydrolyze.
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 unit
In chemistry, a formula unit is the empirical formula of any ionic or covalent network solid compound used as an independent entity for stoichiometric calculations. It is the lowest whole number ratio of ions represented in an ionic compound. E ...
s (in this case, discrete molecules) per
unit cell
In geometry, biology, mineralogy and solid state physics, a unit cell is a repeating unit formed by the vectors spanning the points of a lattice. Despite its suggestive name, the unit cell (unlike a unit vector, for example) does not necessaril ...
, giving a density of 3.50 g·cm
−3.
The fluorine atoms are arranged in the
hexagonal close packing
In geometry, close-packing of equal spheres is a dense arrangement of congruent spheres in an infinite, regular arrangement (or lattice). Carl Friedrich Gauss proved that the highest average density – that is, the greatest fraction of space occu ...
.
In liquid and gas phase, MoF
6 adopt
octahedral molecular geometry with
point group ''
Oh''. The Mo–F
bond length
In molecular geometry, bond length or bond distance is defined as the average distance between nuclei of two bonded atoms in a molecule. It is a transferable property of a bond between atoms of fixed types, relatively independent of the rest of ...
is 1.817 Å.
Applications
Molybdenum hexafluoride has few uses. In the nuclear industry, MoF
6 occurs as an impurity in
uranium hexafluoride
Uranium hexafluoride (), (sometimes called "hex") is an inorganic compound with the formula UF6. Uranium hexafluoride is a volatile white solid that reacts with water, releasing corrosive hydrofluoric acid. The compound reacts mildly with alumin ...
since molybdenum is a
fission product of
uranium
Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...
. It is also an impurity in
tungsten hexafluoride, which is used in the semiconductor industry. MoF
6 can be removed by reduction of a WF
6-MoF
6 mixture with any of a number of elements including
hydrogen iodide at moderately elevated temperature.
[US-Patent 6896866]
Method for Purification of Tungsten Hexafluoride
, 24 May 2005.
References
{{fluorine compounds
Molybdenum(VI) compounds
Hexafluorides
Molybdenum halides
Octahedral compounds