Definitions

Hexafluoride

Hexafluoride

[hek-suh-floor-ahyd, -flawr-, -flohr-]
A hexafluoride is a chemical compound with the general formula XF6. Sixteen elements are known to form stable hexafluorides. Nine of these elements are transition metals, three are actinides, and four are nonmetals or metalloids.

Most hexafluorides are molecular compounds with low melting and boiling points. Four hexafluorides (S, Se, Te, and W) are gases at room temperature (25 °C) and a pressure of 1 atm, one is a liquid (Re), and the others are volatile solids. The p-block and group 6 hexafluorides are colorless, but the other hexafluorides have colors ranging from yellow to orange, red, brown, and black.

The molecular geometry is generally octahedral, with the exception of XeF6, which is a fluxional molecule with a distorted octahedral structure which, according to valence shell electron pair repulsion theory, is caused by the non-bonding lone pair. In the solid state, XeF6 has a complex structure involving tetramers and hexamers. According to quantum chemical calculations, ReF6 and RuF6 should have tetragonally distorted structures (where two of the bonds along one axis are longer or shorter than the other four), but this has not been verified experimentally.

The hexafluorides have a wide range of chemical reactivity. Sulfur hexafluoride is nearly inert and non-toxic. It has several applications due to its stability, dielectric properties, and high density (it is the densest non-toxic gas). Selenium hexafluoride is nearly as unreactive as SF6, but tellurium hexafluoride is toxic, not very stable and can be hydrolyzed by water within 1 day. In contrast, metal hexafluorides are corrosive, readily hydrolyzed and may react violently with water. Some of them can be used as fluorinating agents. The metal hexafluorides have a high electron affinity, which makes them strong oxidizing agents. Platinum hexafluoride in particular is notable for its ability to oxidize the dioxygen molecule, O2, to form dioxygenyl hexafluoroplatinate, and for being the first compound that was observed to react with xenon (see xenon hexafluoroplatinate).

Some metal hexafluorides find applications due to their volatility. Uranium hexafluoride is used in the uranium enrichment process to produce fuel for nuclear reactors. Fluoride volatility can also be exploited for nuclear fuel reprocessing. Tungsten hexafluoride is used in the production of semiconductors through the process of chemical vapor deposition.

The table below lists the main physical and structural properties of the hexafluorides.

Compound m.p (°C) b.p. (°C) MW solid ρ (g cm−1) Bond (pm) Color
Sulfur hexafluoride −50.54 −63.8 (sublimes) 146.06 1.88 (−50 °C) 156.4 colorless
Selenium hexafluoride −35 (2 atm) −47 (sublimes) 192.9534 167–170 colorless
Tellurium hexafluoride −38 −39 241.59 184 colorless
Xenon hexafluoride 49.25 75.6 245.28 3.56 colorless
Molybdenum hexafluoride 17.4 34 209.94 3.50 (−140 °C) 181.7 colorless
Technetium hexafluoride 37.4 55.3 (212) 3.58 (−140 °C) 181.2 yellow
Ruthenium hexafluoride 54 215.07 3.68 (−140 °C) 181.8 dark brown
Rhodium hexafluoride 70 216.91 3.71 (−140 °C) 182.4 black
Tungsten hexafluoride 1.9 17.1 297.85 4.86 (−140 °C) 182.6 colorless
Rhenium hexafluoride 18.5 33.7 300.2 4.94 (−140 °C) 182.6 yellow
Osmium hexafluoride 33 304.2 5.09 (−140 °C) 182.9 yellow
Iridium hexafluoride 44 53 306.2 5.11 (−140 °C) 183.4 yellow
Platinum hexafluoride 61.3 69.1 309.1 5.21 (−140 °C) 184.8 deep red
Uranium hexafluoride 64 352.0 white
Neptunium hexafluoride 54.7 (358) orange
Plutonium hexafluoride 52 (356) brown

References

  • Drews, T; Supeł, J; Hagenbach, A; Seppelt, K (2006). "Solid state molecular structures of transition metal hexafluorides". Inorganic chemistry 45 (9): 3782–3788.
  • Kirss, Rein U., Lamartine Meda. "Chemical Vapor Deposition of Tungsten Oxide." Applied Organometallic Chemistry 12 (1998): 155–160.
  • Bartlett, N. "The Oxidizing Properties of the Third Transition Series Hexafluorides and Related Compounds". Angewandte Chemie International Edition in English 7 (6): 433-439.
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