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|
|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|