Technetium hexafluoride or technetium(VI) fluoride (
Tc F6) is a yellow
inorganic compound with a low
melting point
The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depen ...
. It was first identified in 1961.
In this compound, technetium has an
oxidation state
In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
of +6, the highest oxidation state found in the technetium
halides. In this respect, technetium differs from rhenium, which forms a heptafluoride,
ReF7. Technetium hexafluoride 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 ...
, as technetium 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 ...
(
spontaneous fission in
natural uranium, possible contamination from
induced fission inside the reactor in
reprocessed uranium
Reprocessed uranium (RepU) is the uranium recovered from nuclear reprocessing, as done commercially in France, the UK and Japan and by nuclear weapons states' military plutonium production programs. This uranium makes up the bulk of the material s ...
). The fact that the boiling point of the hexafluorides of uranium and technetium are very close to each other presents a problem in using
fluoride volatility
Fluoride volatility is the tendency of highly fluorinated molecules to vaporize at comparatively low temperatures. Heptafluorides, hexafluorides and pentafluorides have much lower boiling points than the lower- valence fluorides. Most difluor ...
in
nuclear reprocessing.
Preparation
Technetium hexafluoride is prepared by heating technetium metal with an excess of F
2 at 400 °C.
:Tc + 3 →
Description
Technetium hexafluoride is a golden-yellow solid at room temperature. Its melting point is 37.4 °C and its boiling point is 55.3 °C.
Technetium hexafluoride undergoes a solid
phase transition
In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states o ...
at −4.54 °C. Above this temperature (measured at 10 °C), the solid structure is
cubic.
Lattice parameters are ''a'' = 6.16 Å. There are two
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 3.02 g·cm
−3. Below this temperature (measured at −19 °C), the solid structure is
orthorhombic space group ''Pnma''.
Lattice parameters are ''a'' = 9.55
Å, ''b'' = 8.74 Å, and ''c'' = 5.02 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 3.38 g·cm
−3. At −140 °C, the solid structure is still orthothombic, but the lattice parameters are now ''a'' = 9.360
Å, ''b'' = 8.517 Å, and ''c'' = 4.934 Å, giving a density of 3.58 g·cm
−3.
The TcF
6 molecule itself (the form important for the liquid or gas phase) has
octahedral molecular geometry, which has point group (''
Oh''). The Tc–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 1.812 Å.
Its
magnetic moment
In electromagnetism, the magnetic moment is the magnetic strength and orientation of a magnet or other object that produces a magnetic field. Examples of objects that have magnetic moments include loops of electric current (such as electromagne ...
has been measured to be 0.45
''μ''B.
Properties
Physical
TcF
6 is
octahedral
In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet a ...
, as shown by
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
and
Raman spectra. Its low-temperature
orthorhombic form converts to the higher symmetry
body-centred cubic
In crystallography, the cubic (or isometric) crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals.
There are three main varieties of ...
form at room temperature, like other metal hexafluorides such as RhF
6 and OsF
6. Preliminary measurements of
magnetic moment
In electromagnetism, the magnetic moment is the magnetic strength and orientation of a magnet or other object that produces a magnetic field. Examples of objects that have magnetic moments include loops of electric current (such as electromagne ...
yield a value of 0.45
µB, which is lower than expected for a d
1 octahedral compound.
Chemical
TcF
6 reacts with alkaline
chloride
The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride sa ...
s in
iodine pentafluoride
Iodine pentafluoride is an interhalogen compound with chemical formula IF5. It is one of the fluorides of iodine. It is a colorless liquid, although impure samples appear yellow. It is used as a fluorination reagent and even a solvent in special ...
(IF
5) solution to form hexafluorotechnetates. TcF
6 disproportionates on
hydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile.
Biological hydrolys ...
with aqueous
NaOH
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions .
Sodium hydroxide is a highly caustic base and alkali ...
to form a black precipitate of
TcO2.
In
hydrogen fluoride solution, TcF
6 reacts with
hydrazinium fluoride to yield N
2H
6TcF
6 or N
2H
6(TcF
6)
2.
References
{{fluorine compounds
Technetium compounds
Hexafluorides
Octahedral compounds