Technetium hexafluoride

Technetium hexafluoride or technetium fluoride is a yellow inorganic compound with a low melting point. It was first identified in 1961. In this compound, technetium has an oxidation state 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, as technetium is a fission product of uranium. 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 in nuclear reprocessing.

Preparation

Technetium hexafluoride is prepared by heating technetium metal with an excess of F₂ at 400 °C.

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 at −4.54 °C. Above this temperature, the solid structure is cubic. Lattice parameters are a = 6.16 Å. There are two formula units per unit cell, giving a density of 3.02 g·cm⁻³. Below this temperature, 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 units per unit cell, 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 TcF₆ molecule itself has octahedral molecular geometry, which has point group. The Tc–F bond length is 1.812 Å. Its magnetic moment has been measured to be 0.45 μ_B.

TcF₆ is octahedral, as shown by infrared and Raman spectra. Its low-temperature orthorhombic form converts to the higher symmetry body-centred cubic form at room temperature, like other metal hexafluorides such as RhF₆ and OsF₆. Preliminary measurements of magnetic moment yield a value of 0.45 μB, which is lower than expected for a d¹ octahedral compound.

TcF₆ reacts with alkaline chlorides in iodine pentafluoride solution to form hexafluorotechnetates. TcF₆ disproportionates on hydrolysis with aqueous NaOH to form a black precipitate of TcO₂. In hydrogen fluoride solution, TcF₆ reacts with hydrazinium fluoride to yield N₂H₆TcF₆ or N₂H₆₂.