Selenium tetrafluoride explained

Selenium tetrafluoride (SeF₄) is an inorganic compound. It is a colourless liquid that reacts readily with water. It can be used as a fluorinating reagent in organic syntheses (fluorination of alcohols, carboxylic acids or carbonyl compounds) and has advantages over sulfur tetrafluoride in that milder conditions can be employed and it is a liquid rather than a gas.

Synthesis

The first reported synthesis of selenium tetrafluoride was by Paul Lebeau in 1907, who treated selenium with fluorine:^[1]

Se + 2 F₂ → SeF₄A synthesis involving more easily handled reagents entails the fluorination of selenium dioxide with sulfur tetrafluoride:^[2]

SF₄ + SeO₂ → SeF₄ + SO₂An intermediate in this reaction is seleninyl fluoride (SeOF₂).

Other methods of preparation include fluorinating elemental selenium with chlorine trifluoride:

3 Se + 4 ClF₃ → 3 SeF₄ + 2 Cl₂

Structure and bonding

Selenium in SeF₄ has an oxidation state of +4. Its shape in the gaseous phase is similar to that of SF₄, having a see-saw shape. VSEPR theory predicts a pseudo-trigonal pyramidal disposition of the five electron pairs around the selenium atom. The axial Se-F bonds are 177 pm with an F-Se-F bond angle of 169.2°. The two other fluorine atoms are attached by shorter bonds (168 pm), with an F-Se-F bond angle of 100.6°. In solution at low concentrations this monomeric structure predominates, but at higher concentrations evidence suggests weak association between SeF₄ molecules leading to a distorted octahedral coordination around the selenium atom. In the solid the selenium center also has a distorted octahedral environment.

Reactions

In HF, SeF₄ behaves as a weak base, weaker than sulfur tetrafluoride, SF₄ (K_b= 2 X 10⁻²):

SeF₄ + HF → SeF₃⁺ + HF₂⁻; (K_b = 4 X 10⁻⁴)

Ionic adducts containing the SeF₃⁺ cation are formed with SbF₅, AsF₅, NbF₅, TaF₅, and BF₃.^[3] With caesium fluoride, CsF, the SeF₅⁻ anion is formed, which has a square pyramidal structure similar to the isoelectronic chlorine pentafluoride, ClF₅ and bromine pentafluoride, BrF₅.^[4] With 1,1,3,3,5,5-hexamethylpiperidinium fluoride or 1,2-dimethylpropyltrimethylammonium fluoride, the SeF₆²⁻ anion is formed. This has a distorted octahedral shape which contrasts to the regular octahedral shape of the analogous SeCl₆²⁻.^[5]

References

• Selenium: Inorganic Chemistry Krebs. B., Bonmann S., Eidenschink I.; Encyclopedia of Inorganic Chemistry (1994) John Wiley and Sons

See also

• Selenium hexafluoride
• Sulfur tetrafluoride
• Tellurium tetrafluoride

External links

• WebBook page for SeF4

Notes and References

1. Action of Fluorine on Selenium Tetrafluoride of Selenium. . Comptes Rendus de l'Académie des Sciences de Paris. 1907 . 144 . 1042.
2. Konrad Seppelt, Dieter Lentz, Gerhard Klöter "Selenium Tetrafluoride, Selenium Difluoride Oxide (Seleninyl Fluoride), and Xenon Bis[Pentafluorooxoselenate(VI)]" Inorg. Synth., 1987, vol. 24, 27-31.
3. Selenium tetrafluoride adducts. II. Adducts with boron trifluoride and some pentafluorides. R. J. Gillespie. A. Whitla . Can. J. Chem.. 1970. 48 . 4. 657–663. 10.1139/v70-106. free.
4. Vibrational Spectra and Force Constants of the Square-Pyramidal Anions SF₅⁻, SeF₅⁻, and TeF₅⁻. K. O. Christe . E. C. Curtis . C. J. Schack . D. Pilipovich . Inorganic Chemistry. 1972 . 11 . 7. 1679–1682. 10.1021/ic50113a046 .
5. Reactions of the Naked Fluoride Ion: Syntheses and Structures of SeF₆²⁻ and BrF₆⁻ . Ali Reza Mahjoub . Xiongzhi Zhang . Konrad Seppelt . Chemistry: A European Journal. 1995 . 1 . 4 . 261–265. 10.1002/chem.19950010410 . free.