Selenium tetrafluoride
Identifiers
CAS number	13465-66-2
PubChem	123311
Properties
Molecular formula	SeF4
Molar mass	154.954 g/mol
Appearance	colourless liquid
Density	2.77 g/cm3
Melting point	-13.2 °C
Boiling point	101 °C
Hazards
NFPA 704	3 2 W
Related compounds
Other anions	selenium dioxide, selenium(IV) chloride, selenium(IV) bromide
Other cations	sulfur tetrafluoride, tellurium(IV) fluoride
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Selenium tetrafluoride (SeF₄) is a chemical compound. It is a colourless liquid that reacts readily with water. It can be used as a fluorinating reagent in organic syntheses and has advantages over sulfur tetrafluoride in that milder conditions can be employed and it is a liquid rather than a gas. Selenium tetrafluoride was first synthesized by the reaction of selenium with fluorine by Paul Lebeau in 1907.^[1]

Se + 2 F₂ → SeF₄

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

3Se + 4ClF₃ → 3SeF₄ + 2Cl₂

and reacting sulfur tetrafluoride with selenium dioxide:

SF₄ + SeO₂ → SeF₄ + SO₂

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 in accordance with VSEPR theory which predicts pseudo-trigonal pyramidal disposition of the five electron pairs around the selenium atom. The axial Se-F bonds are 177pm with an F-Se-F bond angle of 169.2°. The two other fluorine atoms are attached by shorter bonds (168pm), with an F-Se-F bond angle of 100.6°. In solution at low concentrations this monomeric structure predominates but at higher concentrations there is evidence for weak association between SeF₄ molecules leading to a distorted octahedral coordination around the sulfur atom. In the solid the sulfur atom also has a distorted octahedral environment.
In HF SeF₄ is 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 are formed with SbF₅, AsF₅, NbF₅, TaF₅, and BF₃ that contain the SeF₃⁺ cation ^[2]. 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 ^[3] 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 SeCl₆^{2− [4]}.

[edit] References

• Selenium: Inorganic Chemistry Krebs. B., Bonmann S., Eidenschink I.; Encyclopedia of Inorganic Chemistry (1994) John Wiley and Sons ISBN 0-471-93620-0