Selenium tetrafluoride

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Selenium tetrafluoride

Identifiers
CAS number	[13465-66-2]
Properties
Molecular formula	SeF₄
Molar mass	154.954 g/mol
Appearance	colourless liquid
Melting point	-10°C
Boiling point	101°C
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Selenium tetrafluoride (Se F₄) 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 + ClF₃ → 3SeF₄ + 2Cl₂

and reacting sulfur tetrafluoride wih 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 bond 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 SeBr₅⁻ 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 SeCl₆²⁻ ^[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

^ Paul Lebeau (1907). "Action of Fluorine on Selenium Tetrafluoride of Selenium". Comptes Rendus Acad. Sci., Paris 144: 1042.
^ R. J. Gillespie and A. Whitla (1970). "Selenium tetrafluoride adducts. II. Adducts with boron trifluoride and some pentafluorides". Can. J. Chem. 48 (4): 657–663. doi:10.1139/cjc-48-4-657.
^ KO Christe, EC Curtis, CJ Schack, D Pilipovich (1972). "Vibrational Spectra and Force Constants of the Square-Pyramidal Anions SF₅⁻, SeF₅⁻, and TeF₅⁻". Inorganic Chemistry 11 (7): 1679.
^ Ali Reza Mahjoub, Xiongzhi Zhang, Konrad Seppelt (1995). "Reactions of the Naked Fluoride Ion: Syntheses and Structures of SeF₆²⁻ and BrF₆⁻". Chemistry - A European Journal 1 (4): 216. doi:10.1002/chem.19950010410.