Selenium tetrafluoride

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.


 * 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 (Kb= 2 X 10&minus; 2):
 * SeF4 + HF → SeF3+ + HF2&minus;; (Kb = 4 X 10&minus; 4)

Ionic adducts are formed with SbF5, AsF5, NbF5, TaF5, and BF3 that contain  the SeF3+ cation . With caesium fluoride, CsF, the SeBr5&minus; anion is formed, which has a square pyramidal structure similar to the isoelectronic chlorine pentafluoride, ClF5 and bromine pentafluoride, BrF5 With 1,1,3,3,5,5-Hexamethylpiperidinium fluoride or 1,2-dimethylpropyltrimethylammonium fluoride the SeF62 &minus; anion is formed. This has a distorted octahedral shape which contrasts to the regular octahedral shape of the analogous SeCl62 &minus; .