Selenium tetrafluoride (SeF<sub>4</sub>) 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.
The first reported synthesis of selenium tetrafluoride was by Paul Lebeau in 1907, who treated selenium with fluorine:
A synthesis involving more easily handled reagents entails the fluorination of selenium dioxide with sulfur tetrafluoride:
An intermediate in this reaction is seleninyl fluoride (SeOF<sub>2</sub>).
Other methods of preparation include fluorinating elemental selenium with chlorine trifluoride:
Selenium in SeF<sub>4</sub> has an oxidation state of +4. Its shape in the gaseous phase is similar to that of SF<sub>4</sub>, 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 169.2ð. The two other fluorine atoms are attached by shorter bonds (168 pm), with an F-Se-F bond angle of 100.6ð. In solution at low concentrations this monomeric structure predominates, but at higher concentrations evidence suggests weak association between SeF<sub>4</sub> molecules leading to a distorted octahedral coordination around the selenium atom. In the solid the selenium center also has a distorted octahedral environment.
In HF, SeF<sub>4</sub> behaves as a weak base, weaker than sulfur tetrafluoride, SF<sub>4</sub> (K<sub>b</sub>= 2 X 10<sup>âÂÂ2</sup>):
Ionic adducts containing the SeF<sub>3</sub><sup>+</sup> cation are formed with SbF<sub>5</sub>, AsF<sub>5</sub>, NbF<sub>5</sub>, TaF<sub>5</sub>, and BF<sub>3</sub>. With caesium fluoride, CsF, the SeF<sub>5</sub><sup>âÂÂ</sup> anion is formed, which has a square pyramidal structure similar to the isoelectronic chlorine pentafluoride, ClF<sub>5</sub> and bromine pentafluoride, BrF<sub>5</sub>. With 1,1,3,3,5,5-hexamethylpiperidinium fluoride or 1,2-dimethylpropyltrimethylammonium fluoride, the SeF<sub>6</sub><sup>2âÂÂ</sup> anion is formed. This has a distorted octahedral shape which contrasts to the regular octahedral shape of the analogous SeCl<sub>6</sub><sup>2âÂÂ</sup>.