Selenium tetrafluoride

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:

Se 2 F₂ → SeF₄

A synthesis involving more easily handled reagents entails the fluorination of selenium dioxide with sulfur tetrafluoride:

SF₄ SeO₂ → SeF₄ SO₂

An 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. 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. 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⁻.