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
- 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 10−2):
- SeF4 + HF → SeF3+ + HF2−; (Kb = 4 X 10−4)
Ionic adducts are formed with SbF5, AsF5, NbF5, TaF5, and BF3 that contain the SeF3+ cation . 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  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− .
- 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.