Silicon tetrafluoride

Silicon tetrafluoride or tetrafluorosilane is the chemical compound with the formula SiF4. This colorless compound is notable for having a narrow liquid range: its boiling point is only 4 °C above its melting point. It was first synthesized by John Davy in 1812.[2] It is a tetrahedral molecule.

Silicon tetrafluoride
Names
IUPAC names
Tetrafluorosilane
Silicon tetrafluoride
Other names
Silicon fluoride
Fluoro acid air
Identifiers
3D model (JSmol)
ECHA InfoCard 100.029.104
RTECS number
  • VW2327000
UNII
UN number 1859
Properties
SiF4
Molar mass 104.0791 g/mol
Appearance colourless gas, fumes in moist air
Density 1.66 g/cm3, solid (−95 °C)
4.69 g/L (gas)
Melting point −90 °C (−130 °F; 183 K)
Boiling point −86 °C (−123 °F; 187 K)
decomposes
Structure
tetrahedral
0 D
Hazards
Main hazards toxic, corrosive
Safety data sheet ICSC 0576
NFPA 704 (fire diamond)
Flammability code 0: Will not burn. E.g. waterHealth code 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasReactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g. white phosphorusSpecial hazard W: Reacts with water in an unusual or dangerous manner. E.g. sodium, sulfuric acid
0
3
2
Lethal dose or concentration (LD, LC):
69,220 mg/m3 (rat, 4 hr)[1]
Related compounds
Other anions
 Silicon tetrachloride
Silicon tetrabromide
Silicon tetraiodide
Other cations
 Carbon tetrafluoride
Germanium tetrafluoride
Tin tetrafluoride
Lead tetrafluoride
Related compounds
 Hexafluorosilicic acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Preparation

SiF
4 is a by-product of the production of phosphate fertilizers, resulting from the attack of HF (derived from fluorapatite protonolysis) on silicates, which are present as impurities in the phosphate rock. In the laboratory, the compound is prepared by heating BaSiF
6 above 300 °C, whereupon the solid releases volatile SiF
4, leaving a residue of BaF
2. The required BaSiF
6 is prepared by treating aqueous hexafluorosilicic acid with barium chloride.[3] The corresponding GeF
4 is prepared analogously, except that the thermal "cracking" requires 700 °C.[4] SiF
4 can in principle also be generated by the reaction of silicon dioxide and hydrofluoric acid, but this process tends to give hexafluorosilicic acid:

6 HF + SiO2 → H2SiF6 + 2 H2O

Uses

This volatile compound finds limited use in microelectronics and organic synthesis.[5]

Comparison with other SiX4 compounds
SiH4SiF4SiCl4SiBr4SiI4
b.p. (˚C)[6]-111.9-90.356.8155.0290.0
m.p. (˚C)[6]-185-95.0-68.85.0155.0
Si-X bond length (Å)>0.74 [7]1.552.022.202.43
Si-X bond energy (kJ/mol)[8]384582391310234

Occurrence

Volcanic plumes contain significant amounts of silicon tetrafluoride. Production can reach several tonnes per day.[9] Some amounts are also emitted from spontaneous coal fires.[10] The silicon tetrafluoride is partly hydrolysed and forms hexafluorosilicic acid.

References
  1. "Fluorides (as F)". Immediately Dangerous to Life and Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
  2. John Davy (1812). "An Account of Some Experiments on Different Combinations of Fluoric Acid". Philosophical Transactions of the Royal Society of London. 102: 352–369. doi:10.1098/rstl.1812.0020. ISSN 0261-0523. JSTOR 107324.
  3. Hoffman, C. J.; Gutowsky, H. S. (1953). "Silicon Tetrafluoride". Inorganic Syntheses. Inorganic Syntheses. 4. pp. 145–6. doi:10.1002/9780470132357.ch47. ISBN 9780470132357.CS1 maint: uses authors parameter (link)
  4. Hoffman, C. J.; Gutowsky, H. S. (1953). Silicon Tetrafluoride. Inorganic Syntheses. 4. pp. 147–8. doi:10.1002/9780470132357.ch48.CS1 maint: uses authors parameter (link)
  5. Shimizu, M. "Silicon(IV) Fluoride" Encyclopedia of Reagents for Organic Synthesis, 2001 John Wiley & Sons. doi:10.1002/047084289X.rs011
  6. Silicon Compounds, Silicon Halides. Collins, W.: Kirk-Othmer Encyclopedia of Chemical Technology; John Wiley & Sons, Inc, 2001.
  7. https://www.answers.com/Q/What_is_the_bond_length_of_the_H-H_bond
  8. Ebsworth, E. A. V. In Volatile Silicon Compounds; Taube, H.; Maddock, A. G.; Inorganic Chemistry; Pergamon Press Book: New York, NY, 1963; Vol. 4.
  9. T. Mori; M. Sato; Y. Shimoike; K. Notsu (2002). "High SiF4/HF ratio detected in Satsuma-Iwojima volcano's plume by remote FT-IR observation" (PDF). Earth Planets Space. 54 (3): 249–256. doi:10.1186/BF03353024. S2CID 55173591.
  10. Kruszewski, Ł., Fabiańska, M.J., Ciesielczuk, J., Segit, T., Orłowski, R., Motyliński, R., Moszumańska, I., Kusy, D. 2018 – First multi-tool exploration of a gas-condensate-pyrolysate system from the environment of burning coal mine heaps: An in situ FTIR and laboratory GC and PXRD study based on Upper Silesian materials. Science of the Total Environment, 640-641, 1044-1071; DOI: 10.1016/j.scitotenv.2018.05.319


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