Selenium trioxide[1]
Structural formula of the monomer as found in the gas phase
Structural formula of the monomer as found in the gas phase
Space-filling model of the monomer as found in the gas phase
Space-filling model of the monomer as found in the gas phase
3D model (JSmol)
ECHA InfoCard 100.033.972 Edit this at Wikidata
  • InChI=1S/O3Se/c1-4(2)3 checkY
  • InChI=1/O3Se/c1-4(2)3
  • monomer: O=[Se](=O)=O
  • cyclic tetramer: O=[Se]0(=O)O[Se](=O)(=O)O[Se](=O)(=O)O[Se](=O)(=O)O0
Molar mass 126.96 g/mol
Appearance white hygroscopic crystals
Density 3.44 g/cm3
Melting point 118.35 °C (245.03 °F; 391.50 K)
Boiling point sublimes
very soluble
GHS labelling:[3]
GHS06: ToxicGHS09: Environmental hazard
H301, H331, H373, H410
NFPA 704 (fire diamond)
Lethal dose or concentration (LD, LC):
7 mg/kg (rat, oral)
7.08 mg/kg (mouse, oral)
5.06 mg/kg (guinea pig, oral)
2.25 mg/kg (rabbit, oral)
13 mg/kg (horse, oral)[2]
13 mg/kg (pig, oral)
9.9 mg/kg (cow, oral)
3.3 mg/kg (goat, oral)
3.3 mg/kg (sheep, oral)[2]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Selenium trioxide is the inorganic compound with the formula SeO3. It is white, hygroscopic solid. It is also an oxidizing agent and a Lewis acid. It is of academic interest as a precursor to Se(VI) compounds.[4]


Selenium trioxide is difficult to prepare because it is unstable with respect to the dioxide:

2 SeO3 → 2 SeO2 + O2

It has been generated in a number of ways despite the fact that the dioxide does not combust under normal conditions.[4] One method entails dehydration of anhydrous selenic acid with phosphorus pentoxide at 150–160 °C. Another method is the reaction of liquid sulfur trioxide with potassium selenate.

SO3 + K2SeO4 → K2SO4 + SeO3


In its chemistry SeO3 generally resembles sulfur trioxide, SO3, rather than tellurium trioxide, TeO3.[4] The substance reacts explosively with oxidizable organic compounds.[5]

At 120 °C SeO3 reacts with selenium dioxide to form the Se(VI)-Se(IV) compound diselenium pentaoxide:[6]

SeO3 + SeO2 → Se2O5

It reacts with selenium tetrafluoride to form selenoyl fluoride, the selenium analogue of sulfuryl fluoride

2SeO3 + SeF4 → 2SeO2F2 + SeO2

As with SO3 adducts are formed with Lewis bases such as pyridine, dioxane and ether.[4]

With lithium oxide and sodium oxide it reacts to form salts of SeVIO54− and SeVIO66−:[7] With Li2O, it gives Li4SeO5, containing the trigonal pyramidal anion SeVIO54− with equatorial bonds, 170.6–171.9 pm; and longer axial Se−O bonds of 179.5 pm. With Na2O it gives Na4SeO5, containing the square pyramidal SeVIO54−, with Se−O bond lengths ranging from range 172.9 → 181.5 pm, and Na12(SeO4)3(SeO6), containing octahedral SeVIO66−. SeVIO66− is the conjugate base of the unknown orthoselenic acid (Se(OH)6).


In the solid phase SeO3 consists of cyclic tetramers, with an 8 membered (Se−O)4 ring. Selenium atoms are 4-coordinate, bond lengths being Se−O bridging are 175 pm and 181 pm, non-bridging 156 and 154 pm.[7]

SeO3 in the gas phase consists of tetramers and monomeric SeO3 which is trigonal planar with an Se−O bond length of 168.78 pm.[8]


  1. ^ Lide, David R. (1998). Handbook of Chemistry and Physics (87 ed.). Boca Raton, Florida: CRC Press. pp. 4–81. ISBN 0-8493-0594-2.
  2. ^ a b "Selenium compounds (as Se)". Immediately Dangerous to Life or Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
  3. ^ "C&L Inventory". Retrieved 16 December 2021.
  4. ^ a b c d Egon Wiberg, Arnold Frederick Holleman (2001) Inorganic Chemistry, Elsevier ISBN 0123526515
  5. ^ Schmidt, Bornmann & Wilhelm 1963.
  6. ^ Z. Žák "Crystal structure of diselenium pentoxide Se2O5" Zeitschrift für anorganische und allgemeine Chemie 1980, volume 460, pp. 81–85. doi:10.1002/zaac.19804600108
  7. ^ a b Handbook of Chalcogen Chemistry: New Perspectives in Sulfur, Selenium and Tellurium, Franceso A. Devillanova, Royal Society of Chemistry, 2007, ISBN 9780854043668
  8. ^ Brassington, N. J.; Edwards, H. G. M.; Long, D. A.; Skinner, M. (1978). "The pure rotational Raman spectrum of SeO3". Journal of Raman Spectroscopy. 7 (3): 158–160. Bibcode:1978JRSp....7..158B. doi:10.1002/jrs.1250070310. ISSN 0377-0486.

Further reading