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Selenium trioxide

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Title: Selenium trioxide  
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Subject: Selenium dioxide, Sulfur trioxide, Selenium, Oxides, Selenium compounds
Collection: Interchalcogens, Oxides, Oxidizing Agents, Selenium Compounds
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Selenium trioxide

Selenium trioxide[1]
Structural formula of the monomer as found in the gas phase
Space-filling model of the monomer as found in the gas phase
ChemSpider  Y
Jmol-3D images Image
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
not listed
Lethal dose or concentration (LD, LC):
LD50 (Median dose)
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).
 N  (: Y/N?)

Selenium trioxide is the 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.[3]


  • Preparation 1
  • Reactions 2
  • Structure 3
  • References 4
  • Further reading 5


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.[3]One method entails dehydration of anhydrous selenic acid with phosphorus pentoxide at 150-160 °C. 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.[3]

At 120 °C SeO3 reacts with selenium dioxide to form the Se(VI)-Se(IV) compound diselenium pentoxide:[4]

SeO3 + SeO2 → Se2O5

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

SeO3 + SeF4 → SeO2F2

As with SO3 adducts are formed with Lewis bases such as pyridine, dioxane and ether.[3]

With lithium oxide and sodium oxide it reacts to form salts of SeVIO54− and SeVIO66−:[5] 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 1.729 → 1.815 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 181pm, non-bridging 156 and 154 pm.[5]

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.[6]


  1. ^ Lide, David R. (1998). Handbook of Chemistry and Physics (87 ed.). Boca Raton, FL: CRC Press. pp. 4–81.  
  2. ^ a b "Selenium compounds (as Se)". Immediately Dangerous to Life and Health.  
  3. ^ a b c d Egon Wiberg, Arnold Frederick Holleman (2001) Inorganic Chemistry, Elsevier ISBN 0123526515
  4. ^ 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
  5. ^ a b Handbook of Chalcogen Chemistry: New Perspectives in Sulfur, Selenium and Tellurium, Franceso A. Devillanova, Royal Society of Chemistry, 2007, ISBN 9780854043668
  6. ^ 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.  

Further reading

  • Schmidt, Prof. Dr. Max; Dr. P. Bornmann; Dr. Irmgard Wilhelm (1963-10-02). "The Chemistry of Selenium Trioxide". Angewandte Chemie International Edition in English 2 (11): 691–692.  
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