Selenium trioxide

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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
CAS number 13768-86-0 N
PubChem 115128
ChemSpider 103019 YesY
Jmol-3D images Image 1
Molecular formula SeO3
Molar mass 126.96 g/mol
Appearance white hygroscopic crystals
Density 3.44 g/cm3
Melting point 118.35 °C
Boiling point sublimes
Solubility in water very soluble
Crystal structure tetragonal
EU classification not listed
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 N (verify) (what is: YesY/N?)
Infobox references

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.[2]


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.[2]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.[2]

At 120 °C SeO3 reacts with selenium dioxide to form the Se(VI)-Se(IV) compound diselenium pentoxide:[3]

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.[2]

With lithium oxide and sodium oxide it reacts to form salts of SeVIO54– and SeVIO66–:[4] 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.[4]

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.[5]


  1. ^ Lide, David R. (1998). Handbook of Chemistry and Physics (87 ed.). Boca Raton, FL: CRC Press. pp. 4–81. ISBN 0-8493-0594-2. 
  2. ^ a b c d Egon Wiberg, Arnold Frederick Holleman (2001) Inorganic Chemistry, Elsevier ISBN 0123526515
  3. ^ 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
  4. ^ a b Handbook of Chalcogen Chemistry: New Perspectives in Sulfur, Selenium and Tellurium, Franceso A. Devillanova, Royal Society of Chemistry, 2007, ISBN 9780854043668
  5. ^ 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. doi:10.1002/jrs.1250070310. ISSN 0377-0486. 

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