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 (SeO3) is a chemical compound used in the manufacture of photoelectric cells and solar energy devices. It is hygroscopic, a strong oxidizing agent and a Lewis acid.[2]


Selenium trioxide can be prepared in a number of ways:[2]

The reaction of anhydrous selenic acid and phosphorus pentoxide at 150-160 °C

Selenium metal with oxygen in a high frequency glow discharge

2Se + 3O2 → 2SeO3

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] It is thermodynamically unstable with respect to selenium dioxide and above 185 °C it begins to decompose:

SeO3 → SeO2 + 12 O2;

At 120 °C SeO3 reacts with selenium dioxide to form diselenium pentoxide, Se2O5

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 reaction in a sealed tube forms selenate(VI) compounds containing SeVIO54– and SeVIO66–:[3]

1 mole of SeO3 and 2 moles of Li2O gives Li4SeO5 ;trigonal pyramidal ion SeVIO54– with equatorial bonds , 170.6-171.9 pm; and axial bonds longer 179.5 pm
1 mole SeO3 and 2 moles Na2O gives Na4SeO5 square pyramidal containing the SeVIO54– , with bond lengths ranging from range 1.729 → 1.815 pm
4 moles SeO3 and 6 moles Na2O gives Na12(SeO4)3(SeO6) containing octahedral SeVIO66–


In the solid phase SeO3 consists of cyclic tetramers, with an 8 membered (Se-O)4 ring. Selenium atoms are 4 coordinate, bond lengths are Se-O bridging are 175 pm and 181pm, non-bridging 156 and 154 pm.[3]

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.[4]


  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. ^ a b Handbook of Chalcogen Chemistry: New Perspectives in Sulfur, Selenium and Tellurium, Franceso A. Devillanova, Royal Society of Chemistry, 2007, ISBN 9780854043668
  4. ^ 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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