Selenium trioxide

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^[1]

Structural formula of the monomer as found in the gas phase Space-filling model of the monomer as found in the gas phase
Identifiers
CAS Number	13768-86-0 N
3D model (JSmol)	monomer: Interactive image cyclic tetramer: Interactive image
ChemSpider	103019 Y
ECHA InfoCard	100.033.972
PubChem CID	115128
CompTox Dashboard (EPA)	DTXSID50895004
InChI InChI=1S/O3Se/c1-4(2)3 Y Key: VFLXBUJKRRJAKY-UHFFFAOYSA-N Y InChI=1/O3Se/c1-4(2)3 Key: VFLXBUJKRRJAKY-UHFFFAOYAC
SMILES monomer: O=[Se](=O)=O cyclic tetramer: O=[Se]0(=O)O[Se](=O)(=O)O[Se](=O)(=O)O[Se](=O)(=O)O0
Properties
Chemical formula	SeO3
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
Solubility in water	very soluble
Structure
Crystal structure	tetragonal
Hazards
GHS labelling:[2]
Pictograms
Signal word	Danger
Hazard statements	H301, H331, H373, H410
NFPA 704 (fire diamond)	4 0 2 OX
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)[3]
LC50 (median concentration)	13 mg/kg (pig, oral) 9.9 mg/kg (cow, oral) 3.3 mg/kg (goat, oral) 3.3 mg/kg (sheep, oral)[3]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Preparation

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

Reactions

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 Se^VIO₅⁴⁻ and Se^VIO₆⁶⁻:^[7] With Li₂O, it gives Li₄SeO₅, containing the trigonal pyramidal anion Se^VIO₅⁴⁻ 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 Se^VIO₅⁴⁻, with Se−O bond lengths ranging from range 172.9 → 181.5 pm, and Na₁₂(SeO₄)₃(SeO₆), containing octahedral Se^VIO₆⁶⁻. Se^VIO₆⁶⁻ is the conjugate base of the unknown orthoselenic acid (Se(OH)₆).

Structure

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]

References

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. "C&L Inventory". echa.europa.eu. Retrieved 16 December 2021.
3. "Selenium compounds (as Se)". Immediately Dangerous to Life or Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
4. 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. Handbook of Chalcogen Chemistry: New Perspectives in Sulfur, Selenium and Tellurium, Francesco 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

• Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
• Schmidt, Prof. Dr. Max; Bornmann, Dr. P.; Wilhelm, Dr. Irmgard (1963-10-02). "The Chemistry of Selenium Trioxide". Angewandte Chemie International Edition in English. 2 (11): 691–692. doi:10.1002/anie.196306913.