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.

Selenium trioxide

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:
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)
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)
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. 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. The substance reacts explosively with oxidizable organic compounds.

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

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.

With lithium oxide and sodium oxide it reacts to form salts of Se^VIO₅⁴⁻ and Se^VIO₆⁶⁻: 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.

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.

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.