Selenium trioxide is the

inorganic compound An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistry''. Inorgan ...

with the formula Se O₃. It is white, hygroscopic solid. It is also an

oxidizing agent An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ''electron donor''). In ot ...

and a

Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any ...

. It is of academic interest as a precursor to Se(VI) compounds.Egon Wiberg, Arnold Frederick Holleman (2001) ''Inorganic Chemistry'', Elsevier

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 Phosphorus pentoxide is a chemical compound with molecular formula Phosphorus, P4Oxygen, O10 (with its common name derived from its empirical formula, P2O5). This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desic ...

at 150–160 °C. Another method is the reaction of liquid

sulfur trioxide Sulfur trioxide (alternative spelling sulphur trioxide) is the chemical compound with the formula SO3. It has been described as "unquestionably the most conomicallyimportant sulfur oxide". It is prepared on an industrial scale as a precursor to ...

with potassium selenate. :SO₃ + K₂SeO₄ → K₂SO₄ + SeO₃

Reactions

In its chemistry SeO₃ generally resembles

, SO₃, rather than tellurium trioxide, TeO₃. The substance reacts explosively with oxidizable

organic compounds Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

. At 120 °C SeO₃ reacts with selenium dioxide to form the Se(VI)-Se(IV) compound diselenium pentaoxide:Z. Žák "Crystal structure of diselenium pentoxide Se₂O₅" Zeitschrift für anorganische und allgemeine Chemie 1980, volume 460, pp. 81–85. :SeO₃ + SeO₂ → Se₂O₅ It reacts with

selenium tetrafluoride Selenium tetrafluoride ( Se F4) is an inorganic compound. It is a colourless liquid that reacts readily with water. It can be used as a fluorinating reagent in organic syntheses (fluorination of alcohols, carboxylic acids or carbonyl compounds) an ...

to form selenoyl fluoride, the selenium analogue of

sulfuryl fluoride Sulfuryl fluoride (also spelled ''sulphuryl fluoride'') is an inorganic compound with the formula SO2F2. It is an easily condensed gas and has properties more similar to sulfur hexafluoride than sulfuryl chloride, being resistant to hydrolysis ...

:2SeO₃ + SeF₄ → 2SeO₂F₂ + SeO₂ As with SO₃ adducts are formed with

Lewis base A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...

s such as

pyridine Pyridine is a basic (chemistry), basic heterocyclic compound, heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom . It is a highly flammable, weak ...

, dioxane and

ether In organic chemistry, ethers are a class of compounds that contain an ether group, a single oxygen atom bonded to two separate carbon atoms, each part of an organyl group (e.g., alkyl or aryl). They have the general formula , where R and R� ...

. With

lithium oxide Lithium oxide (Lithium, Oxygen, O) or lithia is an Inorganic compound, inorganic chemical compound. It is a white or pale yellow solid. Although not specifically important, many materials are assessed on the basis of their Li2O content. For examp ...

and

sodium oxide Sodium oxide is a chemical compound with the formula . It is used in ceramics and glasses. It is a white solid but the compound is rarely encountered. Instead "sodium oxide" is used to describe components of various materials such as glasses and f ...

it reacts to form salts of Se^VIO₅⁴⁻ and Se^VIO₆⁶⁻:Handbook of Chalcogen Chemistry: New Perspectives in Sulfur, Selenium and Tellurium, Francesco A. Devillanova, Royal Society of Chemistry, 2007, 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.

Preparation

Reactions

Structure

References

Further reading