Selenium trioxide is the

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

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 ). In other words, an oxi ...

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 sp ...

. 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 P4 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 desiccant and dehydra ...

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

sulfur trioxide Sulfur trioxide (alternative spelling sulphur trioxide, also known as ''nisso sulfan'') is the chemical compound with the formula SO3. It has been described as "unquestionably the most important economically" sulfur oxide. It is prepared on an ind ...

with

potassium selenate Potassium selenate, , is an odorless, white solid that forms as the potassium Salt (chemistry), salt of selenic acid. Preparation Potassium selenate is produced by the reaction of selenium trioxide and potassium hydroxide. : Alternatively, it ...

. :SO₃ + K₂SeO₄ → K₂SO₄ + SeO₃

Reactions

In its chemistry SeO₃ generally resembles

, SO₃, rather than

tellurium trioxide Tellurium trioxide ( Te O3) is an inorganic chemical compound of tellurium and oxygen. In this compound, tellurium is in the +6 oxidation state. Polymorphs There are two forms, yellow-red α-TeO3 and grey, rhombohedral, β-TeO3 which is less reac ...

, TeO₃. At 120 °C SeO₃ reacts with

selenium dioxide Selenium dioxide is the chemical compound with the formula SeO2. This colorless solid is one of the most frequently encountered compounds of selenium. Properties Solid SeO2 is a one-dimensional polymer, the chain consisting of alternating seleniu ...

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 Selenoyl fluoride, selenoyl difluoride, selenium oxyfluoride, or selenium dioxydifluoride is a chemical compound with the formula SeO2F2. Structure The shape of the molecule is a distorted tetrahedron with the O-Se-O angle being 126.2°, the O-Se ...

, 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 eve ...

: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 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, weakly alkaline, water-miscible liquid w ...

, dioxane and

ether In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again ...

. With

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

and

sodium oxide Sodium oxide is a chemical compound with the formula Na2 O. 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 gla ...

it reacts to form salts of Se^VIO₅⁴⁻ and Se^VIO₆⁶⁻:Handbook of Chalcogen Chemistry: New Perspectives in Sulfur, Selenium and Tellurium, Franceso 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