Selenium trioxide

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.