Selenium trioxide

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

In its chemistry SeO₃ generally resembles sulfur trioxide, SO₃, rather than tellurium trioxide, TeO₃. The impure 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).

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.