Selenium trioxide

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. 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.

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

SeO₃ + SeO₂ → Se₂O₅

It reacts with selenium tetrafluoride to form selenoyl fluoride, the selenium analogue of sulfuryl fluoride

SeO₃ + SeF₄ → SeO₂F₂

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 1.729 → 1.815 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 181pm, 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.