Selenium Di Oxide

Solid SeO₂ is a one-dimensional polymer, the chain consisting of alternating selenium and oxygen atoms. Each Se atom is pyramidal and bears a terminal oxide group. The bridging Se-O bond lengths are 179 pm and the terminal Se-O distance is 162 pm.The relative stereochemistry at Se alternates along the polymer chain (syndiotactic). In the gas phase selenium dioxide is present as dimers and other oligomeric species, at higher temperatures it is monomeric.The monomeric form adopts a bent structure very similar to that of sulfur dioxide with a bond length of 161 pm.The dimeric form has been isolated in a low temperature argon matrix and vibrational spectra indicate that it has a centrosymmetric chair form. Dissolution of SeO₂ in selenium oxydichloride give the trimer [Se(O)O]₃.Monomeric SeO₂ is a polar molecule, with the dipole moment of 2.62 D pointed from the midpoint of the two oxygen atoms to the selenium atom.

The solid sublimes readily. At very low concentrations the vapour has a revolting odour, resembling decayed horseradishes. At higher concentrations the vapour has an odour resembling horseradish sauce and can burn the nose and throat on inhalation. Whereas SO₂ tends to be molecular and SeO₂ is a one-dimensional chain, TeO₂ is a cross-linked polymer.

SeO₂ is considered an acidic oxide: it dissolves in water to form selenous acid. Often the terms selenous acid and selenium dioxide are used interchangeably. It reacts with base to form selenite salts containing the SeO²⁻
₃ anion. For example, reaction with sodium hydroxide produces sodium selenite:

SeO₂ + 2 NaOH → Na₂SeO₃ + H₂O