Selenium trioxide
Selenium trioxide is the inorganic compound with the formula SeO. It is white, hygroscopic solid. It is also an oxidizing agent and a Lewis acid. It is of academic interest as a precursor to Se(VI) compounds. Preparation. Selenium trioxide is difficult to prepare because it is unstable with respect to the dioxide: 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. Reactions. In its chemistry SeO generally resembles sulfur trioxide, SO, rather than tellurium trioxide, TeO. At 120 °C SeO reacts with selenium dioxide to form the Se(VI)-Se(IV) compound diselenium pentaoxide: It reacts with selenium tetrafluoride to form selenoyl fluoride, the selenium analogue of sulfuryl fluoride As with SO adducts are formed with Lewis bases such as pyridine, dioxane and ether. With lithium oxide and sodium oxide it reacts to form salts of SeO and SeO: With LiO, it gives LiSeO, containing the trigonal pyramidal anion SeO with equatorial bonds, 170.6–171.9 pm; and longer axial Se−O bonds of  179.5 pm. With NaO it gives NaSeO, containing the square pyramidal SeO, with Se−O bond lengths ranging from range 172.9 → 181.5 pm, and Na(SeO)(SeO), containing octahedral SeO. SeO is the conjugate base of the unknown orthoselenic acid (Se(OH)). Structure. In the solid phase SeO consists of cyclic tetramers, with an 8 membered (Se−O) ring. Selenium atoms are 4-coordinate, bond lengths being Se−O bridging are 175 pm and 181 pm, non-bridging 156 and 154 pm. SeO in the gas phase consists of tetramers and monomeric SeO which is trigonal planar with an Se−O bond length of 168.78 pm