Selenium trioxide, SeO

Selenium trioxide, SeO, is white, crystalline, and hygroscopic. It can be prepared by the action of sulfur trioxide on potassium selenate or of phosphorous pentoxide on selenic acid. It forms selenic acid when dissolved in water. The pure trioxide is soluble in a number of organic solvents. A solution in Hquid sulfur dioxide is a selenonating agent. It is stable in very dry atmospheres at room temperature and on heating it decomposes first to selenium pentoxide [12293-89-9] and then to selenium dioxide. 

The chemical properties of selenium fall between sulfur and tellurium. Thus, selenium reacts with oxygen similarly to sulfur, forming two oxides, selenium dioxide, Se02 and trioxide, SeOs. The metal combines with halogens forming their halides. With nonmetals, selenium forms binary compounds exhibiting oxidation states +4 and -i-6. 

Selenium also forms a trioxide, SeOs. In excess oxygen the product mixture may contain both dioxide and trioxide. The trioxide is unstable. 

Selenium trioxide shows the typical properties of a Lewis acid and forms 1 1 complexes with organic bases such as pyridine, y-picoline, quinoline, morpholine, and triethylamine in liquid SO2 or nitromethane. The complexes were found to be of donor-acceptor type, and IR spectroscopy suggests that selenium is coordinated via the nitrogen atom of the tertiary base. It has also been confirmed that the tetrameric ring of SeOs is retained in the adducts. 

Selenium trioxide melts at (392.5 1 1.5) K, [62JER/MEN], [63MIJ/GER], to a colourless liquid. The decomposition of SeO ,(l) on heating was studied by Jerschkewitz and Menning [62JER/MEN] and found to be dependent on the purity of the specimen. The presence of a selenate salt, selenium dioxide, or water greatly enhances the rate of the decomposition reaction ... 

BRI/OGD] Brlsdon, A. K., Ogden, J. S., Matrix isolation and mass spectrometric studies on selenium trioxide vapour The characterization of monomeric SeOs, J. Mol Struct., 157, (1987), 141-153. Cited on page 128. 

SeO SeOj, selenium trioxide SeOj, trioxidoselenate( l-) SeO, trioxidoselenate(2-) selenite SeOg , trioxidoselenato(2—) selenito... 

SeO, SeOg, selenium trioxide SeOg ", trioxidoselenate( l—) SeOg ", trioxidoselenate(2—) selenite SeOJ", trioxidoselenato(2-) selenito... 

The affinity of selenium for oxygen is less than that of sulphur. The dioxide, SeQ2, is well defined. The trioxide, Se03, is much less stable than its sulphur analogue, and until recently all attempts to isolate it have been unsuccessful (see p. 331). Selenium sesquioxide, Se203, has not been definitely isolated.1 An oxide of composition Se304 has been described,2 but its existence needs confirmation. The existence of a lower oxide, SeO, postulated to explain the characteristic odour of selenium when vaporised in air, is improbable.3... 

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