Selenium trioxide

Selenium trioxide cannot be prepared directly in a pure state by reaction between the elements. Dehydration of selenic acid by P4O10 under stringent conditions [5ITOU/DOS] and reaction of sulphur trioxide with potassium selenate [52LEH/KRU], followed by vacuum distillation, are two preferred methods of preparation. It is an extremely reactive solid at room temperature. 

The entropy of Se03(cr) was estimated by Hart [52HAR] to be 5°(Se03, cr, 298.15 K) = 84.1 J-K -mol from an extrapolation within the periodic table. This datum will be given zero weight. 

Kondrat ev, Strizhov, and Doronin [84KON/STR] measured the heat capacity of Se03(cr) in the temperature range 55 to 307 K and at 12 and 46 K. Extrapolation to 0 K was made by a graphical method, but no details of the extrapolation procedure are presented. The results were  

The uncertainties have been added by the review as twice the estimated relative standard deviations indicated by the authors. No other low temperature studies of Se03(cr) have been found and the above results are selected. 

Trzil [58TRZ] calculated from experimental mean heat capacity measurements by Tuma [57TUM] (original data not available) in the temperature range 293 to 373 K  

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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. 

Selenium trioxide is unstable with respect to the dioxide. 

Potassium hexaoxoxenonate-xenon trioxide, 4674 Selenium trioxide, 4852... 

Compounds Selenium dioxide selenium trioxide selenium oxychloride sodium selenite sodium selenate hydrogen selenide selenic acid selenium sulfide selenium disulfide... 

Solid sulfur trioxide exists in two well-defined modifications. Orthorhombic y-SOs consists of cyclic trimers (Figure 12.38a)/ which are converted by traces of water to the one-dimensional polymeric structure of monoclinic jl-SOi, (Figure 12.38b). In contrast, selenium trioxide forms a cyclic tetramer (Figure 12.38c). ... 

Oils Edible Safflower Potassium Binoxalate Ammonium Chloride Salicylic Acid Ammonium Chloride Lead Acetate Potassium Binoxalate Sodium Silicofluoride Ammonium Carbonate Fluocilicic Acid P-Dichlorobenzene Ammonium Phosphate Sec-Butyl Acetate Sec-Butyl Alcohol Calcium Phosphate Selenium Trioxide Selenium Dioxide Selenium Dioxide Selenium Dioxide Selenium Trioxide Antimony Trioxide Calcium Hypochlorite Carbaryl Cyclohexanone Charcoal... 

The acids and salts derived from the trioxidcs arc well defined, although selenium trioxide itself has not been isolated, and tellurium... 

Chloroselenic Acid, (Se03.HGl)2, has been described as a pale yellow liquid obtained wrhen dry hydrogen chloride and selenium trioxide (see p. 331) are allowed to combine in a cooled vessel.1 It has a density of 2-26, and solidifies at —46° C. It fumes in air owing to the escape of hydrogen chloride, which gas is also evolved on heating, selenium and selenium dioxide remaining. 

Selenium Trioxide or Selenic Anhydride, SeOa.—All attempts to prepare this compound by methods analogous to those used for sulphur trioxide have failed, and it has been found impossible to isolate it by dehydration of selenic acid. 

A repetition of the foregoing work by Meyer and Pawletta,4 however, failed to confirm the production of selenium trioxide. According to these investigators, the dissolution of selenium in selenium oxychloride results in the formation of selenious chloride, and the precipitate obtained by the action of ozone is a mixture of this salt with selenium dioxide. Furthermore, substitution of carbon tetrachloride or glacial acetic acid for selenium oxychloride did not lead to the separation of the tri oxide. 

According to Worsley and Baker8 the compounds described by them as selenium trioxide and chloroselenic acid (q.v.) both react with hydrogen peroxide to produce a perselenic acid. 

In Group VIA (16) the same phenomenon is encountered. Selenium trioxide is thermodynamically unstable relative to sulfur trioxide and tellurium trioxide. The enthalpies of formation of SF6, SeF6, and TeFft are -1210. — 1117, and — 1320 kj mol 1, respectively. This indicates comparable bond energies for S—F and Te—F bonds (317 and 330 kj mol-1, respectively), which are more stable than Se—F bonds (285 kj mol 1). 

Potassium hexaoxoxenonate—xenon trioxide, 4669 Selenium trioxide, 4846... 

Monomeric selenium trioxide (Se03) and tellurium trioxide (Te03) have a trigonal planar structure in the gas phase. In the solid state, Se03 forms cyclic tetramers (Se03)4, in which each Se atom connects two bridging O atoms and two terminal O atoms, with Se-Ob 177 pm and Se-Ot 155 pm (AX4 type). 

Selenium trioxide is less stable than the dioxide, so it dissociates readily ... 

Selenium trioxide is made by dehydration of H2Se04 by P205 at 150 to 160°C it is a strong oxidant and is rapidly rehydrated by water. Selenium trioxide dissolves in liquid HF to give fluoroselenic acid, FSe03H, a viscous fuming liquid. 

The stmcture of the trioxide consists of tetrameric cyclic molecules (8003)4 of 5 4 symmetry. The mean 8e-0 distances are 1.55 A for nonbridging oxygen and 1.77 A for bridging oxygen. Electron diffraction and mass spectrometry of the vapor indicates that there is appreciable dissociation to the monomer at 120 °C. The 8e distances in the monomeric 8e03 molecule are 1.69 A. In the molten state, selenium trioxide is probably polymeric. ... 

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. 

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