Selenium-Oxygen Compounds

Selenium-Oxygen Compounds.—Knudsen effusion measurements have been performed on powdered selenium dioxide at temperatures between 374 and 427 K, and the vapour pressure over this range has been determined.295 The enthalpy of formation of Se02(g) was computed to be AHT(298.i5) — -26.20 kcal mol-1 this leads to a value for an average bond energy of 99.80 kcal moF1, which is comparable with the spectroscopic dissociation energy. A vibrational analysis of the 3130 A absorption system of selenium dioxide has been carried out.296 

The reaction of Se02 with CIF has been shown to give SeOF2 and, with excess CIF, Sep4 the latter is also formed in the reaction of SeCU with CIF. The 

Meunier, S. Andersson, and A. Astrom, J. Solid State Chem., 1975, 13, 142. 

Timoshchenko, N. Fortunatov, and Z. A. Fokina, Nauchn. Tr. Gos. Nauchno-Issled. Proektn. Inst. Redkomet. Prom.-sti. 1974, 58, 119. 

Khalifan, J. Inorg. Nuclear Chem., 1975, 37, 297. 

The uncertainty about the constitution of the vapour of SeOs may have been settled. It has been claimed that only tetramer molecules are present. A redetermination of the vapour pressure gave a straight line in the Van t Hoff plot, indicating that there is no anomalous behaviour for SeOj. New values for the heat of sublimation and heat of evaporation were found. Freezing points have been determined in the SeOs—H2O system, and a maximum freezing point of 61.950 °C was found for the pure acid. Water behaves as a strong electrolyte and SeOs as a weak electrolyte in this system. 

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In alkaline medium it is hydrolyzed to ammonia, elemental selenium, and selenium-oxygen compounds. 

Selenium-oxygen bonds. The Se chemical shifts of some inorganic selenium-oxygen compounds are listed in the Tables 2 and 3. It is apparent that Se (Vl)-species are much more shielded (5 = 944-1076 Table 2) than the Se (IV)-species (5 = 1263-1559 Table 3). This holds as well for SeO3, the anhydride of selenic acid (H2SeO4), the halogen derivatives and its alkyl esters and amides. 

As expected, when comparing vibrational spectra of sulfur and selenium compounds, the absorptions due to bonds to selenium occur at lower frequencies than the corresponding sulfur modes. This frequency shift is normally 50-150 cm" but may be more for selenium-oxygen compounds when compared to sulfur-oxygen compounds ... 

Selenium and its compounds are strong poisons. Selenium halides, hydrogen selenide, and the oxygen compounds of selenium soluble in water are the most toxic. 

Selenium and tellurium belong to the same group as S in the Periodic Table, and their oxygen compounds, analogous to sulfuric acid, are called... 

The bond parameters of the oxygen compounds of selenium compared to those of sulfur have been investigated. The... 

PP-j) Diorgano Oxygen Sulfur (or Selenium) Tellurium Compounds... 

Table 2. Se chemical shifts in various selenium(VI)-oxygen compounds...

Elemental selenium is an excellent catalyst for fhe carbonylation of a variety of nucleophiles such as amines, alcohols, water, and carbon nucleophiles in the presence of oxygen under mild conditions [114]. In fhe synfhesis of ureas from primary amines and CO, for example (Scheme 15.42), the turnover number reaches ca. 10", giving ureas in almost quantitative yields. Successful applications of this Se/CO system include not only synfhetic reactions with a variety of carbamates, carbonates, and sulfur- or selenium-containing compounds but also metallurgical refining of selenium and several important processes as exemplified by isocyanate synfhesis, fhe water-gas shift reaction, and separation of carbon monoxide [115]. 

Compounds. — The compounds of tellurium resemble those of sulfur and selenium quite closely, but it is somewhat more metallic than either of these elements. It seems logical to expect the oxygen compounds of tellurium to be easily reduced, but it is found that the compounds of selenium are more easily reduced than either the sulfur or tellurium compounds.7... 

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