Selenium, and Tellurium

Elemental selenium can be prepared by reaction 15.23. By substituting PhgPSe in this reaction by PhgPS, rings of composition Se Sg (n = 1-5) can be produced (see end-of-chapter problem 2.31). 

Aspects of the chemistry of water have already been covered as follows  

Tellurium has only one crystalline form which is a silvery-white metallic-looking solid and is isostructural with grey selenium. The red allotropes of 8e can be obtained by rapid cooling of molten 8e and extraction into C82. The 

Although cyc/o-Teg is not known as an allotrope of the element, it has been characterized in the salt Cs3[Te22] which has the composition [Cs+]3[Te5 ][Teg]2. 

Selenium and Tellurium. - Simple alkyl and aryl selenoaldehydes 

N-(phenylseleno)succinimide, combined with a suitable Lewis acid.  

The oxidative rearrangement of allylic selenides is a useful method for synthesising allylic amines. Recent developments, both in terms of improved procedures and synthetic applications, are documented. 

A useful review of tellurium reagents in organic synthesis 

Interestingly, the nonpolymer bound form of (287) shows no activity, and possible reasons for this difference have been discussed 

Selenium and tellurium compounds are poisonous The experiments are demonstrated by the instructor  

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

Symptoms of poisoning are irritation of the respiratory tracts and eyes, a protracted cold in the head, and headaches. 

As regards the nature of their action on a human organism, tellurium and its compounds are similar to the inorganic compounds of selenium and arsenic. Hydrogen telluride is the most toxic. Tel-lurium(IV) oxide and aqueous solutions of the salts of tellurous and telluric acids are also toxic. Only tellurium ( metallic ) is not toxic if it gets into an organism. 

Indications of poisoning by tellurium salts are an odour of garlic of the exhaled air, a headache, accelerated breathing and pulse, a feeling of fatigue and dizziness. 

Selenium and Tellurium.- This area has seen continued progress this year and, in addition to new developments in selenium chemistry, the synthetic utility of tellurium is now beginning to establish 

Allenes also undergo free-radical selenosulphonation with PhSeS0 Ph 

The chemistry of selenoxides has also seen some advances this year. The first case of an asymmetric oxidation of an achiral 

1 1 (R OH) gives good yields of dialkyl ethers, ROR. B-Elimination 

PhSeO H, are both shown to effect the mild oxidation of indolines 

The reactions of Se2Ph2 and Te2Ph2 with Bi2Ph4 produce Ph2BiEPh (E = Se, Te), which can react with CH2N2 to form Ph2BiCH2EPh/ The kinetics of exchange between RSeH and RSeSeR, and between thiols and disulfides in D2O have been studied by Reaction is slow for sulfur, and occurs by an 

8elenium possesses several allotropes. Crystalline, red monoclinic selenium exists in three forms, each containing 8eg rings with the crown conformation of Sg (Fig. 16.6c). Black selenium consists of larger polymeric rings. The thermodynamically stable allotrope is grey selenium which contains infinite, helical chains (8e-8e = 237 pm), the axes of which lie parallel to one another. 

Elemental selenium can be prepared by reaction 16.24. By substituting Ph3P8e in this reaction by Ph3P8, rings 

Tellurium has only one ctystalline form which is a silvery-white metallic-looking solid and is isostructural with grey selenium. The red allotropes of Se can be obtained by rapid cooling of molten Se and extraction into CS2. The photoconductivity of Se (see Box 16.1) and Te arises because, in the solid, the band gap of 1.66 eV is small enough for the influence of visible light to cause the promotion of electrons from the filled bonding MOs to the unoccupied antibonding MOs (see Section 6.8). 

Although less reactive, Se and Te are chemically similar to sulfur. This resemblance extends to the formation of cations 

Persulfoxide intermediates, R2SOO, have been proposed for the photosensitized oxidation of sulfides and the oxygen-atom transfer of R2SOO to H-and 0-labeled shown to involve a linear 

R= t-Bu was found to be -3JK mor and raises the possibility that reaction at selenium with nucleophiles may not take place by a simple process and may involve electron transfer. 

TeCU adds to olefins in CDCI3/CH3CN to give mixtures of syn-anti addition products, whereas 2-naphthyl-TeCl3 gives exclusively antiaddi-tion. The latter is consistent with an ionic mechanism via a telluronium intermediate, whereas the former is more complex, though with the ionic mechanism ruled out. 

The reaction of bis(morpholinoselenocarbonyl)triselenide with iodine is a first-order process from a 1 1 charge transfer precursor complex and gives /  

In excess iodine, the reaction is more complex. Mahdi and Miller have shown that [p-EtOC6H4Te]2 and I2 react to give p-EtOC6H4TeI, at a rate which is first order in each reagent, via a proposed square Te2l2 transition state. With excess I2, a complex is formed, as in equation (39), = 

There are a few, relatively early, studies of Se and Te adsorption on metals. Selenium is found to adsorb at the high coordination (hollow) sites on the low Miller index surfaces Ni(100) and Ag(lOO). On the most open surface to have been examined, Ni(J10), the bond distance to the Ni atom in the second substrate layer (2.35 A) is slightly shorter than that to the top layer (2.42 A), suggesting the formation of a Ni-Se bond to the second substrate layer. However, it should be noted that the LEED studies of Se adsorption on metals originate before 1975, whilst more recent studies (1982) were by photo-electron diffraction only. Consequently, detailed substrate distortions, of the type seen in more recent studies of O and S adsorption on metals, have not been searched for. 

Like Se, Te is found to adsorb at the high coordination (hollow) sites on the low Miller index surfaces of Cu and Ni(100). Again, detailed substrate distortions, of the type seen in more recent studies of O and S adsorption, have not been considered. 

Although the sulfur-gold bond has been most investigated, the Group 16 elements selenium and tellurium have also attracted attention and are discussed in detail here (polonium has not received attention due to its radioactivity). 

Amines have been utilized to bind SAMs to gold surfaces and nanoparticles. Venkataraman et al. [197] found that in terms of molecular electronic junctions. 

Chen et al. [201] investigated the single-molecule conductance of alkanes terminated with either dicarboxylic-acid, diamine, or dithiol anchoring groups. The contact resistance was again found to be dependent on the anchoring group, which varies in the order Au—S Au—NH2 Au—COOH. 

Discussion. This gravimetric determination depends upon the separation and weighing as elementary selenium or tellurium (or as tellurium dioxide). Alkali selenites and selenious acid are reduced in hydrochloric acid solution with sulphur dioxide, hydroxylammonium chloride, hydrazinium sulphate or hydrazine hydrate. Alkali selenates and selenic acid are not reduced by sulphur dioxide alone, but are readily reduced by a saturated solution of sulphur dioxide in concentrated hydrochloric acid. In working with selenium it must be remembered that appreciable amounts of the element may be lost on warming strong hydrochloric acid solutions of its compounds if dilute acid solutions (concentration 6M) are heated at temperatures below 100 °C the loss is negligible. 

With tellurium, precipitation of the element with sulphur dioxide is slow in dilute hydrochloric acid solution and does not take place at all in the presence of excess of acid moreover, the precipitated element is so finely divided that it oxidises readily in the subsequent washing process. Satisfactory results are obtained by the use of a mixture of sulphur dioxide and hydrazinium chloride 

A process for the gravimetric determination of mixtures of selenium and tellurium is also described. Selenium and tellurium occur in practice either as the impure elements or as selenides or tellurides. They may be brought into solution by mixing intimately with 2 parts of sodium carbonate and 1 part of potassium nitrate in a nickel crucible, covering with a layer of the mixture, and then heating gradually to fusion. The cold melt is extracted with water, and filtered. The elements are then determined in the filtrate. 

Determination of tellurium Procedure. The solution should contain not more than 0.2 g tellurium in 50 mL of 3M hydrochloric acid (ca 25 per cent by volume of hydrochloric acid). Heat to boiling, add 15 mL of a freshly prepared, saturated solution of sulphur dioxide, then 10 mL of a 15 per cent aqueous solution of hydrazinium chloride, and finally 25 mL more of the saturated solution of sulphur dioxide. Boil until the precipitate settles in an easily filterable form this should require not more than 5 minutes. Allow to settle, filter through a weighed filtering crucible (sintered-glass, or porcelain), and immediately wash with hot water until free from chloride. Finally wash with ethanol (to remove all water and prevent oxidation), and dry to constant weight at 105 °C. Weigh as Te. 

In the alternative method of reduction, which is particularly valuable for the determination of small amounts of tellurium, the procedure is as follows. Treat the solution containing, say, up to about 0.01 g Te in 90 mL with 10 mL of 1 3-sulphuric acid, then add 10 g sodium hypophosphite (phosphinate), and heat on a steam bath for 3 hours. Collect and weigh the precipitated tellurium as above. 

In potassium pentathionate hemitrihydrate, the [S(S20s)J - ion has been found to occur in the trans configuration, with the terminal SO3 groups on opposite sides of the S3 central plane, in contrast to the m configurations found in three earlier studies. The ion thus has an approximate twofold axis, The outer S-S distances are 2.124 and 2.110 A, and the inner 2.021 and 2.036 A. The S-O distances range from 1.408 to 1.457 A. 

Since many of the crystal structures reported for selenium contain tellurium also, it is convenient to consider these elements together. 

A neutron diffraction study of selenous acid, HaSeOg, has confirmed its representation as OSe(OH)2. The selenium atom has pyramidal geometry with Se=0 at 1.643 A and Se-OH at 1.735 and 1.743 A. O-Se-O Angles are 96.39, 98.86, and 104.47°. The two selenite ions in acid lithium selenite  

A number of tellurium-containing mixed-oxide structures have been described. In all but one, tellurium is in the (+4) state. In barium tellurite monohydrate, [BaTe08,Ha0], the tellurium geometry is three-co-ordinate pyramidal with Te-O distances of 1.847, 1.858, and 1.859 A. These values are smaller than the sum of the covalent radii, and imply considerable multiple bonding. The next closest oxygen atoms are at distances of 3.035 and 3.322 A. 

Ditellurium pentoxide contains Te and Te atoms. The geometry of the former is the usual four-co-ordinate pyramidal with Te-O in the range 1.85—1.97 A. The Te atom site has octahedral geometry with Te-O at 1.89—2.08 A. This geometry is also found for both independent Te atoms 

RTeTeR with HjOj in aqueous THF involves a radical pathway. For reaction in an inert atmosphere, the mechanism involving reactions (50)-(52) is proposed. In the present of O2 the reaction is slowed down due to equilibrium (53). The reactivity of some related sulfur, selenium, and tellurium compounds has been reported.  

A number of systematic trends have been identified in the magnetic shielding of these nuclei, but their interpretation in the terms of Chapter 3 is often not 

Straightforward. It is reasonably well established that electron withdrawal from selenium or tellurium will lead to decreased shielding, as exemplified by series such as MeSe , Mc2Se, MejSe , and also by the low-frequency shifts of R3PE, which can be attributed to predominance of the R3P -E canonical form (E = Se or Te). In the series CF3SeX (X = C1, Br, CN, H, Ag) ( Se) correlated well with the electronegativity of and in various aromatic derivatives there are usually 

Other evidence of the dependence of 5 E) upon the electron withdrawing ability of substituents comes from work on tellurium salts and the species R2TeX2, and the substantial increase in resonance frequency that occurs when R2E forms a metal complex by lone pair donation. In addition, estimates of charge densities at selenium based on CNDO calculations in IX, X and other heterocycles support this 

Nonetheless it must be emphasized that there are significant deviations from this behavior, notably in some species p-XC6H4TeCl3 where greater electron release by the aryl group (as predicted by the characteristics ofX) increases in the pat- 

An exception to the generalization that 5 E) increases upon coordination of selenium or tellurium to a metal is provided by / -C5H5(C0)3 WP(Se) Ph2 with ( Se)= 160 ppm. When this is converted into XV there is a huge increase in shielding to 5 = —910 ppm which is clearly due to formation of the three-membered ring, since when Bu3PSe coordinates to mercury (II) via Se the shielding decreases by 147 ppm.  

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Selenium and tellurium occur naturally in sulphide ores, usually as an impurity in the sulphide of a heavy metal. They are recovered from the flue dust produced when the heavy metal sulphide is roasted. 

At high temperatures oxygen reacts with the nitrogen in the air forming small amounts of nitrogen oxide (p. 210). Sulphur burns with a blue flame when heated in air to form sulphur dioxide SO2, and a little sulphur trioxide SO3. Selenium and tellurium also burn with a blue flame when heated in air, but form only their dioxides, Se02 and Te02. 

With concentrated nitric acid, selenium and tellurium form only their +4 oxoacids, H2Se03 and H2Te03 respectively, indicating a tendency for the higher oxidation states to become less stable as the atomic number of the element is increased (cf. Group V, Chapter 9). 

Selenium and tellurium react similarly, forming selenides and selenates(IV), and tellurides and tellurates(IV) respectively. Like the sulphide ion, S , the ions Se and Te form polyanions but to a much lesser extent. 

Sulphur is less reactive than oxygen but still quite a reactive element and when heated it combines directly with the non-metallic elements, oxygen, hydrogen, the halogens (except iodine), carbon and phosphorus, and also with many metals to give sulphides. Selenium and tellurium are less reactive than sulphur but when heated combine directly with many metals and non-metals. 

These closely resemble the corresponding sulphides. The alkali metal selenides and tellurides are colourless solids, and are powerful reducing agents in aqueous solution, being oxidised by air to the elements selenium and tellurium respeetively (cf. the reducing power of the hydrides). 

The elements, sulphur, selenium and tellurium form both di- and tri-oxides. The dioxides reflect the increasing metallic character of... 

Oxygen halides are dealt with in Chapter 11, p. 334. Sulphur, selenium and tellurium form many halides, and only a brief introduction to the subject is given here. 

Ligands Other than Oxygen and Sulfur. See Sec. 3.1.7, Coordination Compounds, for acids containing ligands other than oxygen and sulfur (selenium and tellurium). 

Sihcon and boron bum ia fluorine forming siUcon tetrafluoride [7783-61-17, SiF, and boron trifluoride [7637-07-2] respectively. Selenium and tellurium form hexafluorides, whereas phosphoms forms tri- or pentafluorides. Fluorine reacts with the other halogens to form eight interhalogen compounds (see Fluorine compounds, inorganic-halogens). 

Selenium occurs in the slimes as intermetallic compounds such as copper silver selenide [12040-91 -4], CuAgSe disilver selenide [1302-09-6], Ag2Se and Cu2 Se [20405-64-5], where x < 1. The primary purpose of slimes treatment is the recovery of the precious metals gold, silver, platinum, palladium, and rhodium. The recovery of selenium is a secondary concern. Because of the complexity and variabiUty of slimes composition throughout the world, a number of processes have been developed to recover both the precious metals and selenium. More recently, the emphasis has switched to the development of processes which result in early recovery of the higher value precious metals. Selenium and tellurium are released in the later stages. Processes in use at the primary copper refineries are described in detail elsewhere (25—44). 

Soda. Ash Roasting. Some of the first processes to recover selenium on a commercial basis were based on roasting of copper slimes with soda ash to convert both selenium and tellurium to the +6 oxidation state. Eigure 1 shows flow sheets for two such processes. Slimes are intensively mixed with sodium carbonate, a binder such as bentonite, and water to form a stiff paste. The paste is extmded or peUetized and allowed to dry. Care in the preparation of the extmdates or pellets is required to ensure that they have sufficient porosity to allow adequate access to the air required for oxidation. 

The roasted pellets or extmdes are ground and leached in water. The hexavalent selenium dissolves as sodium selenate [13410-01 -0] Na2Se04. Sodium teUurate, being highly insoluble in the now very strongly alkaline solution, remains in the residue. The separation between selenium and tellurium is readily achieved, provided all tellurium is oxidized to the hexavalent state. 

Sulfation Roasting. Acid roasting technology (Fig. 2) rehes on differences in the volatiUty of the tetravalent oxides of selenium and tellurium at roasting temperatures of 500—600°C to selectively volatilise selenium from slimes. Acid roasting uses sulfuric acid as the oxidant for the conversion of selenium/selenides and tellurium/teUurides to their respective tetravalent oxides. Typical oxidation reactions are as foUow ... 

Losses of selenium and tellurium from the solution are negligible, provided the reactor is equipped with a reflux condenser. The wet chlorination is easily controlled. The reaction is rapid, allowing fast turnover of the precious metals in the slimes and yielding all the selenium and tellurium in soluble form. 

Purifying Selenium and Tellurium. Selenium recovery processes generally yield a metal product which contains some tellurium, and, correspondingly, recovered tellurium generally contains some small amount of selenium (37,41). 

Lea.d nd Le d Alloys. Selenium is reported to lower the surface tension of lead. The addition of 0.1% selenium and tellurium to solder improves its fluidity. 

A. A. Kudryavtsev, The Chemistry and Technology of Selenium and Tellurium, CoUets (Pubhshers) Ltd., London and WeUingborough, 1974. 

N. D. Sindeeva, Mineralogy andTypes of Deposits of Selenium and Tellurium, Interscience Pubhshers, a division of John Wiley Sons, Inc., New York, 1964. 

A. M. Lansche, Selenium and Tellurium—-A. Materials Survey, U.S. Bureau of Mines Information Circular 8340, Government Printing Office, Washington, D.C., 1967. 

S. M. Jasinski, Proceedings of the Fifth International Symposium on Industrial Uses of Selenium and Tellurium, Brussels, Belgium, May 1994, S elenium—T ehurium Development Association, Grimbergen, Belgium, pp. 13—20. 

D. H. Jennings, R. T. McAndrew, and E. S. Stratigakos, A Hydrometallurgical Methodfor Recovering Selenium and Tellurium from Copper Refinery Slimes, paper no. A 68-69, TMS, Warrendale, Pa., 1968. 

Nitrogen and sodium do not react at any temperature under ordinary circumstances, but are reported to form the nitride or azide under the influence of an electric discharge (14,35). Sodium siHcide, NaSi, has been synthesized from the elements (36,37). When heated together, sodium and phosphoms form sodium phosphide, but in the presence of air with ignition sodium phosphate is formed. Sulfur, selenium, and tellurium form the sulfide, selenide, and teUuride, respectively. In vapor phase, sodium forms haHdes with all halogens (14). At room temperature, chlorine and bromine react rapidly with thin films of sodium (38), whereas fluorine and sodium ignite. Molten sodium ignites in chlorine and bums to sodium chloride (see Sodium COMPOUNDS, SODIUM HALIDES). 

Tellurium is stUl recovered in some copper refineries by the smelting of slimes and the subsequent leaching of soda slags which contain both selenium and tellurium. The caustic slags are leached in water and, using the controlled neutralization process, tellurium is recovered as tellurium dioxide. 

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