Alkali tellurides

Alkali tellurides, among which sodium telluride is the most widely employed, are powerful nucleophilic reagents and therefore react easily with alkylating agents. 

The normal low reactivity of aryl halides towards nucleophilic reagents is not generally observed in their reaction with alkali tellurides. The observed reactivity seems to be... 

Crystalline Tellurium. — Molten tellurium solidifies to a brittle, silvery, crystalline mass, which is easily powdered. The crystalline modification can also be obtained by sublimation of the element or by its slow formation, for example in the gradual decomposition of hydrogen telluride5 or in the slow atmospheric oxidation of an aqueous solution of an alkali telluride.6 When obtained of appreciable size the crystals are generally found to be prismatic, of the trigonal system, and isomorphous with metallic selenium (a c=l 1-3298 a=86-8°).7... 

Scheme 2 Generation of the most important alkali tellurides used in organic synthesis. ...

Alkali tellurides 13 and 14, or ditellurides 15, are potent nucleophiles and react rapidly with alkyl halides. Table 1 presents some representative examples of such reactions leading to dialkyl tellurides 3 and dialkyl ditellurides 4. Alkali organotellurolates 2 are also potent nucleophiles and are used to prepare symmetrical and unsymmetrical tellurides (Table 2). 

The most commonly used photocathodes are metallic or alkaline (alkali halides, alkali antimonide or alkali telluride) cathodes. The quantum efficiency rj = is defined as the ratio of the rate of photoelectrons to the rate of incident photons... 

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). 

However, simple stoichiometry often conceals structural complexity as in the many alkali metal tellurides MTe (jc = 1, 1.5, 2.5, 3,... 

No other oxide phases below MO2 have been established but a yellow hydroxide , precipitated by alkali from aqueous solutions of chromium(II), spontaneously evolves H2 and forms a chromium(III) species of uncertain composition. The sulfides, selenides and tellurides of this triad are considered on p. 1017. 

All the dialkali monosulfides are soluble in water and give alkaline solutions. The tellurides are instantly decomposed by air. They are soluble in water, but the solutions are easily oxidized to red polytellurides. The alkali metal tellurides are strong reducing agents which reduce tellurites to metallic tellurium. 

Sheldrick WS, Wachhold M, Jobic S, Brec R, Canadell E (1997) New low-dimensional solids tellurium-rich alkali metal tellurides. Adv Mater 9 669-675 Zheng Zh, Greedan JE (2004) Rare earth elements and materials. In Meyers RA (ed) Encyclopedia of Physical Science and Technology - Inorganic Chemistry. Elsevier Science, URL ... 

Alkali-metal salts, 24 145 Alkali-metal tellurides, 24 417 Alkali-metal thermal electric conversion (AMTEC), 22 773-774 Alkali-metal titanates, 25 43 44 Alkaline autoclaving, selenium recovery via, 22 81... 

For example, no less than 12 different formula types are now known for the alkali metal tellurides MxTey [9, 10]. As the total bond order for Te-Te bonds and their opposite secondary Te---Te interactions must remain constant at 1.0, distances di/di will increase in logarithmic dependence on their individual bond order as d3/d4 shorten. Values of d-y/di can range from those of typical single bonds (269-287 pm [10]) to 304 + 9 pm for symmetrical and modestly distorted 3-center 4-electron interactions. 

As will be discussed in Section 2.7.3.1, a number of duster-like non-valence precise Te moieties (n = 4, 6) have also been identified in alkali metal tellurides but these alleviate their electron surplus by participating in asymmetric linear 3-... 

Sulphides. The partially ionic alkali metal sulphides Me2S have the anti-fluorite-type structure (each Me surrounded by a tetrahedron of S, and each S atom surrounded by a cube of Me). The NaCl-structure type (6/6 coordination) is adopted by several mono-sulphides (alkaline earth, rare earth metals), whereas for instance the cubic ZnS-type structure (coordination 4/4) is observed in BeS, ZnS, CdS, HgS, etc. The hexagonal NiAs-type structure, the characteristics of which are described in 7.4.2.4.2, is observed in several mono-sulphides (and mono-selenides and tellurides) of the first-row transition metals the related Cdl2 (NiAs defect-derivative) type is formed by various di-chalcogenides. Pyrite (cP 12-FeS2 type see in 7.4.3.13 its description, and a comparison with the NaCl type) and marcasite oP6-FeS2 are structural types frequently observed in several sulphides containing the S2 unit. 

Selenides, tellurides andpolonides. Se, Te and Po react easily with most metals and non-metals to form binary compounds (selenides and tellurides are common mineral forms of these elements). Non-stoichiometry is frequently observed in the compounds with the transition elements many of these compounds may be described as metallic alloys. The compounds of the metals of the first two groups may be considered the salts of the acids H2Se, H2Te, etc. The alkali metal selenides... 

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