Alkali metal selenides

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). 

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... 

Alternatively, the compound may be prepared by treating an alkali metal selenide with hydrochloric acid during mild heating ... 

A similar reduction of selenium with a metal naphthalenide provides an in situ synthesis of alkali metal selenides and diselenides.3... 

The enthalpies of formation of the alkali metal selenides were re-evaluated by the review from the enthalpy changes of Reactions 1 and 2 exemplified for lithium in Table A-3. The temperature of the experiments was about 288 K. The reactions combine to Reaction 3 from which the enthalpy of formation of the alkali metal selenide was calculated with selected auxiliary data at 298.15 K. A correction for the formation of some Se in Reaction 2 cannot be made due to lack of data. This also pertains to the correction of the data from the experimental temperature to 298.15 K. The results are assumed to be valid at standard conditions within the error limits and the following val-... 

The synthesis of hexamethyldisilathiane from sodium sulfide and chloro-trimethylsilane is described here. The present method is based on the convenient in situ syntheses of alkali metal selenides and diselenides. Commercial sodium sulfide or lithium sulfide are reported to be poor substitutes for in situ generated sulfides in this reaction. For example, in 1961 Abel reported that disodium sulfide reacts with chlorotrimethylsilane in pressure vessels at 250°C for 20 h to produce I. Our procedure is very convenient, utilizing readily available starting materials and apparatus under mild conditions. The yields are t3q)ically 80-88% at 0.3-mol scale. However, it can be improved to 90-95% on small scale ( 50-mmol) reactions. This procedure can be applied to the synthesis of various disilathianes. 

Discrete polyselenide chains Se without significant secondary intra- or inter-molecular Se- Se contacts have been structurally characterized for n = 2-8. Crystal structures are known in addition to K5Sc3 for the following binary alkali metal selenides A2Sen in the range n = 2-5 A2Sc2 for A = Na, K,... 

Alkali met hydrides, superactive -, reactions with - 43, 34 Alkali metal selenides -, in s/tu-preparation 43, 529 Alkene oxides s. Oxido compds. Alkenes s. Ethylene derivs. 

In each of the composition diagrams in Fig. 14.2, the numbers represent a series of reactions run at a defined composition and temperature. These are isometric sulfur slices through three-dimensional K/P/RE/S quaternary phase diagrams. As just one example of what we have studied. Table 14.1 identifies the compositions at each point and the resulting phase(s). We have rigorously studied how phase formation is dependent upon the compositions of reactions for the rare-earth elements Y, Eu, and La and we have also discovered key structural relationships between the rare-earth elements, indicating a significant dependence on rare-earth and alkali-metal size for sulfides and selenides. 

Selenium is stable in water and in aqueous solutions over the entire pH interval in the absence of any oxidizing or reducing agent. Selenium can be electrochemically reduced to hydrogen selenide or to selenides that are unstable in water and aqueous solutions. It can be oxidized to selenous acid or selenites and further (electrolyti-cally) to perselenic acid (H2Se20s). Selenic and selenous acids and their salts are stable in water. The selenides, selenites, and selenates of metals other than the alkali metals are generally insoluble. 

Layered dichalcogenides, such as sulfides or selenides of Ti, Mo, W, Zr, Ta, Nb, V, and Cr, are able to dissolve certain alkali metal ions and in some cases posttransition (pseudo-alkali) metal ions (Cu, Ag ), via a mechanism in which the guest is inserted between the dichalcogenide layers. Lithium ion intercalation into TiS2, which among layered dichalcogenides has been one of the most prominent cathode... 

The analogous selenium and tellurium species are best formed directly as their alkali-metal or ammonium salts, since the acids themselves tend to be much more unstable towards decomposition than the dithio-phosphorus acids, and are hence difficult to store and handle. Diselenophosphates can be prepared from refluxing a suspension of phosphorus(V) selenide in alcohol followed by treatment with potassium hydroxide or ammonia gas to give the... 

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. Selenium forms compounds with most elements. Binary compounds of selenium with 58 metals and 8 nonmetals, and alloys with three other elements have been described (55). Most of the selenides can be prepared by a direct reaction. This reaction varies from very vigorous with alkali metals to sluggish and requiring high temperature with hydrogen. 

Binary Selenides. Most binary selenides are formed by heating selenium in the presence of the element, reduction of selenites or selenates with carbon or hydrogen, and double decomposition of heavy-metal salts in aqueous solution or suspension with a soluble selenide salt, eg, Na2Se or (NH Se [66455-76-3]. Atmospheric oxygen oxidizes the selenides more rapidly than the corresponding sulfides and more slowly than the tellurides. Selenides of the alkali, alkaline-earth metals, and lanthanum elements are water soluble and readily hydrolyzed. Heavy-metal selenides are insoluble in water. Polyselenides form when selenium reacts with alkali metals dissolved in liquid ammonia. Metal (M) hydrogen selenides of the M HSe type are known. Some heavy-metal selenides show important and useful electric, photoelectric, photo-optical, and semiconductor properties. Ferroselenium and nickel selenide are made by sintering a mixture of selenium and metal powder. 

The selenides of the alkali metals are white when pure, but generally appear pink owing to the presence of free selenium or of polyselenides, as also do the alkali hydroselenides moreover they readily oxidise in air, with separation of selenium. The selenides of the alkaline earth metals are described as pink, but it is questionable whether they have yet been obtained in a pure condition. 

Phosphorus Monoselenide, P2Se, may be obtained by heating one atomic proportion of selenium with two atomic proportions of phosphorus in a current of hydrogen. It is formed with incandescence as a clear red solid, which is combustible and slightly soluble in carbon disulphide. It is decomposed by boiling alkalis witli liberation of phosphine. With metal selenides it forms double selenides. 

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