Selenides formation

Thermodynamic analyses of metal sulphide formation from thiosulphate [52] and thiourea [53] and metal selenide formation from selenosulphate [13,54]... 

Increase in pH (= increase in [OH ]) shifts the equilibrium to the left, resulting in a lower concentration of free Pb ions and thus a slower reaction to give PbSe. This means that, in contrast to the deposition from a selenourea bath described earlier, the rate is dependent on Pb concentration and possibly independent of hydroxide concentration at a constant free-Pb concentration. This would then suggest that the opposite mechanism, i.e., a complex decomposition, is effective for the selenosnlphate bath. It is stressed that these conclusions on selenide formation... 

Nuttall KL. 1987. A model for metal selenide formation under biological conditions. Med Hypotheses 24 217-221. 

By varying the concentration of sodium hypophosphite and of the copper salt, the molecular weight distribution of the polymer may be controlled readily. Among the copper salts suggested are the sulfate, chloride, selenide, formate, and lactate, as well as the acetate. 

Another application concerns the preparation of selenides and tellurides from diaryl diselenides ° or ditellurides. While selenide formation is carried out in refluxing toluene by using a large excess of diselenides, diaryl ditellurides give the best results by utilizing visible irradiation at 35 C. Aryl, benzyl, methyl, and dicyanogen selenides can be prepared in this way. 

Thus a second method was envisaged, the reaction of a nitrile, hydrogen selenide, and an a-halogenated ketone in the presence of a condensation catalyst, which can be POCl, or POCI3 with a Lewis acid such as PCI3 or anhydrous ZnCl. The use of fresh AICI3 leads to the formation of tarry side-products. 

According to Hofman-Bang carbon sulfide selenide, CSSe, catalyzes the iodine-azide reaction but is at the same time decomposed with the formation of selenium. Experiments, in both this laboratory and that of Hofman-Bang have shown that carbon diselenide reacts with sodium azide (in aqueous or aqueous-alcoholic solution) with immediate precipitation of red selenium even at — 20° C. //a selena-triazole is formed in this reaction it must be extremely unstable. 

The use of (TMSlsSiH with acyl selenides can also yield new C-C bond formation, as shown with the a,/l-unsaturated lactam ester (Reaction 67). The resulting ketone can be envisaged as potentially useful for the synthesis of 2-acylindole alkaloids. Both the effects of H-donating ability and steric hindrance by the silicon hydride are evident. 

As discussed earlier the whole process is a redox reaction. Selenium is reduced using sodium borohydride to give selenide ions. In the above reaction, the metal ion reacts with the polymer (PVP or PVA) solution to form the polymer-metal ion solution. Addition of the selenide ion solution to the polymer-metal ion solutions resulted in instantaneous change in the colour of the solutions from colourless to orange (PVA) and orange red (PVP). This indicates the formation of CdSe nanoparticles. The addition of the selenide solution to the polymer - metal ion solution resulted in gradual release of selenide ion (Se -) upon hydrolytic decomposition in alkaline media (equation 4). The released selenide ions then react with metal ion to form seed particles (nucleation). 

In this reaction, the complexation of the metal ion by the polymer is expected to play a significant role in the formation of the nanocrystalline selenides. During the reaction, the... 

The synthesis of a-substituted phosphonates 89, via the electrophilic addition of phosphorylated C-radicals 88 (generated by reaction of BujSnH to the readily accessible a-phosphoryl sulfides (or selenides)) and electrophilic addition to electron rich alkenes, has been described [57] (Scheme 26). A large excess of alkene is necessary to minimize the competitive formation of the undesired compound 90 resulting from direct reduction of the initial radical 88. The ratio 89/90 has been measured for each example. The synthesis of the a-mono- or a,a-di-substituted (R or phosphonates 89 shows that the free radical approach... 

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. 

The induced co-deposition concept has been successfully exemplified in the formation of metal selenides and tellurides (sulfur has a different behavior) by a chalcogen ion diffusion-limited process, carried out typically in acidic aqueous solutions of oxochalcogenide species containing quadrivalent selenium or tellurium and metal salts with the metal normally in its highest valence state. This is rather the earliest and most studied method for electrodeposition of compound semiconductors [1]. For MX deposition, a simple (4H-2)e reduction process may be considered to describe the overall reaction at the cathode, as for example in... 

The value of this method lies in the fact that formation of elemental selenium is unlikely to occur since the high-valency species such as Se(IV) that could oxidize the selenide ions are absent from solution. The SeSO and SOj ions (or their protonated forms) do not oxidize Se , while any free Se that may be formed would redissolve in sulfite giving selenosulfate again, since the latter is prepared by dissolving Se in excess sulfite. 

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