Metallic silicon

If an excess of magnesium is used, magnesium silicide, Mg2Si, is also produced.) The silicon obtained is a light brown hygroscopic powder. Crystalline or metallic silicon is obtained industrially by the reduction of silica with carbon in an electric arc furnace ... 

The Peterson reaction has two more advantages over the Wittig reaction 1. it is sometimes less vulnerable to sterical hindrance, and 2. groups, which are susceptible to nucleophilic substitution, are not attacked by silylated carbanions. The introduction of a methylene group into a sterically hindered ketone (R.K. Boeckman, Jr., 1973) and the syntheses of olefins with sulfur, selenium, silicon, or tin substituents (D. Seebach, 1973 B.T. Grdbel, 1974, 1977) illustrate useful applications. The reaction is, however, more limited and time consuming than the Wittig reaction, since metallated silicon derivatives are difficult to synthesize and their reactions are rarely stereoselective (T.H. Chan, 1974 ... 

Fumed silicas (Si02). Fumed silicas are common fillers in polychloroprene [40], natural rubber and styrene-butadiene rubber base adhesives. Fumed silicas are widely used as filler in several polymeric systems to which it confers thixotropy, sag resistance, particle suspension, reinforcement, gloss reduction and flow enhancement. Fumed silica is obtained by gas reaction between metallic silicon and dry HCl to rend silica tetrachloride (SiCU). SiC is mixed with hydrogen and air in a burner (1800°C) where fumed silica is formed ... 

Silicon analogues of imidazole-2-ylidenes are stable carbenes that form adducts where the metal-silicon bond is relatively weaker than that between metal and carbon atoms. 

Aluminum, silicon, and sulfur are close together in the same row of the periodic table, yet their electrical conductivities are widely different. Aluminum is a metal silicon has much lower conductivity and is called a semiconductor sulfur has such low conductivity it is called an insulator. Explain these differences in terms of valence orbital occupancy. 

An interesting variant of metal-silicon bond formation is the combination of metal halides with silyl anions. Since silyl dianions are not available, only one metal-silicon bond can be formed directly. The silylene complexes are then accessible by subsequent reaction steps [113], An example of this approach is given by the reaction of cis-bistriethylphosphaneplatinumdichloride 25 with diphenylsilylli-thium, which yields, however, only dimeric platinadisilacyclosilanes 26a-c [114]. 

The oxidative addition of silanes (with silicon-hydrogen bonds) to coordinatively unsaturated metal complexes is one of the most elegant methods for the formation of metal-silicon bonds. Under this heading normally reactions are considered which yield stable silyl metal hydrides. However, in some cases the oxidative addition is accompanied by a subsequent reductive elimination of, e.g., hydrogen, and only the products of the elimination step can be isolated. Such reactions are considered in this section as well. 

Compared to the sum of covalent radii, metal-silicon single bonds are significantly shortened. This phenomenon is explained by a partial multiple bonding between the metal and silicon [62]. A comparison of several metal complexes throughout the periodic table shows that the largest effects occur with the heaviest metals. However, conclusions drawn concerning the thermodynamic stability of the respective M —Si bonds should be considered with some reservation [146], since in most cases the compared metals show neither the same coordination geometries nor the same oxidation states. 

For a theoretical consideration of the metal-silicon interaction in silylene complexes, the fragment orbital description proves to be very useful [148], This approach has been extensively used in the organometallic chemistry of carbon and allows a basic understanding of the interrelations also by means of a qualitative description. 

Carbon forms ionic carbides with the metals of Groups 1 and 2, covalent carbides with nonmetals, and interstitial carbides with d-block metals. Silicon compounds are more reactive than carbon compounds. They can act as Lewis acids. 

The high temperatures of coal char oxidation lead to a partial vaporization of the mineral or ash inclusions. Compounds of the alkali metals, the alkaline earth metals, silicon, and iron are volatilized during char combustion. The volatilization of silicon, magnesium, calcium, and iron can be greatly enhanced by reduction of their refractory oxides to more volatile forms (e.g., metal suboxides or elemental metals) in the locally reducing environment of the coal particle. The volatilized suboxides and elemental metals are then reoxidized in the boundary layer around the burning particle, where they subsequently nucleate to form a submicron aerosol. 

Metal/silicon mixer 40 gm 300 gm feed channel width depth Slit depth in steel 60 pm housing... 

Karthaus, O., Terayama, T. and Hashimoto, Y. (2006) Selective dewetting of polymers on metal/silicon micropatterns. e-J. Surf. Sci. Nanotechnol., 4, 656-660. 

This method offers a new route to the formation of transition metal silicon bonds. 

Transition-metal chemistry is currently one of the most rapidly developing research areas. The record of investigation for compounds with metal silicon bonds is closely comparable to that for silicones it was in 1941 when Hein discovered the first metal silicon complex, followed by Wilkinson in 1956. A milestone in the development of this chemistry was Speier s discovery of the catalytic activity of nobel metal complexes in hydrosilylation reactions in 1977. Hydrosilylation is widely used in modem organic syntheses as well as in the preparation of organo functionalized silicones. Detailed investigations of the reaction mechanisms of various catalysts continue to be subject of intense research efforts. 

The importance of metal silicon compounds is demonstrated by a steadily increasing number of applications in organic synthesis, catalysis and material science. 

The preparative potential of silyl- or stannyl-substituted polynuclear complexes is currently far from being exploited. Due to the reactivity of silyl ligands, selective cleavage of metal-silicon bonds is possible. In some cases this was observed during the reaction of an anionic silyl complex with metal... 

Synthesis and Characterization of New Transition Metal-Silicon Compounds... 

Summary Many cyclic and linear transition metal-silicon compounds have been obtained by the elimination of alkali halides. The structures of these complexes were determined by INEPT and INEPT-INADEQUATE NMR spectroscopy. 

Transition metal-silicon compounds are thought to be precursors in metal silicide CVD processes. The main advantage compared with common precursors is the defined silicon to metal ratio. 

New Chemistry for Reactive Transition-Metal Silicon Bonds... 

These new silyl complexes are monomeric in benzene solution, but are dimeric in the solid state. The crystal structure of the samarium analogue (Figure 1) shows that dimers form via intermolecular Sm-CH3-Si interactions. The Sm-Si distance, 3.052 (8) A, is to our knowledge the longest metal-silicon distance known. As with other complexes that display Ln-CH3-Si interactions in the solid state, evidence for these interactions in solution is not observed in NMR spectra. The [H NMR chemical shifts for the neodymium... 

All alkali metal hypersilanides display highly pyramidal central silicon centers with Si-Si-Si angles around 100°. Remarkably, these angles do not differ in the contact or separated ion pairs, further indicating the large degree of charge transfer from the alkali metal cation to the silanide anion. The pyramidalization is also an expression of the increased />-character of the Si-Si bond, and consequent increase in the alkali metal-silicon bond. 

In a further improvement, the NTD Ge spider-web bolometer developed by the Caltech/JPL group [69], and shown in Fig. 15.10, the absorber is a metallized silicon nitride spider-web structure, and the thermometer is a very small NTD germanium crystal [14], The spacing of the web pattern is much smaller than the wavelength, so it acts... 

T. Shibata, A. Wakita, T.W. Sigmon, and James F. Gibbons, Metal-Silicon Reactions and Silicide... 

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