Covalent silicon

A desirable glass melts at a reasonable temperature, is easy to work with, and yet is chemically inert. Such a glass can be prepared by adding a third component that has bonding characteristics intermediate between those of purely ionic sodium oxide and those of purely covalent silicon dioxide. Several different components are used, depending on the properties desired in the glass. 

This picture is reasonably valid for covalent silicon bnt rather simplistic for many of the semiconductors common in CD, which are usually mixed covalent and ionic. However, it serves to give a feeling for size quantization. For those readers who would prefer a more realistic interpretation for semiconductors with considerable ionic character, it is suggested that they construct a similar scheme for purely ionic materials and then imagine the required combination of ionic and covalent character. 

Pauling, in one of his last scientific contributions, pointed out that, based on the observed Si—C bond length, there is a covalent silicon-carbon (of the toluene) bonding in the triethylsilicenium-toluene complex 272. The bond number, n, is 0.35, according to equation 4,... 

Another point which needs to be clarified from the start is the nomenclature of metalated silanes We will frequently use the term silyl anion in this chapter when we talk about metalated silanes. Although the term anion defines, literally taken, an ionic compound, this expression, when used by us, does not necessarily imply that the compound in question is of ionic nature, but covers, as well, in analogy to the use of the term carbanion , silicon compounds with a polarized covalent silicon-metal bond. 

Indeed, the majority of the higher-covalent silicon compounds studied which have an increased coordination number are formed by bi-, tri-, and tetradentate ligands similar effects are apparent with other Group IVA elements. It is likely that the structural factors are responsible for the existence of pentacoordinate pentavalence carbon species expanding the valence shell to 10 (Sect. 3.2). For the same reason that there are about twenty structures of the organotin complexes with seven- and eight-coordination... 

It is known that proton chemical shifts are largely determined by electronic effects The coordinate Si<-D interaction, weakening the covalent silicon bonds (increasing their ionic character), enhances the electron density on the atoms of a hydrocarbon substituent which results, as a rule, in an increase of proton shielding. This is usually accompanied by a drahielding of protons of the donor fragment This chemical shift contribution, however, is often insignificant and masked by other effects, in particular, when the silicon atom has a 7t-donor substituent. 

We have found a linear correlation of the Si-H coupling constant with the Si-H stretching frequency for covalent silicon compounds bearing the 2-(dimethylamino-methyl)phenyl substituent (A-L) [6] (Fig. 3). The values of (obtained in the solid state) and (obtained in CjDj) of 4 (point M) fit this correlation. 

Silicon is more electropositive than carbon (and even more if compared to oxygen and nitrogen) and the covalent silicon-carbon bonds in the sp hybridization state, are... 

The covalent silicon carbide, SiC, is known as carborundum, a name derived from carbon + corundum. (Corundum, as discussed on p. 398, is an extremely hard mineral consisting of aluminum oxide, AI2O3.) SiC is a giant molecule like diamond, is extremely hard, and finds use as an abrasive, in cutting tools, and as a refractory. 

Typical results for a semiconducting liquid are illustrated in figure Al.3.29 where the experunental pair correlation and structure factors for silicon are presented. The radial distribution function shows a sharp first peak followed by oscillations. The structure in the radial distribution fiinction reflects some local ordering. The nature and degree of this order depends on the chemical nature of the liquid state. For example, semiconductor liquids are especially interesting in this sense as they are believed to retain covalent bonding characteristics even in the melt. 

There are hundreds of semiconductor materials, but silicon alone accounts for tire overwhelming majority of tire applications world-wide today. The families of semiconductor materials include tetraliedrally coordinated and mostly covalent solids such as group IV elemental semiconductors and III-V, II-VI and I-VII compounds, and tlieir ternary and quaternary alloys, as well as more exotic materials such as tire adamantine, non-adamantine and organic semiconductors. Only tire key features of some of tliese materials will be mentioned here. For a more complete description, tire reader is referred to specialized publications [6, 7, 8 and 9]. 

The group IV semiconductor materials are fourfold coordinated covalent solids from elements in column IV of tire periodic table. The elemental semiconductors are diamond, silicon and gennanium. They crystallize in tire diamond lattice. 

The oxidation state -1-4 is predominantly covalent and the stability of compounds with this oxidation state generally decreases with increasing atomic size (Figure 8.1). It is the most stable oxidation state for silicon, germanium and tin, but for lead the oxidation state +4 is found to be less stable than oxidation state +2 and hence lead(IV) compounds have oxidising properties (for example, see p. 194). 

The concept of oxidation states is best applied only to germanium, tin and lead, for the chemistry of carbon and silicon is almost wholly defined in terms of covalency with the carbon and silicon atoms sharing all their four outer quantum level electrons. These are often tetrahedrally arranged around the central atom. There are compounds of carbon in which the valency appears to be less than... 

Silicon, germanium, tin and lead can make use of unfilled d orbitals to expand their covalency beyond four and each of these elements is able (but only with a few ligands) to increase its covalency to six. Hence silicon in oxidation state -f-4 forms the octahedral hexafluorosilicate complex ion [SiFg] (but not [SiCl] ). Tin and lead in oxidation state -1-4 form the hexahydroxo complex ions, hexahydroxostannate(IV). [Sn(OH) ] and hexahydroxoplum-bate(IV) respectively when excess alkali is added to an aqueous solution containing hydrated tin(IV) and lead(IV) ions. 

Silicon, unlike carbon, does notiorm a very large number of hydrides. A series of covalently bonded volatile hydrides called silanes analogous to the alkane hydrocarbons is known, with the general formula Si H2 + 2- I uf less than ten members of the series have so far been prepared. Mono- and disilanes are more readily prepared by the reaction of the corresponding silicon chloride with lithium aluminium hydride in ether ... 

All Group IV elements form tetrachlorides, MX4, which are predominantly tetrahedral and covalent. Germanium, tin and lead also form dichlorides, these becoming increasingly ionic in character as the atomic weight of the Group IV element increases and the element becomes more metallic. Carbon and silicon form catenated halides which have properties similar to their tetrahalides. 

Silicon tetrafluoride is a colourless gas, b.p. 203 K, the molecule having, like the tetrahalides of carbon, a tetrahedral covalent structure. It reacts with water to form hydrated silica (silica gel, see p. 186) and hexafluorosilicic acid, the latter product being obtained by a reaction between the hydrogen fluoride produced and excess silicon tetrafluoride ... 

Silicon tetrachloride is a colourless liquid, b.p. 216.2 K, and again the molecule has a covalent structure. Silicon tetrachloride is hydrolysed by water ... 

The covalent carbides These include boron carbide B4C and silicon carbide SiC the latter is made by heating a mixture of silica and coke in an electric furnace to about 2000 K ... 

A completely dehydroxylated surface consists essentially of an array of oxygen atoms the Si-0 linkages are essentially covalent so that the silicon atoms are almost completely screened by the much larger oxygen atoms. Such a surface represents the extreme case and, even on samples ignited at 1100°C, a minute residue of isolated hydroxyl groups will be present. 

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