Coordination number of silicon

The relative ease of these transsilylations is because of the mobility of silyl groups, which is a consequence of the coordination number of silicon of 6, as discussed in the subsequent section. 

Crystalline Silica. Silica exists in a variety of polymorphic crystalline forms (23,41—43), in amorphous modifications, and as a liquid. The literature on crystalline modifications is to some degree controversial. According to the conventional view of the polymorphism of silica, there are three main forms at atmospheric pressure quartz, stable below about 870°C tridymite, stable from about 870—1470°C and cristobalite, stable from about 1470°C to the melting point at about 1723°C. In all of these forms, the structures are based on Si04 tetrahedra linked in such a way that every oxygen atom is shared between two silicon atoms. The structures, however, are quite different in detail. In addition, there are other forms of silica that are not stable at atmospheric pressure, including that of stishovite, in which the coordination number of silicon is six rather than four. 

Mechanisms for the substitution at silicon in solution have been a subject of several reviews (7-15). Since expansion of the coordination number of silicon is a common feature, particular interest is directed to mechanisms involving intermediates or transition states with silicon having a coordination number of 5 or 6. So far, little attention has been devoted in the review literature to mechanistic pathways that do not invoke extracoordination. Knowledge of these mechanisms, however, has often become necessary for the understanding of chemical behavior of organosilicon compounds. In this article we discuss mechanistic pathways involving heterolytic cleavage... 

In polymerizations of cyclic siloxanes, because of the higher coordination number of silicon the active centre can be formed by the decomposition of the transition complex with pentavalent silicon [208]... 

While complete X-ray analysis will establish the structure in the solid state, it is useful to have NMR data on the solution state that illustrate the increase of the coordination number of silicon. It would seem that NMR spectroscopy of nuclei participating directly in donor-acceptor interaction is especially important in investigating silicon compounds with an expanded coordination sphere. This requires the use of Si NMR spectroscopy since the electron shell of the silicon atom, the bond angles and lenghts are strongly affected upon complexation. Valuable information could also be obtained with by " N, N, 0, F NMR data since these elements act as donors. Chemical shifts of nuclei other than hydrogen are determined by various factors and not yet understood well anough to provide easily applied correlations of other physical properties of the molecules. 

On increasing the coordination number of silicon bonded to three and more fluorine atoms, the Jsip value decreases regularly (Table 15). The ratio between coupling constants in the spectra of SiF, SiF , and SiFg (1.6 1.35 1) corresponds to a regular decrease of s- and p-character of the bonding hybride silicon AOs in molecules of these compounds (1.5 1.2 1). 

A number of reviews concerning all aspects, of Si NMR is available they include but a brief note about silicon complexes. All experimental data for compounds in which the coordination number of silicon rises above four display the... 

Scheme 22 Synthons of variable coordination numbers of silicon and nitrogen...

A further increase in the coordination number of silicon from 4 to 5 (e.g. in silatranes) or 6 causes a significant increase in Si nuclear shielding. This is shown in Scheme 7 by the change of about 100 ppm in the Si values of silicates bearing five or four organyl groups. ... 

This unexpected difference between two apparently similar reactions raises the question of whether the N->Si coordinated octahedral 3 can also, under certain conditions, dissociate to form pentacoordinate siliconium chloride salts. To answer this question we have re-investigated the chemistry of 3 in some detail. We now report on the reversible ionic dissociation of 3 to siliconium salts, and the resulting control of the coordination number of silicon by various means temperature,... 

The equilibrium position is strongly affected by a variety of factors, and can be shifted to either extreme under suitable conditions. It is thus possible to effectively control the coordination number of silicon to be either 5 or 6, in a reversible manner, and to modify the complex s properties accordingly. This may prove useful in the future design of molecular switches. 

Her advanced the hypothesis that the polymerization mechanism may involve the temporary expansion of the coordination number of silicon from four to five or six. just as in the case of OH" ion. It is not known whether the HF-catalyzed reaction is third order as in the H -catalyzed reaction in the absence of fluoride below pH 2. or... 

The coordination number of silicon is a much debated quantity in the literature, which we discuss further later. The large discrepancy between different experimental calculations of the radial distribution function and of the density leads to large variations in the calculated coordination number. We discuss this in detail in Section VI. 

The +4 oxidation state (z = 4) is the only important one in the chemistry of silicon in naturally occurring systems [10], and the coordination number of silicon, N, is most often four. Compared to transition metals discussed in the previous chapter, silicon is generally less electropositive, e.g., the partial positive charge on silicon nucleophilic attack, and since N = z, coordination expansion does not spontaneously occur with nucleophilic reagents. These factors make the kinetics of hydrolysis and condensation considerably slower than observed in transition metal systems or in Group III systems. 

The preferred coordination number of silicon is 4, and is thus equal to its valence (-I-IV). For metals, especially transition metals, on the other hand, the preferred coordination number is higher than their valence. The increase of the coordination number beyond the valence is reached by interaction with any nucleophilic (Lewis basic) entity in the system, as will be discussed in more detail below. 

Although the normal coordination number of silicon is four, the atom still acts as a Lewis acid, especially if the silicon is bound to electronegative ligands and hence coordination numbers of five and six are... 

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