Electronegativity, carbon compared with silicon

Figure 6 shows relative orbital energies of trimethylvinyl-silane and allyltrimethylsilane compared with ethylene. The fact that IP of trimethylvinylsilane is lower than that of ethylene indicates the electron-donating effect of the triaUcylsilyl group that is due to the lower electronegativity of silicon than carbon. But at the same time, the LUMO of trimethylvinylsilane is lowered. The HOMO of allyltrimethylsilane is raised relative to ethylene. These facts give the grounds that trimethylvinylsilane is susceptible to nucleophilic attack, whereas allyltrimethylsilane is reactive towards electrophiles. 

As shown in this table, carbon, boron, and silicon have comparable electronic structure. They also have some of the smallest atoms. Silicon and boron are similar elements which can be considered borderline cases between metals and nonmetals. They also have lower electronegativity than carbon and, by convention, their compounds with carbon can be called carbides (see Sec. 2.0 of Ch. 2). The differences between the atomic structure, electronegativity, and atomic radius of these three elements are not as significant as those between carbon and the transition metals (see Ch. 3, Table 3.8). 

The hypothesized delocalization of lone pair electrons in the above silicon compounds is supported by the lowered basicity of the silyl compounds as compared to the corresponding carbon compounds. This reduced basicity is contrary to that expected on the basis of electronegativity effects operating through the a system since silicon is less electronegative than carbon. It is consistent with an internal Lewis acid-base interaction between the nitrogen and oxygen lone pairs and empty acceptor d orbitals on the silicon. Experimentally this reduced basicity is shown by the absence of disiioxane adducts with BF3 and BO ... 

Different elective affinity of elements as compared to carbon. Electropositive elements (Si, B, AI, P) have a considerably larger affinity to electronegative elements than carbon. In other words, silicon, boron, aluminum, phosphorus and other elements form weaker bonds with electropositive elements (H, Si, B, Al, As, Sb, Bi, etc.), and stronger bonds with electronegative elements (O, N, Cl, Br, F, etc.) than carbon. 

We should compare the S reaction at silicon with the S 2 reaction at carbon. There are some iportant differences. Alkyl halides are soft electrophiles but silyl halides are hard electrophiles. Alkyl halides react only very slowly with fluoride ion but silyl halides react more rapidly with fluoride [than with any other nucleophile. The best nucleophiles for saturated carbon are neutral and/or based on elements down the periodic table (S, Se, I). The best nucleophiles for silicon are charged and based on highly electronegative atoms (chiefly F, Cl, and O). A familiar example is the reaction of enolates at carbon with alkyl halides but at oxygen with silyl chlorides (Chapter 21). 

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