Comparison of silicon and carbon

Silicon and carbon command dominant positions in inorganic chemistry (silicates) and organic chemistry (hydrocarbons and their derivatives), respectively. Although they have similar valence electronic configurations, [He]2s22p2 for C and [Ne]3s23p2 for Si, their properties are not similar. The reasons for the difference between the chemistry of the two elements are elaborated below. 

The electronegativity of C is 2.54, as compared with 1.92 for Si. Carbon is strictly nonmetallic whereas Si is essentially a non-metallic element with some metalloid properties. 

Unlike carbon, the valence shell of the silicon atom has available d orbitals. In many silicon compounds, the d orbitals of Si contribute to the hybrid orbitals and Si forms more than four 2c-2e covalent bonds. For example, Sib s- uses sp3d hybrid orbitals to form five Si-F bonds, and SiF62 uses sp3d2 hydrid orbitals to form six Si-F bonds. 

The term catenation is used to describe the tendency for covalent bond formation between atoms of a given element to form chains, cycles, layers, or 3D frameworks. Catenation is common in carbon compounds, but it only occurs to a limited extent in silicon chemistry. The reason can be deduced from the data listed in Table 14.4.1. 

Inspection of Table 14.4.1 shows thatE(C-C) E(Si-Si),. E(C-H) (Si-H) and E(C-C) E(C-O), but (Si-Si) (Si-0). Thus alkanes are much more stable than silanes, and silanes react readily with oxygen to convert the Si-Si bonds to stronger Si-O bonds. Although silanes do not exist in nature, some compounds with Si-Si and Si=Si bonds have been synthesized in the absence of air and in non-aqueous solvents. The silanes Si H2 +2 (n = 1 — 8), cyclic silanes Si H2n (n = 5,6), and some polyhedral silanes are known. The structures of tetrahedral Si Si ButU, trigonal-prismatic Si6(2,6- PrC6H3)6, and cubane-like Sis(2,6-Et2C6H3)8 are shown below  

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