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Anomalous Behaviour of Carbon

Carbon has the unique characteristics of forming compounds with multiple bonds such as C = C, C = O, C C, C N, etc.
It also exhibits the property of catenation, i.e. forming chains of identical atoms. This property is related to the strength of the C—C bond. The higher the bond enthalpy, the greater will be the tendency to form chains (C—C bond enthalpy 348 kJ mol-1 , Si—Si bond enthalpy 222 kJ mol-1).

The covalency of carbon is limited to four because of the non-availability of d orbitals. The other elements of the group are capable of forming compounds in which they attain a covalency higher than four.

The melting and boiling points of carbon are exceptionally high as compared to those of silicon and other elements of the group.

Which of the bond in each of the following pairs is stronger?
(a) C—C and Si—Si
(b) C—O and Si—O

(a) The C—C single bond is stronger than the Si—Si bond. This is due to the smaller size of C leading to the good overlap between the orbitals involved in the bond formation.
(b) The Si—O bond is much stronger than the C—O bond. It is due to the formation of pπ —dπ bond between Si and O atoms.

Describe the structural aspects of trimethylamine and trisilylamine.

Trimethylamine has a pyramidal structure whereas trisilylamine has a planar structure.

In trisilylamine, (H3Si)3N, nitrogen is sp2 hybridized, leaving a filled 2p orbital, which overlaps appreciably with a empty silicon 3dxz (or 3dyz) orbital. Thus, a dative pπ —dπ bond is established. This linkage provides additional bond strength in each Si—N bond. Because of this linkage the skeleton NSi3 forms a planar configuration.

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