Comparison of Carbon with Silicon

Diamond is an insulator, but silicon is a semiconductor. This is because the band gap in Si is smaller than that in C.

There is no Si analogue of graphite; the larger size if the Si atom means that the overlap of the Si p-orbitals is poor, and hence that Si pπ-pπ bonding is poor.

Si forms strong bonds to more electronegative elements, eg. Si-F and Si-O, whereas C forms strong bonds with less electronegative elements, eg. C-H. Hydrocarbons are more stable than silicon hydrides both with respect to decomposition to the elements and combustion to the dioxide and water. Silane (SiH4) is also spontaneously flammable (it is labile) whereas methane requires sparking to burn (it is inert).

The structure of the simple compounds is determined by the ability of the Si d-orbitals to contribute to bonding. For example, (CH3)20 has a C-O-C angle of 109o, but (SiH3)2O has a Si-O-Si angle of 144o. This is due to a Si dπ-O pπ bonding interaction leading to a more linear arrangement of the Si-O-Si system. Also, (SiH3)3N is planar whereas (CH3)3N is pyramidal, and this is due to the same type of interaction, where the overlap is greatest when a planar arrangement is adopted.

Si forms long lived compounds, eg. SiF62-, in which the coordination number of Si is greater than 4, whereas C has coordination numbers which do not exceed 4. Neutral Si compounds such as SiCl4 are Lewis acids; this is due to the ability of the low lying vacant d-orbitals to act as electron acceptors. The formation of compounds by the Lewis acid-base interaction of the vacant d-orbitals is known as valence shell expansion.

Multiple bonding (pπ-pπ in addition to ordinary σ-bonds) only occurs in Si at high temperature, or in inert matrices at low temperature. These can be stabilized at room temperature by the use of bulky ligands, eg. R2Si=SiR2, where R is a substituted phenyl group. B(E=E)<2B(E-E) for both Si and C, but the double bond in the alkene is kinetically stabilized, eg. the polymerization of ethene requires a catalyst. As a result of this Si forms n single bonds rather than a bond of order n; eg. SiO2 is a macromolecular solid formed from vertex shared SiO4 tetrahedra, while CO2 is a molecular gas.

Silicon (and Germanium) Halides: Silicon and Germanium halides are mild Lewis acids, and the most important is SiCl4. They react with Lewis base ligands to give five- and six-coordinate complexes. This formation of hypervalent complexes (the valence shell expansion) leads to the much faster hydrolysis of Si/Ge halides compares to CX4. The water lone pair can bond to the vacant d-orbitals on Si/Ge to form  a six-coordinate species, MX4(OH2)2, followed by elimination of HX to leave the dioxide, MO2. This mechanism is not possible in CX4, and hence it is hydrolyzed more slowly.