SIGMA o BONDS

All single bonds between two atoms are sigma (o) bonds. Pi bonds can only be formed after a sigma bond has already been formed. Therefore a double bond contains one o and one n bond, and a triple bond contains one o and two k bonds. 

Sigma (o) bonds are formed by the end to end overlap of two orbitals. This overlap can take place between s orbitals, p orbitals or hybrid orbitals. 

For example, in the methane molecule (CH4), the four sp3 hybrid orbitals of the carbon atom overlap end to end with one Is orbital from each hydrogen atom to form four C — H bonds. Those bonds are all o bonds. 

When carbon atoms undergo sp3 hybridization, the hybrid orbitals form sigma bonds. 

Pi (ti) bonds are formed by the side by side overlap of two parallel p orbitals. In the n bond, the electron cloud lies above and below the plane formed by o bonds, n bonds are weaker than o bonds. 

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In alcohols, the oxygen of the -OH group is attached to carbon by a sigma (o-) bond formed by the overlap of a sp hybridised orbital of carbon with a sp hybridised orbital of oxygen. Fig. 11.1 depicts structural aspects of methanol, phenol and methojqmiethane. 

Sigma (o) bonds Bonds formed by the head-on overlap of sp, sp2, or sp3 hybrid orbitals with each other or with hydrogen 1s orbitals. 

Formation of a er bonding MO. When the Is orbitals of two hydrogen atoms overlap in phase, they interact constmctively to form a bonding MO. The electron density in the bonding region (between the nuclei) is increased. The result is a cylindrically symmetrical bond, or sigma (o-) bond. 

The electrons shared in a covalent bond result from an overlap of atomic orbitals to give a new molecular orbital. Electrons in the Is- and 2s-orbitals combine to give sigma (o-) bonds. 

Sigma (o-) bonds are strong bonds formed by head-on overlap of two atomic orbitals. 

Sigma (o) bond A bond resulting from electron occupation of a sigma molecular orbital. 

Fig. 9.11. In each of these bonds, the electron pair is shared in an area centered on a line running between the atoms. This type of covalent bond is called a sigma (o ) bond. In the ethylene molecule, the cr bonds are formed using sp orbitals on each carbon atom and the I5 orbital on each hydrogen atom.

Sigma (o) bond (7.6) A chemical bond in which the electron density is concentrated along the line of centers of the two atoms, such as occurs through the interaction of two s orbitals or the end-to-end interaction of twop orbitals. 

How, then, do we account for the bonding of the C atoms As Figure 4.14(a) shows, each carbon atom uses the three sp hybrid orbitals to form two sigma (o-) bonds with the two hydrogen Is orbitals and one tr bond with the sp hybrid orbital of the adjacent C atom. In addition, the two unhybridized 2p orbitals of the C atoms form a pi (it) bond by overlapping sideways [Figure 4.14(b)]. 

A sigma (o-) bond is formed from the end-on overlap of two orbitals. A pi (jt) bond is formed by lateral overlap of p orbitals on adjacent atoms. 

Figure 8-5 A schematic representation of the formation of a carbon-carbon double bond. Two sp -hybridized carbon atoms form a sigma (o) bond by overlap of two sp orbitals green) and a pi (tt) bond by overlap of properly aligned p orbitals (Woe). All orbitals are fatter than shown here.

Sigma (o) Bond The bond resulting from the sigma orbitai, this orbitai being a molecular orbital produced by overlap along the line of axes, such as in a carbon molecule. 

The hybridization model used for main group elements, such as carbon (Chapter 14), can also be applied to d-block elements an empty hybrid orbital on the transition metal centre can accept a pair of electrons from a ligand to form a sigma (o) bond, for example, in octahedral complexes of chromium(iii) as described previously. The atomic orbitals required for an octahedral complex are the 3d 2, 3d,j2 2, 4s, 4pj., 4p, and 4p. These orbitals must be unoccupied so as to be available to accept six pairs of electrons from the ligands. A process of hybridization occurs between the two empty 3d orbitals and the empty 4s and 4p orbitals to form six identical d sp hybrid orbitals (Figure 13.30). 

Sigma (o) bonds are generally stronger than jc bonds (Section 1.9). 

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