Sigma and Pi Bond - Formation, Examples, Overlapping with FAQs

Formation Of Sigma And Pi Bond

The hybridization helps in predicting such complex models of squeezing electrons into the same space of a sphere by concluding all the Sigma Bonds. The structure of ethane consists of C-C single bonds and C-H single bonds, while the structure of ethene has a double bond between C-C and a single bond between C-H, and so the geometry of such a covalent compound is planar. In the above figure, the hybridization found is sp² where whereas in the case of ethane hybridization was sp³.

Thus, we can see that in ethene generation of 2p_z unhybridized orbital with each orbital containing one electron, which is capable of forming a covalent bond. The bonding in ethene can be summarized as follows: The three sp² hybrids overlap with other orbitals of the carbon atom that are identical. The remaining two hybrid orbitals form the bond with the hydrogen atom by overlapping with the 1s orbital. Hence, the 2pz on the carbon atom forms the other bond at a sideways of one another.

The Sigma (σ) Bond

The single bond formed is termed a Sigma Bond. A sigma bond can be represented as "σ". A sigma (σ) bond is the strongest type of covalent bond formed by the head-on (axial) overlap of atomic orbitals. The bond formed by the cation of end-to-end overlap of sigma bonds, where the concentrated density of electrons was found in the nuclei of atoms and the bonding atoms. Sigma bonds are formed end-to-end, which means the overlapping is done coaxially. Such type of overlapping is called axial overlapping.

Example Of Sigma Bond

A sigma bond can be seen in the formation of a hydrogen molecule. This can be seen in the picture depicted below, where hydrogen is bonded singly as a sigma bond.

The Pi (π) Bond

The pi bond is the second bond that is formed in between the C-C atoms and is elongated both above and below the plane of the molecule. According to Lewis dot structure, a double bond, which is formed between molecules, can be represented by a single dash. Pi bond can be represented as π. In a pi bond, the electron density is concentrated on both above and below the plane of the nuclei of the bonding atoms. Therefore, it is noted that the two bonds are formed sigma bond and a pi bond, in the covalent molecule of ethene. In general, when a single bond is present between the two atoms, it is a sigma bond, whereas double bonds consist of one sigma bond and one pi bond. In the case of triple bonds, one sigma and two pi bonds are present.

Examples Of Pi bonds

Ethyne bonding is the ones such example of pi bonding, which is singly bonded with one hydrogen atom and triple bonds are present mong carbon atoms. The bonds are formed by electron transfer of specific wavelength of 2s electron to 2p_z. Here, one 2s orbital will combine with one 2p orbital to form a sigma bond and sp hybridization, whereas the remaining 2py and 2pz form the pi bond.

What Are Sigma And Pi Bonds?

The names are derived from the Greek letters. The sigma and pi bonds are both used to predict the molecule's behaviour as according to molecular orbital theory. In a sigma bond, the orbitals are as similar as according to “s” where whereas in a pi bond, the orbital is similar to the p orbitals. It is found that sigma bonds are stronger in comparison with pi bonds.

Types Of Overlapping

Direct overlapping can be seen in sigma bonds. The electrons that participate in bond formation are termed sigma electrons. Singly bonded electrons are formed according to the combination of atomic orbitals as follows:

s-s Overlapping

In such a type of overlapping, one‘s orbital will participate along the internuclear axis, which means head-on overlapping. This can be represented as follows:

s-p Overlapping

In such a type of overlapping occurs along the internuclear axis when one half-filled p orbital overlaps with one half-filled orbital, and forms the covalent bond. Such a condition can be represented as follows:

p-p Overlapping

In such a type of overlapping occurs along the internuclear axis when one half-filled orbital undergoes head-on overlapping. Such a condition can be represented as follows:

Organometallic Compounds

Sigma-bonded organometallic compounds are defined as those compounds that are formed with the help of metal ‘M’ and carbon of the ligands containing a covalent sigma bond, and are termed as sigma-bonded organometallic compounds. Here, the metal can be bonded to any alkyl family, aryl family, or pi-bonded ligands. These organometallic compounds are basically divided according to the metal-carbon bond as follows:

• Sigma-bonded organometallic compounds

• Pi-bonded organometallic compounds

Sigma-Bonded Organometallic Compounds

The sigma-bonded organometallic compounds are those compounds that contain a metal-carbon covalent sigma bond, so they are termed as sigma-bonded organometallic compounds. Examples of such sigma-bonded organometallic compounds are Grignard reagents, which have a general structure as R-Mg-X, where R is an organic group and X is a halogen compound. In this example covalent sigma bond is present in metal, that is, magnesium, and R is any aryl or alkyl.

Pi-Bonded Organometallic Compounds

Taking an example of ferrocene compound, in which the two Fe- (C₂H₅) bonds. The compound Fe is bonded in a pi bond system; thus, ferrocene is in pi-bonded organometallic compound.

Sigma Bond vs Pi Bond:

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Some Solved Examples

Question.1 XeO₄ molecular is tetrahedral having:

1) Two $p \pi-d \pi$ bonds

2) One $p \pi-d \pi$ bond

3) Four $p \pi-d \pi$ bonds

4) Three $p \pi-d \pi$ bonds

Solution:

One s and three p orbital undergo sp³ hybridization. Four sp³ hybrid orbitals form four σ bonds with oxygen atoms. They are σ sp³ – p. Four pπ – dπ bonds are also formed with oxygen atoms by the unpaired electrons.

Hence, the answer is option (3).

Question.2 Among $\mathrm{P}_2 \mathrm{O}_5$ and $\mathrm{P}_4 \mathrm{O}_{10}$ the compound having more number of $\sigma$ and $\pi$ bond respectively are :

1) $\mathrm{P}_2 \mathrm{O}_5, \mathrm{P}_4 \mathrm{O}_{10}$

2) $\mathrm{P}_4 \mathrm{O}_{10}, \mathrm{P}_2 \mathrm{O}_5$

3) $\mathrm{P}_4 \mathrm{O}_{10}, \mathrm{P}_4 \mathrm{O}_{10}$

4) None of these

Solution:

Hence, the answer is option (4).

Question.3 Which of the following molecules has two sigma $(\sigma)$ and two $\mathrm{pi}(\pi)$ bonds ?

1) $\mathrm{C}_2 \mathrm{H}_4$

2) $N_2 F_2$

3) $\mathrm{C}_2 \mathrm{H}_2 \mathrm{Cl}_2$

4) HCN

Solution:

$\mathrm{C}_2 \mathrm{H}_4$ has $5 \sigma$ and one $\pi$ bond
$\mathrm{N}_2 \mathrm{~F}_2$ has $3 \sigma$ and one $\pi$ bond
$\mathrm{C}_2 \mathrm{H}_2 \mathrm{Cl}_2$ has $5 \sigma$ and one $\pi$ bond
HCN has $2 \sigma$ and two $\pi$ bond
Hence, the answer is option (4).

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