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What is  pi (π) bond?

A  pi (π) bond is a type of covalent bond characterized by preventing the free rotation movement of atoms and by originating between a pair of pure-type atomic orbitals, among other peculiarities. There are bonds that can be formed between atoms by their electrons, which allow them to build larger and more complex structures: molecules.

These bonds can be of different varieties, but the most common in this field of study are the covalents. Covalent bonds, also called molecular bonds, are a type of bond where the atoms involved share pairs of electrons.

This can occur due to the need of the atoms to seek stability, thus forming most of the known compounds. In this sense, covalent bonds can be single, double or triple, depending on the configuration of their orbitals and the number of pairs of electrons shared between the atoms involved.

This is why there are two types of covalent bonds that are formed between atoms based on the orientation of their orbitals: sigma (σ) bonds and pi (π) bonds.

How is the pi bond formed?

In order to describe the formation of the pi bond, the hybridization process must first be discussed, since it intervenes in some important bonds.

Hybridization is a process in which hybrid electronic orbitals are formed; that is, where s and p atomic sublevel orbitals can get mixed up. This causes the formation of sp, sp 2 and sp 3 orbitals , which are called hybrids.

In this sense, the formation of pi bonds occurs thanks to the overlap of a pair of lobes belonging to an atomic orbital on another pair of lobes that are in an orbital that is part of another atom.

This orbital overlap occurs laterally, whereby the electronic distribution is mostly concentrated above and below the plane formed by the bonded atomic nuclei, and causes the pi bonds to be weaker than the sigma bonds.

Formation of pi bonds in different chemical species

As pi bonds are always accompanied by one or two more bonds (one sigma or another pi and one sigma), it is relevant to know that the double bond that is formed between two carbon atoms (made up of one sigma and one pi bond) has lower bond energy than that corresponding to twice the sigma bond between the two.

This is explained by the stability of the sigma bond, which is greater than that of the pi bond because the overlap of atomic orbitals in the latter occurs in a parallel way in the regions above and below the lobes, accumulating the electronic distribution in a more distant way. of atomic nuclei.

Despite this, when pi and sigma bonds are combined, a stronger multiple bond is formed than the single bond itself, which can be verified by observing the bond lengths between various single and multiple bond atoms.

There are some chemical species that are studied for their exceptional behavior, such as coordination compounds with metallic elements, in which the central atoms are only linked by pi bonds.

Characteristics of the pi bond

Does not allow free rotation movement of atoms

The characteristics that distinguish pi bonds from other kinds of interactions between atomic species are described below, beginning with the fact that this bond does not allow free rotational movement of atoms, such as carbon. For this reason, if there is rotation of the atoms, the bond breaks.

Overlap between orbitals by two parallel regions

Likewise, in these bonds, the overlap between the orbitals occurs through two parallel regions, achieving that they have a greater diffusion than sigma bonds and that, for this reason, they are weaker.

Between a pair of pure atomic orbitals

On the other hand, as mentioned above, the pi bond is always generated between a pair of pure atomic orbitals; This means it is generated between orbitals that have not undergone hybridization processes, in which the density of the electrons is concentrated mostly above and below the plane formed by the covalent bond.

In this sense, between a pair of atoms more than one pi bond can occur, always being accompanied by a sigma bond (in the double bonds).

Similarly, a triple bond can occur between two adjacent atoms, which is formed by two pi bonds in positions that form planes perpendicular to each other and a sigma bond between both atoms.

Examples of pi bonds

Ethylene molecule

As previously stated, molecules made up of atoms joined by one or more pi bonds always have multiple bonds; that is, double or triple.

An example of this is the ethylene molecule (H 2 C = CH 2 ), which is made up of a double bond; that is, one pi and one sigma bond between its carbon atoms, in addition to the sigma bonds between the carbons and hydrogens.

Acetylene molecule

For its part, the acetylene molecule (H – C≡C – H) has a triple bond between its carbon atoms; that is, two pi bonds forming perpendicular planes and one sigma bond, in addition to their corresponding carbon-hydrogen sigma bonds.

Benzene and derivatives

Pi bonds are also present between cyclic molecules, such as benzene (C 6 H 6 ) and its derivatives, whose arrangement results in an effect called resonance, which allows the electron density to migrate between atoms and grant it, among other things, greater stability to the compound.

Dicarbon

To exemplify the exceptions mentioned previously, the cases of the dicarbon molecule (C = C, in which both atoms have a pair of paired electrons) and the coordination compound called hexacarbonyl iron (represented as Fe 2 (CO) 6 , the which is formed only by pi bonds between its atoms).

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