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 assemble larger and more complex structures: molecules.
These bonds can be of different varieties, but the most common in this field of study are covalent. The covalent bonds , also called molecular, are a type of link where the atoms involved shared electron pairs.
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 form between atoms based on the orientation of their orbitals: sigma (σ) bonds and pi (π) bonds.
It is important to differentiate between the two bonds, since the sigma bond occurs in single bonds and pi in multiple bonds between atoms (two or more electrons are shared).
How is it formed?
In order to describe the formation of the pi bond, the process of hybridization must first be discussed, as it is involved 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 electron 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.
When talking about the orbital symmetry of this type of union, it should be mentioned that it is equal to that of the p-type orbitals as long as it is observed through the axis formed by the bond. Furthermore, these unions are mostly made up of p orbitals.
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 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 manner 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 combine, 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 united only by pi bonds.
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 those of carbon. For this reason, if there is rotation of the atoms, the bond breaks.
Likewise, in these bonds the overlap between the orbitals occurs through two parallel regions, achieving that they have a greater diffusion than the sigma bonds and that, for this reason, they are weaker.
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 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, there can be a triple bond 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.
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.
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.
There are also pi bonds 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.
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).
- Wikipedia. (sf). Pi bond. Recovered from en.wikipedia.org
- Chang, R. (2007). Chemistry, Ninth edition. Mexico: McGraw-Hill.
- ThoughtCo. (sf). Pi Bond Definition in Chemistry. Recovered from thoughtco.com
- Britannica, E. (nd). Pi bond. Retrieved from britannica.com
- LibreTexts. (sf). Sigma and Pi Bonds. Recovered from chem.libretexts.org
- Srivastava, AK (2008). Organic Chemistry Made Simple. Recovered from books.google.co.ve