Phosphorus Pentachloride or PCl5 is a compound formed by chemical elements Phosphorus (Atomic number: 15, symbol: P) and Chlorine (Atomic number: 17, symbol: Cl).
A Phosphorus Pentachloride molecule consists of 1 atom of phosphorus for 5 atoms of chlorine.
If we talk about the physical appearance of the compound, it is sensitive to water and moisture in its solid form, along with being colorless.
While it being colorless, there have come to exist commercial specimen green and yellow in color upon being contaminated by Hydrogen Chloride (HCl).
It is also found in its liquid and gaseous state wherein it depicts neutral properties. When in solid form, it has a crystalline salt-like formation and an irritating odor.
According to the mass action law, Phosphorus Pentachloride gasifies almost devoid of any separation of phosphorus trichloride or of chlorine gas in the atmosphere.
PCl5 has a molecular weight of 208.24 grams per mol. Its boiling point is higher than that of water at 166.8-degree Celsius while its melting point is as high as 160.5 -degrees Celsius.
It also exhibits properties of Lewis acidity due to its familiarity in chlorination, hydrolysis, etc.
In Chemistry, PCl5 comes into existence through the process of self-ionization. Its chemical equations of equilibrium are as follows:
PCl4+ + Cl- ⇌ PCl5
Lewis Structure of PCl5
Lewis structure of a compound is the arrangement of its underlying atom’s valence shell electrons.
Lewis structures make the use of dots to represent electrons and bonds between different electrons are represented through a straight line, marked at the end of which is a set of electrons.
The ultimate purpose of designing a Lewis structure is to assess and come to the formation of a configuration that holds the foremost arrangement of electrons and therefore equilibrium.
This must be done while considering the relevance of the octet rule and the concepts of formal charges.
The Lewis structure of a compound does not deal with the 3-dimensional representation of its elements in space, nor its molecular design and geometry.
Drawing the Lewis structure of PCl5
Step 1: Count the number of valence electrons in a PCl5 molecule. We can refer to the periodic table for this. We come to understand that PCl5 is made up of Phosphorous and Chlorine.
Phosphorus, having atomic number 15, has an electron composition of 2, 8, 5. Therefore, it has 5 electrons in its outermost shell.
Chlorine has 7 electrons in its outermost shell, owing to its atomic number 17 and resultant placement 2,8,7.
Step 2: To attain stability, each of the 5 Chlorine atoms will form a bond with Phosphorus.
Step 3: Phosphorus lends its 5 valence shell electrons, one to each of the Chlorine atoms.
Step 4: Next task is to check if the atoms are stable. While Chlorine atoms have received the one needed electron, Phosphorus’s valency is 3.
This could have been a problem, but it can hold the 5 Chlorine atoms, due to its empty 3d orbital.
Step 5: Visualizing the diagram, we come up with a Phosphorus in the center, housed by 5 Chlorine atoms.
Molecular Geometry of PCl5
Molecular geometry is an extension of the 2-dimensional diagram as in the below image.
The molecular geometry, in addition to being a 3-dimensional representation of the data at our disposal, is also essential to observe and subsequently infer the reason behind the specific properties a compound exhibits.
It even depicts the correct bond lengths and angles such as the bond angle and the torsion angle between two atoms.
The molecular representation also helps in understanding the factors that cause an element to take a specific arrangement and shape at the atomic level.
Properties such as magnetism, resistance, reactivity, potency, alignment, and physical traits such as color, shape, odor can be explained by this 3-dimensional model.
Quite clearly, these properties eventually establish the probable utility a compound has, and how it will react when introduced to foreign or homogeneous substances.
Molecular models are classified into the following different types, each having its own properties:
- Trigonal Planar
- Trigonal Pyramidal
If we talk about PCl5, the central atom, P gives it’s 5 electrons to each of the 5 Chlorine atoms. The 5 Cl atoms contribute 5 electrons, one for each atom.
This makes the valence shell electrons 10. Total valence shell electron pairs are 5. The PCl5 structure has 2 different kinds of P-Cl bonds.
All the Phosphorus-Chlorine equatorial bonds make 90 degrees and 120 degrees bond angles, two each, with the further bonds in the atom.
The second type of bond is the axial bond. Each of these bonds between P and Cl makes 3 90 degrees and 180 degrees bond angles with the supplementary bonds.
Hybridization of PCl5
The first and foremost understanding of VSPER theory and hybridization is the need for a compound to be stable and in equilibrium.
This concept states that orbitals of atoms that have equal or similar energy can fuse with each other thereby giving rise to new, degenerate orbitals, hybrid in nature.
These hybrid orbitals also influence the molecular geometry, reactivity, and bonding traits of a compound.
Hybridization, in tandem with quantum mechanics, is a widely researched topic of modern science.
The new hybrid orbitals are different from the original ones on account of energy and arrangement of the outermost orbit of electrons in a compound.
Each atomic orbital has a different level of energy and the merger of orbitals is expected to lead to a balance of the charges.
Both fully filled and half-filled orbitals can participate in the process, varying on the presence of the underlying elements on the periodic table.
Different kinds of hybridizations are as follows:
Due to their position in the periodic table, Phosphorus and Chlorine structures consist of s, p, and d orbitals.
The 3d orbitals hold similar energy to 3p and 3s orbitals as well as to 4p and 4s orbitals. What this does is that the hybridization has a wide range of orbitals to choose from, which is 3s or 3p or 3d or 4s or 4p.
While 3d orbitals have a similar or comparable amount of energy, the energy difference between 4s and 3p orbitals means that 3d, 3p, and 4s orbitals cannot participate in the hybridization.
The shape of the PCl5 molecule is Trigonal bipyramidal. Its hybridization is SP3D.
Let us see how this happens:
Step 1: All the 1s, 1d, 3p orbitals are ready to become hybrid. So, PCl5 can obtain 5 SP3D orbitals that are hybridized, each at one corner of the trigonal bipyramidal structure.
Step 2: The different types of bonds have different bond angles. In Phosphorus Chloride, there are 5 different SP3D orbits of Phosphorus that overlap with the p orbitals of Chlorine.
These P orbitals are solely occupied and the five bonds between Phosphorus and Chlorine are sigma bonds.
The PCl5 compound is non-polar in nature, which is because of the symmetric distribution of electron region in the compound’s atoms.
Atomic Bonds of PCl5
- Axial Bonds: 2 out of the 5 Phosphorus-Chlorine bonds are Axial bonds. One bond lies above the equatorial level and the other one lies below it. Both the bonds make an angle of 90 degrees with the plane.
- Equatorial Bonds: The remaining 3 bonds are equatorial in nature. The 3 bonds lie in the same plane, making an angle of 120 degrees with one another.
As the pairs with axial bonds must withstand higher and more arduous repulsiveness from the second type of bonds, the equatorial pairs, the axial bonds between pairs are somewhat elongated.
This increase in the distance leads to weaker bonds.
Therefore, equatorial bonds are stronger and more reactive than axial bonds.
Molecular Orbital Theory and MO diagram of PCl5
Molecular Orbital theory makes use of Molecular Orbital diagrams to showcase a clear picture of the state of electrons in an atom.
While the Valence Bond theory and VSPER give an idea of an atom’s properties, it is not useful in the case of certain molecules.
MO diagram depicts chemical and physical traits of a molecule like bond length, bond energy, bond angle, shape, etc.
Following are the steps to design the MO diagram of PCl5 :
Step 1: Identify the valence electrons of each atom. In PCl5, it is 5 for P and 7 for every 5 atoms of Cl.
Step 2: Check if the molecule is heteronuclear or homonuclear. PCl5 is heteronuclear.
Step 3: Next is to fill orbitals with bonding and energy properties of overlapping orbitals.
Step 4: Higher number of nodes means higher MOs. In the case of PCl5, it is 5 due to SP3D.
Step 5: Once the diagram is drawn, MOs can be filled with electrons.
Uses of PCl5
It is widely used in the capacity of a chlorinating agent and is among the most vital chlorides of phosphorus, the others being POCl3 and PCl3.
Its varied nature makes it highly useful in the manufacturing of essential commodities such as antibiotics, electrolytes for lithium-ion batteries. It also has few more uses in the industry as mentioned below:
- Being a good catalyst, it is used in the making of dyestuff, organic chemicals, and intermediates.
- It is also used as a catalyst in the production of acetyl cellulose. It is the plastic projection film on which movies/videos are printed.
- It is used in the pharmaceutical industry as well in the production of cephalosporin and penicillin.
- It is also a vital part of organic chemistry and is used in the production of acid chlorides.
- Another use of PCl5 is as a catalyst for cyclization and condensation reactions.
The physical and reactive properties of PCl5 and its industry-wide uses can be well understood through the concepts of Lewis Structure, Molecular Geometry, Hybridization, and Molecular Orbital theory.
PCl5 is used in laboratories, pharmaceutical companies, industries, and its uniform arrangement enable these wide uses.
It is also used as a catalyst in chemical reactions and even undergoes a subsequent equilibrium in circumstances of greater concentration:
PCl4+ + PCl6- ⇌ 2PCl5