Phosphorus pentachloride or PCl5 is a compound formed by the chemical elements phosphorus (atomic number: 15, symbol: P) and chlorine (atomic number: 17, symbol: Cl).
A molecule of phosphorus pentachloride consists of 1 phosphorus atom and 5 chlorine atoms.
If we talk about the physical appearance of the compound, in its solid form it is sensitive to water and moisture and is also colorless.
Although colorless, there are commercial specimens in green and yellow colors when contaminated with hydrogen chloride (HCl).
It also occurs in liquid and gaseous states, in which it has neutral properties. In solid form, it presents formation of crystals similar to salt and an irritating odor.
According to the law of mass action, phosphorus pentachloride is gasified in the atmosphere with almost no separation of phosphorus trichloride or chlorine gas.
PCl5 has a molecular weight of 208.24 grams per mole, its boiling point is 166.8 degrees Celsius, higher than that of water, and its melting point is 160.5 degrees Celsius.
It also exhibits Lewis acidic properties due to its familiarity with chlorination, hydrolysis, etc.
In chemistry, PCl5 is formed by the process of autoionization. Their chemical equilibrium equations are as follows:
PCl4+ + Cl- ⇌ PCl5
Lewis structure of PCl5
The Lewis structure of a compound is the arrangement of electrons in the valence shell of the underlying atom.
Lewis structures use dots to represent electrons, and bonds between different electrons are represented by a straight line with a group of electrons marked at the end.
The ultimate goal of designing a Lewis structure is to evaluate and achieve formation of a configuration that preserves the principal arrangement of electrons and thus equilibrium.
This must be done keeping in mind the relevance of the octet rule and the concepts of formal charges.
The Lewis structure of a compound is not concerned with the three-dimensional representation of its elements in space, nor with its design and molecular geometry.
Draw the Lewis structure of PCl5
Step 1: Count the number of valence electrons in a PCl5 molecule. We can use the periodic table for this. We understand that PCl5 is composed of phosphorus and chlorine.
Phosphorus with 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, due to its atomic number 17 and resultant position 2,8,7.
step 2: For reasons of stability, each of the 5 chlorine atoms forms a bond with phosphorus.
stage 3: Phosphorus donates its 5 valence electrons, one for each of the chlorine atoms.
Level 4: The next task is to check if the atoms are stable. While the chlorine atoms received the required electron, the valence of phosphorus is 3.
This could have been a problem, but it can accommodate all 5 chlorines due to its empty 3D orbital.
step 5: Viewing the diagram, we can imagine a match in the center hosted by 5 chlorine atoms.
Molecular geometry of PCl5
Molecular geometry is an extension of the two-dimensional diagram as in the image below.
Molecular geometry is not only a three-dimensional representation of the data we have, but also essential for observing and later deriving the ratios of the specific properties of a compound.
It even represents the correct bond lengths and angles, such as B. the bond angle and the twist angle between two atoms.
Molecular representation also helps to understand the factors that cause an element to adopt a specific arrangement and shape at the atomic level.
This three-dimensional model can explain properties such as magnetism, resistance, reactivity, power, orientation and physical properties such as color, shape and smell.
These properties clearly establish the likely utility of a compound and how it will respond when presented with foreign or homogeneous substances.
Molecular models are classified into the following different types, each with its own characteristics:
- trigonal plane
- Trigonal Pyramid
When we talk about PCl5, the central atom, P donates its 5 electrons to each of the 5 chlorine atoms. The 5 atoms of Cl contribute 5 electrons, one for each atom.
This makes the valence shell electrons 10. The total valence shell electron pairs are 5. The PCl5 structure has 2 different types of P-Cl bonds.
All the equatorial phosphorus-chlorine bonds form bond angles of 90 degrees and 120 degrees, two each, with the extra bonds in the atom.
The second type of connection is the axial connection. Each of these bonds between P and Cl forms 3 bond angles of 90 degrees and 180 degrees with the complementary bonds.
The first and most important understanding of VSPER theory and hybridization is the need for a connection to be stable and balanced.
This concept states that the orbitals of atoms of equal or similar energies can merge, creating new degenerate orbitals of a hybrid nature.
These hybrid orbitals also affect the molecular geometry, reactivity, and bonding properties of a compound.
Hybridization, along with quantum mechanics, is a much explored topic in modern science.
The new hybrid orbitals differ from the original ones because of the energy and arrangement of the outermost orbital of electrons in a compound.
Each atomic orbital has a different energy level, and merging the orbitals is expected to result in charge balancing.
Fully filled and half filled orbitals can participate in the process depending on the presence of the underlying elements in the periodic table.
The different types of hybridizations are as follows:
Due to their position in the periodic table, the structures of phosphorus and chlorine consist of s, p, and d orbitals.
The 3d orbitals are similar in energy to the 3p and 3s orbitals and the 4p and 4s orbitals. What this means is that hybridization has a wide range of orbitals to choose from, which are 3s or 3p or 3d or 4s or 4p.
Although the 3d orbitals have a similar or comparable amount of energy, the difference in energy between the 4s and 3p orbitals means that the 3d, 3p and 4s orbitals cannot participate in hybridization.
The shape of the PCl5 molecule is trigonal-bipyramidal. Its hybridization is SP3D.
Let's see how this happens:
Step 1: All 1s, 1d, 3p orbitals are ready to become hybrids. Thus, PCl5 can have 5 hybridized SP3D orbitals, each in a corner of the trigonal bipyramidal structure.
step 2: Different types of bindings have different binding angles. In phosphorus chloride there are 5 different SP3D orbitals of phosphorus that overlap the p orbitals of chlorine.
These P orbitals are clearly occupied, and the five bonds between phosphorus and chlorine are sigma bonds.
The PCl5 compound is inherently non-polar, due to the symmetrical distribution of electron space in the atoms of the compound.
PCl5 Atomic Bonds
- axial connections: 2 of the 5 phosphorus-chlorine bonds are axial bonds. One limb is above the equator, the other below. Both connections form a 90 degree angle with the plane.
- equatorial loops: The remaining 3 links are equatorial in nature. The 3 links are in the same plane and form an angle of 120 degrees to each other.
Since axially bonded pairs have to withstand greater and more difficult repulsion than the second type of bonds, equatorial pairs, the axial bonds between the pairs are somewhat elongated.
This increase in distance leads to weaker connections.
Therefore, equatorial bonds are stronger and more reactive than axial bonds.
Molecular orbital theory and MO diagram of PCl5
Molecular orbital theory uses molecular orbital diagrams to show a clear picture of the state of electrons in an atom.
Although valence bond theory and VSPER provide information about the properties of an atom, they are not useful for certain molecules.
The MO diagram represents the chemical and physical properties of a molecule, such as 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 there are 5 for P and 7 for every 5 atoms of Cl.
step 2: Check whether the molecule is heteronuclear or homonuclear. PCl5 is heteronuclear.
stage 3: Next, the orbitals are filled with superimposed energy and binding properties of the orbitals.
Level 4: A larger number of nodes means a larger MO. In the case of PCl5 it is 5 due to SP3D.
step 5: Once the diagram is drawn, the MOs can be filled with electrons.
Use of PCl5
It is often used as a chlorinating agent and, along with POCl3 and PCl3, is one of the most important phosphorus chlorides.
Its diverse nature makes it very useful in manufacturing basic products such as antibiotics, electrolytes for lithium-ion batteries. It also has some more uses in the industry as mentioned below:
- As a good catalyst, it is used in the manufacture of dyes, organic chemicals and intermediates.
- It is also used as a catalyst in the manufacture of acetylcellulose. It is the plastic projection film on which films/videos are printed.
- It is used in the pharmaceutical industry, as well as in the manufacture of cephalosporins and penicillins.
- It is also an important ingredient in organic chemistry and is used to make acid chlorides.
- Another use of PCl5 is as a catalyst for cyclization and condensation reactions.
The physical and reactive properties of PCl5 and its use in industry can be well understood through the concepts of Lewis structure, molecular geometry, hybridization and molecular orbital theory.
PCl5 is used in laboratories, pharmaceutical companies and industry and its uniform arrangement allows these wide applications.
It is also used as a catalyst in chemical reactions and even enters another equilibrium in higher concentration:
PCl4+ + PCl6- ⇌ 2PCl5