PCl5 Hybridization: Formation, Types, Polarity and Properties

PCl₅ hybridization was first prepared by English chemist Humphry Davy in 1808 but it was accurate and hence the first accurate analysis of Phosphorous Pentachloride was given by French chemist Pierre Louis Dulong in 1816. PCl₅ (Phosphorous Pentachloride) is a chemical compound having five molecules of chloride attached with one phosphorous molecule. Apart from PCl₃ and POCl₄, PCl₅ is also one of the important phosphorous chlorides and finds its usage in chlorinating reagents. PCl₅ is moisture sensitive and water sensitive and is colorless in nature.

Key Terms- Phosphorus Pentachloride, hybridization, chemical compounds, POCl₄, chemical bonds, polarity

Also check: Difference Between Electrophile and Nucleophile

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What is PCl₅ Hybridization?

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Phosphorus comprises 5 electrons of valence. This is the reason why PCl₅ molecules have sp³d hybridization. Therefore it possesses a trigonal bipyramidal structure, with 1 electron each is shared by chlorine atom. It has a pyramidal structure in which phosphorus is hybridised with sp³d.

For hybridization, the five orbitals are free namely 1s, 3p, and 1d. A set of 5sp³d hybrid orbitals guided to the 5 corners of a trigonal bipyramidal, therefore (VSEPR theory) can be obtained. The 5sp³d phosphorus orbitals in PCl₅ overlap with the p orbitals of chlorine atoms, and P orbitals are occupied separately.

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Structure of PCl₅ Hybridization

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The structure of PCl₅ is trigonal bipyramidal having hybridization of sp³d. there are three equatorial P-Cl bonds of equal length and while the two axial bonds are longer as compared to equatorial bonds. 

The five orbitals - 1s, 3p, and 1d are open for hybridization and are placed at the 5 corners of the structure and hence the structure is of a set of 5 sp₃d hybrid orbitals is obtained. The bond angles between each bond are not the same in the trigonal bipyramidal. These 5 sp₃d hybrid orbitals overlap with singly occupied p orbitals of Chlorine.

Also Read: Monomers

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Preparation of PCl₅ Hybridization

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PCl₅ is obtained by the chlorination of PCl₃ i.e., PCl₅ is obtained when dry chlorine in excess is passed into liquid trichloride. The chlorine reacts with phosphorous trichloride and hence phosphorous pentachloride is obtained. this reaction is used to produce 10,000 tonnes of PCl₅ per year.

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Bonds in PCl₅ Hybridization

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There are two types of bonds formed during the PCl₅ hybridization. The two bonds are Equatorial Bond and Axial Bond.

Equatorial Bonds: Equatorial bonds are perpendicular to the axis or the bond that lies along the equator of the structure is an equatorial bond. three bonds of PCl bond lie in one plane making an angle of 120 degrees with each other.

Axial Bonds: Axial bonds are those that make an angle of 90 degrees with the other bond in the structure. In PCl₅ hybridization, there are 2 PCl bonds. each of PCl bond lies on each side of the equatorial bond-forming an angle of 90 degrees.

Since the axial bonds inhibit more repulsive interaction as compared to equatorial bond pairs, axial bonds become a bit longer. This results in a more reactive PCl₅ molecule.

Also Read:Boiling Point Elevation

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Hybridization of PCl₅ Central Atom

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In the Solid Phase, the PCl₅ molecule remains as an ion pair of PCl₄ and PCl₆. In PCl₅, the central atom P makes 4 bonding electron pairs and has zero lone pair and hence is sp₃ hybridized, forming a tetrahedral shape. Whereas in PCl₆ the central atom i.e., p makes six electron pairs and zero lone pairs giving it an octahedral shape.

In Gaseous Phase, PCl₅ retains its original compound. The central atom P forms five sigma electron bond pairs with Chlorine leaving zero lone pair electron and is hence sp₃d hybridized, giving it a trigonal bipyramidal shape.

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Hybridization Calculation

One of the simplest ways to calculate the hybridization is to count the atoms that surround, leaving the primary atom. However, the hybridization can also be calculated with the below formula:

H=1/2(V+M-C+A)

where,

H = hybridization

V = number of valence electrons

M = is the number of surrounding monovalent atoms

C = Positive charge of Cationic charge

A = Anionic or negative charge

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Polarity of PCl₅

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PCl₅ is polar, and the structure is not covalent, it is ionic. Meaning, PCl₄⁺/Cl^-. Phosphorus trichloride (PCl₃) is a significantly less polar liquid, possessing a boiling point of 73°. If we are to form an acid chloride (for instance, p-chlorobenzene acid with a melting point of 242°), we had better not try heating either with thionyl chloride SOCl₂ or PCl₃. It is due to not being enough polar to dissolve the high-melting crystals even at their respective boiling points. In order to make that acid chloride, we need to melt the PCl₅ compound and the chlorobenzene acid together ~170°, and the conversion will become smooth on continued heating.

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General Properties of PCl₅

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• Phosphorous Pentachloride is a pale yellow crystalline solid
• It has a pungent smell
• Phosphorous Pentachloride strongly reacts with moist air because of its affinity of water.

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Reaction with Water

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Phosphorous Pentachloride reacts highly with water. It is decomposed by water in two stages. In the first stage phosphorus oxychloride (POCl₃) is formed while in the second stage phosphoric acid (H₃PO₄) is formed.

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Things to Remember

• The first accurate analysis of Phosphorous Pentachloride was given by French chemist Pierre Louis Dulong in 1816.
• PCl₅ is sp₃d hybridized.
• PCl₅ reacts violently with water and can be fatal if inhaled.
• There are two bonds formed in Phosphorous Pentachloride- Equatorial Bond and Axial Bond.
• Hybridization is calculated by using the following formula: H=1/2(V+M-C+A)