Phosphorus Pentachloride ($PCl_5$)
The $sp^3d$ Geometry, Ionic Solid State Anomaly, and Hydrolysis Reactions.
Unlike Nitrogen, which is restricted to a maximum covalency of 4, Phosphorus possesses vacant 3d-orbitals. By promoting a 3s electron into the 3d subshell, Phosphorus can expand its octet to form five covalent bonds, leading to the creation of Phosphorus Pentachloride ($PCl_5$). It is a yellowish-white powder, fiercely reactive with water, and harbors a massive structural secret when in the solid state.
1. Preparation of $PCl_5$
A. Action of Chlorine on $PCl_3$
$PCl_5$ is easily prepared by passing an excess of dry chlorine gas through liquid phosphorus trichloride ($PCl_3$).
B. Direct Chlorination of White Phosphorus
Alternatively, it is obtained by the action of an excess of dry chlorine on white phosphorus. (Recall that a limited supply of chlorine yields $PCl_3$).
2. The Structural Anomaly: Gas vs. Solid Phase
This is arguably the most frequently tested concept regarding $PCl_5$ in competitive exams. The molecule completely changes its bonding nature depending on its physical state.
In gaseous and liquid states, $PCl_5$ exists as a discrete, neutral covalent molecule with $sp^3d$ hybridization.
- Geometry: Trigonal Bipyramidal (TBP).
- Bond Angles: Three equatorial bonds at $120^\circ$, and two axial bonds at $90^\circ$ to the equatorial plane.
- Crucial Fact: The two axial P-Cl bonds are longer and weaker than the three equatorial bonds. This is because the axial bond pairs suffer greater repulsion from the three equatorial bond pairs.
In the solid state, $PCl_5$ completely abandons its covalent nature and exists as an ionic solid!
- The Cation $[PCl_4]^+$ is Tetrahedral ($sp^3$ hybridized).
- The Anion $[PCl_6]^-$ is Octahedral ($sp^3d^2$ hybridized).
- Because it is an ionic compound in the solid state, it can conduct electricity when fused (melted).
Figure 1: To relieve the strain of the axial bonds, $PCl_5$ disproportionates in the solid state into stable tetrahedral and octahedral ions.
3. Chemical Properties & Key Reactions
A. Action of Heat (Sublimation & Decomposition)
When heated, $PCl_5$ sublimes. Upon stronger heating, it decomposes into phosphorus trichloride and chlorine gas. The weakness of the two axial bonds is the exact reason it decomposes so readily.
B. Hydrolysis (Partial vs. Complete)
Like $PCl_3$, it reacts violently with water. However, the reaction occurs in two distinct steps depending on the amount of water available.
It first forms Phosphorus oxychloride ($POCl_3$).
With excess water, the $POCl_3$ reacts further to yield Orthophosphoric acid ($H_3PO_4$).
Overall Reaction: $PCl_5 + 4H_2O \rightarrow \mathbf{H_3PO_4} + 5HCl$
C. As a Powerful Chlorinating Agent
$PCl_5$ reacts vigorously with organic compounds containing -OH groups, converting them into chloro-derivatives.
(Ethanol $\rightarrow$ Ethyl Chloride)
(Acetic acid $\rightarrow$ Acetyl chloride)
D. Action on Finely Divided Metals
When heated with finely divided metals, $PCl_5$ acts as a chlorinating agent, forming metal chlorides and getting reduced to $PCl_3$.
$Sn + 2PCl_5 \xrightarrow{\Delta} SnCl_4 + 2PCl_3$
Mastery Check: Phosphorus Pentachloride
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