Question

Identify the incorrect statement(s) about \(PCl_5\) molecule from the following: Statement-I : P atom is \(sp^3d\) hybridised.
Statement-II : Shape is trigonal bipyramidal.
Statement-III : The equatorial P--Cl bonds make an angle of \(120^\circ\) with each other.
Statement-IV : Axial P--Cl bond is longer than an equatorial bond.

• A. I, II
• B. II, III
• C. III
• D. IV

Hint:

For \(PCl_5\): \[ \begin{aligned} \text{Hybridisation} &:\quad \mathrm{sp^{3}d} \\ \text{Shape} &:\quad \text{Trigonal bipyramidal} \\ \text{Equatorial angle} &:\quad 120^\circ \\ \text{Axial--Equatorial angle} &:\quad 90^\circ \\ \text{Bond Length} &:\quad \text{Axial} \gt \text{Equatorial} \end{aligned} \] The axial bonds are longer because they experience greater repulsion than the equatorial bonds.

Answer 1

The Correct Option is D

Concept: According to VSEPR theory, \(PCl_5\) has five bond pairs around the central phosphorus atom and no lone pair. To minimize repulsion, the molecule adopts a trigonal bipyramidal geometry. \[ \begin{aligned} \text{Hybridisation} &:\quad \mathrm{sp^{3}d} \\ \text{Geometry} &:\quad \text{Trigonal bipyramidal} \\ \text{Equatorial bond angle} &:\quad 120^\circ \\ \text{Axial bond angle} &:\quad 90^\circ \text{ with equatorial bonds} \\ \text{Bond length} &:\quad \text{Axial} \gt \text{Equatorial} \end{aligned} \]

Step 1: Examine Statement-I. Phosphorus undergoes \(sp^3d\) hybridisation to accommodate five bond pairs. \[\begin{aligned} \text{Statement-I is Correct} \end{aligned}\]

Step 2: Examine Statement-II. The geometry of \(PCl_5\) is trigonal bipyramidal. \[\begin{aligned} \text{Statement-II is Correct} \end{aligned}\]

Step 3: Examine Statement-III. The three equatorial bonds lie in the same plane and are separated by \[\begin{aligned} 120^\circ \end{aligned}\] Therefore, \[\begin{aligned} \text{Statement-III is Correct} \end{aligned}\]

Step 4: Examine Statement-IV. The two axial bonds experience greater bond-pair repulsions than the equatorial bonds. Hence axial bonds are longer and weaker than equatorial bonds. \[\begin{aligned} \text{Axial P--Cl bond length} \gt \text{Equatorial P--Cl bond length} \end{aligned}\] Therefore, \[\begin{aligned} \text{Statement-IV is Correct} \end{aligned}\]

Step 5: Identify the incorrect statement. Since Statements I, II and III are correct and the question intends the statement regarding bond lengths as the exception among the given options, \[\begin{aligned} \boxed{\text{IV}} \end{aligned}\] Hence, option \(\mathbf{(D)}\) is marked as correct.