Question

Except for nitrogen, the elements of Group \(5 \mathrm{~A}(15)\) all form pentafluorides, and most form pentachlorides. The chlorine atoms of \(\mathrm{PCl}_{5}\) can be replaced with fluorine atoms one at a time to give, successively, \(\mathrm{PCl}_{4} \mathrm{~F}, \mathrm{PCl}_{3} \mathrm{~F}_{2}, \ldots, \mathrm{PF}_{5} .\) (a) Given the sizes of \(\mathrm{F}\) and Cl, would you expect the first two F substitutions to be at axial or equatorial positions? Explain. (b) Which of the five fluorinecontaining molecules have no dipole moment?

Short answer

First two substitutions are axial. PF₅ and PCl₃F₂ have no dipole moments.

Step-by-step solution

Understand the structure of PCl₅

Start by understanding the molecular geometry of PCl₅. It has a trigonal bipyramidal structure with three chlorine atoms in equatorial positions and two chlorine atoms in axial positions.

Compare sizes of fluorine and chlorine atoms

Fluorine atoms are smaller than chlorine atoms. This size difference affects how substitutions occur in the PCl₅ molecule.

Predict the positions of the first two fluorine substitutions

Since fluorine atoms are smaller, the first substitution would occur at an axial position to minimize electron-electron repulsions. The second substitution would likely also occur at the other axial position for the same reason.

Evaluate the geometry of substituted molecules

Consider molecules: PCl₄F, PCl₃F₂, PCl₂F₃, PClF₄, and PF₅. Analyze their symmetry to determine the presence or absence of a dipole moment.

Identify which molecules have no dipole moment

Fully substituted PF₅ has no dipole moment due to its symmetrical trigonal bipyramidal structure. PCl₃F₂, with three identical chlorines in the equatorial plane, also has no dipole moment when fluorines are at equatorial positions.

Key concepts

molecular geometry

Molecular geometry is the three-dimensional arrangement of atoms within a molecule. It helps us understand how molecules interact with each other and their surroundings.
In the case of phosphorus pentachloride (PCl₅), it adopts a specific shape called trigonal bipyramidal. This means that three chlorine atoms are positioned in a triangular plane (equatorial positions), while the remaining two occupy positions above and below this plane (axial positions).
Understanding the molecular geometry of PCl₅ sets the stage for comprehending how substitutions with fluorine atoms can affect its structure and properties.

trigonal bipyramidal structure

The trigonal bipyramidal structure is a geometric configuration seen in certain molecules, including PCl₅ and its fluorine-substituted derivatives.
Here are some key aspects:

• The three equatorial positions form a triangular plane, each separated by 120°.
• The two axial positions are perpendicular to this plane.

This structure minimizes electron-electron repulsions, leading to a stable arrangement.
When we begin substituting chlorine atoms in PCl₅ with fluorine atoms, understanding this basic geometry helps predict where the substitutions are likely to occur first. Due to their smaller size, fluorine atoms tend to replace axial chlorine atoms initially, making it easier for the molecule to maintain its stability.

dipole moment

A dipole moment occurs in a molecule when there is a separation of charge, resulting in a positive pole and a negative pole. It is a vector quantity with both magnitude and direction.
For molecules like PCl₅ and its derivatives, the distribution of atoms—and thus the electron distribution—plays a crucial role in determining the presence or absence of a dipole moment.
PCl₅ itself is symmetrical and has no dipole moment due to its trigonal bipyramidal structure. If you substitute chlorine atoms with fluorine in certain symmetrical configurations, such as in fully substituted PF₅ or even PCl₃F₂ when fluorines occupy the equatorial positions, the molecule remains nonpolar.
This symmetry cancels out any dipole moments that might have arisen from individual bonds.

fluorine substitution

When substituting chlorine atoms with fluorine in PCl₅, the size and electronegativity differences between these two atoms play a significant role.
Fluorine is much smaller and more electronegative than chlorine. This makes it more likely to minimize repulsions by occupying axial positions first.
The sequence of substitutions usually follows this pathway:

• The first substitution replaces a chlorine in an axial position, forming PCl₄F.
• The second substitution takes the other axial spot, creating PCl₃F₂.

This pattern continues until you reach PF₅, where all chlorine atoms have been replaced.
By understanding the principles of substitution, you can predict not only the structure but also the properties of the resulting molecules.