Question

In the trigonal bipyramidal arrangement, why does a lone pair occupy an equatorial position rather than an axial position?

Short answer

Lone pairs are in equatorial positions due to minimized repulsion in a trigonal bipyramidal shape.

Step-by-step solution

Understanding the Trigonal Bipyramidal Geometry

In a trigonal bipyramidal arrangement, atoms or groups of atoms are positioned around a central atom in two distinct types of locations: equatorial and axial. Equatorial positions are in a plane with 120° angles between them, while axial positions are perpendicular to the equatorial plane and are 180° apart.

Analyzing Lone Pair Repulsion

Lone pairs of electrons take up more space than bonding pairs because they are held by only one nucleus and tend to spread out more. This means that any lone pair will create greater repulsion forces compared to bonded atoms.

Evaluating Equatorial Versus Axial Positions

In the equatorial position, a lone pair experiences 120° angles between adjacent pairs, allowing for some distribution of its repulsion forces over a larger area. Conversely, if positioned axially, the lone pair would be aligned with two other atoms forming 90° angles, maximizing repulsion and destabilizing the molecule.

Determining Lone Pair Optimal Position

Because lone pairs are more repulsive and prefer greater separation from other electron pairs, they are placed in equatorial positions in a trigonal bipyramidal arrangement. This minimizes overall steric repulsion and stabilizes the structure by allowing more space between the lone pair and bonded atoms.

Key concepts

Lone Pair Repulsion

In trigonal bipyramidal geometry, understanding lone pair repulsion is crucial. Lone pairs of electrons exert a stronger repulsive force than bonded pairs. This happens because lone pair electrons are attracted to only one nucleus. Thus, they spread out more in space compared to bonding pairs, which are more localized between two nuclei. This increased spread creates significant repulsion, impacting the molecular structure. - **Greater Spread**: Lone pairs tend to occupy more space. - **Greater Repulsion**: They exert more repulsive force than bonding pairs. Leveraging the knowledge of these repulsion forces helps in predicting the optimal positioning of lone pairs in molecular geometries.

Equatorial Position

In a trigonal bipyramidal structure, equatorial positions are located in a plane with 120° angles separating them. These positions contribute to the distinct shape of the molecule. - **Wider Angles**: At 120°, equatorial positions offer wider separation between atoms and lone pairs. - **Distribution of Repulsion**: Positioning lone pairs equatorially distributes repulsive forces over a larger area. This distribution is less disruptive to the molecule, making equatorial positions preferable for lone pairs seeking to minimize interference with bonded atoms.

Axial Position

Axial positions in a trigonal bipyramidal structure are perpendicular to the equatorial plane and form 180° across from each other. Situated above and below the equatorial plane, they provide a linear arrangement. - **Closer Angles**: Axial positions feature 90° angles with equatorial angles. - **Maximized Repulsion**: This close proximity enhances repulsive interactions, particularly with lone pairs. Due to these alignment implications, axial positioning is less favorable for lone pairs, as it escalates repulsion and can destabilize molecular structures.

Steric Repulsion

Steric repulsion refers to the resistance encountered due to close spatial interactions between electron pairs. This is a key factor in determining the configuration of molecules. - **Increasing Separation**: Molecules try to position electron pairs to maximize distance and minimize repulsion. - **Lone Pair Influence**: Lone pairs significantly increase steric repulsion due to their greater spatial demand. In trigonal bipyramidal geometries, selecting equatorial positions for lone pairs helps reduce overall steric repulsion. This choice allows for a stable molecular arrangement, reducing potential destabilization from overly close electron interactions.