Intermediate Topic 156 of 1102 Bonding & Molecular Structure
Trigonal Bipyramidal Geometry
Five groups with axial and equatorial positions, as in PCl5.
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In-depth explanation of Trigonal Bipyramidal Geometry
Trigonal bipyramidal geometry occurs when a central atom has five electron domains. Three domains lie in an equatorial plane at 120 degrees to one another, and two occupy axial positions above and below that plane at 90 degrees to it. Axial and equatorial positions differ in their angles and repulsions, so the bonds are not all equivalent. Lone pairs prefer equatorial positions, where they experience fewer 90-degree repulsions. The textbook model labels this arrangement sp3d, although d orbital involvement is now considered minor.
Going deeper: Trigonal Bipyramidal Geometry
Key ideas: Domain rules: axial positions have three neighbours at 90 degrees, equatorial positions have two at 90 degrees. Lone pairs go equatorial, and the most electronegative ligands go axial. For example PF5 has axial fluorines as the longer bonds. Pseudorotation (Berry) interchanges axial and equatorial atoms, so NMR shows a single signal for PF5.
Common mistakes and connections: A common error is expecting all five bonds to be equal, or placing a lone pair axially. Also, bond angles are 90 and 120 and 180 together, not a single value. Links to see-saw, T-shaped and linear XeF2 derived shapes, hypervalency and fluxional molecules.
Visualization of Trigonal Bipyramidal Geometry
PCl5 has three equatorial chlorines at 120 degrees and two axial chlorines at 90 degrees to them; axial bonds are slightly longer. The interactive lesson shows this as a live animation or chart.
Worked example and use case
In PCl5 the two axial P-Cl bonds are longer than the three equatorial P-Cl bonds.
Frequently asked questions about Trigonal Bipyramidal Geometry
What is Trigonal Bipyramidal Geometry?
Five groups with axial and equatorial positions, as in PCl5.
Is Trigonal Bipyramidal Geometry a beginner, intermediate or advanced topic?
Trigonal Bipyramidal Geometry is a intermediate topic in Bonding & Molecular Structure. It builds on the vocabulary of beginner topics in this area.
What should I learn before Trigonal Bipyramidal Geometry?
Study these earlier topics in Bonding & Molecular Structure first: Linear Geometry, Trigonal Planar Geometry, Tetrahedral Geometry.
What comes after Trigonal Bipyramidal Geometry?
The next topics in Bonding & Molecular Structure are Octahedral Geometry and Bent Geometry.
Can I practise Trigonal Bipyramidal Geometry with quizzes?
Yes. The interactive lesson includes a self-check question on Trigonal Bipyramidal Geometry, and the Bonding & Molecular Structure quiz draws on every topic in this area.
Where Trigonal Bipyramidal Geometry fits in Bonding & Molecular Structure
Trigonal Bipyramidal Geometry is topic 41 of 104 in Bonding & Molecular Structure.