Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025
Definition
Trigonal bipyramidal is a molecular geometry where a central atom is surrounded by five atoms or groups of atoms. This shape consists of three atoms in an equatorial plane and two atoms in axial positions above and below this plane.
5 Must Know Facts For Your Next Test
The bond angles in a trigonal bipyramidal structure are 90° between axial and equatorial positions, and 120° between equatorial positions.
Trigonal bipyramidal geometry is often observed with molecules that have five bonding pairs of electrons around the central atom, such as phosphorus pentachloride (PCl$_5$).
Lone pairs on the central atom occupy equatorial positions to minimize electron-electron repulsion according to VSEPR theory.
This molecular shape can be distorted if there are lone pairs present, leading to structures like seesaw or T-shaped geometries.
Examples of molecules with trigonal bipyramidal geometry include PF$_5$, PCl$_5$, and Fe(CO)$_5$.
Review Questions
Related terms
VSEPR Theory: Valence Shell Electron Pair Repulsion (VSEPR) theory helps predict the shape of molecules based on minimizing repulsions between electron pairs around a central atom.
Seesaw Geometry: A molecular shape derived from trigonal bipyramidal when one equatorial position is occupied by a lone pair, resulting in four bonded atoms forming a seesaw-like shape.
T-shaped Geometry: A molecular structure derived from trigonal bipyramidal when two equatorial positions are occupied by lone pairs, resulting in three bonded atoms forming a T-shape.