Trigonal Bipyramidal
Definition and meaning of Trigonal Bipyramidal in chemistry.
Trigonal bipyramidal is a molecular shape in which a central atom is bonded to five other atoms. Three of the bonds lie in a flat triangle around the middle, and the other two point straight up and down, giving two different kinds of positions.
In more detail
This geometry appears when a central atom is surrounded by five regions of electrons and no lone pairs, according to VSEPR theory. Five groups cannot all sit at the same angle from one another, so the shape splits into two sets. The three equatorial positions form a flat triangle with 120-degree angles between them.
The two axial positions sit above and below that triangle at 90 degrees to it. Phosphorus pentachloride, PCl5, is the standard example. The central phosphorus atom bonds to five chlorine atoms, three in the equatorial plane and two along the axis.
Because the central atom holds more than eight electrons, it is said to have an expanded octet, which is possible for atoms in period 3 and beyond. The hybridization is usually written as sp3d. The two kinds of positions are not equal.
Axial bonds are slightly longer and experience more repulsion because each axial atom is squeezed by three equatorial neighbors at 90 degrees. Equatorial positions are less crowded. For this reason, when a lone pair is present it always goes into an equatorial spot, where it has more room and causes less strain.
Removing atoms and replacing them with lone pairs produces related shapes. Four bonds and one lone pair give a seesaw shape, three bonds and two lone pairs give a T-shape, and two bonds and three lone pairs give a linear molecule. All of these are built on the same trigonal bipyramidal arrangement of five electron regions, so recognizing the parent shape makes the others easier to predict.
Key facts
| Field | General Chemistry |
|---|---|
| Electron regions | 5 bonding, 0 lone pairs |
| Equatorial angle | 120 degrees |
| Axial angle | 90 degrees to the plane |
| Hybridization | sp3d |
| Example | PCl5 |
| Octet | expanded (10 electrons) |
| Lone pairs go | equatorial positions |
Phosphorus pentachloride, PCl5, in the gas phase shows trigonal bipyramidal geometry. Three chlorine atoms sit in a plane 120 degrees apart, while two more sit at the top and bottom, 90 degrees from the equatorial plane.
Frequently asked questions
Why are there two types of positions?
Five electron regions cannot all sit at equal angles. Three settle into an equatorial triangle at 120 degrees, and two occupy axial spots at 90 degrees above and below, so the positions differ in crowding.
Why do lone pairs prefer equatorial positions?
Equatorial positions are less crowded because a lone pair there faces only two close neighbors at 90 degrees instead of three. Placing lone pairs equatorially minimizes repulsion and strain.
What shapes come from lone pairs in this geometry?
One lone pair gives a seesaw, two give a T-shape, and three give a linear molecule. Each keeps the same five-region electron arrangement but shows fewer atoms.
Why can phosphorus form five bonds?
Phosphorus is in period 3 and can hold more than eight valence electrons, an expanded octet. In the common model, this uses sp3d hybrid orbitals to accommodate the fifth bond.