Trigonal bipyramidal molecules
What is here?
You can see ball-and-stick models of phosphorus pentafluoride, sulfur tetrafluoride and chlorine trifluoride:
- all three molecules have five pairs of outer electrons around their central atom, so
- they all have a trigonal bipyramidal arrangement of electron pairs around the central atom.
phosphorus pentafluoride, PF5
sulfur tetrafluoride, SF4
chlorine trifluoride, ClF3
There are five pairs of outer electrons around the central atom in each of these molecules. These pairs of electrons repel each other. In terms of the relative strength of repulsion:
|strongest||lone pair – lone pair|
|⇣||lone pair – bond pair|
|weakest||bond pair – bond pair|
The F—P—F bond angles in PF5 are:
- 90° between an atom in the axial position and an atom in the equatorial position
- 120° between two atoms in the equatorial position.
These angles are obtained when all five pairs of outer electrons repel each other equally. The bond angles in SF4 and in ClF3 are less than these values because of the stronger repulsion by their lone pairs of electrons.
Summary of bond angles and shapes
|PF5||5||0||90° / 120°||trigonal bipyramidal|
|SF4||4||1||86.5° / 102°||see-saw|
triiodide ion, I3–
The triiodide ion, I3–
This is not a molecule but it is an interesting addition to this page. The central iodine atom has five pairs of outer electrons but it has just two bonding pairs of electrons. Its three lone pairs of electrons occupy the equatorial position.
These lone pairs repel each other and the two bonding pairs so that the other two iodine atoms occupy the axial positions. This produces a linear structure, even though there is a trigonal bipyramidal arrangement of electron pairs.