Molecules with Trigonal Bipyramidal Electronic Symmetry

Electronic Structure: AB5
Electronic Geometry: trigonal bipyramidal
Hybridization at central atom: sp3d

A premise of VSEPR theory is that groups of electrons surrounding a central atom repel one another and try to get as far away from one another as possible. However, electrons participating in a covalent atom-atom bond and non-bonding electrons (lone-pair electrons) do not repel one another with the same strength. The repulsion between two groups of non-bonding electrons, for example, is greater than the repulsion between two groups of bonding electrons. The sequence is:

Strongest Repulsion
non-bonding pair -- non-bonding pair
non-bonding pair -- atom-atom bond
atom-atom bond -- atom-atom bond
Least Strong Repulsion

You can see the effects of this unequal repulsion in the models below. In the PF5 molecule, the axial (up and down positions) F atoms are separated by exactly 180°. In the SF4 molecule, however, one of the equatorial F atoms has been replaced by a non-bonding electron pair. This non-bonding pair exerts a greater repulsion on its neighbors than another F atom would. The result is that all the F atoms shift a bit further away from the non-bonding pair, and the axial F atoms are no longer separated by exactly 180°. This trend continues in the BrF3 molecule. The iodine atoms in the I3- ion are, however, once again separated by exactly 180°. Why is this so?

AB5
Trigonal
Bipyramidal
 		Phosphorus Pentafluoride
(PF5)
Non-bonding Electron Pairs
  None on central atom.
 
Polyhedral Framework
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AB4E
See-Saw
 		Sulfur Tetrafluoride
(SF4)
Non-bonding Electron Pairs
  Show:     Hide:
 
Polyhedral Framework
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AB3E2
T-shaped
 		Bromine Trifluoride
(BrF3)
Non-bonding Electron Pairs
  Show:     Hide:
 
Polyhedral Framework
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AB2E3
Linear
 		Triiodide ion
(I3-)
Non-bonding Electron Pairs
  Show:     Hide:
 
Polyhedral Framework
  Show:     Hide:

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