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Octahedral molecules

What is here?

You can see ball-and-stick models of sulfur hexafluoride, iodine pentafluoride and xenon tetrafluoride:

  • all three molecules have six pairs of outer electrons around their central atom, so
  • they all have an octahedral arrangement of electron pairs around the central atom.
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chemdoodle.png atom labels
ball and stick model
space-filling model
perspective view

sulfur hexafluoride, SF6

iodine pentafluoride, IF5

xenon tetrafluoride, XeF4

Shapes

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—S—F bond angles in SF6 are all 90°. These angles are obtained when all six pairs of outer electrons repel each other equally.

The bond angles in IF5 are less than this because of the stronger repulsion by the lone pair of electrons in the axial position.

Summary of bond angles and shapes

Molecule
Bonding pairs
Lone pairs
Bond angle(s)
Shape
SF6 6 0 90° octahedral
IF5 5 1 82° square pyramidal
XeF4 4 2 90° square planar
ICl4 4 2 90° square planar

The central xenon atom in XeF4 has two lone pairs of electrons in the axial positions. The four fluorine atoms occupy the equatorial positions, giving a square planar shape.

tetrachloroiodate ion, ICl4

The tetrachloroiodate ion, ICl4

This is not a molecule but it is an interesting addition to this page. The central iodine atom has six pairs of electrons but it has just four bonding pairs of electrons. Its two lone pairs of electrons occupy the axial positions.

These lone pairs repel each other and the four bonding pairs so that the four chlorine atoms occupy the equatorial positions. This produces a square planar structure, even though there is an octahedral arrangement of electron pairs.