The Foundations of Chemistry

OCTAHEDRAL ELECTRONIC GEOMETRY: AB 6 , AB 5 U,

AND AB 4 U 2

A. Experimental Facts and Lewis Formulas

The Group VIA elements below oxygen form some covalent compounds of the AB 6 type
by sharing their six valence electrons with six other atoms. Sulfur hexafluoride, SF 6 (mp

51°C), an unreactive gas, is an example. Sulfur hexafluoride molecules are nonpolar octa-
hedral molecules. The hexafluorophosphate ion, PF 6
, is an example of a polyatomic ion
of the type AB 6.

B. VSEPR Theory

In an SF 6 molecule we have six valence shell electron pairs and six F atoms surrounding
one S atom. Because the valence shell of sulfur contains no lone pairs, the electronic and
molecular geometries in SF 6 are identical. The maximum separation possible for six elec-
tron pairs around one S atom is achieved when the electron pairs are at the corners and
the S atom is at the center of a regular octahedron. Thus, VSEPR theory is consistent
with the observation that SF 6 molecules are octahedral.

8-12

F F

F

F

S

F F

8-12 Octahedral Electronic Geometry: AB 6 , AB 5 U, and AB 4 U 2 335

In this octahedral molecule the FXSXF bond angles are 90° and 180°. Each SXF
bond is quite polar, but each bond dipole is canceled by an equal dipole at 180° from it.
So the large bond dipoles cancel and the SF 6 molecule is nonpolar.
By similar reasoning, VSEPR theory predicts octahedral electronic geometry and octa-
hedral molecular geometry for the PF 6
ion, which has six valence shell electron pairs
and six F atoms surrounding one P atom.

S

F

F

F

F F

F

In a regular octahedron, each of the

eight faces is an equilateral triangle.