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December 8, 2011

Compounds With Octahedral Shape

For compounds whose central atoms possess six bonding electron pairs, their bonded atoms are oriented towards the corners of an octahedron.

The best examples are the PF6- ( phosphorus hexafluoride or hexafluorophosphate) ion and the SF6 ( sulfur hexafluoride ) molecule whose Lewis structures are shown below.





The ground state electronic configuration of the two central atoms:
P1s2 2s2 2p6 3s2 3px1 3py1 3pz1
S1s2 2s2 2p6 3s2 3px2 3py1 3pz1


In the excited state of the two central atoms above, their valence electrons are assumed to be distributed this way:
1s2 2s2 2p6 3s1 3px1 3py1 3pz1 3dz21 3dx2 - y21


Since six equal orbitals are required, it is assumed that sp3d2 hybridization is used to form six hybrid orbitals:

1s2 2s2 2p6 (sp3d2)1 (sp3d2)1 (sp3d2)1 (sp3d2)1 (sp3d2)1 (sp3d2)1
Phosphorus acquires an extra electron for its sixth orbital.



Geometry of PF6-



  • shape of ion: octahedral
  • bond angle of axial atoms: 180°
  • bond angle of equatorial atoms: 90°
  • bond angle between an equatorial atom and an axial atom: 90°


Axial atoms are in white circles; equatorial atoms are in black circles.





An octahedron is superimposed on the PF6- ion.







Geometry of SF6



  • shape of molecule: octahedral
  • bond angle of axial atoms: 180°
  • bond angle of equatorial atoms: 90°
  • bond angle between an equatorial atom and an axial atom: 90°