|Indiana University Molecular Structure Center|
An octahedron is a regular solid with eight identical faces. Octahedral coordination of a metal occurs when six bonded atoms of coordinating ligands occupy sites that would correspond to the six vertices (corners) of a regular octahedron surrounding the metal atom.
A regular octahedron is shown on the left, octahedral coordination in the middle, and the two are superimposed in the figure on the right.
Each of the figures above can be rotated by holding the left mouse button and dragging the mouse in the frame.
The following shows a molecule of FeCl2(C15H15N7) on the left with an ideal octahedral geometry on the right. Use the mouse to rotate the images and identify the atoms that make up the octahedral coordination in the molecule.
Type "h" after the mouse is clicked in the applet above to turn off or on the hydrogen atoms. You can see the numbering scheme used by typing "l" (for label). You can use the geometry options in the Reciprocal Net JaMM2.0 applet to see how much the angles differ from their "idea" values.
A second example of octahedral coordination is the [Fe(thiosal)3]3- ion shown in the applet below. Note that in this case the molecule is initially oriented showing three-fold symmetry. If you rotate the image of the octahedron in the first image at the top of this page, you will see that it actually posses many symmetry elements including 2-fold, 3-fold, 4-fold rotational as well as mirror symmetry. You can examine the geometry of the [Fe(thiosal)3]3- ion by using JaMM2.0 for [Fe(thiosal)3]3-. (Netscape or Mozilla -- IE does not support JAVA properly)
This illustrates a very important point. The coordination of the metal in the two complexes above is octahedral while the molecular symmetry is not! The reason we call it octahedral coordination is that the bonded atoms of the ligand(s) are present at what would be the verticies of a regular octahedral solid surrounding the metal.
To learn more about regular solids, visit the Regular Solids page.
Quiz yourself on this material!