# Determine symmetry group

1. Feb 23, 2007

### greisen

Hi,

I have to determine the symmetry group of cyclopentadienyl(assumed to be planar and symmetrical) and I should use Huckel theory. Is there a systematic way to determine the symmetry group and is it possible to use software to determine the symmetry group.

2. Feb 23, 2007

### Gokul43201

Staff Emeritus
Yes, there is a systematic way to find the point group that the molecule belongs to.

See Chemical Applications of Group Theory by Albert Cotton, or Physical Chemistry by Atkins.

For the above complex, you can find the symmetry group by inspection.

3. Feb 24, 2007

### greisen

I draw the molecule and determine the different groups;

E - the identity
C2 rotation of 180 degrees
C2 horizontal 180 degrees

I know the molecule should have D5h but I have some trouble finding the rest of the symmetri axis.

Any help appreciated thanks in advance

4. Feb 24, 2007

### Gokul43201

Staff Emeritus
Looks like you've got the wrong structure to begin with. It is not the cyclopentadiene molecule (C5H6), but the (cyclopentadienyl) anionic ligand (C5H5-) that you need to draw the structure for.

Example: The cyclic groups depicted http://content.answers.com/main/content/wp/en-commons/thumb/9/93/200px-Zirconocene-dichloride-3D-sticks.png [Broken] are cyclopentadienyl rings.

Furthermore, even if you were finding the symmetry group for the C5H6 molecule, your third "symmetry element" makes no sense. What does "C2 horizontal" mean? That is not a standard symmetry element. The symmetry elements of the C5H6 molecule are $E,~C_2, ~\sigma _v,~ \sigma _{v'}$, making it a $C_{2v}$ molecule.

Yes, it is D5h, but you will get that only if you start with the right structure.

Last edited by a moderator: May 2, 2017
5. Feb 24, 2007

### GCT

The following are webpages that I've found useful during my undergraduate years and in the inorganic chemistry course.

http://chemistry.umeche.maine.edu/Modeling/symmetry.html

http://www.reciprocalnet.org/edumodules/symmetry/

http://www.ch.ic.ac.uk/vchemlab/symmetry/

http://www.phys.ncl.ac.uk/staff/njpg/symmetry/ [Broken]

Last edited by a moderator: May 2, 2017