Assigning Point Group for XeO2F4

[ochemnerd]

Homework Statement

Had this question on a test today and now I'm having second thoughts. We were asked to assign the point group of symmetry for the compound XeO2F4.

Homework Equations

None

The Attempt at a Solution

I had initially thought that it fell under D4h because it contains 1 C4 axis, 4 C2 axes, 4 σv planes and 1 σh plane. I know that the compound XeF4 is D4h and my rationale for assigning D4h to XeO2F4 was that it is identical to XeF4 except that instead of 2 lone pairs on the Xe there are 2 double bonded O's.

However, is the correct assignment for XeO2F4 octahedron (Oh)? It is a octahedral shape according to VSEPR but I'm not sure if all 6 substituents on the central atom have to be identical on Oh point groups.

Can someone please shed some light on this?

Cheers!

 

[sjb-2812]

Homework Statement

Had this question on a test today and now I'm having second thoughts. We were asked to assign the point group of symmetry for the compound XeO2F4.

Homework Equations

None

The Attempt at a Solution

I had initially thought that it fell under D4h because it contains 1 C4 axis, 4 C2 axes, 4 σv planes and 1 σh plane. I know that the compound XeF4 is D4h and my rationale for assigning D4h to XeO2F4 was that it is identical to XeF4 except that instead of 2 lone pairs on the Xe there are 2 double bonded O's.

However, is the correct assignment for XeO2F4 octahedron (Oh)? It is a octahedral shape according to VSEPR but I'm not sure if all 6 substituents on the central atom have to be identical on Oh point groups.

Can someone please shed some light on this?

Cheers!

yes, all 6 have to be the same. Don't forget there are also C3 axes in octahedral symmetry, which are not present in your proposed structure.

 

[ochemnerd]

Thanks for the clarification, sjb-2812!
I fail to see where the C3 axes would be though in a molecule of octahedral symmetry...

 

[sjb-2812]

Consider the shape at http://en.wikipedia.org/w/index.php?title=File:Octahedron.svg&page=1

Now, rotate one opposing pair of faces about an axis through their centres 120°...

 

[DeeNos]

Your initial thought of assigning D4h for XeO2F4 is correct. The compound does indeed have 1 C4 axis, 4 C2 axes, 4 σv planes and 1 σh plane, making it a D4h point group. The fact that it is similar to XeF4, which is also a D4h point group, further supports this assignment.

As for the consideration of Oh point group, it is not applicable in this case. The Oh point group is only applicable for molecules with six identical substituents around the central atom, which is not the case for XeO2F4. Therefore, D4h is the correct assignment for XeO2F4.