I recently read an article on Wikipedia about pentagonal planar geometry.

There are only two molecules with this kind of geometry $\ce{XeF5-}$ and $\ce{IF5^2-}$ and it has two electron pairs at the axial positions.

As soon as I looked at this, I thought that hexagonal molecular geometry can also exist much the same way i.e. 6 substituents bonded to the central atom and two lone pairs at the axial positions opposite to each other.

On the page, its given that arrangement of 6 atoms in a plane is found in higher coordination states but that does not help either as they have not given any examples of that.

Do hexagonal planar molecules really exists, and if they do, what are they?

  • $\begingroup$ I'd suspect that sterically theres just no chance of getting groups around a central atom $\endgroup$ – NotEvans. Jun 25 '17 at 22:20
  • 1
    $\begingroup$ You could say that in an fcc or hcp metal has atoms hexagonally bonded to any central atom, if you look in the proper plane. $\endgroup$ – Oscar Lanzi Jun 25 '17 at 22:37

I think it's nearly impossible to find or synthesize a "canonical" complex with hexagonal molecular geometry, but in the field of host-guest supramolecular chemistry there are numerous examples of "unusual" geometry. Probably the most well-established class of such compounds are torands ("hosts") incorporating alkali metal cations ("guests").

Check out, for example: (1) Bell, T. W.; Cragg, P. J.; Drew, M. G. B.; Firestone, A.; Kwok, D.-I. A. Angew. Chem. Int. Ed. Engl. 1992, 31 (3), 345–347, DOI 10.1002/anie.199203451.

Here is an example of the structure with potassium ion from (1), Tri-n-butyltorand-potassium picrate clathrate, which I quickly sketched in Olex2:

Top view: enter image description here

Side view: enter image description here

Unit cell and packing: enter image description here

  • 1
    $\begingroup$ They're fascinating complexes, thinking about it more, I guess that this is somewhat an extension to 18-c-6 bonding with a cation, I suspect that adopts a similar geometr $\endgroup$ – NotEvans. Jun 25 '17 at 23:32
  • 1
    $\begingroup$ Yep, they inherit a lot from crown ethers, though I picked potassium and this particular class of torands because their combination allows for a planar 6-coordinated geometry. Na would be too small to properly coordinate, Rb is sticking out from the plane quite a lot. Classic crowns are very flexible, and therefore achieving planar geometry (what OP required) using them would also be tricky. $\endgroup$ – andselisk Jun 25 '17 at 23:57

It looks as though hexagonal planar molecules are currently unknown, but you can find a hexagonal plane in higher coordination structures like a hexagonal bipyramid.

One example of a structure with hexagonal bipyramidal geometry is $\ce{UO_2(OAc)_3}$, which I found here on a site which gives similar info to your link, but includes example structures for known geometries.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.