Do molecules with a hexagonal planar geometry exist?

Question

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 exist, and if they do, what are they?

I'd suspect that sterically theres just no chance of getting groups around a central atom — NotEvans., Jun 25 '17 at 22:20
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. — Oscar Lanzi, Jun 25 '17 at 22:37

score 12 · Accepted Answer · edited Jun 26 '23 at 18:49

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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:

Side view:

Unit cell and packing:

edited Jun 26 '23 at 18:49

Mithoron

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answered Jun 25 '17 at 23:13

andselisk

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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 – NotEvans. Jun 25 '17 at 23:32
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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. – andselisk Jun 25 '17 at 23:57

score 11 · Answer 2 · edited Jun 10 '20 at 14:04

In October 2019 a series of new transition metal complexes has been reported in an article published in Nature by Garçon et al. [1]. Among these, two palladium complexes have a near-perfect hexagonal arrangement of three hydride and three magnesium-diisopropylphenyl ligands according to single-crystal x-ray diffraction studies:

tris([2,6-di(propan-2-yl)phenyl][4-{[2,6-di(propan-2-yl)phenyl]imino}pent-2-en-2-yl]amido)-trihydrido-palladium-tri-magnesium n-hexane solvate (compound 1a, CCDC 1909687);
tris([(2,4,6-trimethylphenyl){4-[(2,4,6-trimethylphenyl)imino]pent-2-en-2-yl}amido)-trihydrido-tri-magnesium-palladium (compound 1b, CCDC 1909688).

Interatomic distance were also supported by DFT calculations, which also showed that $\ce{Pd–Mg}$ bond is primarily ionic in nature.

Fig. 1 […] Single-crystal X-ray derived model of 1a, annotated with selected experimental and calculated bond lengths.

Some news agencies such as C&EN called this structure "first hexagonal planar crystal structure", however the authors of the article make an important addition:

The hexagonal planar coordination environment is known, but it is restricted to condensed metallic phases, the hexagonal pores of coordination polymers, or clusters that contain more than one transition metal in close proximity. […] Here we report the isolation and structural characterization of a simple coordination complex in which six ligands form bonds with a central transition metal in a hexagonal planar arrangement.

References

Garçon, M.; Bakewell, C.; Sackman, G. A.; White, A. J. P.; Cooper, R. I.; Edwards, A. J.; Crimmin, M. R. A Hexagonal Planar Transition-Metal Complex. Nature 2019, 574 (7778), 390–393. DOI: 10.1038/s41586-019-1616-2.

score 3 · Answer 3 · answered Jun 25 '17 at 22:32

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.

Do molecules with a hexagonal planar geometry exist?

3 Answers3

References