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Endohedral fullerene is the name given to a brand new chemical species with the following attributes:

A metal, usually a transition metal, is "trapped" inside the fullerene, like $\ce{C60}$ and $\ce{C82}$. These transition metals are often La, Sc, Ce, Y, or, Ba and Sr etc.

Without any "binding" forces, the atom couldn't be possibly trapped inside the fullerene.

Here it's been discussed that the bonding is the type of "donor-acceptor". Later within, it was explained that

Though there isn't empirical proof, It's believed for these species to have metallic structure.

I have two questions:

  • Is there another simpler name for this "donor-acceptor" bonding? My thoughts lead to wierd names!
  • How "close" is this type of bonding to metallic bonding?

Edit: Citation: The second article:

Publisher: [AIP]; German journal of chemical physics. Volume 115, number 15, page 7215. (15 Oct 2001) (Juelich GMBH or something; Couldn't figure the name out, maybe German fellows would help.)

DOI: 10.1063/1.1406500

Authors: R. Klingeler, G. Kann, I. Wirth, S. Eisebitt, P. S. Bechthold, M. Neeb, and W. Eberhardt (Dr. Rudiger Klingeler is the main author)

Title: $\ce{La$@$C60}$: A metallic endohedral fullerene.

Unfortunately the other mirror links are behind paywalls. (But this article is available in Researchgate, in case you've got institutions to be able to create an account)

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  • $\begingroup$ Is this a clathrate, similar to methane hydrate? A skim of the PDF did not find use of the word "clathrate", but that is another name for "caged" molecules. See en.wikipedia.org/wiki/Methane_clathrate $\endgroup$
    – DrMoishe Pippik
    Commented Feb 1, 2015 at 3:59
  • $\begingroup$ "Cage", "trapping", "inside the lattice". These terms are applied for both, I can see. But the article mentioned a "donor-acceptor" bond with some metallic characters. I'm even more confused now, not to mention more eager to hear answers. $\endgroup$
    – M.A.R.
    Commented Feb 1, 2015 at 16:23
  • $\begingroup$ :) Not that "type"! I mean, there should be a chemical name for the force that allows La to stay there. Unless, it's like a ball in a box that's in rapid motion "hitting" C atoms. (And that isn't mentioned in several articles I skimmed) $\endgroup$
    – M.A.R.
    Commented Feb 2, 2015 at 18:01
  • $\begingroup$ ""Unless, it's like a ball in a box that's in rapid motion "hitting" C atoms"" What else? Whether the motion is "rapid", do You have figures? $\endgroup$
    – Georg
    Commented Feb 2, 2015 at 18:25
  • $\begingroup$ That's what's bothering me. You can ignore my upper comment. It's a guess. I haven't seen anywhere anything that can even be close to proving it. If La is somehow remaining there it's either because of a force or because "it simply can't get out". I was making up chemistry for what it would have been if the second was true. :) $\endgroup$
    – M.A.R.
    Commented Feb 2, 2015 at 18:30

2 Answers 2

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That the question says: "Without any 'binding' forces, the atom couldn't be possibly trapped inside the fullerene", shows you are misunderstanding fullerenes.

Atoms such as Helium and $H_2$ can be confined within the interior of the fullerene.

There are even reports of extraterrestrial helium trapped in fullerenes during astroid or comet impact.

The helium can move around within the fullerene. It is sterically confined rather than bonded.

In the case of La, the metal ion does not remain neutral, but loses about 3 electron to the fullerene. So, in addition to being sterically confined in the cage, there is the interaction between postively and negatively charged species.

As calculated in a later article Computed Structure and Energetics of La@C60 International Journal of Quantum Chemistry, Vol 104, 272–277 (2005), the La has a charge of somewhere betwen 2+ and 3+. In the lowest energy state, the La is off-center with respect to the fullerene, located between the center and the center of a hexagon face. Also, the fullerene cage is distorted from icosahedral by Jahn-Teller distortion.

The La ion is located 2.62 Angstroms from the nearest carbon atom (4.6 Angstroms from the furthest), when at the potential miniumun, but is expected to move within the fullerene unless the temperature is very low. The reference also says "there is obviously a relatively large negative charge on some carbon atoms of the cage", without specifying how this charge is distributed.

Overall I would say, whereas in the case of helium and dihydrogen being confined in the fullerene is completely because of van der Waal repulsive force of avoiding the walls, in the case of La, there is attractive electrostatic force (ionic interaction) between the La3+ and negatively charged carbon atoms.

See also Theoretical investigation of structural and thermodynamic properties of lanthanum carbides LaCn (n = 2–6) which finds strong ionic interactions between La and Cn due to charge transfer.

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It is unlikely to find an uncharged metal atom hovering in the centre of a fullerene cage.

$\ce{C60}$ is a good electron acceptor. This has been widely demonstrated in a lot of studies on photoinduced electron transfer in donor-acceptor dyads, where the donor was a metalloporphyrine attached to the fullerene. Here, excitation of the donor led to an intramolecular redox reaction in which the fullerene radical anion $\ce{C60^{-\cdot}}$ was formed.

If you're interested in this particular topic, look for some earlier works by Devens Gust, Thomas A. Moore and Ana L. Moore.

Taking that into account, full electron transfer, resulting in caged ion pair, or at least some charge transfer is conceivable.

The term "donor-acceptor" thus seems appropriate.

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