organic chemistry - What's the type of bonding in La@C60?

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?

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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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Answer

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.