Endohedral metallofullerenes (EMFs) are able to encapsulate up to four metal atoms. In EMFs, metal atoms are positively charged because of the electron transfer from the endohedral metal atoms to the carbon cage. It results in the strong Coulomb repulsion between the positively charged ions trapped in the confined inner space of the fullerene. At the same time, in many EMFs, such as Lu2@C76, Y2@C79N, M2@C82 (M = Sc, Y, Lu, etc.), Y3@C80, or Sc4O2@C80, metals do not adopt their highest oxidation states, thus yielding a possibility of the covalent metal–metal bonding. In some other EMFs (e.g., La2@C80), metal–metal bonding evolves as the result of the electrochemical or chemical reduction, which leads to the population of the metal-based LUMO with pronounced metal–metal bonding character. This article highlights different aspects of the metal–metal bonding in EMFs. It is concluded that the valence state of the metal atoms in dimetallofullerenes is not dependent on their third ionization potential, but is determined by their ns2(n − 1)d1 → ns1(n − 1)d2 excitation energies. Peculiarities of the metal–metal bonding in EMFs are described in terms of molecular orbital analysis as well as topological approaches such as Quantum Theory of Atoms in Molecules and Electron Localization Function. Interplay of Coulomb repulsion and covalent bonding is analyzed in the framework of the Interacting Quantum Atom approach.

Dear Tobias Krämer sorry to see that your interesting technical question has received only a copy-and-paste answer which has not much to do with your original question. We are synthetic inorganic chemists, so I'm absolutely no expert in this area of theoretical chemistry. However, in my experience it often pays off to search the "Publications" and "Questions" section of RG for relevant literature references and closely related technical questions. This way I found for example the following potentially useful article:

On the presence of non-nuclear attractors in the charge distributions of Li and Na clusters

Article On the presence of non-nuclear attractors in the charge dist...

This paper is freely accessible as public full text on RG. I'm not even sure how relevant this is to your question, but I think you should give it a try. Many relevant references have been posted by RG members, some of them even as public full texts.

Good luck with your work and best wishes, Frank Edelmann

Dear Frank T. Edelmann thanks for your kind response and advise. I have not made much use of the questions here on this platform, so I am quite a novice.

The good news is that I have in the meantime received clarification on my above question from another source, which has solved the problem. The paper that you have linked is most useful, and I believe I have already stumbled over it during my own literature search on this topic (a most interesting field I must say and unknown to me up to now). Once again, many thanks for your help in this matter.

Kind regards,

Tobias