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How can a metal atom be enclosed inside a fullerene carbon cage?

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cluster Figure 1: The four stable geometric structures of the seven-atom cluster of argon, in order of increasing energy: (A) A pentagonal bipyramid. (B) A regular octahedron with one face capped by the seventh atom. (C) A regular tetrahedron with three of its faces capped by other atoms. (D) A trigonal bipyramid with two of its faces capped by other atoms; although this has the highest energy of the four structures, it is very close in energy to the tricapped tetrahedron.

Metal atoms can be enclosed inside a fullerene carbon cage to form what are known as endohedral species, designated by the formula M@C $_{60}$ .1

The primary method for producing these complexes involves vaporizing graphite disks or rods that have been impregnated with the selected metal. This process typically utilizes an electric arc within a helium atmosphere to vaporize the carbon and the metal simultaneously, allowing the fullerene cage to form around the metal atom.1

While this technique is effective for trapping various metals, including alkali metals, alkaline earth metals, and early lanthanoids, other substances like helium can be encapsulated by heating C $_{60}$ in helium vapour under high pressure. The size of the encapsulated atom is a critical factor, as it determines the minimum size required for the surrounding fullerene cage.1