Chemical modification of fullerenes such as the C60 molecule is a prom
ising way to generate novel materials with tailored properties. Exohed
ral insertion of atoms in the interstitial sites of the C60 solid has
created a new class of superconductors. Encapsulation of atoms in the
fullerene cage is expected to lead to stable M@C60 endohedral complexe
s with a very interesting optical response. We study the stability of
MxC60 solids and M@C60 molecules using a Born-Haber cycle. Our analysi
s not only provides the formations energies, but also elucidates the s
tability trends across the periodic table.