The X@C28 compounds form the family of the smallest endohedral fullerenes. Currently, they consist of C28, which is endohedrally doped with group-4 elements (Ti, Zr, Hf) and U. We have studied the electronic properties, equilibrium geometries, binding energies and other properties of several X@C28 at the PBE/def2-TZVP level. These were compared with the tetraanion C284−. For this study we have chosen elements from groups 4, 14 and Sc as endohedral dopants for being many of them tetravalent. In addition, we studied by the first time the entire group-10 (Ni, Pd and Pt) as endohedral dopant of C28. To elucidate the stability of these ionic compounds, we have studied their binding energies. Our results show that a big positive binding energy is necessary but not enough to determine the stability of these endohedral fullerenes. We propose that a HOMO–LUMO gap around 2eV results in less reactive compounds, property only shown by group-4 compounds. Some compounds have aromatic character in all rings according with their NICS(0)iso and reparametrized HOMA indexes. In addition, the charge distribution have been studied with NBO and Hirshfeld population analysis as well as electrostatic potential maps in order to find similarities between them and that shown by the tetraanion C284−. The most stable compounds are the group-4 X@C28 which form compounds: energetically favorable, less reactive, ionic and aromatic. Our results are compared with available experimental information about the formation of these compounds.