The bonding in trimetallic nitride template (TNT) endohedral metallofullerenes is widely described by the ionic model Sc₃N⁶⁺@C_k^6-. The ability of the fullerene cage to accept six electrons determines its capacity to encapsulate the TNT unit (see picture). Only free fullerene isomers with a large (LUMO+3)-(LUMO+4) gap can capture and stabilize these four-atom units.