The bonding in trimetallic nitride template (TNT) endohedral metallofullerenes is widely described by the ionic model Sc3N6+@Ck6-. 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.
![General rule for the stabilization of fullerene cages encapsulating trimetallic nitride templates](https://www.iciq.org/wp-content/uploads/2014/03/General-rule-for_eng_noticies_articles_2005_13.gif)