We analyze the electronic structure of carbide endohedral metallofullerenes of the type Sc2C2@C-82 and study the possibility of rotation of the encapsulated Sc2C2 moiety in the interior of the cage. Moreover, we rationalize the higher abundance of M2C2@C-82 (M = Sc, Y) in which the metal-carbide cluster is encapsulated in the C-3 nu-C-82:8 carbon cage with respect to other carbides of the same family on the basis of the formal transfer of four electrons from the cluster to the cage and sizeable (LUMO-3)-(LUMO-2) gap in the empty cages. This rule also applies to all those endohedral metallofullerenes in which the encapsulated cluster transfers four electrons to the carbon cage as, for example, the reduced [M@C-82](-) systems (M = group 3 or lanthanide metal ion).