Electronic and nonlinear optical properties of endohedral metallofullerenes are presented. The endohedral metallofullerenes contain transition metal encapsulated in inorganic fullerenes X12Y12(X = B, Al & Y = N, P). The endohedral metallofullerenes (endo-TM@X12Y12) possess quite interesting geometric and electronic properties, which are the function of the nature of the atom and the size of fullerene. NBO charge and frontier molecular orbital analyses reveal that the transition metal encapsulated Al12N12fullerenes (endo-TM@Al12N12) are true metalides when the transition metals are Ni, Cu and Zn.Endo-Cr@Al12N12andendo-Co@Al12N12are at the borderline between metalides and electrides with predominantly electride characteristics. The other members of the series are excess electron systems, which offer interesting electronic and nonlinear optical properties. The diversity of nature possessed byendo-TM@Al12N12is not prevalent for other fullerenes.Endo-TM@Al12P12are true metalides when the transition metals are (Cr-Zn). HOMO-LUMO gaps (EH-L) are reduced significantly for these endohedral metallofullerenes, with a maximum percent decrease in EH-Lof up to 70%. Many complexes show odd–even oscillating behavior for EH-Land dipole moments. Odd electron species contain large dipole moments and small EH-L,whereas even electron systems have the opposite behavior. Despite the decrease in EH-L, these systems show high kinetic and thermodynamic stabilities. The encapsulation of transition metals is a highly exergonic process. Theseendo-TM@X12Y12possess remarkable nonlinear optical response in which the first hyperpolarizability reaches up to 2.79 × 105au forendo-V@Al12N12.This study helps in the comparative analysis of the potential nonlinear optical responses of electrides, metalides and other excess electron systems. In general, the potential nonlinear optical response of electrides is higher than metalides but lower than those of simple excess electron compounds. The higher non-linear optical response and interesting electronic characteristics ofendo-TM@Al12N12complexes may be promising contenders for potential NLO applications.