Many insoluble metallofullerenes are called missing metallofullerenes. These metallofullerenes have not yet been isolated despite their detection in raw soot using mass spectrometry. They have been anticipated to show unique structures and properties that differ considerably from those of soluble metallofullerenes. Herein, a missing metallofullerene, La@C-80, was extracted and isolated as a derivative, La@C-80(C6H3Cl2). The structure of La@C-80(C6H3Cl2) was determined unambiguously using single-crystal X-ray crystallographic analysis. In fact, La@C-80 has a novel C-80 cage, C-2v-C-80 (80:3), which has not been reported. The calculation of La@C-80 with C-2v-C-80 (80:3) cage shows that La@C-2v-C-80 (80:3) has a radical character as well as small ionization potential (Ip) and electron affinity (Ea), which are indicative of high reactivity. On the basis of those results, La @ G C80 (80:3) is expected to interact strongly and bond with amorphous carbon or other fullerenes in soot that is made insoluble in common organic solvents.