Structure and stability of endohedral fullerene Sc3N@C-80 were studied by temperature-dependent Raman and infrared spectroscopy as well as by quantum-chemical [density-functional-based tight-binding] calculations. The material showed a remarkable thermal stability up to 650 K. By both theory and experiment, translational and rotational Sc3N modes were found. These modes give a direct evidence for the formation of a Sc3N-C-80 bond which induces a significant reduction of the ideal I-h-C-80 symmetry. From their splitting pattern a crystal structure with more than one molecule in the unit cell is proposed. According to our results: (i) a significant charge transfer from the Sc3N cluster to the C-80 cage; (ii) the strength of three Sc-N bonds; (iii) the chemical bond between triscandium nitride cluster and C-80 cage; and (iv) a large HOMO-LUMO gap are responsible for the high stability and abundance of Sc3N@C-80.