Stimulated by the recent preparation and characterization of the first [50]fullerene derivative, decachlorofullerene[50] (Science 2004, 304, 699), we have performed a systematic density functional study on the electronic and spectroscopic properties of C-50, its anions and derivatives such as C50Cl10 and C50Cl12. The ground state of C-50 has D-3 symmetry with a spheroid shape, and is highly aromatic; the best D-5h C-50 singlet is nonaromatic. Both D-3 and D-5h isomers of C-50 have high electron affinities and can be reduced easily. Due to the unstable fused pentagon structural features, C-50 is chemically labile and subject to addition reactions such as chlorination, dimerization and polymerization. The equatorial pentagon-pentagon fusions of D-5h C-50 are active sites for chemical reactions; hence, D-5h C-50 may behave as a multivalent group. The computed IR, Raman, C-13 NMR and UV-vis spectra of the D-5h C50Cl10 molecule agree well with the experimental data. Finally, D-5h C50Cl10 is predicted to have a high electron affinity and, hence, might serve as an electron-acceptor in photonic/photovoltaic applications. The geometry and C-13 NMR chemical shifts of C50Cl12 were computed to assist further isolation experiments.