A systematic investigation on the molecular structures of silicon- substitued fullerenesC~5~9Si and C~6~9Si has been performed using semiempirical MNDO,AM1 and PM3 methods. The equilibrium geometrical structures, heats of formation, HOMO-LUMO gap energies, heats of atomization, ionization potentials, electron affinities, absolute electronegativities and globle hardnesses of C~5~9Si and C~6~9Si have been studied in this paper. The calculation results obtained indicate that both C~5~9Si and C~6~9Si are less stable compared with C~6~0 and C~7~0, but the stability of C~5~9Si is higher than those of the already successfully synthesized C~5~8N~2 and C~5~8B~2, which means that it can be synthesized experimentally. For C~6~9Si, both the heat of formation and the HOMO-LUMO plitting suggest that the isomer E, in which the silicon atom is located in the equator, is in the ground state. The heterofullerenes C~5~9Si and C~6~9Si have smaller ionization potentials (IP) and electron affinities (EA) as compared with their all--carbon analogues. Thus the redox characteristics of C~6~0 and C~7~0 can be enhanced by doping. The reactivities of the silicon substituted fullerenes towards an electrophile or a nucleophile are considered based on the frontier molecular orbital (FMO) theory and the HSAB principle. The absolute eletronegativities(X) and global hardnesses (η) of the silicon substituted fullerenes are both smaller than those of C~6~0 and C~7~0. Thus the abilities of C~5~9Si and C~6~9Si to attract electrons to themselves are relatively weaker, and the resistance of the chemical potential to change in the number of electrons are also weader as compared with their all carbon analogues.

Key words: FULLERENES, GEOMETRICAL ISOMERISM, STABILITY