The electronic structures of the C-60, (H3P)(2)Pt(eta(2)-C2H4), and (H3P)(2)Pt(eta(2)-C-60) are calculated by using the EHT-MO method with the fragment analysis. We have modified the EHT parameters so as to yield the energy level correlation and optical transition gap for the previous theoretical result of C-60. In Pt-derivatives, our FMO results with the modified parameters show that the carbon-carbon double bonds of C-60 and ethene react like those of electron-poor arenes and alkenes. Back donation is much stronger in the C-60 complex than in the ethylene complex : the amount of charge transfer is about two times larger and the stabilization energy is about 0.24eV larger.