The electronic and structural properties of Si doped fullerenes, obtained from C₆₀ by replacing up to 12 C atoms with Si atoms, are studied by means of first principles density functional theory calculations using numerical atomic orbitals as basis sets. We have analyzed the relative stability of several C_60-xSi_x isomers, with x=1,2,3,6 and 12. We find that, for x=3 and x=6, the Si atoms prefer strongly to be first neighbors of each other, in order to minimize the number of Si-C bonds. However, these configurations of lower energy present a few relatively weak Si-C and Si-Si bonds that could indicate possible breaking paths. For a better comparison with the experimental measurements, we have also considered some positively charged ions, and report here the differences between properties of these ions and the corresponding neutral molecules.