Si-doped heterofullerenes C59Si and C58Si2, obtained from C-60 by replacing one and two C atoms with Si atoms, are investigated via first principles calculations. Static geometry optimizations show that structural deformations occur in the vicinity of the dopant atoms and give rise to Si-C bonds significantly larger than the ordinary C-C bonds of the fullerene cage. In the case of C58Si2, the lowest energy isomer has two Si atoms located at distances corresponding to third nearest neighbors. The electronic structure of these heterofullerenes, although globally close to that of C-60, is characterized by a strong localization of both the HOMO's and the LUMO's on the Si sites. Charge transfer occurs from the dopant atoms to the nearest neighbor C atoms, contributing to the formation of polar Si-C bonds. A detailed analysis of the charge localization, based on the electron localization function and maximally localized Wannier function approaches, reveals that the bonding of Si in the fullerene cage consists of two single and one weak double bond, thus preserving the conjugation pattern of the undoped C-60. Beside the charge localization along the bonds, we observe a peculiar region of charge localization outside the cage above each Si atom. These features are discussed in comparison with the corresponding patterns exhibited by the C-60 system.