Using first principles calculations, we find that novel M@SWSiNTs are stabilized due to their endohedral transition metal atoms, and that their structures are more flexible than SiNTs with hexagonal prism structures [A. K. Singh, V. Kumar, Y. Kawazoe, J. Mat. Chem. 14 (2004) 555]. Because of the strong hybridization between metal and Si orbitals the charge flows freely, as in a crystal, making all M@SWSiNTs studied metallic. Many energy levels are occupied by electrons in hybridized orbitals, resulting in a quasicontinuum energy band. All M@SWSiNTs are good electron acceptors, and most of them are good electron donors. M@SWSiNTs open the possibility of developing novel silicon nanomaterials that could be of interest for industrial applications. (C) 2009 Elsevier B.V. All rights reserved.