It is well known that (formula presented) bonding in carbon can result in stable cage structures, but pure Si clusters with similar cage structures are unstable. Using first-principles calculations, we show that a dodecahedral cage of silicon can be stabilized dynamically as well as energetically by doping with Ba, Sr, Ca, Zr and Pb atoms to create structures of silicon similar to that of the smallest carbon fullerene. The stability and bonding in such cages shed light on Si clathrates in which (formula presented) is the basic building block of the structure. Moreover, the charge distributions and highest-occupied-lowest-unoccupied molecular orbital gaps for these cage structures can be tuned by changing the metal atom. This allows additional freedom for the design of nanomaterials involving Si. {\textcopyright} 2002 The American Physical Society.