We report the results of a density functional study of the encapsulation of halide anions into the dodecahedral cage of the perhydrogenated silicon fullerene Si20H20. The HOMO-LUMO energy gap of the endohedral complexes X-@Si20H20 (X = F, Cl, Br, I) is only ∼0.5 eV larger than that computed for the empty fullerene. The amount of charge that is being transferred from the encapsulated anion to the fullerene increases from F- (∼10%) to I- (>70%). Only the encapsulation of Br- does not alter the cage thus indicating that this spherical anion might be employed in the anion-templated synthesis of the elusive Si20H20 cluster. {\textcopyright} 2005 Elsevier B.V. All rights reserved.