Ab initio, Hartree-Fock (HF) calculations were carried out to determine the structures and energies of the endohedral complexes of C-58 cage with H-2 or CO. It was demonstrated that the formation of these complexes is endothermic with destabilization energies of 3.3 kcal/mol for H-2 and 18.6 kcal/mol for CO. Furthermore, the H-2 and CO molecules have different orientations in the C-58 cage, namely, the orientation of the molecular axis of the former is normal to the face of the 7-member ring, while that of the latter is parallel to that face. In addition, the H-H bond of the H-2 molecule is shortened inside the cage, whereas the length of the C-O bond remains unchanged. (C) 2004 Elsevier B.V. All rights reserved.