The ab initio density functional theory (DFT) method was used to carry out self-consistent calculations of the electronic structure of "mixed" C-28-based fullerenes: (1) heterofullerenes C24N4, C24B4, (2) exohedral complexes C28M4 (M = H, F, Cl, Br), (3) endohedral complexes M@C-28 with 2p- (M = B, C, N, O), and (4) endohedral complexes M@C-28 with 3d-atoms (M = Sc, Ti, V, Cr, Fe, Cu). The electronic structure, charge distributions, chemical bonding and comparative stability in the series of these complexes are analyzed. Studying different types of C-28 functionalization, we established that the optimum chemical stability conditions are met for (1) C24B4 from the series of heterofullerenes, (2) C28F4 from the series of exocomplexes, (3) B@C-28 from the series of endocomplexes with participation of 2p-atoms, and (4) Ti@C-28 from the series of endocomplexes with participation of 3d-atoms. Possible mechanisms of C28 stabilization are discussed depending on the method of functionalization and the type of a "guest" atom. (C) 2001 Elsevier Science B.V. All rights reserved.