To date, the successful synthesis of boron-substituted fullerenes has been achieved. In fact Lewis acid/base adducts of C59B- - -NH3 have been identified. Thus far, no unambiguous or absolute identification of the analogous nitrogen-substituted fullerenes has been reported. In this work we discuss limitations in the stability of monosubstituted heterohedral fullerenes. Both quantum and molecular mechanical approaches are used to understand the limits in bonding and structural characteristics of these materials. Both the electronic structure calculations and the molecular mechanics study suggest that greater inherent stability exists in the monosubstituted boron fullerene compared with the equivalent nitrogen analog. This is somewhat consistent with the sparse experimental information provided for N-doped fullerenes. In addition, structural comparisons show significantly greater distortion of the fullerene framework for the nitrogen-substituted material. Electronic structure calculations predict greater disruption of bonding near the substitution site for C59N in comparison with C59B, and the energy trends from these calculations suggest that the stability of the C59N is considerably lower than that of C59B. Furthermore, the electronic structure calculations suggest that C59N+ may be more stable than the parent neutral material. Disubstituted heterohedral fullerenes have been analyzed utilizing similar electronic requisites for stability. This discussion complements a recent report that describes the mass spectral and electronic absorption and emission spectral data for isomers of C58N2.