Small hydrocarbon complexes (X@cage) incorporating cage-centered endohedral atoms and ions (X = H+, H, He, Ne, Ar, Li-0,Li-+, Be-0,Be-+,Be-2+, Na-0,Na-+, Mg-0,Mg-+,Mg-2+) have been studied at the B3LYP/6-31G(d) hybrid HF/DFT level of theory. No tetrahedrane (C4H4, T-d) endohedral complexes are minima, not even with the very small hydrogen atom or beryllium dication. Cubane (C8H8, O-h) and bicyclo[2.2.2]octane (C8H14, D-3h) minima are limited to encapsulating species smaller than Ne and Na+. Despite its intermediate size, adamantane (C10H16, T-d) can enclose a wide variety of endohedral atoms and ions including H, He, Ne, Li-0,Li-+, Be-0,Be-+,Be-2+, Na-0,Na-+, and Mg2+. In contrast, the truncated tetrahedrane (C12H12, T-d) encapsulates fewer species, while the D-4d symmetric C16H16 hydrocarbon cage (see Table of Contents graphic) encapsulates all but the larger Be, Mg, and Mg+ species. The host cages have more compact geometries when metal atoms, rather than cations, are inside. This is due to electron donation from the endohedral metals into C-C bonding and C-H antibonding cage molecular orbitals. The relative stabilities of endohedral minima are evaluated by comparing their energies (E-endo) to the sum of their isolated components (E-inc = E-endo - E-cage - E-x) and to their exohedral isomer energies (E-isom = E-endo - E-exo). Although exohedral binding is preferred to endohedral encapsulation without exception (i.e., E-isom is always exothermic), Be2+@C10H16 (T-d; -235.5 kcal/mol), Li+@C12H12 (T-d; 50.2 kcal/mol), Be2+@ C12H12 (T-d; -181.2 kcal/mol), Mg2+@C12H12 (T-d; -45.0 kcal/mol), Li+@C16H16 (D-4d; 13.3 kcal/mol), Be+@C16H16 (C-4v; 31.8 kcal/mol), Be2+ @C16H16 (D-4d, -239.2 kcal/mol), and Mg2+@C16H16 (D-4d, -37.7 kcal/mol) are relatively stable as compared to experimentally known He@C20H20 (I-h), which has an E-inc = 37.9 kcal/mol and E-isom = -35.4 kcal/mol. Overall, endohedral cage complexes with low parent cage strain energies, large cage internal cavity volumes, and a small, highly charged guest species are the most viable synthetic targets.