The addition reaction of M(Cl)(CO)(PPh3)(2) = Rh, Ir) and M(PPh3)(2) (M = Pd, Pt) fragments with X@C-60 (X = 0, Li+) were characterized by density functional theory (DFT) and the artificial force-induced reaction (AFIR) method. The calculated free energy profiles suggested that the eta(2)[6:6] addition is the most favorable reaction, which is consistent with the experimental observations. In the presence of Li+ ion, the reaction is highly exothermic, leading to eta(2)[6:6] product of L4IrLiVC60. In contrast, an endothermic reaction was observed in the absence of, a Li+ ion. The encapsulated Li+ ion can enhance the thermodynamic stability of the eta(2)[6:6] product. The energy decomposition analysis showed that the interaction between metal fragment and X@C-60 fragment is the key for the thermodynamic stability. Among the group IA and HA metal cations, Be2+ encapsulation is the best candidate for the development of new fullerene-transition metal complexes, which will be useful for futhre potential applications such as solar cells, catalysts, and electronic devices.