The potential-energy surfaces of the metallo-ene reactions of allyl-MgH and allyl-MgCl with ethylene were studied using ab initio molecular-orbital (MO) methods. The concerted path and the stepwise path of the metallo-ene reactions of allyl-MgH and allyl-MgCl with ethylene were identified and it was determined that the energy barriers on concerted paths of the metallo-ene reactions of allyl-MgH and allyl-MgCl with ethylene are lower than those on the stepwise paths. Furthermore, the concerted path of the metallo-ene reaction of allyl-MgCl with ethylene is more favorable than that of the allyl-MgH reaction system. The reaction mechanisms were analyzed using a CiLC method on the basis of CASSCF MOs. The driving force for the concerted path reactions arises from the migration process of the metal. The difference between the reactivity of allyl-MgH and allyl-MgCl can be explained with the reaction mechanism on the basis of the driving force.