Ion bombardment on graphene sheets can produce atomic vacancies that can trap metal atoms. In this paper, we demonstrated that these trapped metal atoms can effectively bind other molecules with heteroatoms, making them chemisorbed to the graphene. The trapped cobalt atom can bind sarin molecule through fluorine atom with dissociation energy significantly higher than the one bonded via oxygen atom. This suggests that it can displace water molecule and therefore pledge for sarin chemisorbent in atmospheric environment. Our investigations also revealed that metallic character is enhanced upon sarin adsorption unlike the bonding of water molecule with trapped metal atom in graphene lattice which causes an opening of small (0.02 eV) band gap. Present findings can have promising application towards detecting the presence of toxic sarin molecules.