A fundamental understanding of the properties of various metal/graphene nanostructures is of great importance for realising their potential applications in electronics and spintronics. The electronic and magnetic properties of three metal/graphene adducts (metal = Li, Co or Fe) are investigated using first-principles calculation. It is predicated that the metal/graphene adducts have strong affinity to aromatic molecule 1,2-dichlorobenzene (DCB), and the resultant DCB-metal/graphene sandwich structures are much more stable than the simple DCB/graphene adduct. Importantly, it is found that the adsorption of DCB slightly enhances the magnetic moment of the Co/graphene, but turns the Fe/graphene from magnetic to nonmagnetic. A detailed theoretical explanation of the different magnetic properties of the DCB/Co/graphene and DCB/Fe/graphene is achieved based on their different d-band splitting upon DCB adsorption. In addition, the transport property study indicates that the Fe/graphene is a better sensing material for DCB than the pristine graphene.