Using first-principles calculations we demonstrate sizable exchange coupling between a magnetic molecule and a magnetic substrate via a graphene layer. As a model system we consider cobaltocene (CoCp2) adsorbed on graphene deposited on Ni(111). We find that the magnetic coupling between the molecule and the substrate is antiferromagnetic and varies considerably depending on the molecule structure, the adsorption geometry, and the stacking of graphene on Ni(111). We show how this coupling can be tuned by intercalating a magnetic monolayer, e.g. Fe or Co, between graphene and Ni(111). We identify the leading mechanism responsible for the coupling to be the spatial and energy matching of the frontier orbitals of CoCp2 and graphene close to the Fermi level, and we demonstrate the role of graphene as an electronic decoupling layer, yet allowing spin communication between molecule and substrate.PACS numbers: 71.15. Mb, 75.50.Xx, 81.05.ue The emerging field of organic spintronics capitalizes on the novel functionalities achieved when organic molecules are adsorbed on magnetic substrates. The ability to manipulate and tune these functionalities is an important goal. Several problems remain however, before such systems can be incorporated into new technological devices. One in particular is the capability to adsorb molecules on surfaces without any detrimental effects being caused to either the structural or magnetic properties of the molecule. For this reason it is vital to choose molecules with maximum structural robustness upon adsorption [1][2][3]. To this end, the phthalocyanine and porphyrin families are popular choices due to their planar geometry [4][5][6][7][8][9]. However, in some cases, the strong interaction between the metal ion of such flat molecules and the substrate can modify its electronic states and even quench its magnetic moment [10].The use of non-planer molecules, such as metallocenes, can minimize this effect. Metallocenes are composed of a 3d transition-metal ion sandwiched between two cyclopentadienyls (Cp). Depending on the metal ion species, both non-magnetic and paramagnetic behavior can be found [11]. The spin of the metal ion is shielded from the surface by the cage formed by the two Cp rings, reducing the possibility that it will be modified substantially after adsorption. Unfortunately, the deposition of metallocenes on metallic surfaces is a difficult process [12] and, in some cases, complete dissociation of the molecule occurs [13,14].The intercalation of a graphene spacer layer between the reactive surface and the metallocene can reduce the possibility of molecular dissociation during deposition. Additionally, evidence of charge transfer at moleculegraphene-Ni(111) interfaces [15,16] and the theoretical prediction of large charge transfer from cobaltocene (CoCp 2 ) to graphene [17] would suggest that a magnetic coupling between cobaltocene and the Ni(111) surface through the graphene layer is still achievable.In this Letter, we predict, by first principles electronic structure metho...