  We address the interaction of graphene with 3d transition metal adatoms and
the formation of localized magnetic moments by means of first-principles
calculations. By comparing calculations within the generalized gradient
approximation (GGA) to GGA+U we find that the electronic configuration and the
adsorption geometries can be very sensitive to effects of local Coulomb
interactions U in the transition metal d-orbitals. We find high-spin
configurations being favorable for Cr and Mn adatoms independent of the
functional. For Fe, Co and Ni different electronic configurations are realized
depending on the value of the local Coulomb interaction strength U. Chemical
control over the spin of the adatoms by hydrogenation is demonstrated: NiH and
CoH adsorbed to graphene exhibit spin S=1/2 and S=1, respectively.
