We investigated the atomic and electronic structures of metal-functionalized graphene using density functional theory calculations. As dopants, Fe, Co, Ni, and Cu atoms were considered in the form of a single transition metal (TM) atom and a two-atom pair. Our results show that a single Co atom and a Co-Co pair are the most promising
for graphene functionalization, compared with Fe, Ni, and Cu atoms. Among the heteronuclear pair structures, the Co-Ni pair is energetically favorable. The density of states analysis shows hybridization between the TM 3d and C or N 2p orbitals. The spin magnetic moments of the heteronuclear TM pairs may disappear or decrease, depending on which type of TM impurityis paired. As in metalloporphyrins, the structure has a lower formation energy when four nitrogen atoms are replaced as neighbors of the transition metal atom. The spin magnetic moment is found to be in the order Fe > Co > Ni > Cu, which correlates with the d-orbital filling of the metal atom. We believe that our work will shed light on the design of spintronic devices based on TM-functionalized graphene.