Abstract: Graphitic carbon nitride(g-C₃N₄) loaded by metal atom has been applied as an effective single atom catalyst. In the work, the complexation behaviors of g-C₃N₄(CN) with metal ions have been examined by relativistic density functional theory. The formed ［M(CN)］^z+(M=Sc, Y, La, Ac and U; z=2 and 3) have been explored for their structures, metal-ligand interactions and redox properties. It is found that the metal ion has been rendered by six M-N dative bonds, each of which has the bonding strength ranging from -0.79 to -0.47 eV. Regarding Sc complexes, one-electron reduction mainly occurs around the metal center, while La and U complexes have metal-modified ligand reduction mechanism; both metal and ligand in Y complex are approximately equally reduced; in contrast, the ligand of Ac complex gets the reducing electron, and the metal is redox-innocent. This agrees with the fact that there are no experimental reports on Ac(Ⅱ) complexes. Electronic-structure analyses indicate that ［U(CN)］³⁺ bears three high-lying single U(5f) occupied orbitals, suggesting the trivalent uranium center. However, despite having four single U(5f) occupied orbitals, ［U(CN)］²⁺ still possesses considerable ligand contribution, leading to the uranium oxidation state mediating between  Ⅱ and Ⅲ.