Adopting ligand occupation strategy and the principle of molecular self-assembly, a mononuclear cobalt ( Ⅲ) complex [CoⅢ (DIEN)(N3 )3 ] (1, DIEN = diethylenetriamine) was synthesized by solution method using diethylenetriamine as the organic ligand and Co3 + as the central metal ion at room temperature. Single crystal of complex 1 was obtained. The structural characterization and electronic structure analysis were carried out through X-ray single crystal diffraction, elemental analysis, and quantum chemical calculations. Single crystal X-ray diffraction analysis reveals that complex 1 is triclinic, space group of P1 with a =0. 825 3(2) nm, b =0. 892 0(3) nm, c = 0. 898 7(3) nm, α = 106. 497(4)°, β = 90. 281(4)°, γ = 113. 861(4)°, V = 0. 574 7(3) nm3 . The central ion Co(Ⅲ) has an elongated octahedral configuration. Each complex is composed of Co3 + cation, three azide anions and one diethylenetriamine ligand. A disordered azide anion has a site occupancy of 50% , and 1D chain-like supermolecular was formed by N—H…N hydrogen bond and π…π packing interations. In addition, based on crystal structure of complex 1 determined by X-ray crystallographic analysis, the full geometry optimized and frequency calculations have been performed by density functional theory. The single-point energy, atomic charges and frontier molecular orbital were analyzed. The theoretical calculation results indicate that the complex configuration is stable and consistent with the experimental results.