•The M–M bonds in [CpM(CO)3]2 (M = Cr, Mo, and W) cannot be classified as typical covalent single bond.•The strengths of M–M bond increase in sequence of M = Cr, Mo, and W.•The M–M bonds in [CpM(CO)3]2 belong to metallic bonds and have partially covalent characters.The nature of metal–metal bonding in [CpM(CO)3]2 (M = Cr, Mo, W; Cp = C5H5) was characterized within the framework of the atoms in molecules theory, electron localization function, and molecular formation density difference. Calculated results show that both the electrostatic and the orbital interaction play important roles in the [CpM(CO)3]2, and the electrostatic proportions increase in the sequence of M = Cr, Mo, and W. Moreover, the strengths of M–M bond are weaker than single bond. Therefore, the M–M bonds in [CpM(CO)3]2 (M = Cr, Mo, and W) cannot be classified as typical covalent single bond. Among them, the Cr–Cr interaction is weak and that Mo–Mo and W–W interactions have moderate strength. They all belong to metallic bonds and have partially covalent characters, the metallic nature of the M–M bond increases in the sequence of Cr, Mo, and W.The M–M interactions in [CpM(CO)3]2 (M = Cr, Mo, and W) are not typical covalent single bonds. They all belong to metallic bonds and have partially covalent characters, the metallic nature of the M–M bond increases in the sequence of Cr, Mo, and W.