Abstract: Photoelectron velocity map images of Cu(CO)₃⁻ have been experimentally recorded in the 700–1100 nm range. The infrared-inactive Cu-C symmetric stretching modes for Cu(CO)₃ (\nu₂ ≈ 367 cm⁻¹) and Cu(CO)₃⁻ (\nu₂ ≈ 408 cm⁻¹), as well as the electron affinity (1.03±0.01 eV) of Cu(CO)₃, are accurately determined from high resolution photoelectron spectra. In combination with quantum chemical calculations and bonding analyses, the coordination bonds in both Cu(CO)₃⁻ and Cu(CO)₃ are found to be due to back-donation π bonding type, formed via electron promotion from Cu’s 4s orbital to the 4p orbital, which is consequently donated to the unoccupied anti-bonding π* orbitals of the carbonyl groups. The attachment of an additional electron to Cu(CO)₃ strengthens the Cu-CO coordination, making Cu(CO)₃⁻ more stable. The intramolecular interactions between the Cu/Cu⁻ and carbonyl groups are found to be primarily governed by electrostatic forces and orbital interactions.