Density functional theory (DFT) calculations have been carried out to explore the potential energy surface (PES) associated with the gas-phase reactions between ethylenediamine (En) and Cu+. The structures and bonding characteristics of the different stationary points of this PES have been investigated at the B3LYP/6-311G(d,p) level. Final energies were obtained by means of B3LYP/6-311+G(2df,2p) single point calculations. En strongly binds Cu+ by forming a chelated structure in which the metal cation binds to both amino groups. Different mechanisms leading to the loss of H-2, NH3, and CuH are analyzed in terms of the topology of the PES. The most favorable mechanism corresponds to the loss of H2, through a process in which the transition metal cation acts as a carrier, connecting a hydrogen atom from a methylene group with a hydrogen atom of one of the amino groups. The product ion is a five-membered ring in which Cu+ bridges between N atoms of the H2N-CH2-CH-NH moiety. The loss of ammonia is less favorable, because all mechanisms involve higher activation barriers. The most favorable of these mechanisms implies hydrogen shift between the two methylene groups that triggers a spontaneous hydrogen shift between the two amino groups, favored by the existence of a strong intramolecular hydrogen bond. These mechanisms explain the experimental results involving fully C-deuterated species, where only a loss of HD and NH3 are observed. The loss of HCu is also discussed.