Reactions of copper atoms with dinitrogen have been studied in argon and dinitrogen matrices at 10 K by UV-visible and FTIR spectroscopies. The co-condensation of copper vapor in neat dinitrogen matrices produces only the metal dimer 2:1 and 2:2 species, i.e., the Cu-2(N-2) and Cu-2(N-2)(2) complexes. Density functional calculations carried out on these complexes indicate that Nz coordinates to Cut in end-on way. No reactivity occurs in argon dilute matrices when the dinitrogen-argon rate is lower than 50%. The reactivity of copper is greatly affected by the presence of CO in the matrix giving rise to mixed copper CO/N-2 species. The cooperative effect of CO is examined by DFT calculations. From both our experimental and theoretical results it is evident that only copper dimers exhibit a marked reactivity toward the Nz molecule, whereas the naked Cu(0) atoms need the "help" of a reactive co-ligand like CO to promote the coordination of dinitrogen.