This paper analyses the interaction of Co- (d8, 1G, and 3F) and Co2- (d7, 4F) cations with (glycyl)nglycine (n = 1-3) oligomers. The structure, relative energies, and binding energies of the complexes formed have been theoretically determined by means of density functional methods. For all Co- complexes the ground spin state is the triplet one and the most stable structures show tricoordinated geometries. In contrast, for Co 2+ systems the lowest energy structures are tricoordinated (n = 1), tetracoordinated (n = 2), and pentacoordinated (n = 3). For both cobalt cations, interaction energies increase with the peptide length. Differences in the coordination properties of the ligands are discussed according to their length as well as to the electronic configuration of the metal cation, and results are compared to those previously obtained for the analogous Cu-/2- systems. The IR spectra of the most stable and low energy conformers have been simulated, and a discussion of the main vibrational features is provided. © 2009 American Chemical Society.