In this paper we analyze using Density Functional Theory (DFT) the electronic structure of the Creutz-Taube ion, [(NH3)(5)Ru-pyrazine-RU(NH3)(5)](5+), a classical example of a mixed-valence complex. A complete structural and bonding analysis, description of the low-lying excited states, and a vibronic coupling calculation along both the symmetric and the antisymmetric Ru-pyrazine-Ru stretching mode are presented. The results are in very good agreement with the reported experimental data and we are able to assign unambiguously this compound as a Class III system, following the Robin and Day classification. In this work we demonstrate that DFT does offer a unique tool for handling the localization-delocalization of electrons in mixed-valence systems and therefore we expect that this approach can play an important role in characterizing electron-transfer systems. Localized Ru-N(pyrazine) stretching in the range 254-275 cm(-1) has been estimated from our calculations, which should be used in future model Hamiltonians which aim to include the Ru-pyz bond vibration to describe the spectral features of the Creutz-Taube ion.