Quantum chemical study of the electrochemical reduction of the [Co(H2O)6]2+ and [Co(NH3)5(H2O)]2+ ions

The electrochemical discharges of metallic ions onto the electrode surface require two fundamental steps: the charge transfer reaction and partial or/and total desolvation. Nevertheless, it is still unknown which comes first. A full (molecular) understanding of how these steps occur is still lacking. Since this phenomenon occurs at the molecular and electronic level, we have performed a quantum chemical study for the specific case of Co(II) ions (forming pH-dependent complexes [Co(H2O)6]2+ and [Co(NH3)5(H2O)]2+). We compute the energetics of possible reduction routes for these Co ion complexes, to find out whether the reduction or desolvation takes place first for each complex and to assess the influence of the coordination sphere along the processes. We have performed the electronic structure calculations at the PM3 semiempirical, HartreeFock pseudopotential ab initio and density functional theory levels. Our results, for all calculations, suggest that the coordination sphere may aid in the electron transfer and that charge transfer and reduction before desolvation are energetically more favorable for these complexes.