Quantum chemical analysis of Со²⁺ aqua complexes electrochemical reduction

Abstract

Based on the analysis of quantum chemical calculations results (GAMESS, density functional theory, B3LYP method) as to [Co(H2O)n]z(H2O)6–n clusters for z = 0, 1, 2 and n=1÷6, it has been demonstrated that electrochemical reduction of [Co(H2O)6]2+ aqua complexes runs stage-wise. At the first stage, an electron injected into the [Co(H2O)6]2+ complex is entirely located in the orbital of the central atom, as z(Co) herewith changes from +1.714 е to +0.777 е. The weakening of Со–ОН2 bonds leads to decomposition of resulting [Co(H2O)6]+ particles into two energetically related forms – [Co(H2O)4]+ and [Co(H2O)3]+. Further reduction of these intermediates runs differently. Electron injection into the [Co(H2O)3]+ intermediate terminatesthe transition of Со2+-ions to Со0 z(Co)= –0.264 е. This process is accompanied by rapid decomposition of [Co(H2O)3]0 product into monohydrate atom of cobalt Со(Н2О). On the contrary, electron injection into the [Co(H2O)4]+ intermediate leads to emergence of a specific structure – [Co+(H2O–)(Н2О)3]¹0, whereby the electron is located in the atoms of cobalt only by 28%, and by 72% in cobalt-coordinated water molecules, clearly focusing on one of the. In this molecule, z(H2O) changes from +0.148 е to –0.347 е. There is an assumption that a non-equilibrium [Co+(H2O–)(Н2О)3]0¹ form transits to [Co(ОH)(Н2О)3]0 hydroxo-form, which further disproportionates turning into Co(ОH)2 hydroxide. In order to reduce the impact of this unfavorable reaction pathway on the overall reaction rate Со2+ + 2ē = Со0, we suggest raising the temperature to ensure complete dissociation of [Co(H2O)4]+ to [Co(H2O)3]+.

Keywords

• cobalt aqua complexes, electrochemical reduction, quantum chemical modeling