Ligand field effects on the aqueous Ru(III)/Ru(II) redox couple from an all-atom density functional theory perspective

The [RuCl6](4-)(aq)->[RuCl6](3-)(aq) + e(-) and [Ru(CN)(6)](4-)(aq) -> [Ru(CN)(6)](3-)(aq) + e- half redox reactions are investigated using density functional based ab initio molecular dynamics methods. The aim is to understand at a microscopic level how the difference in pi-bonding of these ligands is reflected in the redox chemistry. To this end, we have computed the redox and reorganization free energies using a method derived from the Marcus theory of electron transfer. The resulting estimate of the free energy change of the full redox reaction between the two coordination complexes is compared to experiment. Our findings indicate that ligand character has an important effect on the vertical ionization chemistry but less on the relaxation of the system after removal or addition of electrons. This enables us to correlate the redox free energies with the HOMO energy levels of the combined solute + solvent system and analyze the redox chemistry in terms of the corresponding energy level diagram.