Formation and Release of NO from Ruthenium Nitrosyl Ammine Complexes [Ru(NH3)5(NO)]2+/3+ in Aqueous Solution: A Theoretical Investigation.

In this article, density functional theory in conjunction with Monte Carlo statistical mechanical simulation was used to investigate the electronic structure, reduction potential, solvation, and solvent effects on the electronic spectra of nitrosyl ammine complexes using [Ru(NH3)5(NO)]2+/3+ as model compounds. In addition, ligand exchange reactions with solvent water molecules were also investigated. It is shown that the complexes are involved in strong hydrogen bonds in aqueous solution, with mean average energies of -13.5 ± 0.4 and -22.4 ± 0.4 kcal mol-1 for Ru(II) and Ru(III), respectively. Interestingly, for all the complexes studied, the NO ligand is not involved in hydrogen bonding interactions in aqueous solution. These strong hydrogen bonds are responsible for the high stability of these complexes in aqueous solution, showing formation constants Kf greater than 1021. The complex [Ru(NH3)5(NO)]3+ can easily be reduced by biological reducing agents in both the singlet and triplet states; however, the reduction is easier in the triplet state, which has a positive reduction potential of 1.70 V. The formation of [Ru(NH3)5(NO)]3+ in its most stable singlet state may take place through at least two singlet-triplet surface crossings leading to nonadiabatic effects. The existence of the minimum-energy crossing points makes the release of NO from the triplet state more favorable, with an activation energy almost seven times lower (∼6 kcal mol-1).