The π-donor/acceptor trans effect on NO release in ruthenium nitrosyl complexes: a computational insight

Abstract

Nitric oxide (NO) molecules are associated with diverse biochemical routes. Ruthenium complexes are widely used as model structures to control NO availability. In this study, the NO release mechanism in trans-[RuCl(NO)(NH₃)₄]²⁺ and trans-[Ru(NO)(NH₃)₄(P(OEt)₃)]³⁺ complexes, containing trans ligands with a π-donor (Cl⁻) or π-acceptor (P(OEt)₃) character, respectively, is elucidated. Typically, a chemical reduction reaction is necessary to facilitate the NO release from these metal complexes. The protonation of trans-[RuCl(NO)(NH₃)₄]⁺, and not the aquation of the chloride ligand, appears as decisive to promote the NO release. The HNO ligand substituted with one external water molecule can promote the Cl⁻ release, explaining the presence of the anion Cl⁻ in the reaction environment, as evidenced by experimental results. The HNO compound can reduce the remaining metal complex and protonate an external H₂O molecule to produce the NO molecule in an aqueous medium. The reduced π-acceptor–Ru–NO complex can readily release the NO ligand. Complexes containing trans π-acceptor ligands have lower activation energy barriers for NO release than those containing trans π-donor ligands, which is totally in line with the experimental data available. The protonation of the NO ligand in π-acceptor–Ru–NO complexes does not decrease the energy barrier related to the NO release reaction. These data show relevant aspects for the development of structures capable of donating the NO molecule.