Can the relative positions (cis–trans) of ligands really modulate the coordination of NO in ruthenium nitrosyl complexes?

Abstract

Nitric oxide is involved in a series of biological processes. Ruthenium tetraammine complexes are model structures to control the NO availability. The influence of ligands is critical to determine the Ru–NO bond stability. Herein, the ligands of different natures and charges, namely, σ-donors (NH₃ and H⁻), (ii) π-donors (H₂O and NH₂⁻), and (iii) σ-donors/π-acceptors (CO and CN⁻) were evaluated relative to the effect promoted by same ligands not only in the cis position to the NO group, but also in the trans position to NO. The energy decomposition analysis shows linear Ru–NO⁺ and bent Ru–NO⁰ bonds in ruthenium tetraammine complexes independent of ligands in cis or trans positions with regard to the NO group. Overall, the substitution of one σ-donor ligand with one π-donor ligand in the cis position to the NO group not stabilized the Ru–NO bond. These substitutions involving ligands in the trans position to NO stabilized the Ru–NO bond. Complexes with σ-donor/π-acceptor ligands compared to compounds with π-donor ligands in the cis or trans position to the NO group destabilized the Ru–NO bond. Charge distribution investigation realized from the Voronoi deformation density (VDD) method presents the Ru–NO bond more stabilized by negatively charged σ-donor, π-donor and σ-donor/π-acceptor ligands, in the cis or trans position to NO. These findings provide crucial information to the rational design of new NO storage–release systems with potential to deliver NO to desired targets in a controlled manner.