Tracking the role of trans-ligands in ruthenium–NO bond lability: computational insight

Abstract

Nitric oxide (NO) is an important endogenously produced molecule. Ruthenium–NO tetraamine complexes appear as model structures to investigate the control of nitric oxide bioavailability. NO release typically occurs through the trans-[Ru^II(NH₃)₄L_trans(NO⁺)]^3+/2+ + e⁻ → trans-[Ru^II(NH₃)₄L_trans(NO⁰)]^2+/+ reduction reaction, followed by the trans-[Ru^II(NH₃)₄L_trans(NO⁰)]^2+/+ + H₂O → trans-[Ru^II(NH₃)₄L_trans(H₂O)]^2+/+ + NO⁰ hydrolysis reaction. The choice of the NO trans ligand, L_trans, is fundamental to control the Ru–NO bond stability. Here, the nature and charge influences of L_trans were evaluated considering the following ligands L_trans = σ-donors (NH₃ and H⁻), π-donors (H₂O and NH₂⁻), and π-donors and π-acceptors (CO and CN⁻) through the ZORA-BP86/TZ2P computational model. Energy Decomposition Analysis in conjunction with the Natural Orbitals for Chemical Valence methodology (EDA-NOCV) showed that molecules with L_trans = H₂O and NH₂⁻ promoted larger Ru–NO stabilization than complexes with L_trans = NH₃ and H⁻, respectively, mainly due to the interaction orbital energy (ΔE_oi). The opposite trend was observed in compounds with L_trans = CO and CN⁻ compared to structures with L_trans = H₂O and NH₂⁻. The study of the charge distribution, performed using the Voronoi Deformation Density (VDD) method, and the topological analysis of the electron density, realized using the Quantum Theory of Atoms in Molecules (QTAIM), on the investigated complexes indicated that L_trans with negative charge promoted, in general, predominantly a decrease of the electron flux in the L_trans(σ) → Ru(d_σ*) ← NO(σ) process.