The nature of the Ru-NO bond in ruthenium tetraammine nitrosyl complexes

Quantum chemical calculations at the DFT level have been carried out for trans-[Ru^II(NH₃)₄(L)NO]^q and trans-[Ru^II(NH₃)₄(L)NO]^q-1 complexes, where L = NH₃, Cl^-, and H₂O. The equilibrium geometries and the vibrational frequencies are reported not only for the ground state (GS) but also for light-induced metastable states MS1 and MS2. The nature of the Ru-NO⁺ and Ru-NO° bonds has been investigated by means of the energy decomposition analysis (EDA). The nature of the Ru-NO bond has been analyzed for the three states GS, MS1, and MS2, considering two different situations: before and after one-electron reduction. The results suggest that not only the orbital term but also the ΔE_Pauli term is responsible for weakening of the Ru^II-NO° bond, the ΔE_Pauli term increasing in comparison with the Ru ^II-NO⁺ bonds, thus making the NO° ligand more susceptible to dissociation in comparison with NO⁺. Calculations of the Ru^III-NO° species show that in this case the bonds are mainly covalent, but the electrostatic stabilization also plays an important role. Among the orbital interactions, the π-back-donation is the most important term.