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- [RuII(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 MS I and MS2. The nature of the Ru-NO+ and Ru-NOo bonds has been investigated by means of the energy decomposition analysis (EDA). The nature of the Ru-NOo 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 Delta E-Pauli term is responsible for weakening of the Ru-II-NOo bond, the Delta E-Pauli term increasing in comparison with the Ru-NO+ bonds, thus making the NOo ligand more susceptible to dissociation in comparison with NO+. Calculations of the Ru-III-NOo 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 T-back-donation is the most important term.