The effects of N-heterocyclic ligands on the nature of the Ru-(NO) bond in ruthenium tetraammine nitrosyl complexes

Quantum chemical calculations at the DFT level have been carried out to analyze quantitatively the Ru^II-(NO) ⁺, Ru ^III-(NO) ⁰ and Ru^II-(NO) ⁰ bonds in trans-[Ru^II (NH₃)₄ (L)(NO)] ^q and trans-[Ru^II (NH₃)₄ (L)(NO)] ^q-1 complexes, where L = 4-picoline (4-pic), C-bound imidazole (imC), N-bound imidazole (imN), nicotinamide (nic), pyridine (py), and pyrazine (pz). Equilibrium geometries and the vibrational frequencies are reported for the ground state GS and light-induced metastable states, MS1 and MS2, presenting good agreement with the experimental data. The nature of the Ru ^II-(NO) ⁺ and Ru^II- (NO) ⁰ bonds was investigated by means of the energy decomposition analysis, EDA. The Ru-(NO) bonding situation has been analyzed in two different situations: prior and after one-electron reduction at the NO⁺ group. The EDA results for the complexes prior to the reduction of the NO⁺ indicate that the metalligand π-orbital interactions between NO⁺ and the [Ru ^II (NH₃)₄ (L)] ^q-1 are the most important term and that the trans-ligands imN and nic contribute to an increase in the π-donor strength of the metal centre towards NO⁺. For Ru^III-(NO)⁰ bonds, the smallest values of ΔE _int, ΔE_Pauli, ΔE_elstat, and D _e are observed when L = imC or L = nic, independent of the state under consideration, GS or MS1, indicating that when L = imC or nic the Ru ^III-(NO) ⁰ bond in GS or in MS1 states is more labile. After the reduction of the NO+ group, the Ru^II-(NO) ⁰ becomes more labile when the trans-ligand is imC, which agrees with the experimental rate constants of NO⁰ dissociation.