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–(NO), Ru–(NO) and Ru–(NO) bonds in trans-[Ru(NH3)4(L)(NO)] and trans[Ru(NH3)4(L)(NO)] 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–(NO) and Ru– (NO) 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 -orbital interactions between NO and the [Ru(NH3)4(L)] are the most important term and that the trans-ligands imN and nic contribute to an increase in the -donor strength of the metal centre towards NO. For Ru–(NO) bonds, the smallest values of Eint, EPauli, Eelstat, and De 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–(NO) bond in GS or in MS1 states is more labile. After the reduction of the NO group, the Ru–(NO) becomes more labile when the trans-ligand is imC, which agrees with the experimental rate constants of NO dissociation.