Molecular geometries, vibrational analysis, bonding and molecular orbitals of the three isomers of [RuCl(qn)2NO] (Hqn=2-methyl-8-quinolinol or 2-chloro-8-quinolinol): density functional theory studies

The geometries of the three isomers of [RuCl(qn)2NO] {Hqn=2-methyl-8-quinolinol (H2mqn) or 2-chloro-8-quinolinol (H2cqn)} were calculated with the restricted Hartree–Fock and different DFT methods. The B3PW91 method gave the best performance in the calculation for predicting the structure and vibrational spectrum of this series of complexes. The deviation between the calculated and the observed NO harmonic stretching frequencies was about 1% for these complexes. Furthermore, the IR bands containing the vibration of RuN, RuO and RuCl of the complexes were successfully assigned with the help of theoretical calculations. The electronic structures of the complexes were calculated at B3PW91/Lanl2dz level. The different bonding characteristic of the RuNO for the cis types of the 2cqn and 2mqn complexes was determined by the NBO population analysis. For both the 2mqn and 2cqn complexes, the HOMO is best described as a ligand based orbital which contains a small amount of Ru and NO character, while the LUMO is best represented by an antibonding overlap of the Ru(d) and π* NO(p) orbitals. There is a large degree of mixing between the NO and the metal d orbitals in the frontier orbitals, indicating that the peculiarity of the {RuIINO+} group affects the synthesis, structure and reactivity of nitrosylruthenium(II) complexes containing 8-quinolinol and its derivatives. The DFT calculations suggest that the trans isomers are more difficult to prepare than the respective cis isomers, which explains the experimental observations.