Spectroscopic properties and electronic structure of pentammineruthenium(II) dinitrogen oxide and corresponding nitrosyl complexes:: Binding mode of N2O and reactivity

The spectroscopic properties and the electronic structure of the only nitrous oxide complex existing in isolated form, [Ru(NH3)(5)(N2O)]X-2 (1, X = Br-, BF4-), are investigated in detail in comparison to the nitric oxide precursor, [Ru(NH3)(5)(NO)]X-3 (2). IR and Raman spectra of 1 and of the corresponding (NNO)-N-15 labeled complex are presented and assigned with the help of normal coordinate analysis (NCA) and density functional (DFT) calculations. This allows for the identification of the Ru-N2O stretch at similar to300 cm(-1) and for the unambiguous definition of the binding mode of the N2O ligand as N-terminal. Obtained force constants are 17.3, 9.6, and 1.4 mclyn/Angstrom for N-N, N-O, and Ru-N2O, respectively. The Ru(II)-N2O bond is dominated by pi back-donation, which, however, is weak compared to the NO complex. This bond is further weakened by Coulomb repulsion between the fully occupied t(2g) shell of Ru(II) and the HOMO of N2O. Hence, nitrous oxide is an extremely weak ligand to Ru(II). Calculated free energies and formation constants for [Ru(NH3)(5)(L)](2+) (L = NNO, N-2, OH2) are in good agreement with experiment. The observed intense absorption at 238 nm of 1 is assigned to the t(2g) --> pi* charge transfer transition. These data are compared in detail to the spectroscopic and electronic structural properties of NO complex 2. Finally, the transition metal centered reaction of nitrous oxide to N2 and H2O is investigated. Nitrous oxide is activated by back-donation. Initial protonation leads to a weakening of the N-O bond and triggers electron transfer from the metal to the NN-OH ligand through the 7 system. The implications of this mechanism for biological nitrous oxide reduction are discussed.