Abstract
Complexation reactions of ruthenium-nitrosyl complexes in HNO(3) solution were investigated by density functional theory (DFT) calculations in order to predict the stability of Ru species in high-level radioactive liquid waste (HLLW) solution. The equilibrium structure of [Ru(NO)(NO(3))(3)(H(2)O)(2)] obtained by DFT calculations reproduced the experimental Ru-ligand bond lengths and IR frequencies reported previously. Comparison of the Gibbs energies among the geometrical isomers for [Ru(NO)(NO(3)) (x) (H(2)O)(5-x) ]((3-x)+/-) revealed that the complexation reactions of the ruthenium-nitrosyl complexes with NO(3) (-) proceed via the NO(3) (-) coordination to the equatorial plane toward the Ru-NO axis. We also estimated Gibbs energy differences on the stepwise complexation reactions to succeed in reproducing the fraction of Ru-NO species in 6 M HNO(3) solution, such as in HLLW, by considering the association energy between the Ru-NO species and the substituting ligands. Electron density analyses of the complexes indicated that the strength of the Ru-ligand coordination bonds depends on the stability of the Ru species and the Ru complex without NO(3) (-) at the axial position is more stable than that with NO(3) (-), which might be attributed to the difference in the trans influence between H(2)O and NO(3) (-). Finally, we demonstrated the complexation kinetics in the reactions x = 1 → x = 2. The present study is expected to enable us to model the precise complexation reactions of platinum-group metals in HNO(3) solution.