Complexation and bonding studies on [Ru(NO)(H(2)O)(5)](3+) with nitrate ions by using density functional theory calculation

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.