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.