Density Functional Theory (DFT)-Based Bonding Analysis Correlates Ligand Field Strength with Ru Mössbauer Parameters of Ruthenium-Nitrosyl Complexes.

We applied density functional theory calculations to ruthenium-nitrosyl complexes, which are known to exist in high-level radioactive waste generating during reprocessing of spent nuclear fuel, to give a theoretical correlation between Ru Mössbauer spectroscopic parameters and ligand field strength for the first time. The structures of the series of complexes, [Ru(NO)L] (L = Br, Cl, NH, CN), were modeled based on the corresponding single-crystal X-ray coordinates. The comparisons of the geometries and total energies between the different spin states suggested that the singlet spin state of [Ru(II)(NO)L] complexes were the most stable. This result was supported by the benchmark calculations of the Ru Mössbauer isomer shift (δ) and quadrupole splitting (Δ) values. The calculated results of both the δ and Δ values reproduced the experimental results by reported previously and increased in the order of L = Br, Cl, NH, CN. Finally, we estimated the ligand field strength (Δ) based on molecular orbitals, assuming symmetry and showed the increase of Δ values in that order, being consistent with well-known spectrochemical series of ligands. The increase attributes to the strengthening of the abilities of σ-donor and π-acceptor of the L-ligands to the Ru atom, resulting in the increase of the δ values. Furthermore, the increase of the σ-type donation into Ru orbital and the π-type back-donation from Ru , orbitals in that order caused the decrease of the electron density along the Ru-NO axis, resulting in the increase of the Δ values. This study is expected to contribute to the ligand design for the ruthenium-nitrosyl complexes, leading to the drug design for NO carrier and the decontamination of radioactive ruthenium from the ecological system, as well as the recovery of platinum-group metals from high-level radioactive waste.