Density Functional Theory (DFT)-Based Bonding Analysis Correlates Ligand Field Strength with ⁹⁹Ru Mössbauer...

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 ⁹⁹Ru Mössbauer spectroscopic parameters and ligand field strength for the first time. The structures of the series of complexes, [Ru(NO)L₅] (L = Br^-, Cl^-, NH₃, 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⁺)L₅] complexes were the most stable. This result was supported by the benchmark calculations of the ⁹⁹Ru Mössbauer isomer shift (δ) and quadrupole splitting (ΔE_Q) values. The calculated results of both the δ and ΔE_Q values reproduced the experimental results by reported previously and increased in the order of L = Br^-, Cl^-, NH₃, CN^-. Finally, we estimated the ligand field strength (Δ_o) based on molecular orbitals, assuming C_4v symmetry and showed the increase of Δ_o 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 d orbital and the π-type back-donation from Ru d, d orbitals in that order caused the decrease of the electron density along the Ru-NO axis, resulting in the increase of the ΔE_Q 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.