Computational Study on Mössbauer Isomer Shifts of Some Organic-neptunium (IV) Complexes

Relativistic DFT calculations are applied to some organo-neptunium (IV) complexes, Cp3NpIVX (Cp = η5-C5H5; X = BH4, Cl, OtBu, Ph, nBu), in order to understand their bonding properties between Np and the ligands. We employ scalar-relativistic ZORA Hamiltonian with all-electron basis set (SARC). The calculated electron densities at Np nucleus position in the complexes at B2PLYP / SARC theory strongly correlate to the experimental Mössbauer isomer shifts of 237Np system. The result of bond overlap population analysis indicates that the bonding strength decreases in order of X = BH4, Cl, OtBu, Ph and nBu. The tendency depends on the degree of the covalent interaction between Np 5f-electron and X ligand. It is suggested that it is important to estimate the bonding contribution of 5f-orbital to understand the electronic state for organoactinide complexes.