Bonding Study on Trivalent Europium Complexes by Combining Mössbauer Isomer Shifts with Density Functional Calculations

Abstract

Scalar-relativistic density functional calculations were applied to some trivalent europium complexes to discuss the covalent interactions in their coordination bonds. Five Eu (III) complexes whose ¹⁵¹Eu Mössbauer isomer shifts vary from −1.77 to 0.57 mm s⁻¹ were modeled by referring previously reported Mössbauer experiments. Geometrical optimizations of their complexes using BP86 functional reproduced the experimental coordination structures. Single-point calculations were applied to their optimized geometries at three different density functionals, namely, BP86, B3LYP, and B2PLYP, to obtain their electron densities at Eu nucleus. A comparison of the linearity between the electron densities and the corresponding ¹⁵¹Eu Mössbauer isomer shifts revealed that the B2PLYP functional shows the best linearity. We also performed electron population and bond analyses of the molecular orbitals in their complexes obtained by single-point calculations using Mulliken’s procedure. As a result, the electron densities of both d- and f-orbital electrons of Eu ion in the complexes were found to be correlated to the experimental ¹⁵¹Eu Mössbauer isomer shifts. This result indicated that the d- and f-orbital electrons in the Eu ion are involved in the covalent interaction of the coordination bond between the Eu ion and the ligands.