Conformation and Bonding of Lanthanide(III) Trihalides LnX3 (Ln = La–Lu; X = F, Cl, Br): A Relativistic Local Vibrational Mode Study

This study employed relativistic methods to investigate the connection between the conformation and bonding properties of 45 lanthanide trihalides LnX₃ (Ln: La–Lu; X:F, Cl, Br). Our findings reveal several insights. The proper symmetry exhibited by open-shell LnX₃ requires the inclusion of spin–orbit coupling, achieved with 2-component relativistic Hamiltonians. Fluorines (LnF₃) primarily exhibit pyramidal structures, while chlorides and bromides tend to yield planar conformations. For a given halide, the strength of Ln–X bonds increases across the lanthanide series, another outcome of the lanthanide contraction. Both strength and covalency of Ln–X bonds decrease upon the halide, i.e., LnF₃ > LnCl₃ > LnBr₃. We introduced a novel parameter, the local force constant associated with the dihedral β(X–Ln–X–X), k^a(β), which quantifies the resistance of these molecules to conformational changes. We observed a correlation between k^a(β) and the covalency of the Ln–X bond, with higher k^a(β) values indicating a stronger covalent character. Finally, the degree of pyramidalization in the LnX₃ structures is connected to (i) the extent of charge donation within the molecule and (ii) the greater covalency of the Ln–X bond. These findings provide valuable insights into the interplay between the electronic structure and molecular geometry in LnX₃.