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

Abstract

This study employed relativistic methods to investigate the connection between the conformation and bonding properties of 45 lanthanide trihalides LnX 3 (Ln: La−Lu; X:F, Cl, Br). Our findings reveal several insights. The proper symmetry exhibited by open-shell LnX 3 requires the inclusion of spin−orbit coupling, achieved with 2-component relativistic Hamiltonians. Fluorines (LnF 3 ) 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., LnF 3 > LnCl 3 > LnBr 3 . 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 LnX 3 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 LnX 3 .