Ionic versus covalent character in lanthanide complexes. A hybrid density functional study

The geometric structures and harmonic vibrational frequencies of La, Gd and Lu trihalides have been investigated by a hybrid density functional/Hartree-Fock approach coupled with a relativistic effective core potential. The adequacy of this electronic protocol is confirmed by the fairly good agreement with experimental data and with those obtained by more expensive post-HF computations. A detailed analysis of the electronic density has been performed using the natural bond localization procedure and its recent extension to natural electronic resonance theory to understand the role of charge transfer in the bonding of lanthanide complexes. This approach underlines the role of electrostatic interactions in the lanthanide-halogen bond, even if a charge transfer mechanism plays a role, especially in the more weakly bonded bromide and iodide derivatives.