DFT modeling of ligands in lanthanide chemistry: Is Ln[N(SiH3)2]3 a model for Ln[N(SiMe3)2]3?

DFT (B3PW91) calculations with large core ECPs for the whole series Ln[N(SiH3)2]3 have been carried out. The calculations find two types of geometries dependant on the silicon basis set. In the absence of d orbitals on Si, the calculations reproduce the helicoidal shape of the molecules and the absence of any agostic interaction. When d orbitals are added to the Si basis set, a corolla structure is obtained as a minimum, in addition to the less stable helicoidal structure. The greater stability of the corolla family is due to the presence of three Ln···Si–H agostic interactions, whose stabilising influence is overestimated by the modeling of SiMe3 by SiH3 in the vicinity of an electron-deficient lanthanide center. These results suggest that the coordination sphere of the lanthanide center could be highly sensitive to the level of calculation and that great caution should be taken in representing ligands in lanthanide chemistry.