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(Si H3)(2)](3) have been carried out. The calculations find two types of geomet ries 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 bas is set, a corolla structure is obtained as a minimum, in addition to the le ss 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 sugg est that the coordination sphere of the lanthanide center could be highly s ensitive to the level of calculation and that great caution should be taken in representing ligands in lanthanide chemistry.