The participation of 4f electrons in the bonding of the lanthanide complexes Ln(NR2)(3); R = H, SiH3, has been investigated at the DFT level. Structural parameters obtained with small core (f electrons in the valence) and large core (f electrons in the core) effective core potentials (ECPs) suggest the nonparticipation of the f electrons to the Ln-N bonding. A methodological study has been carried out on the lanthanide contraction with various ab initio methods using large core ECPs. The calculated lanthanide contraction (0.180 Angstrom) is in excellent agreement with the experimental value (0.179 Angstrom). Comparison of calculated structural parameters with available X-ray data shows that calculations with large core ECPs and density functional methods quantitatively reproduce the bonding at the lanthanide.