[en] Full text: In case of lanthanide (Ln) compounds the bis(trimethylsilyl)amido (btmsa) ligand is considered as a steric and electronic equivalent of the bis(trimethylsilyl)methanido (btmsm) group . Because of their extreme air and moisture sensitivity the chemistry of Ln(btmsm)₃ is very poor, whereas that of comparatively stable Ln(btmsa)₃ is much more developed. As we wish to extend the chemistry of Ln(btmsm)₃ and plan to derive and to parameterize the crystal field (CF) splitting patterns of the target molecules, it is recommended to study in advance the electronic structures of the corresponding compounds derived from Ln(btmsa)₃ . Beside (btmsa)₃ Nd(m-Cl)Li(THF)₃ a number of mono base adducts [(btmsa)₃ Ln.B] with sterically less demanding bases such as OPPh₃ , NCtBu or CNtBu have been described and were partly structurally characterized. Considering only the immediately coordinating atoms, the effective CF is of C_3v symmetry. According to recent spectroscopic investigations, Nd(btmsa)₃ dissolved in Tetrahydrofuran (THF)/MeTHF (ratio 1:3) adds quantitatively THF and forms a transparent hydrocarbon glass at low temperatures. On the basis of absorption measurements run at ambient and at low temperatures the CF splitting pattern of (btmsa)₃Nd.THF was derived. Subsequently the parameters of an empirical Hamiltonian were fitted to the experimental CF splitting pattern to give an r.m.s. deviation of 26.5 cm -1 for 76 assignments. The parameters obtained are compared with those of base-free Nd(btmsa)₃ and (btmsa)₃ Nd(λ-Cl)Li(THF)₃