Homoleptic Bis trimethylsilyl amide Complexes

The introduction of the btsa ligand into lanthanide chemistry by Bradley led to the isolation of the homoleptic compounds Ln(btsa)3 [7], interpreted as the first 3-coordinate lanthanide complexes. This result was indeed spectacular and exploration of the chemical and physical properties of this simple system is continuing [104-106]. 

However, the molecular structure of the Ln(btsa)3 complexes was the most interesting feature. Herein the lanthanide metals adopt a pyramidal geometry, independent on the size of the metal as well the state of matter [22a, 108, 112-115]. In contrast, main and d-transition metals exhibit planar geometry [19c]  

The trigonal pyramidal molecular structure has been subjected to numerous interpretations. In Sc(btsa)3 the pyramidal ScN3 fragment was ascribed to crystal packing because the nonplanarity is lost in the gas phase [114a]. Electric dipole moment measurements confirmed a planarity in dilute benzene solutions [106a]. Zero dipole moments are also obtained for the larger lanthanide metals 

Another interesting feature of the solid state structure is the long range packing of the Ln(btsa)3 molecules (Fig. 9). The presence of large hexagonal spaces within the crystal lattice might induce a facile phase transition, e.g., reorientation of the molecules in the crystalline lattice [115] and inclusion of solvent molecules [99b, 108], and therefore be responsible for the lack of quality of the solid state structure determinations. 

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