The synthesis as well as the spectroscopic and structural characterization of a new class of C-2-symmetric. mononuclear metallocene complexes of the lanthanide elements is described. Heteroleptic lanthanidocene silylamide complexes have been obtained ate-complex-free according to silylamine elimination reactions of complexes Ln[N(SiHMe2)(2)](3)(THF)(x) (x = 1, Ln = Sc; x = 2, Ln = Y, La, Nd, Lu) and linked, substituted cyclopentadiene and indene systems, The molecular structure of [Me2Si(C5Me4)(2)]La[N(SiHMe2)(2)] (95% isolated yield) has been determined by X-ray crystallography. Brintzinger-type, indenyl-derived metallocene complexes have been isolated in racemic yields as high as 72%. TR and multinuclear NMR spectroscopy (H-1, C-13, Si-29, Y-89) reveals the presence of an unprecedented strong diagostic interaction between the electron-deficient metal centers and the SiH moiety of the bis(dimethylsilyl)amide ligand: Sill stretching vibrations as low as 1759 cm(-1) and (1)J(Si.H) coupling constants as low as 133 Hz indicate a distinct weakening of the SiH bonding. X-ray structure analyses of the compounds rac-[Me2Si(2-Me-Benz-Ind)(2)]Ln[N(SiHMe2)(2)] (Ln = Y, Lu) and rac-[Me2Si(2-Me-C9H5)(2)]Ln[N(SiHMe2)(2)] (Ln = Y, Lu) show that the structural features of both the chelating ancillary ligands and such agostically fused metallacycles depend on the type of the ligand and the size of the metal: bite angles Omega of approximately "U-shaped" ansa-ligands as large as 68.2(2)degrees and Ln-Si and Ln-H contacts as close as 3.028(1) and 2.37(3) Angstrom, respectively, have been detected, the latter forcing very large Si-N-Si angles up to 160.1(2)degrees. The isolation of reaction intermediates such as Y[N(SiHMe2)(2)](3)(THF) and partly exchanged [Me2Si(2-Me-4-Ph-Ind)(2)H]Y[N(SiHMe2)(2)](2) provides mechanistic details of this peculiar silylamine elimination reaction. Additionally the pK(a) values of various protonated ligands including new 9-(SiHMe2)-fluorene, 3-(SiHMe2)-indene, and 3-(SiHMe2)-2-Me-indene determined according to the method of Fraser give evidence of a thermodynamically controlled ligand exchange.