Amidometallate von Seltenerdelementen. Synthese und Kristallstrukturen von [Na(12-Krone-4)~2~][M{N(SiMe~3~)~2~}~3~(OSiMe~3~)] (M = Sm, Yb), [Na(THF)~3~Sm{N(SiMe~3~)~2~}~3~(C≡C-Ph)], [Na(THF)~6~][Lu~2~(μ-NH~2~)(μ-NSiMe~3~){N(SiMe~3~)~2~}~4~] sowie von [NaN(SiMe~3~)~2~(THF)]~2~. Anwendungen der Seltenerdkomplexe als Polymerisationskatalysatoren