Abstract1,2‐Diaza‐3‐silacyclopent‐5‐ene – Synthesis and ReactionsThe dilithium salt of bis(tert‐butyl‐trimethylsilylmethylen)ketazine (1) forms an imine‐enamine salt. 1 reacts with halosilanes in a molar ratio of 1:1 to give 1,2‐diaza‐3‐silacyclopent‐5‐enes. Me3SiCH=CCMe3 [N(SiR,R′)‐N=C‐C]HSiMe3 (2‐7). (2: R,R′ = Cl; 3: R = CH3, R′ = Ph; 4: R = F, R′ = CMe3; 5: R = F, R′ = Ph; 6: R = F, R′ = N(SiMe3)2; 7: R = F, R′ = N(CMe3)SiMe3). In the reaction of 1 with tetrafluorosilane the spirocyclus 8 is isolated. The five‐membered ring compounds 2‐7 and compound 9 substituted on the silicon‐fluoro‐ and (tert‐butyltrimethylsilyl) are acid at the C(4)‐atom and therefore can be lithiated. Experiments to prepare lithium salts of 4 with MeLi, n‐BuLi and PhLi gave LiF and the substitution‐products 10‐12. 9 forms a lithium salt which reacts with ClSiMe3 to give LiCl and the SiMe3 ring system (13) substituted at the C(4)‐atom. The ring compounds 3‐7 and 10‐12 form isomers, the formation is discussed. Results of the crystal structure and analyses of 8, 10, 12, and 13 are presented.