[Me2SiNBFN(SiMe3)2]3 (1) is synthesized in the reaction of trilithiated hexamethylcyclotrisilazane with F2BN(SiMe3)2. The mono- and dilithium derivs. of the six-membered ring (Me2SiNH)2Me2SiNSiFMe2 react with the F2B-substituted cyclotrisilazane F2B(NSiMe2)3(SiFMe2)2 in the molar ratio 1:1 or 1:2 to give BF-coupled rings, e.g., I. The crystal structure anal. of 1 and I and pyrolysis of 1 and I with formation of Si B carbonitride and its crystn. to composite powders at very high temps. are reported.