A series of five-, six-, and seven-membered cyclic silylhydrazines have been prepared from the reactions of 1,2-bis(bromosilyl)ethane and 1,3-bis(bromosilyl)propane with 1,1-dimethylhydrazine [leading to 1-(dimethylamino)-1-aza-2,5-disilacyclopentane (3) and 1-(dimethylamino)-1-aza-2,6-disilacyclohexane (4)] and 1,2-dimethylhydrazine [leading to 1,2-dimethyl-1,2-diaza-3,6-disilacyclohexane (7) and 1,2-dimethyl-1,2-diaza-2,7-disilacycloheptane (8)] in the presence of triethylamine, respectively. The compounds with endocyclic Si-N-Si units (3, 4) are found to be stable for long periods of time, while those with Si-N-N-Si units (7, 8) decompose within a few days at ambient temperature. Compounds 3 and 4 have been reacted with the Lewis acid BH3 to give the dimethylamine-borane adducts 5 and 6. All compounds have been fully characterized by spectroscopic data [IR, MS, NMR (H-1, C-13, N-15, Si-29)]. Single crystals of 5 [6] grown from the melt and studied by low-temperature X-ray diffraction analyses are orthorhombic, space group Pbca (No. 61), with a = 11.385(1) [13.300 (1)] Angstrom, b = 9.938(1) [9.837(1)] Angstrom, c = 17.156(1) [16.364(1)] Angstrom, d(calc) = 1.096 (1.081) g cm(-3), and Z = 8 [8]. In both compounds, the BH3 unit is bound to the nonsilylated nitrogen atom, indicating the reduction of the basicity of nitrogen by Si substitution. The silylated nitrogen atoms show planar coordination, while the borylated amine unit is tetrahedrally coordinated. From a comparison of the ring geometries of 5 and 6 with known open-chain structures, it appears that the C2Si2N ring system of 5 is clearly more strained than that of 6 (C3Si2N) This argument also offers an explanation for the preferred formation of the compounds 1,6-diaza-2,5,7,10-tetrasila-[4.4.0]bicyclodecane (1) and bi(1-aza-2,6-disilacyclohexyl) (2) as compared to their isomers with different ring sizes. The relative stabilities of these isomers in question have been quantified by ab initio (MP2(fc)/6-31G*) calculations of geometries and energies of the systems [(CH2)(n)(SiH2)]N-N[(SiH2)(CH2)(n)] versus the annelated molecules (CH2)(n)(SiH2)N-2(CH2)(n)(SiH2) with n = 1-3. These results show the annelated isomers (ring enlarged) to be lower in energy for n = 1 and 2, while for n = 3 the N-N bridged nonannelated isomer is preferred.