STRUCTURE OF THE HIGHLY CROWDED ALKYNE DI[TRIS(TRIMETHYLSILYL)METHYL]ACETYLENE AND THE OCTASILANE HEXAKIS(TRIMETHYLSILYL)DISILANE

(I) 2,2,7,7-Tetramethyl-3,3,6,6-tetrakis(trimethylsilyl)-2,7-disilaoct-4-yne, C22H54Si6, M(r) = 487.2, triclinic, P1BAR, a = 9.0383 (10), b = 9.3596 (9), c = 11.855 (3) angstrom, alpha = 68.207 (13), beta = 74.837 (14), gamma = 59.858 (9)degrees, V = 801.8 (2) angstrom3, Z = 1, D(x) = 1.009 g cm-3, lambda(Mo Kalpha) = 0.71073 angstrom, mu = 2.62 cm -1, F(000) = 270, T = 297 K, final R = 0.035 for 3484 unique observed reflections. The molecule is centrosymmetric. The two very bulky (Me3Si)3C groups in the molecule have no unusually close methyl-methyl distances between them, the shortest such C...C distance being 4.043 (2) angstrom, i.e. approximately equal to the sum of the van der Waals radii for two methyl groups. The triple bond [1.199 (2) angstrom] is not significantly longer than expected for an alkyne. Si-C(methyl) distances range 1.860 (2)-1.873 (2) angstrom, while the C-Si distances of the CSi3 group range 1.915 (1)-1.923 (2) angstrom. (II) 2,2,5,5-Tetramethyl-3,3,4,4-tetrakis(trimethylsilyl)-2,3,4,5-tetrasilahexane, C18H54Si8, M(r) = 495.3, trigonal, R3cBAR, hexagonal axes a = 16.154 (2), c = 22.859 (3) angstrom, V = 5166.1 (13) angstrom3, Z = 6, D(x) = 0.955 g cm-3, lambda(Cu Kalpha) = 1.54184 angstrom, mu = 30.11 cm-1, F(000) = 1644, T = 296 K, final R = 0.042 for 889 unique observed reflections. The molecule lies on a site of 32 symmetry. The effect of the six bulky Me3Si groups is to lengthen the central Si-Si bond to 2.403 (2) angstrom, although the peripheral Si-Si bond lengths [2.3738 (9) angstrom] are not unusually long. The three Si-C distances range 1.855 (4)-1.876 (3) angstrom. The closest C...C distance between methyl groups in opposite halves of the molecule is 3.580 (5) angstrom, significantly shorter than the sum of the van der Waals radii.