Synthesis and Structure of Lithium Tris (trimethylsilyl) silanide . 1,5 DME Abstract. Lithium tris(trimethylsilyl}silanide' 1,5 DME 2a synthesized from tetrakis(trimethylsilyl)silane 1 [6] and methyllithium in 1,2-dimethoxyethane 1 ), crystaIlizes in the monocIinic space group P2 1 /c with following dimensions of the unit cell determined at a temperature of measurement of -120 ± 2°C: a = 1072.9(3); b = 1408.3(4}; c = 1775.1(5} pm; ß = 107.74(2)°; 4 formula units (Z = 2). An X-ray structure determination (R w = 0.040) shows the compound to be built up from two [lithium tris(trimethylsilyl)silanide] moieties which are connected via a bridging DME moleeule. Two remaining sites of each four-coordinate lithium atom are occupied by a chelating DME ligand. The Li-Si distance of 263 pm is considerably longer than the sum oi covalent radii; further charactcristic mean bond lengths and angles are: Si-Si 234, Li-O 200, O-C 144, 0···0 (bite) 2G4pm; Si-Si--Si 104°, Li--Si-Si 107° to 126°; O-Li-O (inside the chelate ring) 83°. Unfortunately, di(tert-butyl)bis(trimethylsilyl)silane 17 prepared from di(tert-bntyl)dichlorsilane 15, chlorotrimethylsilane and lithium, does not react with alkyllithinm compounds to give the analogons silanide.

The structures of dimethyldichalkogenanes (H 3 C) 2 E 2 liquid at room temperature have been determined by single-crystal x-ray diffraction {E ϭ S, α-form 1a, P2 1 /c; Z ϭ 4; a ϭ 860.9(5); b ϭ 648.0(4); c ϭ 845.9(4) pm; β ϭ 94.23(4)°; Ϫ141 ± 2°C; β-form 1b, P2

The molecular and crystal structures of the isotypic compounds tetramethyldiphosphane 1 and -diarsane 2 have been determined by single crystal X-ray diffraction (monoclinic; C2/m; Z = 2; 1/2: -155/-144 °C\a = 540.6(2)/555.8(2); b = 1131.0(6)/l 136.7(6); c -602.5(2)/612.8(2) pm;β = 97.31(3)/95.24(3)°;P-P 221.2(1)/As-As 242.9(1) pm). In accordance with the crystallographically imposed symmetry 2/m, the molecules adopt antiperiplanar conformation. Their crystal structures are closely related to those of the homologues tetramethyldistibane 3 [1] and -dibismuthane 4 [3]. As found for the latter compounds, the E -E units (E = P, As) are also aligned in linear chains, but the intermolecular E···E contacts (P···P 381/As···As 370 pm) do not allow any extended interaction between molecules, which otherwise would result in thermochromic effects.

The title compound 1 was prepared by reacting methyl-bis(dimethylamino)difluorophosphorane with butyllithium at -95 °C. The crystalline product is characterized by its NMR and IR spectra. The results of the X-ray crystal and molecular structure analyses are reported.