UNSATURATED SILICON COMPOUNDS. VII. PREPARATION AND STRUCTURE OF A SILAETHENE TETRAHYDROFURANATE

Article abstract of DOI:10.1016/0022-328X(84)85190-6

The compound ^tBu2SiF-CLi(SiMe3)2*4THF, accessible by a multistep synthesis, decomposes in diethyl ether at room temperature to give initially Me2SiF-CLi(SiMe3)(SiMe^tBu2)*nTHF, and finally a colourless crystalline tetrahydrofuran adduct of the silaethene Me2Si=C(SiMe3)(SiMe^tBu2).The adduct may be sublimed (with partial decomposition, the gas phase containing both discrete silaethene and THF).The crystals are orthorhombic, space group P2₁2₁2₁ with lattice constants a 10.000(5), b 15.386(8), c 15.509(8) Angstroem, V 2386.23 Angstroem³ and d_calcd 1.038 g/cm³ for Z = 4 (T = -45 deg C).Full-matrix least-squares refinements of 208 parameters based on 1991 observed reflections (F_o <*> 4.0 ?(F_o)) converged at R = 0.052.The THF molecule is clearly coordinated to the Si atom, which thereby adopts a distorted tetrahedral geometry.The trigonal planar sp² geometry expected for both the Si and C atoms ofthe silaethene molecule is retained only for the latter.However, the bond distance between these atoms (1.747(5) Angstroem) is the shortest Si-C bond ever observed.The dative Si-O bond from the THF molecule, with a length of 1.878(4) Angstroem, is significantly longer than ordinary SiO bonds involving two- or three-coordinated oxygen.The implications of the structural parameters for the description of the bonding are discussed, and it is concluded that the adduct may be regarded as a zwitterion the formal charges of which are being located at the oxygen and the sp² carbon atoms, respectively.Strong support for this concept is provided by a comparison with relevant structural data for silaethenes and phosphorus ylids.