Silicon-containing trigonal

Silicon is usually found in oxides and silicates. With very electropositive metals it forms silicides. The compound CaSi2 can be viewed as containing Ca2+ and Si ions. The crystal structure is trigonal, Dgft, R3m, a = 3.8259, and c = 15.904 A with three molecules in the unit cell. The Si ion has the same valence shell as As, and layers of... 

While interactions of alkali metals or magnesium with elemental silicon and germanium (or their oxides) can give various silicides or germanides, most of these products do not contain true anions and some are semiconductors. However, in Li12Si7 there are Si5 rings and trigonal planar Si-centered Si, units in the lattice. 

Both experimental data and theoretical calculations show that the transition of a trigonal bipyramidal into a square pyramidal structure for compounds of penta-coordinate silicon does not require high energy consumption. Up to now the attempts of experimental separation of diastereomers of organosilicon compounds with penta-coordinate silicon atom were unsuccessful. For spirocyclic bisacetylaceto-nates, XYSi(Acac)2, containing a hexa-coordinate silicon atom, however, geometric isomerism has been observed... 

In contrast to compounds 10-16 SiO C framework), the X Si-silicates 17 and 18 contain a S1O5 framework. The latter compounds were obtained by reaction of Si(OMe)4 with two mole equivalents of the respective a-hydroxycarboxylic acid and one mole equivalent of 2-(dimethylamino)ethanol in boiling acetonitrile (reaction time 50 h yield 61 % (17) or 77 % (18)). Recrystalli2ation of 18 from dimethylformamide yielded the 18 DMF solvate which was structurally characterized by single-crystal X-ray diffraction [11]. As can be seen from Fig. 6, the coordination polyhedron surrounding the silicon atom of 18 is a distorted trigonal bipyramid (Berry distortion [8] 13.9 %), in which the carboxylate oxygen atoms occupy the axial sites. 

In spite of the fact that in many cases the larger alkaline earth metals can be exchanged for divalent rare earth metals, the latter ones form different varieties of three-dimensional or layered triply bonded disilicides with either the a-XhSi2 or the AIB2 structure. These two structure types are clearly dominated by a trigonal prismatic arrangement of the cations and contain silicon in exclusively trigonal planar coordination (Fig. le and If). 

Cai4Sii9 is an overlooked phase in the binary system with nearly an even number of (3b)- and (2b)-Si atoms per formula unit [31, 48]. It contains a novel two dimensional polyanion of silicon which is displayed in Fig. 6a. The central unit is a 3,3,3-barrelane Sin which is linked via six Si3 bridges to six neighboring barrelanes. The whole polyanion contains seven layers of silicon atoms and has a width of ca. 900 pm. The barrelane part is shown in Fig. 6b with its trigonal planar (3b)-silicon atoms. The calcium atoms are distributed not only between but also inside the layers. 

Silicon is immediately below carbon in the periodic table and the most obvious similarity is that both elements normally have a valency of four and both form tetrahedral compounds. There are important differences in the chemistry of carbon and silicon—silicon is less important and many books are devoted solely to carbon chemistry but relatively few to silicon chemistry. Carbon forms many stable trigonal and linear compounds containing n bonds silicon forms few. The most important difference is the strength of the silicon-oxygen o bond (368 kj mol ) and the relative weakness of the silicon-silicon (230 kjmor ) bond. Together these values account for the absence, in the oxygen-rich atmosphere of earth, of silicon analogues of the plethora of structures possible with a carbon... 

In addition to the head-to-tail hydrosilylation described above we could show that monomeric vinyl-functionalized hypervalent silanes can be used for the preparation of polycarbosilanes containing silicon atoms in a trigonal-bipyramidal environment [7b, 8] Treatment of... 

As we have discussed previously, many structures of molecules which contain intramolecular dative bonds to silicon are known. The reader is referred to the review article of Tandura and colleagues for a comprehensive listing26. The extent to which a trigonal bipyramidal geometry is adopted by the central silicon atom will depend on the strength of the coordinate bond. Examples of relatively strong 0->Si and N->Si interactions are provided by 299409 and 300410 for which respective dative bond distances of 1.918(6) and... 

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