Si coordinations

Brand-new results show the existence of heptacoordinated silicon as described in some of the following papers of this chapter, which also contribute to the discussion of mechanistical pathways in the course of nucleophilic substitution reactions at silicon. From these results one may speculate whether compounds with octa- and nonacoordinated silicon may be characterised in the near future. Although it is a problem to assign coordination numbers in -w-bound systems, it is worthwhile to note Jutzi s dccamethylsilicocene with a formal Si-coordination number ten in the oxidation state +2 in this context. With respect to Si(U)-compounds it should be stated that there are further derivatives with the... 

The chiral sites which are able to rationalize the isospecific polymerization of 1-alkenes are also able, in the framework of the mechanism of the chiral orientation of the growing polymer chain, to account for the stereoselective behavior observed for chiral alkenes in the presence of isospecific heterogeneous catalysts.104 In particular, the model proved able to explain the experimental results relative to the first insertion of a chiral alkene into an initial Ti-methyl bond,105 that is, the absence of discrimination between si and re monomer enantiofaces and the presence of diastereoselectivity [preference for S(R) enantiomer upon si (re) insertion]. Upon si (re) coordination of the two enantiomers of 3-methyl-l-pentene to the octahedral model site, it was calculated that low-energy minima only occur when the conformation relative to the single C-C bond adjacent to the double bond, referred to the hydrogen atom bonded to the tertiary carbon atom, is nearly anticlinal minus, A- (anticlinal plus, A+). Thus one can postulate the reactivity only of the A- conformations upon si coordination and of the A+ conformations upon re coordination (Figure 1.16). In other words, upon si coordination, only the synperiplanar methyl conformation would be accessible to the S enantiomer and only the (less populated) synperiplanar ethyl conformation to the R enantiomer this would favor the si attack of the S enantiomer with respect to the same attack of the R enantiomer, independent of the chirality of the catalytic site. This result is in agreement with a previous hypothesis of Zambelli and co-workers based only on the experimental reactivity ratios of the different faces of C-3-branched 1-alkenes.105... 

Figure 1.16 Newman projections of energy minimum conformations for (ff)-3-methyl-l-pentene and (,S>3-mclhyl-l -pentenc upon coordination on a site of the kind of Figure 1.14 or 1.15. Energy minimum conformations for si coordination are shown on right while those for re coordination are on left.

Figure 1.23 Transition states for secondary insertion of styrene into secondary growing chain presenting si chirality (that is, generated secondary insertion of. si-coordinated styrene), (a) Model for unlike (syndiospecific) propagation includes fluxional site of R chirality at metal atom, which imposes re-propene coordination, while (b) model for like (isospecific) propagation includes fluxional sites of S chirality at metal, which imposes. si-propene coordination. Syndiospecific transition state (a) is favored because smallest substituent on C atom of chain, the H atom, can be pointed toward Cp ligand, whereas isospecific transition state (b) is of higher energy because Cp of growing chain is oriented toward Cp ring.

Due to re- and si-coordination of prochir-al substrates at a catalyst with C2-sym-metric chiral ligands two diastereomeric catalyst-substrate complexes emerge. In the case of C,-symmetric ligands already four stereoisomer intermediates result. 

Si coordinated by stronger donor solvents => random coil/globule-like... 

CH3CN, 33-11, 33 H20, 34, 35-jCH3CN, and 37jCH3CN represent nearly ideal or (more or less) distorted square pyramids, with the carbon atom Cl in the apical position. In most cases, the geometries of the Si-coordination polyhedra in the crystal are located on the Berry-pseudorotation coordinate (pivot atom Cl). 

Fig. 11. Fourier transformations of PtLj-edge SEXAFS data and simulations for 2.5 ML and 5 ML of Pt deposited onto Si(lll), and PtSi standard silicide, The peak B is fitted by assming a local Pt Si coordination...

Figure 34. Alkali zeolites projected into a portion of the Na-K-Si coordinates. Anal = analcite Ph = phillipsite solid solution Ze = alkali zeolites undifferentiated Alb = albite KF = potassium feldspar Q = quartz Si = amorphous silica, a) low, b) medium, and c) high temperature facies. Shaded areas are two-phase fields.

This is by for the most common coordination number. With certain ions si -coordinate complexes are predominant. For example. chromhim(lll) and cobah(MI) are almost exclusively octahedral in their complexes. It was this large series of octahedral Cr(III) and Co(III) complexes which led Werner to formulate his theories of coordination chemistry and which, with square planar plalinum(II) complexes, formed the basis for almost all of the classic work on complex compounds. Before discussing the various isomeric possibilities for octahedral complexes, it is convenient to dispose of the few nonoctahedral geometries. 

The most widely studied compounds possessing N —Si coordination are silatranes, azasilatranes and their analogs75. This family of chelates warrants a separate discussion, which can be found elsewhere75. 

Stable N—Si coordinated complexes generally contain a chelate ring with a covalent bond to silicon on one side, and a dative N - Si bond on the other. Only few such bidentate ligand types have been used, and the resulting families of related chelates are depicted in structures 90-10076 -95. 

In an attempt to stabilize low-valent silicon compounds, a class of compounds was studied in which N -> Si coordination served to stabilize a double bond between silicon and either sulfur114,115 phosphorus116, nitrogen116, oxygen114,115,117 or a transition metal113,118-121 (e.g. 107). In these compounds the silicon is formally pentavalent, though it is coordinated to only four atoms. This topic belongs more appropriately to the chapter on silylenes, and is summarized here briefly for the sake of completeness. 

TABLE 22. 29Si chemical shifts for bis-catecholato silicon complexes with N—Si coordination (CD2G2)... 

Remarkably facile thermal generation of a silylene has recently been demonstrated for the pentacoordinate alkoxydisilane 92416. Intramolecular N—>-Si coordination in 92 was shown by X-ray crystallography. This disilane underwent thermolysis at 110°C in toluene, or 90 °C in DMF, producing the silylene 93, which was trapped with 2,3-dimethylbutadiene and diphenylacetylene (Scheme 26). The 2 1 adduct with diphenylacetylene was shown to have one nitrogen intramolecularly coordinated to silicon, even though the silicon atom lacks any electronegative substituents. 

Clusters of dissolved species combine to form unit cells. To form si coordination numbers of 20 and 24 are required for 512 and 51262 cavities, while sll requires coordination numbers of 20 and 28 for the 512 and 51264 cavities. Nucleation is facilitated if labile clusters are available with both types of coordination numbers for either si (e.g., CH4 + CO2 mixtures) or sll(e.g., CH4+C3H8 or most unprocessed natural gases). If the liquid phase has clusters of only one coordination number, nucleation is inhibited until the clusters can transform to the other size, by making and breaking hydrogen bonds. 

Using careful NMR monitoring, the reaction starting with amide or lactams was confirmed to be multistage including a previously non-described 0-[(dimethylchlor-osilyl)methyl]imidate showing a N —> Si coordination. The chelated imidate is formed under kinetic control whereas the chelated amide is formed under thermodynamic control.44... 

X-ray studies show the silicon to be trigonal bipyramidal coordinated with the oxygen and chlorine in the apical positions.46 48 Variations in the substitution at silicon are reflected in variations in the strength of the O — Si coordination bond, sometimes resulting in the loss of the pentacoordination at silicon and formation of a salt.49 These studies have been reviewed.50... 

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