Five coordinate silicon

In conclusion, all these observations suggest the presence of a five-coordinate silicon intermediate both in nucleophilic displacements at silicon and in the racemization of halosilanes as previously suggested (5, 59). The ability to form a pentacoordinate silicon adduct is not controlled by the electronegativity of the substituents at silicon, but by the tendency of the Si—X bond to be stretched under the influence of a nucleophile ... 

Compounds 4.75 and 4.76 formally contain tetravalent silicon, which is not electron-deficient. It is, however, a strong Lewis acid, and on reaction with F , five-coordinate silicon compounds are obtained. In the case of 4.75, the fluoride anion is localised mainly on the boron atom, although it does display dynamic behaviour involving hopping between boron and silicon. Compound 4.76 as the KF adduct contains two five-coordinate silicon atoms that chelate the F anion. The coordination sphere of the K+ counter-ion is completed by a molecule of [18] crown-6 (Figure 4.32a). 

The first evidence was provided by Yoder et al., who demonstrated (X-ray analysis) that the result of the reaction between A,A-bis(tri methyl si I yljacetamide with chlor-omethyldimethylchlorosilane was not N,N-bis(chloromethyldimethylsilyl)acetamide as erroneously reported,38 but a five-membered ring containing a five-coordinated silicon atom.39 This synthesis was extended to other model molecules.40 41... 

The most widely studied of the neutral five-coordinate silicon compounds are the so-called sUatranes of general stmcture (47). They are of interest both because of their unusual structure, which incorporates a strong transannular N Si bond, and because of their apparent physiological properties. ... 

The addition of a fluoride ion to various fluorosUanes gives salts with a five-coordinate silicon. Thus R4NF addition to SiF4 affords [R4N][SiFs] (R = EtorPr). A crystallographic study of [PhCH2NMe3][SiFs] shows the silicon to have trigonal bipyramidal geometry. 

Various theoretical studies (2, 3) reveal that nucleophilic substitution on silicon always proceeds through a five-coordinate silicon adduct intermediate. Thus the Sn2 mechanism on Si never resembles that of carbon. 

The constitution and compositions of 6-11 have been proven by NMR spectroscopy, mass spectrometry, and correct elemental analyses. Previous studies show that the Si NMR chemical shift is susceptible to the coordination number around the silicon center (between +24 and -76 ppm for four-coordinate silicon and -47 to -110 ppm for five-coordinate silicon) [7], and this can be... 

COFj will react with three equivalents of MegSiCFg In MeCN, in the presence of KF upon work up, followed by acidification with concentrated sulfuric acid, (CF 3)30011 is isolated in ca. 75% yield [1159aaj. The reaction is believed to proceed by the mechanism illustrated in Fig. 13.11, involving a five-coordinate silicon intermediate, [Me3SiF(CF3)]. ... 

No optically active five-coordinated silicon compounds have been obtained. However a chiral hexacoordinate silicon trisacetylacetonate cation (23) has been resolved by Dhar, Doron, and Kirschner (40) through the separation of diaste-reomeric dibenzoyl-d-tartrate salts (eq. [21]). 

Are the nucleophilic substitutions at silicon consistent with the occurrence of the rate-determining formation of a five-coordinate silicon intermediate ... 

In associative ligand exchange, the intervention of hexacoordinated intermediates has been demonstrated in various intermolecular ligand exchange processes at five-coordinated silicon compounds (286,355) (eq. [114]). 

The question whether or not ligand permutations via Berry pseudorotation in five-coordinate silicon compounds occur is a very provocative one. With the present state of knowledge, no definitive evidence has been obtained for such a process to occur, even for SiFs". It does, however, represent a possible isomerization pathway. From the results obtained, it appears to involve an activation... 

Interesting results were obtained for a series of silatranones103 which showed chemical shifts consistent with five-coordinate silicon for the silatran-3-ones and silatran-3,7-diones. The silicon atom in the silatran-3,7,10-triones, however, is a good electron acceptor and binds electron-donor solvents such as DMSO. For these compounds chemical shifts in the —130 to —150 ppm region, characteristic of six-coordinate silicon, were observed. 

DePuy and coworkers " have established the same order of acidity for alkanes by examining the preferred modes of decomposition of the 5-coordinate silicon anions formed by the reaction of OH with (CH3)3SiR. By analogy with the reaction of F the initial step is believed to be formation of the complex 21 which can either expel CH3 or R. The exothermicity of the initial addition reaction is insufficient to allow expulsion of an alkyl anion and heterolysis of an Si—C bond results in the solvated ion 22a or 22b (Scheme 1). Rapid proton transfer within this ion-molecule complex leads to the irreversible formation of RH or CH4. Molecular orbital calculations show the five-coordinate silicon anion 21 (R = CH3) to be at a minimum and the transition structure for decomposition of 21 has CH3 almost completely removed while the proton of the OH group is only slightly perturbed. 

Reacting la,b with MeLi in HMPA as active solvent and in the presence of MeOH as trapping agent, the attack of a methyl anion at the silicon atom of the silacyclobutane is the first reaction step and gives a pentacoordinated silicon anion (Scheme 2). Such five-coordinated species are discussed as intermediates during the ring opening polymerization of silacyclobutenes, -butanes, and -pentenes [2]. Furthermore, five-coordinated silicon species are well described to be stable compounds [3]. 

Stebbins JF, McMillan P (1993) Compositional and temperature effects on five-coordinated silicon in ambient pressure sihcate glasses. J Non Cryst Sol 160 116-125 Stebbins JF, Xu Z (1997) NMR evidence for excess non-bridging oxygen in an aluminosilicate glass. Nature 390 60-62... 

FIGURE 3.18 Side view of 100% Ca + depleted surface of wollastonite 100 showing the cluster formation and densification of leached surface due to connection between two side by side run ehains. A five-coordinated silicon is shown as pentagon, Si(4) as gray tetrahedra, calcium as gray ball, hydroxides aas ball and stick, and oxygen is red hydrogen as white. 

Martin has examined further the mechanisms of the nucleophile-catalyzed racemization at the four-coordinate silicon of (7), which has been shown to be first, and never second, order in nucleophile and to involve one, and not two, nucleophile molecules. The kinetics of the inversion at the five-coordinate silicon in (8), where Nu = P-XC6H4CHO (X = H, 3-CFj,... 

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