Neutral pentacoordinate silicon complexes

By this method also the zwitterionic silicates 9-15 were obtained The geometry at silicon in these compounds is TBP, like in anionic and neutral pentacoordinate silicon complexes. A typical crystal structure is shown in Figure 5 for compound 9. This structure apparently also exists in solution (CD3CN), as the 29Si chemical shift for 9 in this solvent (—122.9 ppm) compares well with the solid state CP-MAS shift of -121.0 pm28 31. 

New neutral pentacoordinate silicon complexes with a dative O Si bond (207, 208) have been prepared and studied. The NMR spectral data and crystal structures were reported239. 

Ligand Exchange via Coordinative Si-N Bond Cleavage and Pseudorotation in Neutral Pentacoordinate Silicon Complexes... 

Another case of bridging was also reported recently in a neutral poly-silicon complex, and is discussed in the section on neutral pentacoordinate complexes (Section III.C.2). 

Table 19 presents Si chemical shifts and Si—H coupling constants for some of the cationic pentacoordinate silicon complexes and for some of their precursors. No clear trend can be seen for the Si chemical shifts of these compounds some are shifted downfield and others upheld, relative to their pentacoordinated neutral precursors. However, a trend seems to emerge from the one-bond Si—H coupling constants in the cationic pentacoordinate complexes V(Si—H) is generally greater than for neutral pentacoordinate complexes. This is possibly the result of the greater s-character in the Si—H bonds, in the doubly coordinated (sp -hybridized) siUcenium cations . 

Scheme 24), the hydride migration from silicon to an adjacent unsaturated imino carbon atom leads to a pentacoordinated silicon complex 89 as final product [170]. For the intermediate 88 a dynamic equilibrium between two conformers 88a and 88b with pentacoordinated Si atom was observed by NMR spectroscopy. For related compounds with hexacoordinated Si atom within a (0,N>2SiMe(H) coordination sphere, the authors observed reversible neutral dissociation of the N-Si dative bond, i.e., an equilibrium between hexa- and pentacoordinated hydrido complexes of silicon [235]. 

The weakness of the Si—Br bond, which is reflected by its ionic dissociation upon silicon hexacoordination, is also manifested in neutral pentacoordinated Si complexes. As shown by the related compoimds in Scheme 44, the gradual approach of the additimial donor moiety (O atom) causes a stepwise dissociation of the Si X bond, which is reflected in the umbrella inversion of the equatorial Si-bound alkyl groups [338, 339]. In this course, a very long Si—Br bond (3.12 A) has been observed for 172. 

The study of compounds containing pentacoordinate silicon atoms currently represents one of the main areas of research in silicon chemistry. This is evident from the numerous reviews and proceedings published on this topic in recent years.112 Most of the pentacoordinate silicon compounds described in the literature are either salts with A5.S7-silicate anions or neutral silicon complexes with a 4+1 coordination to silicon. This review deals with a completely different class of pentacoordinate silicon compounds zwitterionic A S /-silicatcs. These molecular compounds contain a pentacoordinate (formally negatively charged) silicon atom and a tetracoordinate (formally positively charged) nitrogen atom. 

III. PENTACOORDINATE NEUTRAL SILICON COMPLEXES A. Chelates with Nitrogen-Silicon Coordination... 

A novel (3,3) sigmatropic rearrangement of a hexacoordinate allyl-silicon complex (neutral tetraoxyspirosilicate) to a pentacoordinate complex was recently described242. The allyl group migrates from silicon to the a-carbon of a tropolone ligand242. 

We make a distinction between two types of ionic pentacoordinate complexes those which are in dynamic equilibrium with neutral hexacoordinate complexes have been dealt with in Sections III.A.4, III.A.5.ii, and III.B.2. The second group includes those pentacoordinate siliconium-ion salts which are formed as such and are stable and do not equilibrate (to a noticeable extent) with their hypothetical neutral hexacoordinate counterparts. The present section discusses this group of persistent salts of pentacoordinate silicon cations. 

Table 6. Bond Distances (pm) for Pentacoordinate Silicon at the Expense of the Formation of Five-Membered Chelate Ring (Neutral Complexes)...

The general synthetic method for the preparation of penta- [4] and hexacoordinate [5] neutral silicon complexes from 0-silylated hydrazides 2 has been described previously. The syntheses used in the present study are shown in Scheme 1. In both reactions the pentacoordinate complex is an intermediate on the way to the corresponding hexacoordinate species. However, the equilibrium condition is sufficiently favorable to allow the isolation of 4 as the major product simply by using an excess of 3. By contrast, the intermediate 6 is unstable, and is transformed spontaneously in solution at 300 K within 2-3 hours to 7. At lower temperatures the solution NMR spectra of 6 could be studied for several hours before it completely transformed to 7. 

Binuclear Ethylene-Bridged Silicon Chelates Equilibrium between Neutral Hexacoordinate and Ionic Pentacoordinate Siliconium Complexes... 

Hexamethylphosphoramide (HMPA) adducts of tetrachlorosilane (SiCU) were investigated with NMR spectroscopy in solution and solid-state structures [93]. In solution, the meridional and facial isomers of the hexacoordinated cationic silicon complex [SiCl3(HMPA)3] Cl predominate at all HMPA concentrations and are in equilibrium with the hexacoordinated neutral tram- and cA-[SiCL((HMPA)2] complexes, as well as the pentacoordinated cationic Si-complex cis- [SiCl3(HMPA)2] "Cr [93]. 

The reaction conditions for the synthesis of [Si2(pz )g] and [Si(pz )4] have been investigated in detail in order to obtain suitable precursor compoxmds for the synthesis of the hitherto unknown Janus-head ligand tris(3,5-dimethylpyrazolyl) silanide ([Si(pz )3] [241]. The X-ray stmctures of several pentacoordinated disilanes of type 95 with X = Cl or 3,5-dimethylpyrazolyl have been presented in this work. The chemistry of 3,5-dimethylpyrazolyl silicon complexes has been further explored. Although pyrazolide is an anionic substituent, its second N atom (the imine N atom of N-silylated pyrazole) also coordinates to silicon, thus exhibiting features of a neutral nitrogen donor as well. 

The main methods for the synthesis of hexacoordinate silicon compounds are similar to those for pentacoordinate complexes and were outlined in a recent review6. These methods include (a) addition of nucleophiles (neutral or anionic) to tetracoordinate silanes (b) intermolecular or intramolecular coordination to an organosilane (c) substitution of a bidentate ligand in a tetrafunctional silane. The following discussion focuses mainly on new complexes, reported since the recent reviews6,7 were published. 

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