Neutral Pentacoordinate 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). 

A second method for the preparation of neutral pentacoordinate complexes consists of an exchange reaction between chloromethylchlorosilanes (ClCH2SiMe Cl3 , n = 0-2) and N-trimethylsilylated compounds, as shown for example in equation 2989. The compounds of types 96-99 were prepared by this method89-93. The same reagent was used extensively for the synthesis of O Si complexes, and this is discussed in more detail in the corresponding section (III.B.l). 

TABLE 12. Barriers for pseudorotation and Si—N cleavage in neutral pentacoordinate complexes... 

Hexacoordinate fluorosilicates were obtained from neutral pentacoordinate complexes by addition of K[18-crown-6]F191,192. In this way compounds 168-173 were prepared from their pentacoordinate precursors. These compounds, in addition to being fluorosilicates, contain in each chelate ring a dative Si—N bond. 

Selected Bond Lengths and Angles for the Neutral Pentacoordinate Complexes... 

In contrast, the reaction of 1 with polyfluorosilanes (26) proceeded slowly at room temperature (over several hours).16 Indeed, in this reaction the neutral pentacoordinate complex resulting from a single ligand exchange (4 a-c,h) was formed as a stable intermediate and could be observed for several hours in the NMR spectra (Eq. 16). 

Figure 6. Selected geometrical parameters and NMR chemical shifts of neutral R3SiX compounds and neutral pentacoordinated complexes (S)R3SiX calculated at the HF/6-3lG(d) and IGLO-HF/[7s6p2d/5s4pld/3slp]//HF/6-3lG(d) level of theory. Bond lengths in A, angles in deg, NMR chemical shifts relative to TMS. Values in parentheses refer to experimental results. [41]...

In the presence of bulky X ligands, a facile methyl halide elimination reaction is observed (Eq. 2) [3]. In this elimination the siliconium ion complex 2, with its two N—>Si dative bonds, is converted into a neutral pentacoordinate complex 3, with only one remaining dative bond (Fig. 1, Table 1). The reaction is probably driven by partial release of steric interaction, caused by the removal of one of the A-methyl groups. This is indicated by a decrease in elimination rate in the presence of less bulky ligands, cyclohexyl and isobutyl, and the failure to observe elimination when X = methyl. The reactivity order of the halide ions follows their nucleophilicities F > Br > CF, while the less nucleophilic ttiflate ion does not react at all. 

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 . 

Roewer et al. have used salene-type ligands for the preparation of neutral penta- and hexacoordinated silane complexes.834-836 Interestingly, the reaction of the acid form of the salene ligand reacts with organotrichlorosilanes in the presence of diethylamine to form pentacoordinated enamine silane complexes 849-851, whereas the disodium salt of the salene ligand reacts with phenyltrichlorosilane providing the hexacoordinated chlorosilane complex 852 (Scheme 119).834 The pentacoordinated complex 849 was also obtained when the hexacoordinated complex 852 was reacted with triethylamine (Scheme 119). 

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. 

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. 

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

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. 

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. 

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. 

The reactions of dihydrobilin (1,19-dideoxybiladiene-a, c) with transition metals are strongly influenced by the nature of the metal ion. Thus with Mn(OAc)3 or FeClj the corresponding metallocorrolates have been obtained in high yield, in the presence of chromium or ruthenium salts the reaction product isolated has been the metal free macrocycle, while coordination of rhodium requires the presence of an axial ligand such as a phosphine, arsine or amine [21]. Neutral pentacoordinated rhodium complexes have thus been obtained. Although analysis of the electronic spectra of the reaction mixtures demonstrated that cyclization of the open-chain precursor and formation of metallocorrolates occur even in the absence of extra ligands, no axially unsubstituted rhodium derivative has been reported. 

Among neutral pentacoordinate intramolecular germanium complexes, compounds containing a C,0-chelating ligand have been most extensively studied. 

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. 

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