3 penta-coordinate silanes

Tamao et al,83 found that a higher coordinated silylene 119 can be formed from penta-coordinated silane 118 (Scheme 31). Warming a solution of 118 in toluene or dimethylformamide in the presence of diphenylacetylene or 2,3-dimethyl-l,3-butadiene resulted in the formation of silylene-trapping products 120 and 121. Interestingly, no 1 1 reaction product between the silylene and the acetylene was isolated. Thus, it must be concluded that the insertion of silylene 119 into a Si-C bond of initially formed silacyclopro-pene is faster than the addition to the triple bond of the acetylene so that the silacyclopropene cannot be isolated under the reaction conditions. 

Figure 9. Reaction of alcohol on penta-coordinated metal-silane complex.

Hydrolysis of letraa I koxy silane (TMOS or TEOS) is generally performed in the presence of a catalyst which can be an acid, a base or a nucleophile. This is also the case for the hydrolysis of R/Si(OMe)350. In the case of TMOS and TEOS, the acid catalysis is due to the reversible protonation of the alkoxy group which converts it to a better leaving group. However, the nucleophilic attack of the oxygen atom of water is still a key step (equation 17). In the case of basic catalysis, nucleophilic attack of the OH- anion at the silicon centre leads to a penta-coordinated intermediate, followed by the elimination of the RO group (equation 18). For nucleophilic catalysis (promoted by F, HMPA, imidazole, 7V,7V-dimethylaminopyridine as well as OH ) the formation of a penta-coordinated species (equation 19) increases the reactivity of the silicon atom towards the nucleophilic attack of water that leads to an hexa-coordinated intermediate, which finally leads to the product of hydrolysis or condensation. 

Frye, C. L., G. E. Vogel, and J. A. Hall Triptych-siloxazolidines Penta-coordinate Bridgehead Silanes resulting from transannular interaction of Nitrogen and Silicon. J. Amer. chem. Soc. 83, 996—997 (1961). 

Organosilicon spirocyclic compounds consisting of two five-membered rings linked through the silicon atom, readily form rather stable complexes with bases the central silicon atom in such complexes is penta-coordinate. Thus, bis(ethylene-l,2-dihydroxy )silane forms adducts with alkali metal alkoxides 232.233) which event is not... 

The role of 3d AOs in the formation of silane and difluorosilane adducts has been examined. It has been shown that, although these orbitals do not play an essential role, their participation considerably increases the stability of SiHj and FjSiHj". This is especially pronounced in the case of fluorosilane complexes. Participation of 3d AOs decreases the energy of the system with penta-coordinate silicon stronger than that of the system with tetra-coordinate silicon. 

In general the five- and six-coordinate silicon compounds have chemical shifts at unusually low frequency from — 50 to — 200 ppm, with the six-coordinate compounds appearing at lower frequency from —130 to —200 ppm while the five-coordinate compounds are in the —50 to —150 ppm region. Silicon-29 NMR provides a good method for studying the equilibria between normal four- coordinate silanes and higher-coordinate species in solution as well as characterizing these compounds as penta- or hexacoordinate silicon compounds. 

Table 27 lists the 29Si chemical shifts for the high coordination complexes. Triphenyl-silane serves as a tetracoordinate model for the complexes 197 and 200, for comparison. It can be seen that the increased coordination in the latter complexes effects a small upheld shift, much smaller than is normally observed upon going from tetra- to penta- (Table 11, Section III.A.2.b) and hexacoordination (Table 24, Section VI.A.2). This is in accord with the crystallographic results, which show that no significant change in hybridization takes place in these compounds, in contrast to TBP and octahedral complexes.

In this special type of reaction, three-coordinated or square-planar complexes of certain transition metals (Rh, Ir, Pt) add silanes to form penta- or hexa-coordinate complexes, respectively, both the oxidation number and coordination number of the transition metal increasing by two,... 

Like in the other types of pentacoordinate complexes, the 2 Si NMR signal in O—Si chelates is shifted to high field relative to its tetracoordinate analogs (Tables 14 and 15). Therefore the difference between the 2 Si chemical shifts of penta- vs. tetracoordinate silanes (the coordinative shift ) serves as a measure for the extent of coordination (Table 15). It can be seen from the data in Table 14 that in series 117 the silicon signal is shifted downfield as the electron-withdrawing power of the para-substituent on the aromatic ring increases, resulting in weaker coordination . This information can also be seen in the N chanical shifts, and to a lesser extent in the carbonyl— C shifts listed in Table 14. Similar trends in the coordinative 2 Si shifts as well as in the carbonyl C-and 0-chemical shifts for complexes of type 118 were reported by Pestunovich and coworkers . 

---