Silicates with Coordination Numbers Four and Six

It has been previously theorized that in aqueous silicate solution silicon is coordinated with six oxygen atoms. Such views of Weyl (76) and Treadwell and Wieland (77) were discussed by Her (78). However, more recent spectroscopic studies of silicate solutions seem to indicate that the silica is 4-coordinated with oxygen and that the Si(OH)s ion, for example, does not exist, at least in solution. Fortnum and Edwards (79) concluded that Si(OH)g was not present in solution because Raman lines analogous to those of SiF , PF , and Te(OH) were not found. The four low intensity lines found at 448, 607, 777, and 935 cm suggested that the ion is (H0)2Si02 since it is similar to (HO)2P02 , which gives lines at 401, 511, 885, and 1050 cm . 

Similar conclusions were reached by Early et al. (80), who also noted the similarity of Raman spectra of (H0)2Si02 and (H0)2S02. Using H2O and D2O, Freund (81) examined silicate solutions with Raman-laser spectra and identified (HO)3SiO , (H0)2Si02 , and (H0)3Si02 , the latter only at very high alkali concentrations. Equilibrium was obtained rapidly and completely. There was no evidence for the presence of dimer species. 

In this octamer, referred to as a double-four-ring structure there are eight silicon s 

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