Silicon complexes tetravalent

TABLE 27. X-ray crystallographic and 29Si NMR data for hepta- and octacoordinate silicon complexes and tetravalent model compounds... 

Si NMR offers a unique method for characterization of these complexes since their chemical shifts deviate greatly from the normal range found for most tetravalent silicon compounds. A number of different types of extracoordinate silicon complexes have been prepared and characterized (125) and their Si NMR chemical shifts determined. The chemical shift data are in Table XVIII. 

The value of Si NMR in characterizing extracoordinate silicon complexes should be evident from the data in Table XVIII. Additionally this method can be used to establish whether a given ligand will form an extracoordinate complex with silicon or remain tetracoordinate. For example, a reaction mixture of SiCU and a bidentate ligand can be examined for resonances outside the normal Q region to determine whether an extracoordinate complex or a normal tetravalent compound is formed. 

Bridged silylene complexes are the subject of a recent comprehensive review by Ogino and Tobita338. These complexes can be classified into three types A, B and C (Scheme 9). In type A complexes there is no metal-metal bonding, the silicon is essentially tetravalent, and the bonding is similar to that in mononuclear metal-silyl complexes. In type C complexes, the bonding is best described as /j2-coordination of the Si—H bond to the metal, or alternatively as a metal-hydrogen-silicon 3-center 2-electron bond. 

In contrast to the sulfide of silicon (p. 187). the sulfides of tetravalent germanium (white) and tetravalent tin (yellow) are stable in water, but form complexes in the presence of excess sulfide ion. The sulfides of divalent tin and lead (both black) dissolve neither in strong base, in excess sulfide, nor in dilute acids. Note that PbS has the same structure as sodium chloride and is probably the most nearly covalent salt known having this structure, its color and metallic lustre setting it apart from the structurally similar ionic halides and oxides. 

Intramolecular complexes of penta-coordinate silicon formed by only one five-membered ring tre listed in Table 6. For identical substituents, the axial bound distances are longer than the equatorial ones and both are pronouncedly elongated in comparison with distances in tetravalent silicon compounds. A change in the covalent bond lengths upon the formation of the cycle points to the tendency to acquire an aromatic character. 

Unlike carbon, tetravalent silicon displays a pronounced acceptor ability and is capable of extending its valence shell to contain 10 or even 12 electrons. Such penta- or hexacoordinated species may either be anions or neutral coordination complexes in which the valence shell expansion is achieved by either intra- or intermolecular interaction with donor atoms belonging, as a rule, to the first (N, O and F) and second (P, S and Cl) rows. However, the Lewis acid strength of tetravalent silicon is weaker than that of the heavier members of Group 14, as dramatically illustrated by a comparison of the solid... 

In order to explain the mechanism of the polymerization of silicic acid, we would have to start with the fact that the tetravalent silicon is still unsaturated coordinaiively. Both of the strongly defined secondary valences which are active in the fluoride complexes of silicon must also play a role in the hydrated oxfde. 

There are no chalcogenoether crown complexes with silicon acceptor units. Within group 14, F NMR spectroscopic evidence was reported for the formation of the tetravalent [GeF4(K -[9]aneS3)] in solution at low temperatures. Although this compound was not stable enough to... 

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