Silicon extracoordinate compounds

Another important feature of the extracoordinate silicon compounds (Scheme 7.14) is the increase in natural atomic charge at the central atom compared to the tetracoordinate precursors [69]. The counter-intuitive increase in the positive charge on silicon, which becomes even more substantial in the case of anionic nucleophiles, such as F , is compensated by a more negative character of the surrounding groups (X), and this results in an enhanced ionic nature of the Si-X bond. This polarization then favors intermolecular charge-dipole interaction, which results in an increased Lewis acidity of the hypercoordinate silicon [70]. 

Characterization of these extracoordinate compounds has depended mostly on elemental analysis and differences in IR and UV spectra compared with typical tetravalent silicon compounds. (124)... 

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. 

As the chemistry of hypervalent or extracoordinate silicon compounds is covered elsewhere in this volume, we shall concentrate on some interesting, but less well surveyed aspects of silicon complexing. This brief survey is limited to direct participation of the silicon atom in coordination. The influence of silicon on neighbouring atom coordination is mentioned in Section III.A on basicity. 

Mechanisms for the substitution at silicon in solution have been a subject of several reviews (7-15). Since expansion of the coordination number of silicon is a common feature, particular interest is directed to mechanisms involving intermediates or transition states with silicon having a coordination number of 5 or 6. So far, little attention has been devoted in the review literature to mechanistic pathways that do not invoke extracoordination. Knowledge of these mechanisms, however, has often become necessary for the understanding of chemical behavior of organosilicon compounds. In this article we discuss mechanistic pathways involving heterolytic cleavage... 

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. 

---