Silicon, extended coordination

To a large extent the chemical shifts of carbon and silicon run parallel, but the chemistry of the two elements is somewhat different. Thus silicon can have extend its valence shell beyond the coordination number of 4. A few stable or-ganosilicon compounds in which silicon is divalent are known (the silylenes), and compounds with a silicon-silicon double bond also exist (the disilenes). 

Photoswitching has allowed Kawashima et al. to control the configuration of the N-N double bond and the coordination number of potassium 18-crown-6 2-(phenylazo)phenyltetrafluorosilicates 836 and 837, which are accessible by the reaction of 2-(phenylazo)phenyltrifluorosilane 838 with KF/18-crown-6 (Scheme 116).830 In the (Z)-form, the silicon atom of 837 is pentacoordinated, whereas in the (E)-form, one nitrogen atom of the azo group of 836 intramoleculary coordinates to the silicon atom, thus extending its coordination sphere to six ligands. Almost... 

The ability of the silicon atom to extend its coordination number even when it is bonded to three or four carbon atoms was demonstrated by the preparation and structural studies of 2,8,9-tricarbasilatranes 158-160398 -400. 

The first evidence was provided by Yoder et al., who demonstrated (X-ray analysis) that the result of the reaction between A,A-bis(tri methyl si I yljacetamide with chlor-omethyldimethylchlorosilane was not N,N-bis(chloromethyldimethylsilyl)acetamide as erroneously reported,38 but a five-membered ring containing a five-coordinated silicon atom.39 This synthesis was extended to other model molecules.40 41... 

Fig. 4a. Structure of Caj jLij jSij (skew [001] view) with planar silicon polyanions of para-linked flat six-membered rings the trigonal prismatic cation array is indicated by the polyhedral representation trigonal prisms and extended trigonal prismatic coordination of the Zintl anion (CN6 (Ca) light blue, CN6 (Li, Ca) green, CN7 (Li, Ca) purple) trigonal prisms coordinating the central part of the anion are exclusively composed of Ca atoms...

The tandem silylformylation-allylsilylation of a diallylsilyl ether has been reported by Leighton, using [Rh(acac)(CO)2] as the catalyst (equation 11). This method was optimized and extended to propargylic systems where good diastere-oselectivity can be achieved using the isopropyl derivative. A mechanism is proposed for the stereoselectivity involving coordination of the silicon by both of the oxygen atoms (equation 12). 

R. Tacke, O. Seiler, Higher-coordinate silicon compounds with SiOs and SiOe skeletons, in Silicon Chemistry From the Atom to Extended Systems (Eds. P. Jutzi, U. Schubert), Wiley-VCH, Weinheim, 2003, p. 324 - 337. 

Silicon compounds with coordination number larger than four are the object of many studies first with respect to their application as catalysts in organic and inorganic syntheses and second as starting materials for the preparation of a broad variety of organosilicon compounds [1]. Additionally, hypervalent silicon hydride compounds can successfully be used as model compounds to study, for instance, the mechanism of nucleophilic substitution reactions, which is of great interest since the silicon atom is able to easily extend its coordination number [1]. Moreover, hypervalent silanes are suitable as starting materials for the synthesis and stabilization of low-valent silanediyl transition metal complexes [2-5]. 

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... 

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