Hexacoordinated Si compounds

Penta- and Hexacoordinated Si Compounds with Heavier Halides (Br, I)... 

Si coordination spheres have been reported (with weak S Si interactiOTis indicated by interatomic separations of 3.05-3.48 A), thiocarbamate was shown to form a noticeably shorter Si—S bond in 186 (2.70 A) and the animi of the thiourea derivative methimazole was shown to be capable of forming a trisilane (187) with two adjacent octahedral Si coordination spheres, one of which comprises four Si—S bonds (with separations ranging between 2.35 and 2.48 A). Notably, this trisilane with a central Si(Si2S4) skeleton is one out of only few crystallographically characterized hexacoordinated Si compounds which are devoid of first and second row elements in the octahedral Si coordination sphere of one of their Si atoms (further examples can be found with P and Cl in the coordination sphere, vide infra). The kind of tetrahedral capping by rather long Si S separations of about 3.17-3.50 A is encotmtered with various methimazolyl-substituted silanes such as 188 [349]. 

Unlike Si and Ge acetylacetonates (acac), in which the central M atom is hexacoordinated, the coordination number of the Sn atom in RSn(acac)3 is 7173. Hexacoordinated Si, Ge, Sn, Pb complexes with tropolone (Tp) are known. Tp4M molecules, where the metal atom is octacoordinated for M = Sn, Pb or hexacoordinated for M = Si, Ge, are of particular interest. The latter compounds correspond to the structure Tp3M+Tp- and exist as salts or ion pairs174. 

Molecular and Electronic Structure of Penta- and Hexacoordinate Silicon Compounds Table 13. C Chemical Shifts (ppm) for Si-Substituent of Silatranes in CDCI3 93-69s,774-778)... 

The molecular structure analyses of 3a and 3b (Fig. 3) confirm the hexacoordination of the Si atom. The coordination sphere around the Si atom is distorted octahedral. In both cases the monodentate ligands (phenyl group as well as benzoate and picrate) are situated in the trans position. This is probably typical for Salen-Si complexes [3, 4], but their configuration is quite different from that of the hexacoordinate tin compound 2c. 

Disubstitution of a silicon center by the 2-dimethylaminomethyl-4,6-dimethylphenyl substituent may result in hexacoordinated silicon compounds. This is manifested in the X-ray structure of difluorosilane 4, which shows a weak Si-N interaction of both amino side chains resulting in Si-N distances of 282 pm and 287 pm (Fig. 4). Both fluorine substituents occupy positions trans to the attacking nitrogen donor atoms, thereby forming N-Si-F angles of 173° and 172°. Thus, the silicon center is weakly hexacoordinated showing a coordination sphere, that may be described as a bicapped distorted tetrahedron [4]. 

This review is not the first to discuss the structural chemistry of organosilicon compounds. There have been a few earlier reviews discussing various aspects of the structural chemistry of organosihcon compounds. The stereochemistry of elements of Group 15 and 16 bonded to silicon was reviewed in 1973 °. In 1985 the geometry of silatranes was reviewed and simulation of the reaction pathway for Sis/2 substitution reactions at tetrahedral silicon using structural data was published. Later on, in 1986, the structural chemistry of tricoordinate silicon was reviewed and the X-ray and NMR studies on penta- and hexacoordinate silicon compounds were summarized. The most comprehensive review on the structural chemistry of organosilicon compounds was published in 1989 by W. S. Sheldrick ... 

Oxidative addition to a related silylene (Scheme 15) afforded a hexacoordinated silicon complex with two Si—I bonds (72), the first crystallographically evidenced hexacoordinated iodosilicon complex so far [203]. Furthermore, addition of chalcogens E (S, Se, Te) afforded pentacoordinated Si compounds with Si=E bonds (73) [204],... 

Scheme 24), the hydride migration from silicon to an adjacent unsaturated imino carbon atom leads to a pentacoordinated silicon complex 89 as final product [170]. For the intermediate 88 a dynamic equilibrium between two conformers 88a and 88b with pentacoordinated Si atom was observed by NMR spectroscopy. For related compounds with hexacoordinated Si atom within a (0,N>2SiMe(H) coordination sphere, the authors observed reversible neutral dissociation of the N-Si dative bond, i.e., an equilibrium between hexa- and pentacoordinated hydrido complexes of silicon [235]. 

This is encountered with various silanides, which can bridge two or more countercations with their Si-located lone pair (e.g., in compounds 149—151) [314-317]. In a similar fashion, some silanides can p -bridge assemblies of three transition metals (compounds 152 and 153), which are stabilized by the additional ligand functionalities of the silanide, thus rendering the sdanide Si atom formally hexacoordinated (Scheme 41) [318, 319], The Si NMR shifts of 152 and 153 clearly distinguish them from hexacoordinated Si complexes with six lone pair donors in the coordination sphere. 

Scheme 41 Compounds with penta- and hexacoordinated Si atoms due to the presence of lone pair acceptors around the silanide Si atom. For compounds (152) and (153) the Si NMR shifts are listed below the molecular formula...

This variety of hypercoordinated bromosilicon compounds is in sharp contrast to hypercoordinated iodosilicon compounds, which are represented by far less crystallographicaUy evidenced examples. In addition to some pentacoordinated monoiodosilicon compounds with tridentate chelators (compound 177, Si—I bond lengths ranging between 2.74 and 2.82 A) [95, 188, 190], the first crystallographi-cally characterized hexacoordinated iodosilicon compound (with two Si—I bonds, 2.64 and 2.66 A) [203] has been reported very recently (compound 72, Scheme 15). 

In the case of neutral systems the geometry corresponds to a bicapped tetrahedron or, in other words, to a tetrahedron which undergoes two nucleophilic coordinations. They have been observed [5] to be stable in solution. A careful H variable temperature NMR study shows that these compounds undergo an intramolecular isomerization which takes place without cleavage of Si-N bonds. The AGt of this so-called "Baylar Twist" isomerization has been estimated at between 12 and 18 kcal/mole depending on the substituents at the silicon. We can conclude that this work supports strongly hexacoordinated silicon structures as possible intermediates in the usual course of nucleophilic reactions at silicon. 

Si-N bonds in compounds with hexacoordinate silicon and tricoordinate nitrogen atoms... 

All Si—N bonds in compounds where hexacoordinate silicon atom is bonded to tricoordinate nitrogen atoms are dative bonds, where the nitrogen atom provides its lone-pair electrons to the bond. The average Si — N bond was calculated from 31 individual values to be 1.969 A (s.d. 0.05 A and s.m. 0.008 A). 

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