Hexacoordinate silicon complexes intermolecular

The main methods for the synthesis of hexacoordinate silicon compounds are similar to those for pentacoordinate complexes and were outlined in a recent review6. These methods include (a) addition of nucleophiles (neutral or anionic) to tetracoordinate silanes (b) intermolecular or intramolecular coordination to an organosilane (c) substitution of a bidentate ligand in a tetrafunctional silane. The following discussion focuses mainly on new complexes, reported since the recent reviews6,7 were published. 

An interesting example of an intermolecular complex is the trisilicon complex 194, in which only the central silicon is coordinated to the bidentate donor molecule225. The structure is a regular octahedron, with two tetrahedral termini. The silicon nitrogen bonds are rather short (2.012 and 1.991 A), and are comparable to those of octahedral intramolecular complexes (Table 23). 194 permits a comparison of Si—Cl bonds in a tetrahedral silicon moiety (2.03 to 2.07 A) with Si—Cl bonds trans to the dative bond in a hexacoordinate silicon (2.39 and 2.21 A). As expected, the latter are substatntially longer than the regular covalent bonds. 

Polyhalosilanes form neutral intermolecular hexacoordinate complexes with donor molecules such as pyridine, triethylamine, 2,2 -bipyridine and 1,10-phenanthroline. This topic has recently been reviewed6. It was demonstrated that electronegative substituents on the silicon are essential for the formation of intermolecular complexes. Thus, while SiCLt and Cl2CHSiCl3 react with 1,10-phenanthroline and with 2,2 -bipyridine to form hexacoordinate chelates, MeSiCl3 does not react224,225. For completion we discuss here a few examples, and compare some of the properties of intermolecular complexes with those of the intramolecular complexes. 

In summary, we conclude that in contrast to phosphinomethanides A, anionic phosphines of type B can only form intra- or intermolecular phosphorus silicon donor-acceptor complexes under particularly favorable circumstances. These are seen primarily in suitable ligand geometries. Multidentate phosphine ligands which strongly impose coordination numbers higher than four to silicon seem to be especially suitable. In accord with previous findings [4], chlorine atoms as further silicon substituents favor the phosphine coordination additionally. With some caution we also state that hypervalent silicon with phosphorus donor atoms seems to prefer hexacoordination over pentacoordination. 

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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