Hexacoordinate silicon compounds neutral complexes

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. 

Molecular and Electronic Structure of Penta- and Hexacoordinate Silicon Compounds TaHe 18. Si Chemical Shifts (ppm) of Some Neutral Penta-coordinate Silicon Complexes... 

Summary Treatment of Si(NCO)4 or Si(NCS)4 with 4-aminopent-3-en-2-ones yielded novel neutral hexacoordinate silicon(IV) complexes with an S/O2N4 framework, compounds 3-6. These silicon(IV) complexes were characterized in the solid state by single-crystal X-ray diffraction and Si VACP/MAS NMR spectroscopy. Compounds 3-5 crystallized as the (OC-6-12)-isomer, and 6 was isolated as the rranj-isomer. 

In contrast to the well-established chemistry of anionic and cationic hexacoor-dinate silicon(IV) complexes with SiOe skeletons, neutral hexacoordinate silicon(IV) species with SiOe frameworks are significantly less well explored. Compounds and 60 are examples of this type of compound. 

Recent studies on hydrazide-based hypercoordinate silicon complexes demonstrated the unusual flexibility of these compounds their tendency to reversibly transform between penta- and hexacoordinate compounds [1, 2] on the one hand and to irreversibly rearrange to more stable complexes [3] on the other. Thus, neutral hexacoordinate bis-chelate complexes (1) undergo reversible ionization in solution (Eq. 1), which is strongly dependent on a variety of factors temperature (ionization is enhanced at low temperatures), solvent (ionization takes place in hydrogen-bond donor solvents such as CHCI3, CH2CI2, and CHFCI2), the nature of the anion, the... 

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]. 

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