Neutral hexacoordinate complexes

The initial intermediate 33 is the normal bis-(N — Si)-coordinated complex, which is obtained in analogy to numerous other neutral hexacoordinate complexes 30-38 (Eq. 15), prepared from SiC and various trichlorosilanes and substituted O-trimethylsilylated hydrazides. The final product 70 is the result of a novel molecular rearrangement, by which a chloride has been displaced by the dimethylamino-nitrogen and has migrated from carbon to silicon, accompanied by ring expansion and conversion of the N -> Si to O -> Si coordination. 

We make a distinction between two types of ionic pentacoordinate complexes those which are in dynamic equilibrium with neutral hexacoordinate complexes have been dealt with in Sections III.A.4, III.A.5.ii, and III.B.2. The second group includes those pentacoordinate siliconium-ion salts which are formed as such and are stable and do not equilibrate (to a noticeable extent) with their hypothetical neutral hexacoordinate counterparts. The present section discusses this group of persistent salts of pentacoordinate silicon cations. 

Keywords ionic dissociation, equilibrium reaction, siliconium compounds, neutral hexacoordinate complexes, negative entropy... 

Roewer et al. have used salene-type ligands for the preparation of neutral penta- and hexacoordinated silane complexes.834-836 Interestingly, the reaction of the acid form of the salene ligand reacts with organotrichlorosilanes in the presence of diethylamine to form pentacoordinated enamine silane complexes 849-851, whereas the disodium salt of the salene ligand reacts with phenyltrichlorosilane providing the hexacoordinated chlorosilane complex 852 (Scheme 119).834 The pentacoordinated complex 849 was also obtained when the hexacoordinated complex 852 was reacted with triethylamine (Scheme 119). 

Hexacoordinate fluorosilicates were obtained from neutral pentacoordinate complexes by addition of K[18-crown-6]F191,192. In this way compounds 168-173 were prepared from their pentacoordinate precursors. These compounds, in addition to being fluorosilicates, contain in each chelate ring a dative Si—N bond. 

VI. NEUTRAL HEXACOORDINATE SILICON COMPLEXES A. Intramolecular Coordination... 

Relatively few ligand types have been used for the formation of neutral hexacoordinate silicon complexes, resulting in several complex types 182-193208 -218. Acetylacetonato (acac) chelates [182, (acac SiXY] were prepared directly from the reaction of... 

TABLE 23. X-ray structural data for neutral hexacoordinate (N—Si) chelate complexes... 

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. 

The most widely studied group of hydrazide-based silicon complexes is the group of neutral hexacoordinate bis-chelates, with coordination of two dimethylamino donors to silicon. 

Si NMR Chemical Shifts of Neutral Hexacoordinate Bis-(N Si) Silicon Complexes with the N-NMe2 Donor Group [CDC13, 300 K, ( /, Hz)]... 

A subgroup of the neutral hexacoordinate silicon complexes discussed above is the family of binuclear complexes (55a,c,g,j). 

Comparative absorption spectrophotometry has been used in the studies of lanthanide /J-diketonatc complexes in solids as well as in solutions. The neutral hexacoordinated lanthanide tris diketonate on dissolution in a polar nonaqueous solvent, increased its coordination to eight by accepting two solvent molecules [204]. The addition of water or other oxygenated solvent to Nd(diket)3 in solution resulted in significant changes in the shape and intensity of the band due to 4l9/2 —> 4Gis/2, transition. These changes have been attributed to an increase in coordination number of Nd(III) from 6 to 8 by the coordination of two solvent molecules [238-241]. 

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... 

Synthesis and Structural Characterization of Novel Neutral Hexacoordinate Silicon(IV) Complexes with 5 i02N4 Skeletons... 

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. 

Scheme 1. Syntheses of the neutral hexacoordinate silicon(IV) complexes 3-6.

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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