Hexacoordinate silicon complexes ionic

Picoline was found to react in chloroform solution with bis(dichlorosilyl)amine to form two hexacoordinate silicon complexes the ionic [H2Si(3-MeC5H4N)4]2+-2Cl and a neutral H2Si(3-MeCsH4N)2Cl2. These complexes are in equilibrium with each other in chloroform solution. The crystal structure of the ionic complex was reported247. 

Summary The first ionic dissociation of the Si-Cl bond in neutral hexacoordinate silicon complexes is reported. An equilibrium reaction between the ionic siliconium chloride and its neutral precursor (dissociation-recombination) is observed. The population ratio can be controlled by temperature or by replacement of the chloro ligand by a triflate group. The reaction enthalpy and entropy of the dissociation are both negative, suggesting that solvent organization facilitates dissociation at low temperature. 

A convenient method for the preparation of neutral bis(N->Si) hexacoordinate silicon complexes has been developed and reported recently, consisting of ligand exchange between a polychlorosilane (1) and 0-trimethylsilyl derivatives of hydrazides (2, Eq. 1) [2]. An attempt to utilize this synthetic route for the preparation of isomeric 0->Si coordinated chelates did not lead to the expected hexacoordinate complexes, but to ionic siliconium chloride salts stabilized by two (O—>Si) dative bonds (5, Eq. 2) [3]. 

Summary Binuclear hexacoordinate silicon complexes with two N-+Si dative bonds (per Si atom) have been prepared. The crystal stracture conforms to a distorted octahedron, with the N— Si bonds trans to each other. The binuclear complex undergoes Si-Cl dissociation of one Si Cl bond in CD2CI2 and CDCI3 solution, to form a binuclear, monosiliconium chloride salt, in reversible equilibrium with its precursor. The dissociation and equilibrium reactions are observed by variable-temperature Si NMR spectra. The extent of ionic dissociation increases as the temperature is decreased. The equilibrium population ratio is shifted completely to the di-ionic side at room temperature, by replacement of the chloride by the less nucleophilic triflate anion. The crystal structure of a disiliconium ditriflate shows a) well separated ions, b) that the geometry about silicon is almost an exact square pyramid, and c) that the N- Si bonds are among the shortest coordination bonds of this kind. 

Arguments for the intermediacy of penta- or hexacoordinate silicon intermediates have been discussed in detail in earlier reviews (10-13,247), but little attention has so far been devoted to Eq. (40). There is, however, a body of evidence that nucleophilic displacement at silicon may follow pathway (40). Evidence for this mechanism includes the following (1) the common existence of compounds postulated to be intermediates or modeling the intermediates i.e., positively charged ionic silane-nucleophile complexes containing tetracoordinate silicon (2) the dynamic behavior of these compounds when mixed with their components, and the behavior... 

The first ionic hexacoordinate complexes, silicon-tris-acetylacetonate cations (176), were reported as early as 1903 by Dilthey196a and Rosenheim and coworkers196b. Subsequently many other /i-diketonate complexes were studied197-199, and the subject was extensively reviewed7,200 and will not be discussed further here. 

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. 

Binuclear Ethylene-Bridged Silicon Chelates Equilibrium between Neutral Hexacoordinate and Ionic Pentacoordinate Siliconium Complexes... 

In search for new oligomers and polymers containing hypervalent silicon chelates, the reactions of bis(trichlorosilyl)ethane (1) with O- and N-trimethylsilylated hydrazides (2 and 3, respectively) have been studied. The reaction of 1 with 3 was reported to lead to ionic binuclear disiliconium dichloride salts (4, Eq. 1) [1]. It was not obvious how the reaction of 2 with 1 might proceed would it form a neutral binuclear hexacoordinate complex, like the mononuclear analogues [2], or would it form ionic complexes, in analogy to 4 ... 

The weakness of the Si—Br bond, which is reflected by its ionic dissociation upon silicon hexacoordination, is also manifested in neutral pentacoordinated Si complexes. As shown by the related compoimds in Scheme 44, the gradual approach of the additimial donor moiety (O atom) causes a stepwise dissociation of the Si X bond, which is reflected in the umbrella inversion of the equatorial Si-bound alkyl groups [338, 339]. In this course, a very long Si—Br bond (3.12 A) has been observed for 172. 

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