Pentacoordinate siliconium ions

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. 

However, in the case of more weakly interacting counterion such as TPFPB the NMR data are in line with the formation of a pentacoordinated siliconium ion. [124] It was concluded that the stability of the pentacoordinated ion strongly depends on solvent and counterions. Specifically, methanol seems to stabilize the tetracoordinated form extensively. [124]... 

Steric Effect on the Formation, Structure, and Reactions of Pentacoordinate Siliconium Ion Salts... 

The isolation and structural characterization of pentacoordinated siliconium ions bearing the 2-(dimethylaminomethyl)phenyl substituent proves that the geometry of this substituent is well suited for a twofold coordination of a trigonal planar silicon center [8],... 

The data are interpreted by a reversible formation of a pentacoordinate silicon intermediate followed by attack of a second molecule of nucleophile in the rate-determining step, involving either a symmetrical octahedral intermediate 208 or a pentacoordinate siliconium ion 209 (Scheme 62). In either case, racemization takes place. 

No firm evidence is available to exclude either type of the intermediates (C) and (D). Stable examples of both are now well-documented in phosphorus159 as well as in silicon chemistry (Section II). The participation of pentacoordinate siliconium ions in chlorosila-cyclobutane isomerization160 and of analogous cationic species in the racemization of triorganotin halides161 has been supported. The possible intervention of hexacoordinate intermediates in substitution reactions at phosphorus has often been considered162. 

Kalikhman 1, Gostevskii B, Sivaramakrishna A, Kost D, Kocher N, Stalke D (2005) Steric effect on the formation, structure, and reactions of pentacoordinate siliconium ion salts. In Auner N, Weis J (eds) Organosilicon chemistry VI from molecules to materials. Wiley, Weinheim, pp 297-302... 

A special case of reversible ionization of a hexacoordinate silicon complex has been described as a novel tautomeric equilibrium.41 It differs from the formation of siliconium-ion salts in that the positive charge resides on nitrogen, in a dimethylammonium cation, and not on silicon. The transsilylation of lg with 12 in equimolar concentrations leads to the pentacoordinate zwitterionic complex 13 (Eq. (10), Section II.B.5). However, when the molar ratio was 2 1, respectively, an equilibrium mixture of tautomers (58, 59) was obtained, as shown in Eq. (21). The same mixture was also obtained when a second mole-equivalent of lg was added to 13. 

Free tricoordinate silicon cations (silicenium) are exceedingly unstable, and have only recently been first realized.74,75 A silicenium ion can be stabilized by coordination with two intramolecular donors, to form a stable (pentacoordinate) siliconium complex. These have been reported with... 

In the presence of bulky X ligands, a facile methyl halide elimination reaction is observed (Eq. 2) [3]. In this elimination the siliconium ion complex 2, with its two N—>Si dative bonds, is converted into a neutral pentacoordinate complex 3, with only one remaining dative bond (Fig. 1, Table 1). The reaction is probably driven by partial release of steric interaction, caused by the removal of one of the A-methyl groups. This is indicated by a decrease in elimination rate in the presence of less bulky ligands, cyclohexyl and isobutyl, and the failure to observe elimination when X = methyl. The reactivity order of the halide ions follows their nucleophilicities F > Br > CF, while the less nucleophilic ttiflate ion does not react at all. 

Swain et al. [79] compared the hydrolysis behavior of triphenylmethyl fluoride (TMF) and triphenylsilyl fluoride (TSF) under similar conditions. They found that, whereas the hydrolysis of TMF was consistent with a positively charged carbonium ion intermediate, the hydrolysis of TSF was not consistent with a positively charged siliconium ion intermediate but rather an intermediate in which silicon is less positively charged than in the original molecule. They concluded that pentacoordinate intermediates are easy pathways for displacements on silicon that are not available for carbon which cannot expand its valence to include more than eight electrons. 

Using Si NMR and Raman spectroscopy, Jonas [68], Artaki et al. [81], and Zerda and Hoang [71] have investigated the hydrolysis of TMOS (pH - 5-7.5) at pressures up to 5 kbar. Their observations that pressure increases the rates of reaction without affecting the distributions of hydrolyzed and/or condensed species are consistent with an associative mechanism involving a pentacoordinate intermediate (transition-state volume, AF, is negative) rather than a dissociative mechanism in which the rate-determining step is the formation of a tricoordinated siliconium ion plus alcohol (AF positive). 

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