Pentacoordinate silicon complexes cationic

The standard group valence is 4. For Si, Ge, and Sn, however, coordination numbers exceeding 4 occur regularly. The central atoms in such species are thus hypervalent, that is, they have more than an octet of valence electrons. For example, 2,2-bipyridyl reacts with triphenylchlorosilane to yield a relatively stable pentacoordinate silicon complex. Observe that, although the Si carries a -1 formal charge, the complex as a whole is cationic ... 

Table 19 presents Si chemical shifts and Si—H coupling constants for some of the cationic pentacoordinate silicon complexes and for some of their precursors. No clear trend can be seen for the Si chemical shifts of these compounds some are shifted downfield and others upheld, relative to their pentacoordinated neutral precursors. However, a trend seems to emerge from the one-bond Si—H coupling constants in the cationic pentacoordinate complexes V(Si—H) is generally greater than for neutral pentacoordinate complexes. This is possibly the result of the greater s-character in the Si—H bonds, in the doubly coordinated (sp -hybridized) siUcenium cations . 

Scheme 2 Cationic pentacoordinated silicon complexes formed upon ionic dissociation of a Si—Cl bond...

Scheme 22 8 2 Cationic pentacoordinated silicon complex 86 (left) and schematic representation of a Berry pseudorotatirai process (right) [221]...

The first few pentacoordinate cationic silicon complexes (153-155) were discussed in an earlier review2. The first two were described as trivalent silicenium ions stabilized by mtermolecularcoordination183 184, while 155 is an intramolecular silicenium ion21a. Since then, a few other compounds were reported. Oxidation of the dihydrido-hexacoordinate chelate 156 by the addition of excess iodine produced 157185. The X-ray structure of 157 showed a slightly distorted TBP for the cation, with a well separated (Si—I distance 5.036 A) I8 2 anion for every two cations. The nitrogens are in the apical positions, and both Si-N distances (2.08 and 2.06 A) are longer than covalent bonds but well within the coordination distance. 

In contrast, syn products are formed through a boat-like transition state, also involving a cationic silicon complex. In this complex, however, the silicon atom is pentacoordinated and one phosphoramide only is bound to the silicon atom. 

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. 

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. 

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... 

For several of the siliconium salts crystal structure analyses were obtained, confirming the pentacoordination and the ionic nature of the compounds (well separated cations and anions). The crystal structures for 90a(OTf), 90c(OTf), 91a(OTf), 91a(AlCl4), and 93a(OTf) are depicted in Figs. 47-51, respectively. Further structural support is found in the 29Si NMR chemical shifts (Table XXVI). A remarkable observation in Table XXVI is the nearly equal 29Si chemical shifts of siliconium salts sharing the same silicon complex, but with different anions [e.g. 91a(OTf), 91a(Br), and 91a(AlCl4)] the equal shifts are the evidence that the siliconium cations are essentially independent of... 

To the best of our knowledge, cationic pentacoordinate silicon(IV) complexes with SiOs skeletons have not yet been described, whereas the chemistry of cationic hexacoordinate silicon(IV) species with SiO(> skeletons is well established. 

Hexamethylphosphoramide (HMPA) adducts of tetrachlorosilane (SiCU) were investigated with NMR spectroscopy in solution and solid-state structures [93]. In solution, the meridional and facial isomers of the hexacoordinated cationic silicon complex [SiCl3(HMPA)3] Cl predominate at all HMPA concentrations and are in equilibrium with the hexacoordinated neutral tram- and cA-[SiCL((HMPA)2] complexes, as well as the pentacoordinated cationic Si-complex cis- [SiCl3(HMPA)2] "Cr [93]. 

A formally pentavalent cationic silicenium complex, with intramolecular base-stabilization, is the silylene-iron 167, with a Fe=Si double bond. The crystal structure of 167 shows that silicon is essentially tetrahedral in this compound, and the solution 29 Si chemical shift is 8 = 118.3 ppm, compatible with the double bond character rather than with pentacoordination at silicon189. 

The silicon atom in the crystal of cationic complex [Ph3Si(bipy)] I is pentacoordinated but the bond lengths are not determined Geometrical distortion from the TBP to the SPY configuration expresses from the sum of dihedral angle method desaibed in Refs. Two crystallo-... 

For the cationic pentacoordinate tropolonates there appears to be an effect of the nature of the non-chelating group R on chemical shift. Those tropolone complexes in which the silicon atom is bonded to an sp hybridized carbon in R ([47] and [48]) exhibit a chemical shift of — 141 ppm compared with hexacoordinate T3Si (<3 = — 139 ppm). 

B2 Preparation of Cationic Silicon(IV) Complexes Starting From Pentacoordinate Carbene Adducts of SiCU 348... 

PREPARATION OF CATIONIC SILICON(IV) COMPLEXES STARTING FROM PENTACOORDINATE CARBENE ADDUCTS OF SiCU... 

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