Pentacoordinate silicon compounds cationic complexes

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 . 

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. 

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. 

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... 

---