Si -cation

The sharply increasing interest in the chemistry of monovalent silyliumylidene ion derivatives of the type RSi stimulated a great number of experimental studies in this field, which recently culminated in the synthesis of the very remarkable (Tl -Me5C5)Si+ cation 25+ TPFPB- (Scheme 2.21). A stable 25+ TPFPB- was obtained by the oxidation of decamethylsilicocene 26 with (Me5C5H2) TPFPB in CH2CI2 (Scheme 2.21). 

In the solid state, the (T -Me5C5)Si cation in 25 TPFPB displayed only a weak interaction with the anionic part, featuring an almost ideal pentagonal-pyramidal geometry (Figure 2.1). 

The fundamental structural unit of all crystalline silicates is a tetrahedron that has an anion at each of its four corners and a Si" cation in the center. The silicate tetrahedra can be arranged in a variety of ways giving rise to a great variety of crystalline silicate minerals. Most of these arrangements involve some degree of sharing of the corner 0 anions between adjacent silicate tetrahedra. 

Figure 15-11 Schematic structure of glass, which consists of an irregular network of Si04 tetrahedra connected through oxygen atoms. = O, = Si, = cation. Cations such as Li+, Na+, K+. and Ca2+ are coordinated to the oxygen atoms. The silicate network is not planar. This diagram is a projection of each tetrahedron onto the plane of the page. [Adapted tram G. A Perley, Glasses for Measurement of pH," Anal. Chem. 1949,21,394.]...

The characteristic structural features of two-coordinate low-valent germanium cation 276 are similar to those of the analogous Si cation 268 discussed above. The average C—N and C—C bond distances (1.348 A and 1.392 A, respectively) and the essentially planar ring indicate aromatic delocalization.691 In contrast, however, the y-CH proton (8 H 4.23) is more shielded. The aromatic substituents are perpendicular to the GeN2C3 plane. 

Figure 3. IMS findings (Ref. [8]) for Si cations examples of raw spectra (left) and summary plot exhibiting a transition to compact structures once clusters grow beyond n = 24 (right).

The greatly diminished stabilization of the silylenium compared to the carbenium structure by it substituents is connected with the known low effectiveness of 2p-3p (it) conjugation. Calculations were made on the =Si+ cation substituted with a phosphorus group with the hope of showing a superior 3p-3p (tt) conjugation of P—Si+ moiety (26,28). A smaller inductive destabilization by phosphorus than by electronegative substituents of the second period (N, O) was also expected. However, all... 

Ammonia interacting with a surface, as sketched in Fig. 4.57, will form NH4 ions coordinated to the bridging oxygen atom. Most oxide surfaces behave very similarly to the MgO surfaces. An exception is the surface generated on silica. In Si02 the Si cations are tetrahedrally coordinated. This coordination can be restored on the surface when the Si02 surface becomes exposed to H2O. Silanol groups develop, as sketched in Fig. 4.58. 

The calculated Si chemical shifts of alkyl substituted Si cations are about 350 ppm, and only depend somewhat on the basis sets and level of geometry optimization [40]. The experimentally obtained shifts on "silyl cation" complexes can be compared with those calculated shifts for model structures. 

A similar approach may be used in discussing the substitution of cations in silicate structures (Jensen, 1980 Chapter 8). Thus, when an anion is placed in the field of an Si cation, its electron cloud is tightened and its polarizability decreases. Also, as each of the oxygens of the SiO/ anion is placed in the field of a second Si + cation, its polarizability further decreases, as does that of each of the remaining unshared oxygens of the tetrahedral anion. This leads to the prediction that the polarizability or softness of the oxygen donor atoms will decrease in the order ... 

Appearance Opaque tan solid Solubility Dispersible (Hot) pH, 1% solution 4.0-5.0 Ionic nature Si. Cationic Odor None Viscosity Solid Specific Gravity . 90-.93 (75C)... 

Recent studies by Xu et al. (1999a,b 2000) have demonstrated that two structural transitions occur within the Lii cAli-xSii+ c04 system at room temperature. When x i > -0.3, the Ar and Si cations disorder over the tetrahedral sites, and when xsi > 0.65, the... 

Structural fragment of SAPO. Ten of the 12 O anions are located by the Monte Carlo method starting from a partial map containing the four symmetry independent Si cations and using prior knowledge of their coordination polyhedra. 

The study of the interaction mechanism of thin films of BACY prepolymer on different surface states of Si and oxidized Al employing advanced techniques such as XPS, UPS, MIES, IR reflection spectroscopy, and AFM was undertaken by Possart and Dieckhoff [364]. The trioxy triazine was the only moiety identified to have adhesive interaction with the substrate. On a Si surface, the mechanism was identified as donor-acceptor interaction where the lone pairs of electrons on N and O atoms of trioxy triazine were involved in the electron donor process for the Si cation. On aluminum oxide, the Lewis acidic OH groups act as electron acceptors, withdrawing electron density from the lone pairs of O and N of the trioxy triazine. Back donation of electron density from Al metal to the organic layer is operative beneath the oxide layer. The dicyanate monomer doesn t adhere at all and desorbs quickly out of the interphase region on the substrate. It was concluded that thermosetting reaction of the prepolymer is thus hampered and the resulting network will be less dense near the substrate than in the bulk. 

Catalytic alkylation of alkylsilanes with olefinic and acetylenic compounds using solid catalysts was examined in a closed recirculation reactor at 373 - 473 K. Alkylation of diethylsilane(E2) with these compounds took place smoothly on silica-alumina (SA) and S03/Zr02 catalysts but not on alumina, which means protonic solid acid catalyzed the reaction. n-Alkylated products were the main products and the /so-alkylated ones were the minor products regardless the type of olefins. The product distribution indicates the reaction takes place via a nucleophilic attack of olefins on a Si cation. 

Both ionic and covalent natures of the Si-0 bond contribute to the preference for (Si04)4- tetrahedron formation in both crystalline and glassy silicas. In addition, each O anion is coordinated by two Si cations, corresponding to comer sharing of the oxide tetrahedra, preventing the close-packing of anion layers and resulting in relatively open structures [5],... 

The entire series of small H Si+ cations (n= 1-5) was studied by Pople and Curtiss using MP4 theory and a series of extended basis sets3078,c. Ionization potentials and heats of formations were calculated (Table 37) and evidence was presented that the theoretical results are accurate to about 2 kcal mol-1. 

Fig. 19.15 The structure of the [(q -C5Me5)Si] cation present in the compound [(q -C5Me5)Si][B(C6Fs)4], determined by X-ray diffraction [P. Jutzi et al. (2004) Science, vol. 305, p. 849]. Hydrogen atoms are omitted colour code Si, pink C, grey.

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