Silicon halides

   Many fluorides and chlorides of Si are known, but we confine our discussion to SiF₄ and SiCl₄  and some of their derivatives. Silicon and Cl₂ react to give SiCl₄, and SiF₄ can be obtained by fluorination of SiCl₄ using SbF₃, or by reaction 1.1.

                       (1.1)

Both SiF₄ and SiCl₄ are molecular with tetrahedral structures. They react readily with water, but the former is only partially hydrolysed (compare equations 1.2 and 1.3). Controlled hydrolysis of SiCl₄ results in the formation of (Cl₃Si)₂O, through the intermediate SiCl₃OH.

               (1.2)

                                              (1.3)

    The reaction between equimolar amounts of neat SiCl₄ and SiBr₄ at 298K leads to an equilibration mixture of SiCl₄, SiBrCl₃, SiBr₂Cl₂, SiBr₃Cl and SiBr₄ which can be separated by fractional distillation. The Lewis base N-methylimidazole (MeIm) reacts with SiCl₄ and SiBr₂Cl₂ to give trans-[SiCl₂(MeIm)₄]_2‏ (Figure 1.1a) as the chloride and bromide salts respectively. This provides a means of stabilizing an [SiCl₂]₂‏ cation. The formation of [SiF₆]^2-, the hexafluorosilicate ion (Figure 1.1b), illustrates the ability of Si to act as an F^- acceptor and increase its coordination number beyond 4.

Hexafluorosilicates are best prepared by reactions of SiF₄ with metal fluorides in aqueous HF; the K‏ and Ba²⁺‏ salts are sparingly soluble. In aqueous solution, fluorosilicic acid is a strong acid, but pure H2SiF6 has not been isolated. The [SiF5]_ ion (Figure 1.1c) is formed in the reaction of SiO2 with aqueous HF, and may be isolated as a tetraalkylammonium ion. Silicon tetrachloride does not react with alkali metal chlorides, although lattice energy considerations suggest that it might be possible to stabilize the [SiCl₆]^2- ion using a very large quaternary ammonium cation.

Fig. 1.1 Solid state structures (X-ray diffraction) of (a) trans-[SiCl₂(MeIm)₄]₂‏ from the salt [SiCl₂(MeIm)₄]Cl₂:3CHCl₃ (MeIm = N-methylimidazole) [K. Hensen et al. (2000) J. Chem. Soc., Dalton Trans., p. 473], (b) octahedral [SiF₆]^2-, determined for the salt [C(NH₂)₃]₂[SiF₆] [A. Waskowska (1997) Acta Crystallogr., Sect. C, vol. 53, p. 128] and (c) trigonal bipyramidal [SiF₅]^-, determined for the compound [Et₄N][SiF₅] [D. Schomburg et al. (1984) Inorg. Chem., vol. 23, p. 1378]. Colour code: Si, pink; F, green; N, blue; C, grey; Cl, green; H, white.