Halohydrides of silicon and germanium

Of compounds of the type SiH_nX_4-n (X= halogen, n = 1–3), SiHCl₃ is of particular importance in the purification of Si in the semiconductor industry. The success of the second step in below scheme depends on the precursor being volatile. SiHCl₃ (mp 145 K, bp 306K) is ideally suited to the process, as is SiH₄ (mp 88K, bp 161 K).

  Another application of SiHCl₃ is hydrosilation a method of introducing an SiCl₃ group and an entry to organosilicon chemistry.

   The halo-derivatives SiH₂X₂ and SiH₃X (X=Cl, Br, I) can be prepared from SiH₄  and some reactions of SiH₃Cl (bp 243 K) are shown in Figure 1.1. The ease with which SiH_nX_4-n compounds hydrolyze releasing HX means that they must be handled in moisture-free conditions. The preparation and reactivity of GeH₃Cl resemble those of SiH₃Cl. The structures of trisilylamine, N(SiH₃)₃, and disilyl ether, (H₃Si)₂O, are shown in Figure 1.1.

Fig. 1.1 Representative reactions of SiH₃Cl. The structures of N(SiH₃)3 (determined by Xray diffraction at 115 K) and (H₃Si)₂O (determined by electron diffraction).

The NSi₃ skeleton in N(SiH₃)₃ is planar and the N^_Si bond distance of 173pm is shorter than the sum of the covalent radii, ∑r_cov  similarly, in (H₃Si)₂O, the Si^_O^_Si bond angle of 1448 is large (compare 1118 in Me₂O) and the Si^_O bonds of 163pm are shorter than ∑r_cov. Trigermylamine is isostructural with N(SiH₃)₃, but P(SiH₃)₃ is pyramidal with P^_Si bonds of length 225 pm. In (H₃Si)₂S, the Si^_S^_Si bond angle is 978 and the Si^_S bond distances (214 pm) are consistent with a bond order of 1. For many years, these data have been taken as an indication that N and O take part in (p–d)π-bonding with Si there being no corresponding interactions in Si^_P or Si^_S bonds. However, recent arguments centre around the planarity of N(SiH₃)₃ (and related strengthening of Si^_N and Si^_O bonds) being due to n(N)→σ^*(Si^_H) electron donation, where n(N) represents the non-bonding (lonepair) electrons of the N atom. This is so-called negative hyperconjugation,† and is analogous to the donation of electrons from a d-block metal centre to a σ^*-orbital of a PR₃ ligand that we describe in Section 20.4. A stereoelectronic effect also contributes to N(SiH₃)₃ being planar. The polarity of the N^_Si bonds (X ^P(Si)=1.9, X ^P (N) = 3.0) is such that there are significant long-range electrostatic repulsions between the SiH3 groups. These are minimized if the NSi₃-skeleton in N(SiH₃)₃ adopts a trigonal planar, rather than pyramidal, geometry. The possibility of (p–d)π-bonding in N(SiH₃)₃ should not be confused with the (p–p)π-bonding which occurs in, for example, Si=N bonds (with a formal bond order of 2) in compounds such as tBu₂Si=NSitBu₃. Notice that the nitrogen atom is in a linear environment and can be considered to have a stereochemically inactive lone pair, possibly involved in π-interactions.

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