Binary hydrides The group 14 elements

   Silane, SiH₄, is formed when SiCl₄ or SiF₄ reacts with Li[AlH₄] and is a source of pure Si (equation 1.1) for semiconductors. Silanes Si_nH_2n+‏2 with straight or branched chains are known for 1 ≤ n ≤ 10, and Figure 1.1 compares the boiling points  of the first five straight-chain silanes with their hydrocarbon analogues. Silanes are explosively inflammable in air (equation 1.2).

               (1.1)

        (1.2)

     A mixture of SiH₄, Si₂H₆, Si₃H₈ and Si₄H₁₀ along with traces of higher silanes is obtained when Mg₂Si reacts with aqueous acid, but the non-specificity of this synthesis renders it of little practical value. By irradiating SiH₄ with a CO₂ laser, SiH₄ can be converted selectively into Si₂H₆. Silane is a colourless gas which is insoluble in water, reacts rapidly with alkalis (equation 1.3) and forms compounds of the type M[SiH₃] with Na, K (equation 1.4), Rb and Cs.The crystalline salt K[SiH₃] possesses an NaCl structure and is a valuable synthetic reagent, e.g. equation 1.5.

                                (1.3)

                   (1.4)

          (1.5)

   Germanes Ge_nH_2n‏+2 (straight and branched chain isomers) are known for 1 ≤ n ≤ 9. GeH₄ is less reactive than SiH₄; it is a colourless gas (bp 184 K, dec 488 K), insoluble in water, and prepared by treating GeO₂ with Na[BH₄] although higher germanes are also formed. Discharges of various frequencies are finding increased use for this type of synthesis and have been used to convert GeH₄ into higher germanes, or mixtures of SiH₄ and GeH₄ into Ge₂H₆, GeSiH₆ and Si₂H₆. Mixed hydrides of Si and Ge, e.g. GeSiH₆ and GeSi₂H₈, are also formed when an intimate mixture of Mg₂Ge and Mg₂Si is treated with acid. Reactions between GeH₄ and alkali metals, M, in liquid NH₃ produce M[GeH₃], and, like [SiH₃]^-, the [GeH₃]^- ion is synthetically useful. The reaction of SnCl₄ with Li[AlH₄] gives SnH₄ (bp 221 K) but this decomposes at 298K into Sn and H₂; note the variation in reactivities: SiH₄ > GeH₄ < SnH₄. Plumbane, PbH₄, is poorly characterized and may not actually have been isolated. Significantly, however, replacement of H atoms by alkyl or aryl substituents is accompanied by increased stability.

Silanes Si_nH_2n‏+2 with straight or branched chains are known for 1 ≤ n ≤ 10, and Figure 1.1 compares the boiling points of the first five straight-chain silanes with their hydrocarbon analogues. Silanes are explosively inflammable in air (equation1.2).

  A mixture of SiH₄, Si₂H₆, Si₃H₈ and Si₄H₁₀ along with traces of higher silanes is obtained when Mg₂Si reacts with aqueous acid, but the non-specificity of this synthesis renders it of little practical value. By irradiating SiH₄ with a CO₂ laser,SiH₄ can be converted selectively into Si₂H₆.

Fig. 1.1 Boiling points of the straight-chain silanes, Si_nH_2n‏+2, and hydrocarbons C_nH_2n‏+2.

Silane is a colourless gas which is insoluble in water, reacts rapidly with alkalis (equation 1.3) and forms compounds of the type M[SiH₃] with Na, K (equation 1.4), Rb and Cs. The crystalline salt K[SiH₃] possesses an NaCl structure and is a valuable synthetic reagent, e.g. equation 1.5.

Germanes Ge_nH_2n‏+2 (straight and branched chain isomers) are known for 1 ≤ n ≤ 9. GeH₄ is less reactive than SiH₄; it is a colourless gas (bp 184 K, dec 488 K), insoluble in water, and prepared by treating GeO₂ with Na[BH₄] although higher germanes are also formed. Discharges of various frequencies are finding increased use for this type of synthesis and have been used to convert GeH₄ into higher germanes, or mixtures of SiH₄ and GeH₄ into Ge₂H₆, GeSiH₆ and Si₂H₆. Mixed hydrides of Si and Ge, e.g. GeSiH₆ and GeSi₂H₈, are also formed when an intimate mixture of Mg₂Ge and Mg₂Si is treated with acid.

    Reactions between GeH₄ and alkali metals, M, in liquid NH₃ produce M[GeH₃], and, like [SiH₃]^-, the [GeH₃]^- ion is synthetically useful. The reaction of SnCl₄ with Li[AlH₄] gives SnH₄ (bp 221 K) but this decomposes at 298K into Sn and H₂; note the variation in reactivities: SiH₄ > GeH₄ < SnH₄. Plumbane, PbH₄, is poorly characterized and may not actually have been isolated. Significantly, however, replacement of H atoms by alkyl or aryl substituents is accompanied by increased stability.

Fig. 1.2 Representative reactions of SiH₃Cl. The structures of N(SiH₃)3 (determined by Xray diffraction at 115 K) and (H₃Si)₂O (determined by electron diffraction).