Silicon is ordinarily rather unreactive. It is attacked by halogens giving tetrahalides, and by alkalis giving solutions of silicates. It is not attacked by acids except hydrofluoric ; presumably the stability of SiF62- provides the driving force here.
Germanium is somewhat more reactive than silicon and dissolves in concentrated H2SO4 andHNO3. Tin and lead dissolve in several acids and are rapidly attacked by halogens. They are slowly attacked by cold alkali, rapidly by hot, to form stannates and plumbites. Lead often appears to be more noble and unreactive than would be indicated by its standard potential of more noble and unreactive than would be indicated by its standard potential of -0.13 V. This low reactivity can be attributed to a high overvoltage for hydrogen and also. In some instances, to insoluble surface coatings. Thus lead is not dissolved by dilute H2so4 and concentrated HCI
COMPOUNDS OF GROUP IV ELEMENTS
15-3
Hydrides,MH4
These are colorless gases. Only monosilane, SiH4 is of any importance. This spontaneously flammable gas is prepared by the action of LiAIH4 on Si02 at 150-170° or by reduction of SiCI4 with LiAIH4 in an ether. Although stable to water and dilute acids, rapid base hydrolysis gives hydrated SiO2 and H2.
Substituted silanes with organic groups are of great importance, as are some closely related tin compounds (Chapter 29). The most importance reaction of compounds with Si−H bonds, such as HISiCI3 or HSiCH3 , is the Speier or hydrosilation reaction of alkenes:
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This reaction, which employs chloroplatinic acid as a catalyst, is commercially important for the synthesis of precursors to silicones.
15-4
Chlorination of the group IV elements on heating gives colorless or yellow (PbCI4) liquids. They are hydrolyzed by water eventually to the hydrous oxides but partial hydrolysis can give oxochlorides. In presence of aqueous HCI the chlorides of Ge, Sn, and Pb give chloroanions,and GeCI4 differs from SiCI4 in that it can be distilled and separated from concentrated HCI. The principle uses of SiCI4 and GeCI4 are in synthesis of the pure elements and for silicon and tin, in the synthesis of organo compounds (Chapter 29)