The Group IV Elements Si, Ge, Sn, Pb

There is no more striking example of an enormous discontinuity in general properties between the first and the second row elements followed by a relatively smooth change toward more metallic character thereafter than in this group. Little of the chemistry of silicon can be inferred from that of carbon. Carbon is strictly non-metallic silicon is essentially non-metallic germanium is a metalloid tin and especially lead are metallic. Some properties of the elements are given in Table 11-1. 

Element Electronic M.p. B.p. Ionizations potentials (eV) Electro- nega- tivities Covalent radius (A) 

Catenation. While not as extensive as in carbon chemistry, catenation is an important feature of Group IV chemistry in certain types of compound. Extensive chains occur in Si and Ge hydrides (up to Si6H14. and Ge9H20), 

On the whole, however, there is a decrease in the tendency to catenation in the order C Si Ge Sn Pb. This general, if not entirely smooth, decrease in the tendency to catenation may be ascribed partly to diminishing strength of the C—C, Si—Si, Ge—Ge, Sn—Sn and Pb—Pb bonds (Table 11-2). 

0 Data derived mainly from MX4-type compounds which are unstable or non-existent when M = Pb. Pb—C in PbEt4 = 128.8 kJ mol 1 see C. F. Shaw and A. L. Allred, Orga-nometallic Chem. Reu., A, 1970, 5, 96. 

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The occurrence of homoatomic anions [En] of the elements of groups IV (C, Si, Ge, Sn, Pb) and V (P, As, Sb, Bi) as constituents of Zintl phases is well knownJ Despite numerous attempts, just a few of the En clusters have been isolated as uncharged molecules E Rx from the Zintl phases by reaction with suitable agents such as alkyl chlorides. 

In this oxidation state the titanium atom has formally lost its 3d and 4s electrons as expected, therefore, it forms compounds which do not have the characteristics of transition metal compounds, and which indeed show strong resemblances to the corresponding compounds of the lower elements (Si, Ge, Sn, Pb) of Group IV—the group into which Mendeleef put titanium in his original form of the periodic table. 

Discuss this statement as it applies to the Group IV elements, C, Si, Ge, Sn, Pb, indicating any properties of carbon which appear anomalous. Illustrate your answer by considering ... 

The rates of redistribution reactions for given pairs of substituents on a central atom or moiety of Group IV, under comparable conditions, increases in the order Siexchange rates increase upon reducing the sites available for exchange. Exchange reactions proceed faster on the alkylsilicon moiety,... 

The analogous phosphorus heterocyclic systems that include selenium or tellurium are discussed in detail in Chapter 6.12, and the systems incorporating phosphorus and arsenic or antimony are covered in Chapter 6.14. In addition, numerous examples of five-membered heterocyclic systems with phosphorus and elements of group III (B, Al, Ga, In) and group IV (Si, Ge, Sn, Pb) are included in Chapters 6.16 and 6.17 of this volume. The systems with five heteroatoms in a five-membered ring (e.g., pentaphospholide anion and tetrazaphospholium cation) and the fused heterocyclic rings (see Volumes 9-11) are beyond the scope of this chapter and are not covered here. 

The arsenic acids have no structural parallel in the heavier elements of Group IV (Si, Ge, Sn, Pb) but appear to function as a carboxylic acid would in ring-closure reactions. The contrast in behavior of an As—Cl bond toward electrophilic attack on a benzene ring versus Si—Cl reactivity should be noted. No clear-cut examples of a Friedel-Crafts reaction involving a silicon-chlorine bond have been reported 107). 

Also alkyl- and arylmetal fulminates have been prepared, among them the triphenylmetal fulminates of the group IV b elements (CgH j MCNO (M = Si, Ge, Sn, Pb) by reaction of the corresponding halides with silver or alkali fulminate. The termal stability increases from the silicon to the lead compound (Table III),... 

What forms are taken by the dioxides of the Group IV elements C, Si, Ge, Sn and Pb How do these structures reflect trends in bonding down the Group ... 

Sihcon is a Group 14 (IV) element of the Periodic Table. This column iacludes C, Si, Ge, Sn, and Pb and displays a remarkable transition from iasulatiag to metallic behavior with increasing atomic weight. Carbon, ia the form of diamond, is a transparent iasulator, whereas tin and lead are metals ia fact, they are superconductors. SiUcon and germanium are semiconductors, ie, they look metaUic, so that a poHshed siUcon wafer is a reasonable gray-toned mirror, but they conduct poorly. Traditionally, semiconductors have been defined as materials whose resistance rises with decreasiag temperature, unlike metals whose resistance falls. 

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