The Inert Pair Effect

While the chemistry of aluminum is largely that of the trivalent state, the monovalent state becomes increasingly stable as one goes down group 13 for thallium, the monovalent state is the common state. Trivalent T1 salts are reactive and used as oxidants in organic chemistry, as shown below for the thallation of benzene  

For group 14, whereas divalent carbon and silicon (carbenes and silylenes) are typically highly reactive, divalent germanium is considerably more stable. Divalent tin salts are quite stable, although still relatively reducing. By contrast, divalent lead salts are very stable and tetravalent lead compounds are potent oxidants the cleavage of vicinal diols by lead tetraacetate is a good example of the latter  

The situation in group 15 is qualitatively similar. Thus, whereas the pentavalent state is clearly the most stable for phosphorus, the trivalent state becomes more stable as one goes down the group. Like trivalent thallium and tetravalent lead compounds, pentavalent 

As we explore p-block chemistry, we will encounter a wide variety of hypervalent molecules used as reagents in organic synthesis in quite a few of these, the inert pair effect is believed to play a role. 

The Sn2 paradigm provided a starting point for our discussion, from which the following key points are worth remembering  

Note well the italicized sentence. It implies that the inert pair effect is a relative phenomenon. It emerges only through a comparison with other Groups. For example, the atoms of the Group II elements have configurations of the type [FIS] ns, and in the +2 oxidation state, their ionic configuration is [FIS]. For a Group II element, the process 

Rewrite this last sentence so that it applies to the loss of the outer s pair in Group 111. 

We have spent some time on the inert pair effect because isotopes of some of the elements 113-116 have recently been made. Because of the island of stability , their half-lives are relatively long and it may be possible to study their chemistry. 

We expect them to be typical elements lying beneath the elements Tl, Pb, Bi, Po, At and Rn in the Periodic Table. As the inert pair effect increases down Groups III, IV and V, it should influence the chemistry of these new elements in an especially marked way. 

The inert pair effect is considered on one of the CD-ROMs accompanying this Book. 

Various names are given to this effect. In addition to inert-pair effect, it has been called the 6s inert-pair effect and the inert-s-pair effect. No matter what we call this idea, it states that the valence ns electrons of metallic elements, particularly those and 6V pairs that follow the second- and third-row transition metals, are less reactive than we would expect on the basis of trends in effective nuclear charge, atomic sizes, and ionization energies. This translates into the fact that In, Tl, Sn, Pb, Sb, Bi, and, to some extent, Po do not always show their expected maximum ojddation states but sometimes form compounds where the oxidation state is 2 less than the expected group valence. This effect is more descriptive and less readily explained than the first three ideas in our network, but we are learning not to be content with just descriptions. How, then, can we explain or at least partially rationalize this effect  

Ionization Energies and Bond Energies of the Group 3A Elements 

Frost diagram for manganese under acid and base conditions. [Deveioped by Rene T. Boere, University of Lethbridge, Alberta, Canada, copyright (2000, 2013). Used with permission.] 

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Although both aluminum and indium are in Group 13/III, aluminum forms A1J+ ions, whereas indium forms both In3+ and In+ ions. The tendency to form ions two units lower in charge than expected from the group number is called the inert-pair effect. Another example of the inert-pair effect is found in Group 14/IV tin forms tin(IV) oxide when heated in air, but the heavier lead atom loses only its two p-electrons and forms lead(II) oxide. Tin(II) oxide can be prepared, but it is readily oxidized to tin(IV) oxide (Fig. 1.56). Lead exhibits the inert-pair effect more strongly than tin. 

The inert-pair effect is the tendency to form ions two units lower in charge than expected from the group number it is most pronounced for heavy elements in the p block. 

Identify which of the following elements experience the inert-pair effect and write the formulas for the ions that they form (a) Sb (b) As (c) Tl (d) Ba. 

Many metallic elements in the p and d blocks, have atoms that can lose a variable number of electrons. As we saw in Section 1.19, the inert-pair effect implies that the elements listed in Fig. 1.57 can lose either their valence p-electrons alone or all their valence p- and s-electrons. These elements and the d-block metals can form different compounds, such as tin(II) oxide, SnO, and tin(IV) oxide, Sn02, for tin. The ability of an element to form ions with different charges is called variable valence. 

Group 13/III is the first group of the p block. Its members have an ns np1 electron configuration (Table 14.5), and so we expect a maximum oxidation number of +3. The oxidation numbers of B and A1 are +3 in almost all their compounds. However, the heavier elements in the group are more likely to keep their s-electrons (the inert-pair effect, Section 1.19) so the oxidation number +1 becomes increasingly important down the group, and thallium(I) compounds are as common as... 

In the past 15 years a large number of polonium compounds have been prepared in visible quantities for the first time and as a result of these investigations it has been shown that polonium behaves chemically very much as would be expected from its position in the Periodic Table, with the inert-pair effect, likely to be more marked in polonium than in tellurium, still little in evidence. 

In acidic solution In and Tl have + 1 states, consistent with the inert pair effect that affects the heavier elements of Groups 13 to 15. 

Valencies and oxidation slates that vary from those expected from the operation of the octet rule were explained hypervalency and the inert pair effect were described. 

The formation of compounds in the formal oxidation state of VI is well established for all four elements, for example, the sexivalent fluorides and the TeF ion. However, oxidation to this highest valency state becomes progressively more difficult as group VIB is descended since the inert pair effect causes the elements to behave as though two of their valence electrons are absent. Some examples of the compounds formed by the group VIB elements and their stereochemistries are described in Table 1. 

Finally, the heavier posttransition metals have group number oxidation states corresponding to < 10 configurations indium(lll), thallium(III), tin(IV), lead(lV), anti-mony(V). bismuth(V), etc. However, there is an increasing tendency, termed the inert pair effect," for the metals to employ p electrons only and thus to exhibit oxidation states two less than those given above (see Chapter 18). 

It can readily be shown that there is no exceptional stability (in an absolute sense) of the s electrons in the heavier elements. Table 18.3 fists the ionization energies of the valence shell s electrons of the elements of Groups IIIA (13) and IVa (14). Although the ts electrons are stabilized to the extent of -300 U mol-1 (3 cV) relative to the 5s electrons, this cannot be the only source of the inert pair effect since the 4s electrons of Ga and Ge have even greater ionization energies and these elements do not show the effect—the lower valence Gaff) and Geffl) compounds are obtained only with difficulty. 

The inert-pair effect is due in part to the different energies of the valence p- and s-electrons. In the later periods of the periodic table, valence s-electrons are very low in energy because of their good penetra-... 

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