The Inert s-Pair Effect

In books on inorganic chemistry, the marked increase in the stability of the lower oxidation state (by two units) of heavier elements descending the main groups of the periodic Table is often explained by the inert s-pair effect (see J. E. Huheey U)). For example, elements like In and Sn may use only 1 or 2 electrons for the formation of bonds instead of 3 or 4 (group number), leaving one electron pair in the outer valence shell inert . The electron pair is assumed to occupy an s-orbital. This classification does not very much contribute to the understanding of bonding first... 

While the contraction resulting from the poor shielding of 4/ electrons ceases at hafnium, the relativistic effect continues across the sixth row of the periodic table. It is largely responsible for the stabilization of the 6. orbital and the inert s pair effect shown by the elements Hg-Bi. It also stabilizes one40 of the 6p orbitals of bismuth allowing the unusual i-l oxidation state in addition to +3 and + 5.4 ... 

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 ... 

Another force that can result in distorted coordination polyhedra is the inert (lone) pair effect. The inert pair effect refers to the reluctance of the heavy post transition elements from groups 13 -15 to exhibit the highest possible oxidation state, by retaining their pair of valence s electrons. The lone pair of electrons on these elements can be stereochemically active and take the place of an anion in the coordination sphere of a cation, or squeeze between the anions and the metal causing distortion of the polyhedra. 

The concept of an atom s oxidation state see Oxidation Number) can provide fundamental information about the stmcture and reactivity of the compound in which the atom is found. In fact, it can be argued that oxidation states provided the basis for Medeleev s initial organization of the periodic table. For the main group elements, the relative stability of lower oxidation states within a given group increases as the atomic number increases. This trend in the periodic table see Periodic Table Trends in the Properties of the Elements) is generally attributable to the presence of an inert s pair see Inert Pair Effect) caused by relativistic effects see Relativistic Effects). 

Inert r-pair effect The tendency of the two outermost s electrons to remain nonionized or unshared in compounds characteristic of the post-transition metals. 

It is because of this resistance of the outer s pair for bonding that the phenomenon is called the inert pair effect. 

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... 

Another notable difference in properties down groups is the inert psiir effect > as demonstrated by the chemical behaviour of Tl, Pb and Bi. The main oxidation states of these elements are + I, + 2 and + 3, respectively, which are lower by two units than those expected from the behaviour of the lighter members of each group. There is a smaller, but similar, effect in the chemistry of In, Sn and Sb. These effects are partially explained by the relativistic effects on the appropriate ionization energies, which make the achievement of the higher oxidation states (the participation of the pair of s-electrons in chemical bonding) relatively more difficult. 

The counterion is quite important to the outcome of this reaction. Ion pairs, which form between alkali metal ions and the complex, induce CO lability which aids in the substitution process.1 1 Large charge-delocalized cations such as PPM are much less effective in forming ion pairs. The sulislilution process shown in Eq. 15.95 occurs readily when the counterion is Na but foils when it is PPN. Another good example of this effect can be seen by comparing NaICo CO)J and PPN [ Co[Pg.356]

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. 

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