The Inert Electron Pair

In his seminal article of 1916 Lewis remarked that the heavier elements in Groups 13 and 14 form a class in which the atomic kernel is probably not neither uniquely determined nor invariable during chemical change. By the time that Sidgwick published his book The Electronic Theory of Valency in 1927, our understanding of the electronic structure of atoms was much further advanced, and the distinction between s- and p-electrons had been drawn [67], We now realize that the first two electrons in any group [...] correspond to the pair of electrons [...] in helium, and can have a certain completeness of their own [...]. We might thus anticipate that under some conditions the first two valence electrons in an element could [...] refuse either to ionize, or to form covalencies, or both. Why this inertness [...] should appear precisely where it does in the periodic table, we cannot say.  

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The harder (in Pearson s model ) or the more electronegative the ligand, the more pronounced the stereochemical activity of the inert electron pair localized at the chalcogen atom. 

It is not possible to obtain a stable modification of Pb02 by replacing the inert electron pair by an additional oxygen atom, but if we push the layers somewhat apart and place an additional anion above each lone electron pair, we end up with the layer structure of BiOF or PbFCl [240]. 

Thallous halides offer a unique possibility of studying the stereochemistry of the (chemically) inert electron pair, since their structures and their pressure and temperature-dependent phase transitions have been well established. Thallium (1) fluoride under ambient conditions, adopts an orthorhombic structure in the space group Pbcm which can be regarded as a distorted rocksalt structure (Fig. 2.4). In contrast to TIF, the thallium halides with heavier halogens, TlCl, TlBr and Til, adopt the highly symmetric cubic CsCl structure type under ambient conditions [46]. Both TlCl and TlBr, at lower temperatures, undergo phase transitions to the NaCl type of structure [47]. 

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... 

The term inert pair is often used for the tendency of the 6s2 electron pair to remain formally unoxidized in the compounds of Pb(n) [and also in the case of T1(I) and Bi(m) etc.]. As discussed above, this tendency can be related to relativity. Figure 59 shows the relativistic and non-relativistic valence orbital energies for Sn and Pb. The relativistic increase of the s-p gap leads to a 6s2 inert pair in the case of Pb. However, the situation is more complex if the local geometry at the heavy atom (Pb) is discussed. There are examples for both, stereochemically inactive and stereochemically active s2 lone pairs. 

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 ... 

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 methyl hydrogens of toluene are relatively inert to the attack of these charged particles. In contrast, the polarizable 7t-electron system of the aromatic nucleus can be easily perturbed by the approach of the bromine cation. In a simplified form, their interaction can be described in the sequence of the following steps. The attacking cation pulls an electron pair of the aromatic system towards itself to form a C-Br bond. A concerted shift of the next electron pair leads to the development of a positive charge on the methylbearing para-carbon atom. Loss of a proton from this c-complex 13 leads to the formation of bromotoluene with restoration of the aromaticity in the system. Besides the para isomer of bromotoluene 8, the corresponding ortho isomer is also formed, but in lesser amounts. 

Thus, the main relativistic effects are (1) the radical contraction and energetic stabilization of the s and p orbitals which in turn induce the radial expansion and energetic destabilization of the outer d and f orbitals, and (2) the well-known spin-orbit splitting. These effects will be pronounced upon going from As to Sb to Bi. Associated with effect (1), it is interesting to note that the Bi atom has a tendency to form compounds in which Bi is trivalent with the 6s 6p valence configuration. For this tendency of the 6s electron pair to remain formally unoxidized in bismuth compounds (i.e. core-like nature of the 6s electrons), the term inert pair effect or nonhybridization effect has been often used for a reasonable explanation. In this context, the relatively inert 4s pair of the As atom (compared with the 5s pair of Sb) may be ascribed to the stabilization due to the d-block contraction , rather than effect (1) . On the other hand, effect (2) plays an important role in the electronic and spectroscopic properties of atoms and molecules especially in the open-shell states. It not only splits the electronic states but also mixes the states which would not mix in the absence of spin-orbit interaction. As an example, it was calculated that even the ground state ( 2 " ) of Bij is 25% contaminated by Hg. In the Pauli Hamiltonian approximation there is one more relativistic effect called the Dawin term. This will tend to counteract partially the mass-velocity effect. 

The oxidation number of A is two less than the group valence , so that there is a lone pair of electrons in addition to the six bonding pairs. The structure of the IFg ion is not yet known. Careful studies of the crystal structures of (NH4)4(Sb Br6) (Sb Brg) and of (NH4)2TeCl6 and KjTeBrg show that the ions under discussion form undistorted octahedra in spite of the presence of the seventh electron pair. Thus the latter does not occupy a bond position but is a stereochemically inert pair. 

Second, the spatial requirement of "inert" electron pairs is easily deduced from the table, namely from the entries of In+/In +, T1+/T1 +, Sn +/Sn +, Pb +/Pb +, and Sb +/Sb +, being of the order of 10 cm /mol, about the size of the hydrogen anion, H. Later studies have shown that the latter anion, a strongly polarizable ion depending on the binding partner, also varies greatly in its incremental volume [34]. 

The properties and behavior of p-block metals are less homogenous than those of the s-metals because of their atomic and ionic sizes and other characteristics such as their unusual crystalline structures and the presence of the pair of electrons on the s orbital (s ) of the outermost electron shell. These s orbital electrons do not participate in chemical interactions (covalent or ionic) (inert electron pair). This influence increases with atomic number within a group. 

Metals have positive oxidation numbers (Table 3.1). Generally, the representative metals have oxidation numbers equal to the group A number. The metals from the p-block can have a second oxidation number, two units smaller than the group A number. In the case of these elements (e.g., Tl, Sn, and Pb) the stability of the highest oxidation state decreases down a group due to the ns inert electron pair. The penetration effect is greater for ns orbitals than np orbitals. As a consequence, the ns electrons are more attracted to the nucleus and more inert. 

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