Stereochemical inert pair 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. 

For both of these complexes the lone-pair electrons are not stereochemically active with respect to the polyhedral coordination, but stabilize the 6s and 5s level, respectively (inert pair effect). The interactions between metals and arenes are rather weak but significantly influence the coordination sphere in comparison to BiCl3 or SbCl3. 

If the presence of a lone pair of electrons influences the shape of a molecule or ion, the lone pair is stereochemically active. If it has no effect, the lone pair is stereochemically inactive. The tendency for the pair of valence s electrons to adopt a non-bonding role in a molecule or ion is termed the stereochemical inert pair effect. 

Stereochemical inert pair effect mentioned in Section 2.8. Similar effects are seen for Pb (group 14) and Bi (group 15), for which the most stable oxidation states are -1-2 and - -3 respectively, rather than - -4 and - -5. The inclusion in Table 13.1 of ° values for the and M /M... 

The preference for the +2 over +4 oxidation state increases down the group, the change being due to relativistic effects that make an important contribution to the inert pair. The inert pair concept holds only for the lead ion Pb2+(aq), which could have a 6s2 configuration. In more covalent Pb11 compounds and most Sn11 compounds there are stereochemically active lone pairs. In some MX2 (M = Ge or Sn) compounds Ge and Sn can act as donor ligands. 

Another commonly observed decomposition pathway involves C-H bond activation. For example, the reaction of (l,3-But2Gp)2ZrMe2 with B(C6F5)3 yields a C-H activated 77s,771- tuck-in cation 710 (Scheme 173), which is inert with respect to ethylene polymerization. There are also significant metal-alkyl group effects on the thermodynamic stability and stereochemical mobility of the B(C6F5)3-derived Zr and Hf metallocenium ion pairs.538... 

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