Stereochemically inactive electron pair

Cubane a common geometry adopted by many inorganic cluster complexes of formula A4B4 where A and B represent two atom types which occupy alternating vertices of a cube. These atoms may have ligands or other functionalities attached Stereochemcially active lone pair of electrons a complex whose observed structure includes a coordination site which is occupied by a lone pair of electrons. Conversely, a complex which contains a stereochemically inactive lone pair of electrons is one in which the geometry does not show a specific vacant site where the electron pair should reside... 

To what point group do the (ShBr ) and (ShBr p ions belong if they possess regular octahedral structures Explain why one of these ions possesses a stereochemically inactive lone pair of electrons, while the other ion has no Sh-centred lone pair. 

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. 

In the vast majority of cases in which six coordination is observed, the bonding can be viewed as arising from the interaction of all three cr -orbitals with a halide anion, i.e., all three in S. Because the three orbitals are all trans to the primary E-X bonds, such a situation leads naturally to octahedral coordination. Moreover, in cases in which the primary and secondary bonds are the same length, i.e., where A = 0 and a three-center, four-electron bonding model is appropriate, a regular octahedron is the result. Such a structure is clearly at odds with simple VSEPR theory, which is predicated on the lone pair(s) occupying specific stereochemical sites, but stereochemical inactivity of the lone pair tends to be the rule rather than the exception in six-coordinate, seven-electron pair systems Ng and Zuckerman (102) have reviewed this topic for p-block compounds in general. 

Octahedral EX ions of Selv and Te1 with Cl" and Br have long been known and have been of interest because of their octahedral structures despite the presence of a valence shell electron pair. The stereochemical inactivity of this pair of electrons indicates that they occupy the valence shell ns orbital. These EXj- complexes are generally obtained by saturating aqueous solutions of E02 in KX with HX, and adding appropriate large cations. However, the equilibria in these solutions are very complex and species such as SeOBrJ and TeCL OH)" also form. An interesting case where the lone pair is stereochemically active is the (/ octahedral TeFf ion in BaTe2F10.33... 

Figure 2 Examples of various geometries based on VSEPR rules. Also given is the N-X-L notation (Section 2.3). Note that the lone pair of electrons in SeCle is in a stereochemically inactive 4s orbital...

It is interesting to note that the coordination environment in the halidotellurate anions is an almost regular octahedron in apparent violation of the VSEPR rules for a 14 electron AXgE system. It has been suggested that the lone electron pair of tellurium resides in the stereochemically inactive 6s orbital and the six Te-X bonds form three pairs of 3c-4e bonds involving the three 6p orbitals (for a more detailed discussion, see Ref 81). 

Co4(CO) 12 may be described in exactly the same way, except that the site occupied in the iron compound by a face-bridging CO group, and thus by four metal and two ligand electrons, is now to be occupied by (symmetry-correct combinations of) three cobalt lone pairs. The basal cobalt atoms are five-coordinate (if as usual we discount bridged metal-metal bonds) with the sixth site of the coordination octahedron occupied by a lone pair and the 18-electron count achieved by straight, stereochemically inactive cobalt-cobalt bonds. This is exactly the situation required by the straight bond description of Co2(CO)g (H(8) above). 

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. 

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