Anion delocalization

If a pair of cations is separated by two anions, delocalization superexchange is still possible, though very much weakened, since the cation d orbitals spread out over the entire anion with which they form a partial covalent bond. Correlation superexchange is also weakened by the anion-anion correlation factor. Polarization super-exchange may be competitive in this case, but it is cooperative. It follows that the rules for the sign of 180° cation-anion-anion-cation interactions are the same as those for 180° cation-anion-cation interactions, but the magnitude is reduced, probably by an order of magnitude. 

We get the full value when there are no nitro groups to take the brunt of the negative charge. This vinylic substitution (an unusual reaction ) has a p value of +9.0. It cannot be an S>j2 reaction or it would have a small p value and it cannot be an S>j 1 reaction or it would have a negative p value (fewer electrons in the transition state). It must be an addition-elimination mechanism through a benzylic anion delocalized round both benzene rings. 

We can draw five equivalent resonance structures for the cyclopentadienyl anion, delocalizing the negative charge over every carbon atom of the ring. 

What is the nature of this intermediate Well, in essence it is an anion delocalized over five sp2 hybridized carbons of a six-membered ring (the sixth, the point at which the nucleophile attacked, is sp hybridized). It s possible to make a simple homologue of such a species by deprotonating cyclohexadiene. Delocalizing the anion generates the three structures below. 

Other delocalization properties which lead to a high / factor are delocalization of the radical anion over a few identical C-X linkages as in CH4, CH2CI2, and so on [4b, 32b, 46]. Delocalization of the odd electron in X , as for example in PhCH2 leads to a large / factor. The relationship between the size of / and the extent of radical anion delocalization has been retrieved recently with the ab initio VB computations df the curve crossing diagram [12 a] for H + CH4, and the results are discussed later in this chapter. 

In this series of anions delocalization of charge is produced by extended p-d TT bonding. 

---