Meisenheimer complexes from carbanions

In crystals of salts of oxygen-containing carbanions, such as [(OgN)2 C—C(N02)2]2, (2) (3), or the Meisenheimer complexes [C6H2(N02)3 (OR)(OR )]- (41), (where R and R are alkyl groups), the alkali metal is found to have oxygen neighbours from different anions there are no short M—C contacts. 

The SnAR mechanism is an addition-elimination mechanism that proceeds through a carbanion with delocalized electrons (a Meisenheimer complex). The following steps show the mechanism for the formation of p-nitrophenol from p-nitroiodobenzene. 

The nucleophile attacks the carbon bearing the leaving group from a trajectory that is nearly perpendicular to the aromatic ring. (Recall from Section 9.5 that groups caimot be displaced from sp carbon atoms by back-side attack.) Nucleophilic attack forms a resonance-stabilized carbanion intermediate called a Meisenheimer complex. 

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