Benzonitrile, resonance structures

In the nitrobenzonitriles, such resonant structures will be mutually suppressed and consequently no large deviations from the additivity rule occur, such as exist when the two groups are of opposite charge so that the resonance is augmented by additional valence bond structures. The dipole moment of />-nitrobenzonitrile is zero, of o-nitrobenzonitrile, 6 19, D and m-nitro-benzonitrile, 3-78, D. Similar behaviour is observed with the di-substituted halogen derivatives of benzene and with the phenylenediamines. 

A cyano group is similar to a carbonyl for analogous reasons involving contributing resonance structures of the type shown for benzonitrile. 

Chromium (and tungsten) imino carbene complexes 113 are found to be synthetic equivalents for nitrile ylides on the basis of the resonance structure (Scheme 5.23) [34]. The reaction of the imino carbene complex 113 with alkyne 114 affords pyrrole 115. Imidazoles 116 and 120, pyrrolidines 118, and oxazoline 119 are also obtained by the [3+2] cycloaddition of the imino carbene complexes 113 with benzonitrile, alkenes 117, and benzaldehyde, respectively. Although the regiochemical outcome of these reactions is unpredictable, the regioselectivity can be correlated with the resonance structure in the case of the reaction with 3-hexyn-2-one. 

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