Nitrile ylide

Nitrile ylides also known as nitrilium ylides or nitrilium methylides, are generally reactive intermediates formally consisting of a carbanion of an alkyl or similar group bonded to the nitrogen atom of a cyanide unit. With a few exceptions, they cannot be isolated. However, a structure has been determined on a particularly stable nitrile ylide by X-ray crystallography. Another nitrile ylide has been captured under cryogenic conditions.

As ylides, they possess a negative charge and a positive charge on adjacent atoms. However, they also have resonance, including 1,3-dipole contributing structures:

The most appropriate resonance structure is dependent upon the substituent pattern (the identity of the R and R′ groups). The 3-dimensional structure of the nitrilium ylide itself may also provide a clue as to the most appropriate resonance structure, with a linear R–C≡N–C unit supportive of the charge distribution indicated for resonance structures 1a & 1b and also consistent with the nomenclature ''nitrilium yl**ide'''''. As resonance structures 1c and 1d become more important the nitrilium ylide distorts its geometry from linear in favor of a different valence tautomer **2''' that is distinctly bent:

Nitrile ylides are isoelectronic with nitrile oxides: \ce{R-C#}\overset{\oplus}\ce{N}\ce{-O^\ominus}