Aromaticity of the chelate rings. The case of bis(ethylenediamine)nickel(II)

- Chelate rings in Ni(II) complexes of several diamines show large negative NICS.
- Smaller the diamine, greater the aromaticity and more negative is NICS.
- The aromaticity is of σ type.
- Ease of formation of Curtis macrocyle is justified by its σ-aromaticity.
- 4n + 2 rule does not hold for σ-aromatic systems.

The aromaticity of the chelate rings in [Ni(en)2]2+, [Ni(pn)2]2+, [Ni(bn)2]2+, [Zn(en)2]2+ and [Pd(en)2]2+ (en = 1,2-diaminoethane, pn = 1,3-diaminopropane and bn = 1,4-diamino-butane) are studied by means of nucleus independent chemical shift (NICS) calculations. Comparing the data with those of benzene and cyclopropane, it is concluded that the chelate rings in [Ni(en)2]2+, [Ni(pn)2]2+ and [Ni(bn)2]2+ are σ-aromatic while the ones in [Zn(en)2]2+ and [Pd(en)2]2+ are non aromatic. The order of aromaticity is [Ni(en)2]2+ > [Ni(pn)2]2+ > [Ni(bn)2]2+. The trend is correlated with the ease of formation of Curtis macrocycle from bis diamine complexes of Ni(II). The aromaticity in [Zn(en)2]2+ is lost because of breaking of sigma conjugation at the metal. It seems that the sigma aromatic systems do not follow the 4n + 2 rule.

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