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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