Are Chelate Rings Aromatic? Calculations of Magnetic Properties of Acetylacetonato and o-Benzoquinonediimine Chelate Rings

The aromaticity of the chelate rings of acetylacetonato (acac) and o-benzoquinonediimine (bqdi) ligands was investigated theoretically by calculating nucleus-independent chemical shifts (NICS). The calculations were done for the complexes with various metals and various other ligands. The results show that acac chelate rings in none of the complexes satisfy this magnetic criterion for aromaticity. According to the results for bqdi chelate rings, there is only the Ru2+−bqdi chelate ring with large negative NICS values, indicating possible aromaticity by magnetic criterion.