Stacking vs. CH–π interactions between chelate and aryl rings in crystal structures of square-planar transition metal complexes

The crystal structures of square-planar transition-metal complexes from the Cambridge Structural Database (CSD) with close contacts between planar chelate rings and aryl rings containing six carbon atoms (C6-aryl) were analyzed. Most of the chelate rings in these structures are fused with aromatic or other π-delocalized chelate rings. The results show that planar chelate rings can be involved in stacking and CH–π interactions with organic aryl rings. However, the number of stacking interactions is a few times larger than the number of CH–π interactions. The analysis also shows that in almost all cases CH–π interactions are formed only when stacking interactions are prevented by voluminous substituents. Hence, between planar chelate rings and C6-aryl rings stacking interactions are preferred to CH–π interactions.