Stacking vs. CH-pi 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 (C-6- aryl) were analyzed. Most of the chelate rings in these structures are fused with aromatic or other pi-delocalized chelate rings. The results show that planar chelate rings can be involved in stacking and CH-pi interactions with organic aryl rings. However, the number of stacking interactions is a few times larger than the number of CH-pi interactions. The analysis also shows that in almost all cases CH-pi interactions are formed only when stacking interactions are prevented by voluminous substituents. Hence, between planar chelate rings and C-6- aryl rings stacking interactions are preferred to CH-pi interactions.