A geometrical analysis has been performed on [small pi]-[small pi] stacking in metal complexes with aromatic nitrogen-containing ligands based on a Cambridge Structural Database search and on X-ray data of examples in the recent literature. It is evident that a face-to-face [small pi]-[small pi] alignment where most of the ring-plane area overlaps is a rare phenomenon. The usual [small pi] interaction is an offset or slipped stacking, i.e. the rings are parallel displaced. The ring normal and the vector between the ring centroids form an angle of about 20[degree] up to centroid-centroid distances of 3.8 A. Such a parallel-displaced structure also has a contribution from [small pi]-[sigma] attraction, the more so with increasing offset. Only a limited number of structures with a near to perfect facial alignment exists. The term [small pi]-[small pi] stacking is occasionally used even when there is no substantial overlap of the [small pi]-ring planes. There is a number of metal-ligand complexes where only the edges of the rings interact in what would be better described a C-H [small pi] attraction.
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