A critical account on pi-pi stacking in metal complexes with aromatic nitrogen-containing ligands

A geometrical analysis has been performed on pi-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 pi-pi alignment where most of the ring-plane area overlaps is a rare phenomenon. The usual 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 degrees up to centroid-centroid distances of 3.8 Angstrom. Such a parallel-displaced structure also has a contribution from 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 pi-pi stacking is occasionally used even when there is no substantial overlap of the 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 . . . pi attraction.