Analysis of geometrical parameters in the crystal structure of square-planar complexes, with and without chelate rings, of all transition metals from Cambridge Structural Database shows that there are stacking interactions between the phenyl ring and the chelate ring with delocalized π-bonds. Short distances between the metal and carbon atom of the phenyl ring and parallel orientation of phenyl ring with respect to the coordination plane were found in the crystal strucutres of complexes with chelate rings. There is a correlation of the distances between the centers of chelate and phenyl rings and the metal-carbon distances showing that the chelate and phenyl rings tend to overlap. In the crystal structures, there are mutual slipped-parallel orientations of the phenyl and chelate rings. The data show that the geometry of the stacking interaction between phenyl and chelate rings is similar to the geometry of the stacking interaction of two benzene rings, indicating that chelate rings can behave similarly to organic aromatic rings.

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Analysis of the geometrical parameters in crystal structures of square-planar copper(II) complexes from the CSD shows that the short noncovalent copper(II)−phenyl carbon distances are a consequence of the interaction between the phenyl ring and the chelate ring with delocalized π-bonds.

Evidence of chelate−chelate stacking interactions was obtained by analyzing crystal structures of square-planar transition-metal complexes from the Cambridge Structural Database. The analysis showed that chelate−chelate stacking interactions occur in a large number of the crystal structures of neutral square-planar complexes. We found 955 structures with 1866 chelate−chelate stacking interactions. In most of the structures, chelate rings are fused with other chelate rings or with organic aromatic rings; however, chelate−chelate stacking interactions of isolated rings were also found. In order to describe the geometry of the chelate−chelate stacking interactions, several geometric parameters were analyzed. In most of the interactions, two chelate rings are in parallel or antiparallel orientation. The normal distances are similar to the distances of stacking interactions observed in other systems, while the offset of two interacting chelate rings can be different from that observed in other systems.

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In the crystal structure of the bis(acetone‐1‐naphthoylhydrazinato)copper(II) complex there are interactions of neighbouring molecules via naphthyl groups; the naphthyl group of one molecule interacts with the copper centre and CH3 group of another molecule. The geometry of the crystal structure and dispositions of charges in the naphthyl and chelate rings indicate that there are stacking interactions between the aromatic ring and, not only the copper atom, but also the whole chelate ring. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

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