Evidence of Chelate−Chelate Stacking Interactions in Crystal Structures of Transition-Metal Complexes

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.