The first systematic study of π interactions between non-aromatic rings, based on the authors' own results from an experimental X-ray charge-density analysis assisted by quantum chemical calculations, is presented. The landmark (non-aromatic) examples include quinoid rings, planar radicals and metal-chelate rings. The results can be summarized as: (i) non-aromatic planar polyenic rings can be stacked, (ii) interactions are more pronounced between systems or rings with little or no π-electron delocalization (e.g.quinones) than those involving delocalized systems (e.g.aromatics), and (iii) the main component of the interaction is electrostatic/multipolar between closed-shell rings, whereas (iv) interactions between radicals involve a significant covalent contribution (multicentric bonding). Thus, stacking covers a wide range of interactions and energies, ranging from weak dispersion to unlocalized two-electron multicentric covalent bonding (`pancake bonding'), allowing a face-to-face stacking arrangement in some chemical species (quinone anions). The predominant interaction in a particular stacked system modulates the physical properties and defines a strategy for crystal engineering of functional materials.

A series of four novel mononuclear complexes of copper(II) with chloranilic acid and 2,2′-bipyridine were prepared and structurally characterised by X-ray structure analysis and IR spectroscopy. The complexes exhibit square-planar, square-pyramidal and octahedral coordination. The chloranilate dianion coordinates the Cu^II atom in a terminal bidentate o-quinone-like mode forming a mononuclear complex species. The crystal packing of the aqua complex and the complexes with crystal water molecules are defined by hydrogen bonds. However, significant contributions are from interactions involving five-membered chelate rings with both types of ligands and intermolecular π-systems (bipyridine and chloranilate rings). The crystal packing of the complex with square planar Cu^II coordination is dominated by interactions of the chelate rings with π-delocalised bonds and intermolecular π-systems. The occurrence of dimorphism could be related to the different types of these interactions in the stacks. The crystal packing of the octahedral complexes of the mono- and dihydrate solvates reveal 3D-networks with pores occupied by non-coordinated bipyridine molecules.