Supramolecular phenomena are determined by energies, but a different property, geometry, is commonly measured, and abundant data are available. The sum of van der Waals radii, the conventional distance criterion in assessing geometrical data, needs to be related to the relevant intermolecular potential, with cognisance of the chemistry of the molecular surface. The relationship between distance distributions in crystals and the distance scale of the intermolecular potential is evaluated, and the relevant concepts (and mis-concepts) are elaborated. The position of the minimum in the intermolecular potential, that is the most stabilising distance, is ca. 0.4 Å larger than the van der Waals distance. In crystals a small number of destabilising contacts less than the van der Waals distance can be enforced by a much larger number of longer stabilising distances. Crystal packing analyses with cut-offs at the van der Waals distance are likely to miss key supramolecular features.

The 1,10-phenanthroline (phen) ligand in metal complexes commonly forms offset face-to-face (OFF) motifs, and less frequently edge-to-face (EF) motifs. An investigation of the 335 M(phen) complexes, 159 M(phen)₂ complexes, and 33 M(phen)₃ complexes in the Cambridge Structural Database has revealed that in crystals these primary OFF and EF motifs combine to form concerted motifs, and extended motifs, in a variety of ways. Stacks of phen ligands engaging OFF motifs on both faces are very common for complexes M(phen) and M(phen)₂. Even more common for M(phen)₂ are zigzag chains in which each phen ligand links to neighbours in the chain with an OFF motif. The parallel fourfold aryl embrace (P4AE, comprised of one OFF and two EF) occurs for complexes M(phen)₂ and M(phen)₃, with some variety in geometry between a single OFF at one extreme to (EF)₂ at the other. This variability in the P4AE is a consequence of the larger surface area of the phen ligand, compared with those of 2,2′-bipyridyl (bipy) ligands or phenyl groups (which also form this motif), and has been evaluated by calculations of the supramolecular attractive energies. The P4AE associate further, maximising the use of phen surfaces, to form chains of P4AE, chains of P4AE·OFF, and compact two-dimensional nets propagated by both P4AE and OFF motifs. There are examples of three-dimensional nets using these motifs. There is a notable absence of sixfold aryl embraces (6AE, comprised of concerted (EF)₆) amongst M(phen)₃ complexes, and a clear difference with M(bipy)₃ complexes where 6AEs are prevalent. Various M(phen)₂ and M(phen)₃ complexes pack in crystals to form tight hydrophobic domains, often as slabs, segregated from hydrophilic domains containing hydrogen bonding components and anions. The crystallisation and crystal packing of [Co(phen)₃][BF₄]₂·H₂O·EtOH, which exemplifies this pattern, are reported. The implications for crystal engineered enantioselection by [M(phen)₃] complexes are discussed.

We evaluate prospects for the crystal engineering of anionic halocuprate complexes [Cu_nX_m]^z−, X = Cl, Br, I, crystallised with polyatomic cations that possess alkyl or aryl coated surfaces and do not form hydrogen bonds. In solution, halocuprate complexes are involved in kinetically fast dissociative and associative equilibria, as well as redox variation between Cu(I), mixed Cu(I)/Cu(II) and Cu(II), and so the species that crystallise need not be those that predominate in solution: we describe instances of this complication, and the trapping of complexes which are not evident in solution. Our survey includes the relationships between cation properties and the identity and structure of the crystallised halocuprate complex, and the occurrence of polymorphism and de-facto polymorphism. A key aspect is the packing arrangement of anions and cations in these crystals: we illustrate the primary influence of electrostatic energies in determination of crystal packing, and the secondary but significant influences of cation··cation motifs and embraces in controlling details of crystal structure. In the context of crystal engineering, for which fundamental understanding and control of the crystallisation process and of the crystal structure are prerequisites, we conclude that even a chemically simple coordination system such as [Cu_nX_m]^z− needs additional knowledge and physico-chemical insight before there can be confident design and reliable fabrication of desired crystals. Progress will be aided by the publication (in crystal structure papers) of more information about crystallising solutions and the effects of crystallisation variables, as well as ability to repeat crystallisation.

We review the main types of intermolecular embrace motifs adopted by molecules with arylated surfaces, and draw attention to some recent interesting developments.

The atom-centred FeMo cofactor of nitrogenase most likely contains N, resting at the [NMoFe₇]¹⁸⁺ redox level, inserted from N₂, and subsequently restricting the modes of binding of substrate to the NFe₆ core.