At the molecular level, the enantiomerically pure square-planar organoplatinum complex (SP-4-4)-(R)-[2-(1-aminoethyl)-5-fluorophenyl-κ²C¹,N][(R)-1-(4-fluorophenyl)ethylamine-κN](isocyanato-κN)platinum(II), [Pt(C₈H₉FN)(NCO)(C₈H₁₀FN)], and its congener without fluorine substituents on the aryl rings adopt the same structure within error. The similarities between the compounds extend to the most relevant intermolecular interactions, i.e. N—H…O and N—H…N hydrogen bonds link neighbouring molecules into chains along the shortest lattice parameter in each structure. Differences between the crystal structures of the fluoro-substituted and parent complex become obvious with respect to secondary interactions perpendicular to the classical hydrogen bonds; the fluorinated compound features short C—H…F contacts with an F…H distance of ca 2.6 Å. The fluorine substitution is also reflected in reduced backbonding from the metal cation to the isocyanate ligand.

Polymorphism is a challenging phenomenon and the competitive packing alternatives which are characteristic for polymorphs may be encountered for essentially rigid molecules. A second crystal form of the well known compound o-nitrobenzoic acid, C₇H₅NO₄, an important intermediate in the production of dyes, pharmaceuticals and agrochemicals, is described. Although obtained serendipitously, its intra- and intermolecular features match expectations from database searches and theoretical calculations. O—H…O hydrogen-bonded carboxylic acid dimers represent the building blocks in both polymorphs. For steric reasons and in agreement with a calculated potential energy surface, the carboxylic acid and nitro groups cannot simultaneously be coplanar with the benzene ring but have to tilt. In the well established crystal form, this out-of-plane torsion is more pronounced for the nitro substituent. In contrast, the new polymorph is characterized by a major tilt of the carboxylic acid group. The molecules in both alternative crystal forms achieve a similar compromise with respect to acceptable intramolecular O…O contacts.

In the solid obtained from N-(6-methylpyridin-2-yl)mesitylenesulfonamide and acetic acid, the constituents interact via two N—H...O hydrogen bonds. The H atom situated in one of these short contacts is disordered over two positions: one of these positions is formally associated with an adduct of the neutral sulfonamide molecule and the neutral acetic acid molecule, and corresponds to a cocrystal, while the alternative site is associated with salt formation between a protonated sulfonamide molecule and deprotonated acetic acid molecule. Site-occupancy refinements and electron densities from difference Fourier maps suggest a trend with temperature, albeit of limited significance; the cocrystal is more relevant at 100 K, whereas the intensity data collected at room temperature match the description as cocrystal and salt equally well.

Reactions of calcium bromide with enantiopure and racemic proline in aqueous solution lead to two solids in which the zwitterionic amino acid acts as a bridging ligand between neighbouring cations. Depending on the chirality of the amino acid, topologically very different products are obtained. With racemic proline, bromide acts as a simple uncoordinated counter-anion for the cationic heterochiral chains in catena-poly[[aquacalcium(II)]-μ-aqua-μ₃-DL-proline-μ₂-DL-proline], {[Ca(C₅H₉NO₂)₂(H₂O)₂]Br₂}_n. In agreement with chemical intuition, only carboxylate and aqua O atoms coordinate the alkaline earth cation in a low-symmetry arrangement. In contrast, L-proline affords the two-dimensional network poly[dibromidobis(μ₂-L-proline)calcium(II)], [CaBr₂(C₅H₉NO₂)₂]_n, with an unexpected CaBr₂ unit in a more regular coordination sphere.

In catena-poly[copper(II)-di-μ-chlorido-μ-proline-κ²O:O′], [CuCl₂(C₅H₉NO₂)]_n, two symmetry-independent metal cations adopt distorted octahedral coordination, typical for d⁹ Jahn–Teller systems. Each chloride bridge is involved in both a short and a very long interaction with a Cu^II centre. The centrosymmetric crystal structure contains homochiral chains of opposite handedness which extend along the shortest lattice parameter (i.e. a). The O:O′-bridging coordination mode of proline, although a common motif for such complexes in general, is remarkable for Cu^II; the vast majority of amino acid derivatives of this cation are characterized by N,O-chelation.

Hydrogen bonds are considered a powerful organizing force in designing supramolecular architectures because they are directional, selective and reversible at room temperature. trans-Dithiocyanatotetrakis(4-vinylpyridine)nickel(II) is a popular host for the inclusion of small molecules and 2,3,5,6-tetrafluoro-1,4-diiodobenzene (TFDIB) represents a strong halogen-bond donor. These constituents cocrystallize in a 1:1 stoichiometry, [Ni(NCS)₂(C₇H₇N)₄]·C₆F₄I₂, in the tetragonal space group I4₁/a. Both residues occupy special positions, i.e. the pseudo-octahedral Ni^II complex is located on a twofold axis and the TFDIB molecule sits about a crystallographic centre of inversion. The components interact via a short S...I contact of 3.2891 (12) Å between the thiocyanate S atom of the host and the iodine substituent at the perhalogenated aromatic ring of the smaller guest molecule. This interaction meets the commonly accepted criteria for a halogen bond. Such halogen bonds to sulfur are significantly less common than to smaller electronegative atoms.

3-(4-Pyridyl)-acetylacetone (HacacPy) acts as a pyridine-type ligand towards CdX₂ (X = Cl, Br, I). Chain polymers with six-coordinated metal cations are obtained from CdCl₂ and with alternating five- and six-coordinated Cd centers from CdBr₂. In either case, the formation of these compounds does not depend on the precise stoichiometry. In contrast, two different reaction products form with the heavier congener CdI₂, namely a ligand-rich molecular complex CdI₂(HacacPy)₂ and a ligand-deficient one-dimensional polymer [CdI₂(HacacPy)]¹_∞. Interconversion between these two iodo derivatives is possible via thermal degradation and mechanochemical synthesis. The acetylacetone moiety in HacacPy may be deprotonated and chelated to Fe^III, and the resulting complex Fe(acacPy)₃ reacts analogously to a bridging polypyridine ligand towards the same Cd halides as the molecule HacacPy itself. With CdCl₂ and CdBr₂, isomorphous chain polymers are obtained in which the Cd cations adopt distorted octahedral coordination and one of the peripheric pyridyl groups remains uncoordinated. With CdI₂, the iron complex acts as a -Fe(acacPy)₃ bridge between tetrahedral Cd centers and gives rise to a ladder structure.

Under accelerated and controlled evaporation, chain polymers crystallize from aqueous solutions of Ca^II and Mn^II halides with enantiopure L-alanine or racemic DL-alanine. In all ten solids thus obtained zwitterionic amino acid ligands bridge neighbouring cations. The exclusively O-donor-based coordination sphere around the metal cations is completed by aqua ligands; the halides remain uncoordinated and act as counter-anions for the cationic strands. Despite the differences in ionic radii and electronic structure between the main group and the transition metal cation, their derivatives with L-alanine share a common structure type. In contrast, the solids derived from DL-alanine differ and adopt structures depending on the metal cation and the halide. Homochiral chains of either chirality or heterochiral chains with different arrangements of crystallographic inversion centres along the polymer strands are encountered. On average, the six-coordinated Ca^II cations, devoid of any ligand field effect, show more pronounced deviation from idealized octahedral geometry than the d-block cation Mn^II.