N-H...X (X = Cl and Br) hydrogen bonds in three isomorphous 3,5-dichloropyridinium salts.

Specific short contacts are important in crystal engineering. Hydrogen bonds have been particularly successful and together with halogen bonds can be useful for assembling small molecules or ions into crystals. The ionic constituents in the isomorphous 3,5-dichloropyridinium (3,5-diClPy) tetrahalometallates 3,5-dichloropyridinium tetrachloridozincate(II), (CHClN)[ZnCl] or (3,5-diClPy)ZnCl, 3,5-dichloropyridinium tetrabromidozincate(II), (CHClN)[ZnBr] or (3,5-diClPy)ZnBr, and 3,5-dichloropyridinium tetrabromidocobaltate(II), (CHClN)[CoBr] or (3,5-diClPy)CoBr, arrange according to favourable electrostatic interactions. Cations are preferably surrounded by anions and vice versa; rare cation-cation contacts are associated with an antiparallel dipole orientation. N-H...X (X = Cl and Br) hydrogen bonds and X...X halogen bonds compete as closest contacts between neighbouring residues. The former dominate in the title compounds; the four symmetrically independent pyridinium N-H groups in each compound act as donors in charge-assisted hydrogen bonds, with halogen ligands and the tetrahedral metallate anions as acceptors. The M-X coordinative bonds in the latter are significantly longer if the halide ligand is engaged in a classical X...H-N hydrogen bond. In all three solids, triangular halogen-bond interactions are observed. They might contribute to the stabilization of the structures, but even the shortest interhalogen contacts are only slightly shorter than the sum of the van der Waals radii.