Abstract: | In the crystal structures of the conformational isomers hydrogen {phosphono(pyridin‐1‐ium‐3‐yl)amino]methyl}phosphonate monohydrate (pro‐E), C6H10N2O6P2·H2O, (Ia), and hydrogen {phosphono(pyridin‐1‐ium‐3‐yl)amino]methyl}phosphonate (pro‐Z), C6H10N2O6P2, (Ib), the related hydrogen {(2‐chloropyridin‐1‐ium‐3‐yl)amino](phosphono)methyl}phosphonate (pro‐E), C6H9ClN2O6P2, (II), and the salt bis(6‐chloropyridin‐3‐aminium) hydrogen bis({2‐chloropyridin‐1‐ium‐3‐yl(0.5+)]amino}methylenediphosphonate)] (pro‐Z), 2C5H6ClN2+·C12H16Cl2N4O12P42?, (III), chain–chain interactions involving phosphono (–PO3H2) and phosphonate (–PO3H?) groups are dominant in determining the crystal packing. The crystals of (Ia) and (III) comprise similar ribbons, which are held together by N—H...O interactions, by water‐ or cation‐mediated contacts, and by π–π interactions between the aromatic rings of adjacent zwitterions in (Ia), and those of the cations and anions in (III). The crystals of (Ib) and (II) have a layered architecture: the former exhibits highly corrugated monolayers perpendicular to the 100] direction, while in the latter, flat bilayers parallel to the (001) plane are formed. In both (Ib) and (II), the interlayer contacts are realised through N—H...O hydrogen bonds and weak C—H...O interactions involving aromatic C atoms. |