Key indicators: single-crystal X-ray study; T = 173 K; mean (C-C) = 0.004 Å; R factor = 0.037; wR factor = 0.081; data-to-parameter ratio = 12.3.The title compound, [Zn(C 2 H 8 NO 6 P 2 ) 2 (H 2 O) 4 ]Á4H 2 O, was synthesized by the reaction of ZnCl 2 with 1-aminoethane-1,1-diyldiphosphonic acid in aqueous solution. The asymmetric unit contains one-half of the complex and two water molecules of solvation. The Zn atom occupies a special position on an inversion centre. This results in a slightly distorted octahedral coordination environment, which consists of the O atoms from two phosphonic acids and four water molecules. The crystal structure displays N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonding, which creates a three-dimensional network. Related literature

The asymmetric unit of the second polymorph of the title compound, C2H9NO6P2, contains one mol­ecule existing as a zwitterion. The N atom of the ammonio group is protonated and one of the phospho­nic acid groups is deprotonated. Bond lengths and angles are similar in both polymorphs. Besides the differences in cell parameters, the most significant structural difference between this structure and that of the first polymorph [Dudko, Bon, Kozachkova, Tsarik & Pekhno (2008 ▶), Ukr. Khim. Zh. 74, 104–106] is the presence of strong symmetric hydrogen bonds between neighbouring phospho­nate groups. H atoms involved in these hydrogen bonds are located at inversion centres and O⋯O distances are observed in the range 2.458 (5)–2.523 (5) Å. These bonds and additional O—H⋯O and N—H⋯O hydrogen bonds inter­link the mol­ecules, giving a three-dimensional supromolecular network.

The title compound, NH4 +·C2H8NO6P2 −·2H2O, was obtained by the reaction between 1-amino­ethane-1,1-diyldiphospho­nic acid and ammonium hydroxide (1:1) in an aqueous solution. The asymmetric unit contains one anion with two H atoms transferred from the phospho­nic acid groups to the amino group of the anion and to an ammonia mol­ecule, giving an ammonium cation. The structure displays N—H⋯O and O—H⋯O hydrogen bonding, which creates a three-dimensional network.

In the title compound, [Co(C2H8NO6P2)2(H2O)2]·9H2O, the CoII atom has a slightly distorted octa­hedral coordination environment consisting of four deprotonated phospho­nate O atoms of two independent 1-amino­ethyl­idendiphospho­nate anions and complemented by the O atoms of two water mol­ecules in cis positions. The anions exists in the zwitterionic form (protonated amino group and two deprotonated phospho­nate O atoms) and constitute two six-membered chelate rings. The crystal structure also contains nine partly disordered uncoordinated water mol­ecules, which create an extensive three-dimensional network of strong O—H⋯O and N—H⋯O hydrogen bonds.

In the asymmetric unit of the title coordination polymer, {[CoNa(C10H12N2O8)(H2O)2]·2H2O}n, the CoII ion is coord­inated in a distorted octa­hedral environment, defined by two N atoms and four carboxyl­ate O atoms. Two CoII ions and two 2-({2-[(1,2-dicarboxyl­atoeth­yl)amino]­eth­yl}amino)­butane­dio­ate (EDDS) ligands form a dimeric complex dianion [Co2(EDDS)2]. These dimeric units are connected via Na+ ions, forming a three-dimensional polymeric structure. In the crystal, the ligand N—H groups and the coordinated and solvent water mol­ecules are involved in inter­molecular N—H⋯O and O—H⋯O hydrogen bonding, reinforcing the three-dimensional polymeric structure.