[Mg(H2P2O6)(H2O)4] is the first alkaline earth hypodiphosphate to be structurally determined. It consists of (H2P2O6)2− anions that are bridged by Mg2+ cations into a chain structure. Water mol­ecules complete the octa­hedral coordination sphere of the metal and built up a three-dimensional hydrogen-bonding network.

The calcium salts Ca2P2O6·2H2O (1) and [Ca(H2O)3(H2P2O6)]·0.5(C12H24O6)·H2O (2) were prepared and structurally characterized by single‐crystal X‐ray diffraction. Compound 1 crystallizes in the orthorhombic space group Pbca and compound 2 in the monoclinic space group P21/n. The crystal structure of compound 1 consists of chains of edge‐sharing [CaO7] polyhedra linked by hypodiphosphate(IV) anions to form a three‐dimensional network. The crystal structure of compound 2 consists of alternated layers of crown ether and water molecules and respective ionic units. Within the layers of ionic units the Ca2+ cations are octahedrally coordinated by three monodentate dihydrogenhypodiphosphate(IV) anions and three water molecules. The IR/Raman spectra of the title compounds were recorded and interpreted, especially with respect to the [P2O6]4– and [H2P2O6]2– groups. The phase purity of 2 was verified by powder diffraction measurements.

Three polymorphs of barium dihydrogen‐hypodiphosphate(IV)‐dihydrate, BaH2P2O6·2H2O (A, B and C), were obtained and structurally characterized by single‐crystal X‐ray diffraction. A crystallizes in the monoclinic space group P21/n (no. 14) with a = 7.459(1) Å, b = 8.066(1) Å, c = 12.460(2) Å, β = 91.27(1) ° and Z = 4. B crystallizes in the monoclinic space group C2/c (no. 15) with a = 11.049(8) Å, b = 6.486(3) Å, c = 10.956(6) Å, β = 106.89(5) ° and Z = 4. C crystallizes in the orthorhombic space group C2221 (no. 20) with a = 9.193(3) Å, b = 6.199(2) Å, c = 12.888(4) Å and Z = 4. Discrete [H2P2O6]2– units, barium cations and water molecules, held together by intermolecular hydrogen bonds of the type O–H···O, build up the structures of the three polymorphs. The phase purity of A and C was verified by powder diffraction measurements.

Dedicated to Professor Thomas M. Klapötke on the Occasion of his 60th Birthday Two new alkaline-earth metal dihydrogenhypodiphosphates, AE[H 2 P 2 O 6 ] • n H 2 O (AE = Ca, Sr and n = 1.5, 2) have been prepared and structurally characterized by single crystal X-ray diffraction. Calciumdihydrogenhypodiphosphate sesquihydrate, Ca[H 2 P 2 O 6 ] • 1.5 H 2 O (1) (lattice parameters:in the orthorhombic space group Pbcn. The crystal structure of compound 1 consists of chains of edge-and face-sharing of [CaO 6 ] and [CaO 8 ] polyhedra linked by dihydrogenhypodiphosphate(IV) anions to form a three-dimensional network. The structure of 2 consists of alternating layers of chains of edge-sharing [SrO 8 ] polyhedra and respective anionic units. The different water-content compositions result from the corresponding alkaline-earth metal coordination polyedra and by the preferred number of water molecules in their coordination sphere, respectively. The IR/Raman spectra of the title compounds have been recorded and interpreted, especially with respect to the [H 2 P 2 O 6 ] 2À group. The phase purity of 2 was verified by powder diffraction measurements.

The cover picture shows a projection of the unit cell of [Ca(H2O)3(H2P2O6)] ∙ 0.5(C12H24O6) ∙ H2O along the c axis. The compound crystallizes in the monoclinic space group P21/n with four formula units in the unit cell. The asymmetric unit contains one [Ca(H2O)3(H2P2O6)] unit, one half of a 18‐crown‐6 molecule and one water molecule. The crystal structure consists of alternated layers of crown ether and water molecules and respective ionic units involved in an extended hydrogen‐bonding network. Within the layer of the ionic units, [Ca(H2O)3(H2P2O6)], each Ca2+ cation is bridged by three anions. The Ca2+ ion is octahedrally coordinated by O atoms of three [H2P2O6]2– anions and three water molecules. In the anionic unit, [H2P2O6]2–, the tetravalent phosphorus atoms are surrounded tetrahedrally by three oxygen atoms and one additional phosphorus atom with a P – P distance of 2.186(1) Å and P – O bond lengths from 1.493(2) to 1.581(1) Å. The discrete dihydrogen hypodiphosphate anion in staggered conformation for the P2O6 skeleton has the two H atoms in cis position and exhibits, therefore, C2 symmetry and is located at the center of inversion. The structure exhibits within the layer of the ionic units a hydrogen‐bonded network, in which the [H2P2O6]2– units are joined to ribbons linked by short O – H ∙∙∙ O hydrogen bonds with distances 2.548(1) Å and 2.587(1) Å. The O – H ∙∙∙ O distances between water molecules and [H2P2O6]2– ions range from 2.745(1) to 3.033(1). Furthermore, the hydrogen‐bond lengths between the 18‐crown‐6 ether and water molecule range from 2.857(1) to 2.899(1) Å. Details are discussed in the article by Madeline Haase and Mimoza Gjikaj on page http://onlinelibrary.wiley.com/doi/10.1002/zaac.201700111/abstract.