Cover Picture: Preparation, Crystal Structure and Vibrational Spectra of Ca 2 P 2 O 6 ·2H 2 O and [Ca(H 2 O) 3 (H 2 P 2 O 6 )]·0.5(C 12 H 24 O 6 )·H 2 O (Z. Anorg. Allg. Chem. 15/2017)

The cover picture shows a projection of the unit cell of [Ca(H 2 O) 3 (H 2 P 2 O 6 )] ∙ 0.5(C 12 H 24 O 6 ) ∙ H 2 O along the c axis. The compound crystallizes in the monoclinic space group P2 1 / n with four formula units in the unit cell. The asymmetric unit contains one [Ca(H 2 O) 3 (H 2 P 2 O 6 )] 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(H 2 O) 3 (H 2 P 2 O 6 )], each Ca 2+ cation is bridged by three anions. The Ca 2+ ion is octahedrally coordinated by O atoms of three [H 2 P 2 O 6 ] 2– anions and three water molecules. In the anionic unit, [H 2 P 2 O 6 ] 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 P 2 O 6 skeleton has the two H atoms in cis position and exhibits, therefore, C 2 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 [H 2 P 2 O 6 ] 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 [H 2 P 2 O 6 ] 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 962 ff .