Cover illustration: Part of the crystal structure of 2-amino-4,6-bis(benzyloxy)-5-nitrosopyrimidine, showing the hard hydrogen bonds and the long N-H(arene) contact. See Quesada, Low, Melguizo, Nogueras & Glidewell [Acta Cryst. (2002), C58, o355-o358].
The crystal structure of Li2BaP2O7 has been refined in monoclinic space group C2/c. Tunnels parallel to the (010) plane house Li atoms. A previous report of the structure in orthorhombic space group Cmcm included no R factor or positional parameters.
The crystal structures of the title compounds consist of acidic pyrophosphate–metallate(II) layers joined by K+ ions and hydrogen-bridging bonds. The Zn2+/Mn2+ ions are octahedrally surrounded by four pyrophosphate O atoms and by two water molecules. The (HP2O7)3− anions exhibit eclipsed conformations.