Crystal Structures of Hexaaquamagnesium Diliturate Dihydrate, Mg(H2O)6(2+)(C4H2N3O5-)2.2H2O and the Isomorphous Calcium Salt.

The crystal structures of the two isomorphous title compounds were determined by single-crystal X-ray diffraction. The space group for the monoclinic crystals is P2(1)/c, No. 14, Z=2. At 293 K, a=12.039(1), b=12.096(1), c=6.741(2) Angstrom, beta=97.86(1)degrees and V=972.4(5) Angstrom(3) for hexaaquamagnesium 5-nitrobarbiturate dihydrate. The cell parameters for the isomorphous calcium compound are a=12.349(1), b=12.167(1), c=6.818(1) Angstrom, beta=97.08(1)degrees, V=1016.6(3) Angstrom(3). In the crystals the cations and the anions are stacked in alternating layers. The crystal structures are extensively stabilized by hydrogen bonds some of which link the anions to infinite chains. The hydrogen bonds in these chains are significantly shorter when the cation is hexaaquamagnesium.