Chemical preparation, calorimetric studies, crystal structure, and IR absorption spectra are given for a new metal-organic diphosphate. From a structural point of view the title compound (\mathrm{Cu}{2}\left(\mathrm{C}{2} \mathrm{~N}{2} \mathrm{H}{10}\right)\left(\mathrm{HP}{2} \mathrm{O}{7}\right){2}\left(\mathrm{C}{2} \mathrm{~N}{2} \mathrm{H}{8}\right){2} \cdot 3 \mathrm{H}{2} \mathrm{O}) is an intricate stacking of copper(II) atoms, ethylenediammonium groups, (\mathrm{HP}{2} \mathrm{O}{7}^{4-}) anions, and both ethylenediamine and water molecules. This layered arrangement has a monoclinic unit cell with (a=18.808(8) \AA, b=) 9.631(2) (\AA, c=14.019(8) \AA), and (\beta=109.63(5)^{\circ}). The space group is (C 2 / c) and (z=4). As in all atomic arrangements including acidic diphosphate groups one observes the formation of an infinite network of anions connected by strong (\mathrm{H})-bonds, infinite (\left(\mathrm{HP}{2} \mathrm{O}{7}\right){n}) chains parallel to the (b) direction, for the present structure. The copper atom has a square-pyramidal fivefold coordination built by two nitrogen atoms, two oxygen atoms, and one water molecule. Another interesting fcature of the title compound is the coexistence of both ethylenediammonium groups and ethylenediamine moleculcs in the atomic arrangement. A chelated complex of the formula (\left[\mathrm{Cu}\left(\mathrm{C}{2} \mathrm{H}{8} \mathrm{~N}{2}\right)\left(\mathrm{HP}{2} \mathrm{O}{7}\right)\left(\mathrm{H}_{2} \mathrm{O}\right)\right]^{-})is observed. 1994 Academic Press. Inc.