Hydrogen bonding in diaqua­tetra­kis(urea-[kappa]O)M^II dinitrates, with M = Ni and Co

The isomorphous title compounds, [Ni{(NH₂)₂CO}₄(H₂O)₂](NO₃)₂ and [Co{(NH₂)₂CO}₄(H₂O)₂](NO₃)₂, feature discrete centrosymmetric cations in octa­hedral coordinations, formed by four urea mol­ecules linked via their O atoms to the central ion in equatorial positions and two water mol­ecules in trans positions. The complexes are packed in a pseudo-hexa­gonal manner separated by the nitrate counter-ions. All H atoms are involved in moderate hydrogen bonds of four types: N-H...O=C, N-H...O-N, O-H...O-N and N-H...O-H. Graph-set analysis was applied to distinguish the hydrogen-bond patterns at unitary and higher level graph sets.