Abstract[Ni(C2H5N4O2)2] · HCl · H2O, C4H10N8NiO4 · HCl · H2O, forms triclinic crystals, Mr = 347.36, P1̄, a = 7.219(2), b = 7.316(1), c = 11.797(3) Å, α = 73.89(2), β = 86.37(2), γ = 85.71(2)°, V = 596 Å3 , Z = 2, dc = 1.93 Mgm-3 ; final Rw = 0.028 for 1957 reflections. The planar complex molecules form equidistant stacks along b, with the molecular planes inclined at ~29° to the stacking axis. Molecules of adjacent stacks are linked along a by an intermolecular H bridge coexistins with the usual intramolecular H bridges. The analogous Pt complex, C4H10N8O4Pt · HCl · H2O, Mr = 483.74, a = 6.480(4), b = 16.115(5), c = 12.194(9) Å, β = 101.27(4)°, V = 1249 Å3 , Z = 4, dc = 2.57 Mgm-3 , P21/a, crystallizes in a columnar structure with a Pt-Pt separation of ~a/2 ≈ 3.24 Å. Due to twinning, faking orthorhombic symmetry, a complete structure determination was not possible.