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The complex [Ni(H2O)3(phen)(C4H2O4)] ⋅ H2O (1), which was obtained by reaction of phenanthroline, Ni(NO3)2 ⋅ 6H2O, and maleic acid in CH3OH/H2O at pH = 7.05, crystallized in the monoclinic space group P21 (no. 4) with cell dimensions: a = 9.350(1) Å, b = 7.631(1) Å, c = 12.821(1) Å, β = 106.25(1), and Dcalc = 1.607 g/cm3 for Z = 2. The Ni atoms are each octahe drally coordinated by one chelating phen ligand, three H2O molecules and one monodentate maleato ligand to form [Ni(H2O)3(phen)(C4H2O4)] complex molecules with d(Ni–O) = 2.038–2.090 Å, d(Ni–N) = 2.066, 2.089 Å. The formed complex molecules are, via the intermolecular hydrogen bonds, assembled into columnar 1D chains. Interdigitation of the chelating phen ligands of the neighboring chains leads to 2D layers and the crystal H2O molecules are hydrogen bonded to the oxygen atoms of the maleate not coordinated to the Ni atom. However, reaction of NiCO3, phen, and maleic acid in CH3OH/H2O at pH = 6.33 afforded [Ni(H2O)2(phen)(C4H2O4)] ⋅ 2H2O (2), which crystallized in the triclinic space group P (no. 2) with cell dimensions: a = 7.971(1) Å, b = 8.237(1) Å, c = 13.304(1) Å, α = 81.005(6)°, β = 87.877(8)°, γ = 78.322(8)°, and Dcalc = 1.671 g/cm3 for Z = 2. The Ni atoms are each octahedrally coordinated by two N atoms of one phen ligand and four O atoms of two H2O molecules and two bis–monodentate maleato ligands with d(Ni–O) = 2.041–2.120 Å and d(Ni–N) = 2.095 Å. The Ni atoms are bridged by the maleato ligands to generate 1D ∞1 [Ni(H2O)2(phen)(C4H2O4)2/2] chains along . The supramolecular assemblies of the 1D chains via π–π stacking inter- actions result in thick 2D layers parallel to (001), between which the noncoordinating H2O molecules are sandwiched. The paramagnetic [Ni(H2O)2(phen)(C4H2O4)2/2] ⋅ 2H2O (2) obeys the Curie–Weiss law χm(T-Θ) = 1.139cm3 ⋅ mol−1 ⋅ K with the Weiss constant Θ = −0.95 K.