o-Phthalic acid (H2pht) is successfully used for creating an extensive range of coordination compounds with different dimensionalities: monomers, clusters, and coordination polymers. The phthalate ligand can adopt 54 coordination modes with metal atom in complexes [1]. Monodentate coordination mode of phthalate residue leads to formation of mononuclear complexes. The presence of an uncoordinated COOH group in the complexes, coordinated and uncoordinated water molecules as well as hydrogen-bond donors in aromatic amines creates conditions to form various kinds of supramolecular networks. The combinations of the phthalate ligand and N-donor aromatic amines result in organization of a series of mononuclear complexes with the common formula [M(pht)(amine)(H2O)3] · n(H2O), where M = Co(II), Ni(II); amine = (py)2, (2-MeIm)2, (-Pic), bpy, and phen; n = 0, 1, 3 [2]. In these compounds the discrete complex molecules are found to be of a very similar shape. The phthalate ligand occupies only one place in the metal octahedron, and the coordination sphere is completed by N-atoms of aromatic amines and oxygen atoms of water molecules. Discrete complexes in all these compounds are connected by hydrogen bonds in double-chains. Depending on the nature of aromatic amines, the arrangement of these chains differs between crystals. Another example of a mononuclear complex is [Ni(Hpht)2(H2O)4]·2(H2O), where the Ni(II) ion is coordinated by two oxygens of monodentate Hpht and four oxygen atoms of H2O forming a fairly regular octahedron [3]. The coordination octahedra are linked by intermolecular H2O…Ocarb hydrogen bonds organizing a layer structure. Here we report new mononuclear complexes [M(pht)2(H2O)4](H2pip)∙8(H2O), where M = Cu(II), Co(II) or Ni(II), pip = piperazine). These structures are assembled from the same moieties ([M(pht)2(H2O)4]2 anion, the protonated piperazine molecule and 8 solvate water molecules, crystallize in the same space group P-1 but have different unit cell parameters. Three crystallographically non-equivalent Co atoms and Ni atom in corresponding compounds have a slightly distorted octahedral coordination alike in [3] with average distances of 2.09 Å for CoO and 2.06 Å for NiO. In complex anion [Cu(pht)2(H2O)4]2 six oxygen atoms situate in the vertex of squared bipyramid. The mean CuO in base plane is in the region of 1.97 Å and the corresponding distances are 2.41 Å in apical positions. The presence of numerous hydrogen atom results in extensive and similar intramolecular and intermolecular hydrogen bonding in these compounds. Coordinated water molecules form strong hydrogen bonds with the oxygen atoms of the carboxylate group of adjacent complexes to generate the chain. The chains are packed via N-H…O and O-H…O in a 3D network as shown on Figure. It is necessary to note that solvate water molecules organize water clusters with alternation of 4- and 6-membered rings.figure