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The crystal structure of the title compound, [Zn2(C8H7O2)4(H2O)2]·[Zn2(C8H7O2)4(C6H6N2O)2]·2H2O, consists of two kinds of dinuclear ZnII complexes (complex A and complex B) and uncoordinated water mol­ecules. In complex A, [Zn2(C8H7O2)4(H2O)2], each Zn cation is chelated by a 4-methyl­benzoate (PMB) anion and coordinated by a water mol­ecule, and is further bridged by two PMB anions in a trigonal-bipyramidal geometry. In complex B, [Zn2(C8H7O2)4(C6H6N2O)2], each ZnII cation is coordinated by a monodentate PMB anion and one nicotinamide (NA) ligand, and is further bridged by two PMB anions in a tetra­hedral geometry. Weak intra-mol­ecular π–π stacking between adjacent benzene rings is observed in complex B, the centroid–centroid distance being 3.710 (2) Å. Extensive O—H⋯O and N—H⋯O hydrogen bonding and weak C—H⋯O hydrogen bonding is present in the crystal structure. The crystal studied was a racemic twin; the minor twin component refined to 38%.

In the title centrosymmetric binuclear complex, [Cu2(C8H7O2)4(C6H6N2O)2], the Cu atoms [Cu⋯Cu = 2.6375 (6) Å] are bridged by four 4-methyl­benzoate (PMB) ligands. The four nearest O atoms around each CuII ion form a distorted square-planar arrangement, and the distorted square-pyramidal coordination is completed by the pyridine N atom of the isonicotinamide (INA) ligand. Each CuII ion is displaced by 0.2633 (1) Å from the plane of the four O atoms, with an average Cu—O distance of 1.974 (2) Å. The dihedral angles between carboxyl­ate groups and the adjacent benzene rings are 7.88 (19) and 9.68 (10)°, while the benzene rings are oriented at a dihedral angle of 85.90 (9)°. The pyridine ring is oriented at dihedral angles of 8.59 (7) and 83.89 (9)° with respect to the benzene rings. In the crystal structure, inter­molecular N—H⋯O hydrogen bonds link the mol­ecules into a three-dimensional network. π–π contacts between the benzene rings and between the pyridine and benzene rings, [centroid–centroid distances = 3.563 (2) and 3.484 (2) Å, respectively] may further stabilize the crystal structure.

In the centrosymmetric title compound, [Cu(C8H7O2)2(C6H6N2O)2]·2H2O, the CuII ion is located on a crystallographic inversion center. The asymmetric unit is completed by one 4-methyl­benzoate anion, one isonicotinamide (INA) ligand and one uncoordinated water mol­ecule; all the ligands are monodentate. The two O and the two N atoms around the CuII ion form a slightly distorted square-planar arrangement. The dihedral angle between the carboxyl­ate group and the attached benzene ring is 13.86 (9)°, while the pyridine and benzene rings are oriented at a dihedral angle of 86.08 (5)°. The uncoordinated water mol­ecules are linked to the INA ligands by O—H⋯O hydrogen bonds. In the crystal structure, inter­molecular O—H⋯O, N—H⋯O and C—H⋯O hydrogen bonds link the mol­ecules into a three-dimensional network.

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