The reactions of barium 1,2,4,5-benzenetetracarboxylate with CoSO 4 produced the dinuclear species Co(H 2O) 5(μ-C 6H 2(COO) 4)Co(H 2O) 5 which was characterized by crystallographic methods. The results have shown that the compound belongs to the monoclinic C2/ c space group and it crystallized with six molecules of water in the lattice. The crystal is stabilized by a very elaborate H-bonded 3D network. The molecule contains an inversion center. The average Co–OH 2 distance is 2.0710(8) Å and the sixth Co–O bond is 2.0356(7) Å. The reaction of MnCl 2 with benzenetetracarboxylate ions produced a similar dinuclear species, Mn(H 2O) 5(μ-C 6H 2(COO) 4)Mn(H 2O) 5·H 2O ( 2). The crystallographic results have shown that there are two independent molecules, which each contains an inversion center. This compound crystallized with only one molecule of water, and again the H-bonding system is very elaborate. The aqueous reaction of K 2Mn(C 6H 2(COO) 4)·3H 2O with CoSO 4 produced a compound containing two dinuclear species {Mn(H 2O) 5(μ-C 6H 2(COO) 4)Mn(H 2O) 5}{Co(H 2O) 5(μ-C 6H 2(COO) 4)Co(H 2O) 5}·2H 2O, which was characterized by X-ray diffraction methods. The results have shown that the latter compound is isomorphous with the Mn(II) compound. One of the Mn(II) dimer in 2 has been replaced by a Co(II) dimer.