The reactions of barium 1,2,4,5-benzenetetracarboxylate with CoSO 4 produced the dinuclear species Co(H 2 O) 5 (m-C 6 H 2 (COO) 4 )Co(H 2 O) 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) A , and the sixth Co-O bond is 2.0356(7) A , . The reaction of MnCl 2 with benzenetetracarboxylate ions produced a similar dinuclear species, Mn(H 2 O) 5 (m-C 6 H 2 (COO) 4 )Mn(H 2 O) 5 •H 2 O (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 2 Mn(C 6 H 2 (COO) 4 )•3H 2 O with CoSO 4 produced a compound containing two dinuclear species {Mn(H 2 O) 5 (m-C 6 H 2 (COO) 4 )Mn(H 2 O) 5 }{Co(H 2 O) 5 (m-C 6 H 2 (COO) 4 )Co(H 2 O) 5 }•2H 2 O, 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 One of the Mn(II) dimer in 2 has been replaced by a Co(II) dimer.