title: Terephthalate salts of dipositive cations.
abstract: The crystal structures of M(C(8)H(4)O(4))(H(2)O)(2), M = Mg, Mn, Fe and Co, have been determined by applying Monte Carlo simulated annealing techniques to synchrotron powder diffraction data and refined by the Rietveld method using both synchrotron and laboratory powder data. These isostructural compounds crystallize in the monoclinic space group C2/c, with 18.2734 (9) <or= a <or= 18.7213 (13), 6.5186 (13) <or= b <or= 6.5960 (4), 7.2968 <or= c <or= 7.4034 (6) A, 98.653 (2) <or= beta <or= 99.675 (1) degrees and Z = 4. The structure consists of alternating layers (perpendicular to a) of terephthalate anions and octahedrally coordinated metal cations. The octahedra are isolated; each carboxylate bridges two metal cations. The equatorial metal coordination consists of four terephthalate O atoms and there are two axial water molecules. Both water-molecule H atoms participate in normal-strength hydrogen bonds to carboxylate O atoms. Quantum chemical calculations (using CASTEP) were used to determine the H-atom positions and analyze the hydrogen bonding and the metal coordination. Both the atomic charges and the M-O bond-overlap populations indicate that, despite the fact that these compounds are isostructural, the metal-terephthalate bonding is different. The bonding in the Mg complex is essentially ionic, while the M-O bonds in the Mn, Fe and Co complexes have significant covalent character. Comparison of a new Rietveld refinement of the structure of copper(II) terephthalate trihydrate with the reported single-crystal structure provides an opportunity to assess the accuracy and precision that can be expected from structures of aromatic carboxylates determined using X-ray powder data. The average difference between the bond distances in the two structures is 0.03 A and the average difference in bond angles is only 1.1 degrees.