The reactions of M(ClO4)2·6H2O (M = Mn2+, Co2+ and Ni2+) with N-(2-pyridylmethyl)-iminodiacetic acid (H2pmida) gave three discrete polynuclear complexes {Na[Mn3(pmida)3(H2O)3]}ClO4 (1), {Na[Co3(pmida)3(H2O)3](H2O)}ClO4 (2) and [Co(H2O)6][Co3(pmida)3(H2O)3]2(ClO4)2·4H2O (3) and two 1D polymers {[Co(pmida)(H2O)]·H2O}n (4) and {[Ni(pmida)(H2O)]·H2O}n (5). All complexes were characterized by elemental analysis, infrared (IR) spectroscopy and single-crystal X-ray diffraction analysis. Complexes 1 and 2 show similar structures with an unprecedented trinuclear unit [M3(pmida)3(H2O)3] in which the three six-coordinated divalent metal ions are joined by three coplanar syn-anti bridging carboxylate groups of three ligands into a closed planar twelve-membered ring with one Na+ ion located on the C3 axis. Complex 3 possesses two kinds of coordination units: mononuclear cation [Co(H2O)6]2+ and trinuclear unit [Co3(pmida)3(H2O)3]. The Co2+ and Ni2+ ions in complexes 4 and 5, respectively, present distorted octahedral geometries and are bridged by anti–anti bridging carboxylate groups of the ligands to form 1D chains. Compounds 1–5 display a structure variation from discrete polynuclear to 1D coordination polymer along Mn(II), Co(II) to Ni(II), and a dependence of the formation of discrete compound or coordination polymer on the identity of the metal ion. The magnetic investigations of 1–3 were also carried out. The negative Weiss constants and coupling constants obtained from two kinds of fitting models indicate the presence of dominant antiferromagnetic exchanges mediated by the syn–anti bridging carboxylate groups between the metal centers of three compounds.
