Novel cobalt-pyrazine-2,3-dicarboxylate complexes with 1,10-phenanthroline (phen), [Co(pzdca)(-phen)(2)](2)center dot 11H(2)O (1), N,N,N',N'-tetramethylethylenediamine (tmen), (H(2)tmen)[Co(pzdca)(2)(tmen)]center dot 9H(2)O (2) and 2,2-dimethylpropane-1,3-diamine (dmpen), [Co(CO3)(dmpen)(2)](pzdca)(0.5)center dot H2O (3) have been synthesized and characterized by elemental and thermal analyses, spectroscopic (IR and UV-vis) and X-ray diffraction techniques. In 1 and 2, pyrazine-2,3-dicarboxylate ligand coordinated to the Co(II) ions through one nitrogen atoms of pyrazine ring and oxygen atoms of carboxylate group as a bidentate ligand and distorted octahedral geometries of 1 and 2 are completed by phen and tmen ligands, respectively. In 2, the tmen molecules exhibit chemically different functions: it coordinated to the Co(II) ion as a bidentate ligand and in the other form it protonated and acts as counter-ion. In complex 3, Co(III) ion is coordinated by four nitrogen atoms of dmpen and two oxygen atoms of CO3 ligand and the pzdca behaves as a counter-ion. Furthermore, structures of 1 and 2 contain extensive hydrogen bonding between crystal water molecules to form infinite 2D water layers and I D water chains, respectively. In vitro antimicrobial activities of new complexes were tested against selected wild type and clinical microorganisms by MIC. Complexes exhibited antimicrobial activity at high concentrations against the bacteria, fungi and clinical isolate tested. (C) 2009 Elsevier B.V. All rights reserved.