Three metal complexes based on the flexible bis(benzimidazole) derivatives, namely [Co 2 (L1) 4 (pydc) 2 ]·2H 2 O (1), {[Cu(L1)(pydc)]·0.75H 2 O} n (2), and [Co 2 (L2) 2 (pydc) 2 ]·8H 2 O (3), [L1 = 1,2-bis(5,6-dimethylbenzimidazol-1-ylmethyl)benzene, H 2 pydc = 2,6-pyridinedicarboxylic acid, L2 = 1,4-bis(5-methylbenzimidazol-1-ylmethyl)benzene] were hydrothermally synthesized and characterized by IR spectra, elemental analysis, UV/Vis diffuse refraction spectroscopy, and X-ray single-crystal diffraction. Complexes 1 and 3 exhibit a dinuclear structure composed of two central cobalt atoms * Dr. G. H. Cui Fax: +86-0315-2592170 E-Mail: tscghua@126.com [a] College of Chemical EngineeringComplex 3 was synthesized by a similar procedure to that of 1, except that L1 ligand was replaced by L2 (38.0 mg, 0.1 mmol). The block-