Synthesis, crystal structure, magnetic behavior and thermal property of three polynuclear complexes : [M(dca)2(H2O)2]n · (hmt)n [M = Mn(II), Co(II)] and [Co(dca)2(bpds)]n [dca, dicyanamide; hmt, hexamethylenetetramine; bpds, 4,4'-bipyridyl disulfide]