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]