Isotypic one-dimensional coordination polymers: catena-poly[di­chlorido­cadmium]-μ-5,6-bis­­(pyridin-2-yl)pyrazine-2,3-di­carboxyl­ato-κ2N5:N6] and catena-poly[di­chlorido­mercury(II)]-μ-5,6-bis­­(pyridin-2-yl)pyrazine-2,3-di­carboxyl­ato-κ2N5:N6]

The isotypic title one-dimensional coordination polymers, [CdCl2(C18H14N4O4)]n, (I), and [HgCl2(C18H14N4O4)]n, (II), are, respectively, the cadmium(II) and mercury(II) complexes of the dimethyl ester of 5,6-bis­(pyridin-2-yl)pyrazine-2,3-di­carb­oxy­lic acid. In both compounds, the metal ions are located on a twofold rotation axis and a second such axis bis­ects the Car—Car bonds of the pyrazine ring. The metal ions are bridged by binding to the N atoms of the two pyridine rings and have an MN2Cl2 bisphenoidal coordination geometry. The metal–Npyrazine distances are much longer than the metal–Npyridine distances; the difference is 0.389 (2) Å for the Cd—N bonds but only 0.286 (5) Å for the Hg—N bond lengths. In the crystals of both compounds, the polymer chains are linked via pairs of C—H⋯Cl hydrogen bonds, forming corrugated slabs parallel to the ac plane.