Gravar-mail: Tetra-μ-aqua-octaaqua­bis(μ-4-chloro­pyridine-2,6-dicarboxyl­ato)bis­(4-chloro­pyridine-2,6-dicarboxyl­ato)tri­cobalt(II)disodium(I) bis­[triaqua­bis(4-chloro­pyridine-2,6-dicarboxyl­ato)cobalt(II)] hexa­hydrate