catena-Poly[[[diaqua-terbium(III)]-μ-6-carboxy-nicotinato-μ-pyridine-2,5-di-carboxyl-ato] dihydrate].

The title compound, {[Tb(C(7)H(3)NO(4))(C(7)H(4)NO(4))(H(2)O)(2)]·2H(2)O}(n), is isotypic with the analogous Tm(III) compound [Li, Zhang, Wang & Bai (2009). Acta Cryst. E65, m411]. The Tb(III) atom is octa-coordinated by two water mol-ecules and by four carboxyl-ate O atoms and two pyridyl N atoms from two pyridine-2,5-dicarboxyl-ate (2,5-pydc) and two 6-carboxy-nicotinate (2,5-Hpydc) ligands. The 2,5-pydc and 2,5-Hpydc ligands bridge Tb(III) atoms, generating helical coordination polymers along [001]. An extensive network of O-H⋯O hydrogen bonds is formed between the coordination polymers and the uncoordinated water mol-ecules. The refined Flack parameter of 0.54†(2) suggests inversion twinning.