The title compounds. [Ln(PDC)(HPDC)](infinity) (H2PDC = pyridine-3,4-dicarboxylic acid, Ln = La, Cc, Pr), have been prepared and have been characterised by IR, MS, TGA, X-ray crystallography and powder diffraction. These isostructural complexes display both bridging and chelating carboxylate binding modes including a highly unusual chelation in which both carboxylate substituents bind to the same Pr atom, imparting chirality around the metal despite the non-chiral crystal setting. This system represents the first fully characterised metal complex containing the N-protonated ligand derivative (N-HPDC-). A polymorph of the ligand precursor, pyridine-3,4-dicarboxylic acid is also reported. (c) 2006 Elsevier Ltd. All rights reserved.