5,6-Dioxo-1,10-phenanthroline-2,9-dicarboxylic acid dihydrate (H(2)pdda·2H(2)O), C(14)H(6)N(2)O(6)·2H(2)O, was obtained by carbonylation of 1,10-phenanthroline-2,9-dicarboxylic acid. Its first coordination compound, [MnCl(C(14)H(6)N(2)O(6))(H(2)O)(2)]Cl·2H(2)O, in which the H(2)pdda ligand remains protonated, was synthesized in aqueous acetic acid. H(2)pdda chelates one water molecule via hydrogen bonds in H(2)pdda·2H(2)O, whereas in the coordination compound it chelates one heptacoordinate Mn(II) atom via coordination bonds. The N atoms in H(2)pdda·2H(2)O and the Cl atoms in the coordination compound are also involved in hydrogen bonds. Extensive hydrogen bonding results in supramolecular networks in both compounds.