The title structure, C(12)H(9)N(2) (+)·C(4)H(5)O(6) (-)·3H(2)O, shows that one of the protons of d-tartaric acid has been transferred to 1,10-phenanthroline. The d-hydrogen tartrate anions are joined together in a head-to-tail fashion via a short hydrogen bond with donor-acceptor distance of 2.4554 (12) Å, unsymmetrical O-H distances of 1.01 (4) Å and 1.45 (4) Å, and a 174 (4)° O-H-O bond angle. The phenanthrolinium rings are π-stacked with an average separation of 3.58 (11) Å. The structural report corrects a previous report in the literature [Wang et al. (2006 ▶). Acta Cryst. E62, o2508-o2509] of the isostructural l-hydrogen tartrate enanti-omer in which the proton transfer and short hydrogen bond were missed.

The title compound, (C(13)H(11)N(2))(2)[Ni(C(7)H(3)NO(4))(2)]·3H(2)O, consists of a mononuclear anionic complex, two 9-amino-acridinium cations and three uncoordinated water mol-ecules. Two pyridine-2,6-dicarboxyl-ate (pydc) ligands are bound to the Ni(II) ion, giving an NiN(2)O(4) bonded set. The coordination geometry around the Ni(II) atom is distorted octa-hedral. There are two types of robust O-H⋯O hydrogen-bond synthons, namely R(6) (6)(24) and R(2) (4)(8), which link the complex anions and water mol-ecules to each other. N-H⋯O hydrogen bonds connect the stacks of anions and cations in the structure. Other inter-molecular inter-actions, including weak C-H⋯O hydrogen bonds, π-π [shortest centroid-centroid distance = 3.336 (7) Å] and C-O⋯π [O⋯centroid distance = 3.562 (10) Å] inter-actions, connect the various components.

The asymmetric unit of the title compound, 2C(14)H(13)N(2) (+)·2C(4)H(5)O(6) (-)·9H(2)O, contains two cations and two anions in addition to nine mol-ecules of water. Each of the hydrogen tartrate anions is hydrogen bonded to itself by translation along [100] in a head-to-tail fashion via a short hydrogen bond with donor-acceptor distances of 2.473 (4) and 2.496 (4) Å. A large number of inter-molecular O-H⋯O, N-H⋯O and C-H⋯O hydrogen-bonding inter-actions, as well as π-π stacking [centroid-centroid distances in the range 3.642 (3) to 3.866 (3) Å], play an important role in the crystal structure.

The title compound, C(13)H(10)N(+)·C(7)H(4)NO(4) (-)·H(2)O or (acrH)(+)(pydcH)(-)·H(2)O, is a monohydrate of acridinium cations and a mono-deprotonated pyridine-2,6-dicarb-oxy-lic acid. The structure contains a range of non-covalent inter-actions, such as O-H⋯O, O-H⋯N and N-H⋯O hydrogen bonds, as well as π-π stacking [range of centroid-centroid distances = 3.4783 (5)-3.8059 (5) Å]. The N-H⋯O hydrogen bond between the donor acridinium cation and the carboxyl-ate acceptor is particularly strong. The average separation between the π-stacked acridinium planes is 3.42 (3) Å.

The title compound, (C(4)H(8)N(3)O)[Cr(C(7)H(3)NO(4))(2)]·C(7)H(5)NO(4)·6H(2)O, was obtained by the reaction of Cr(NO(3))(3)·9H(2)O with pyridine-2,6-dicarboxylic acid (pydcH(2)) and creatinine (creat) in aqueous solution (molar ratio 1:2:2). The cation is a protonated creatinine (creatH(+)) while the anion is a bis-pydc(2-) Cr(III) complex. The Cr(III) is coordinated by four oxygen and two nitro-gen atoms of two (pydc)(2-) groups and has a disorted octa-hedral coordination environment. The structure also contains a neutral mol-ecule of pydcH(2) that is hydrogen bonded to the creatH(+) and six mol-ecules of water. Extensive inter-molecular inter-actions, including seventeen classical hydrogen bonds, two weak C-H⋯O bonds, and C-O⋯π stacking inter-actions, with O⋯centroid distances of 3.211 (13) and 3.300 (12) Å, connect the various components in the crystal structure.

The reaction of copper(II) nitrate hexa-hydrate with pyridine-2,6-dicarboxylic acid (pydcH(2)) and 2,4,6-triamino-1,3,5-triazine (melamine) in aqueous solution in a 1:2:2 molar ratio gave the title compound, (H(5)O(2))(C(3)H(7)N(6))(3)[Cu(C(7)H(3)NO(4))(2)](2)·C(7)H(5)NO(4)·6H(2)O. The hydroxonium hydrate (H(5)O(2))(+), also known as the Zundel cation, resides on a twofold rotation axis. The O-H distance is 1.274 (14) Å, the O⋯O distance is 2.518 (5) Å, and the O-H-O angle is 162 (8)°. One of the melamine H(+) cations, the uncoordinated pydcH(2), and two water mol-ecules also reside on crystallographic twofold axes. The Cu(II) atom has a tetra-gonally distorted octa-hedral coordination environment. The structure features extensive hydrogen bonding, with 21 distinct inter-actions. There is also a centrosymmetric C=O⋯π inter-action with an O⋯centroid distance of 3.288 (3) Å. The structure is similar to a mixed-valence manganese(II/III) structure but shows inter-esting differences in the metal-atom coordination. One of the water molecules is equally disordered with respect to a twofold axis.