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In the crystal structure of the title salt, C22H21N2O6 +·Br−·2H2O, pairs of betaine mol­ecules are bridged by protons (the bridging proton is disordered), forming strong and symmetrical O—H⋯O hydrogen bonds, leading to an infinite chain along the b axis. The water mol­ecules are linked to the betaine mol­ecule and the bromide ion through O—H⋯O and O—H⋯Br inter­actions. The central ring, located on an inversion centre, makes dihedral angles of 1.2 (2)° with the outer rings. One of the carboxylic acid groups is deprotonated.

The title centrosymmetric coordination polymer, {[Cu(C22H20N2O6)(H2O)2]Br2}n, formed by the reaction of the flexible double betaine ligand 2,2′-{[p-phenyl­enebis(oxymethyl­ene)]bis­(pyridine-3,1-di­yl)}diacetic acid with CuBr2, contains a Cu(II) atom ( symmetry) which is surrounded by two water molecules and bridged by two anions in a square-planar coordination. In the crystal, polymeric zigzag chains are linked via O—H⋯Br inter­actions, forming a two-dimensional network extending parallel to (011).

In the title polymeric coordination complex, {[Cd(C22H20N2O6)(H2O)4](NO3)2·0.5H2O}n, obtained from the self-assembly of the flexible double betaine 3,3′-[p-phenyl­enebis(oxymethyl­ene)]bis­(1-pyridinioacetate) with cadmium nitrate, both the octa­hedrally coordinated CdII cation and the substituted betaine ligand lie on inversion centres. The chains constructed through the trans-related acetate groups of the ligand are inter-connected via O—H⋯O hydrogen bonds involving coordinated aqua ligands, the nitrate anions and the partial-occupancy (0.25) water mol­ecule of solvation, forming a three-dimensional structure.