The structures of three related complexes of general formula M(pds)(nab)₂ [pds is the peroxodisulfate anion and nab is an nitrogen-containing aromatic base], viz. bis(2,9-dimethyl-1,10-phenanthroline-κ₂N,N′)(peroxodisulfato-κ²O,O′)cadmium, [Cd(S₂O₈)(C₁₄H₁₂N₂)₂], (V), bis­(3,4,7,8-tetramethy-1,10-phenanthroline-κ²N,N′)(peroxodisulfato-κ²O,O′)zinc, [Zn(S₂O₈)(C₁₆H₁₆N₂)2], (VI), and bis­(3,4,7,8-tetramethy-1,10-phenanthroline-κ²N,N′)(peroxodisulfato-κ²O,O′)cadmium, [Cd(S₂O₈)(C₁₆H₁₆N₂)2], (VII), present the same topological coordination, with three chelating ligands in an MN₄O₂ polyhedron. The main difference resides in the fact that the first two complexes are bisected by a crystallographic twofold axis, thus providing a symmetrical environment to the cation, while in the third one this symmetry is disrupted into a clearly unsymmetrical disposition, probably by way of an unusually strong intra­molecular C–H⋯O hydrogen bond. The situation is compared with similar interactions in the literature. The structure of (V) is based on a redetermination in the correct space group C2/c of the structure originally described in the Cc space group [Harvey et al. (2001). Aust. J. Chem. 54, 307-311; Marsh (2004). Acta Cryst. B60, 252-253.]