Three phenanthroline–metal complexes with topologically similar but geometrically different conformations

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-κ(2) N,N′)(peroxodi­sulfato-κ(2) O,O′)cadmium, [Cd(S(2)O(8))(C(14)H(12)N(2))(2)], (V), bis­(3,4,7,8-tetra­methy-1,10-phenanthroline-κ(2) N,N′)(peroxodi­sulfato-κ(2) O,O′)zinc, [Zn(S(2)O(8))(C(16)H(16)N(2))(2)], (VI), and bis­(3,4,7,8-tetra­methy-1,10-phenanthroline-κ(2) N,N′)(peroxodi­sulfato-κ(2) O,O′)cadmium, [Cd(S(2)O(8))(C(16)H(16)N(2))(2)], (VII), present the same topological coordination, with three chelating ligands in an MN(4)O(2) 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].