The experimental electron-density distributions in crystals of five chain polymers [M(mu-X)2(py)2] (M = Zn, Cd; X = Cl, Br; py = 3,5-substituted pyridine) have been obtained from high-resolution X-ray diffraction data sets (sin theta/lambda > 1.1 A(-1)) at 100 K. Topological analyses following Bader's 'Atoms in Molecules' approach not only confirmed the existence of (3, -1) critical points for the chemically reasonable and presumably strong covalent and coordinative bonds, but also for four different secondary interactions which are expected to play a role in stabilizing the polymeric structures which are unusual for Zn as the metal center. These weaker contacts comprise intra- and inter-strand C-H...X-M hydrogen bonds on the one hand and C-X...X-C interhalogen contacts on the other hand. According to the experimental electron-density studies, the non-classical hydrogen bonds are associated with higher electron density in the (3, -1) critical points than the halogen bonds and hence are the dominant interactions both with respect to intra- and inter-chain contacts.