The reactions of 4,7-phenanthroline (4,7-phen) with a variety of metal salts have been investigated and the products structurally characterised by single crystal X-ray diffraction. Reaction of CoX₂ (X = Cl, Br) or M(NO₃)₂ (M = Cd, Zn, Ni) salts with 4,7-phen all afford one-dimensional coordination polymers, [CoX₂(4,7-phen)]_∞ (1, 2) or [M(NO₃)₂(4,7-phen)(H₂O)]_∞ (M = Cd 3, Zn 4, Ni 5), in which the metal centres are linked via 4,7-phen ligands which act in a bridging bidentate manner. The remaining metal coordination sites are occupied by either halide (1, 2) or NO₃⁻ and H₂O ligands (3–5). In{[Ni(NO₃)₂(4,7-phen)(H₂O)]·(4,7-phen)}_∞, 5 an additional, uncoordinated, molecule of 4,7-phen is included in the structure, hydrogen bonded to a coordinated H₂O molecule. In contrast, reaction of CuX₂ (X = Cl, Br) or Cu(NO₃)₂ with 4,7-phen leads to the formation of discrete complexes, [CuX₂(4,7-phen)₂] (X = Cl 6, Br 7, NO₃8) in which the N-donor ligand coordinates in a monodentate fashion and π–π interactions between coordinated 4,7-phen molecules generate an extended structure in the solid state. Investigation of the phase purity of 4 and 5 revealed that in the case of 4 the coordination polymer formed only in the presence of a previously reported hydrogen bonded structure, while 5 was shown to be contaminated with an unidentified phase. The rôle and diversity of the interactions adopted by 4,7-phen in forming coordination polymers and related species are discussed.