Two group-12 phenanthroline complexes, namely, mercury (II) chloride (HgCl2)(1,10-phenanthroline) (complex 1) and [cadmium iodide (CdI2)(1,10-phenanthroline)] n (complex 2), have been prepared through solvothermal reactions and structurally characterised by single-crystal X-ray diffraction. Complex 1 features an isolated (zero-dimensional) structure, and the mercury (II) (Hg2+) ion is surrounded by two nitrogen atoms and two chloride ions to form a tetrahedron. Complex 1 crystallises in the space group P21/c of the monoclinic system with four formula units in a cell: a = 3·9712(5), b = 17·847(2), c = 16·688(2) Å, β = 91·938(2)°, V = 1182·1(3) Å3. Complex 2 is characterised by a one-dimensional motif, and the cadmium (II) ion is bound by four iodide ions and two nitrogen atoms to give an octahedron. Complex 2 crystallises in the space group I2/a of the monoclinic system with four formula units in a cell: a = 7·7299(6), b = 11·1309(10), c = 16·3011(12) Å, β = 100·6880(10)°, V = 1378·23(19) Å3. Photoluminescence results reveal that complex 1 shows an emission in the green region, while complex 2 displays an emission in the red region. This theoretical study discovered that the emission of both complexes originates from the ligand-to-ligand charge-transfer transition.