4(3H)-quinazolinone may act as ligand for metal ions in different coordination modes – coordinating through nitrogen atom para to the quinazolinone oxygen atom (mode 1), or through nitrogen atom ortho to the quinazolinone oxygen atom (mode 2), after hydrogen atom migration (tautomerization). Earlier crystal structural investigations have shown that in the reaction of cadmium chloride or bromide with 4(3H)-quinazolinone, this ligand interacts with Cd(II) cations via para nitrogen atoms (mode 1), and the octahedral coordination around the metal is completed by two ligands in a trans axial geometry. New cadmium(II) and mercury(II) coordination polymers were obtained via reaction of (3H)-quinazolinone with CdBr₂ and HgCl₂, respectively. Single crystal X-ray structural analysis reveals that coordination compounds crystallize in monoclinic P2₁/c and triclinic P-1 space groups, featuring halide-bridged 1D chain polymers based on momonuclear [M(X₂)(L)] subunits ((M = Cd, Hg, X = halide ion and L = 4(3H)-quinazolinone). Each metal ion is coordinated to one quinazolinone ligand, via the nitrogen atom ortho to the quinazolinone oxygen atom, with similar coordination geometries for metal ions in both coordination polymers.