A series of new polymeric mercury(II) complexes, [Hg(4-bpdb)(SCN)2]n (1), [Hg(3-bpdb)(SCN)2]n (2), [Hg(4-bpdb)Br2]n (3), [Hg(3-bpdb)Br2]n (4) and [Hg2(2-bpdb)Br4]n (5) {4-bpdb = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene, 3-bpdb = 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene and 2-bpdb = 1,4-bis(2-pyridyl)-2,3-diaza-1,3-butadiene} was prepared from reactions of mercury(II) thiocyanate or bromide with three organic nitrogen donor-based ligands under thermal gradient conditions using the branched tube method. All these compounds were structurally characterized by single-crystal X-ray diffraction. The thermal stabilities of compounds 1–5 were studied by thermal gravimetric (TG) and differential thermal analyses (DTA).