Cleavage reactions of six phenylene- and polyphenylene-mercury heterocycles with the mercuric halides and mercuric acetate in suitable refluxing solvents readily produce 1,2-C6X4(HgX)2; 2,2′-C6H4C6H4(HgX)2 and 2,2″-C6H4C6H4C6H4(HgX)2 (X = Cl, Br, I or CH3CO2). Addition of silver acetate to 2,2′-C6H4C6H4(HgBr)2 gives 2,2′-C6H4C6H4(HgO2CCH3)2 which reacts with concentrated hydrochloric acid to form 2,2′-C6H4C6H4(HgCl)2. Treatment with sodium iodide, or in some cases simply the application of heat under vacuum, causes symmetrization of these cleavage products back to the original heterocycles in high yield. The terphenylenemercury dimer, (C6H4C6H4C6Hg)2, forms solvates with a wide variety of donor and non-donor solvents. Pyrolysis of mercuric 2,3,4,5-tetrafluorobenzoate at 190°C yields 2-mercurio-3,4,5,6-tetrafluorobenzoate and 2,3,4,5-tetrafluorobenzoic acid; at 300°C the former gives tetrafluorophenylenemercury, (C6F4Hg)3.