Octachlorocycloheptatriene, C 7 Cl 8 , 3, reacts with a variety of organometallic precursors, e.g., Fe 2 (CO) 9 , Co 2 (CO) 8 , [(C 5 H 5 )Fe(CO) 2 ] − , to give dodecachloroheptafulvalene, C 14 C 12 ,12, in yields ranging from 54 to 68%. There is no evidence to support the intermediacy of carbene complexes of the type C 7 Cl 6 =ML n , and it is suggested that the reaction proceeds by the dechlorination of C 7 Cl 8 to give hexachlorotropylidene, which dimerizes to the observed product. Extended Hückel molecular orbital calculations are used to show that the chlorinated carbene, C 7 Cl 6 , should be more stable than the corresponding parent system, C 7 Cl 6 . The EHMO calculations also reveal that planar heptafulvalenes are electronically disfavored and should exist preferentially in the S-anti conformation, as is the case for C 14 H 12 . The molecule C 14 Cl 12 ,12, is shown by X-ray crystallography to be formed only as the syn conformer, which isomerizes to the anti structure only at high temperature. The conformations of the two seven-membered rings in 12 closely resemble that found in octachlorocycloheptatriene itself. C 7 Cl 8 , 3, crystallizes in the orthorhombic space group Pnma with a = 7.140(1) Å, b = 13.329(3) Å, c = 12.595(3) Å, and V = 1198.7(4) Å 3 for Z = 4.3 adopts a severely bent boat structure in which the planes C(1)-C(7)-C(6) and C(2)-C(3)-C(4)-C(5) make angles of 51.8° and 32.4°, respectively, with the C(1)-C(2)-C(5)-C(6) plane. Key words: octachlorocycloheptatriene, heptafulvalenes, metal-mediated dimerization.