Octachlorocycloheptatriene, C7Cl8, 3, reacts with a variety of organometallic precursors, e.g., Fe2(CO)9, Co2(CO)8, [(C5H5)Fe(CO)2]−, to give dodecachloroheptafulvalene, C14C12,12, in yields ranging from 54 to 68%. There is no evidence to support the intermediacy of carbene complexes of the type C7Cl6=MLn, and it is suggested that the reaction proceeds by the dechlorination of C7Cl8 to give hexachlorotropylidene, which dimerizes to the observed product. Extended Hückel molecular orbital calculations are used to show that the chlorinated carbene, C7Cl6, should be more stable than the corresponding parent system, C7Cl6. 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 C14H12. The molecule C14Cl12,12, 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. C7Cl8, 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) Å3 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.