or mercury bis[(diazomethyl)-carbonyl] compounds (10a–c or 12a–c) with 5-chloro-5H-dibenzo[a,d]cycloheptene (6) furnishes the 5(diazomethyl)-5H-dibenzo(a,d)cycloheptenes(8a–c or 13a–c). Copper(II)acetylacetonate catalysed decomposition of these compounds in toluene results, in all cases, in the formation of dibenzosemibullvalenes (17a–f). In addition, the dibenzo[a,e]cyclooctenes 16a–e and, in the