Under very mild conditions, (5-alkynyl-5H-dibenzo-[a,d]cycloheptatrien-5-ol)Co 2 (CO) 6 clusters 1 lose a carbon monoxide ligand and form the corresponding η 2 -alkene-μ-alkyne-Co 2 (CO) 5 complexes 2 (the first isolated examples of the initial phase of the Pauson-Khand reaction). This behaviour is not paralleled by the homologue in which an additional methylene group has been inserted within the seven-membered ring, as in the [5-alkynyl-6H-dibenzo[a,e]cyclooctatrienol]Co 2 -(CO) 6 system 8, for which several X-ray crystal structures are reported. The apparent additional flexibility of this molecular framework does not translate into conformations appropriate for the formation of η 2 -alkene-μ-alkyne-Co 2 (CO) 5 complexes. The relative inertness of the ethynyl complex 1a, compared [a] 2051 ing of the seven-membered ring towards the cluster, such that this distance, d, shortens to 2.04 Å. In contrast, in the eightmembered ring complex 8a, not only is the corresponding bond-centroid-to-cobalt distance, d, considerably longer (ca. 4.2 Å), but the C11=C12 double bond is also twisted away from the horizontal, as depicted in Figure 6b.

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