A zwitterionic triosmium cluster bearing a metallated azulene ligand coordinated perpendicularly as an alkylidene bridge

The cluster [Os3(CO)10(MeCN)2] reacts readily with azulene in refluxing cyclohexane to give the oxidative addition product [Os3(μ-H)(μ2-η1-C10H7)(CO)10] 1 which was shown by its X-ray crystal structure to contain the C5 ring of the azulenyl ligand bonded through a single carbon atom at the 1-position. We propose that the compound is zwitterionic, with the 7-membered ring a tropylium cation and the 5-membered ring coordinated as a μ-alkylidene to the metal cluster, which carries a formal negative charge. Thermal loss of one CO ligand leads by further oxidative addition to the known cluster [Os3(μ-H)2(μ3-η1:η1:η1-C10H6)(CO)9] 2.