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.