Facile Changes in Bonding Modes of Azulene in Di- and Triruthenium Clusters upon Interaction with CO

Carbonylation of the known cluster [Ru3(CO)6(μ-CO)(μ3-η5:η3:η3−C10H8)] (1) gives the metastable trinuclear derivative [Ru3(CO)8(μ3-η5:η2:η1-C10H8)] (2). On further carbonylation of 1 the stable product [Ru2(CO)5(μ2-η5:η3-C10H8)] (3) is obtained. X-ray diffraction studies and full spectroscopic assignment of 3 show that the azulene ligand is bonded through a π-Cp to one metal and via a π-allyl moiety to the other. Compound 3 reacts reversibly with CO to yield [Ru2(CO)6(μ2-η5:η1-C10H8)] (4), which maintains the π-Cp bond while the heptadienyl ring contributes to the bonding only through one σ-C−Ru interaction.