Facile changes in bonding modes of azulene in di- and triruthenium clusters upon interaction with CO

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