Hexanuclear and pentanuclear benzene carbido clusters: Synthesis, characterisation and molecular structures of [Os₆C(Co)₁₄(η⁶- C₆H₆)]

Reaction of the pentaosmium-carbido dianion [Os5C(CO)14]2-1 with [Os(C6H6)(MeCN)3]2+gave [Os6C(CO)14(η6-C6H6)] 2 in fair yield; while the corresponding reaction with [Ru(C6H6)(MeCN)3]2+gave [RuOs5C(CO)14(η6-C6H6)] 3 in moderate yield. Treatment of 3 with carbon monoxide produces an unstable species [RuOs5C(CO)15(η6-C6H6)] 4a, which on standing in CH2Cl2undergoes fragmentation to give the species [RuOs4C(CO)12(η6-C6H6)] 4b or decarbonylation to 3. Clusters 2, 3 and 4b have been fully characterised by both spectroscopic and crystallographic methods. The X-ray structure analysis shows that both 2 and 3 are isomorphous. Both molecules contain an octahedral cavity accommodating an interstitial C(carbide) atom, with the η6-C6H6coordinated to the Ru or Os atom in an apical position. Cluster 4b is an unprecedented pentanuclear osmium - ruthenium mixed-metal carbido cluster containing an η6-coordinated benzene ligand.