Reduction of the tetraosmium cluster [Os₄H₄(CO)₁₂] with K–Ph₂CO gave the cluster dianion [Os₄H₄(CO)₁₁]^2–1 in quantitative yield. This dianion was treated with [Ru(C₆H₆)(MeCN)₃]²⁺ to give [RuOs₄H₄(η⁶-C₆H₆)(CO)₁₁]2 in fair yield, while the corresponding reaction with [Os(C₆H₆)(MeCN)₃]²⁺ gave [Os₅H₄(η⁶-C₆H₆)(CO)₁₁]3 and [Os₅H₄(η⁶-C₆H₆)(CO)₁₂]4 in similar yields (ca. 25% each). Clusters 2–4 have been fully characterised by both spectroscopic and crystallographic methods. The crystal structures of 2 and 3 are isomorphous and both the molecules are isostrctural. Both contain a trigonal bipyramidal metal-core geometry with the η⁶-C₆H₆ co-ordinated to the Ru or Os atom in an equatorial position. Cluster 4 contains an Os₄ tetrahedron with one Os–Os edge bridged by an ‘Os(η⁶-C₆H₆)(CO)’ unit. Complex 3 is not convertible into 4 under thermolytic and photolytic conditions in the presence of CO, or 4 into 3 in the absence of CO.