Reactions of [H2Os3(CO)10] with a series of diynes, RC2C2R′ (1: R = Ph, R′ = CH 2OH; 2: R = Ph, R′ = C(O)Ph; 3: R = R′ = C(OH)Me 2), have been studied. It was found that upon coordination to the triosmium cluster, the nucleophilic oxygens of the R′ substituents of 1 and 2 take part in intramolecular cyclization reactions to give [HOs 3(CO)10{μ-η1:η2-PhCH 2-(C=CH-C=CH-O)}] (5) and [HOs3(CO) 10{μ-η1:η1-Ph(C=CH-C=CO)CPh}] (6), respectively, both of which contain furan rings coordinated to the cluster core. On heating of the latter compound, the furan moiety remains intact, but a carbonyl group dissociates from the cluster, leading to the formation of [HOs3(CO)9(μ3-η1:η 3:η1Ph(C=CH-C=C-O)CPh)] (7) with a closed "C 3Os3" pentagonal pyramidal structure. Reaction of [H2Os3(CO)10] with 3 does not lead to cyclization of the diyne; instead, the clusters [Os3(CO) 10{μ3-η2-(RCH=CH-C2R)}] (8) and [Os3(CO)10{μ3-η2-(RC 2C2R)}] (9) are formed. Deuterium labeling of the starting compounds has been used in the reaction of [H2O