The reaction of the heteronuclear cluster RuOs3(μ‐H)2(CO)13 with saturated and unsaturated C5 rings led to the aromatization of the rings to afford the clusters RuOs3(μ‐H)(CO)9(μ‐CO)2(η5‐C5R4R′). However, the reaction with cyclopentene gave the cluster RuOs3(μ‐H)3(CO)11(μ,η1:η2‐C5H7) via C–H activation of an sp2 carbon instead. Thermolysis of two of the products, RuOs3(μ‐H)(CO)9(μ‐CO)2(η5‐Cp′) and RuOs3(μ‐H)(CO)9(μ‐CO)2(η5‐C5H4SiMe3), led to their isomerization to butterfly clusters.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)