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A theoretical analysis is presented on the structure, bonding nature, and rotational barrier of the Os3(CO)9(CeH6) complex, which has a benzene molecule in a new face-capping u3-y2'-v2'-V2 coordination mode. Using RHF geometry optimization and MP2 calculations, we have estimated the interaction energy between the Os3(CO)9 fragment and the benzene molecule, as well as the benzene rotational barrier with respect to the OS3 triangle (15.9 kcal/mol). An MO analysis leads to an interpretation of the interaction between Os3(CO)9 and benzene in terms of donation and back-donation which are enhanced by the CH bending. The origin of the KekulS distortion of benzene has been attributed to an increase in the back-donation and a decrease in exchange repulsion due to benzene x electrons. The rotational barrier is caused by a decrease in the back-donation and an increase in exchange repulsion due to benzene a electrons. A comparison has been made among the interactions of this polymetallic compound with benzene and ethylene as well as those of a model monometallic fragment with ethylene and benzene.

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