AB-INITIO MOLECULAR-ORBITAL STUDY OF THE ELECTRONIC-STRUCTURE AND THEROTATIONAL BARRIER OF BENZENE IN THE HELICOPTER COMPLEX OS3(CO)9(C6H6)

A theoretical analysis is presented on the structure, bonding nature, and rotational barrier of the 083(CO)9(C6H6) Complex, which has a benz ene molecule in a new face-capping mu3-eta2:eta2:eta2 coordination mod e. Using RHF geometry optimization and MP2 calculations, we have estim ated the interaction energy between the Os3(CO)9 fragment and the benz ene molecule, as well as the benzene rotational barrier with respect t o the Os3 triangle (15.9 kcal/mol). An MO analysis leads to an interpr etation of the interaction between Os3(CO)9 and benzene in terms of do nation and back-donation which are enhanced by the CH bending. The ori gin of the Kekule distortion of benzene has been attributed to an incr ease in the back-donation and a decrease in exchange repulsion due to benzene ir electrons. The rotational barrier is caused by a decrease i n the back-donation and an increase in exchange repulsion due to benze ne a electrons. A comparison has been made among the interactions of t his polymetallic compound with benzene and ethylene as well as those o f a model monometallic fragment with ethylene and benzene.