Formation of metallacyclopentadiene from two molecules of acetylene was theoretically investigated.When Cp_2Zr species is used as a metal moiety,this coupling reaction easily takes place with nearly no barrier and significantly large exothermicity.When Pt(Ph_3)_2 is used as a metal moiety,this reaction needs considerably large activation barrier,while this reaction is exothermic.The important interaction in this coupling reaction is the charge-transfer from the occupied d orbital of the metal center to the bonding overlap between pi* orbitals of two acetylenes.The very high reactivity of Cp_2Zr comes from the fact that Cp_2Zr has an occupied d orbital at high energy,which facilitates the coupling reaction.
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