Dependence of the Structure and Stability of Cyclocumulenes and Cyclopropenes on the Replacement of the CH2 Group by Titanocene and Zirconocene: A Density Functional Theory Study

Density functional theory (DFT) studies of the energetics of the isomers of C5H4cyclocumulene (1), ethynylcyclopropene (2), diethynylmethane (3), and their organometallic analogues obtained by replacing the CH2 groups by Cp2Ti and Cp2Zr (4−6)show that the relative energies of the hydrocarbons are dramatically altered in the transition-metal analogues. The metallacyclocumulenes are comparable in energy to 5 and 6. The relative energies also explain the experimental observation that the metallacyclopropene (5) is more favorable for the Ti complex. Substituents on the carbon skeleton help to fine-tune the energetics. The bonding situations in 4 and the recently characterized five-membered zirconacyclopentyne complex 9 are very similar and are in tune with their structures.