Theoretical study of the formation of a benzene cobalt complex from cobaltacyclopentadiene and acetylene

With the B3LYP theoretical method, the reaction of cobaltacyclopentadiene complex with acetylene in singlet and triplet states leading to benzene cobalt complex was studied in detail. In the most favorable path in the sin.-let state, an acetylene molecule attacks cobaltacyclopentadiene from the side, where the vacant d orbital extends over, so that [4 + 2] cycloaddition gives a eta(4)-benzene complex without any activation energy, called the collapse mechanism. The reaction in the triplet state passes through a single transition state with an activation barrier of 14.1 kcal/mol, leading to the eta(6)- benzene complex. The reactant of cobaltacyclopentadiene and the product of the benzene complex in the triplet state are more stable than those in the singlet state, whereas a substantial activation energy is required in the triplet state, suggesting that the spin may change during the reaction. Calculations of the crossing points between the singlet and triplet states showed that in the most favorable reaction path, the spin changes to the singlet state before passing through the triplet transition state, and that the collapse mechanism in the singlet state is followed. The energy required to lead to the crossing point for this spin change was calculated to be 7.0 kcal/mol, which is lower than the activation barrier.