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 singlet state, an acetylene molecule attacks cobaltacyclopentadiene from the side, where the vacant d orbital extends over, so that [4 + 2] cycloaddition gives a η4-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 η6-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.

The transformation of bis(acetylene)cobalt complex to cobaltacyclopentadiene complex was studied using a hybrid density functional theory method. B3LYP calculations showed that the reaction of an unsubstituted system, bis(η2-acetylene)cobalt complex, on a singlet potential energy surface is an easy reaction with a small activation energy of 11.2 kcal/mol and an exothermicity of −19.2 kcal/mol. The low activation barrier was as expected for a symmetry-allowed reaction. Because the product of cobaltacyclopentadiene has a low-lying unoccupied orbital, the two Co–Cα bonds are different in distance due to the second-order Jahn–Teller effect, and the triplet cobaltacyclopentadiene is more stable than the singlet cobaltacyclopentadiene, different from the reactant and transition state. In addition, we performed calculations for the reactions of acetylenes substituted by methyl and/or methoxycarbonyl groups, in order to investigate the factors that control the regioselectivity observed in this type of reaction. The calculations for the mono- and disubstituted reactions showed that these substituents prefer α-carbon to β-carbon. We analyzed the origin of this regioselectivity based on the relative stability of the products, to find that it is closely related to the site preference in the substituted butadienes. This suggests that the site preference of substituents is an important factor of regioselectivity.

Isomerization from η 4 -pyridine complex, 5aS to the more stable one, 5bS.As shown in Figure 1, the acetonitrile molecule keeps the interaction with the coordinatively unsaturated cobaltacyclopentadiene to lead to 5aS, in which the C5 and N are the center of the coordination. However, there is more stable isomer of η 4 -pyridine complex 5bS. Thus, we investigated the isomerization of 5aS to 5bS. There are three sound paths for this isomerization. (i) The first path of isomerization.The optimized structures in this path and their ZPE-corrected energies relative to 2S and free acetonitrile are shown in Figure S1.As shown in Figure S1, 5aS isomerizes to another η 4 -pyridine complex 5cS via TS4a, with the activation barrier of 16.3 kcal/mol. TS4a can be considered as an η 2 -pyridine complex and pyridine ring has almost planar structure. The vibration mode with the imaginary frequency in TS4a is for pyridine ring rotation. Starting from TS4a, the IRC calculations, followed by optimization in the reactant and product directions lead to the two η 4 -pyridine complexes 5aS and 5cS with the four atoms of the pyridine ring coordinating to the cobalt atom (N, C1, C4 and C5 4 Figure S1. Profile for the first path of isomerization of 5aS to the most stable isomer η 4 -pyrdine Co(I) complex 5aS. All energies are ZPE-corrected and relative to 2S + CH 3 CN. All bond lengths are in Å.

O−CN bond cleavage of cyanates (ROCN) has been achieved at room temperature in the reaction of ROCN with a methyl Fe, Mo, or W complex. A mechanistic investigation involving DFT calculations revealed that silyl migration from Mo to the CN nitrogen gave an N-silylated η 2 -imidato Mo complex. This intermediate analogue was isolated and characterized by X-ray analysis. Catalytic O−CN bond cleavage was achieved using Cp(CO) 3 MoMe under thermal conditions.