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.

Using hybrid density functional theory calculations with the B3LYP functional, the reaction mechanisms for cleavage of R 2 N−CN (R  H, Me) and MeO−CN bonds in the presence of an unsaturated iron(II) silyl complex, CpFe(CO)SiMe 3 , were studied. The following sequence of reactions was shown to be favorable: (i) coordination of a nitrile through the lone pair of electrons on the nitrile N atom (N CN ) to form an end-on complex, (ii) isomerization of the endon complex to a side-on complex, (iii) migration of the silyl group to N CN facilitated by the hypervalent character of the Si atom and its electrostatic attraction with N CN to form a stable Fe−C−N CN three-membered-ring intermediate with an Fe−N CN dative bond, (iv) dissociation of the N CN atom from Fe and coordination of an amino N atom (N NR 2 ) or methoxy O atom to Fe leading to an Fe−C−N NR 2 or Fe−C−O three-membered-ring intermediate, and (v) cleavage of the R 2 N−C or MeO−C bond to form a silyl isocyanide ligand.Step iv possesses the largest activation energy in the sequence of reactions. The activation energies for the reactions of H 2 NCN, Me 2 NCN, and MeOCN were calculated to be 29.9, 28.0, and 19.1 kcal/mol, respectively, on the basis of potential energies with zero-point energy correction. This accounts for the experimental observation that the intermediates formed by silyl group migration can be isolated. The effects of the amino and methoxy groups are discussed by comparing their reaction profiles with that for the reaction of acetonitrile. Localized orbital analysis showed that in the three-membered-ring intermediates formed in step iv, the R 2 N−C and MeO−C bonds are activated by ring strain, whereas the Me−CN bond is activated by interaction of the Me−C bond with the vacant coordination site that is produced in the dissociation of N CN .

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Step for the formation of active catalyst TpIr(η 2 -HCCH) 2 from the pre-catalyst complex.Experimental results for the oxidative coupling of bis(alkyne)Tp Me2 Ir complex to iridacyclopentadiene in the presence of excess water in cyclohexane have shown that the two ethylene ligands or 2,3-dimethylbutadiene ligand in the pre-catalyst Ir complex are substituted by two 1,4-dimethyl-2-butyne-1,4-dioate molecules at temperature in the range of −20 to 25 °C. Furthermore, the use of a higher temperature of 60 °C led to the formation of the substituted iridacyclopentadiene. These results therefore demonstrate that the TpIr(η 2 -acetylene) 2 complex is initially formed as an intermediate during the formation of iridacyclopentadiene by the substitution of the two ethylene or 2,3-dimethylbutadiene ligand(s) in the TpIr(η 2 -alkene) 2 pre-catalyst with two acetylene ligands. These results also suggest that this substitution reaction is a facile process. 21 In actual fact, the substitution reaction for the replacement of butadiene and 2,3-dimethylbutadiene with two acetylene molecules were calculated to be exothermic by 0.8, 3.7 and 4.4 kcal/mol and 0.0, 3.0 and 5.4 kcal/mol, respectively, at the B3LYP/I, B3LYP/II and M06/II levels. * TpIr(η 2 -HCCH) 2 in C s symmetry as a TS and its reactant and product (1a and 1a').The coordination of the HCCH ligand to TpIr in 1a(C s ) resulted in elongation of the C−C triple bond of HCCH from 1.205 Å to 1.273 Å, representing an increase of 0.086 Å (Figure S1). The two Ir−C distances in 1a(C s ) are 2.066 and 2.106 Å, which indicates that one C atom of the alkyne ligand, called α-C atom, is coordinated more strongly to the Ir atom than the other C atom, called β-C atom. Notably, the distance between the two β-C atoms (2.569 Å) was found to be much shorter than the distance between the two α-C atoms (3.021 Å) (Figure S1).The results of frequency and IRC calculations starting from 1a(C s ) showed that this structure is the transition state between 1a and the corresponding enantiomer 1a', which are shown in Figures 1 and S1, respectively. The key difference between the structures of 1a(C s ) and 1a or 1a' can be easily seen in the coordination of the two HCCH ligands to the metal center. While the two HCCH

Using hybrid density functional theory calculations with the B3LYP functional, the reaction mechanisms for cleavage of R2N–CN (R = H, Me) bonds in the presence of unsaturated molybdenum(II) silyl catalyst, Cp(CO)2MoSiMe3 (Cp = η5-C5H5), were studied. The catalytic cycle takes place in two stages; the first involves cleavage of the R2N–CN bond. The favorable sequence of reactions for this stage is as follows: (i) coordination of a nitrile through the lone pair of electrons on the nitrile nitrogen atom (NCN) to give an end-on complex; (ii) isomerization of the end-on complex to a side-on complex; (iii) migration of the silyl group to NCN to form a stable Mo–C–NCN three-membered-ring intermediate with an Mo–NCN dative bond; (iv) dissociation of NCN from Mo and coordination of an amino N atom (NNR2) to Mo, leading to an Mo–C–NNR2 three-membered-ring intermediate; and (v) cleavage of the R2N–C bond to form a silylisocyanide complex. The second stage involves the regeneration of the active catalyst through two σ-metathesis steps. In the first, Cp(CO)2MoNR2 reacts with HSiMe3 to give Cp(CO)2MoH and R2NSiMe3, and in the second, σ-metathesis of Cp(CO)2MoH with HSiMe3 regenerates Cp(CO)2MoSiMe3. Step (iv) in the first stage possesses the largest activation energy and is the rate-determining step. The activation energies for this step for the reactions of H2NCN and Me2NCN were calculated to be 36.4 and 38.3 kcal/mol, respectively, based on potential energies with zero-point energy correction. After dissociation of the silylisocyanide ligand from the silylisocyanide complex, it will be isomerized to silylcyanide, as in previous studies. The catalytic cycle for the cleavage of R2N–CN bond is compared with that of MeO–CN bond. The effects of the metal atoms are also discussed.