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DFT calculations on the transformation reaction of the internal alkynes [CpM(PhCCC 6 H 4 R-p)-(dppe)] + (M = Fe, Os; R = OMe, CO 2 Et, Cl) to the corresponding vinylidenes were carried out. It was found that the isomerization of all complexes of group 8 elements studied in the present work, as well as the Ru complex that had already been reported in our previous study, proceeds via a direct 1,2-aryl shift. For the Fe complex, two types of direct 1,2-aryl shifts (paths 1 and 2), which depend on the orientation of the alkyne/vinylidene parts, were found, and the activation free energy of path 2 is smaller than that of path 1. As for the Os complex, path 1 and another direct 1,2-shift, path 3, were obtained, and path 3 has smaller activation free energy, which is the case for the Ru complex. Therefore, the isomerization reaction of internal alkynes to vinylidenes proceeds through path 2 for the Fe complexes and path 3 for the Ru and Os complexes. The 1,2-migration reactions via path 2 for the Fe complex and path 3 for the Os complex were found to be nucleophilic, which is based on an orbital interaction corresponding to an electron transfer from a carbon on the migrating group to the atom being migrated, as well as for the Ru complexes. To evaluate the stability of the alkyne and vinylidene complexes, orbital interaction energies between an organometallic complex part and an internal alkyne or a vinylidene moiety were calculated by natural bond orbital (NBO) analysis. It was revealed that the Os complex has the strongest interaction, followed by the Ru and Fe complexes. Namely, both the internal alkyne complexes and the vinylidene complexes are more stabilized in a heavier metal complex. The activation free energy for migration of the aryl or phenyl group is actually the lowest for the Fe complex among the three metals. These findings contribute to the development of the synthetic strategy of vinylidenes from internal alkynes. ■ INTRODUCTIONVinylidene is a high-energy tautomer of an alkyne and exhibits higher reactivity in comparison to the parent alkyne. This reactivity facilitates the use of vinylidene as intermediates in catalytic alkyne transformations. It is well-known that vinylidene is stabilized by coordination to a transition metal, and the relative stability of alkyne and vinylidene isomers in the coordination sphere of a transition metal is reversed in many cases. 1 The alkyne/vinylidene isomerization on a transitionmetal complex has been exploited in the synthesis of a wide range of vinylidene complexes. 2 For the development of the use of vinylidene, it is essential to establish a method for synthesizing various vinylidene complexes and to elucidate reaction mechanisms. Great effort has been devoted to both experimental and theoretical approaches to determine the mechanism underlying the transformation of terminal alkynes into the corresponding vinylidene complexes. 3 Three general pathways are suggested for the terminal alkyne/vinylidene rearrangement (Scheme 1).