The ring contraction of six-membered metallabenzynes to five-membered metal–carbene complexes in comparison with the corresponding rearrangement of benzyne to cyclopentadienylidene carbene have been studied by computational quantum mechanical calculations. The rearrangement of benzyne to a cyclopentadienylidene carbene is endothermic by 27.8 kcal mol−1. The instability of the cyclopentadienylidene carbene is attributed to the electron deficient singlet carbene carbon atom. However, the isolobal replacement of a sp-hybridized carbon atom in benzyne by a 14 valence electron transition metal fragment (M(PH3)2Cl2, M = Fe, Ru and Os) makes the corresponding rearrangement feasible. The first row transition metal iron shows thermodynamic (exothermic by 19.1 kcal mol−1) and kinetic preference (energy barrier of 1.8 kcal mol−1) towards the carbene complex. However, the preference reduces down the group. The coordination number of the metal as well as the low-lying in-plane M–C π*-MO play a crucial role for this rearrangement. The conversion of metallabenzynes to metal–carbene complexes is also associated with a ring plane rotation to attain an effective overlap between the metal d-orbital and the p-orbital on the carbene carbon atom.
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