Ring contraction of six-membered metallabenzynes to five-membered metal-carbene complexes: a comparison with organic analogues.

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.