Effect of Transition Metal Fragments on the Reverse Fritsch–Buttenberg–Wiechell Type Ring Contraction Reaction of Metallabenzynes to Metal–Carbene Complexes

Metallabenzynes (1M), contrary to their organic analogues, benzynes, undergo ring-contraction to metal–carbene complexes (2M) via a reverse Fritsch–Buttenberg–Wiechell (FBW) type rearrangement. A detailed computational quantum mechanical study has been carried out to understand the effect of different third row transition metal fragments (ML₂L′₂; M = W, Re, Os, Ir, Pt; L/L′ = PH₃, Cl, CO) on the stability of metallabenzynes and their reactivity toward reverse FBW type rearrangement. Our results indicate that the late transition metal fragments Ir­(PH₃)­Cl₃ and PtCl₄ prefer 16 VE metal–carbene complex (2M), while the middle transition metal fragments W­(PH₃)₄, Re­(PH₃)₃Cl, and Os­(PH₃)₂Cl₂ prefer metallabenzyne (1M). This can be attributed to the reduced overlap between the transition metal fragment ML₂L′₂ and organic fragment C₅H₄ in metallabenzyne 1M when M changes from W to Pt. Furthermore, the presence of a π-accepting ligand CO on the metal fragment makes the conversion of 1M to 2M more feasible.