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.