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)Cl-3 and PtCl4 prefer 16 VE metal-carbene complex (2M), while the middle transition metal fragments W(PH3)(4), Re(PH3)(3)Cl, and Os(PH3)(2)Cl-2 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 pi-accepting ligand CO on the metal fragment makes the conversion of 1M to 2M more feasible.