Halogenation of the cluster Os3(CO)10(μ-SbPh2)2, 2, with PhICl2 or I2 affords the five-membered metallacyclic rings Os3(Cl)2(CO)10(μ-SbPh2)2, 3d and Os3(I)2(CO)10(μ-SbPh2)2, 3d-I, respectively, via oxidative addition across an Os-Os bond. The reaction of 2 with SbPh2Cl or SbPh2H gives the 3,5-fused rings Os3(X)(CO)9(μ-SbPh2)3, (X = Cl, 4a; H, 4b), respectively. Halogenation of 4 with PhICl2 gives the five-membered metallacyclic rings Os3(Cl)(X)(CO)9(SbPh2Cl)(μ-SbPh2)2 (X = Cl, 5a; H, 5b) via oxidative addition across an Os-Sb bond. The analogous reaction with I2 also results in oxidative addition across an Os-Sb bond, but this is followed by displacement of SbPh2I to afford a five-membered metallacycle containing a μ-I ligand, viz., Os3(μ-I)(X)(CO)9(μ-SbPh2)2 (X = Cl, 7a, H, 7b). © 2016 Elsevier B.V. All rights reserved.