TMNO-activated ligand substitution in the fused ring compounds Os3(μ-SbPh2)2(CO)10, 1, and Os3(μ-SbPh2)3(Cl)(CO)9, 2, in acetonitrile affords the mono- and disubstituted CH3CN-derivatives. The initial product of CH3CN substitution in 1 is an equatorial isomer, which can isomerize to the axial isomer upon heating; they are the first examples in which both isomers have been characterized crystallographically. Subsequent reaction of the CH3CN-derivatives of 2 with two-electron donor ligands L do not always result in the displacement of the acetonitrile ligands; displacement of a carbonyl ligand can also occur and appears to depend on the stereoelectronic properties of L. This is suggestive of an associative mechanism. © 2016 Elsevier B.V.