The reactions of bis(triphenylphosphine)nitrogen(1+) (PPN) salts of chloride, bromide, and iodide with Os3(CO)12 in the presence of trimethylamine N-oxide give the monoanionic clusters PPN[Os3X(CO)11], where X = Cl, Br, and I, respectively. PPN(N3) reacts directly with Os3(CO)12 to give the isocyanato-containing cluster PPN[Os3(NCO)(CO)11]. Single-crystal X-ray crystallographic analyses of PPN[Os3Br(CO)11] [P21/c space group, a = 14.191 (32) Å, b = 17.986 (12) Å, c = 18.160 (7) Å, β = 90.36 (10)°, Z = 4] and PPN[Os3I(CO)11] [P21/c space group, a = 14.696 (14) Å, b = 19.584 (14) Å, c = 16.604 (11) Å, β = 95.17 (7)°, Z = 4] revealed that the basic Os3L12 structure is preserved. The bromide was located in an axial position, whereas the iodide is coordinated in the equatorial plane. 13C NMR spectroscopy at low temperature revealed that the solid-state structures of [Os3Br(CO)11]- and [Os3I(CO)11]- are maintained in solution and that in both [Os3(NCO)(CO)11]- and [Os3Cl(CO)11]- the anionic ligand resides in an axial position. The lowest temperature fluxional process for X = Cl, Br, and NCO involved complete exchange of all the equatorial carbonyls. This was followed at higher temperatures by a series of trigonal twists and/or pairwise bridge-terminal CO-exchange steps that allowed all carbonyls to exchange with one another. In the iodo cluster, all carbonyl resonances were found to uniformly broaden and coalesce into one average resonance. It is proposed that this occurs when a rate-determining trigonal twist of the OsI(CO)2 group moves the iodide out of the equatorial plane.