We elucidate here the nature of the emissive states in Rb 7 Sb 3 Cl 16 nanocrystals (NCs) for which we report a synthesis. These NCs have a crystal structure comprising both isolated [SbCl 6 ] 3− octahedra and isolated [Sb 2 Cl 10 ] 4− dimers of octahedra. The optical properties of Rb 7 Sb 3 Cl 16 NCs are compared with those of Sb-doped Rb 3 InCl 6 NCs, the latter containing only isolated [SbCl 6 ] 3− octahedra. The remarkably similar behaviors of the two systems indicate that the optical emission in both materials originates from the isolated octahedra. Density functional theory calculations suggest that the [SbCl 6 ] 3− octahedra are optically active in emission because the local arrangement of the Rb + ions around the octahedra limits the elongation of the Sb−Cl bonds upon excitation, helping to confine the self-trapped exciton in them. Conversely, in the [Sb 2 Cl 10 ] 4− dimers the constraining effect of the surrounding Rb + ions is less marked and the Sb−Cl bonds fully break upon photoexcitation, opening up an efficient nonradiative channel for the self-trapped excitons.