The new series of oxyhalides M(2)REN(b6)Cl(17)O (M = monovalent cation, RE = rare earths) with 16 valency electrons per Nb-6 cluster is isotypic with MRENb(6)Cl(18) (trigonal, space group R (3) over bar). The crystal structure of Cs2LuNb6Cl17O, a = 9.3863(6) Angstrom, c = 25.664(5) Angstrom, Z = 3, refined from single crystal X-ray diffraction data, indicates that it is based on [((Nb6Cl11Oi)-O-i)Cl-6(a)](5-) units in which one oxygen is in statistical occupancy on the twelve inner positions. The observed interatomic distances are correlated with the VEC, the charge of the unit, and the matrix effect owing to both the oxygen and chlorine ligands surrounding the Nb-6 cluster.