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New A x REM6X18 (A = monovalent cation; RE = rare-earth metal; M = Nb, Ta; X = Cl, Br; x = 0, 1, 2), A2REM6X18-y O y (y = 1, 3) and REM6X13O3 containing edge-bridged octahedral (M6Xi 12)Xa 6 units are obtained in sealed silica tubes at 700 °C. The structures of CsErTa6Cl18 and Cs2UTa6Cl15O3 were established by single-crystal X-ray diffraction. They both crystallize in the P3̄1c space group (a = 9.239(2) Å, c = 17.233(7) Å and a = 9.1824(5) Å, c = 17.146 (2) Å, respectively). Features which act to stabilize this type of cluster compound are analyzed. Density functional theory (DFT) calculations were carried out in order to learn more about the relationships that exist between their structural arrangement and the number of electrons available for metal−metal bonding in M6 clusters. In particular, DFT results show that valence electron counts (VEC) from 14 to 16 are possible for the same octahedral M6Xi 12Xa 6 arrangement because of a nonbonding (M−M bonding and M−Xi antibonding) MO lying in the middle of a large energy gap separating a bonding set of MOs from an antibonding set of MOs. Replacing one X ligand by a less-hindered oxygen ligand does not modify much the electronic structure of these species. Stable M6X17O units with different electron counts are theoretically possible. In contrast, a larger number of oxygen ligands perturbs the electronic structure, and 14-electron species are likely to be trapped, as experimentally observed in the case of compounds containing M6X15O3 units.