Detailed structural and theoretical studies of the bonding in edge-bridgedhalide and oxyhalide octahedral niobium and tantalum clusters

New A(x)REM(6)X(18) (A = monovalent cation; RE = rare-earth metal; M = Nb, Ta; X = CL Br; x = 0, 1, 2), A(2)REM(6)X(18-y)O(y) (y = 1, 3) and REM6X13O3 containing edge-bridged octahedral (M6X12i)X-6(a) units are obtained in se aled silica tubes at 700 degrees C. The structures of CsErTa6Cl18 and Cs2UT a6Cl15O3 were established by single-crystal X-ray diffraction. They both cr ystallize in the P (3) over bar 1c space group (a = 9.239(2) Angstrom, c = 17.233(7) Angstrom and a = 9.1824(5) Angstrom, c = 17.146 (2) Angstrom, res pectively). Features which act to stabilize this type of cluster compound a re analyzed. Density functional theory (DFT) calculations were carried out in order to learn more about the relationships that exist between their str uctural arrangement and the number of electrons available for metal-metal b onding in M-6 clusters. In particular, DFT results show that valence electr on counts (VEC) from 14 to 16 are possible for the same octahedral (M6X12X6 a)-X-i arrangement because of a nonbonding (M-M bonding and M-X-i antibondi ng) MO lying in the middle of a large energy gap separating a bonding set o f MOs from an antibonding set of MOs. Replacing one X ligand by a less-hind ered oxygen ligand does not modify much the electronic structure of these s pecies. Stable M6X17O units with different electron counts are theoreticall y possible. In contrast, a larger number of oxygen ligands perturbs the ele ctronic structure, and 14-electron species are likely to be trapped, as exp erimentally observed in the case of compounds containing M6X15O3 units.