Group VB transition metal oxide clusters M4O n −/0 (M = Nb, Ta; n = 8–11): structural evolution and chemical bonding

Abstract

The structural and electronic properties of two series of Group VB transition metal oxide clusters, M4O − and M4O n (M = Nb, Ta; n = 8–11), are investigated using density functional theory calculations. Generalized Koopmans’ theorem is applied to predict the vertical detachment energies and simulate the photoelectron spectra. Large highest occupied molecular orbital–lowest unoccupied molecular orbital gaps are observed for these two stoichiometric M4O10 clusters and estimated to be 3.98 and 4.38 eV for M = Nb and Ta, respectively. The M4O 10 −/0 (M = Nb, Ta) clusters are polyhedral cage structures with high symmetry (T d for the neutral and D 2d for the anion) in which each metal atom joints three bridging and one terminal O atoms. For the Nb oxide species, Nb4O 8 −/0 and Nb4O 9 −/0 can be viewed as removing two and one terminal O atoms from Nb4O 10 −/0 , respectively. The Ta species follow the same rule to the Nb species, except that the anionic Ta4O8 − is formed by removing one terminal and one bridging O atoms from Ta4O10 −. The Ta4O9 containing a localized Ta3+ site can readily react with O2 to form the Ta4O11 which can also be viewed as replacing a terminal oxygen atom in Ta4O10 by a peroxo O2 unit, whereas the added oxygen atom is found to be a bridging one in the O-rich clusters Nb4O 11 −/0 and the anionic Ta4O11 −. Molecular orbital analyses are performed to analyze the chemical bonding in the tetra-nuclear metal oxide clusters and to elucidate their structural and electronic evolution.