Structure and bonding in dinuclear oxoanions of V, Nb, Ta, Mo, and W

The structure and bonding in [M2O7](n-) anions of the early transition metals V, Nb, Ta, Mo, and W have been investigated by density-functional methods. Several molecular conformations have been tested in geometry optimizations, and systems with a linear M-O-M bridge have been the only structures obtained for V, Nb, and Ta, and the most stable configurations for Mo and W. Molecular-orbital analysis has indicated that multiple bonds are formed between the metal and both bridging (O-b) and terminal (O-t) oxygen atoms. However, it has been found that M-O-b interactions are characterized by bond lengths and bond orders that are typical of a single M-O bond. The results from population analysis (Mulliken charges and Mayer bond indices) have suggested that the repulsive interactions between the ends of the molecules may be a more important structural factor in determining the configuration of the bridge than M-O-b pi bonding is.