The electron density distribution for a cross-section of the NbF3 and NbO2F unit cells is shown in Fig. . As expected, the charge density is more delocalised in NbO2F due to the more covalent character of the Nb-O bonds. The highly electronegative fluoride, however, results in a more localised charge density for NbF3. The increasing covalency as x decreases corresponds to an increasing anionic contribution to the states near the Fermi level, driving the reduction in minority spin band gap.