Applications to metal oxides and other inorganic solids

When considering transition metals, the presence of partly filled d orbitals is an additional factor, which must be considered. In some cases the d orbitals are able to overlap to form a band or bands and the material could have a high conductivity. The other situation is when the d orbital overlap is poor and the orbitals are effectively localized on the atoms. This latter situation is shown well by stoichiometric NiO which has low conductivity and there is no evidence suggesting there is reasonable overlap of d orbitals.

The high conductivity situation is shown by TiO and VO. These both have the rock salt structure like NiO, but here the d_xy, d_xz, d_yx orbitals overlap strongly (fig 18) to form a broad t_2g band. This band is partly filled, hence they show almost metallic conductivity. The t_2g band (capacity for 6 electrons) in NiO must be full and the 2 extra electrons of Ni²⁺ are in eg levels, d_z² and d_x2-y2. These orbitals point directly at the oxide ions (fig 18) and due to these oxide ions the eg orbitals on adjacent Ni²⁺ ions are unable to overlap to form a band, hence remain localized on the ions.