The Mott-Hubbard gap and breakdown of the band model

The simple band approach says that a compound with partially filled d orbitals should form a metallic solid. However, this is not the case; a large number of halides, oxides and compounds with less electronegative ligands form non-metallic solids, which have magnetic and spectroscopic properties associated with partially filled levels.

A good example is NiO. Pure NiO is green and shows d-d transitions associated with octahedral ligand-field splitting of the 3d orbitals, as is the case with an isolated ion, eg Ni(H₂O)₆²⁺ (fig. 11)

The magnetic properties reveal two unpaired electrons on each Ni²⁺ ion. There is a strong interaction between neighbouring ions which leads to antiferromagnetic ordering of the spins at lower temperatures. In compounds of this type the d orbitals appear localized as apposed to forming a conduction band like in metallic compounds. This non-metallic behaviour stems from strong electron repulsion between electrons in d orbitals. The band model relies on a simple approximation to deal with electron repulsion which isn’t good enough for many transition metal compounds.