Valence tautomerism (VT) in transition metal complexes is a fascinating area ... two or more forms that differ in the distribution of electrons and oxidation states, often leading to different ...

The observation of a Mott transition from a metal to an insulator ... This theory relies on the notion of a perfect crystal in which the valence electrons interact with the periodic potential ...

We find that it is the 4s orbital electrons that are ionised first before the 3d orbitals. We have to conclude that, ironically for the d-block elements, the 4s orbitals are in fact higher in energy ...