It behaves for all the world as if the two metals have plus two and a half charges each even though charges only come in units of one! This compound gave rise to a whole field of "mixed valence" ...

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 ...