Transition metal oxidation states

[Moogie]

Hi

Why does Ni only form NiO with oxidation state of +2 whereas the other transition metals form a wide range of oxides with variable oxidation states

Thanks

 

[DrDu]

It behaves the same way as do Zn and Copper which are its neighbours.
The reason is that the d-electrons shield each other only badly from the nuclear charge. Hence the effective nuclear charge increases (Google for Slater rules) when the number of d-electrons increases and the energy of the d-electrons relative to the valence s-electrons drops when going in the period from left to right so that they participate no longer in bonding at the end of the period.