The lowest oxide of a transition metal is basic whereas the highest is amphoteric or acidic.
Some of the valence electrons of the metal atom are not engaged in bonding in the low oxidation state of the metal. As a result, it has the ability to transfer electrons and act as a base. Valence electrons, on the other hand, are engaged in bonding and are not available in higher oxidation states of the metal. Instead, because the effective nuclear charge is large, it may take electrons and act as an acid.
