Transition metals can form stable ions with different oxidation states. But I am confused why doesn't only the most stable state exist. Let me clarify my question more:
$$\ce{Ti^{2+} -> Ti^{3+} + e^-} $$ With $E^0 = +0.5~\mathrm{V}$.
This just shows that $\ce{Ti^{3+}}$ is more stable. So my question is that why doesn't $\ce{Ti^{3+}}$ only exist in its most stable form?