Highest oxidation states of transition metals are typically observed only in their:

Oxygen and fluorine are both small, very electronegative and strong oxidising agents. They can stabilise high formal positive charges on the metal by drawing electron density away. Consequently the highest oxidation states (e.g. $\mathrm{Mn^{+7}}$ in $\mathrm{KMnO_4}$, $\mathrm{Os^{+8}}$ in $\mathrm{OsO_4}$) are observed in oxides and fluorides, not in compounds with less electronegative elements.