Transition metals can form multiple bonds, including $\pi$-bonding, with oxygen. This strong $\pi$-bond overlap stabilizes higher oxidation states via oxo ligands. Despite fluorine's high electronegativity, it only forms single $\sigma$-bonds and does not engage in $\pi$-bonding with metals. Consequently, oxo ions, such as CrO$_4^{2-}$, are more effective at stabilizing Cr(+6) than their fluoride counterparts.