Question

Why is the ability of oxygen greater than fluorine to stabilise higher oxidation states of transition metals?

Hint:

$\pi$-bonding: O $>$ F for stabilising high oxidation states.

Answer 1

Oxygen can form multiple bonds ($\pi$-bonding) with transition metals.
This effective $\pi$-bond overlap stabilises higher oxidation states through oxo ligands.
Fluorine, although very electronegative, forms only single $\sigma$-bonds and does not form $\pi$-bonds with metals.
Therefore, oxo ions like CrO$_4^{2-}$ stabilise Cr(+6) more than fluorides can.