# Fluoride anion as a strong field ligand

In the case of $$\ce{[Fe(H_2O)_6]^{3+}}$$ there are $$5$$ unpaired electrons as water acts as a weak field ligand. But instead of flouride acting as a weak field ion in the $$\ce{[MnF_6]^4-}$$ , why does it act as a strong field ligand? In what conditions does it act as a strong field ligand and a weak field ligand?