Concept:Weak field ligands cause small crystal field splitting
(Δ) and form high-spin complexes.
Explanation:According to the spectrochemical series, ligands are arranged from weak field to strong field:
I−<Br−<SCN−<Cl−<F−<OH−<C2O42−<H2O<NCS−<EDTA≈NH3<en<NO2−<CN−<COF− lies on the left side of the series, so it is a weak field ligand.
NH3 and EDTA are moderate to strong field ligands.
CO is one of the strongest field ligands.
Therefore, among the given options, fluoride ion is the weak field ligand.
Answer:C. F−