Arrange the following ligands in increasing order of their field strength:

(A) $SCN^-$
(B) $F^-$
(C) $H_2O$
(D) $CO$

Choose the correct answer from the options given below:

(A), (B), (C), (D)

The correct answer is Option (3) → (A), (B), (C), (D)

This arrangement is based on the Spectrochemical Series, which ranks ligands in increasing order of their ability to split d-orbitals (Crystal Field Splitting Energy, $\Delta_o$).

The Spectrochemical Series

The series is determined experimentally by measuring the absorption spectra of coordination complexes. The general order for these ligands is:

$SCN^-$ (A) < $F^-$ (B) < $H_2O$ (C) < $CO$ (D)

Detailed Breakdown

• (A) $SCN^-$ (Thiocyanate): When bonded through sulfur, it is a weak field ligand and sits at the lower end of the series.
• (B) $F^-$ (Fluoride): Being a halide, it is a weak field ligand. Halides generally cause minimal splitting because they are $\pi$-donors.
• (C) $H_2O$ (Water): Positioned in the middle of the series, water is often considered the boundary between weak field and strong field ligands.
• (D) $CO$ (Carbon Monoxide): This is a strong field ligand. It is at the extreme right of the series due to its ability to engage in $\pi$-backbonding, which greatly increases the crystal field splitting energy.