Use the spectrochemical series

The field strength of ligands is compared using the spectrochemical series. A commonly used order from weak field to strong field is:

Important points:

• $\mathrm{CO}$ and $\mathrm{CN^-}$ are very strong field ligands.
• $\mathrm{H_2O}$ is stronger than $\mathrm{F^-}$.
• $\mathrm{F^-}$ is stronger than $\mathrm{S^{2-}}$.
• $\mathrm{OH^-}$ is stronger than $\mathrm{F^-}$ but weaker than $\mathrm{NH_3}$.

Check each option

Option A

$\mathrm{NCS^- > EDTA^{4-} > CN^- > CO}$

This is incorrect because $\mathrm{CN^-}$ and $\mathrm{CO}$ are among the strongest ligands, so they cannot come after $\mathrm{NCS^-}$.

Option B

$\mathrm{CO > H_2O > F^- > S^{2-}}$

From the spectrochemical series: $\mathrm{CO}$ is very strong field, $\mathrm{H_2O > F^- > S^{2-}}$ Hence this order is correct.

Option C

$\mathrm{S^{2-} > OH^- > EDTA^{4-} > CO}$

Incorrect, because $\mathrm{S^{2-}}$ is a weak field ligand, whereas $\mathrm{CO}$ is one of the strongest.

Option D

$\mathrm{OH^- > F^- > NH_3 > CN^-}$

Incorrect, because actual order is: $\mathrm{F^- < OH^- < H_2O < NH_3 < CN^-}$ So $\mathrm{NH_3}$ and $\mathrm{CN^-}$ should be stronger than $\mathrm{OH^-}$ and $\mathrm{F^-}$.

Final answer

The correct decreasing order of field strength is:

$\boxed{\mathrm{CO > H_2O > F^- > S^{2-}}}$

Therefore, the correct option is B.