The correct sequence of ligands in the order of decreasing field strength is:

This question asks us to identify the correct order of ligand field strength (spectrochemical series) from the given options.

Key Concept: The Spectrochemical Series

The spectrochemical series ranks ligands by their ability to cause crystal field splitting ($$\Delta$$). A "strong field" ligand causes a large splitting, while a "weak field" ligand causes a small splitting. The well-established order (from strongest to weakest) is:

$$CO > CN^- > NO_2^- > en > NH_3 > NCS^- > H_2O > OH^- > F^- > SCN^- > Cl^- > S^{2-} > I^-$$

Checking each option against this series:

Option 1: $$NCS^- > EDTA^{4-} > CN^- > CO$$

This places $$NCS^-$$ as the strongest ligand, which is incorrect. In the spectrochemical series, $$NCS^-$$ is a weaker field ligand than $$CN^-$$ and $$CO$$. Also, $$CO$$ should be at the top, not the bottom. This order is wrong.

Option 2: $$CO > H_2O > F^- > S^{2-}$$

Let us verify each comparison: $$CO$$ is the strongest field ligand in the series. $$H_2O$$ is a moderate field ligand, weaker than $$CO$$. $$F^-$$ is a weak field ligand, weaker than $$H_2O$$. $$S^{2-}$$ is a very weak field ligand, weaker than $$F^-$$. This exactly follows the spectrochemical series. This order is correct.

Option 3: $$S^{2-} > OH^- > EDTA^{4-} > CO$$

This places $$S^{2-}$$ as the strongest, which is incorrect since $$S^{2-}$$ is actually one of the weakest field ligands. This order is wrong.

Option 4: $$OH^- > F^- > NH_3 > CN^-$$

This places $$OH^-$$ above $$NH_3$$ and $$CN^-$$, which contradicts the series where $$CN^- > NH_3 > OH^-$$. This order is wrong.

The correct answer is Option 2: $$CO > H_2O > F^- > S^{2-}$$.