Correct order of field strength of given ligands is:

\(H_2O > OH^- > SCN^- > S^{2-}> I^-\)

The correct answer is option 3. \(H_2O > OH^- > SCN^- > S^{2-}> I^-\).

The field strength of ligands affects the splitting of the d-orbitals in metal complexes and is crucial for understanding the color and reactivity of these complexes. Here’s a detailed explanation of the order \(H_2O > OH^- > SCN^- > S^{2-} > I^-\):

Water (\(H_2O\)):

Water is a neutral ligand and is generally considered a weak field ligand.

Effect on d-Orbitals: It causes a moderate splitting of the d-orbitals in the metal ion. It is stronger compared to some anionic ligands like iodide due to its ability to form stronger bonds through lone pair donation.

Hydroxide (\(OH^-\)):

Hydroxide is a weak field ligand. This is due to its negative charge and smaller size, which allows it to interact more strongly with the metal center.

Effect on d-Orbitals: The negative charge of \(OH^-\) increases its field strength relative to neutral water, causing greater splitting of the d-orbitals.

Thiocyanate (\(SCN^-\)):

Thiocyanate can be a strong field ligand, especially when it coordinates through the sulfur atom. It is often considered weaker than hydroxide in terms of field strength.

Effect on d-Orbitals: The field strength can vary depending on whether the sulfur or nitrogen end of the thiocyanate is bound to the metal. In general, \(SCN^-\) is weaker compared to \(OH^-\) when bound through sulfur.

Sulfide (\(S^{2-}\)):

Sulfide is a weak field ligand. The \(S^{2-}\) ion has a high electron density and donates electron pairs readily to the metal ion.

Effect on d-Orbitals: Due to the weak field strength, sulfide causes significant splitting of the d-orbitals, often leading to low-spin configurations in complexes.

Iodide (\(I^-\)):

Iodide is the weakest field ligand among those listed. This is due to its large size and low electronegativity.

Effect on d-Orbitals: The weak field strength of iodide results in minimal splitting of the d-orbitals, making it a weak field ligand compared to the others.

The order \(H_2O > OH^- > SCN^- > S^{2-} > I^-\) reflects the increasing field strength of these ligands:

Water, being neutral, has the highest field strength among these options. Hydroxide, with its negative charge, has a weaker field effect than water. Thiocyanate, which is a weak field ligand, is generally weaker than hydroxide in many cases. Sulfide, with its high electron density and negative charge, has weak field strength among these ligands just higher than iodine. Iodide, due to its large size and low charge density, has the weakest field strength.