Question

Find the order of field strength
\( \text{CN}^- \), \( \text{SCN}^- \), \( \text{NCS}^- \), \( \text{S}^{2-} \), \( \text{OH}^- \)

Hint:

A simplified version of the spectrochemical series to memorize is:
\( \text{I}^- < \text{Br}^- < \text{Cl}^- < \text{F}^- < \text{OH}^- < \text{H}_2\text{O} < \text{NH}_3 < \text{en} < \text{CN}^- < \text{CO} \).
Use donor atom types (Halogen \( < \) O \( < \) N \( < \) C) as a rule of thumb.

Answer 1

Step 1: Understanding the Concept:
The field strength of ligands is experimentally determined and arranged in a series called the spectrochemical series. It ranks ligands based on their ability to split the d-orbitals of a central metal ion (\( \Delta_o \)).

Step 2: Key Formula or Approach:
Recall the general order of the spectrochemical series, which loosely follows the donor atom: Halogens \( < \) Sulfur donors \( < \) Oxygen donors \( < \) Nitrogen donors \( < \) Carbon donors.

Step 3: Detailed Explanation:
Let's analyze the given ligands based on their donor atoms and position in the spectrochemical series:
1. \( \text{S}^{2-} \): A pure sulfur donor. It is a very weak field ligand, typically placed near the halogens.
2. \( \text{SCN}^- \) (Thiocyanate): An ambidentate ligand coordinating through Sulfur. Sulfur donors are weak.
3. \( \text{OH}^- \) (Hydroxide): An oxygen donor. It is stronger than sulfur donors but generally considered a weak to intermediate field ligand.
4. \( \text{NCS}^- \) (Isothiocyanate): The same ambidentate ligand, but coordinating through Nitrogen. Nitrogen donors form stronger fields than oxygen donors.
5. \( \text{CN}^- \) (Cyanide): A carbon donor. Carbon donors form very strong \( \pi \)-bonds (synergic bonding) with the metal.
Following the rule (\( \text{S} < \text{O} < \text{N} < \text{C} \)):
\( \text{S}^{2-} < \text{SCN}^- < \text{OH}^- < \text{NCS}^- < \text{CN}^- \).

Step 4: Final Answer:
Increasing order: \( \text{S}^{2-} < \text{SCN}^- < \text{OH}^- < \text{NCS}^- < \text{CN}^- \).